Rozprawy doktorskie na temat „Organic/polymeric Solar Cells”

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 108-110).
The technical and commercial prospects of polymer solar cells were evaluated. Polymer solar cells are an attractive approach to fabricate and deploy roll-to-roll processed solar cells that are reasonably efficient (total PV system efficiency>10%), scalable and inexpensive to make and install (<100 $/m2). At a cost of less than 1$/Wp, PV systems will be able to generate electricity in most geographical locations at costs competitive to coal's electricity (at 5-6 cents/KWh) and will make electricity available to more people around the world (-20% of the world population is without electricity). In this chapter, we explore organic polymer solar cell technology. The first chapter discusses the potential impact of solar cells on electricity markets and the developing world and its promise as a sustainable scalable low carbon energy technology. The second chapter discusses some of the complexity in designing polymer solar cells from new materials and the physics involved in some detail. I also discuss the need to develop new solution processed transparent conductors, cost effective encapsulation and long life flexible substrates. The third chapter discusses polymer solar cells cost estimates and how innovative designs for new modules could reduce installation costs. In the final chapter I discussed the prospects for commercialization of polymer solar cells in several niche markets and in grid electricity markets; the commiseration prospects are dim especially with the uncertainty in the potential improvement in polymer solar cell stability.
by Burhan K. Saif Addin.
M.Eng.

This thesis deals with the device physics of organic solar cells. Organic photovoltaics (OPV) is a field of applied research which has been growing rapidly in the last decade leading to a current record value of power-conversion efficiency of 10 percent. One major reason for this boom is a potentially low-cost production of solar modules on flexible (polymer) substrate. Furthermore, new application are expected by flexible or semitransparent organic solar cells. That is why several OPV startup companies were launched in the last decade. Organic solar cells consist of hydrocarbon compounds, deposited as ultrathin layers (some tens of nm) on a substrate. Absorption of light leads to molecular excited states (excitons) which are strongly bound due to the weak interactions and low dielectric constant in a molecular solid. The excitons have to be split into positive and negative charges, which are subsequently collected at different electrodes. An effective dissociation of excitons is provided by a heterojunction of two molecules with different frontier orbital energies, such that the electron is transfered to the (electron) acceptor and the positive charge (hole) remains on the donor molecule. This junction can be realized by two distinct layers forming a planar heterojunction or by an intermixed film of donor and acceptor, resulting in a bulk heterojunction. Electrodes are attached to the absorber to collect the charges by providing an ohmic contact in the optimum case. This work focuses on the electrical processes in organic solar cells developing and employing a one-dimensional drift-diffusion model. The electrical model developed here is combined with an optical model and covers the diffusion of excitons, their separation, and the subsequent transport of charges. In contrast to inorganics, charge-carrier mobilities are low in the investigated materials and charge transport is strongly affected by energy barriers at the electrodes. The current-voltage characteristics (J-V curve) of a solar cell reflect the electrical processes in the device. Therefore, the J-V curve is selected as means of comparison between systematic series of simulation and experimental data. This mainly qualitative approach allows for an identification of dominating processes and provides microscopic explanations. One crucial issue, as already mentioned, is the contact between absorber layer and electrode. Energy barriers lead to a reduction of the power-conversion efficiency due to a decrease in the open-circuit voltage or the fill factor by S-shaped J-V curve (S-kink), which are often observed for organic solar cells. It is shown by a systematic study that the introduction of deliberate barriers for charge-carrier extraction and injection can cause such S-kinks. It is explained by simulated electrical-field profiles why also injection barriers lead to a reduction of the probability for charge-carrier extraction. A pile-up of charge carriers at an extraction barrier is confirmed by measurements of transient photocurrents. In flat heterojunction solar cells an additional reason for S-kinks is found in an imbalance of electron and hole mobilities. Due to the variety of reasons for S-kinks, methods and criteria for a distinction are proposed. These include J-V measurements at different temperatures and of samples with varied layer thicknesses. Most of the studies of this this work are based on experimental data of solar cells comprisiing the donor dye zinc phthalocyanine and the acceptor fullerene C60. It is observed that the open-circuit voltage of these devices depends on the mixing ratio of ZnPc:C60. A comparison of experimental and simulation data indicates that the reason is a changed donor-acceptor energy gap caused by a shift of the ionization potential of ZnPc. A spatial gradient in the mixing ratio of a bulk heterojunction is also investigated as a donor(acceptor)-rich mixture at the hole(electron)-collecting contact is supposed to assist charge extraction. This effect is not observed, but a reduction of charge-carrier losses at the “wrong” electrode which is seen at an increase in the open-circuit voltage. The most important intrinsic loss mechanism of a solar cell is bulk recombination which is treated at the example of ZnPc:C60 devices in the last part of this work. An examination of the dependence of the open-circuit voltage on illumination intensity shows that the dominating recombination mechanism shifts from trap-assisted to direct recombination for higher intensities. A variation of the absorption profile within the blend layer shows that the probability of charge-carrier extraction depends on the locus of charge-carrier generation. This results in a fill factor dependent on the absorption profile. The reason is an imbalance in charge-carrier mobilities which can be influenced by the mixing ratio. The work is completed by a simulation study of the influence of charge-carrier mobilities and different recombination processes on the J-V curve and an identification of a photoshunt dominating the experimental linear photocurrent-voltage characteristics in reverse bias
Diese Dissertation beschäftigt sich mit der Physik organischer Solarzellen. Die organische Photovoltaik ist ein Forschungsgebiet, dem in den letzten zehn Jahren enorme Aufmerksamkeit zu Teil wurde. Der Grund liegt darin, dass diese neuartigen Solarzellen, deren aktueller Rekordwirkungsgrad bei 10 Prozent liegt, ein Potential für eine kostengünstige Produktion auf flexiblem (Polymer)substrat aufweisen und aufgrund ihrer Vielfältigkeit neue Anwendungsbereiche für die Photovoltaik erschließen. Organische Solarzellen bestehen aus ultradünnen (einige 10 nm) Schichten aus Kohlenwasserstoffverbindungen. Damit der photovoltaische Effekt genutzt werden kann, müssen die durch Licht angeregten Molekülzustände zu freien Ladungsträgern führen, wobei positive und negative Ladung an unterschiedlichen Kontakten extrahiert werden. Für eine effektive Trennung dieser stark gebundenden lokalisierten angeregten Zustände (Exzitonen) ist eine Grenzfläche zwischen Molekülen mit unterschiedlichen Energieniveaus der Grenzorbitale erforderlich, sodass ein Elektron auf einem Akzeptor- und eine positive Ladung auf einem Donatormolekül entstehen. Diese Grenzschicht kann als planarer Heteroübergang durch zwei getrennte Schichten oder als Volumen-Heteroübergang in einer Mischschicht realisiert werden. Die Absorberschichten werden durch Elektroden kontaktiert, wobei es für effiziente Solarzellen erforderlich ist, dass diese einen ohmschen Kontakt ausbilden, da ansonsten Verluste zu erwarten sind. Diese Arbeit behandelt im Besonderen die elektrischen Prozesse einer organischen Solarzelle. Dafür wird ein eindimensionales Drift-Diffusionsmodell entwickelt, das den Transport von Exzitonen, deren Trennung an einer Grenzfläche und die Ladungsträgerdynamik beschreibt. Abgesehen von den Exzitonen gilt als weitere Besonderheit einer organischen Solarzelle, dass sie aus amorphen, intrinsischen und sehr schlecht leitfähigen Absorberschichten besteht. Elektrische Effekte sind an der Strom-Spannungskennlinie (I-U ) sichtbar, die in dieser Arbeit als Hauptvergleichspunkt zwischen experimentellen Solarzellendaten und den Simulationsergebnissen dient. Durch einen weitgehend qualitativen Vergleich können dominierende Prozesse bestimmt und mikroskopische Erklärungen gefunden werden. Ein wichtiger Punkt ist der schon erwähnte Kontakt zwischen Absorberschicht und Elektrode. Dort auftretende Energiebarrieren führen zu einem Einbruch im Solarzellenwirkungsgrad, der sich durch eine Verringerung der Leerlaufspanung und/oder S-förmigen Kennlinien (S-Knick) bemerkbar macht. Anhand einer systematischen Studie der Grenzfläche Lochleiter/Donator wird gezeigt, dass Energiebarrieren sowohl für die Ladungsträgerextraktion als auch für die -injektion zu S-Knicken führen können. Insbesondere die Tatsache, dass Injektionsbarrieren sich auch negativ auf den Photostrom auswirken, wird anhand von simulierten Ladungsträger- und elektrischen Feldprofilen erklärt. Das Aufstauen von Ladungsträgern an Extraktionsbarrieren wird durch Messungen transienter Photoströme bestätigt. Da S-Knicke in organischen Solarzellen im Allgemeinen häufig beobachtet werden, werden weitere Methoden vorgeschlagen, die die Identifikation der Ursachen ermöglichen. Dazu zählen I-U Messungen in Abhängigkeit von Temperatur und Schichtdicken. Als eine weitere Ursache von S-Knicken werden unausgeglichene Ladungsträgerbeweglichkeiten in einer Solarzelle mit flachem Übergang identifiziert und von den Barrierefällen unterschieden. Weiterer Forschungsgegenstand dieser Arbeit sind Mischschichtsolarzellen aus dem Donator-Farbstoff Zink-Phthalozyanin ZnPc und dem Akzeptor Fulleren C60. Dort wird beobachtet, dass die Leerlaufspannung vom Mischverhältnis abhängt. Ein Vergleich von Experiment und Simulation zeigt, dass sich das Ionisationspotenzial von ZnPc und dadurch die effektive Energielücke des Mischsystems ändern. Zusätzlich zu homogenen Mischschichten werden Solarzellen untersucht, die einen Gradienten im Mischungsverhältnis aufweisen. Die Vermutung liegt nahe, dass ein hoher Donatorgehalt am Löcherkontakt und ein hoher Akzeptorgehalt nahe des Elektronenkontakts die Ladungsträgerextraktion begünstigen. Dieser Effekt ist in dem hier untersuchten System allerdings vergleichsweise irrelevant gegenüber der Tatsache, dass der Gradient das Abfließen bzw. die Rekombination von Ladungsträgern am “falschen” Kontakt reduziert und somit die Leerlaufspannung erhöht. Der wichtigste intrinsische Verlustmechanismus einer Solarzelle ist die Rekombination von Ladungsträgern. Diese wird im letzten Teil der Arbeit anhand der ZnPc:C60 Solarzelle behandelt. Messungen der Leerlaufspannung in Abhängigkeit von der Beleuchtungsintensität zeigen, dass sich der dominierende Rekombinationsprozess mit zunehmender Intensität von Störstellenrekombination zu direkter Rekombination von freien Ladungsträgern verschiebt. Eine gezielte Variation des Absorptionsprofils in der Absorberschicht zeigt, dass die Ladungsträgerextraktionswahrscheinlickeit vom Ort der Ladungsträgergeneration abhängt. Dieser Effekt wird hervorgerufen durch unausgeglichene Elektronen- und Löcherbeweglichkeiten und äußert sich im Füllfaktor. Weitere Simulationsergebnisse bezüglich des Einflusses von Ladungsträgerbeweglichkeiten und verschiedener Rekombinationsmechanismen auf die I-U Kennlinie und die experimentelle Identifikation eines Photoshunts, der den Photostrom in Rückwärtsrichtung unter Beleuchtung dominiert, runden die Arbeit ab

One of the key challenges of organic solar cells is their relatively low power conversion efficiency. One way to improve the efficiency of these cells is to develop donor materials with improved photon harvesting capabilities, well-located highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels, good hole transport characteristics and good processability. In this thesis, the design, synthesis and characterization of fifteen low band gap donor-acceptor type polymers are described. Two different acceptor moieties, 3,6- bis(thien-2-yl)-2,5-di-N-alkylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and 2,1,3- benzothiadiazole (BT) were used in our polymer designs and the polymers were synthesised using the palladium-catalysed Stille cross-coupling method. The first series of polymers were random co-polymers of DPP and dithienothiophene. By tuning the solubility and absorption characteristics of the polymers, we achieved a polymer that gave power conversion efficiencies of up to 4.85 % when applied in solar cells. Low open-circuit voltages were obtained for these cells, hence the next series of polymers was designed with the aim of improving the open-circuit voltages. Although the lower HOMO levels of these polymers resulted in higher open-circuit voltages when applied in solar cells, the low hole mobility of the polymers and poor morphology of the polymer:fullerene films resulted in low solar cell power conversion efficiencies. Finally, a series of benzothiadiazole-oligothiophene polymers were synthesised. These polymers had high hole mobilities and wide absorption spectra. When these polymers were applied in organic thin-film transistors, good hole mobilities of up to 0.20 cm2/Vs were achieved, and when applied in solar cells, power conversion efficiencies of up to 6.2 % were achieved. These results show that benzothiadiazoleoligothiophene systems are promising candidates for both transistor and solar cell applications.

In this thesis I have studied organic solar cells (photovoltaic devices) based on a highly self-organizing polymer, regioregular poly(3-hexyIthiophene) (P3HT), because of its particular crystallization tendency leading to high charge carrier mobilities, good light-harvesting in red parts, and suitable energy band structure for an electron-donor. Prior to organic solar cell study, the pristine P3HT films have been investigated to understand their optical/electrical property and nanocrystal structure changes upon thermal annealing. As an electron-acceptor for organic solar cells, two candidates were employed: One is polymer [poly(poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)), another is big small molecule [[6,6]-phenyl Cgi-butyric acid methyl ester (PCBM)]. The kinds of blends used for organic solar cell fabrication were P3HT:F8BT, P3HT:PCBM, and P3HT:PCBM:F8BT. Organic solar cells were fabricated by spin-coating these blend films onto transparent conductive oxide coated substrates followed by depositing metal electrodes (sometimes inserting LiF). For better understanding of device performance changes, blend films have been examined with optical absorption, photoluminescence including time-resloved system, normal reflection mode x-ray diffraction, grazing incidence x-ray diffraction (Synchrotron), atomic force microscopy, scanning electron microscopy, high resolution transmission electron microscopy with field emission gun, transient absorption spectroscopy, and time-of-flight mobility measurement. As a result, P3HT:F8BT solar cells (maximum external quantum efficiency=~3%) showed poorer efficiency than P3HT:PCBM solar cells (maximum external quantum efficiency=~73%), though both blends have P3HT components, which is attributed to the low electron mobility of F8BT compared to PCBM. The power conversion efficiency of P3HT:PCBM solar cells has reached 4.4-5.5% at 85~8.5mW/cm^ (air mass 1.5 simulated solar illumination), which is ascribed mainly to the formation of vertical phase segregation upon thermal annealing leading to pseudo layered p-n junction. This layered structure might reduce the charge recombination between P3HT positive polaron (radical cation) and PCBM negative polaron (radical anion), a parameter that has been quantitatively analysed using a new model proposed in this work.

This Ph.D. Thesis focuses on the investigation of organic photovoltaic (OPV) technology, especially in aspects of experimental device processing, and optoelectronic and electrical characterization on OPV devices to be readily marketable. More specifically, the topics addressed are the following: origin of recombination current,open-circuit voltage and crystallinity, transport driving force, contact selectivity and interface states, alternative hole transporting layers and oxygen and degradation routes.

L'objectif de ce travail était le développement des nanocomposites d’EVOH/zeolites à base de charges telles que les zéolites pour l’encapsulation des cellules solaires organiques, et l’étude de leur comportement photochimique. Ce travail a porté sur l’étude du mécanisme de photooxydation des copolymères d’EVOH puis sur l'impact des zéolites sur ce mécanisme. Les propriétés fonctionnelles des nanocomposites d’EVOH/zéolites ont été étudiées en prenant en compte le taux de charge et la taille des particules. Les propriétés des copolymères d’EVOH et des nanocomposites d’EVOH/zéolites comme la transparence optique, la morphologie de surface, les propriétés mécaniques et thermiques, et les propriétés d'absorption de l'eau ont été étudiées. Sur la base des résultats obtenus, les meilleurs candidats pour l'encapsulation des cellules solaires organiques ont été proposés. Le mécanisme de photooxydation des copolymères a été proposé, la photostabilité des matériaux et l'impact des zéolites sur le comportement photochimique du polymère ont été étudiés. Le test électrique de calcium et le suivi des performances des cellules solaires organiques encapsulées ont été effectués afin d'évaluer l’efficacité des matériaux étudiés comme candidats potentiels pour une encapsulation efficace et stable des cellules solaires
The goal of this work was to develop EVOH/zeolite nanocomposites based on inorganic fillers such as zeolites for potential encapsulation of OSCs and to investigate their photochemical behaviour. The research was focused on the photooxidation mechanism of pristine EVOH copolymers and on the impact of the filler addition on this mechanism. EVOH/zeolite nanocomposites functional properties were characterised taking into account different particle sizes and filler contents. Properties of EVOH copolymers and EVOH/zeolites nanocomposites such as optical transparency, surface morphology, mechanical and thermal properties, and water uptake properties were investigated. On the basis of obtained results, the best candidate(s) for encapsulation of organic solar cells has been proposed. The chemical degradation mechanism of pristine polymers has been proposed, the materials photostability and the impact of the zeolite particles on the photochemical behaviour of the polymer have been studied. Electrical calcium test and performance of encapsulated OSCs were carried out in order to evaluate the ability of the studied materials to be used as potential candidates for efficient and stable encapsulation coatings for OSCs applications

This thesis is dedicated to developing conjugated polymer and small-molecule donor materials for solution-processable organic solar cells. To begin with, a brief introduction of organic solar cells (OSCs) and an overview of donor materials development were presented in Chapter 1. In chapter 2, we used carbon-carbon triple bands as linkage of the TVT unit to develop a new building block, ATVTA. Small molecules S-03, S-04, and S-05 with ATVTA as building block showed broad absorption spectra and low-lying HOMO energy levels. S-01 with TVT unit and S-02 with AT2 as building block were also synthesized for clear comparison. OSCs devices based on S-01 and S-02 showed a Voc of 0.88 V and 0.89 V, respectively. The device based on S-03 exhibited a high Voc of 0.96 V, leading to a PCE of 2.19%. The devices based on S-04 and S-05 afforded a notable Voc over 1.0 V. The results demonstrate that ATVTA unit is a promising building block for extending π conjugation of the molecules without pulling up their HOMO energy levels. Chapter 3 focused on the development of 2D-conjugated small-molecule donor materials. The 2D-conjugated small molecule S-06 possesses excellent solution processability, broad absorption feature, respectable hole mobility and good film-forming morphology. The conjugated thiophene side chain not only effectively extends the absorption spectrum, but also lowers the HOMO energy level, which is desirable for obtaining high Voc. The BHJ OSCs based on S-06:PC70BM (1:0.5, w/w) afforded a high PCE of 4.0% and a notable FF of 0.63 without any special treatment needed. This preliminary work demonstrates that this kind of 2D-conjugated small molecules offer a good strategy to design new photovoltaic small molecule-based donor materials with high FF and Voc for high-efficiency OSCs. The consistently developed two 2D-conjugated small molecules S-07 and S-08 also possess low-lying HOMO energy levels. OSC device based on S-07:PC60BM (1:3, w/w) afforded a notable Voc of 0.96 V, with a PCE of 2.52%. BHJ devices based on S-08 will be fabricated and tested to investigate its photovoltaic properties in the near future. We developed a series of oligothiophenes with platinum(Ⅱ) as the building block in Chapter 4. These small metallated conjugated small molecules exhibited broad spectra and relatively low-lying HOMO energy levels in the range of –5.27 eV to –5.40 eV. Introducing platinum(Ⅱ) arylene ethynylenes as building block can be considered as an approach to obtain small-molecule donors with satisfactory absorption features and HOMO energy levels. Nevertheless, due to the low FF, the PCEs of these donor materials based devices are lower than 2%. Fine tuning the film morphologies of this kind of metallated small-molecule donor materials should be carried out to improve their photovoltaic performance. We addressed an efficient approach to improve the photovoltaic properties by side chain engineering in 2D-conjugated polymers in Chapter 5. Considering the fact that the Voc of PBDTTT based devices is less than 0.8 V, we introduced alkylthio substituent on the conjugated thiophene side chains of the 2D-conjugated copolymer to further improve the photovoltaic performance of the 2D-conjugated copolymers PBDTTTs. The weak electron-donating ability of the alkylthio side chains effectively down-shifted the HOMO energy level of PBDTT-S-TT by 0.11 eV in comparison to the corresponding polymer with alkyl substitution on the conjugated thiophene side chains. The PSC device based on PBDTT-S-TT showed an enhanced Voc of 0.84 V, which is among the highest one in the reported copolymers based on BDT and TT units, leading to an enhanced PCE of 8.42%. The results indicate that molecular modification by introducing alkylthio side chain will be a promising strategy to broaden the absorption, down-shift the HOMO energy level and increase the hole mobility of the low band gap 2D-conjugated polymers for further enhancing the photovoltaic performance of PSCs. PBDTT-O-TT-C and PBDTT-S-TT-C were developed to further study the conclusion. We found that OSC device based on PBDTT-S-TT-C with alkylthio side chain also demonstrated a high Voc of 0.89 V, with a PCE of 6.85% when processed with 3% DIO additive

Organic materials, such as conjugated polymers, have emerged as a promising alternative for the production of inexpensive and flexible photovoltaic cells. As conjugated polymers are soluble, liquid based printing techniques enable production on large scale to a price much lower than that for inorganic based solar cells. Present day state of the art conjugated polymer photovoltaic cells are comprised by blends of a semiconducting polymer and a soluble derivative of fullerene molecules. Such bulk heterojunction solar cells now show power conversion efficiencies of up to 4-6%. The quantum efficiency of thin film organic solar cells is however still limited by several processes, of which the most prominent limitations are the comparatively low mobility and the high level of charge recombination. Hence organic cells do not yet perform as well as their more expensive inorganic counterparts. In order to overcome this present drawback of conjugated polymer photovoltaics, efforts are continuously devoted to developing materials or devices with increased absorption or with better charge carrier transporting properties. The latter can be facilitated by increasing the mobility of the pure material or by introducing beneficial morphology to prevent carrier recombination. Minimizing the active layer film thickness is an alternative route to collect more of the generated free charge carriers. However, a minimum film thickness is always required for sufficient photon absorption. A further limitation for low cost large scale production has been the dependence on expensive transparent electrodes such as indium tin oxide. The development of cheaper electrodes compatible with fast processing is therefore of high importance. The primary aim of this work has been to increase the absorption in solar cells made from thin films of organic materials. Device construction, deploying new geometries, and evaluation of different methods to provide for light trapping and photon recycling have been strived for. Different routes to construct and incorporate light trapping structures that enable higher photon absorption in a thinner film are presented. By recycling the reflected photons and enhancing the optical path length within a thinner cell, the absorption rate, as well as the collection of more charge carriers, is provided for. Attempts have been performed by utilizing a range of different structures with feature sizes ranging from nanometers up to centimeters. Surface plasmons, Lambertian scatterers, micro lenses, tandem cells as well as larger folded cell structures have been evaluated. Naturally, some of these methods have turned out to be more successful than others. From this work it can nevertheless be concluded that proper light trapping, in thin films of organic materials for photovoltaic energy conversion, is a technique capable of improving the cell performance. In addition to the study of light trapping, two new alternative electrodes for polymer photovoltaic devices are suggested and evaluated.

Thin films of the fullerenes PC60BM and PC70BM and the non-fullerene N2200, three popular electron acceptor materials in organic photovoltaics, have been studied, using both the Kelvin probe method as well as ultraviolet photoelectron spectroscopy. With these methods the work function was measured, as well as the highest occupied molecular orbital (HOMO) onset. Additionally band bending effects were studied by illuminating the samples while measuring the work function with the Kelvin probe so called surface photovoltage. Sample of each material was exposed to either air and simulated sunlight or N2 and simulated sunlight, for different length of time, to observe how the materials work function evolves after exposure to the different conditions. It was observed that, as expected from previous studies, that PC60BM was less photo-stable than PC70BM. Additionally, the work function of PC60BM changed significantly by storage in N2. Each material after exposure for 24h to air and light, was annealed and measured with the Kelvin probe. A restoring effect was observed,  for the non-fullerene material N2200. All three materials developed an increasing surface photovoltage, which suggest increased band bending, when exposed to air and light, indicating that due phot-oxidization, charges are redistributed at the surface of the film. The fullerenes showed a larger surface photovoltage effect than the non-fullerene materials. A difference between the work function values obtained from the Kelvin probe method and the ultraviolet photoelectron spectroscopy could be seen, however the exact reason for this couldn't be isolated within this thesis, but was discussed.

Over the last few decades, bulk-heterojunction organic photovoltaic devices comprising an intimately mixed donor-acceptor blend have gained serious attention due to their potential for being cheap, light weight, flexible and environmentally friendly. In this thesis, APFO-3/PCBM bulk-heterojunction based organic photovoltaic devices with an inverted layer sequence were investigated systematically. Doctor blade coating is a technique that is roll-to-roll compatible and cost efficient and has been used to fabricate the solar cells.

Initial studies focused on optimization of the electrodes. A thin film of the conductive polymer PEDOT:PSS was chosen to be the transparent anode. Different PEDOT:PSS films with respect to the film thickness and deposition temperature were characterized in terms of conductivity and transmission. Decent conductance and transmittance were obtained in the films deposited with wet film thickness setting of 35 μm, The cathode was fabricated from a metal bilayer comprising Al and Ti with an area about 1 cm², and the best-working cathodes contained a 70 nm thick Al layer covered by a thin Ti layer of about 10 -15 nm.

Optimized coating temperature and wet film thickness settings for the active layer and PEDOT:PSS layer were experimentally determined. The highest efficiency of the APFO-3/PCBM based inverted solar cells fabricated by doctor blading was 0.69%, which exceeded the efficiency of spin-coated inverted cells.

A higher efficiency (0.8 %) was achieved by adding a small amount of high molecular weight polystyrene to the active layer. Morphological changes after adding of the polystyrene were observed by optical microscopy and AFM. A coating temperature dependent phase separation of the APFO-3/PCBM/polystyrene blend was found.

 

The cost of the present generation of inorganic silicon solar cells is very high and further breakthroughs in cost and efficiency using traditional materials are becoming less and less likely after over 50 years of development. Next generation organic solar cells offer a solution to the limitations of silicon through the vision of low-cost, liquid-based, large area fabrication technology based on polymer and nanomaterials at room temperature. However, most polymers used in solar cells are dissolved in organic solvents such as xylene, toluene, chloroform, and chlorobenzene. Such solvents are harmful to people and environments, leading to higher costs due to complicated waste disposal processing. This is in conflict with the low cost, green, and renewable energy for which we are aiming. To realize a green organic solar cell, a novel solar cell has been created using an environmentally friendly water-soluble thiophene polymer [(Sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate])] (PTEBS) and nanocrystalline TiO2. This novel system has shown great potential in photovoltaics the work has garnered the attention of the international community.In our innovative solar cells, the water-soluble polythiophene (PTEBS) is used as electron donor. Nanoparticle TiO2 acts as electron acceptor. PTEBS/TiO2 solar cells with various structures including bilayer heterojunctions, bulk heterojunctions and a hybrid of bilayer and bulk heterojunctions have been developed and explored. These results are comparable to the best polymer/metal-oxide solar cells reported by other groups using organic solvents.In summary, this is the first time that green solar cells have been fabricated from environmentally friendly water-soluble polymers. By using water as the solvent and utilizing liquid-based processing, the cost of the energy generated by this type of solar cell will be further lowered. In addition, the flexible polymer offers the ease of fabrication and integration into different devices.

Due to the low cost, lightness and flexibility, Polymer Solar Cell (PSC) technology is considered one of the most promising energy technologies. In the past decades, PSCs using fullerenes or fullerene derivatives as the electron acceptors have made great progress with best power conversion efficiency (PCE) reaching 11%. However, fullerene type electron acceptors have several drawbacks such as complicated synthesis, a low light absorption coefficient and poor tuning in energy levels, which prevent the further development of fullerene-based PSCs. Hence the need to have a new class of electron acceptors as an alternative to conventional fullerene compounds. Non-fullerene acceptors (NFAs) have developed rapidly in the last years and the maximum PCEs have exceeded 14% for single-junction cells and 17% for double-junction tandem cells. By combining an electron-donating backbone, generally with several fused rings with electron-withdrawing units, we can simply construct NFA of the acceptor–donor–acceptor type (A–D–A). Versatile molecular structures have been developed using methods such as acceptor motif engineering and donor motif engineering. However, there are only a few electron-donating backbones that have been proved to be successful. Therefore, it is still necessary to develop promising building blocks to further enrich the structural diversity. An indacenodithiophene (IDT) unit with just five fused rings has a sufficiently rigid coplanar structure, which has been regarded as one of the promising electron-rich units to design high-performance A–D–A NFAs. In this work, performed at the King Abdullah University of Science and Technology in Saudi Arabia, a new nine-cyclic building block (TBIDT) with a two benzothiophene unit was synthesized and used for designing new non-fullerene electron acceptors.

Organic solar cells have great potential for cost-effective and large area electricity production, but their applicability is limited by the relatively low efficiency. In this dissertation I report investigations of novel materials and the underlying principles of organic solar cells, carried out at the University of St Andrews between 2011 and 2015. Key results of this investigation: • The charge carrier mobility of organic semiconductors in the active layer of polymer solar cells has a rather small influence on the power conversion efficiency. Cooling solar cells of the polymer:fullerene blend PTB7:PC₇₁BM from room temperature to 77 K decreased the hole mobility by a factor of thousand but the device efficiency only halved. • Subphthalocyanine molecules, which are commonly used as electron donor materials in vacuum-deposited active layers of organic solar cells, can, by a slight structural modification, also be used as efficient electron acceptor materials in solution-deposited active layers. Additionally these acceptors offer, compared to standard fullerene acceptors,advantages of a stronger light absorption at the peak of the solar spectrum. • A low band-gap polymer donor material requires a careful selection of the acceptor material in order to achieve efficient charge separation and a maximum open circuit voltage. • Metal structures in nanometer-size can efficiently enhance the electric field and light absorption in organic semiconductors by plasmonic resonance. The fluorescence of a P3HT polymer film above silver nanowires, separated by PEDOT:PSS, increased by factor of two. This could be clearly assigned to an enhanced absorption as the radiative transition of P3HT was identical beside the nanowires. • The use of a processing additive in the casting solution for the active layer of organic solar cells of PTB7:PC₇₁BM strongly influences the morphology, which leads not only to an optimum of charge separation but also to optimal charge collection.

The molecular design, synthesis, spectroscopic and photophysical characterization of a new series of organic small molecules and transition metal-containing polymers incorporating different n-conjugated chromophores are discussed. The applications of some of these compounds in bulk heterojunction (BHJ) organic solar cells are also outlined. Chapter 1 contains a brief overview on the background of organic solar cells, their structures and performance in solution-processed organic BHJ devices. Chapter 2 presents the synthetic methodology and characterization of a series of new dipyrrin-based materials and their application in organic solar cells. In this section, four metal-based metallopolymers for organic solar cells have been designed, synthesized and two of them have been fabricated for BHJ organic solar cells. Through the alternation of different metal ions and boron element in the same dipyrrin framework, a series of dipyrrin-based metal complexes and BODIPY-containing compounds have been synthesized. Electrochemical analysis and DFT calculations proved that M4 with BODIPY-based structure is more efficient in optimizing the HOMO-LUMO energy level which further increases the Voc value. A full account of the preparation, characterization, photophysical and thermal properties of a new series of benzo[1,2-b:4,5-b']dithiophene (BDT), cyclopenta[2,1-6:3,4-6']dithiophene (CPT) and triphenylamine (TPA) centered small molecules are presented in chapters 3, 4 and 5, respectively. Different acceptor-donor-acceptor (A-D-A) based materials were prepared and employed in organic solar cells in order enhance the power conversion efficiency (PCE) of the devices. Some of the materials have been found to show higher PCEs of up to 3.91%. Given the excellent solution-processability as well as performance advantage, this work provides us a feasible strategy to develop low-cost and high PCE materials in solar cell applications, which would help small molecular organic solar cells to reach a level of practical applications. In chapter 6, four low-bandgap Pt-containing polymers were synthesized and characterized by a variety of techniques. Among them, the largest λonset of 699 nm in solution and λonset of 736 nm in the thin film of P6 were observed and the corresponding energy gap Eg was estimated to be 1.77 eV and 1.68 eV, respectively. After evaluating these oxidation and reduction potentials, P6 also showed the smallest band gap of 1.65 eV with the corresponding HOMO and LUMO energy levels of -5.17 eV and -3.52 eV, respectively. Also, the molecular weights of these polymers were examined by the GPC method. The highest Mn of 24.0 kDa and Mw of 50.4 kDa with the PDI of 2.10 were observed in P8. Chapter 7 and 8 present the concluding remarks and the experimental details of the work described in Chapters 2-6.

Les cellules photovoltaiques proposent une solution au problème énergétique en raison de leur source inépuisable: le soleil. Plusieurs types de cellules, qu'elle soient inorganiques ou organiques, sont étudiées, avec comme objectif d'obtenir de hauts rendements pour de faibles coûts. Dans ce contexte, ce travail de thèse se propose d'étudier des cellules solaires hybrides nanostructurées à base de nanofils de silicium et de matériaux organiques afin de bénéficier des avantages de ces différents matériaux. La morphologie controlée de la croissance des nanofils de silicium par dépôt chimique en phase vapeur assisté par plasma (PECVD) via un procédé Vapeur-Liquide-Solide est présentée. Le mélange de matériaux organiques est ensuite déposé sur les nanofils de silicium par un procédé d'enduction par centrifugation. Dans ce type de cellules hybrides, les nanofils de silicium jouent le rôles de matériaux accepteurs ou aident à l'absorption de la lumière. Pour améliorer les performance de ces cellules, il est nécessaire d'optimiser la qualité du réseau de nanofils par une gravure chimique visant à éliminer les traces de catalyseur résiduelles ainsi que l'oxyde natif du silicium. Cet effet de la gravure a été largement étudié et discuté. De plus les propriétés d'accepteur d'électrons des nanofils de silicium à base de catalyseurs de Bismuth ont été étudiées. Les résultats montrent clairement le potentiel de ce type de cellules, notamment 1) l'augmentation de la conversion de lumière par l'amélioration de l'efficacité du rendement quantique pour les grandes longueurs d'onde, 2) l'utilisation d'une grande variété de nanofils avec des morphologies et propriétés électriques finement controlées
Solar cells are an exciting alternative energy technology due to the infinite energy source, the Sun. Many types of solar cells based on inorganic or organic materials are currently developed with the objective of higher efficiency and lower cost. In this context, this thesis suggests to study nano-structured hybrid solar cells based on silicon nanowires (SiNWs) and organic active materials to benefit advantages of both materials. SiNWs are grown by PECVD on transparent conducting oxide via Vapor-Liquid-Solid (VLS) mechanism with careful control of their nano-morphology. The organic materials made of polymers or blend polymers are then deposited by spin-coating on top of SiNWs. In these hybrid solar cells the SiNWs are used as light-trapping medium and/or electron acceptor material. For better solar cell performance, the optimization of SiNWs array is carried out by removing residual catalyst and etching parasitic hydrogenated amorphous silicon. Their effects on hybrid solar cells have been fully analyzed and discussed. Furthermore, the electron-acceptor properties of the nano-structured SiNWs have been estimated with Bismuth-doped n-type SiNWs. The results clearly reveal the potential of this type of hybrid solar cells, namely, 1) power conversion efficiency improvement by enhancing external quantum efficiency in longer wavelength regime and 2) variety uses of SiNWs by tuning their electrical property and morphology

This thesis describes studies on nanohybrid systems consisting of single-walled carbon nanotubes (SWNTs) with monolayer coatings of semiconducting polymers. Steady-state and time-resolved optical and high-resolution microscopy experiments were used to investigate the blends. These materials show promise for use in organic photovoltaics (OPVs) owing to the high carrier mobilities and large aspect ratios of SWNTs, the controllable solubilisation of tubes with various polymers and the broad light-harvesting abilities of organic polymers. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising carbon nanotubes in OPV devices, revealing poor performances to date. The experimental methods used during the thesis are detailed in Chapter 3 and the solution processing techniques used to prepare the polymer–nanotube blend samples are described in Chapter 4. Chapter 5 describes a study on a nanotube blend with a thiophene polymer, a system previously unsuccessfully implemented into OPV devices. Ultrafast spectroscopic measurements showed that electrons can transfer on a 400 fs time scale from the polymer to nanotubes and the conditions to allow long-lived free charges to be produced were found. The study is extended in Chapter 6 to show that nanostructures consisting of a nanotube coated in one polymer can then be coated by a second polymer and that these nano-engineered structures could be implemented into OPV devices. The use of a competition binding process to isolate purely semiconducting nanotubes dispersed with any desired polymer is then described in Chapter 7. Finally, Chapter 8 introduces systems consisting of chains of porphyrin units, nature’s light-harvesting systems, bound to nanotubes and the blends were found to exhibit the required electronic alignment for use in OPVs. The work described in this thesis provides an explanation for the poor device behaviour of nanotube–polymer blends to date and, in particular, demonstrates several nanohybrid systems that show particular promise for improved OPV applications.

L’objectif final de la thèse est l'impression de la couche photo-active ternaire d'une cellule solaire organique en utilisant deux approches: l'une concerne l'apport de nanotubes de carbone (SWCNT) pour améliorer les propriétés de transport, l'autre concerne la préparation de mélanges ternaires de matériaux pour contrôler la couleur des cellules. Les encres pour la couche active incluant des SWCNT fonctionnalisés sont composées d’un donneur d'électron (polymère) (poly(3-hexylthiophène), [P3HT]) et d’un accepteur d'électron ( [6,6]-phényl C61-butyrique ester méthylique d'acide [PCBM]) et ont été développées pour la fabrication de cellules inversées. Ces cellules sont réalisées sur substrats de verre pour l'optimisation de leurs performances, puis sur substrats plastiques pour les applications. Diverses couches d'interfaces ont été testées, qui incluent l'oxyde de zinc (ZnO, couches obtenues par pulvérisation ionique (IBS) ou à partir de solutions de nanoparticules) pour la couche de transport d'électrons et le PEDOT:PSS, le P3MEET, le V2O5 et le MoO3 pour la couche de transport de trous. Des essais ont été effectués avec et sans CNT afin d’étudier leur impact sur les performances. Des résultats similaires sont obtenus dans les deux cas. Il était attendu que les CNT améliorent les performances, ce qui n’a pas été observé pour le moment. Des travaux supplémentaires sont donc nécessaires au niveau de la formulation de la couche active.Avec trois polymères de couleur rouge (P3HT), bleu (B1) et vert (G1), nous avons préparé des mélanges ternaires efficaces permettant l'obtention de couleurs jusque là indisponibles . Nous avons fait une étude sur le piégeage et les mécanismes de diodes parallèles associés aux mélanges. En général, nous avons constaté que les mélanges ternaires de polymères bleu et vert peuvent être décrits par une mécanisme de diodes parallèles, sans entrainer de perte de performances, ce qui n'est pas possible pour les systèmes P3HT:B1 :PCBM et P3HT:G1:PCBM qui se piègent mutuellement. L’objectif final du projet est l'impression de la couche photo-active ternaire d'une cellule solaire organique, composites ternaires (polymère:polymères:acceptor) ou dopés avec les SWCNT. Cette étape nécessite encore des développements futurs
Two approaches were followed to achieve increased control over properties of the photo-active layer (PAL) in solution processed polymer solar cells. This was accomplished by either (1) the addition of functionalized single-walled carbon nanotubes (SWCNTs) to improve the charge transport properties of the device or (2) the realization of dual donor polymer ternary blends to achieve colour-tuned devices.In the first component of the study, P3HT:PC61BM blends were doped with SWCNTs with the ambition to improve the morphology and charge transport within the PAL. The SWCNTs were functionalized with alkyl chains to increase their dispersive properties in solution, increase their interaction with the P3HT polymer matrix, and to disrupt the metallic characteristic of the tubes, which ensures that the incorporated SWCNTs are primarily semi-conducting. P3HT:PCBM:CNT composite films were characterized and prepared for use as the photoactive layer within the inverted solar cell. The CNT doping acts to increase order within the active layer and improve the active layer’s charge transport properties (conductivity) as well as showed some promise to increase the stability of the device. The goal is that improved charge transport will allow high level PSC performance as the active layer thickness and area is increased, which is an important consideration for large-area inkjet printing. The use of ternary blends (two donor polymers with a fullerene acceptor) in bulk-heterojunction (BHJ) photovoltaic devices was investigated as a future means to colour-tune ink-jet printed PSCs. The study involved the blending of two of the three chosen donor polymers [red (P3HT), blue (B1), and green (G1)] with PC61BM. Through EQE measurements, it was shown that even devices with blends exhibiting poor efficiencies, caused by traps, both polymers contributed to the PV effect. However, traps were avoided to create a parallel-like BHJ when two polymers were chosen with suitable physical compatibility (harmonious solid state mixing), and appropriate HOMO-HOMO energy band alignment. The parallel diode model was used to describe the PV circuit of devices with the B1:G1:PC61BM ternary blend

Negli ultimi decenni, l’uso dei materiali organici per la realizzazione di dispositive elettronici si è guadagnato l’attenzione di molti gruppi di ricercatori. Questo è dovuto principalmente alla possibilità di usare, con questi materiali, tecniche di fabbricazione a basso costo da fase liquida, adatte anche per lo sviluppo di dispositivi su supporto flessibile, e di poter modificare le proprietà dei materiali stessi secondo le esigenze di applicazione. Particolarmente in optoelettronica, l’uso di questi materiali per la realizzazione di sorgenti luminose (OLED, diodo organico emettitore di luce, o OTFL, laser organici a film sottile), foto-diodi e celle solari è stato già dimostrato. In questo contesto, la combinazione di differenti dispositivi organici potrebbe spianare la strada a nuove applicazioni nel campo della comunicazione dati, sensoristica, digitalizzazione di immagini ed energia solare rinnovabile. I dispositivi foto-voltaici a eterogiunzione-bulk di polimeri coniugati depositati da soluzione liquida rappresentano una promessa nel campo della conversione di energia solare e della comunicazione dati, grazie ad una efficienza di conversione fino al ~5%, e una risposta temporale ad una sorgente ottica a ~200KHz.
In the last few decades, the use of organic materials for the realization of electronic devices has gained the attention of many research groups. This is mainly due to the possibility to use low-cost techniques for fabrication as solution-processing, suitable also to flexible substrates, and to tailor the material properties for specific applications. In the field of optoelectronics, the use of such materials for the realization of light sources (OLED, Organic Light-Emitting Diode, or OTFL, Organic Thin-Film Lasers), photo-diodes and solar cells has already been demonstrated. In this context, the combination of different organic devices for integrated optical systems, can pave the way to new applications in the field of data communication, sensing application, imaging and solar energy. Conjugated polymer bulk-heterojunction photo-voltaic device made from blend solution could be a good promise for solar energy conversion and data communication purpose, with its solar conversion efficiencies up to ~5% and a time-resolved response of ~200KHz to an optical source.

Organic solar cells have shown great promise of becoming a cheaper alternative to inorganic solar cells. Additionally, they can also be made semitransparent. To avoid using expensive indium tin oxide electrodes in organic solar cells the electrodes can be made from conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). However, these so-called PEDOT-PEDOT solar cells are prone to short-circuiting. The work behind this thesis thus aimed to find the cause of these short circuits. The initial working hypothesis assumed the hygroscopic PSS in the bottom electrode could attract water across the active layer when the top electrode layer was applied. This would then swell the bottom electrode and cause the active layer to crack leading to short circuits. Accordingly, swelling was investigated as it was suspected to be the main cause of the shorts. This was achieved by coating reflective substrates with different layers from the solar cell, dropping water on top of the stack and then filming the thin film interference effects. SEM, AFM and IR were also used for further analysis. Although the bottom electrode swells, it was found that water does not cause permanent cracking. Instead, the research unveiled that water causes a formation of blisters, which are suspected to be made of PSS. The exact mechanism for the formation of the shorts remains unclear however.

Polymer photovoltaic systems whose photoactive layer is a blend of a semiconducting polymer with a fullerene derivative in a bulk heterojunction configuration are amongst the most successful organic photovoltaic devices nowadays. The three-dimensional organization in these layers (the morphology) plays a crucial role in the performance of the devices. Detailed characterization of this organization at the nanoscale would provide valuable information for improving future material and architectural design and for device optimization. In this thesis, the results of morphology studies of blends of several polyfluorene copolymers (APFOs) blended with a fullerene derivative are presented. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was combined with dynamic Secondary Ion Mass Spectrometry (dSIMS) for surface and in-depth characterization of the blend films. NEXAFS was performed using two different electron detection methods, partial (PEY) and total (TEY) electron yield, which provide information from different depth regimes. Quantitative compositional information was obtained by fitting the spectra of the blend films with a linear combination of the spectra of films of the pure components. In blends of APFO3 with PCBM in two different blend ratios (1:1 and 1:4 of polymer:fullerene) NEXAFS data show the existence of compositional gradients in the vertical direction for both blend ratios, with clear polymer enrichment of the free surface. A series of APFOs with systematic changes in the side-chains was studied and it was shown that those small modifications can affect polymer:fullerene interaction and induce vertical phase separation. Polymer-enrichment of the free surface was clearly identified, in accordance with surface energy minimization mechanisms, and a compositional gradient was revealed already in the first few nanometers of the surface of the blend films. dSIMS showed that this vertical phase separation propagates throughout the film. It was possible to determine that as the polar character of the polymer increases, and thus the polymer:fullerene miscibility is improved, the tendency for vertical phase separation becomes stronger.

Paper II was not published at the time of the licentiate defence and had the title: NEXAFS spectroscopy study of the surface composition in APFO3:PCBM blend films

Els avenços en les tecnologies d'energia renovable encara permeten obtenir bons rendiments amb un cost menor que els preus dels combustibles fòssils. L'energia fotovoltaica (cèl·lules solars) és la tecnologia d'energia solar més desenvolupada pel fet que converteix directament l'energia de la llum solar en electricitat. Les cèl·lules solars orgàniques basades en polímers: materials de fullerne (PSC) es consideren una font d'energia prometedora de baix cost. Avui dia, els PSC han d'exhibir alta eficiència, llarga vida útil, fabricació de baix cost i qualitats amigables amb el medi ambient per a la seva comercialització a gran escala. Aquí es va descriure la fabricació, el modelatge i la caracterització de cèl·lules solars orgàniques basades en polímers amb arquitectura invertida (iPSCs). La tesi se centra en l'estudi de la millora simultània de l'eficiència i l'estabilitat a llarg termini dels iPSCs basats en polímers: fullerens. Els polímers PTB7 i PTB7-Th es van usar com materials donants d'electrons, mentre que el fullereno PC70BM es va usar com a acceptor d'electrons. Es van usar òxid de zinc (ZnO) i òxid de titani (TiOx) com capes de transport d'electrons (ETL), a més es va usar PFN per a propòsits de comparació. Tots els dispositius iPSC es van caracteritzar per mètodes òptics, elèctrics i fotofísics per tal de comprendre els mecanismes de pèrdua involucrats en el procés de degradació. Es van usar models de circuits equivalents per analitzar les característiques de J-V en la foscor i es van usar dades d'impedància espectroscòpica per identificar l'origen dels mecanismes de pèrdua. En aquesta tesi es van demostrar cèl·lules solars orgàniques d'alta eficiència i estables basades en PTB7: PC70BM i PTB7-Th: PC70BM amb arquitectura invertida. A més, el TiOx utilitzat com a capa de transport d'electrons és crucial per millorar l'eficiència i l'estabilitat dels iPSCs. Finalment, també es va demostrar que les capes de ZnO dipositades mitjançant impressió per raig de tinta es poden aplicar amb èxit a la fabricació de iPSCs d'alta eficiència a escala de laboratori.
Los avances en tecnologías de energía renovable permiten obtener buenos rendimientos con un costo menor que los precios de los combustibles fósiles. La fotovoltaica es la tecnología de energía solar más desarrollada debido a que convierte directamente la energía de la luz solar en electricidad. Las células solares orgánicas basadas en polímeros: materiales de fullereno (PSC) se consideran una fuente de energía prometedora de bajo costo. Hoy en día, PSC deben exhibir alta eficiencia, larga vida útil, fabricación de bajo costo y cualidades amigables con el medio ambiente para su comercialización a gran escala. Aquí se describió la fabricación,modelado y caracterización de células solares orgánicas basadas en polímeros con arquitectura invertida (iPSCs). La tesis se centra en el estudio de mejoras simultáneas de la eficiencia y estabilidad a largo plazo de iPSCs basados en polímeros: fullerenos. Los polímeros PTB7 y PTB7-Th se usaron como materiales donantes de electrones, mientras que el PC70BM se usó como aceptor de electrones. Se usaron óxido de zinc (ZnO) y titanio (TiOx) como capas de transporte de electrones (ETL), además se usó PFN para propósitos de comparación. Todos los dispositivos iPSC se caracterizaron por métodos ópticos, eléctricos y fotofísicos con el fin de comprender mecanismos de pérdida involucrados en el proceso de degradación. Se usaron modelos de circuitos equivalentes para analizar las características de J-V en oscuridad y se usaron datos de impedancia espectroscópica para identificar el origen de los mecanismos de pérdida. En esta tesis se demostraron células solares orgánicas de alta eficiencia estables basadas en PTB7: PC70BM y PTB7-Th: PC70BM con arquitectura invertida. Además, TiOx utilizado como capa transportadora de electrones crucial para mejorar la eficiencia y la estabilidad de los iPSCs. Finalmente, también se demostró que las capas de ZnO depositadas mediante impresión por chorro de tinta se pueden aplicar con éxito para fabricación de iPSCs de alta eficiencia a escala de laboratorio.
The advances on the renewable energy technologies still allow obtaining good performances with lower cost than fossil fuel prices. Photovoltaics (solar cells) is the most developed solar energy technology due to converts directly the sunlight energy into electricity. The organic solar cells based on polymer:fullerne materials (PSCs) are considered as a promising low-cost energy source. Nowadays, the PSCs must exhibit high efficiency, long lifetime, low-cost fabrication, and environmentally friendly qualities for their large-scale commercialization. Herein the fabrication, modelling and characterization of polymer-based organic solar cells with inverted architecture (iPSCs) were described. The thesis focuses on the study of the simultaneous improvement of efficiency and long-term stability of iPSCs based on polymer:fullerenes. The polymers PTB7 and PTB7-Th were used as electron donor materials, whereas the fullerene PC70BM was used as the electron acceptor. Zinc oxide (ZnO) and titanium oxide (TiOx) were used as electron transport layers (ETL), moreover PFN was used for comparison purposes. All iPSC devices were characterized by optical, electrical and photophysical methods in order to understand the loss mechanisms involved in the degradation process. Equivalent circuit models were used to analyze the J-V characteristics in the dark and impedance spectroscopy data was used to identify the origin of the loss mechanisms. High efficient and stable organic solar cells based on PTB7:PC70BM and PTB7-Th:PC70BM with inverted architecture were demonstrated in this thesis. Moreover, TiOx used as an electron transport layer is crucial for improving the efficiency and the stability of iPSCs. Finally, it was also demonstrated that ZnO layers deposited by inkjet printing can be successfully applied to the fabrication of high efficiency iPSCs at laboratory scale.

In this thesis, regioregularpoly( 3-hexylthiophene) (rr-P3HT) polymer was used as a light absorption and electron donating material, while the C60 fullerene and its derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were used as electron acceptor materials. The effect of solvent to control the degree of mixing of the polymer and fullerene components, as well as the domain size and charge transport properties of the blends were investigated in detail using P3HT:C60 films. The photo-physical, structural and electrical transport properties of the polymer blends were carried out according to their ratios. A distinctive photoluminescence (PL) quenching effect was observed indicating a photo-induced electron transfer. In this thesis, the effect of solvents on the crystallization and interchain interaction of P3HT and C60 fullerene films were studied using XRD, UV-vis, PL, Raman and FTIR spectroscopy. The polymer blends formed with non-aromatic solvents exhibited an improved crystallinity and polymer morphology than that formed with aromatic solvents. An improved ordering was demonstrated in the polymer films spin coated from non-aromatic solvents. This indicates that the limited solubility of rr P3HT in a marginal solvent such as non-aromatic solvents can offer a strategy to obtain highly ordered crystal structures and lead directly to optimal morphologies on the films.

Organic solar cells show great potential to become a commercially available technology for renewable clean energy production due to their attractive properties. Inexpensive materials and manufacturing processes, including classical roll-to-roll fabrication, as well as the ability to produce flexible, low weight, semitransparent devices are some of the advantages organic photovoltaics provide. Addressing the most common issues in these new technologies, i.e., the low efficiencies of devices and rapid degradation of materials, could bring a realistic alternative for the photovoltaic industry. In this work, the performance of P3HT:PCBM based bulk heterojunction solar cells modified through low energy gold ion implantation in the hole transporting layer, the PEDOT:PSS, is studied. Reference solar cells without gold were also fabricated and characterized for comparison. Through field emission scanning electron microscopy (FESEM) micrographs, the formation of gold nanoparticles (AuNPs) in the PEDOT:PSS has been shown layer for the highest implantation doses used. Absorbance measurements of PEDOT:PSS films before and after gold implantation further confirmed this result. TRIDYN and SRIM simulation programs estimated shallow gold implantations of ~3 nm underneath the PEDOT:PSS films surface. Current-voltage (JxV) characteristics of reference solar cells under AM 1.5 illumination presented the uncommon S-shaped curves, an abnormal deviation from typical JxV curves. This was attributed to PEDOT:PSS degradation due to oxygen and water exposure, which reduced its work function significantly. As a result, deteriorated parallel and series resistances were obtained in reference devices, which ultimately reduced their field factors and power conversion efficiencies. This abnormal behavior was consistently eliminated with the introduction of AuNPs near the PEDOT:PSS/Active-layer interface, leading to the rectification of the illuminated JxV curves of modified solar cells and the reestablishment of cell parameters. Consequently, outstanding improvements in the field factors and power conversion efficiencies were observed in these devices. This was attributed to enhancement (and prevention from the reduction) of the PEDOT:PSS work function layer due to the presence of AuNPs, which rearranged the energy levels at the interface to a more favorable state: higher electron blocking and lower hole extraction barriers.
Células solares orgânicas têm mostrado grande potencial para se tornar uma alternativa tecnológica na produção de energia limpa e renovável. Baixo custo dos materiais e dos processos de manufatura, e a possibilidade de fabricar dispositivos com baixo peso, flexibilidade e semitransparência, inclusive pelo método clássico de roll-to-roll, são algumas das vantagens oferecidas pela fotovoltaica orgânica. Resolver os problemas mais comuns destes dispositivos, como a baixa eficiência na conversão de energia e a rápida degradação dos materiais, é necessário para sua disponibilização no mercado fotovoltaico atual. Neste trabalho, células solares de heterojunção volumétrica baseadas no polímero P3HT e modificadas através da implantação de íons de ouro de baixa energia na camada de PEDOT:PSS são estudadas. Dispositivos equivalentes sem modificação de ouro também foram fabricados e caracterizados como referência. Imagens obtidas através de um microscópio eletrônico de varredura por emissão de campo (FESEM Field Emission Scannig Electron Microscopy) mostraram a formação de nanopartículas de ouro (AuNPs) na camada de PEDOT:PSS para as doses de implantação mais elevadas. Medidas do espectro de absorbância dos filmes de PEDOT:PSS antes e depois da implantação de ouro confirmam este resultado. Simulações feitas com os softwares TRIDYN e SRIM estimaram o ouro implantado em u ma profundidade de ~3 nm abaixo da superfície do PEDOT:PSS. As curvas de corrente-tensão (JxV) características das células solares de referência sob iluminação AM 1.5 mostraram um comportamento de forma S, que corresponde a um desvio da forma típica das curvas JxV. Isto foi atribuído à degradação dos filmes de PEDOT:PSS devido à exposição ao oxigênio e à água, que reduz sua função trabalho significativamente. Como resultado, deterioraram-se as resistências em paralelo e em série destes dispositivos, o que em última instância, reduziu o Field Factor (FF) e a eficiência na conversão de energia. Este comportamento anormal foi eliminado de forma consistente após a introdução de AuNPs perto da interface PEDOT:PSS/Camada-Ativa. As curvas JxV das células solares modificadas sob iluminação foram retificadas e os valores dos seus parâmetros restabelecidos. Melhorias notáveis no FF e eficiência de conversão de energia foram obtidas para todas as células solares modificadas. Isto foi atribuído ao aumento da função trabalho da camada de PEDOT:PSS pela presença das AuNPs, que reorganizou os níveis de energia na interface para um estado mais favorável: com barreiras de potencial otimizadas para bloquear a extração de elétrons e favorecer a de buracos.

The effect of light exposure in ambient air on thin films made from an electron acceptor polymer poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200), an electron donor polymer Poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1) and their blends, has been studied using UV-vis spectroscopy and Atomic Force Microscopy (AFM). For solutions of TQ1, N2200 and blends, the linearity of the Beer-Lambert law for absorption spectroscopy has been verified. The measured UV-vis spectra show that TQ1 thin films are more sensitive to degradation by simulated sunlight than N2200 films. They also show that among the polymer blends, the N2200-rich blend with volume ratio 1:2 (TQ1:N2200) was less sensitive to degradation by simulated sunlight than blends of ratio 1:1 and 2:1. The AFM images showed a change in roughness between the undegraded and degraded films, where the TQ1, 1:1 and 1:2 films obtained lower roughness after 45 hours of degradation, and the N2200 and the 2:1 films obtained higher roughness.
Effekten av simulerad solljusexponering i omgivande luft på tunna filmer gjorda av en elektronaccepterande polymer poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200), en elektrondonerande polymer Poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1) och deras blandningar, har undersökts genom ultraviolett-synlig-spektroskopi (UV-vis-spektroskopi) och atomkraftsmikroskopi (AFM). Genom lösningar av TQ1, N2200 och blandningar, har det linjära förhållandet i Beer Lamberts lag för absorptionsspektroskopi verifierats. De mätta UV-vis-spektrumen visar att tunna TQ1-filmer är känsligare mot degradering genom simulerat solljus än tunna N2200-filmer. De visar också att den N2200-rika blandningen med ett volymförhållande av 1:2 (TQ1:N2200) var mindre känslig för degradering av simulerat solljus än blandningar med volymförhållandet 1:1 och 2:1. AFM-bilderna visade en förändring i råhet mellan degraderade och icke-degraderade filmer, där TQ1-, 1:1-, och 1:2-filmerna fick en lägre ytråhet efter 45 timmar av degradering, och N2200- och 2:1-filmera fick en högre ytråhet.

2013-2014
The main focus of the present work is addressed to the field of organic electronics, which has attracted increasing interest for the development of flexible, large area and low cost electronic applications, from light emitting diodes to thin film transistors and solar cells. The present work describes initially, the application of low-frequency electronic noise spectroscopy for the characterization of organic electronic devices as an innovative and non-destructive technique. In particular the role of the modification induced by thermal stress on the electronic transport parameters under dark conditions of a bulk-heterojunction polymer solar cell have been investigated in detail. The investigated organic solar cell is based on a blend between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C6l-butyric acid methyl ester (PCBM), representing the classical reference structure regarding the polymer:fullerene type devices. Before the irreversible modification of the active layer, the solar cell has been modeled at low frequencies as a parallel connection between a fluctuating resistance RX(t) and a capacitance CX. Under dc biasing, the carriers injected into the active layer modify the equivalent electrical impedance thus changing the noise spectra. The experimental spectral trace can be interpreted by means of a theoretical model based on the capacitance Cμ, which takes into account the excess of minority carriers in the blend, and the device resistance Rrec. The measured electric noise is of 1/f-type up to a cut-off frequency fX, after which a 1/f3 dependence has been observed. The analysis of fX gives information regarding the recombination lifetime of the electrons in the active layer, while the voltage dependence of the Cμ provides information about the density of states for the lowest unoccupied molecular orbital (LUMO) level in the PCBM material. Furthermore, the voltage fluctuations spectroscopy has been used to detect modifications of the active layer due to thermal stress. The temperature has been identified as one of the external parameters that can accelerate the parameter degradation. The analysis of the flicker and the thermal noise at low frequency reveals a decrease of the charge carrier zero-field mobility after a thermal cycle. This effect has been related to morphological changes of the solar cell active layer and the interface between the metal contact and the blend. Moreover, the influence of the solvent additives during the film preparation stage on the electronic transport in the solar cells has been studied by means of noise spectroscopy, and a detailed comparison of the optoelectronic properties of solar cells prepared with different blends has been made. On one side, a P3HT/PCBM based bulk heterojunction solar cell is one of the most prominent candidates for a polymer solar cell, but on the other side, its conversion efficiency is limited by poor longwavelength absorption. One way to increase the conversion efficiency is to modify the active layer absorption by the addition of materials, that increase the absorption of light in the red and infrared spectral region. One of the most promising materials for this task are inorganic quantum dots (QDs). In the present study we choose InP/ZnS quantum dots with an emission peak wavelength of about 660 nm. ... [edited by Author]
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Des de l'aparició de les cèl·lules solars orgàniques (CSO), hi ha hagut una intensa recerca per a aconseguir fer-les tan rentables com les contaminants fonts d'energia tradicionals. Una solució prometedora és el mètode de la heterounió interdigitada, que permet obtenir dispositius amb una extensa interfície Donador-Acceptador per a una correcta dissociació d'excitons. L'avantatge que aquest mètode té sobre el de la heterounió de tipus bulk és que permet que hi hagin camins directes sense interrupció per a la recol·lecció de portadors lliures als elèctrodes. En aquesta tesi hem desenvolupat un procediment numèric complet per a simular les diferents etapes del procés de conversió fotovoltaica a les CSO. Aquest model està basat en el mètode d'elements finits, el qual ens pot aportar informació de cada magnitud en funció de la posició. Aplicant aquest model de simulació, hem realitzat un estudi sistemàtic de CSO interdigitades per a poder predir quines característiques geomètriques seran més apropiades per a optimitzar-les i com augmentar la seva eficiència. El model ha sigut validat amb resultats experimentals, obtinguts en les nostres instal·lacions utilitzant el mètode de síntesi assistit per plantilles amb alúmina anòdica nanoporosa. Finalment, el procediment també ha sigut adaptat i aplicat en d'altres tipus de dispositius i estructures per a demostrar que pot funcionar correctament en altres situacions a banda de les CSO interdigitades. Aquestes han sigut: reproduir resultats experimentals de cèl·lules solars hibrides amb un elèctrode nanostructurat de TiO2, i reproduir l'efecte plasmònic en piràmides de nanoesferes d'or.
Desde la aparición de las células solares orgánicas (CSO), ha habido una intensa búsqueda para conseguir hacerlas tan rentables como las contaminantes fuentes de energía tradicionales. Una solución prometedora es el método de la heterounió interdigitada, el cual permite obtener dispositivos con una extensa interface Donador-Aceptador para una correcta disociación de excitones. La ventaja de este método sobre el de la heterounió de tipo bulk es que permite caminos directos sin interrupción para la recolección de los portadores libres. En esta tesis hemos desarrollado un procedimiento numérico completo para simular las etapas del proceso de conversión fotovoltaica en las CSO. Este modelo está basado en el método de elementos finitos, el cual nos aporta datos de cada magnitud en función de la posición. Aplicando este modelo de simulación, hemos realizado un estudio sistemático de CSO interdigitadas para poder predecir que características geométricas serán más apropiadas para optimizarlas y aumentar su eficiencia. El modelo ha sido validado con resultados experimentales, obtenidos en nuestras instalaciones utilizando el método de síntesis asistido por plantillas con alúmina anódica nanoporosa. Finalmente, el procedimiento también ha sido aplicado a otro tipo de dispositivos y estructuras para demostrar que puede funcionar correctamente en otros casos a parte del de las CSO interdigitadas. Estos han sido: reproducir resultados experimentales de células solares hibridas con un electrodo nanoestructurado de TiO2, y reproducir el efecto plasmónico en pirámides de nanoesferas de oro.
Since the advent of organic solar cells (OSC), there has been an intense search to make them at least as profitable as traditional polluting energy sources. One promising solution is the interdigitated heterojunction approach. This method provides devices with a widespread Donor-Acceptor interface for a proper exciton dissociation. The advantage over the bulk heterojunction approach is that the interdigitated cells provide uninterrupted direct paths for charge carrier collection to the electrodes. In this thesis we have developed a complete numerical procedure to simulate the different steps of the photovoltaic conversion process in organic solar cells. This model is based on the finite element method, which can give us information of every magnitude as a function of the position. By applying this numerical simulation model, a systematic study of interdigitated OSC has been done to predict which geometrical characteristic will be better to optimize this kind of devices and how to increase their efficiency. The model has been validated with experimental results of real devices, obtained in our facilities by using the template-assisted synthesis method with nanoporous anodic alumina templates. Finally, the simulation procedure has also been adapted and applied to other devices and structures to demonstrate that it can work correctly not only for the case of interdigitated full organic solar cells. The simulation model has been applied to two situations: to reproduce experimental results of hybrid solar cells with a nanostructured TiO2 electrode, and to reproduce the plasmonic effect in gold nanospheres pyramids.

Les cellules solaires organiques à hétérojonction en volume (BHJ en anglais) font l'objet d'un grand intérêt car elles représentent une source d'énergie bon marché et renouvelable. Cependant, à cause des rendements généralement bas, ce type de cellule peine à intégrer le marché. Afin d’en augmenter le rendement, contrôler la morphologie des semi-conducteurs dans la BHJ représente un élément clé. Dans ce contexte, il apparaît, dans la littérature, que les additifs solvant permettent de contrôler cette morphologie et d'augmenter les rendements.Cette thèse a pour but de fournir une étude complète sur l'utilisation des additifs. Le couple de semi-conducteurs étudié est le poly(3-hexylthiophene) (P3HT)/[6,6]-phényl-C61butanoate de méthyle (PC61BM).Une première partie présente une méthode développée pour guider la sélection d'additifs parmi une liste de solvants. Cette méthode emploie les paramètres de solubilité de Hansen des semi-conducteurs. Elle est appliquée au système P3HT/PC61BM et résulte en l'identification de trois nouveaux additifs performants. Ensuite, des caractérisations structurales, électriques et optiques sont menées sur la BHJ et permettent d'identifier les effets des additifs. Les effets de ces additifs se révèlent être différents en fonction de l'architecture des dispositifs. L'origine de telles différences est corrélée aux variations de mobilités des porteurs de charge causées par les additifs. Des tests de photo-stabilité ont été menés et montrent que les additifs sont capables d'augmenter la stabilité des cellules solaires. L’origine de telles améliorations est étudiée. Enfin, l'étude est étendue à deux autres nouveaux polymères semi-conducteurs
Polymeric bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted significant interest as a low cost and renewable technology to harvest solar energy. However, their generally low efficiencies are a barrier for their movement into commercial application. Controlling the BHJ morphology is a key step in the pursuit of higher OSC efficiencies. Processing additives have emerged as effective components for optimizing the BHJ morphology. This thesis provides a comprehensive study on the introduction of additives in the formulation of semiconductors. The semiconductor system studied is based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61 BM). First, a method was developed to guide the selection of additives from a large range of solvents. This method employs the Hansen solubility parameters of the semiconductors and was successfully applied to the P3HT/PC61 BMsystem. It resulted in the identification of three new efficient additives. Next, the mechanistic role of additives in influencing the BHJ morphology is investigated by performing structural, electrical and optical characterizations. Also, the effect of additives on OSC performance was found to depend on the type of the OSC architecture. Such differences were correlated to the variations in charge carrier mobilities caused by the additive. Furthermore, photo-stability tests, performed on different types of OSCs, showed that processing additives can improve the photo-stability. The origin of such improvement is investigated. Finally, the scope of this study is extended to two other donor semiconducting polymers

Conjugated polymers are semiconducting materials that are currently being researched for numerous applications from chemical and biological sensors to electronic devices, including photovoltaics and transistors. Much of the novel research on conjugated polymers is performed in academic settings, where scientists are working to prepare conjugated polymers for commercially viable applications. By offering numerous advantages, inherent in macromolecular materials, conjugated polymers may hold the key to cheap and environmentally friendly manufacturing of future electronic devices. Mechanical flexibility, and solvent-based coating processes are two commonly cited advantages. Transitions in the backbone conformation of polythiophenes (PT) in organic solvents have been widely observed to influence thin-film morphology. However, conformational transitions of water-soluble PT derivatives, with respect to their intramolecular versus intermolecular origin, remain largely obscure. Here, conformational transitions of a water- soluble polythiophene in aqueous ionic surfactants are investigated by means of Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), polarizing optical microscopy (POM), ultraviolet-visible (UV-Vis) absorption and fluorescence spectroscopy, and various X-ray scattering techniques. As-prepared complexes exist as stable hydrogels. Upon dilution, a significant time-dependent chromism occurs spontaneously. A coil-to-rod conformational transition is identified in this mechanism and verified using small-angle x-ray scattering (SAXS). Study into the corresponding kinetics demonstrates an inverse first-order rate law. It is found that the conformational transition is thermally reversible and concentration-independent. The critical transition temperature is largely dependent on the surfactant formulation. A theoretical model is presented to explain this new phenomenon and the mechanisms behind its influence on the optoelectronic and solid-state morphological properties. A relationship between the dilute-solution processing with surfactants and the final properties of the system is substantiated.

The world energy consumption is increasing at an average rate of 2.1 % per year, spurred by the economic growth in most of Asian countries, Europe and Canada. The consequent depletion of fossil fuels reservoirs and the reinforced need of environmental sustainability are making the challenge of clean and renewable energy sources one of the most urgent challenges for humankind. Solar power is among the best candidates for the leading role in the energy revolution, being clean, infinite and well distributed over the planet. For this reason, photovoltaic technologies for electricity production are gaining increasing popularity. Although inorganic silicon solar cells dominate the market of photovoltaics, organic and hybrid materials attract considerable interest since their properties like flexibility, light-weight, transparency and low-cost could make the difference in the raising of solar electricity. So far, these materials could not yet outperform conventional silicon, stimulating intensive scientific research on both the development of new materials and the understanding of the photophysical mechanisms governing the photovoltaic behavior of organic/hybrid semiconductors. In this thesis, a series of new organic and hybrid photoactive materials is studied using Electron Paramagnetic Resonance spectroscopy (EPR). This technique, combined with photoexcitation, allows to unambiguously characterize the photoinduced processes involving the formation of paramagnetic states like radicals and triplet states. As shown in the thesis, EPR can also give useful information about molecular ordering in the materials, which is known to be intimately connected with charge transport properties. Conjugated polymers are known for their semiconducting properties and their blends with strong electron accepting fullerene derivatives are among the best performing organic photovoltaic systems. Donor-acceptor alternating copolymers have been introduced to enhance the light-harvesting properties of the blends. Compared to homopolymers, they usually display a lower crystallinity of the deposited films. Thus, XRD techniques are often not suitable to investigate their molecular ordering features. We apply EPR to the analysis of molecular orientational order in the films of two polymers representative of this class, showing that a consistent degree of preferential orientation occurs with two common deposition methods. Fullerene-free materials for polymer solar cells have been recently introduced and overcome some of the drawbacks of fullerene acceptors like the limited absorption and the poor bandgap tunability. In this framework, we study two blends of electron-donor and acceptor polymers to probe their properties with respect to the common fullerene/donor combination, showing that they avoid charge recombination to triplet states which is an active loss mechanism in fullerene-containing blends. Furthermore, the all-polymer films provide a high degree of orientational order and efficient interaction between the donor and acceptor phases that make them promising alternatives to polymer-fullerene blends. A reduced graphene oxide-triphenylamine covalently-linked nanohybrid is studied as potential photosensitizer for TiO2 in dye-sensitized solar cells, able to improve the conductivity and the stability of the system. EPR shows that efficient photoinduced electron transfer from the sensitizer to the semiconductor occurs, paving the way to this new class of photosensitizers. Finally, we investigate the photoactivity of a supramolecular soft-material, forming a gel, composed of small self-assembling donor and acceptor molecules. In this case, EPR allows to verify the efficiency of charge transport across the supramolecular structures, suggesting appealing semiconducting properties of the material. The results of this thesis show the relevance of EPR for unraveling functional and morphological properties of photovoltaic materials and provide a useful characterization of the photophysics of new systems that may be further explored to bring substantial progresses to the field of organic photovoltaics.
Il consumo mondiale di energia ha un tasso medio di crescita del 2.1 % all’anno, trainato dalla crescita economica di molti Paesi asiatici, dell’Europa e del Canada. Il conseguente depauperamento delle risorse di combustibili fossili e il più stringente bisogno di proteggere l’ambiente stanno facendo della sfida delle energie rinnovabili una delle più urgenti sfide che l’umanità deve affrontare. L’energia solare è tra i migliori candidati a svolgere il ruolo di punta nella rivoluzione energetica, essendo una fonte di energia pulita, infinita e ben distribuita nel pianeta. Per questo motivo le tecnologie fotovoltaiche per la produzione di energia elettrica stanno acquistando crescente popolarità. Sebbene le celle solari a base di Silicio dominino il mercato del fotovoltaico, materiali organici e ibridi sono fonte di crescente interesse grazie alle loro peculiari proprietà, come la flessibilità, la leggerezza e la trasparenza, il basso costo, che ci si aspetta possano fare la differenza nell’affermazione del fotovoltaico. Fino ad ora questi materiali non hanno superato il rendimento dei materiali convenzionali a base di Silicio, stimolando la ricerca scientifica verso lo sviluppo di nuovi materiali e lo studio dei meccanismi fotofisici che governano il comportamento fotovoltaico dei semiconduttori organici e ibridi. In questa tesi, una serie di nuovi materiali fotoattivi, organici e ibridi, è stata studiata utilizzando la spettroscopia di Risonanza Paramagnetica Elettronica (EPR). Tale tecnica, combinata con la fotoeccitazione, permette di caratterizzare i processi fotoindotti che portano alla formazione di stati paramagnetici come radicali e stati di tripletto. Come mostrato nella tesi, la tecnica EPR può essere anche utilizzata per ottenere informazioni circa l’ordine molecolare nei materiali, che è noto essere strettamente collegato alle loro proprietà di trasporto di carica. I polimeri coniugati sono noti per le loro proprietà di semiconduttori e le loro miscele con derivati fullereneci - forti electron-accettori - sono tra i sistemi fotovoltaici organici più efficienti. Copolimeri alternanti composti da unità elettron-accettrici e donatrici sono stati introdotti per aumentare l’efficienza di assorbimento dello spettro solare. Rispetto ai classici omopolimeri, questi mostrano solitamente una minore cristallinità dei film depositati. Pertanto, tecniche diffrattometriche si rivelano spesso inadeguate per caratterizzarne l’ordine molecolare. In questa tesi l’EPR viene utilizzato per analizzare l’ordine orientazionale in due polimeri rappresentativi di questa classe, mostrando che un grado consistente di orientazione preferenziale è presente nei film ottenuti con due diverse tecniche di deposizione. Materiali fullerene-free per le celle solari polimeriche sono stati recentemente introdotti per superare alcuni degli svantaggi degli accettori fullerenici, come il limitato assorbimento della luce solare e la difficoltà nel regolare il bandgap e le proprietà elettroniche. In questo conteso, abbiamo studiato due blend costituiti da polimeri elettron-accettori e donatori al fine di investigarne le proprietà e di compararle a quelle dei convenzionali blend di polimeri donatori con derivati fullerenici, dimostrando che essi eliminano la ricombinazione di cariche a formare stati di tripletto, meccanismo noto come fonte di perdita di efficienza nei materiali contenti fullereni. Inoltre, i film polimerici mostrano un elevato grado di ordine orientazionale e un’efficiente interazione tra le fasi di donatore e di accettore che li rendono promettenti alternative ai blend di polimero e fullerene. Un nanoibrido composto da grafene ossido ridotto e molecole di trifenilammina legati covalentemente, è stato studiato come potenziale colorante per la titania in celle solari sensibilizzate a colorante, capace di migliorare la conducibilità e la stabilità del sistema. L’EPR ha mostrato che un efficiente trasferimento elettronico fotoindotto avviene tra l’ibrido e il semiconduttore, aprendo la strada all’applicazione di una nuova classe di coloranti. Infine, la fotoattività di un materiale supramolecolare, un gel composto da piccole molecole di donatore e accettore che autoassemblano, è stata studiata. In questo caso l’EPR ha permesso di verificare un efficiente trasporto di carica attraverso le strutture supramolecolari, suggerendo interessanti proprietà semiconduttive del materiale. I risultati di questa tesi dimostrano la rilevanza dell’EPR per l’indagine su aspetti funzionali e morfologici di materiali fotovoltaici e forniscono una caratterizzazione della fotofisica di nuovi sistemi che potrebbero essere ulteriormente esplorati per apportare progressi sostanziali nel campo del fotovoltaico organico e ibrido.

This thesis demonstrates a comprehensive study of multifunctional applications of low-cost solution-processable organic semiconducting materials. It presents a series of rationally designed predominantly dye based innovative soft semiconductors with their generic optoelectronic properties. The performance of these materials’ application in various devices, including transistors, solar cells, memory devices and displays, are evaluated through world class collaboration to establish the structure-property relationship. In doing so, we not only developed several high-performance materials but also found that fused ring incorporation into the conjugated backbone is an effective strategy to construct multifunctional semiconductors towards flexible and printed electronics.