Dissertations / Theses on the topic 'Methylammonium'

Hybrid organic-inorganic perovskites has attracted much attention for its diverse optoelectronic applications. Many studies point out that hybrid organic-inorganic perovskites compounds have superior physical properties that can enable these materials to fabricate good performance solar cells. However, there is a lack of repeatable recipe for the fabrication of perovskite transistors with high mobilities. In this work, a detailed investigation has been conducted on the fabrication of Methylammonium-based perovskite compounds transistors on various polymer substrates. A group of methacrylate-based polymers has been chosen as the materials for gate dielectric layers. Generally, we found that the growth of perovskite crystals highly depends on the hydrophobicity of the substrates. More hydrophobic polymer layers yield larger crystal growth, but suppress the adhesion of perovskites crystals. Aromatic groups in methacrylate-based polymers have hydrophobic properties but it still gives better compact perovskite films with larger crystals. Poly(phenyl methacrylate) (PPhMA) enables the growth of the best perovskite films. The best performance of MAPbI3-xClx perovskite transistors was fabricated on PPhMA with an electron mobility µsat = 4.30 cm2 V−1 s−1 at 150 K. Photothermal deflection spectroscopy was used to investigate the subgap optical absorptions of the perovskite films.

Philosophiae Doctor - PhD (Chemistry)
Organic-inorganic hybrid perovskite solar cells have emerged as promising materials for next-generation photovoltaics with certified efficiency of 22.1%. Despite rapid developments, achieving precise control over the morphologies of the perovskite films, enhanced stability and reproducibility of the devices remains challenging. In this work, we employed a low-temperature solution processing technique to attain high efficiency inverted planar heterojunction devices with device architecture ITO/PEDOT:PSS/Perovskite/PCBM/Al (indium doped tin oxide; poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; [6,6]-phenyl-C61-butyric acid methyl ester; aluminium). A perovskite solar cell fabrication technique is developed and opto-electronic characterization of solution-processed planar heterojunction perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbI3-xYx (Y = Cl, Br, I) is presented in this thesis work. By employing lead iodide (PbI2) with various amounts of additional methylammonium halides, perovskite precursor solutions were obtained, which were used in the fabrication of four perovskite systems, MAPbI3, MAPbI3-xClx and MAPbI3-xBrx and MAPbBr3. The absorption and photoluminescence (steady state and temperature-dependent) behavior were explored in this compositional space.
2021-08-31

Hybrid perovskite semiconductors from the MAPbX3 family (MA= CH3NH3; X = I, Br, Cl) can be used to make efficient ( > 22 %) solar cells despite disorder introduced by solution processing. Much remains to be understood about these materials. Optical constants of MAPbX3 single crystals derived from ellipsometry measurements are assigned to inter-band transitions from electronic structure calculations. These are used to simulate the contributions from different optical transitions to a typical transient absorption spectrum. The ellipsometry measurements are further used to show the reversible formation of CH3NH3PbI2·H2O and/or (CH3NH3)4PbI6·2H2O in single crystals thin films and devices upon exposure of MAPbI3 to water vapour, which is an important degradation pathway. Quasi-elastic neutron measurements allowed the dynamics of MA cations to be probed in the material. The dipolar MA+ reorientate between preferred alignments with a room temperature residence time of ~14 ps. Collective realignment of MA+ to screen a device’s built-in potential could reduce photovoltaic performance. However, the timescale for a domain wall to traverse a typical device is roughly estimated to be ~0.1 – 1 ms, faster than most observed hysteresis in MAPbI3 solar cells. Temperature dependent Raman and terahertz spectroscopy measurements indicate that MA+ reorientations are crucial to the transport properties of the material. Most of the vibrational features in MAPbX3 observed between 50 and 3500 cm-1 are assigned to calculated vibrational modes. The presence of additional peaks in the experimental spectra might be due to mode splitting caused by dynamic effects. The spectral linewidths of MAPbX3 indicate unusually short phonon lifetimes, linked to its low lattice thermal conductivity. This suggests that optical rather than acoustic phonon scattering prevails at room temperature in these materials, limiting charge mobility. These findings highlight the central role of disorder and heterogeneity to the optoelectronic properties of MAPbX3 and its impact on device behaviour and stability.

In order to limit global warming to 1.5-2 °C deployed solar photovoltaic (PV) power must increase from today's 0.228 terawatts to 2-10 terawatts installed by 2030, depending on demand. These goals require increasing manufacturing capacity, which, in turn, requires lowering the cost of electricity produced by PV. However, high demand scenarios will require greater cost reductions in order to make PV generated electricity as competitive as it needs to be to enable this growth. It is unclear whether established PV technologies — silicon, CdTe, GaAs, or CuInxGa1-xSe2 — can achieve the necessary breakthroughs in efficiency and price. A newer technology known as the 'perovskite solar cell' (PSC) has recently emerged as promising contender.
     In the last seven years the efficiency of PSCs increased by the same amount covered by established technologies in the last thirty. However, PSCs suffer from chemical instability under operating conditions and hysteresis in current-voltage measurements used to characterize power output. Characterizing the defect structures formed by this material and how they interact with device performance and degradation may allow stabilization of PSCs. To that end, this work investigates defects in perovskite solar cells, the impact of these defects on performance, and the effect of alloying and degradation on the electronically active defect structure. Chapter I gives a brief introduction, motivating research in solar cells generally and perovskites in particular as well as introducing some challenges the technology faces. Chapter II gives some background in semiconductors and the device physics of solar cells. Chapter III introduces the performance and defect characterization methods employed. Chapter IV discusses results of these measurements on methylammonium lead triiodide cells correlating defects with device performance. Chapter V applies the some of the same techniques to a series of CH3NH3Pb(I1-xBrx)3 based perovskites aged for up to 2400 hours to explore the impact of alloying and aging on the defect structure. Chapter VI discusses implications for perovskite development and directions for future research.
     This dissertation includes previously published co-authored material.

Submitted by SILVIA LETICIA FERNANDES null (sy.fernandes@hotmail.com) on 2016-05-31T17:30:17Z No. of bitstreams: 1 Tese_Silvia Leticia Fernandes.pdf: 5734164 bytes, checksum: 5485bc4d14d63bd72484d78e0e3a1907 (MD5)
Rejected by Ana Paula Grisoto (grisotoana@reitoria.unesp.br), reason: Solicitamos que realize uma nova submissão seguindo a orientação abaixo: O arquivo submetido está sem a ficha catalográfica. A versão submetida por você é considerada a versão final da dissertação/tese, portanto não poderá ocorrer qualquer alteração em seu conteúdo após a aprovação. Corrija esta informação e realize uma nova submissão contendo o arquivo correto. Agradecemos a compreensão. on 2016-05-31T20:24:02Z (GMT)
Submitted by SILVIA LETICIA FERNANDES null (sy.fernandes@hotmail.com) on 2016-05-31T22:31:21Z No. of bitstreams: 1 Tese_Silvia Leticia Fernandes.pdf: 5362328 bytes, checksum: 3ad1eb46dfedf8840bc516a67f927a4f (MD5)
Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-06-01T14:17:17Z (GMT) No. of bitstreams: 1 fernandes_sl_dr_bauru.pdf: 5362328 bytes, checksum: 3ad1eb46dfedf8840bc516a67f927a4f (MD5)
Made available in DSpace on 2016-06-01T14:17:17Z (GMT). No. of bitstreams: 1 fernandes_sl_dr_bauru.pdf: 5362328 bytes, checksum: 3ad1eb46dfedf8840bc516a67f927a4f (MD5) Previous issue date: 2016-04-27
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
O desenvolvimento das células solares de perovskita foi acompanhado por uma revolução no campo dos dispositivos fotovoltaicos. Células solares de perovskita atingiram eficiências de conversão de energia maiores que 21% em apenas 5 anos após sua descoberta, colocando-as em competição com as células solares comerciais de silício. Apesar de promissores, os dispositivos de perovskita enfrentam desafios que impedem sua comercialização, sendo o maior deles o problema de estabilidade. Nesse âmbito, a presente tese teve como principal foco o desenvolvimento de células solares de perovskita baseadas em filmes nanoestruturados de Nb2O5 e TiO2, visando melhor compreensão do funcionamento desses dispositivos afim de se obter a solução dos problemas hoje enfrentados. Os resultados obtidos mostram eficiências maiores que 13% para o sistema: filme compacto de Nb2O5/ filme mesoporoso de TiO2/ CH3NH3PbI3; e eficiências tão elevadas quanto 15% para sistema usando filme compacto e mesoporoso de TiO2/ CH3NH3PbI3. As melhores células solares montadas com filmes compactos de Nb2O5 apresentaram correntes de curto circuito de 19 mA/cm2, tensão de circuito aberto de 960 mV, fator de preenchimento de 75% e eficiências de 13%. Para as células formadas com filmes de compactos de TiO2 foram obtidas correntes de curto circuito de 20 mA/cm2, tensão de circuito aberto de 1V, fator de preenchimento de 70% e eficiências de 15%. A estabilidade dos dispositivos e a presença de histerese nas curvas de tensão-corrente foram estudadas variando parâmetros como a composição da camada compacta (TiO2 versus Nb2O5), a espessura dessa camada; assim como o método de síntese utilizado para preparar os filmes de perovskitas (método de deposição sequencial versus método de engenharia dos solventes). De maneira geral, os resultados mostraram que células solares preparadas com filmes de Nb2O5 de 50 nm em conjunto com perovskitas preparada pelo método de deposição sequencial resultaram em dispositivos sem histerese e com maior estabilidade do que os preparados com filmes de TiO2 ou mais espessos que 50nm.
The development of perovskite solar cells was accompanied by a revolution in the photovoltaics field. Perovskite solar cells have reached higher energy conversion efficiencies of 21% in just 5 years after its discovery, putting them in competition with commercial silicon solar cells. Although promising, the perovskite devices face some challenges which delay their commercialization, and one of most important is the stability. In this context, the present thesis intended the development of perovskite solar cells based on nanostructured films of Nb2O5 and TiO2, in order to better understand the functioning of these devices. Efficiencies up to 13% were obtained for the system composed of: compact Nb2O5 / mesoporous TiO2/ CH3NH3PbI3 and efficiencies as high as 15% for compact system using compact TiO2/ mesoporous TiO2/ CH3NH3PbI3. The best solar cells prepared using compact Nb2O5 films showed a short circuit current of 19 mA/cm 2 , open circuit voltage of 900 mV, fill factor of 75% and 13% of efficiency. Devices prepared using compact TiO2 films reached short circuit current of 20 mA/cm2 , open circuit voltage of 1V, fill factor of 70% and 15% of efficiency. The stability of the devices and the presence of current-voltage hysteresis were studied by changing parameters such as the composition and the thickness of the compact layer (TiO2 vs. Nb2O5), as well as the synthesis method used to prepare the perovskite films (sequential deposition method vs solvent-engineering method). Overall, the results showed that solar cells prepared with 50 nm Nb2O5 film in combination with perovskite prepared by sequential deposition method have resulted in devices without hysteresis and greater stability than those prepared with TiO2 films or thicker than 50nm.
FAPESP: 2012-07745-9

The work deals with the theory of preparing perovskite solar cells. How about basic structures and the specific types of training opportunities and reproducibility of results. In the third part describes the complete preparation of the article, which reached the highest efficiency and the procedure for subsequent repetition of the experiment

This thesis studies the degradation of perovskite photovoltaic cells placed in atmospheres of different moisture. Samples with inverse structure: ITO/ PEDOT:PSS/ CH3NH3PbI3–XClX / PC70BM/ Ca/ Al were prepared. Electrical characteristics were measured for 2 months and similar degradation trend was observed for all the samples. Perovskite cell efficiency PCE decreased to 20 % of the initial value in t80= 46±3 days in laboratory, t80=23±1$ days in nitrogen atmosphere and t80=25,7±0,6 dní days in dry atmosphere. For the initial 27 days of the experiment, a faster degradation linked with the decrease of ISC, FF and VOC was observed. After this period, the value of FF has stabilised at 0,777±0,009 % and the value of VOC at 0,70±0,02 % of their original value (in the laboratory atmosphere). Additional fall of PCE resulted only from the decrease of ISC and was slower than in the initial period. From the results acquired, it has emerged that during the experiment, the cell encapsulation provided a sufficient barrier against outer moisture. Residual moisture present in the sctructure, was labeled as the source of the degradation. The moisture is believed to enter with the hygroscopic material PEDOT:PSS during the samples preparation procedure. During the degradation, absorption measurements of photovoltaic cells were executed. The absorption spectra didn't change. That indicates, that the decrease of ISC is not caused by the reduction of light absorption. The thesis also studied the degradation of perovskite solar cells under illumination. The samples were exposed to UV radiation for 55,5 hours. The PCE time of decrease to 20 % of the initial value was t80 = 6±2 days. It was revealed, that UV radiation significantly accelerates the decrease of ISC.

Der Schwerpunkt der vorliegenden Arbeit liegt in der Charakterisierung der elektronischen Eigenschaften von hybriden organisch-anorganischen Perowskit (HOIP)-Schichten während der Schichtbildung und in verschiedenen Umgebungen mittels Photoelektronenspektroskopie (PES). Insbesondere wird der Methylammonium-Blei-Iodid-Chlorid-Perowskit (MAPbI3-xClx) untersucht. Als erstes werden Änderungen in den elektronischen Eigenschaften, der Zusammensetzung, sowie der Kristallstruktur mittels PES, Flugzeit-Sekundärionenmassenspektrometrie, sowie Röntgendiffraktometrie mit streifendem Einfall analysiert. Die Resultate weisen auf die entscheidende Rolle von Chlor im texturierten Wachstum der Perowskitschicht hin. Die auskristallisierte Perowskitschicht weist eine stärkere n-Typ Eigenschaft auf, welche auf die Änderung der Zusammensetzung während der Schichtbildung zurückgeführt werden kann. Außerdem beweisen die Ergebnisse eindeutig die Ablagerung von Chlor an der Grenzfläche zwischen der Perowskitschicht und dem Substrat. Zweitens werden die separaten Einflüsse von Wasser, Sauerstoff, und Umgebungsluft auf die elektronischen Eigenschaften von MAPbI3-xClx-Schichtoberflächen untersucht. Bereits geringste Wassermengen ähnlich wie im Hochvakuum oder in inerter Umgebung können eine reversible Reduzierung der Austrittsarbeit hervorrufen. Höherer Wasserdampf-Partialdruck führt zu einer Verschiebung des Valenzbandmaximums (VBM) weit vom Fermi-Niveau, sowie zu einer Reduzierung der Austrittsarbeit. Im Gegensatz dazu führt eine Sauerstoffexposition zu einer Verschiebung des VBM in Richtung des Fermi-Niveaus und zu einer Steigerung der Austrittsarbeit. Analog kommt es zu einer Verschiebung von bis zu 0.6 eV bei einer Exposition gegenüber Umgebungsluft, was den vorwiegenden Einfluss von Sauerstoff demonstriert. Die vorliegenden Untersuchungen betonen den kritischen Einfluss der Schichtbildung, der Zusammensetzung, sowie der Umgebungsbedingungen auf die elektronischen Eigenschaften von HOIP.
The present thesis aims at characterizing the electronic properties of solution-processed hybrid organic-inorganic perovskites (HOIPs) in general, and the HOIP methyl ammonium (MA) lead iodide-chloride (MAPbI3-xClx) films, in particular, at different stages, namely from its formation to its degradation, by means of photoelectron spectroscopy (PES). Firstly, the formation of MAPbI3-xClx films upon thermal annealing is monitored by a combination of PES, time-of-flight secondary ion mass spectrometry, and grazing incidence X-ray diffraction for disclosing changes in electronic properties, film composition, and crystal structure, respectively. Overall, the results point to the essential mediating role of chlorine in the formation of a highly textured perovskite film. The film formation is accompanied by a change of composition which leads to the film becoming more n-type. The accumulation of chlorine at the interface between perovskite and the underlying substrate is also unambiguously revealed. Secondly, the separate effects of water and oxygen on the electronic properties of MAPbI3-xClx film surfaces are investigated by PES. Already low water exposure – as encountered in high vacuum or inert conditions – appears to reversibly impact the work function of the film surfaces. Water vapor in the mbar range induces a shift of the valence band maximum (VBM) away from the Fermi level accompanied by a decrease of the work function. In contrast, oxygen leads to a VBM shift towards the Fermi level and a concomitant increase of the work function. The effect of oxygen is found to predominate in ambient air with an associated shift of the energy levels by up to 0.6 eV. Overall, the findings contribute to an improved understanding of the structure-property relationships of HOIPs and emphasize the impact of least variation in the environmental conditions on the reproducibility of the electronic properties of perovskite materials.

Ces onze dernières années ont vu apparaitre les pérovskites organiques inorganiques hybrides (HOIPs) comme un passionnant domaine de recherche pour leur application potentielle dans les technologies du photovoltaïque (PV) en raison de leurs exceptionnelles propriétés optoélectroniques et de leur facilité de mise en oeuvre. Cependant, les matériaux HOIPs ont plusieurs inconvénients dont leur manque de stabilité en conditions opérationnelles. Améliorer celle-ci est l'un des plus grands défis à relever avant commercialisation. La formule générale est (A1,A2,A3,A4)Pb(X1,X2)3, où les sites A occupés par une distribution de 1 à 4 cations métalliques/organiques et les sites X par celle d’anions halogénures. Les défauts lacunaires natifs sont considérés comme une cause possible de dégradation des cellules solaires HOIPs. L'objectif de ce travail est de comprendre le rôle des défauts dans la stabilité à long terme des matériaux PV HOIPs. A cette fin, des défauts primaires ont été introduits de manière contrôlée par irradiation avec des électrons de haute énergie (1MeV) dans des lots de couches et cellules solaires (SCs) à base de divers composés HOIPs. Il s'agit notamment du prototype PV HOIPs, MAPbI3 (A1PbX13), et de nouveaux composés mixtes d’halogénures à triple ou quadruple cations, (CsMAFA)Pb(I1-xBrx)3 (A3PbX23) ou (GACsMAFA)Pb(I1-yBry)3 (A4PbX23). Les couches sont fabriquées selon la même procédure que les couches actives SCs et, ensuite, traitées dans des conditions similaires. Pour A1PbX13/A3PbX23, la structure SC est de type p-i-n avec des couches organiques pour le transport des trous et des électrons (HTL/ETL). Les couches sont déposées sur le substrat verre/ITO/HTL (PEDOT:PSS) sans ou avec couche supérieure ETL (PCBM). Pour A4PbX23, la structure SC est de type n-i-p avec des couches ETL inorganiques (TiO2) et HTL organiques (Spiro-OMeTAD). Les couches sont directement déposées sur du verre.La spectroscopie d'annihilation de positons donne une évidence directe de l'existence de défauts lacunaires natifs et induits par irradiation dans chaque composé. Les spectres d’absorbance en fonction de l’énergie montrent que le vieillissement naturel et après irradiation génère différentes populations de défauts dans chaque composé. De plus, celles-ci pour A1PbX13 et A3PbX23 diffèrent selon l'absence ou la présence de la couche supérieure ETL. Les populations de défauts évoluent pendant au moins 3 mois. Le vieillissement modifie (i) la bande interdite, (ii) les queues de bande de conduction/valence et (iii) l'absorption optique via des niveaux électroniques profonds. Les effets d’illumination sous laser varient aussi en fonction du vieillissement. L’asymétrie des pics de photoluminescence (PL) dans chaque composé sous illumination laser continue reflète une superposition de raies d’émission gaussiennes à énergie, FWHM et hauteur évoluant avec le temps d'illumination. Les transitions d'émission impliquent des niveaux électroniques localisés peu profonds dans A3PbX23/A4PbX23 et résonnants dans A1PbX13. De tels effets durent au moins 3 mois dans A4PbX23. Ces niveaux électroniques sont attribués à des populations de défauts spécifiquement induits par illumination. Le vieillissement naturel et après irradiation donne des spectres PL à décroissance temporelle résolue en une ou deux exponentielles. Le nombre et la durée de vie sont fortement influencés par l’irradiation initiale et la composition. Une amélioration frappante du fonctionnement PV pour le type SC p-i-n est induite par le vieillissement dû à l'irradiation. Le rendement quantique externe et les performances PVs ont des valeurs plus élevées pour l’état irradié que de référence durant 6 à 12 mois de vieillissement. Cela prouve que l'ingénierie des défauts par irradiation d'électrons à haute énergie a le potentiel de fournir des voies de traitement innovantes pour améliorer la stabilité à long terme des performances photovoltaïques HOIPs
During the last eleven years, Hybrid Organic Inorganic Perovskites (HOIPs) materials have emerged as an exciting topic of research for potential application in solar cell technologies due to their outstanding optoelectronic properties and processing advantages. However, HOIPs materials suffer from several drawbacks with, in peculiar, their lack of stability under operational conditions (light, bias, environment…). To improve this stability is one of the biggest challenges to be addressed before commercialization. The general formula for HOIPs is (A1,A2,A3,A4)Pb(X1,X2)3, where the A sites can be occupied by a distribution of 1 to 4 metallic/organic cations and X sites with halide anions. The role of native vacancy defects has been questioned as a possible cause for HOIPs solar cells degradation. The aim of this work is to understand the defect role in long term stability of HOIPs materials for photovoltaics. For this reason, primary defects were introduced in a controlled way via high energy electron irradiation (1MeV) in sets of layers and solar cells (SCs) fabricated using various HOIPs compounds. Those include the photovoltaic HOIPs prototype, MAPbI3 (A1PbX13), and emergent triple or quadruple cation mixed halide HOIPs, (CsMAFA)Pb(I1-xBrx)3 (A3PbX23) or (GACsMAFA)Pb(I1-yBry)3 (A4PbX23). The HOIPs layers are fabricated according to the same procedure as the HOIPs active SC layers and, subsequently, treated in similar conditions. For A1PbX13 and A3PbX23, the solar cells are of the p-i-n structure with organic hole and electron transport layer (HTL/ETL). The HOIPs layers are deposited on the glass/ITO/HTL (PEDOT:PSS) substrate without or with the top ETL layer (PCBM). For A4PbX23, the solar cells are of the n-i-p type with inorganic ETL (TiO2) and organic HTL (Spiro-OMeTAD) layers. The layers are directly deposited on glass without the ETL layer.Positron Annihilation Spectroscopy (PAS) gives direct evidence for native vacancy-type defects and irradiation induced ones in layers of each HOIP compound. The energy dependence of absorbance shows that natural and after irradiation ageing generates different defect populations in each HOIP compound. These populations strikingly also differ depending on the absence or presence of the top ETL layer for the A1PbX13 and A3PbX23 compounds. The defect populations evolve over ageing duration as long as 3 months. The prominent effects of ageing include (i) band gap modification, (ii) tailing of conduction/valence band extrema and (iii) optical absorption via deep subgap electronic levels. Illumination effects under laser also vary with ageing for each HOIP compound. Asymmetric photoluminescence (PL) peaks in each compound under continuous laser illumination reflect that radiative emission involves Gaussian emission rays with energy, FWHM and height evolving with illumination time. The emission transitions involve shallow localized electronic levels in A3PbX23 and A4PbX23 and resonant ones in A1PbX13. These electronic levels are attributed to specifically illumination-induced defect populations. Natural and after irradiation ageing result in PL decay lifetime spectra resolved into one or two exponential decay components. The decay components number and lifetime are strongly affected by the initial production of irradiation defects and HOIPs composition. Such effects last over 3 months at least in A4PbX23. The p-i-n solar cells exhibit most striking irradiation ageing induced photovoltaics performance. The External Quantum Efficiency (EQE versus photon energy) and the photovoltaic performance (I-V under illumination) of the irradiated solar cells have higher values than those in the reference SCs after 6 to 12 months of ageing. This gives evidence that defect engineering via high energy electron irradiation has a potential for providing innovative processing pathways to enhance the long-term stability of HOIPs photovoltaic performance

碩士
國立臺灣大學
應用物理研究所
107
Electric-field-induced dipole rotation of the intercalated organic molecules in halide perovskites has been suggested to be one controllable factor for fundamental properties and stabilities in perovskites. However, up to now, how the electric field triggers the dipole rotation of the intercalated organic molecules is still unknown. Here, we record the real-space atomic image and simultaneously probe the corresponding current-voltage (I-V) hysteresis in the methylammonium lead bromide (MAPbBr3) system using cross-sectional scanning tunneling microscopy and spectroscopy. In this work, we addressed the change of topography at specific bias intervals and anomalous I-V hysteresis with four gap-like regions as well as two unusual inflection points at forward 1.68 V and backward -0.87 V under ramp reversal scanning mode. We suppose that the dipole rotation, initiated by an electric field, concludes to two opposite surface dipole moments, creating an electronic transformation between the n-type-like and p-type-like feature. The two inflection points correspond to the critical voltage of dipole rotation. The transformation thus forms an abnormal I-V hysteresis behavior in MAPbBr3.

In this work, the characterization of methylammonium lead iodide (MAPI) layers, which were fabricated from PbI2 and PbCl2 via a sequential (2step) closed space sublimation (CSS) route under high vacuum, is presented. The characterization was carried out by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), UV/VIS absorption spectroscopy, photoluminescence (PL) spectroscopy and in vacuo photoelectron spectroscopy (PES). In addition, the 2step CSS MAPI layers were incorporated into planar solar cells and which were subsequently analyzed. For the transformation of the lead salt layers in the CSS, four substrate temperatures (75 °C, 90 °C, 130 °C, 150 °C) were chosen. The crucible temperatures and transformation times were adjusted to obtain most complete transformations. A high phase purity for the 2step CSS MAPI fabricated from PbI2 and from PbCl2 can be derived from the XRD measurements in the whole substrate temperature range. The SEM measurements show that the morphology of the MAPI layers undergoes significant changes which become more pronounced with increasing substrate temperature and can be separated into three distinct processes taking place simultaneously: the formation of the perovskite by incorporation of MAI into the lead salt grains, the recrystallization of the perovskite grains and an Ostwald ripening like growth of the recrystallized grains. From UV/VIS spectroscopy experiments a band gap of MAPI around 1.58 eV could be derived. The in vacuo PES experiments show the Fermi level pinned to the conduction band minimum. The UV/VIS and PE spectroscopy results appear to be independent on the substrate temperature. Combining the UV/VIS and the PES results, band energy diagrams for PbI2, PbCl2, MAI and MAPI could be created. Those band energy diagrams highlight the importance of a complete reaction of the lead salt on one hand and the avoidance of a MAI capping layer on top of the MAPI absorber, on the other hand. Working solar cells incorporating MAPI layers derived form PbI2 and from PbCl2 could be fabricated for all examined substrate temperatures. However, the efficiencies of the fabricated solar cells were mostly limited to the range of 2-3 %, with few solar cells exceeding 4 %. The reason for this is probably the combination of a hindered charge extraction due to a thin PbI2 interface layer between the MAPI absorber and the FTO/TiO2 electrode with the recombination of photoexcited charge carriers in the MAPI layers. Nevertheless, the relatively large size and the narrow efficiency distribution of the solar cells on one substrate indicate the potential of the 2step CSS process to fabricate solar cells with active areas in the square centimeter regime. Thereby, the unique property of the CSS, the combination of high processing temperatures and a high vacuum environment, is expected to open promising opportunities, especially for research on inorganic perovskite absorber materials.

碩士
國立臺南大學
材料科學系碩士班
104
New Materials for Methylammonium Lead Halide Perovskite Solar Cells and Stability Performance Student：Chih-Feng Tuan Advisor：Ing-Chi Leu Department of Materials Science, National University of Tainan Abstract Organic-inorganic hybrid perovskite solar cells have the characteristics of high power conversion efficiency, low manufacturing cost, and short energy payback time. However, the lifetime of organic-inorganic hybrid perovskite solar cells is about 2000 hours, which cannot compete with crystalline silicon solar cells. There are four parts in this thesis, first part is to simplify the processing procedure by depositing mesoporous SnO2 without compact electron-selective layer. The second part is to improve the power conversion efficiency by adding polymer materials. The third part is to lower the fabrication cost by using CuSCN/conductive graphite composite electrodes instead of noble metal and to improve the stability of the perovskite solar cells. The fourth part is to combine the polymer enhanced perovskite light absorber and conductive graphite electrode to manufacture low cost perovskite solar cells. In this study, we developed crack-free mesoporous SnO2 film and improve the Voc and Jsc. Furthermore, we increased the recombination resistance of perovskite solar cell by adding PEO polymer and obtained a PCE of 14.67%. We replace both noble metal electrode and expensive organic hole transporting layer with conductive graphite electrode. Eventually, we combine PEO polymer enhanced perovskite light absorber and conductive graphite electrode, a solar cell with 74% of PCE retention after 2600 hours is achieved. The stability of perovskite solar cells is improved significantly with the new materials used in this study. Keyword: Perovskite, Tin dioxide , Polymer, Graphite, Stability

碩士
國立成功大學
化學工程學系
103
This study investigated the growth of methylammonium lead halide perovskite material by two step solution processing as well as the influence of transport layer and light absorption layer prepared by different procedure on the performance of solar cell. First, the growth mechanism of methylammonium lead halide in two step solution processing was investigated. By adjusting the reaction time between methylammonium halide solution and lead iodide thin film, the surface morphology of perovskite thin film could be controlled. And, the concentration of methylammonium halide solution was modified to increase the conversion of lead iodide thin film. As a result, CH3NH3PbI3 and CH3NH3PbI3-xClx perovskite thin films were successfully prepared. And then planar perovskite solar cells were fabricated using CH3NH3PbI3 thin film, where, TiO2 compact layer was deposited on FTO glass by hydrolysis of TiCl4 aqueous solution. An optimized efficiency of 8.3% was obtained at a TiO2 compact layer thickness of 100nm. To further investigate the influence of mesoporous transport structure on the efficiency, mesoporous TiO2 structure, prepared by coating TiO2 nanoparticle colloid solution on compact layer, was employed to fabricate mesoporous perovskite solar cell. The results show that lower porosity of mesoporous structure would restrict the diffusion of methylammonium halide leading to the presence of unreacted lead iodide. Therefore, a higher porosity was employed to increase the conversion of lead iodide. An efficiency of 8.0% was obtained via the use of higher porosity mesoporous structure and the deposition of TiO2 compact layer by the oxidization of Ti thin film instead of the hydrolysis of TiCl4 to efficiently suppress the charge recombination at the interface between compact layer and substrate.

This thesis studies the fabrication and characterization of Methylammonium Lead Iodide perovskite thin films for photodetector applications. Unlike, any other perovskite halide material, Methylammonium Lead Iodide perovskite has long range balanced electron and hole transportation lengths. A major challenge is to use such materials to grow fine quality thin films. We tried to grow these perovskite thin films by the Pulsed laser deposition (PLD) technique, which is well known and successful in areas such as metal oxide-based complex perovskite systems. The unique ability to transfer from the bulk to a film with original Stoichiometry is the main strength of this technique. The first chapter provides a brief overview of the halide Perovskites and their properties such as optoelectronic properties, Ferroelectric property and thermal conductivity. In the second chapter, we describe the Pulsed laser deposition (PLD) technique used for the synthesis of the thin films. The invention of PLD technique has helped the fabrication of high-quality uniform thin films over a large area on the substrate. Understanding the pros/cons of PLD technique helps us in identifying the novelty of the thesis work. The second chapter also describes material growth techniques and all the characterization techniques that provide foundation for the entire thesis. In the third chapter, we describe the application of the CH3NH3PbI3 thin films grown by pulsed laser deposition for photodetection applications. With this method, we obtained good perovskite films coverage on fluorine-doped tin oxide coated substrates and observed well developed grains. The films showed no sign of degradation over several months of testing. We investigated the surface morphology and surface roughness of the films. We carried out a study on the solar and infrared photodetection of CH3NH3PbI3 thin films. In the fourth chapter, we describe the effect of change in annealing temperature and annealing time by keeping the thin film fabrication parameters constant. Annealing temperature was varied from 100 ◦C to150 ◦C including room temperature sample. The annealing time which was maintained for 10 min. We investigated structural and optical characterizations for all the samples (with different annealing temperature). Further, we carried out near-infrared photodetection of the CH3NH3PbI3 based thin films using an infrared lamp (IR) of 808 nm source at 0V. In each case the photo response was found to be stable with time. We annealed this sample with different annealing time 10 min, 30 min,50 min 70 min and 90 mins. At the end we have observed that the photo responsivity of the samples was high for a combination of annealing temperature of 150◦ C for 90 mins. In the chapter 5, we used TiO2 as an electron transport layer to enhance the photo response of the device. The TiO2 layer was grown by same technique. The fluorine doped tin oxide glass was used as substrate on that we grown TiO2, and then CH3NH3PbI3. The structural, morphology, cross section and optical properties of the films were studied. The photo response showed higher than previous samples. We also introduced one more layer Spiro-OMeTAD as a hole transport layer, while the idea behind this was to improve the photoresponsivity of the device FTO/TiO2/CH3NH3PbI3/Spiro-OMeTAD. Here again, studies were done in terms of cross-section and morphology of the film. We carried out similar electrical measurements. After introducing the hole transport layer, the photo responsivity increased. Here we compared the photoresponse parameters of both devices FTO/TiO2/CH3NH3PbI3 and FTO/TiO2/CH3NH3PbI3/Spiro-OMeTAD. In the Chapter 6, The thesis concludes with an overall summary of this work and with some future predictions based on the device structures.

碩士
國立交通大學
應用化學系碩博士班
108
Temperature dependent absorption spectra and photoluminescence spectra and their electric field effects have been studied for MAPbBr3 (methylammonium lead bromide) nanocrystalline films. Experimental results and discussion are divided into two parts. The first part is concerned with the absorption measurements. Crystal structure of MAPbBr3 at different temperatures is discussed, based on the measurements of XRD pattern as well as the measurements of electroabsorption and electrophotoluminescence spectra. Applied electric field induces the Stark effect on this material. This effect comes from the change in energy level of the material under the applied electric field, which results from electric dipole moment and polarizability of the excited state and the ground state. By analyzing the electroabsorption spectra, the difference in electric dipole moment and polarizability between the ground state and excited state could be obtained at each temperature in the range of 290-60 K with interval. In the analysis of the observed electroabsorption spectra, integral method was used, and temperature dependence of the electronic structure as well as the shape and position of the exciton band and continuum band in the absorption spectra was confirm. The dependence of the electroabsorption intensity on the modulation frequency of applied ac field was also examined at 290 K and 40 K. Then it is found that the signal increases with increasing modulation frequency and saturates at ~500Hz and 1000Hz at 40 and 290 K, respectively, suggesting the field-induced ion migration at low frequencies. The second part in the thesis is concerned with the photoluminescence of MAPbBr3. Under the application of electric field, the photoluminescence spectrum as well as intensity changes, and photoluminescence quenching whose magnitude depends on temperatures is observed. In the results, electrophotoluminescence spectra which depend on temperature were obtained. The presence of different states such as exciton state and trap state is also shown different temperature. From the electroabsorption and electrophotoluminescence experimental data, the binding energy of exciton of MAPbBr3 could be obtained.

碩士
國立臺北科技大學
光電工程系研究所
105
In this study, the electrical and optical characteristics of the textured crystalline silicon (C-Si) solar cells coated with luminescent down-shifting (LDS) of MAPbBr3 perovskite nanophosphor by spin-on film technique are demonstrated. Due to high reflectance and low spectral response at the ultraviolet (UV) and blue wavelengths (300–450 nm) bands, a higher recombination loss would be exhibited on the surface of photovoltaic devices because the incident photons of higher energy were absorbed within a short distance from the surface. The LDS phosphors materials can absorb high-energy photons and re-emitted lower-energy photons for the applications of solar cells to improve low spectral response at short wavelength band. Otherwise, the large diameter of phosphor had a larger shading and reflecting area to incident lights. The effects will more obvious be presented on the textured solar cell. In this study, the MAPbBr3 nanophosphor layer was appositely deposited by spin-on film technique on the textured C-Si solar cells. The spin methods and the concentration and layer of nanophosphor to achieve high efficiency are also discussed. The samples with nanophosphor concentration of 10 mg/ml and with 1-3 layers of nanophosphor deposited by two-step spinning rate on the textured silicon solar cell with a SiNx anti-reflection coating are prepared for comparing. The SEM analysis, optical reflectance, external quantum efficiency, dark current-voltage and photovoltaic current-voltage measurements of the solar cells with MAPbBr3 nanophosphor layer are measured and compared. The short circuit current density enhancement (ΔJsc) of 3.13% (from 36.48 mA/cm2 to 37.62 mA/cm2) and 4.35% (from 35.83 mA/cm2 to 37.39 mA/cm2), and the conversion efficiency enhancement (Δη) of 3.38% (from 15.08% to 15.59%) and 4.56% (from 15.13% to 15.82%) were obtained for the cells with 1-layer and 2-layer nanophosphor, respectively. However, the performance of current density and conversion efficiency of the textured cells with 3 layers nanophosphor are degraded that ΔJsc of -1.77% (from 37.85 mA/cm2 to 37.18 mA/cm2) and Δη of -1.72% (from 15.13% to 14.87%). The experimental results show that the textured silicon solar cell with 1-2 layer of MAPbBr3 nanophosphor presented a good LDS characteristics. Especially, the efficiency of the cell coated with 2 layers of MAPbBr3 nanophosphor with the concentration of 10 mg/ml is superior to that of the other ones.

To mitigate the adverse environmental effects of burning fossil fuels, it became necessary to explore alternative ‘clean’ renewable energy sources to meet the ever-increasing energy demands. While silicon-based solar cell devices have been at the forefront for decades, recently organic-inorganic hybrid halide perovskites APbX3 [A = methylammonium (MA+), formamidinium (FA+); X = halides] have transpired as a new family of materials as the alternatives, owing to their exceptional optoelectronic properties such as tuneable bandgap, low exciton binding energy, high carrier mobility, high defect tolerance etc. Remarkably, the efficiency of these solar cells with hybrid perovskites as the active layer has shot up from 3.8% in 2009 to exceed 25% at present. However, the environmental stability of the given materials remains elusive, placing a considerable hurdle on the way to its commercialization. Compositional engineering by partially substituting ‘A-site’ (MA+ with FA+) and/or ‘X-site’ (I- with Br-) ions of the perovskite have proven to be one of the successful approaches to enhance the stability of these materials. More recently, reasonable success in increasing environmental stability is achieved by incorporating bulkier and hydrophobic organic cations at the ‘A-site’, resulting in 2D layered counterparts with enhanced bandgap and exciton binding energy. In this thesis work, we have explored the opto-electronic and thermal properties of dimensionally controlled 2D as well as compositionally engineered 3D hybrid halide systems. In addition to the solar energy, hydrogen evolution reaction (HER) has a great significance in promoting electrochemical energy conversion in fuel cells. Being one of the most efficient catalysts for HER, MoS2 – the flagship member of the 2D layered transition metal dichalcogenides family, has gained much attention recently. We have also discussed the electronic structure of MoS2, responsible for such novel applications.

Organic-inorganic hybrid perovskites have emerged as promising photovoltaic materials in the last few years, with the possibility of easy, solution synthesis. In this thesis, we have investigated some intrinsic material properties of the hybrid lead halide perovskites in an attempt to understand factors responsible for the excellent photovoltaic behaviour. The presence of the (CH3NH3)+ or methylammonium (MA) ion with a permanent dipole moment in CH3NH3PbI3 gives rise to the possibility of ferroelectricity. In view of the continued controversy concerning the ferroelectric/non-ferroelectric nature of CH3NH3PbI3, we have addressed the more basic question of whether it is polar or not. We have measured the Second Harmonic Generation (SHG) efficiency, which is a sensitive probe to the presence of centre of inversion in the system and show that SHG efficiency of CH3NH3PbI3, if non-zero, is below the detection limit, strongly indicative of a nonpolar structure; consistent with P-E loop and single crystal XRD measurements. This nonpolar structure is a time-averaged description of the MA dipoles, consistent with many different dynamic behaviours, such as MA units rotating freely or in a correlated manner or frozen randomly. A comparison of temperature dependent dielectric constants of MAPbX3 and CsPbBr3 (without dipolar units) suggests that the MA+ dipoles are rotating freely with time scales much faster than μs. Ab initio molecular dynamics simulations show that these dipoles are randomly oriented with no net dipole moment when averaged over even a few unit cells, with a rotational time scale of ~ 7 ps at 300 K for these dipoles. Further, using pump-probe SHG efficiency measurements in MAPbX3 we have ruled out the possibility of a transient ferroelectric state in presence of photoexcitation. Further, we have carried out detailed investigation of dielectric properties of a larger class of hybrid lead halide perovskites, specifically the formamidinium lead halides (FAPbX3). Although the behaviour of dielectric constants of FAPbCl3 and FAPbBr3 in the low temperature resemble that of the MAPbX3 system, the absence of its strong temperature dependence in contrast to MAPbX3 lead us to conclude that the formamidinium (FA) dipoles are frozen in a glassy state. This is supported by the temperature dependent single crystal XRD results, which reveal disordered FA ions in the room temperature as well as at 100 K. Exciton binding energy is an important parameter in a photovoltaic material since it determines whether the mechanism is dominated by free charge carriers or excitons at room temperature. The exciton binding energy reported for MAPbI3 in the literature varies over a wide range of values. From careful experiments to measure temperature dependent PL spectra of MAPbI3 and MAPbBr3 we have estimated the exciton binding energy. PL intensity of MAPbBr3 films is observed to be sensitive to vacuum, environmental conditions and illumination. Since the penetration depth of the excitation wavelength, 405 nm, is very small in the sample, most part of the PL intensity observed can be considered to be from the near-surface region of the sample. We propose that defects are created at the surface of MAPbBr3 by the evaporative loss of MABr due to dynamic pumping. Considering all these factors, we have obtained the binding energy of MAPbBr3 film to be 79 meV, which corresponds to the intrinsic nature of the surface of MAPbBr3 film in vacuum.

Hybrid perovskite/silicon tandem solar cells offer low-cost alternatives to the commercially established silicon solar cells. In this thesis, we present the material and device optimizations of the subcells that can be used to fabricate tandem cells: Methylammonium Lead Halide (MAPbI3) based perovskite solar cells and Silicon/metal oxide type-II heterojunction based solar cells. Specifically, we focus on (a) improving the Voc of (MAPbI3) solar cells using band-matched polymer hole transporting layers (HTL), (b) improving the intrinsic stability of MAPbI3 by introducing Acetamidinium (AA) cation in the matrix, (c) studying the effect of Magnesium and Bromine substitution in MAPbI3, and (d) developing a Silicon/Cu2O type-II heterojunction solar cell that can act as a bottom cell in the proposed perovskite/silicon tandem solar cell. a) The fermi-splitting (in the absorber) and consequently the Voc of a thin film heterojunction cell depends on the fermi-level of the adjacent transport layers. The most widely used HTL: Spiro-OMeTAD has a HOMO of -5.0 eV, a 0.5 eV valance band maxima (VBM) offset with MAPbI3. In this part, we examine whether a p-type semiconducting polymer: ‘Poly-4-(5-(9,9-dihexyl-7-methyl-9H-fluoren-2-yl)thiophen-2-yl)-5,6-difluoro-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole’ (PF-DTDFBT) having a HOMO level of -5.6 eV: exactly matched to the VBM of MAPbI3 leads to the enhancement of the Voc of the cell. The increased fermi-splitting directly contributed to the improvement of Voc from 1.04 V in standard Spiro-OMeTAD HTL devices to 1.11 V in PF-DTDFBT interlayer devices. In addition, the polymer being hydrophobic leads to an increase in device stability by reducing the seepage of moisture into the active perovskite layer slowing down its degradation. b) One of the major obstacles in the commercialization of perovskites is its instability towards moisture, optical and thermal stimulus. The MAPbI3 structure consists of the organic methyl-ammonium (MA+) cation held in the PbI64- octahedral cage by 3 hydrogen bonds, weakened by the continuous tumbling motion of the C-N spine (in MA+). We report a cation : Acetamidinium, having (i) 4 h-bonds and (ii) restricted C-N bond rotation, that binds more strongly with the PbI64- cage as compared to MA+. Acetamidinium substitution leads to improvement in device performance and stability, which retained 70% of their initial PCE in 480 hours, while the standard cells degrade to ~43 % of their initial PCE in the same time frame. c) The state-of-the-art perovskite devices use the recipe involving weak co-ordination bond between DMSO-PbI2 (solvent-solute binding) broken by an in-situ antisolvent drip (Toluene or Chlorobenzene) to control the nucleation density and grain growth rate. Although for small device areas (~ 1 cm2) this method provides dense compact films with 200-250 nm grain sizes, this method is not suited for depositing films on large area, required for making efficient solar panels. We present the controlled addition of MgCl2/MgI2 in perovskite in the precursor solution that helps in forming films comparable in quality (compactness, grain sizes and carrier lifetime) with the films deposited by anti-solvent drip method. This deposition method neither requires DMSO nor anti-solvent drip making it more commercially attractive. 10% MgCl2/MgI2 addition in MAPbI3 leads to a ~ 80 μs/120 μs increase in carrier lifetime as measured by resonance assisted microwave photoconductance supporting a 150 mV rise in Voc of the devices. d) A silicon/metal oxide heterojunction solar cell is expected to have a lower thermal budget and a higher Jsc as compared to the more popular silicon homojunction cell where a considerable number of incoming photons are lost to the free carrier absorption in the highly doped p++ emitter region. We report the optimization of a type-II n-silicon/Cu2O heterojunction based solar cell. The Si/Cu2O interface is passivated by a 1.3 nm ultra-thin silicon dioxide layer. The passivated devices showed a 200 mV increase in Voc over the unpassivated devices. Further in-situ p-type doping of Cu2O was done by incorporating nitrogen into the Cu2O crystal. This served two purposes : the Voc and FF increased due to enhanced built-in voltage and film conductivity respectively, and the hole fermi-level is now defined by the doped Cu2O layer, providing the relaxation to use lower work function transparent contacts (which otherwise would have decreased the Voc of the cell). The best devices : doped-Cu2O/Si cells with transparent ITO top contact exhibited 5.23% PCE.
Department of Science and Technology, GoI.; Solar Energy Research Institute for India and the UnitedStates (SERIIUS) ; Visvesvaraya PhD Scheme for Electronics & IT program by MeitY, GoI