Dissertations / Theses on the topic 'Hybrid Halide Perovskites'

This thesis details the development of a novel photovoltaic device based on organometal halide perovskites. The initial focus of this thesis begins with the study of lighttrapping strategies in solid-state dye-sensitised solar cells (detailed in chapter 3). While I report enhancement in device performance through the application of near and far-field light-trapping techniques, I find that improvements remain step-wise due to fundamental limitations currently employed in dye-sensitised solar cell technology— notably, the available light-sensitising materials. I found a promising yet under researched family of materials in the methyl ammonium tri-halide plumbate perovskite (detailed in chapter 4). The perovskite light-sensitiser was applied to the traditional mesoscopic sensitised solar cell device architecture as a replacement to conventional dye yielding world-record breaking photo-conversion e!ciencies for solid-state sensitised solar cells as high as 8.5%. The system was further developed leading to the conception of a novel device architecture, termed the mesoporous superstructured solar cell (MSSC), this new architecture replaces the conventional mesoporous titanium dioxide semiconductor with a porous insulating oxide in aluminium oxide, resulting in very low fundamental losses evidenced through high photo-generated open-circuit voltages of over 1.1 V. This development has delivered striking photo-conversion ef- ficiencies of 10.9% (detailed in chapter 6).

Hybrid lead halide perovskites, AMX 3 compounds in which A = CH 3 NH 3 (MA), CH(NH 2 ) 2(FA), Cs; M = Pb,Sn; X = I, Br, Cl, display remarkable performance in solution-processed optoelectronic devices, including > 22% efficient thin film photovoltaic cells. These compounds represent the first class of materials discovered to exhibit properties associated with high performance compound semiconductors, while being formed at or near room temperature using simple solution chemistry techniques. This thesis is focused on the synthesis, structural characterisation and phase behaviour of MAPbI 3 , FAPbI 3 , A-site solid solutions and novel organic metal halide framework materials. The complete atomic structure and phase behaviour of methylammonium lead iodide is elucidated for the first time, including hydrogen positions, using high flux, constant wave-length neutron powder diffraction. At 100 K an orthorhombic phase, space group Pnma, is observed, with the methylammonium cations ordered as the C–N bond direction alternates in adjacent inorganic cages. Above 165 K a first order phase transition to tetragonal, I4/mcm, occurs with the unlocking of cation rotation, which is disordered primarily in the ab plane. Above 327 K a cubic phase, space group Pm3m, is formed, with the cations isotropically disordered on the timescale of the crystallographic experiment. The high temperature phase of formamidinium lead iodide, α-FAPbI 3 is shown for the first time to be cubic, (Pm3m), at room temperature using time-of-flight, high resolution neutron powder diffraction. Polymorphism and the low temperature phase behaviour of FAPbI 3 have been further investigated using reactor and spallation neutron sources with high resolution in temperature. A tetragonal phase, P4/mbm, is confirmed in the temperature range 140-285 K.The composition, structural and optical parameters of ’A’ site solid solutions (MA/FA)PbI 3 have been investigated by single crystal X-ray diffraction, UV-vis spectroscopy and 1 H solution NMR. A composition-dependent transition in the crystal class from tetragonal to cubic(or pseudo-cubic) at room temperature is identified and correlated to trends in the optical absorption. Novel hybrid materials with inorganic frameworks of varying dimensionality from 0D to 2D, including imidazolium lead iodide and piperazinium lead iodide, have been synthesised using various templating organic cations and their atomic structures solved by single crystal X-ray diffraction.

The perovskite family of materials is extremely large and provides a template for designing materials for different purposes. Among them, hybrid organic-inorganic perovskites (HOIPs) are very interesting and have been recently identified as possible next generation light harvesting materials because they combine low manufacturing cost and relatively high power conversion efficiencies (PCEs). In addition, some other applications like light emitting devices are also highly studied. This thesis starts with an introduction to the solar cell technologies that could use HOIPs. In Chapter 2, previously published results on the structural, electronic, optical and mechanical properties of HOIPs are reviewed in order to understand the background and latest developments in this field. Chapter 3 discusses the computational and experimental methods used in the following chapters. Then Chapter 4 describes the discovery of several hybrid double perovskites, with the formula (MA)$_2$M$^I$M$^{III}$X$_6$ (MA = methylammonium, CH$_3$NH$_3$, M$^I$ = K, Ag and Tl, M$^{III}$ = Bi, Y and Gd, X = Cl and Br). Chapter 5 presents studies on the variable presure and temperature response of formamidinium lead halides FAPbBr$_3$ (FA = formamidinium, CH(NH$_2$)$_2$) as well as the mechanical properties of FAPbBr$_3$ and FAPbI$_3$, followed by a computational study connecting the mechanical properties of halide perovskites ABX$_3$ (A = K, Rb, Cs, Fr and MA, X = Cl, Br and I) to their electronic transport properties. Chapter 6 describes a study on the phase stability, transformation and electronic properties of low-dimensional hybrid perovskites containing the guanidinium cation Gua$_x$PbI$_{x+2}$ (x = 1, 2 and 3, Gua = guanidinium, C(NH$_2$)$_3$). The conclusions and possible future work are summarized in Chapter 7. These results provide theoreticians and experimentalists with insight into the design and synthesis of novel, highly efficient, stable and environmentally friendly materials for solar cell applications as well as for other purposes in the future.

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

Über die besonders hohe Effizienz von Halid-Perowskit (HaP)-basierten optoelektronischen Bauteilen wurde bereits in der Literatur berichtet. Um die Entwicklung dieser Bauteile voranzutreiben, ist ein umfassendes und verlässliches Verständnis derer elektronischen Struktur, sowie der Energielevelanordnung (ELA) an HaP Grenzflächen von größter Bedeutung. Demzufolge beschäftigt sich die vorliegende Arbeit mit der Untersuchung i) der Bandstruktur von Perowskit-Einkristallen, um ein solides Fundament für die Darlegung der elektronischen Eigenschaften von polykristallinen Dünnschichten zu erarbeiten, und mit ii) den Einflüssen von Oberflächenzuständen auf die elektronische Struktur der Oberfläche, sowie deren Rolle bei der Kontrolle von ELA an HaP Grenzflächen. Die Charakterisierung erfolgt überwiegend mithilfe von Photoelektronenspektroskopie (PES) und ergänzenden Messmethoden wie Beugung niederenergetischer Elektronen an Oberflächen, UV-VIS-Spektroskopie, Rasterkraftmikroskopie und Kelvin-Sonde. Erstens weist die Banddispersion von zwei prototypischen Perowskit-Einkristallen eine starke Dispersion des jeweiligen oberen Valenzbandes (VB) auf, dessen globales Maximum in beiden Fällen am R-Punkt in der Brillouin-Zone liegt. Dabei wird eine effektive Lochmasse von 0.25 m0 für CH3NH3PbBr3, bzw. von ~0.50 m0 für CH3NH3PbI3 bestimmt. Basierend auf diesen Ergebnissen werden die elektronischen Spektren von polykristallinen Dünnschichten konstruiert und es wird dadurch aufgezeigt, dass eine Bestimmung der Valenzbandkantenposition ausgehend von einer logarithmischen Intensitätsskala aufgrund von geringer Zustandsdichte am VB Maximum vorzuziehen ist. Zweitens stellt sich bei der Untersuchung der elektronischen Struktur von frisch präparierten Perowskit-Oberflächen heraus, dass die n-Typ Eigenschaft eine Folge der Bandverbiegung ist, welche durch donatorartige Oberflächenzustände hervorgerufen wird. Des Weiteren weisen die PES-Messungen an Perowskiten mit unterschiedlichen Zusammensetzungen aufgrund von Oberflächenphotospannung eine Anregungslichtintensitätsabhängigkeit der Energieniveaus von bis zu 0.7 eV auf. Darüber hinaus wird die Kontrolle von ELA durch gezielte Variation der Oberflächenzustandsdichte gezeigt, wodurch sich unterschiedliche ELA-Lagen (mit Abweichungen von über 0.5 eV) an den Grenzflächen mit organischen Akzeptormolekülen erklären lassen. Die vorliegenden Ergebnisse verhelfen dazu, die starke Abweichung der in der Literatur berichteten Energieniveaus zu erklären und somit ein verfeinertes Verständnis des Funktionsprinzips von perowskit-basierten Bauteilen zu erlangen.
Optoelectronic devices based on halide perovskites (HaPs) and possessing remarkably high performance have been reported. To push the development of such devices even further, a comprehensive and reliable understanding of their electronic structure, including the energy level alignment (ELA) at HaPs interfaces, is essential but presently not available. In an attempt to get a deep insight into the electronic properties of HaPs and the related interfaces, the work presented in this thesis investigates i) the fundamental band structure of perovskite single crystals, in order to establish solid foundations for a better understanding the electronic properties of polycrystalline thin films and ii) the effects of surface states on the surface electronic structure and their role in controlling the ELA at HaPs interfaces. The characterization is mostly performed using photoelectron spectroscopy, together with complementary techniques including low-energy electron diffraction, UV-vis absorption spectroscopy, atomic force microscopy and Kelvin probe measurements. Firstly, the band structure of two prototypical perovskite single crystals is unraveled, featuring widely dispersing top valence bands (VB) with the global valence band maximum at R point of the Brillouin zone. The hole effective masses there are determined to be ~0.25 m0 for CH3NH3PbBr3 and ~0.50 m0 for CH3NH3PbI3. Based on these results, the energy distribution curves of polycrystalline thin films are constructed, revealing the fact that using a logarithmic intensity scale to determine the VB onset is preferable due to the low density of states at the VB maximum. Secondly, investigations on the surface electronic structure of pristine perovskite surfaces conclude that the n-type behavior is a result of surface band bending due to the presence of donor-type surface states. Furthermore, due to surface photovoltage effect, photoemission measurements on different perovskite compositions exhibit excitation-intensity dependent energy levels with a shift of up to 0.7 eV. Eventually, control over the ELA by manipulating the density of surface states is demonstrated, from which very different ELA situations (variation over 0.5 eV) at interfaces with organic electron acceptor molecules are rationalized. Our findings further help to explain the rather dissimilar reported energy levels at perovskite surfaces and interfaces, refining our understanding of the operational principles in perovskite related devices.

Philosophiae Doctor - PhD
The organic-inorganic hybrid perovskites such as methyl ammonium lead iodide (MAPbI3) or mixed halide MAPbI3-xClx (x is usually very small) have emerged as an interesting class of semiconductor materials for their application in photovoltaic (PV) and other semiconducting devices. A fast rise in PCE of this material observed in just under a decade from 3.8% in 2009 to over 25.2% recently is highly unique compared to other established PV technologies such as c-Si, GaAs, and CdTe. The high efficiency of perovskites solar cells has been attributed to its excellent optical and electronic properties. Perovskites thin film solar cells are usually deposited via spin coating, vacuum thermal evaporation, and chemical vapour deposition (CVD).

In the last 10 years, the research interest has been drawn towards the hybrid organic-inorganic halide perovskites, an innovative material characterized by remarkable optoelectronic properties and by its simplicity of fabrication; hybrid halide perovskites are currently being employed as active material in solar cells, X-ray photodetectors and light emitting devices. The following thesis presents the characterization of two perovskite-based materials. The first is a methylammonium lead iodide (MAPbI3) thin film solar cell, which has been fabricated and characterized at the University of Konstanz (Germany), with the aim to optimize the deposition procedure. The second material is a methylammonium lead bromide (MAPbBr3) single crystal that have been characterized at the University of Bologna with surface photovoltage and photocurrent spectroscopies, as a function of the deposited dose of X-rays in order to monitor the induced effects of radiation. After the exposure to X-rays, the exciton binding energy, calculated from the surface photovoltage spectra, has been found to increase by 20 meV with respect to the not irradiated sample. A similar result has been found with the photocurrent spectroscopy. The reasons for the increase in binding energy is discussed and attributed to a change in polarizability of the single crystal. The recovery of the crystals has been registered as well and has shown that the material is able to return to the initial condition after just few hours from the last X-ray's deposition.

The aim of the present PhD thesis has been the synthesis of novel materials for photovoltaic (PV) technologies. One possible way to increase the conversion efficiency of silicon solar cells is to shift the incident sunlight spectrum, converting photons poorly exploited by the active PV material into more effective photons. The materials studied as energy converters, consist of thin film layers integrated in the PV panel and made of a host material and an active luminescent species. In the present thesis, the energy conversion systems made of lanthanide doped fluorides (CaF2, NaYF4, NaGdF4, KYF4 and YOF) have been synthetized using MOCVD and sol-gel methods and deeply studied for their potential integration in silicon solar cells. Together with the traditional silicon PV technology, a new class of PV materials, based on inorganic multi-junction technology, new hybrid materials and organic dye-sensitized solar cells (DSSC) is subject of interest in the last years. Hybrid metalorganic/inorganic systems of Eu-complex/NiO films have been developed for the DSSC technology and for the potential improvement of the PV material with new functionalities, that combine the semiconductor behavior of the inorganic part to the luminescent properties of the metalorganic component. The all-inorganic halide perovskite CsPbBr3 has been synthetized through a precipitation method. The perovskite CsPbBr3 has a semiconductor behavior, with an energy band gap useful in the PV application and has the advantages to not have any labile or expensive organic components.

Dans le but de concevoir un dispositif de contrôle du clignotement des nanocristaux, il était nécessaire de créer un composite à l'état solide pouvant s'intégrer dans ce dispositif. Nous avons donc encapsulé des boites quantiques (BQs) à base de cadmium dans une matrice cristalline de pérovskite hybride de bromure de plomb. Ce manuscrit retrace l'ensemble des étapes qui ont été validé pour atteindre la création de ce nouveau composite. Nous avons développé avec succès une synthèse de BQs résistantes à l'encapsulation dans une matrice ionique mais également un échange de ligands inorganiques qui nous a permis d'intégrer de manière efficace les nanocristaux au sein de leur matrice en conservant leurs propriétés de luminescence. Après encapsulation, nous avons pu mettre en avant des preuves montrant une encapsulation efficace et un couplage entre les BQs et la matrice. Ces deux critères sont favorables à l'utilisation de ce composite dans le dispositif de contrôle. Ce dispositif consiste in fine à suivre optiquement la luminescence des BQs et à appliquer un champ électrique pour extraire et évacuer les charges en excès, qui sont à l'origine de l'état non émissif. Le développement de cette partie nous permettra dans le futur d'étudier le phénomène de clignotement mais surtout d'obtenir une source de photons uniques stable et à la demande
To construct a device for controlling the blinking of nanocrystals, it was necessary to create a solid-state active material that can be integrated in such an apparatus. To this end, we have encapsulated cadmium-based quantum dots (QDs) in a crystalline matrix of a hybrid lead-bromide perovskite. This manuscript describes all the steps that have been undertaken to achieve the creation of this new composite. We have developed a synthesis of QDs that are resistant to encapsulation in an ionic matrix by means of an organic-inorganic ligand exchange that allowed us to integrate nanocrystals into the matrix while conserving their luminescence properties. We were thus able to document efficient encapsulation and a coupling between the QDs and the matrix. These two characteristics are favorable for using this composite in a control device which ultimately aims at optically following the luminescence of the BQs and applying an electric field to extract and evacuate the excess charges responsible for the nonemissive state. The successful completion of this step will enable us in the future to study the phenomenon of blinking and, more importantly, to construct a stable on-demand single-photon source

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.

Insight into the nanoscale carrier transport in the rapidly developing class of solutionprocessed semiconductors known as metal halide perovskites is the focal point for these studies. Further advancement in fundamentally understanding photophysical processes associated with charge carrier transport is needed to realize the true potential of perovskites for photovoltaic applications. In this work, we study photogenerated carrier transport to understand the underlying transport behavior of the material on the 10s to 100s nanometer lengthscales. To study these processes, we employ a temporally-resolved and spatially-resolved technique, known as transient absorption microscopy, to elucidate the charge carrier dynamics and propagation associated with metal halide perovskites. This technique provides a simultaneous high temporal resolution (200 fs) and spatial resolution (50 nm) to allow for direct visualization of charge carrier migration on the nanometer length scale. There are many obstacles these carriers encounter between photogeneration and charge collection such as morphological effects (grain boundaries) and carrier interactions (scattering processes). We investigate carrier transport on the nanoscale to understand how morphological effects influence the materials transport behavior. Morphological defects such as voids and grain boundaries are inherently small and traditionally difficult to study directly. Further, because carrier cooling takes place on an ultrafast time scale (fs to ps), the combined spatial and temporal resolution is necessary for direct probing of hot (non-equilibrium) carrier transport. Here we investigate a variety of ways to enhance carrier transport lengthscales by studying how non-equilibrium carriers propagate throughout the material, as well as, carrier cooling mechanisms to extend the non-equilibrium regime. For optoelectronic devices based on polycrystalline semiconducting thin films, grain boundaries are important to consider since solution-based processing results in the formation of well-defined grains. In Chapter 3, we investigate equilibrium carrier transport in metal halide perovskite thin films that are created via the highly desired solution processing method. Carrier transport across grain boundaries is an important process in defining efficiency due to the literary discrepancies on whether the grains limit carrier transport or not. In this work, we employ transient absorption microscopy to directly measure carrier transport within and across the boundaries. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation. The results of this work provide insight into why this defect tolerant material performs so well. Photovoltaic performance (power conversion efficiency) is governed by the ShockleyQueisser limit which can be overcame if hot carriers can be harvested before they thermalize. To convert sunlight to usable electricity, the photogenerated charge carriers need to migrate long distances and or live long enough to be collected. It is unclear whether these hot carriers can migrate a long enough distance for efficient collection. In Chapter 4, we report direct visualization of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy. This work demonstrates three distinct transport regimes. (i) Quasiballistic transport, (ii) nonequilibrium transport, and (iii) diffusive transport. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers of transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites to ultimately overcome the Shockley-Queisser limit. In the next work, we investigated a way to extend non-equilibrium carrier lifetime, which ultimately corresponds to an accelerated carrier transport. From the knowledge of the hot carrier transport work, we showed a proof of concept that the excess kinetic energy corresponds to long range carrier transport. To further develop the idea of harvesting hot carriers, one must investigate a way to make the carriers stay hot for a longer period (i.e. cool down slower). In Chapter 5, we slow down the cooling of hot carriers via a phonon bottleneck, which points toward the potential to overcome the Shockley-Queisser limit. Open questions remain on whether the high optical phonon density from the bottleneck impedes the transport of these hot carriers. We show a direct visualization of hot carrier transport in the phonon bottleneck regime in both single crystalline and polycrystalline lead halide perovskites, more specifically, a relatively new class of alkali metal doped perovskites (RbCsMAFA), which has one of the highest power conversion efficiencies. Remarkably, hot carrier diffusion is enhanced by the presence of a phonon bottleneck, the exact opposite from what is observed in conventional semiconductors such as GaAs. These results showcase the unique aspects of hot carrier transport in hybrid perovskites and suggest even larger potential for hot carrier devices than previously envisioned by the initial results presented in Chapter 4. The final chapter will be divided into two sections, as we summarize and highlight our collaborative efforts towards homogenization of carrier dynamics via doping perovskites with alkali metals and our work on two-dimensional hybrid quantum well perovskites. Further studies on the champion solar cell (RbCsMAFA) were performed to elucidate the role inorganic cations play in this material. By employing transient absorption microscopy, we show that alkali metals Rb+ and Cs+ are responsible for inducing a more homogenous halide (Iand Br- ) distribution, despite the partial incorporation into the perovskite lattice. This translates into improved electronic dynamics, including fluorescence lifetimes above 3 µs and homogenous carrier dynamics, which was visualized by ultrafast microscopy. Additionally, there is an improvement in photovoltaic device performance. We find that while Cs cations tend to distribute homogenously across the perovskite grain, Rb and K cations tend to phase segregate at precursor concentrations as low as 1%. These precipitates have a counter-productive effect on the solar cell, acting as recombination centers in the device, as argued from electron beam-induced current measurements. Remarkably, the high concentration of Rb and Cs agglomerations do not affect the open-circuit voltage, average lifetimes, and photoluminescence distribution, further indicating the perovskite’s notorious defect tolerance. A new class of high-quality two dimensional organic-inorganic hybrid perovskite quantum wells with tunable structures and band alignments was studied. By tuning the functionality of the material, the strong self-aggregation of the conjugated organic molecules can be suppressed, and 2D organic-halide perovskite superlattice crystals and thin films can be easily obtained via onestep solution-processing. We observe energy transfer and charge transfer between adjacent organic and inorganic layers, which is extremely fast and efficient (as revealed by ultrafast spectroscopy characterizations). Remarkably, these 2D hybrid perovskite superlattices are stable, due to the protection of the bulky hydrophobic organic groups. This is a huge step towards the practicality of using perovskites for optoelectronics, since stability is always a huge concern with water-sensitive materials. The molecularly engineered 2D semiconductors are on par with III-V quantum wells and are promising for next-generation electronics, optoelectronics, and photonics.

Hybrydowe perowskity – halogenki ołowiu i kationu organicznego - stanowią nową klasę materiałów, mogących znaleźć zastosowanie jako konwertery energii słonecznej w kolejnej generacji ogniw fotowoltaicznych. Struktura chemiczna tych materiałów opisywana jest wzorem APbX3, gdzie A jest kationem organicznym a X anionem halogenkowym (najczęściej Br-, Cl- lub I-, lub ich kombinacją). Hybrydowe perowskity charakteryzują się efektywną absorpcją światła w szerokim zakresie spektralnym, długimi drogami dyfuzji oraz długimi czasami życia nośników. Własności te przekładają się na wysoką wydajność konwersji fotonów, która w przypadku najlepszych ogniw perowskitowych sięga 22%. Niski koszt wytwarzania oraz szybki rozwój tej klasy materiałów stawia fotowoltaikę opartą na perowskitach wśród potencjalnych rozwiązań zastępujących obecnie wiodącą technologię krzemową. Niniejsza praca poświęcona jest optycznym badaniom własności elektronowych oraz morfologii cienkich warstw hybrydowych perowskitów. Wykorzystano związki oparte na kationie metylamoniowym lub formamidynie, z jodem lub bromem jako anionami dominującymi. Uzyskane wyniki pozwalają określić wpływ składu chemicznego na badane parametry materiału. Na podstawie pomiarów magnetotransmisji bezpośrednio wyznaczono wartość energii wiązania ekscytonu i masy zredukowanej. Energie wiązania ekscytonu w temperaturze T = 2 K wynoszą od 14 do 25 meV. Są to wartości mniejsze lub porównywalne do średniej energii termicznej w temperaturze pokojowej (25 meV). Co więcej, wartości te maleją wraz ze wzrostem temperatury, do 10-24 meV w T = 160 K. Tym samym wnioskujemy, że nośniki wzbudzone światłem w temperaturze pokojowej można uznać za termicznie zjonizowane. Zmierzone wartości masy zredukowanej mieszczą się w zakresie 0.09-0.13 masy spoczynkowej elektronu. Pokazaliśmy również, że zarówno energia wiązania ekscytonu, jak i jego masa zredukowana zależą w przybliżeniu liniowo od wartości przerwy energetycznej. Uzyskane zależności pozwalają w łatwy sposób oszacować wartości tych parametrów dla nowo zsyntetyzowanych związków perowskitowych. Metodą przestrzennie rozdzielonej fotoluminescencji zbadano morfologię warstw perowskitowych z mikrometrową rozdzielczością, co pozwoliło zaobserwować pojedyncze ziarna krystaliczne. Otrzymane mapy luminescencji powierzchni próbek pokazują, że wszystkie badane warstwy składają się z ciemnych i jasnych ziaren. Pokazujemy, że w niskotemperaturowa przemiana z fazy tetragonalnej w ortorombiczną jest niepełna, a pozostałości fazy tetragonalnej są obserwowane nawet w T = 4 K. Zauważając zwiększone występowanie tych inkluzji w okolicach obszarów uszkodzonych strukturalnie, korelujemy obecność niskotemperaturowej fazy tetragonalnej z lokalnymi zubożeniami zawartości halogenków.
The hybrid organo-lead halide perovskites are an emerging class of materials, proposed for use as light absorbers in a new generation of photovoltaic solar cells. The chemical formula for these materials is APbX3, where A is an organic cation and X represents halide anions (most commonly Br-, Cl- or I-, or alloyed combination of these). The hybrid perovskites combine excellent absorption properties with large diffusion lengths and long lifetime of the carriers, resulting in photon conversion efficiencies as high as 22%. Another advantage is the inexpensiveness of the fabrication process. Therefore, with the rapid development of this class of materials, the perovskite photovoltaics has perspectives to outperform the well-established silicon technology. Here, we use optical methods to investigate the basic electronic properties and morphology in the thin films of several representatives of the hybrid perovskites. We study the compounds based on Methylammonium and Formamidinium organic cations; the iodides and wide band-gap bromides, showing how the chemical composition influences the investigated parameters. Using magneto-transmission, we directly determine the values of exciton binding energy and reduced mass. We find that the exciton binding energies at T = 2 K, varying from 14 to 25 meV, are smaller or comparable to the average thermal energy at room temperature (≈25 meV). Moreover, these values fall further at T = 160 K, to 10–24 meV. Based on that we conclude that the carriers photocreated in a perovskite material can be considered to be thermally ionized at room temperature. The measured reduced masses are in the range of 0.09-0.13 of the electron rest mass. We also show that both exciton binding energy and reduced mass depend linearly on the band gap energy. Therefore, the values of these parameters can be easily estimated for the synthesis of new perovskite compounds. Using spatially resolved photoluminescence, we probe the morphology of perovskite films with micrometer resolution, which enables us to observe single crystalline grains. The resulting maps show that all investigated thin films are composed from the dark and bright crystalline grains. We demonstrate that the low temperature phase transition from tetragonal to orthorhombic phase is incomplete in all studied materials, as the remains of the tetragonal phase are found even at T = 4 K. By investigating structurally damaged and photo annealed regions, where the occurrence of the tetragonal phase at low temperatures is enhanced, we attribute its presence to the depleted halide content.

Ink-based semiconductors that come to mind today include conjugated molecules and polymers, colloidal quantum dots, metal halide hybrid perovskites, and transition metal oxides. These materials form an ink (solution/ suspension/ sol-gel) that can be applied and dried in ambient air to form high-quality films for optoelectronic devices. In this study, we will introduce the current understanding of ink-based lead and lead-free hybrid perovskite and perovskite-inspired thin films. Examples will be presented through time-resolved studies of the solidification to link the solid-state microstructure and device figures of merit to the ink’s formulation, drying, and solidification process. The perovskite crystallization kinetics characterized in situ during the solution process indicates an essential role by the inclusion of Cs+ and K+ alkali metal cations in perovskite inks. The film and device characterizations indicate the functions of mixed cation and halides in determining the optoelectronic properties. The further sophisticated design of perovskite inks enables significantly optimized charge dynamics, including exciton separation, inter-grain charge transfer, trap density, charge mobility, and charge collection efficiency. The considerably improved optoelectronic properties lead to higher charge collection efficiency and, therefore, better open-circuit voltage and fill factor for the Cs+-containing 3D perovskite devices in contrast to the control FAPbI3 one. Recent developments in ink formulation and processing that enable scalable ambient fabrication of high-quality perovskite semiconductor films will also be presented. These findings raise the possibility of developing more controlled perovskites for systematically addressing both charge dynamics and degradation mechanisms in concert for the timely commercialization of perovskite solar cells.

Perovskites with the general chemical formula ABX3 can be categorized into oxides and halides depending on the nature of the X anion. 3D organic-inorganic halide perovskites are extensively studied in the context of solar cell and photo- and electro-luminescence applications due to their outstanding optoelectronic properties, while oxide perovskites have attracted a great deal of attention for their many interesting physical properties such as structural, electrical, magnetic, and magnetocaloric effects. More recently, 2D layered organic-inorganic hybrid (OIH) materials have emerged as a new class of materials with rich optoelectronic properties. They exhibit proven advantages over their 3D counterparts due to their large structural diversity and improved environmental stability against heat and moisture. 2D OIH materials have exhibited many interesting physical properties such as high exciton binding energies, intense photoluminescence, ferroelectricity, and chiro-optical properties. While the Lead-based hybrid perovskites have been very well studied for their spectacular optoelectronic properties, lately there have been attempts to design new materials such as Cd2+, Cu2+, Sn2+ -based hybrid halide materials which offer a new playground in the field of photovoltaic research. The work reported in this thesis explores the ferroelectric properties of Cd2+ - and Cu2+ -based halide materials and discusses the possible microscopic mechanisms for the origin of ferroelectricity in these materials. Further, using experimental and theoretical inputs, it is shown that the Cu2+ -based hybrid materials have outstanding chiro-optical properties. In addition, we also explore the interesting magnetic properties of a few transition metal compounds, exhibiting inverse magnetocaloric effect as well as Griffiths phase in some temperature ranges. Chapter 1 briefly introduces various concepts relevant to the investigations reported in subsequent chapters of this thesis. The present status of the research in the field of 2D organic- inorganic hybrid materials with an emphasis on various exciting properties such as ferroelectricity, bandgap modulation, and chiro-optical properties has been discussed. This chapter also presents discussions relevant to the family of oxide perovskites with reference to magnetic properties from fundamental and technological standpoints. Chapter 2 describes different experimental and theoretical methods that were employed to carry out the studies presented in this thesis. Chapter 3 presents a detailed study of the successive structural phase transitions of BA2CdCl4. These results establish that these structural phase transitions are associated with intrinsic ferroelectric transitions, from room temperature paraelectric to intermediate temperature ferroelectric, followed by another low-temperature ferroelectric phase. It was widely believed that ferroelectricity in these 2D organic-inorganic hybrid materials originated due to the order-disorder transition of molecular dipoles associated with the organic spacer cations. However, results in this thesis show that there are dipoles also associated with the inorganic components due to local structural distortions. The thesis presents a combination of experimental and theoretical results suggesting that the dipoles associated with the organic spacers and the dipoles originating from these local structural distortions within the inorganic units both play significant roles in the observed ferroelectricity of BA2CdCl4. Chapter 4 deals with the chiro-optical properties of quasi 2D (R-/S-MBA)2CuBr4 hybrid material. We discussed the role of chiral organic amine cations on the optical properties of these hybrid materials. Few Lead based compounds and one copper-based sample show giant chiro-optical properties but these are chirally active only for wavelengths < 490nm, limited by their large bandgaps. (R-/S-MBA)2CuBr4 shows a relatively large chiro-optical property in the orange-red part of the visible spectrum. Structural analyses of these compounds show that these are made of alternating layers of the chiral organic units and an inorganic layer of isolated CuBr4 units. Such isolated inorganic units distinguish this class of compounds from the more intensely investigated hybrid lead halide systems where the basic PbX6 (X = Cl, Br, or I) units are linked together by corner sharing of the halide ions, making them intrinsically 2D systems. The present Cu-based system would have qualified as a 0D system but for the Cu-Br….Br-Cu interactions that allow the separated CuBr4 units to interact, making the system a quasi-2D system. This subtle structural aspect plays an important role in giving this system remarkable chiro-optical properties. The semi-isolation of the CuBr4 units allows them to be rotated along the 21 screw-axis by the chiral organic units via strong hydrogen bonding, thereby imparting the giant chirality to the entire hybrid system. Simultaneously, the connectivity of the CuBr4 units via Br…Br interactions imparts a quasi-2D character helping to achieve a broadband absorption, thereby extending the chiro-optical properties to longer wavelengths. Chapter 5 discusses tuning of the bandgap while retaining ferroelectricity through halide substitution in Cu2+ -based chiral 2D hybrid materials to obtain small bandgap ferroelectric materials. Search for such small bandgap ferroelectric materials has been popular in the literature, not only because most ferroelectrics tend to have large bandgaps but also because of their obvious applications in solar photovoltaics. The ferroelectric Lead-based hybrid perovskites have been very well studied for their rich optoelectronic properties that are relevant in photovoltaic applications, but all are having a relatively higher bandgap. We have lowered the ligand to metal charge transfer bandgap from ~2.53 to 2.09 eV while retaining ferroelectricity in a copper chloride based low-dimensional hybrid material through partial substitution of chlorine with bromine. Our results show that a complete substitution of Cl- by Br- leads to a bandgap of ~1.62 eV with a loss of the ferroelectric state in the pure bromide material. Chapter 6 discusses the low temperature magnetic state along with the main ferromagnetic ordering at ~200 K and the magnetocaloric effect of double perovskite, Nd2NiMnO6. An earlier study on this compound established that for any applied magnetic field lower than 3 T, these samples show a downturn in M(T), and any field higher than 3 T, shows an upturn in M(T) for the temperature range below 100 K. This has been interpreted as Nd moments experience an effective ~3 T internal magnetic field due to the presence of the ordered Ni-Mn ferromagnetic sublattices. This indicates that the low temperature magnetic state of this compound is easily influenced by an externally applied magnetic field in the tune of 3 T, suggesting possible interesting magnetocaloric effects in this material. This chapter presents a detailed study of the magnetocaloric properties of Nd2NiMnO6. Interestingly, it shows a significant inverse magnetocaloric effect (IMCE) at low temperatures (T < 50 K) together with a significant conventional magnetocaloric effect (CMCE) at the ferromagnetic ordering temperature (Tc ~200 K). IMCE and CMCE correspond to the antiferromagnetic arrangement of Nd and Ni–Mn sublattices and ferromagnetic ordering of Ni–Mn sublattices, respectively. Nd2NiMnO6 with its second order phase transition follows the universal behavior of magnetic entropy change, ΔSM(T); it also shows a power law dependency on the magnetic field as ΔSM ∝ 𝐻𝜂. Chapter 7 deals with Griffiths phase-like magnetic anomalies in disordered La0.85Sr0.15CoO3 induced by chemical doping. In earlier studies on doped LaCoO3 with doping concentration higher than the percolation threshold (18%) shows non-Griffiths phase-like behavior. But no reports are available for the composition just below the percolation limit in this context. So, we chose the 15% doping concentration and explored its magnetic properties carefully. Our results establish that this composition shows typical Griffiths phase-like behavior in the intermediate temperature range and followed by spin glass behavior below 60 K. The existence of nanoscale ferromagnetic clusters below 240 K contributes to the total magnetization of the system for low applied magnetic fields resulting in a downturn of the χ-1 vs. T plot. The extent of this downturn is strongly suppressed by increasing the dc applied magnetic field, a typical signature of Griffiths phase. In the appendix, we present results of investigating Cs- and Na-doped WO3 exhibiting strong absorption in the near-infrared (NIR) and transmittivity in the visible range. Despite several publications, there is a lack of agreement in the community on the origin of this strong optical absorption with competing claims of polaronic and plasmonic origins. We address this controversy by first investigating bulk samples that are relatively free of complications arising from any shape anisotropy; we show by combining experimental and theoretical results that all spectral features in both bulk and nanoparticle samples are consistent with plasmonic excitations, without any need to invoke a polaronic mechanism. Doped WO3 exhibits strong optical absorption primarily due to surface plasmon resonances in colloidal nanoparticles, while their bulk counterparts are dominated by bulk plasmonic features. Investigating systems with different crystal structures and charge doping levels, we established that the complex spectral features of these plasmonic absorption bands for both bulk and nano samples are dominated by the underlying structure-dependent anisotropic electronic properties that determine the plasmonic features

In order to facilitate the human energy needs with renewable energy sources in the future, new concepts and ideas for the electricity generation are needed. Solar cells based on metal halide perovskite semiconductors represent a promising approach to address these demands in both single-junction and tandem configurations with existing silicon technology. Despite intensive research, however, many physical properties and the working principle of perovskite PVs are still not fully understood. In particular, charge carrier recombination losses have so far mostly been studied on pure films not embedded in a complete solar cell. This thesis aimed for the identification and quantification of charge carrier recombination dynamics in fully working devices under conditions corresponding to those under real operation. To study different PV systems, transient electrical methods, more precisely Open-Circuit Voltage Decay (OCVD), Transient Photovoltage (TPV) and Charge Extraction (CE), were applied. Whereas OCVD and TPV provide information about the recombination lifetime, CE allows to access the charge carrier density at a specific illumination intensity. The benefit of combining these different methods is that the obtained quantities can not only be related to the Voc but also to each other, thus enabling to determine also the dominant recombination mechanisms.The aim of this thesis is to contribute to a better understanding of recombination losses in fully working perovskite solar cells and the experimental techniques which are applied to determine these losses
Um künftig den menschlichen Energiebedarf in Zukunft mit erneuerbaren Energiequellen zu decken sind neue Konzepte und Ideen für die Stromerzeugung erforderlich. Solarzellen auf der Basis von hybriden Perowskit-Halbleitern stellen einen vielversprechenden Ansatz dar, um dieser Anforderung – beispielsweise in Tandem-Konfigurationen zusammen mit Silizium– gerecht zu werden. Trotz intensiver Forschung sind viele physikalische Eigenschaften und das Funktionsprinzip dieser neuartigen Solarzellen immer noch nicht vollständig verstanden. Insbesondere wurden die Rekombinationsverluste bisher meist nur an reinen Schichten untersucht, welche nicht in einen kompletten Solarzellenaufbau integriert waren. Die vorliegende Arbeit zielte auf die Identifizierung und Quantifizierung der Ladungsträger-Rekombinationsdynamik in voll funktionsfähigen Solarzellen unter Bedingungen, die denen im realen Betrieb entsprechen, ab. Um verschiedene PV-Systeme zu untersuchen wurden transiente elektrische Methoden, genauer gesagt OCVD, TPV und CE, angewandt. Während OCVD und TPV Informationen über die Rekombinationslebensdauer liefern, erlaubt CE die Berechnung der Ladungsträgerdichte. Die Kombination dieser Methoden hat den Vorteil, dass die erhaltenen Größen miteinander in Verbindung gesetzt werden können und somit umfangreiche Rückschlüsse auf die zugrundeliegende Rekombinationmechanismen ermöglichen. Das Ziel dieser Arbeit ist es, zu einem besseren Verständnis der Rekombinationsverluste in voll funktionsfähigen Perowskit-Solarzellen und der experimentellen Techniken, die zur Bestimmung dieser Verluste angewandt werden, beizutragen

Solarzellen aus organisch-anorganischen hybriden Perowskithalbleitern gelten als mögliche Schlüsseltechnologie zur Erzeugung günstiger und umweltfreundlicher elektrischer Energie und somit als Meilenstein für die Energiewende. Um die weltweit stetig wachsende Nachfrage an elektrischer Energie zu decken, bedarf es Solarzellentechnologien, welche gleichzeitig eine hohe Effizienz nahe dem Shockley-Queisser-Limit als auch eine hinreichend gute Stabilität aufweisen. Während die Effizienz von Solarzellen auf Basis von Perowskithalbleitern in dem letzten Jahrzehnt eine bemerkenswerte Entwicklung erfahren hat, lassen sich die wesentlichen physikalischen Mechanismen dieser Technologie noch nicht vollständig erklären. Die elektronisch-ionische Mischleitfähigkeit ist eine dieser Eigenschaften, welche die Effizienz und besonders die Stabilität der Perowskit-Solarzelle beeinflusst. Zentrales Thema dieser Arbeit ist daher die Untersuchung von mobilen ionischen Defekten und deren Einfluss auf Solarzellenparametern. Es wird gezeigt, dass die Migrationsraten ionischer Defekte in Perowskit breiten Verteilungen unterliegen. Durch die Anwendung eines neu entwickelten Regularisationsalgorithmus für inverse Laplace-Transformationen und verschiedener Messmoden für transiente Störstellenspektroskopie kann somit geklärt werden, warum sich berichtete ionische Defektparameter aus der Literatur für gleiche Defekte stark unterscheiden können. Dieses grundlegende Verständnis kann angewendet werden, um den Einfluss von kleinen stöchiometrischen Variationen auf die Defektlandschaft zu untersuchen und das Zusammenspiel zwischen elektronischen und ionischen Eigenschaften besser zu verstehen. Der Einsatz der Meyer-Neldel Regel ermöglicht ferner eine Kategorisierung ionischer Defekte in Perowskithalbleitern. Im letzten Teil dieser Arbeit wird gezeigt, dass elektrische und optische Methoden wie intensitätsmodulierte Spektroskopie geeignet sind, um Informationen über mobile Ionen in hybriden Perowskiten zu erhalten. Zusätzlich wird das elektronische Rekombinationsverhalten näher untersucht.
Solar cells made of organic–inorganic hybrid perovskite semiconductors are considered as a possible key technology for the conversion of cheap and environmentally friendly electrical energy and thus as a milestone for the turnaround in energy policy. In order to meet the steadily growing global demand for electrical energy, solar cell tech- nologies are required that are both highly efficient, i.e. close to the Shockley–Queisser limit, and sufficiently stable. While the efficiency of solar cells based on perovskite semi- conductors has undergone a remarkable development in the last decade, the essential physical mechanisms of this technology cannot yet be fully explained. The electronic- ionic mixed conductivity is one of these properties, which influences the efficiency and especially the stability of the perovskite solar cell. The central topic of this thesis is therefore the investigation of mobile ionic defects and their influence on solar cell parameters. It is shown that the migration rates of ionic defects in perovskites are attributed to wide distributions. By application of a newly developed regularisation algorithm for inverse Laplace transform and different measurement modes for deep-level transient spectroscopy, it can thus be clarified why reported ionic defect parameters from the literature for the same defects can differ significantly. This basic understanding can be used to study the influence of small stoichiometric variations on the defect landscape and to better understand the interaction between electronic and ionic properties. Us- ing the Meyer–Neldel rule also allows the characterisation of ionic defects in perovskite semiconductors. The last part of this thesis shows that electrical and optical methods such as intensity-modulated spectroscopy are suitable for obtaining information about mobile ions in hybrid perovskites. In addition, the electronic recombination behaviour is examined more closely.

碩士
國立清華大學
光電工程研究所
102
This paper proposed a low temperature, solution process, simple process, a large area of the lead halide perovskite organic/inorganic hybrid solar cell. In this paper, in which the use of lead halide perovskite as the photoactive layer. With the high solubility PbCl2 in DMSO to increase the concentration of the precursor solution, and construct organic / inorganic hybrid solar cell. Our device configuration:Glass/ITO/PEDOT:PSS/Perovskite/PCBM/Al belong to normal structure. Suitably selected the hole and the electron transport layer by spin coating and dried to optimize conditions for the performance of the solar cell of the present paper is better. In this paper, Construction of the solar cell efficiency of up to 7.0 %, short-circuit current of 18.1 mA/cm2 has excellent performance. Lead halide perovskite organic / inorganic hybrid solar cell laden with good efficiency and performance advantages of a large area can be to facilitate the production of large-area components toward future development.

碩士
國立臺灣大學
材料科學與工程學研究所
106
Organic-inorganic hybrid lead halide perovskite solar cells have been developed rapidly because its excellent performance. However, the active material is unstable in ambient air, which limits its practical application. Thermal instability of devices states a more fundamental problem. In this thesis, atomic layer deposited inorganic metal oxides was applied to perovskite solar cells devices in order to solve the problem. We first investigated compatibility of perovskite with a variety of metallic precursors and with oxidants, respectively. We concluded criteria of selecting condition of ALD process and choice of precursors that would not damage perovskite. With optimal parameters, devices with ultra-thin atomic layer deposited Al2O3 or TiO2 direct on top of perovskite showed good performance. However, thermal instability of devices still did not improve due to imperfect coverage of oxides layer resulted from lack of nucleation cite on perovskite surface. To solve this problem, we deposited ALD AZO on organic charge transport layer instead. Device of this architecture reached efficiency of 14.6%, and only dropped to 80% of initial value after 1-day storage in glove box at 85℃. The thermal instability was much improved as efficiency of control devices dropped to less than 50% of initial value.

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.

Perovskite solar cells have garnered significant interest thanks to the impressive rise of their efficiency over the last few years to a power conversion efficiency (PCE) of 25.2% despite being processable using cheap and potentially high-throughput solution coating techniques. Using the two-step conversion process high-quality perovskite films with high quality and uniformity can be produced, however, this process still needs a deeper and fundamental understanding. This thesis has shed light on the ink-to-solid conversion during the two-step solution process of hybrid perovskite formulations. We demonstrated that the conversion of PbI2 to perovskite is largely dictated by the state of the PbI2 precursor film in terms of its solvated states. We used several in situ diagnostic measurments such as grazing incidence wide-angle x-ray scattering (GIWAXS), quartz crystal microbalance with dissipation monitoring (QCM-D), and optical reflectance and absorbance all performed during spin coating, to monitor the nucleation and growth of crystalline phases, the mass deposition at the solid-liquid interface and the rigidity as well as the solution thinning behavior and the changes in optical absorbance of the precursor and perovskite. We compare conversion behaviors from different lead states by using methylammonium iodide (MAI), formamidinium iodide (FAI), and/or mixtures of halides (I, Br) and show that conversion can occur spontaneously and quite rapidly at room temperature without requiring further thermal annealing. We confirm this by demonstrating improvements in the morphology, microstructure and optoelectronics properties of the resulting perovskite films, as well as their impact on the PCE of solar cells using complimentary measurements such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and with steady-state photoluminescence.