The quest for efficient and cost-effective thin-film photovoltaic (PV) materials has recently zeroed in on hybrid lead halide perovskites, owing to their low cost, ease of processing, and exceptional efficiency metrics—peaking at 33.9% when combined with silicon in tandem devices. Nevertheless, there are substantial concerns about the stability, toxicity, and consequential environmental footprint of lead-based perovskites, thereby necessitating rigorous research to identify and develop alternative materials with superior stability profiles and diminished toxicity. Amongst the myriad candidates, chalcogenide perovskites and their related structures, represented by the empirical formula ABQ3 (with A = Ca, Ba, Sr; B = Zr, Hf, Ti; Q = S, Se), have emerged as particularly promising contenders. These materials are distinguished by their optimal optoelectronic properties and robust stability. Notably, barium zirconium sulfide, BaZrS3, has garnered significant attention in the scientific community due to its distinctive perovskite structure and several unique optoelectronic properties, making it a frontrunner in this domain of PV materials research. However, synthetic routes to these materials, especially as colloidal nanomaterials, remain limited, due in part to their high crystallization energy and oxophilicity. In this thesis, we have successfully devised solution-based approaches to synthesize colloidal nanomaterials of BaTiS3 and BaZrS3, including its titanium- and selenium alloyed phases. Our methodology involves utilizing reactive metal amide precursors in oleylamine, with diethylthiourea and trioctylphosphine selenide serving as sources for sulfur and selenium, respectively. Chapter I discusses the general background of current and emerging PV materials. Chapter II delves into various synthetic routes reported for inorganic ternary and binary sulfide and selenide nanomaterials, incorporating transition metals from groups 3, 4, and 5. This section also encompasses our synthetic methods for BaZrS3 and BaTiS3 colloidal nanomaterials. Chapter III provides an in-depth discussion of our developed techniques for producing nanorods and nanoparticles of barium titanium sulfide. In Chapter IV, our focus shifts to the synthesis of colloidal nanoparticles of barium zirconium sulfide perovskites. Additionally, Chapter V explores the synthesis of titanium and selenium alloyed barium zirconium sulfide. Finally, the synthesis of mixed halide lead perovskite nanocrystals, achieved through a postsynthetic anion-exchange method, is discussed in Appendix E.

The present work addresses issues related to the derivation of reduced models of atomistic systems, their statistical calibration, and their relation to atomistic models of materials. The reduced model, known in the chemical physics community as a coarse-grained model, is calibrated within a Bayesian framework. Particular attention is given to developing likelihood functions, assigning priors on coarse-grained model parameters, and using data from molecular dynamics representations of atomistic systems to calibrate coarse-grained models such that certain physically relevant atomistic observables are accurately reproduced. The developed Bayesian framework is then applied in three case studies of increasing complexity and practical application. A freely jointed chain model is considered first for illustrative purposes. The next example entails the construction of a coarse-grained model for a liquid heptane system, with the explicit design goal of accurately predicting a vapor-liquid transfer free energy. Finally, a coarse-grained model is developed for an alkylthiophene polymer that has been shown to have practical use in certain types of photovoltaic cells. The development therein employs Bayesian decision theory to select an optimal CG potential energy function. Subsequently, this model is subjected to validation tests in a prediction scenario that is relevant to the performance of a polyalkylthiophene-based solar cell. / text

In the recent years area organic photovoltaics is generating a lot of interests because whole process of synthesis and fabrication is less energy intensive process as well as it is cost effective compared to conventional inorganic Si based photovoltaic technology. This work mainly deals with the fabrication and optimization of device fabrication conditions for organic photovoltaic materials. In first part of the work, the solar cell fabrication conditions were optimized for the commonly used system P3HT – PCBM. The fabricated device was optimized for the solvents used for the active material, concentration of the active material solution, donor- acceptor ratio of the active material, annealing conditions of the active layer and the metal evaporation conditions for the cathode. All the optimization procedures were carried out in controlled atmosphere to minimize the environmental effect inference during fabrication of the solar cell devices. All the characterization was carried out at ambient conditions. The efficiency of the solar cell was improved from 0.009% to 6.2%. the environmental stability of the fabricated devices were carried out after encapsulating it with epoxy based resin in both ambient conditions as well as extreme conditions like 85% RH at 25°C inside the humidity chamber. It was observed that both the data matches well with each other indicating proper encapsulation required to safe guard the device for the better performance over the period of time. Second part of this work mainly deals with understanding the structure property relationship of thiophene based donor- acceptor- donor molecule 2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone (DTCPA), which is highly crystalline, low band gap organic molecule which absorbs over entire visible region of the solar spectra. DTCPA crystals of various morphologies were prepared by various recrystallization routes. It was observed that macro scale morphology of these crystals differs from each other. Also depending on the method of recrystallization sizes of the crystals also varies. All the recrystallized DTCPA shows strong orientation toward (001) direction. However, it was observed that lattice parameters of these crystals slightly differ from each other owing to the recrystallization methodology. These variations in crystal parameters are more than 0.02 which is significant. It was also observed that the crystallite sizes depend on the recrystallization routes. Slow evaporation of concentrated solution (SEC) grown crystals has the larger crystallite size of 170nm. It was observed that absorption range of these crystals slightly differ from each other owing to the change in the crystallite sizes and crystal parameters. Third part of this work deals with the fabrication and optimization of thermal evaporation process of DTCPA for photovoltaic applications. DTCPA is stable at higher temperatures as well as has sharp melting point which make it ideal candidate for thermal evaporation. In this work films of DTCPA were fabricated for various evaporation rates by thermal evaporation technique. Chemical integrity of the molecules upon evaporation is found to be intact as observed from FTIR spectroscopy. XRD shows that at lower (25 W/m2) as well as higher (40 W/m2) films are oriented to (001), (400) as well as (311) directions, at 30 W/m2 and 35 W/m2 there is a strong orientation towards (311) and (001) directions respectively. Photo luminescence studies indicate that there is strong 410 nm emission for films deposited at the power of 25 W/m2 and 40 W/m2. Microscopic studies confirm that morphology is dependent on the deposition rates as it changes with the change in deposition rate. This in turn reflects in the device characteristics of these films. It was observed that films deposited at high deposition rates show better device characteristics with high VOC and current density values. All these device fabrication and characterizations were carried out in ambient conditions. Fourth part of this work deals with P3HT - DTCPA composites which exhibit wide range of light absorption. It was observed that DTCPA act as nucleating centers for the P3HT molecules and increases crystallinity in the composite. Furthermore, DTCPA helps in exciton separation because of donor and acceptor moieties present in the molecule. It also helps in charge transportation because of its crystalline nature and further it induces molecular ordering in the P3HT matrix. The band diagram of P3HT- DTCPA suggests that the band edges of both materials are ideal for charge separation. In addition, crystalline nature of the DTCPA molecule helps in effective charge transportation. J-V characteristics shows that there is large built in potential in the devices from these blends leading to large Voc. Composites with lower DTCPA loadings show higher efficiency than with higher loadings. These devices were prepared in ambient conditions and needs to be optimized for obtaining better device properties. In the fifth part of the work two types of system were studied to understand the band edge matching on the photovoltaic properties, carbazole based copolymers and DTCPA based copolymers. In the case of carbazole based copolymers it was observed that by copolymerizing carbazole with thiophene based derivatives lowers the band gap and modifies the HOMO and LUMO levels for better suit for the photovoltaic device fabrication. It was observed that that is two orders of improvements in the efficiency by co polymerizing carbazole with benzothiodizole as improves the JSC and VOC. Also the copolymerization of carbazole with both benzothiodiazole and bithiophene results in better light harvesting as the optical band gap was lowered. In the case of DTCPA copolymers with DTBT and DHTBT as both are random copolymers the solubility was low as well as their HOMO band edge was mismatched with the PEDOT: PSS which is a hole transport layer. However, the alternate polymerization of DTCPA with DTBT improved the band edge matching and also the solubility. As a result there was tenfold improvement in the charge collection and hence the efficiency was improved from 0.02% to 2.4%. Many of the conducting polymers have good material property but poor filmability. In the sixth part of this work deals with fabrication of device quality films by alternate deposition technique like pulsed laser deposition. Two types of system were studied in this work (i) polypyrrole- MWCNT nanocomposites and (ii) Poly DTCPA polymer. In both the cases it was observed that chemical integrity of the polymer retained during ablation. PolyDTCPA films were fabricated by pulsed laser deposition by both IR (Nd-YAG) and UV (KrF) laser source. Morphological studies indicate that IR laser ablated films were particulate in nature whereas UV laser ablated films were grown as continuous layers as polyDTCPA absorbs better in UV region. As a result the IV characteristics indicate that IR laser ablated films are resistive in nature and UV laser ablated films are good rectifiers indicating the suitability of the process for fabrication of device quality films.

In recent years, organic photovoltaics (OPVs) have attracted considerable attention as a potential source of renewable energy over traditional materials due to their light weight, low production cost, mechanically stability and compatibility with flexible substrates in roll to roll processing for high volume production. In the OPVs interface plays an important role in determining the performance of the device. Interface signifies formation of efficient contact with electrode, film, and transport of free charge carrier, which results in better performance in the device. Interface engineering also helps in improving mechanical robustness of the device. Hence, understanding of interface, modification and its evaluation is important in fabrication of efficient device. In this thesis interface is modified such that the performance of the device can be improved (chapter 3 and chapter 4). In Chapter 5 and chapter 6 interface is modified such that device can be fabricated on uncommon substrate. Fabrication of device on uncommon substrates (fiber reinforced plastic and flexible glass substrate), has unique challenges. In chapter 5 and chapter 6, we look at how interface is modified to overcome the challenges associated and also understand the role of interface in improving the performance of device on such substrates is discussed. In Chapter 1 we discuss about working of organic solar cells and the challenges associated in device fabrication. Understanding of interface to overcome challenges associated is explained. It also covers brief introduction to the succeeding chapters discussed in the thesis and its recent developments. To understand the properties of interface and to analyze device performance various characterization techniques have been used are discussed in chapter 2. This chapter also covers the materials and general device fabrication techniques used in this thesis. In chapter 3, a narrow bandgap (NBG) polymer used as a near IR sensitizer in P3HT: PCBM blend. Since, P3HT with a band gap of ~1.9 eV, the commonly used p-type material absorbs approximately ~25 % of incident light. Hence, MP2 (NBG polymer) is used along with P3HT: PCBM in active layer to form a ternary blend, which helps in increased absorption. Basic properties of MP2 are evaluated using UV-visible spectroscopy, differential scanning calaorimetry(DSC), thermogravimetric analyser (TGA), gel permeation chromatography (GPC) and photoluminescence (PL) techniques. To evaluate enhanced absorption of ternary UV-visible spectroscopy is carried out. Charge transfer from one moiety to other in ternary blend is evaluated using PL and Ttime resolved microwave conductivity (TRMC). Morphology of the ternary is assessed using atomic force microscope (AFM) and structural characterization is carried out by X-ray diffraction (XRD). Performance of the device is evaluated by current-voltage (J-V) characterizations. Further improved performance is supported by external quantum efficiency (EQE). Charge extraction with linear increasing voltage (CELIV) of the device is done to evaluate the recombination mechanism in the device and to assess the performance of the device. One-dimensional (1D) ZnO nanostructures provide direct paths for charge transport, and also offer large interfacial area to make them an ideal electron transport layer. In chapter 4 highly aligned ZnO nanorods is used as electron transport layer in OPV. Growth of ZnO nanorods is two-step processes, growing seed layer and growing ZnO nanorods from hydrothermal process using an appropriate seed layer. Two different soft-chemical solution- growth methods (upward and downward) are developed to fabricate self-assembled, oriented ZnO nanorods. Substrate mounting, surface properties and optical transmittance are optimized by varying the nanorods growth conditions. Further the ZnO nanorods are UV ozone treated and its effect on performance of nanostructured buffer layer based device is evaluated. In Chapter 5 OPV is fabricated on an opaque FRP substrate. Fabrication of OPV device on opaque substrate plastic is unique and hence understanding various properties is vital. Such devices fabrication require bottom up approach, with transparent electrode as the top electrode and metal electrode on the surface of FRP. FRP has inherent rough surface of about few microns RMS roughness. In order to reduce the roughness of the substrate FRP was planarized. The planarized layer is chosen, such that it chemically binds with the substrate. The chemical interaction between substrate and planarizing coating is evaluated by FTIR and Raman spectroscopy. The binding of planarized layer and FRP is evaluated using nanoscratch technique and surface energies are studied using contact angle measurements. In addition, adhesion properties of the metal electrodes, which are deposited on planarized FRP are evaluated using nanoscratch technique. Fabrication of OPV requires a top transparent electrode. Simple spin coating technique is used to optimize the top electrode. The property of top electrode is evaluated using UV-visible spectroscopy for transmittance, and sheet resistance of the electrode is characterized. OPV device is fabricated on planarized FRP substrate using optimized top transparent electrode and its PV properties is evaluated. Performance of the device is evaluated for two different bottom electrodes and further performance of device is enhanced using buffer layers. Usually flexible OPVs are fabricated on plastic substrate such as PET, PEN. However they are not structurally stable at high temperatures and have high oxygen and moisture Permeability. In Chapter 6 Organic based photovoltaic devices were fabricated on flexible glass. Flexible glass has high strength and it is also known for low oxygen and moisture permeability. Fabrication of device on flexible glass has never been done before and hence, generation of data is necessary for commercialization of the technology. Device fabrication is optimized by using two different transparent conducting layers (ITO- sputter deposited, PEDOT: PSS-solution processed) and device performance was evaluated for both. Since the substrate is flexible in nature understanding the performance of the device during flexing is important. For this 2-parallel plate flexural apparatus is fabricated for in-situ measurements along with current voltage measurements. These devices are flexed cyclically and performance of device is evaluated. Therefore, work discussed in the thesis show by modifying the interface of the device, and understanding various interfaces of the device is crucial for improving the performance of the device. Also by engineering the interface, devices can be fabricated on various types of substrate.

<p>As energy demands for the world increase, the necessity for alternate sources of energy are critical. Just in the United States alone, 92 quadrillion British thermal units (Btu) were used in 2020. As political and geographical pressures surrounding oil increase, along with the growing concern for climate, the drive to explore alternative and renewable means for harvesting energy is on the rise. Solar cells, also known as photovoltaics (PVs), are an attractive renewable source and have been developed as an alternative energy means for over 60 years. When considering losses due to atmospheric absorption and scattering, the Earth’s surface gets about 1000 W/m2 of energy from the sun, which is why there are research efforts around the world trying to maximize the efficiency of solar cells.</p> <p>Organic-inorganic halide perovskites provide for ideal absorbing layers that feature long carrier lifetime and diffusion lengths, strong photoluminescence, and promising tunability. Furthermore, the solution-processing methods used to make these perovskites ensure that the solar cells will remain low-cost and have easy scale-up possibilities. The main problem perovskites is that they degrade in the presence of water, thus leading to decreased device performance.</p> <p>In this work two approaches are investigated to increase moisture stability. The first investigates incorporation of thiols as pseudohalides into the 2D perovskite structure. Instead of the theorized perovskite, two novel 2D compounds were created, Pb₂X(S-C₆H₅)₃ (X= I, Br, Cl) and PbI_1.524(S-C₆H₅)_0.476. While not perovskites, this study gives insight into the effect that the thiol may have on determining structure when comparing –S-C₆H₅ with –SCN groups. Future work will explore more electronegative thiols that will be used to make moisture resistant, tunable 2D perovskites.</p> <p>The second approach is to incorporate longer organic ammonium cations into the perovskite structure to produce quasi-2D perovskite films fabricate them into devices. Adding in electronically insulating ligands leads to a stricter requirement for vertically aligned 2D films and special care must be taken to have efficient charge collection. The current field has successfully incorporated short ligands such as butylammonium (BA) into PVs, however the extension to larger and more beneficially hydrophobic ligands has been very scarce. In this work, a novel solvent engineering system is developed to create vertically aligned quasi-2D perovskite absorbing layers based off of a bithiophene ligand (2T). These absorbing layers are then characterized and incorporated into efficient PV devices. Generalizations to solvent conditions related to ligand choice is discussed herein, creating deep insights into incorporating more conjugated ligands into devices.</p>

Inexhaustible solar energy, which provides a clean, economic and green energy, seems to be an alternative solution, for current and future energy demands. Harvesting solar energy presents a challenge in using eco-friendly, earth abundant and inexpensive materials. Although present CdTe and Cu (In, Ga)Se2 (CIGS) technologies, provide light-to-electricity comparable to silicon technology, toxicity of Cd and scarcity of In limits the widespread utilization. Future tera-watt level module capacity would then be feasible by the low-cost technologies. The chalcogenide thin film technology would therefore provide the exceptional utilization in the large-area module monolithic integrations benefitting from the low material consumption owing to the direct band gap. The current thesis presents the results obtained from the quest of other thin film materials and their utilization to an unconventional Cd-free buffer layer. The films suitability for the future applications was assessed through photovoltaics device studies in a comparative manner. Chapter-1 deals with the motivation for the solar energy and the importance of thin film photovoltaics. Alternative materials which are abundantly available would help to reach the future tera watt level production, where the conventional silicon technology alone cannot satisfy the global energy demand. The utilization of non-conventional thin film based solar cells and their working principles were elucidated. The histories of the copper based alternative materials were introduced. Chapter-2 deals with the versatile thin film growth technique that has been designed fabricated and installed further which can handle the growth of the absorber and the top TCO layers with insitu sulphurisation. The methodology of the absorber deposition was discussed in detail. The experimental details for the co-sputtering of CuInAl alloy were presented. A novel selenization method, assisted by the combination of inert gases was developed for the annealing of CuInAl alloyed precursor films. Chapter-3 deals with the presentation of the results obtained on buffer and window layers. Chemical Bath deposition technique was employed for the growth and optimization of the conventional CdS and non-toxic buffer ZnS buffer layers. A) Cadmium sulphide thin films suitable for the utilization of high efficiency solar cells were optimized. Optimization of the buffer involved the effects of cadmium precursors, ammonia concentration and buffer capsule effect. A green route was presented so as to consume the precursors to the maximum extent possible. B) The alternative non-toxic buffer Zinc Sulphide (ZnS) thin films were successfully grown using the above optimized conditions. Moreover the window layer was also optimized for better device partner. Zinc Oxide was used as a n-type partner for the p-type CIS films. The ZnO films were grown by the RF-sputtering from the single cathode exhibited good crystallinity with Zincite structure (hexagonal ZnS, a= 3.249A0 and c= 5.205A0). All the grown films showed high resistivity. Al: ZnO thin films were optimized in two methods 1) by dc co-sputtering from the elemental cathodes, Zinc and Aluminum, 2) dc-sputtering from the single 2% Al-doped ZnO cathode. Low resistivity Al:ZnO thin films were deposited in both the cases. Effect of Aluminum doping into ZnO crystal lattice upon the optical and electrical properties were discussed. Chapter-4 deals with the synthesis of various absorber materials, characterizations and some properties. Briefly the A) Optimization of the CuIn1-xAlxSe2 phase with better adhesion and better crystallinity. Aluminum doping into the crystal lattice of CuInSe2 aided the wide band gap tuning of CIAS thin films. Morphological investigations were carried out for the different set of thin films before and after selenization. Effects of copper and Aluminum concentrations on the lattice parameter of the selenized thin films were addressed. The present chapter deals with the A) electrical properties of CIAS films and its heterojunction partners. Resistivity measurements and effects of Cu/In ratio and the effect of Al doping were described in detail. The CIAS/ZnO heterostructure, CIAS/Al:ZnO heterostructure junction properties as a function of different sun illuminations were discussed. B) The alternative earth abundant, eco-friendly, non-toxic elements Cu2ZnSnS4, absorber thin films synthesis and characterizations. Photo conductive photo measurements showed CZTS a potential candidate for near infra-red photodectection. C) Cu2CoSnS4 (CCTS) nanostructures and quantum dots were synthesized via simple chemical routes. CCTS quantum dots were tuned to exhibit the red edge effect and cold white phosphors. D) Cu3BiS3 nano rods were synthesized and characterized structurally and optically. The transport properties of Cu3BiS3 nanorods were tailored for showing the metallic to semiconducting transitions. Chapter-5 Discusses the A) Efforts made in understanding the CIAS based solar cells through interfaces such as CIAS/ZnO, Mo/CIAS, CIAS/CdS/i-ZnO/Al:ZnO and improving the open circuit voltage VOC upon a rotating substrate, involving the inline and in situ processes, for fabricating the cell/ module were discussed. The device statistics for various set of cells were analyzed. B) Solar cells of CTS absorber with the non-toxic buffer ZnS were fabricated and device properties were analyzed. C) CCTS quantum dots embedded in the polymer matrix were utilized for making the inverted hybrid solar devices in combination of ITO/AZnO bilayered contact replacing the acidic PEDOT: PSS. D) The solar cells made of CCTS hollow spheres by spin coating the absorber in the configuration SLG/Mo/CCTS/CdS/ iZno-AZnO/Ni-Al-Al showed a lower efficiency of 0.02%. Chapter-6 concludes with the summary of present investigations and the scope for future work.

L’exploitation des couches interfaciales à base de d’oxydes métalliques ouvre des perspectives nouvelles dans le domaine des cellules photovoltaïques organiques (PVOs).Cette thèse s’inscrit dans le développement, la caractérisation et l’analyse de couches interfaciales, à base d’oxyde de zinc (ZnO), déposées par pulvérisation cathodique, dans l’élaboration de dispositifs solaires compatibles avec des substrats flexibles et des procédés « roll-to-roll ». Après un état de l’art du domaine, une première étude sera consacrée à l'apport et à l’optimisation des dépôts par pulvérisation cathodique des films de ZnO sur des couches actives de P3HT:PCBM dans le cas de structures conventionnelles. Une seconde partie mettra en évidence l'importance des procédés de recuit des couches de ZnO déposées sur des substrats flexibles ou rigides à base d’ITO pour des structures inverses de cellules PVOs flexibles. Suite à ces études, l’élaboration de ces dispositifs sera testée et validée à partir de techniques inspirées de la lithographie douce. L’interfaçage des films de ZnO avec de nouveaux matériaux tel que le graphène sera également abordé dans ces recherches. / Exploring interfacial layers based on metal oxides opens new perspectives in the field of organic photovoltaic (OPV). This thesis takes place in development, characterization and analysis of Zinc Oxide (ZnO) based interfacial layers, deposited by sputtering in the preparation of solar cells devices compatible with flexible substrates and "roll-to-roll" process. After a state of art, the first study will focus on contribution and optimization of ZnO films sputtered on active layer (P3HT:PCBM) for conventional structures. A second study will highlight the importance of annealing processes of ZnO layers deposited on flexible and rigid substrates based on ITO for inverse flexible solar cells. Following these studies, the development of the devices will be tested and validated using printing techniques inspired from soft lithography. Finally, new materials as graphene will be interfaced with ZnO layers and discuss in this research.