It has been 64 years since Bell Laboratories built the first silicon solar cell in 1954. The harnessing of the almost unlimited energy from the sun for human civilization seems not an untouchable dream anymore. However, the rapid growth of the global population companied with the growing demand to enable a decent life quality causes the energy issue more challenging than ever. Nowadays silicon solar cells continue to take a leading position, not only offering potential solutions for energy demands but also stimulating the development of various photovoltaic technologies. Among them, solution processible thin film solar cells attract most attentions due to multiple advantages over traditional silicon solar cells. In this dissertation, I focus on two most promising types of them: 1) kesterite solar cells and 2) hybrid organic-inorganic perovskite solar cells. Particularly I work on the grain growth mechanism and processing techniques via nanoscale interface engineering to improve materials thin film properties and device architecture design. In Chapter 3, Cu2ZnSn(S,Se)4 was used as a model system to demonstrate the kinetic control of solid-gas reactions at nanoscale by manipulating the surface chemistry of both sol-gel nanoparticles and colloidal nanocrystals. It was identified that thiourea (commonly used as sulfur sources for metal sulfides) can transform to melamine during the film formation, and melamine would serve as surface ligands for as-formed Cu2ZnSn(S,Se)4 nanoparticles. These surface ligands can affect the solid-gas reactions during the selenization, which enable us to control film morphologies and device performance by simply adjusting the amount of surface ligands. To further enhance Cu2ZnSn(S,Se)4 device performance, a systematic investigation on alkali metal doping effect was conducted. In Chapter 4, alkali metal-containing precursors were used to study influences on Cu2ZnSn(S,Se)4 film morphology, crystallinity and electronic properties. K-doped Cu2ZnSn(S,Se)4 solar cells showed the best device performance. Due to the surface electronic inversion effect, various thickness of CdS buffer layers were tested on K-passivated Cu2ZnSn(S,Se)4 surface for further improving device efficiency. Over 8% power conversion efficiency of K-doped Cu2ZnSn(S,Se)4 solar cell with 35 nm CdS has been reached. Finally, in Chapter 5, the hybrid organic-inorganic perovskite solar cells are introduced. We demonstrated a novel tandem device employing nanoscale interface engineering of Cu(In,Ga)Se2 surface alongside a heavy-doped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] hole transporting layer between the two subcells that preserves open-circuit voltage, and enhanced both fill factor and short-circuit current. As a result, we have successfully doubled the previous efficiency record for a monolithic perovskite/Cu(In,Ga)Se2 tandem solar cell to 22.43% power conversion efficiency, which is the highest record among thin film monolithic tandem photovoltaic devices. The conclusion and future outlooks of my works on kesterite and perovskites solar cells are summarized in Chapter 6.

Thin-film kesterites have been explored as promising absorbers in future photovoltaic devices due to their earth-abundant and non-toxic constituents, which do not impose any future production limitations. However, the current record conversion efficiency of polycrystalline kesterite devices is 12.6%--i.e., at least 2.4% short of the efficiency threshold needed to make this material competitive with chalcogenide-based thin film technologies. This shortage in conversion efficiency has been in part ascribed to the large extent of carrier recombination by defects at the grain boundaries and contact/absorber interfaces. In this work, methods nanoscale compositional and electrical characterization of grain boundaries and contact/absorber interfaces in kesterite solar cells have been developed, using a unique combination of advanced nano-characterization tools including Auger Nanoprobe Microscopy (NanoAuger), Kelvin Probe Force Microscopy (KPFM) and Cryogenic Focused Ion Beam (Cryo-FIB). NanoAuger and KPFM measurements on high-performance CZTSSe thin film PV devices revealed that the presence of SnOx at the grain boundaries is essential to the high VOC. This passivation layer needs to be formed by an air anneal process performed after the film deposition. In contrast to the oxide at the grain boundary, oxide layer on the top surfaces of the grains has been found to be (Sn,Zn),O. A new cross-sectioning method via grazing angle of incidence Cryo-FIB milling, has been developed where smooth cross-sections with at least 10x scale expansion have been prepared. These surfaces were characterized for CIGSe monitor films confirming the presence of MoSe2 interlayer acting as a proper hole contact on the back surface.

The fundamental concept of the book is to explain how to make thin film solar cells from the abundant solar energy materials by low cost. The proper and optimized growth conditions are very essential while sandwiching thin films to make solar cell otherwise secondary phases play a role to undermine the working function of solar cells. The book illustrates growth and characterization of Cu2ZnSn(S1-xSex)4 thin film absorbers and their solar cells. The fabrication process of absorber layers by either vacuum or non-vacuum process is readily elaborated in the book, which helps for further development of cells. The characterization analyses such as XPS, XRD, SEM, AFM etc., lead to tailor the physical properties of the absorber layers to fit well for the solar cells. The role of secondary phases such as ZnS, Cu2-xS,SnS etc., which are determined by XPS, XRD or Raman, in the absorber layers is promptly discussed. The optical spectroscopy analysis, which finds band gap, optical constants of the films, is mentioned in the book. The electrical properties of the absorbers deal the influence of substrates, growth temperature, impurities, secondary phases etc. The low temperature I-V and C-V measurements of Cu2ZnSn(S1-xSex)4 thin film solar cells are clearly described. The solar cell parameters such as efficiency, fill factor, series resistance, parallel resistance provide handful information to understand the mechanism of physics of thin film solar cells in the book. The band structure, which supports to adjust interface states at the p-n junction of the solar cells is given. On the other hand the role of window layers with the solar cells is discussed. The simulation of theoretical efficiency of Cu2ZnSn(S1-xSex)4 thin film solar cells explains how much efficiency can be experimentally extracted from the cells. One of the first books exploring how to conduct research on thin film solar cells, including reducing costs Detailed instructions on conducting research

This book provides recent development in thin-film solar cells (TFSC). TFSC have proven the promising approach for terrestrial and space photovoltaics. TFSC have the potential to change the device design and produce high efficiency devices on rigid/flexible substrates with significantly low manufacturing cost. TFSC have several advantages in manufacturing compared to traditional crystalline Si-solar cells like less requirement of materials, can be prepared with earth’s abundant materials, less processing steps, easy to dispose, etc. Several universities/research institutes/industry in India and abroad are involved in the research area of thin-film solar cells. The book helps the readers to find the details about different thin-film technologies and its advancement at one place. Each chapter covers properties of materials, its suitability for PV applications, simple manufacturing processes and recent and past literature survey. The issues related to the development of high efficiency TFSC devices over large area and its commercial and future prospects are discussed.

The first comprehensive book on thin-film solar cells, potentially a key technology for solving the energy production problem in the 21st century in an environmentally friendly way. It covers a wide range of scientific and technological aspects of thin film semiconductors - deposition technologies, growth mechanisms and the basic properties of amorphous and nano-crystalline silicon - as well as the optimum design theory and device physics of high-efficiency solar cells, especially of single-junction and multi-junction solar cells. The development of large-area solar cell modules using single and multi-junction solar cells is also considered. Examples of recent photovoltaic systems are presented and analysed.

"You, 0 Sun, are the eye of the world You are the soul of all embodied beings You are the source of all creatures You are the discipline of all engaged in work" - Translated from Mahabharata 3rd Century BC Today, energy is the lifeline and status symbol of "civilized" societies. All nations have therefore embarked upon Research and Development pro grams of varying magnitudes to explore and effectively utilize renewable sources of energy. Albeit a low-grade energy with large temporal and spatial variations, solar energy is abundant, cheap, clean, and renewable, and thus presents a very attractive alternative source. The direct conver sion of solar energy to electricity (photovoltaic effect) via devices called solar cells has already become an established frontier area of science and technology. Born out of necessity for remote area applications, the first commercially manufactured solar cells - single-crystal silicon and thin film CdS/Cu2S - were available well over 20 years ago. Indeed, all space vehicles today are powered by silicon solar cells. But large-scale terrestrial applications of solar cells still await major breakthroughs in terms of discovering new and radical concepts in solar cell device structures, utilizing relatively more abundant, cheap, and even exotic materials, and inventing simpler and less energy intensive fabrication processes. No doubt, this extraordinary challenge in R/D has led to a virtual explosion of activities in the field of photovoltaics in the last several years.