The distribution of the electrostatic potential in and between the materials in Cu(In,Ga)Se2 thin-film solar cells has a major impact on their superior performance. This thesis reported on the nanoscale imaging of the electrostatic potential on untreated cross sections of operating Cu(In,Ga)Se2 solar cells using Kelvin probe force microscopy.

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.

Nowadays, thin-film solar cells potentially offer a suitable technology for solving the energy production problem with an environmentally friendly method. Besides, thin film technologies show advantages over their bulk-semiconductor counterparts due to their lighter weight, flexible shape and device fabrication schemes and low cost in large-scale industrial production. Although many books currently exist on general concepts of PV materials and devices, few are offering a comprehensive overview of the fast development in thin film Cu(In, Ga)Se2-based solar cells. "Key Developments in CuInGaSe2 Thin Film Solar Cell" would provide an international perspective on the latest research on this topic. It presents a wide range of scientific and technological aspects on basic properties and device physics of high-efficiency CIGS solar cells from the last research frontier point of view. The book was designed for photovoltaic researchers and scientists, students and engineers, with the mission to provide knowledge of the mechanisms, materials, devices, and applications of CIGS-based technology necessary to develop cheaper and cleaner renewable energy in the coming years.

This book offers a bird’s-eye view of the recent development trends in photovoltaics – a big business field that is rapidly growing and well on its way to maturity. The book describes current efforts to develop highly efficient, low-cost photovoltaic devices based on crystalline silicon, III–V compounds, copper indium gallium selenide (CIGS) and perovskite photovoltaic cells along with innovative, cost-competitive glass/ flexible tubular glass concentrator modules and systems, highlighting recent attempts to develop highly efficient, low-cost, flexible photovoltaic cells based on CIGS and perovskite thin films. This second edition presents, for the first time, the possible applications of perovskite modules together with Augsburger Tubular photovoltaics.