High Performance Thin Film Solar Cells via Nanoscale Interface
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.