Study of the Impact of Microstructures and Interface Energetics in Perovskite and Organic Solar Cell

To deal with the increasing demand on energy and the concerns about fossil fuels, solar energy has become one of the most promising alternative energy. Photovoltaic technology has been developed to harnesses the solar energy. Different types of solar cells depending on the materials and structures of the devices have been developed, such as crystalline Si cells, dye sensitized solar cells, perovskite solar cells and organic photovoltaics. Solar cells with high efficiency, low cost, and excellent stability are desirable for the market.;The first part of this study focuses on the organic-inorganic hybrid perovskite photovoltaics. Solar cells consisting of polycrystalline perovskite thin films have demonstrated a rapid increase of power conversion efficiency (PCE) in the past few years. To further boost the device performance, it is crucial to understand how the microstructures, such as the film texture, grains and grain boundaries, impact the electrical properties of the perovskite thin film. The ramp-annealing treatment is adapted to tailor the texture of perovskite films, where a strong correlation between the device performance and the thin film texture is revealed by X-ray diffraction (XRD) and J-V characteristics. Electrochemical impedance spectroscopy (EIS) further suggests that the enhanced texture structure not only suppresses recombination at the contact but also improves the carrier diffusion length, which ultimately contributes to better device performance.;The other important feature of the polycrystalline thin film is grains and grain boundaries. To investigate the influence of these microstructures on device performance, photo-conducting atomic force microscopy (pc-AFM) and Kelvin probe force microscopy (KPFM) measurements, which provide the nano-scale resolution, are performed on perovskite thin films with columnar structures. Three discrete photocurrent levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain identified by electron backscattering diffraction (EBSD). Local J-V curves measured on these grains further suggest an anti-correlation behavior between short-circuit current (Jsc) and open-circuit voltage (Voc). These results suggest the orientation-dependent carrier mobility in perovskite thin films. In addition, the photoresponse of perovskite films displays a pronounced heterogeneity across grain boundaries, with low-angle boundaries exhibiting even better performance than the adjacent grain interiors. KPFM further reveals the downward band bending at grain boundaries which draws electrons and repels holes. Thus, the low-angle grain boundaries facilitate the electron transport and suppress recombination.;The second part of this study focuses on the interface engineering of organic photovoltaics (OPVs). Organic photovoltaics have attracted a significant amount of attention as they offer potential benefits of low cost and mechanical flexibility. It is known that in OPV devices the energy level alignment at the interfaces between metal electrodes and the photoactive layer is critical in determining the charge collection efficiency. Here, zinc oxide (ZnO) buffer layer is introduced between the bulk heterojunction (BHJ) organic layer and the cathode material. By varying the processing condition of ZnO layer, the energy level alignment at the contact is tuned and thus the device performance. The interfacial energetics is further investigated by KPFM. Schottky barriers with varied widths are identified at ITO/ZnO interfaces. With electrons tunneling through the narrow Schottky barrier, the charge collection efficiency at the cathode is improved.