Strategies for the development of high performance organic-inorganic perovskite solar cells and their novel application

Abstract

Organic-inorganic perovskite thin films have drawn significant attention in last several years because of the tremendous progress in the development of high efficiency solar cells. This impressive progress can be ascribed to the ideal physical properties of these cost-effective perovskite materials such as bandgap tunable, high absorption coefficients, and long carrier diffusion lengths. High quality perovskite films are the key important factor in achieving the high performance of solar cells. The growth techniques of perovskite film have strong impact on the quality of perovskite materials and now common growth processes for the perovskite materials can be roughly classified into solution technique, thermal evaporation (TE) deposition, and solution-vapor hybrid deposition. This thesis covers an overview of recent development of perovskite solar cells (PSCs), fundamental knowledge of the solar cells, characterization techniques as well as different strategies for achieving high performance and novel PSCs. In this thesis, a series of optimized and controllable fabrication techniques for yielding high performance devices were systematically investigated. Firstly, a modified TE method was introduced to prepare high quality CH₃NH₃PbI₃ (MAPI) layers. A uniform and pinhole-free perovskite films can be achieved by using this TE technique. We successfully demonstrate a MAPI-based planar PSCs with power conversion efficiencies (PCEs) as high as 12.5% through the optimization of the thickness of each precursor layer and the number of precursor pairs. To achieve further improvement of device performance, a series of optimized controllable fabrication processes were established for yielding devices with PCEs as high as 15.8%. We conducted systematic investigations of the underlying mechanism of the improvements in the PCEs due to post-deposition oxygen annealing in solution-processed planar MAPI-based PSCs. Further investigation is conducted by introducing O₂ during the perovskite formation stage to enhance the incorporation of oxygen in the perovskite material. Then, we designed a highly versatile hybrid chemical vapor deposition (HCVD) process for the growth of high quality MAPI layers which were crystallized in a well-controlled ambient consisting of N₂/O₂ mixture resulting in high crystallinity, large grain size, good uniformity and low defect density films. We found that a N₂/O₂ mixture carrier gas is effective in passivating traps in the perovskite films. With optimized controllable HCVD perovskite growth process, a champion device with power conversion efficiency of 17.6% is achieved. A proven method to break the S-Q limit of the efficiency of a single junction solar cell is to adopt a tandem solar cell structure which is composed of a low bandgap material and a high bandgap material. Mechanically stacked four-terminal tandem devices offer larger flexibility for different combinations of the top and bottom cells as the two cells are mechanically stacked upon each other with independent connections. Here, combining Si solar cells with optimized perovskite solar cells is an effective strategy to overcome the efficiency bottleneck. To obtain high PCE of four-terminal perovskite/c-Si tandem solar cells, efficient engineering of the optical and electrical properties of the tandem devices are critical. Based on previous optimized fabrication technique of perovskite solar cells, another two effective optical engineering techniques were developed to boost the high performance of four-terminal perovskite/c-Si tandem solar cells: 1. Optical engineering of the transparent electrode (MoO₃/Au/MoO₃) to obtain high transmission at long wavelength for the tandem solar cell applications. 2. Enhancement of light harvesting power achieved by adopting the novel biomimicking elastomeric petals as the light trapping layer. An average PCE of 23.5 % was achieved for perovskite/c-Si four-terminal tandem device with optimized optical features. In addition, we successfully demonstrate a novel tandem structure of co-anode (ca) and co-cathode (cc) photovoltachromic cells (PVCCs) by vertically integrating a semi-transparent perovskite solar cell and electrochromic supercapacitor. The PVCCs exhibit a seamless integration of energy harvesting-storage device, automatic and wide color tunability and enhanced photo-stability of PSCs. As the colored PVCC blocks off most of the illuminated light, it automatically switches off the photo-charging. The perovskite solar cell then operates under a low-power operating state, which prevents the perovskite solar cell from long-time sunlight-exposure and prolongs its working lifetime. These works provide unprecedented advantages over conventional smart window design.