Copper Zinc Tin Sulfide Thin Films For Photovoltaics PDF Download

The world energy consumption has increased rigorously in recent years due to the rapid economic development and the massive global population expansion. Today the world energy supply relies heavily on fossil fuels, known as non-renewable energy resources, which have limited reserves on Earth and do not form or replenish in a short period of time. Burning fossil fuels not only brings environmental pollutions but also results in carbon dioxide and other greenhouse gases, which are to blame for global warming. Therefore, to build a more sustainable and greener future, we have to develop alternative renewable energy resources. Photovoltaic (PV) cell, also commonly known as solar cell, is a very promising renewable energy technology. Here in this dissertation, we have studied two emerging PV materials with earth abundant elements, i.e. copper zinc tin sulfide (CZTS) and organic-inorganic hybrid halide perovskite. Having earth abundant elements means that the raw materials have rich reserves on Earth and the costs are relatively low. It also means that the materials have the potential capability to be produced in large scales in industry. We first explored two different deposition methods for preparing CZTS thin films. In the first method, the CZTS was fabricated by a solution based method with diethyl sulfoxide (DMSO) as the solvent and the effect of spin speed on the properties of CZTS thin films was studied. The results indicated that a higher spin speed was more favorable for attaining a more densely packed and pinhole-free film while no crystallographic differences were observed. In the second method, CZTS was fabricated using sputtered metal precursors followed by a closed-space sulfurization (CSS) technique, which had high manufacturing compatibility and could be applied in industry. After exploring different sulfurization conditions, including temperatures and time, the champion cell was obtained at 590oC for 30min, with a maximum power conversion efficiency (PCE) of 5.2%. We then explored three different organic-inorganic hybrid halide perovskite materials for solar cell applications. The first perovskite material is methylammonium tin triiodide (MASnI3, bandgap ~1.3 eV). It was fabricated by a hybrid thermal evaporation. The as-deposited MASnI3 thin films exhibit smooth surfaces, uniform coverage across the entire substrate, and strong crystallographic preferred orientation along the 100 direction. Our results demonstrate the potential capability of the hybrid evaporation method for preparing high-quality MASnI3 perovskite thin films which can be used to fabricate efficient lead (Pb)-free perovskite solar cells (PVSCs). The second perovskite material is mixed-cation (formamidinium and cesium) lead iodide (FA0.8Cs0.2PbI3). We find that one of the main factors limiting the PCEs of FA0.8Cs0.2PbI3 PVSCs could be the small grain sizes, which leads to relatively short mean carrier lifetimes. We further find that adding a small amount of lead thiocyanate additive can enlarge the grain size of FA0.8Cs0.2PbI3 perovskite thin films and significantly increase the mean carrier lifetime. As a result, the average PCE of FA0.8Cs0.2PbI3 PVSCs increases from 16.18 ± 0.50 (13.45 ± 0.78)% to 18.16 ± 0.54 (16.86 ± 0.63)% when measured under reverse (forward) voltage scans. The best-performing FA0.8Cs0.2PbI3 PVSC registers a PCE of 19.57 (18.12) % when measured under a reverse (forward) voltage scan. The third perovskite material is FA0.8Cs0.2Pb(I0.7Br0.3)3 (bandgap ~1.75 eV). We find that the cooperation of lead thiocyanate additive and a solvent annealing process can effectively increase the grain size of the perovskite thin films while avoiding the undesired excess lead iodide formation. As a result, the average grain size of the FA0.8Cs0.2Pb(I0.7Br0.3)3 perovskite thin films increases from 66 ± 24 nm to 1036 ± 111 nm and the mean carrier lifetime shows a more than 3-fold increase, from 330 ns to over 1000 ns. As a result, the average open-circuit voltage (Voc) of FA0.8Cs0.2Pb(I0.7Br0.3)3 PVSCs increases by 80 (70) mV and the average PCE increases from 13.44 ± 0.48 (11.75 ± 0.34)% to 17.68 ± 0.36 (15.58 ± 0.55)% when measured under reverse (forward) voltage scans. The best-performing wide-bandgap (~1.75 eV) PVSC registers a stabilized PCE of 17.18%, demonstrating its suitability for top cell applications in all-perovskite tandem solar cells.