| dc.description.abstract | Renewable energy sources based on thin film solar cells (TFSC) can effectively contribute in a wide variety of energy field due to their higher light absorbance, shorter energy payback time, and high flexibility. Recently, researches on earth-abundant and non-toxic kesterite based Cu2ZnSn(S,Se)4 (CZTS/Se) solar cell has sparked a lot of attention in the photovoltaic sector. However, large open-circuit voltage deficit (VOC,def) issue in CZTS/Se solar cell remains unsolved even after a decade of research and development. Therefore, in-depth research to solve fundamental bottlenecks are required to address the VOC,def issue. This thesis focuses on investigating the fundamental limitations of CZTSe solar cells, in particular the large VOC,def, by different characterization techniques.
One of the reasons that substantially impairs the photovoltaic (PV) performance of CZTSe is occurrence of intrinsic point/cluster defects. Understanding the nature of and controlling cation disorder remains a crucial challenge for improving their PV performances. Herein, a cation substitution method has been introduced to control and passivate the defect states in bandgap of CZTSe by incorporating Ag. Different x values of (AgxCu1-x)2ZnSnSe4 (ACZTSe) thin film absorbers were synthesized, where x = Ag/(Ag+Cu), i.e., with ratios of x = 0.00, 0.05, 0.10, 0.15, and 0.20 to provide a comprehensive understanding of defect states for ACZTSe solar cell. Intensity-dependent low-temperature photoluminescence measurements show that 10% Ag-alloyed CZTSe provides the shallowest defect states and less nonradiative recombination. It is also illustrated by first-principles calculations that Ag alloying enables the formation and suppresses the beneficial and detrimental defects, respectively. The best power conversion efficiency of 10.2% is achieved for the 10% Ag-alloyed CZTSe cell, along with an enhanced open-circuit voltage.
To determine the relationships among cation disorder, defect concentration, overall long-range crystallographic order, and local atomic-scale structure for (AgxCu1−x)2ZnSnSe4 (ACZTSe) material, the combination of neutron diffraction and synchrotron-based x-ray absorption techniques are implemented. The joint Rietveld refinement technique is used to quantify the concentration of defects in Ag-alloyed stoichiometric and non-stoichiometric Cu2ZnSnSe4 (CZTSe) powder samples. As main outcome, the cation distribution was determined to quantify the intrinsic point defects. This directly shows that Ag effectively inhibit the high concentration of the deep CuZn antisite and promotes shallower defects such as Cu-vacancy (VCu), which is important for improved device performance. Moreover, we studied the atomic-scale structure of ACZTSe as a function of composition using x-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) analyses of the nearest neighbor shell has been performed by simultaneous fitting of all K-edges to determine oxidation states, charge transfer mechanism (reflecting the occupancy of electronic states at/near the probed element) and structural parameters, respectively. | en_US |