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The Study of Polycrystalline CdSexTe1-x Synthesis, Band Engineering, and Electro-Optical Properties
thesisposted on 01.05.2021, 00:00 authored by xin Zheng
Alloying selenium (Se) with cadmium telluride (CdTe) is considered one of the key roles for breaking the efficiency record of solar cells from 16.8% to 22.1%. This research focuses on polycrystalline CdSexTe1-x (px-CdSeTe) thin-film synthesis, TEC glass/MgxZn1-xO (MZO)/gradient CdSeTe/CdTe/Au device fabrication, band engineering, and electro-optical properties analysis of px-CdSeTe in device application. First, px-CdSeTe was deposited on TEC glass using the physical vapor deposition method, including close-spaced sublimation (CSS) and thermal evaporation. The incorporation of Se with different deposition methods had evaluated. CdSeTe/CdTe bilayer devices on TEC glass/MZO substrates were then optimized and compared. The intentional introduction of oxygen in the ambient during Cl passivation resulted in a roll-under feature in the current density-voltage (J-V) character. Varying the MZO buffer and Se/(Se+Te) mole ratio in the front were both used to optimize the device performance. Next, the electrical and optical properties of devices with and without CdSeTe in the same fabrication condition were compared to evaluate the role of a gradient CdSeTe interlayer in devices. Alloying Se in the front substantively improves carrier lifetime and photocurrent collection. In order to examine where and how recombination lifetime improvement impacted device performance, we analyzed the grain structure, composition, and recombination through the thickness of the absorber using electron backscatter diffraction, auger-electron spectroscopy, cathodoluminescence spectrum imaging, and time-resolved photoluminescence microscopy. Despite small CdSeTe grains near the p-n-junction and significantly larger CdTe grains in the rest of the film, both time-resolved photoluminescence and cathodoluminescence revealed that the carrier lifetime in CdSeTe alloy regions is longer than that in CdTe regions. The results indicate that Se both passivates grain boundaries and improves grain-interior carrier lifetime. However, these effects only occur where there is significant alloying, which is essential for the CdSeTe/CdTe bilayer device engineering. To further understand why CdSeTe alloying significantly increase the efficiency of CdTe-based solar technology without a huge open-circuit voltage (Voc) deficit, we built a computational model that compares Se grading profile, carrier lifetimes, band alignment, and carrier concentration contribution to transport, recombination, and performance. We found that the photocurrent gain caused by bandgap narrowing alone is insufficient to describe experimental efficiency gains. Performance can be increased by adjusting Se compositions and band grading depths. However, these performance gains are small relative to the enhanced lifetime contributions by Se alloying, which matched our experimental observations. Similarly, CdSeTe band alignment shifts can significantly increase performance if front interface recombination is prevalent. For a wide range of Se grading profiles, the hole density is a critical component for achieving efficiency greater than 25%. Finally, we sought to understand the necessity of high doping in Se rich regions. We examined arsenic (As) and Se incorporation in px-CdSe/CdTe:As and px-CdSeTe/CdTe:As bilayer through interdiffusion. Results showed a significant Se/As interdiffusion and As accumulation at the interface after activation. No clear dopants segregation in the grain boundary (GB) was observed. Numerical modeling shows that a non-uniform hole density distribution and high interface recombination are the challenges for low Voc and low fill factor. Interface passivation and the As interdiffusion CdSeTe would be interesting areas for further study.