Abstract
Selenium compositional grading in CdTe-based thin-film solar cells substantively improves carrier lifetime and performance. However, where and how recombination lifetime improves has not been studied significantly. Here, we deposit a CdSexTe1−x/CdTe bilayer on MgZnO/SnO2/glass, which achieves a short-circuit current density greater than 28 mA/cm2 and carrier lifetimes as long as 10–20 ns. We analyze 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 pn-junction and significantly larger CdTe grains in the rest of the film, both time-resolved photoluminescence and cathodoluminescence reveal that the carrier lifetime in CdSeTe alloy regions is longer than in CdTe regions. The results indicate that Se both passivates grain boundaries and improves grain-interior carrier lifetime. However, these effects occur only where there is significant alloying, which is important for bandgap engineering.
Highlights
Thin-film polycrystalline CdTe-based solar cells have reached 22.1% cell efficiency.1 More importantly, commercial modules have reached efficiencies of 17%–18% at costs competitive with conventional energy sources, and they have superior temperature coefficient and spectral response relative to silicon and other solar technologies.2,3 Eliminating the CdS window layer and introducing CdSexTe1−x (CdSeTe) bandgap grading contributed to the steady increase in small-area research cell efficiency from 16% to 22.1%
The external quantum efficiency (EQE) in Fig. 2(b) indicates how CdSeTe increases photocurrent by collecting more light in the red spectral region
Auger profiles on CdSeTe/CdTe devices indicate about 8% Se incorporation at the front interface that corresponds with a layer of small CdSeTe grains as revealed by electron backscatter diffraction (EBSD)
Summary
Thin-film polycrystalline CdTe-based solar cells have reached 22.1% cell efficiency. More importantly, commercial modules have reached efficiencies of 17%–18% at costs competitive with conventional energy sources, and they have superior temperature coefficient and spectral response relative to silicon and other solar technologies. Eliminating the CdS window layer and introducing CdSexTe1−x (CdSeTe) bandgap grading contributed to the steady increase in small-area research cell efficiency from 16% to 22.1%. CdSeTe has helped increase photocurrent by lowering the energy bandgap but without commensurate open-circuit voltage (Voc) losses. This is attributed in part to longer carrier lifetimes relative to CdTe-only absorbers.. The regions and physical mechanisms where CdSeTe produces longer carrier lifetime in CdSeTe/CdTe devices are critical to bandgap engineering. After the CdCl2 treatment, Se diffuses into the CdTe region both along the grain boundaries (GBs) and in the grain interior (GI) It is not clear how much the lifetime throughout will be adjusted by varying Se levels, ranging from an alloy at the front to impurity levels (
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