High efficiency polycrystalline CdS/CdTe solar cells on buffered commercial TCO-coated glass
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Multiple polycrystalline CdS/CdTe solar cells with efficiencies greater than 15% were produced on buffered, commercially-available Pilkington TEC GlassTM at EPIR Technologies, Inc. (EPIR) and verified by the National Renewable Energy Laboratory (NREL). n-CdS and p-CdTe were grown by chemical bath deposition (CBD) and close space sublimation, respectively. Samples with sputter-deposited CdS were also investigated. Initial results indicate that this is a viable dry-process alternative to CBD for production-scale processing. Published results for polycrystalline CdS/CdTe solar cells with high efficiencies are typically based upon cells utilizing research-grade transparent conducting oxides (TCOs) requiring high-temperature processing inconducive to low-cost manufacturing. EPIR’s results for cells on commercial glass were obtained by implementing a high resistivity SnO2 buffer layer and optimizing the CdS window layer thickness. The high resistivity buffer layer prevents the formation of CdTe-TCO junctions, thereby maintaining a high open circuit voltage and fill factor; while using a thin CdS layer reduces absorption losses and improves the short circuit current density. EPIR’s best device demonstrated an NREL-verified efficiency of 15.3%. The mean efficiency of hundreds of cells produced with a buffer layer between December 2010 and June 2011 is 14.4%. Quantum efficiency results are presented to demonstrate EPIR’s progress toward NREL’s best-published results.