Modification of electron states in CdTe absorber due to a buffer layer in CdTe/CdS solar cells

Abstract

By application of the ac admittance spectroscopy method, the defect state energy distributions were determined in CdTe incorporated in thin film solar cell structures concluded on ZnO, ZnSe, and ZnS buffer layers. Together with the Mott-Schottky analysis, the results revealed a strong modification of the defect density of states and the concentration of the uncompensated acceptors as influenced by the choice of the buffer layer. In the solar cells formed on ZnSe and ZnS, the Fermi level and the energy position of the dominant deep trap levels were observed to shift closer to the midgap of CdTe suggesting the mid-gap states may act as recombination centers and impact the open-circuit voltage and the fill factor of the solar cells. For the deeper states, the broadening parameter was observed to increase indicating fluctuations of the charge on a microscopic scale. Such changes can be attributed to the grain-boundary strain and the modification of the charge trapped at the grain-boundary interface states in polycrystalline CdTe.

Regimes of current transport mechanisms in CdS/CdTe solar cells

“…Since absorption in the CdS window layer does not contribute to the photocurrent, its thickness also has a profound effect on solar cell performance. Finally, the window layer type has been shown to influence the deep levels present in the absorber, with CdS, ZnS and ZnSe all giving different behaviours [4]. As its thickness is understood to also influence the electrical junction [5], this confirmed the selection of the CdS thickness as an experimental variable in the present work.…”

Solution-processed all-oxide solar cell based on electrodeposited Cu2O and ZnMgO by spray pyrolysis

“…Another factor is an increased deep-level trap density in the Zn 0.88 Mg 0.12 O buffer layer that partly depletes the absorber from free carriers at the heterojunction vicinity. Such effect has been seen in other types of solar cells in the literature, as in CdS/CdTe heterojunctions, grown on different substrate layers [58].…”

Band Alignment for Rectification and Tunneling Effects in Al2O3 Atomic-Layer-Deposited on Back Contact for CdTe Solar Cell

“…Although Si-based solar cell devices currently dominate photovoltaic power production, thin-film solar cell devices based on CdTe have achieved a notable market share as a cost-effective alternative. − For thin-film CdTe solar cells, efficiency improvement is always the main goal. Generally, the common methods for improving efficiencies include designing new device structures, − improving the material composition and related crystalline quality of thin films, , and optimizing the contact interfaces between thin films. − The back contact is a key issue for the performance of CdTe thin-film solar cells because it is challenging to attain low resistance and a stable electrical contact. − Many efforts have been devoted to optimize the back contact based on physical concepts and nanoengineering. − …”