Reduced defect density at the CZTSSe/CdS interface by atomic layer deposition of Al2O3

Abstract

The greatest challenge for improving the power conversion efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)/CdS/ZnO thin film solar cells is increasing the open circuit voltage (VOC). Probable leading causes of the VOC deficit in state-of-the-art CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1 nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature. Capacitance-voltage profiling and quantum efficiency measurements reveal that ALD-Al2O3 interface modification reduces the density of acceptor-like states at the heterojunction resulting in reduced interface recombination and wider depletion width. Indications of increased VOC resulting from the modification of the heterojunction interface as a result of ALD-Al2O3 treatment are presented. These results, while not conclusive for application to state-of-the-art high efficiency CZTSSe devices, suggest the need for further studies as it is probable that interface recombination contributes to reduced VOC even in such devices.

Improving the open‐circuit voltage of Cu2ZnSnSe4 thin film solar cells via interface passivation

“…We note that a couple of papers on passivation of top CZTSe surfaces with a thin Al 2 O 3 layer were very recently published while we were preparing this manuscript . Our work differs from those studies in that we have demonstrated that the chemical role of the Al 2 O 3 passivation layer, that is the suppression of Cu–Cd inter‐diffusion leading to a cleaner CZTSe/CdS junction, is responsible for the improved V oc .…”

P–N Junction Passivation in Kesterite Solar Cells by Use of Solution-Processed TiO2 Layer

“…Erkan et al also used an ALD-coated Al 2 O 3 layer between a CZTSSe absorber layer and CdS layer. It was shown statistically that the passivated solar cells have higher V OC , almost similar J SC and lower FF as compared to the unpassivated solar cells [15]. However, in general the passivated and unpassivated CZTSSe solar cells in their study had lower performance than the state of the art especially due to low FF.…”

“…Recently, several studies focused on passivation of the p-n junction in kesterite solar cells by introducing n-type metal oxide layers such as Al 2 O 3 [15], [16] or TiO 2 [17] layers between the kesterite absorber and CdS buffer layers which did improve V OC . The improvement of V OC using the passivation layers can be explained by reducing the interface recombination by chemical passivation (reduction of interface trap density) and field effect passivation (formation of a fixed charge density and consequently decreasing the charge carrier concentration at the interface).…”

A Review of CZTS Thin Film Solar Cell Technology

“…These encouraging findings suggest that low-cost and environmentally friendly CBD Zn(O, S) films promise to be used as the Cd-free buffer layer for solar cells with kesterite thin film [36]. Table 2 below summarize optical band alignment of different buffer/absorber interfaces below: CdS are widely used as a buffer layer in CZTS-based solar cells [37]. However, the carrier recombination at the interface between CZTS absorber and Cds buffer results in a decrease of Voc.…”