Hybrid chemical bath deposition-CdS/sputter-Zn(O,S) alternative buffer for Cu2ZnSn(S,Se)4 based solar cells

Abstract

To replace the conventionally used CdS buffers in Cu2ZnSn(S,Se)4 (CZTSSe) based thin-film solar cells, sputtered Zn(O,S) buffer layers have been investigated. Zn(O,S) layers with three different [O]/([O] + [S]) ratios (0.4, 0.7, and 0.8)—and a combination of Zn(O,S) and CdS (“hybrid buffer layer”) were studied. In comparison to the CdS reference, the external quantum efficiency (EQE) of the Zn(O,S)-buffered devices increases in the short- and long-wavelength regions of the spectrum. However, the average EQE ranges below that of the CdS reference, and the devices show a low open-circuit voltage (VOC). By adding a very thin CdS layer (5 nm) between the absorber and the Zn(O,S) buffer, the VOC loss is completely avoided. Using thicker intermediate CdS layers result in a further device improvement, with VOC values above those of the CdS reference. X-ray photoelectron spectroscopy (XPS) measurements suggest that the thin CdS layer prevents damage to the absorber surface during the sputter deposition of the Zn(O,S) buffer. With the hybrid buffer configuration, a record VOC deficit, i.e., a minimum difference between bandgap energy Eg (divided by the elementary charge q) and VOC (Eg/q – VOC) of 519 mV could be obtained, i.e., the lowest value reported for kesterite solar cells to date. Thus, the hybrid buffer configuration is a promising approach to overcome one of the main bottlenecks of kesterite-based solar cells, while simultaneously also reducing the amount of cadmium needed in the device.

Interface engineering of p-n heterojunction for kesterite photovoltaics: A progress review

“…ZnO 1-x S x (Zn(O,S)) and Zn 1-x Sn x O y (ZnSnO) are suitable Cd-free alternative buffer layers when considering environmental impact, materials scarcity and bandgap tunability. Recently, replacing the CdS layer with Zn(O,S) or ZnSnO has been intensively studied in kesterite solar cells [44,45,47,49,[68][69][70][71][72]. Though Zn(O,S) shows a large tunable bandgap, kesterite solar cells with this buffer layer still possess low efficiencies (<8%) [68,71].…”

Effect of Post-thermal Annealing on the Structural, Morphological, and Optical Properties of RF-sputtered In2S3 Thin Films

“…It is well known that the high-efficient devices of the CIGS and CZTS-based solar cells generally contain n-type CdS buffers which are prepared by chemical bath deposition method (CBD) [8][9][10]. However, liquid waste resulting from CBD method contains toxic Cd, S and ammonia and it creates environmental problems and hazards for human health [11,12]. Therefore, there has been a great effort to replace CdS buffer by other cadmium-free materials.…”