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

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 (V_OC). By adding a very thin CdS layer (5 nm) between the absorber and the Zn(O,S) buffer, the V_OC loss is completely avoided. Using thicker intermediate CdS layers result in a further device improvement, with V_OC 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 V_OC deficit, i.e., a minimum difference between bandgap energy E_g (divided by the elementary charge q) and V_OC (E_g/q – V_OC) 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.