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.
Currently, the use of Zn(O,S) as buffer material for Cu(In,Ga)Se2 (CIGS) solar cells is intensely studied in order to further boost the performance of these devices. In this context, nondestructive analytical tools are needed that enable the determination of buffer bandgap energies in the complete device. To this end, we developed a spectroscopic approach based on electroreflectance (ER). From a set of measured angle-resolved ER (ARER) spectra, an averaged modulus spectrum is numerically calculated. This method suppresses the commonly observed detrimental line-shape distortions due to interference effects in the layered device structure and thus enables the determination of bandgap energies even for thin buffer layers. To verify the working principle of ARER, we first apply it to CIGS absorber and CdS buffer layers. Then, we utilize it to investigate CIGS solar cells with Zn(O,S) buffers. All ARER results are compared to the results of diffuse ER, a technique previously developed for the suppression of interference fringes. We demonstrate that ARER is the superior ER method for nondestructive bandgap determination of thin buffer layers in complete CIGS solar cells. Moreover, a Cu containing compound was determined as a secondary phase in the Zn(O,S) buffer by combined ARER studies, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy.
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