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Abstract
Ag alloying and the introduction of alkali elements through a postdeposition treatment are two approaches to improve the performance of Cu(In,Ga)Se2 (CIGS) thin film solar cells. In particular, a postdeposition treatment of an alkali metal fluoride of the absorber has shown a beneficial effect on the solar cells performance due to an increase in the open circuit voltage (VOC) for both (Ag,Cu)(In,Ga)Se2 (ACIGS) and CIGS based solar cells. Several reasons have been suggested for the improved VOC in CIGS solar cells including absorber surface and interface effects. Less works investigated how the applied postdeposition treatment influences the ACIGS absorber surface and interface properties and the subsequent buffer layer growth. In this work we employed hard X-ray photoelectron spectroscopy to study the chemical and electronic properties at the real functional interface between a CdS buffer and ACIGS absorbers that have been exposed to different alkali metal fluoride treatments during preparation. All samples show an enhanced Ag content at the CdS/ACIGS interface as compared to ACIGS bulk-like composition, and it is also shown that this enhanced Ag content anticorrelates with Ga content. The results indicate that the absorber composition at the near-surface region changes depending on the applied alkali postdeposition treatment. The Cu and Ga decrease and the Ag increase are stronger for the RbF treatment as compared to the CsF treatment, which correlates with the observed device characteristics. This suggests that a selective alkali postdeposition treatment could change the ACIGS absorber surface composition, which can influence the solar cell behavior.
Highlights
Solar cells based on Cu(In,Ga)Se2 (CIGS) thin film absorbers have shown high and stable efficiency values for both laboratory cells and industrial modules with a recent record cell efficiency of 23.4%.1 To reach closer to the theoretical maximum efficiency of around 33%, further reductions of optical and electrical losses are needed
It is likely that a redistribution of the absorber elements near the surface region may occur during the alkali postdeposition treatment (PDT), and recent studies showed the formation of an Alk−In− Se layer in CIGS solar cells subjected to an alkali metal fluoride
The survey spectra measured at 3 keV, that is, with a lower bulk sensitivity, are very similar for the 18 nm thick CdS samples and show mainly the signals from the CdS buffer (Cd and S signals), while for the 9 nm thick CdS samples, some weak Cu, In, Se, and Ag signals are observed, in addition to the signals from CdS, suggesting that the interface between the ACIGS and CdS is probed at this photon energy for a thin (9 nm) CdS buffer
Summary
Solar cells based on Cu(In,Ga)Se2 (CIGS) thin film absorbers have shown high and stable efficiency values for both laboratory cells and industrial modules with a recent record cell efficiency of 23.4%.1 To reach closer to the theoretical maximum efficiency of around 33%, further reductions of optical and electrical losses are needed. Hard X-ray photoelectron spectroscopy (HAXPES) was employed to study the interfaces between CdS buffers and ACIGS absorbers, which have been exposed to two different alkali metal fluoride PDT processes, prior to the buffer layer deposition.
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