Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films

N Nicoara,N Barreau,L Arzel,Th Lepetit,S Sadewasser,S Harel

doi:10.1038/srep41361

Abstract

Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.

Highlights

Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS)
While most studies to date have focused on the surface and close to surface effect of the KF-PDT, it had been speculated that K might be found at grain boundaries (GBs)[28,29]
To ensure the stability of the surface electronic properties, we monitored the development of the contact potential difference (CPD) with time over a period of ~10 hours by repeated Kelvin probe force microscopy (KPFM) experiments with intermediate calibrations of the atomic force microscope (AFM) tip on an Au/Si reference sample (Fig. S2, Supplementary Information)

Summary

Introduction

Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Despite the abundance of grain boundaries (GBs) in thin-film solar cells based on Cu(In,Ga)Se2 (CIGS), CdTe, and the recent perovskite absorbers, all of these materials have achieved power conversion efficiencies above 22%1,2, within reach of that of single crystalline silicon. The recent efficiency increases in CIGS solar cells have been achieved by a potassium fluoride post-deposition treatment (KF-PDT) applied after the deposition of the CIGS absorber[23,24]. We studied the initial formation of CdS on the CIGS surface and its effect on the surface electronic properties

Results

Discussion

Conclusion