Effect of potassium fluoride post-deposition treatment on Cu(In,Ga)Se2 thin films and solar cells fabricated onto sodalime glass substrates

• Voc loss (Eg/q-Voc) reduced to ~0.44 V after KF-PDT for different Eg.
• K increased in entire CIGS region, however Na seemed no change when KF-PDT is performed on SLG.
• Ga at near surface region increased after KF-PDT.
• EBIC analysis indicated the effective carrier collection of the CIGS solar cells widened toward the Mo back-contact side after KF-PDT.

In this study, we investigated the effect of potassium fluoride post-deposition treatment (KF-PDT) on Cu(In1-xGax)Se2 (CIGS) thin films and solar cells fabricated onto sodalime glass (SLG) substrates. Secondary Ion Mass Spectrometery (SIMS) analysis showed that the K increased in the entire CIGS layer after KF-PDT, however Na seemed no change because of a large amount of K and Na diffused from SLG substrates. X-ray photoelectron spectroscopy (XPS) analysis revealed that the KF-PDT depleted Cu, whereas increased K and Ga at the near surface CIGS layer, before the rinsing process. Removal of native oxides from KF-treated CIGS thin film was observed based on rinsing solutions. Ammonia aqueous solution removed Ga compounds more efficiently from KF-treated CIGS thin film than that of distilled water. Time-resolved photoluminescence (TRPL) revealed that the carrier lifetime and PL intensity greatly increased after KF-PDT, resulting in the reduction of photo-generated carrier recombination. The Voc loss (Eg/q-Voc) decreased from 0.52 to around 0.44 V for KF-treated CIGS solar cells with band gap (Eg) of 1.06–1.23 eV. Electron beam-induced current (EBIC) analysis indicated that the effective carrier collection of the CIGS solar cells widened toward the Mo back-contact side after KF-PDT. This phenomenon could be attributed to the increase in carrier lifetime. C-V measurements suggested that KF-PDT not only increased hole concentration but also passivated bulk defects.

Electron beam induced current (EBIC) images overlaid with the cross-sectional SEM photographs of finished CIGS solar cells (a) without and (b) with KF-PDT. It indicates that the effective carrier collection of CIGS solar cells widened towards the Mo-back-contact side after KF-PDT.Figure optionsDownload as PowerPoint slide