Ag Alloying and KF Treatment Effects on Low Bandgap CuInSe2 Solar Cells

Silver alloying and KF post-deposition treatments are explored as approaches to increase the efficiency of low bandgap CuInSe
₂
(CIS) solar cells. The KF treatment prevents a loss in VOC when CIS has a thinner CdS layer, and also increases the carrier concentration of these devices. Compared with CIS devices, Ag alloyed CIS (ACIS) devices have lower efficiencies due to a lower carrier concentration which decreases V
_OC
. When ACIS is exposed to the KF treatment, the resulting devices have reduced V
_OC
and fill factor (FF) compared with untreated devices; however, thinning the CdS layer with KF-treated ACIS improves V
_OC
. ACIS+KF films show an unidentified compound in Raman spectra and the resulting solar cells have a dominant interface recombination mechanism and light-to-dark crossover. An electron microscopy study of the growth and coverage of CdS reveals that KF-treated samples in this study have faster CdS growth and ACIS+KF has complete CdS coverage with less than half of the normal deposition time. Improved coverage of CdS layers on KF-treated samples is also verified by X-ray photoelectron spectroscopy. Finally, using a heat treatment and an anti-reflection layer enables CIS+KF solar cells to obtain a high 16.0% efficiency for CIS with a V
_OC
= 526 mV and J
_SC
= 41.0 mA/cm
²
.