Impact of substrate temperature during NaF and KF post-deposition treatments on chemical and optoelectronic properties of alkali-free Cu(In, Ga)Se2 thin film solar cell absorbers

The substrate temperature TPDT during the alkali fluoride post-deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers is a critical process parameter. So far, TPDT is optimized empirically for a particular alkali distribution, and literature only reports on the effects of high TPDT (300-350 degrees C). To better understand the influence of TPDT and to close the gap for TPDT below 300 degrees C, a temperature series from 100 to 315 degrees C was studied. Furthermore, most studies on KF PDTs are performed with alkali-containing soda-lime glass substrates, in which the electrical effects of alkalis from the substrate and the PDT on the final device cannot be separated. To circumvent this obstacle, we use alkali-free substrates in our study and compare electrical and chemical effects of NaF and KF PDTs at different TPDT using current-density-voltage and capacitance-voltage measurements, as well as time-of-flight secondary-ion mass spectrometry and ultraviolet photoelectron spectroscopy. The alkali concentrations are quantified as a function of TPDT and discussed in relation to the respective cell parameters. Both, NaF and KF PDTs, improve the cell efficiency significantly, with the highest conversion efficiency of 16.3% obtained for a NaF PDT at TPDT = 315 degrees C. A given TPDT leads to a larger K concentration (after KF PDT) than Na concentration (after NaF PDT). For TPDT above 150 degrees C, similar conversion efficiencies are achieved for NaF and KF PDTs. In contrast, the KF PDT results in a significant decrease of the conversion efficiency at TPDT = 105 degrees C, likely caused by the formation of a Cu-Se secondary phase at the CIGSe surface, which is accompanied by a shift of the valence band maximum towards the Fermi energy.

Automated summary

The influence of post-deposition treatments with NaF and KF on Cu(In,Ga)Se2 thin-film solar cell absorbers was studied at different substrate temperatures. Both treatments significantly improved cell efficiency, with the highest conversion efficiency achieved with NaF PDT at a high temperature. KF treatment led to reduced efficiency at low temperature due to the formation of a Cu-Se secondary phase at the CIGSe surface.