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

Abstract 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°C). To better understand the influence of TPDT and to close the gap for TPDT below 300°C, a temperature series from 100 to 315°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°C. A given TPDT leads to a larger K concentration (after KF PDT) than Na concentration (after NaF PDT). For TPDT above 150°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°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.