Co-evaporated CuInSe2: Influence of growth temperature and Na on solar cell performance

CuInSe₂ films were prepared by a 3-stage-co-evaporation process. For Na-free absorbers we do not observe any influence of the preparation temperature between 330 °C and 510 °C on the solar cell performance with typical efficiencies around 9 %. NaF-PDT leads to an increase of the open-circuit voltage and the efficiency for samples prepared at a growth temperature of 450 °C and 510 °C (around 13 % for such samples). For samples prepared at 330 °C NaF-PDT has no influence on the open circuit voltage and the efficiency although the treatment increases the charge carrier concentration. At higher preparation temperatures, absorbers with Na present during growth (NaF-precursor samples) lead to similar IV parameters as NaF-PDT absorbers. NaF-precursor samples grown at temperatures at 330 °C, show an X-ray diffraction reflex, which is characteristic for the presence of twin stacking faults within the growing thin films. Solar cells with these absorbers show bad performances. In contrast, Na-free grown absorbers do not show twin stacking faults after the absorber growth process independently of the preparation temperature. Capacitance-voltage measurements give charge carrier concentrations of around 3×10¹⁶ cm^-3 (NaF-precursor samples), 5×10¹⁵ cm^-3 (NaF-PDT samples) and 6×10¹⁴ cm^-3 (Na-free samples). From scanning electron microscopy it is deduced that Na, which is present during the growth, leads to films with smaller grain sizes.