Influence of Cu/(Ge + Sn) composition ratio on photovoltaic performances of Cu2Sn1−xGexS3 solar cell

Impact of Cu/(Ge + Sn) composition ratio of Cu2Sn1−xGexS3 (CTGS, x = 0.00-0.07) films on their physical properties and photovoltaic performances is investigated. The CTGS thin films are fabricated by sputtering depositions of Cu/SnS2 stacked precursors and then the sulfurization in S and GeS2 vapors under 550 °C for 1 h in N2 flow. The Cu/(Ge + Sn) ratio is varied by changing Cu precursor thickness. It is revealed that GeS secondary phase is formed in the resulting CTGS films with the Cu/(Ge + Sn) ratios below 1.72, while Cu4SnS4 secondary phase is detected with the Cu/(Ge + Sn) ratios above 1.83. The single phase in bulk of CTGS films are attained with the Cu/(Ge + Sn) ratios in the range of 1.72-1.83. In addition, CTGS solar cells with different Cu/(Ge + Sn) ratios are fabricated. As a result, a 3.77%-efficient CTGS solar cell is obtained with the optimized Cu/(Ge + Sn) ratio of 1.83. An increased open-circuit voltage is attributed to the single phase CTGS film, thus decreasing the carrier recombination, and the enhanced short-circuit current density is feasibly caused by the decrease in carrier density of CTGS absorber.