11.4% Efficiency Kesterite Solar Cells on Transparent Electrode

Thin film solar cells on semitransparent substrates are attracting much attention due to new application scenarios including building-integrated photovoltaics (BIPV). Environmentally-benign element constituted and highly stable kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film solar cells are ideal candidates for such applications. However, the efficiency of kesterite solar cells on semitransparent substrates is far behind that on opaque Mo-based substrates. Here, fabrication of CZTSSe solar cells on fluorine-doped tin oxide (FTO) substrates from molecular solution and how step-by-step absorber engineering improves device performance is reported. A power conversion efficiency of 7.02% is obtained when the absorber is fabricated on bare FTO, which is improved to 9.56% after adding a MoO 3 interfacial layer. Investigations show the enhancement originates from the transformation of MoO 3 to MoSe 2 during film selenization which initiates crystallization at the back contact and at the same time prevents oversize grains at the absorber surface. Na-doping and Ag alloying further facilitate grain growth and mitigate band tailing, resulting in a certified effective area efficiency of 11.43% with all device parameters comparable to that on an Mo-substrate. This is the first time highly efficient kesterite solar cells are demonstrated on transparent electrodes, which opens up new opportunities for these earth-abundant elements composed of thin film photovoltaics.