11.4% Efficiency Kesterite Solar Cells on Transparent Electrode

Abstract 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.