Inkjet-printed indium sulfide buffer layer for Cu(In,Ga)(S,Se)(2) thin film solar cells

We report an environmentally friendly inkjet-printed indium sulfide (In2S3) buffer layer using benign chemistry and processing conditions. A pre-synthesized indium-thiourea compound is dissolved in a mixture of water and ethanol, inkjet printed on a Cu(In,Ga)(S,Se)(2) absorber and annealed in air. The buffer layer shows a beta-In2S3 structure with few organic impurities and band gap in the range of 2.3 eV. An ultraviolet ozone treatment applied to the surface of the absorber prior to inkjet printing of the precursor is used to improve the wettability of the ink and therefore the surface coverage of the buffer on the absorber layer. The device with a fully covering In2S3 layer shows better open circuit voltage and fill factor than the device with a partially covering In2S3 layer. The best In2S3 device showed a light to electric power conversion efficiency similar to the reference cadmium sulfide buffer layer device. Good wettability conditions are therefore essential for higher efficiency solar cells when the buffer layer is inkjet-printed.