Impact of Band‐gap Gradient in Semi‐Transparent and Bifacial Ultra‐Thin Cu(In,Ga)Se2 Solar Cells

Abstract Ultra‐thin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide back contact reduce the material consumption of rare indium and gallium and simultaneously exhibit great potential for semi‐transparent bifacial application. For highly efficient CIGSe solar cells, a steep back Ga grading and Na treatment are expected. However, Na will promote the formation of highly resistive GaOx at the rear interface owing to Ga accumulation. In this work, the three‐stage co‐evaporation process is renewed and the effect of the deposition sequence in the first stage on the Ga distribution as well as the cross‐correlated influence of Na is explored. In particular, the standard deposition sequence of Ga+In is altered to start with In. When a thin In layer is pre‐deposited on the back contact, the fill factor and efficiency increase. The deposition of In+Ga+In in the first stage of CIGSe growth leads to efficiencies 28% (on average) higher than for the standard deposition sequence of Ga+In. Additionally, 2.74% efficiency is reached under rear and 9.32% under simultaneous front and rear illumination. Therefore, adapting the deposition sequence in the first stage of CIGSe growth is identified as a key to improving the device performance on transparent back contact.