Solution-based processing of Cu(In,Ga)Se2 absorber layers for 11% efficiency solar cells via a metallic intermediate

In this work, a low cost solution-based method for the deposition of uniform Cu-In-Ga layers compatible with roll-to-roll processing is described. As ink system we use metal carboxylates dissolved in a mixture of a nitrogen containing base and an alcohol. This solution can be coated homogeneously under inert atmosphere using a doctor blade technique. With this method and appropriate precursor concentrations, crack-free metal layers with dry-film thicknesses of more than 700 nm can be deposited in one fast step. For the controlled film formation during the drying of the solvents a flow channel has been used to improve the evaporative mass transport and the convective gas flows of any unwanted organic species. Due to the absence of organic binders with high molecular weight, this step allows the formation of virtually pure metal
layers. Elementary analyses of the dried thin films reveal less than 5 wt% of carbon residues at 200°C. In situ X-ray diffraction data of the drying step show the formation of Cu-In-Ga alloys. The subsequent processing of Cu(In,Ga)Se2 chalcopyrites with evaporated elemental selenium takes place in a separate tube oven under inert atmosphere. Photoelectric measurements of cells with CdS buffer and ZnO window layer reveal a short-circuit current of 29 mA/cm2, an open-circuit voltage of 533 mV, and a fill factor of 0.69 under standard conditions. Thus efficiencies of up to 11% on 0.5 cm2 area without antireflective coating have been achieved.