Organic planar heterojunction solar cell optimisation with ITO obtained by ion beam sputtering

Abstract

Single planar heterojunction organic solar cells based on CuPc-C60 small molecule active layer have been optimised using ITO anode electrode obtained by ion beam sputtering (IBS). The main interests in IBS process is an easy control of layer thickness and light transmission, and a better roughness of the anode compared to commercially available ITO, which can allow a PEDOT-PSS layer thickness optimisation. Comparison of two identical solar cells with commercially available ITO and our ITO obtained by IBS leads to a power conversion efficiency enhancement from 0.5% to 0.7%. Surprisingly, decreasing the serial resistance by increasing ITO thickness does not lead to further improvements, and this feature has been attributed to an increase of the ITO absorption and a possible cathode breaking due to a too thick cell structure. Better roughness of our ITO has permitted to decrease the PEDOT-PSS layer thickness, leading to an enhancement of the power conversion efficiency up to 1.3%. Optimising the cathode by adding an exciton blocking layer (BCP or LiF) has allowed us to decrease the active layer thickness, leading to a decrease of the cell serial resistance and giving rise to near 1.9% power conversion efficiency.