An underestimated photoactive area in organic solar cells based on a ZnO interlayer

Abstract

Solution-processed ZnO is commonly used as a charge-selective interlayer between an absorber and electrode in organic solar cells. In this work, the impact of the resistance of the sol–gel grown ZnO interlayer on solar cell performance is investigated. We find that the UV-induced doping effect leads to a significantly reduced ZnO resistance, which gives rise to an underestimated photoactive area and thus overestimated short-circuit current density (J_sc) for the solar cell measured without an aperture. Moreover, we show that this so far mostly overlooked issue can be unintentionally triggered during common fabrication and characterization processes, because the UV photons flux from a solar simulator, or from a light source for encapsulating the solar cell, are already sufficient in leading to the too much increased lateral conductivity of the ZnO. Finally, we demonstrate that interlayers with rather high sheet resistance can lead to an overestimation of J_sc (e.g. by 10% for a 10 MΩ per square interlayer in a 2 mm² device). Therefore, the validity of the argument that high-resistance interlayers do not lead to overestimated J_sc should always be carefully evaluated.