ZnO:Al-based recombination layers for polymer tandem solar cells: Influence of acidic or pH-neutral poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) formulations

We report on the fabrication of organic tandem solar cells which utilize sputtered ZnO:Al recombination layers in combination with the common p-type poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) polymer layer. The impact of the layer thickness on the device performance was investigated. Our experiments determined the minimal value of the ZnO:Al layer thickness in order to obtain a fully functional tandem solar cell. Although PEDOT:PSS is acidic and its application will reduce the initial ZnO:Al layer thickness by 10–50 nm, it could be applied to build functional tandem devices when the initial layer was thick enough to compensate for this loss in layer thickness. For comparison, we also investigated an alternative approach using a pH-neutral PEDOT:PSS formulation which lead to similar results regarding device performance. The conductivity and functionality of thicker ZnO:Al layers became sufficient to serve as a middle electrode to characterize the subcells within the tandem device. In this way, the bottom cell was measured after each subsequent top cell coating process step, revealing no alteration.

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► Tandem cells need at least a 45 nm thick ZnO:Al recombination layer.
► Acidic or pH-neutral PEDOT:PSS are utilizable in combination with ZnO:Al.
► Loss of ZnO:Al layer can be compensated by excess layer thickness.
► Works also for ZnO nanoparticles.