The Quantitative Effect of Surface Wetting Layers on the Performance of Organic Bulk Heterojunction Photovoltaic Devices

We use realistic morphological and charge transport modeling to unambiguously quantify the effect of wetting layers on organic photovoltaic device (OPV) performance. Direct measurements of the blend surface of OPVs commonly reveal surface wetting layers that may hinder charge extraction. However, quantifying the effect of these layers on OPV performance experimentally is extremely challenging due to the conjoined nature of the bulk and surface morphology. We show that wetting layers of both the wrong and the correct material can adversely affect performance, the latter effect being due to subsurface features. The results are generalized to show that it is the peak composition of wrong material en route to an electrode, not the surface composition, which determines the effect of wetting layers. The predictions are in good agreement with available data, suggesting that our model can be used to assess when seemingly ubiquitous wetting layers will adversely affect OPV performance.