The effect of donor layer thickness on the power conversion efficiency of organicphotovoltaic devices fabricated with a double small-molecular layer

Abstract

In organic photovoltaic (OPV) devices fabricated with a double small-molecular layer,the power conversion efficiency strongly depends on the thickness of the organicdonor layer (here, copper phthalocyanine). In other words, the power conversionefficiency increases with the donor layer thickness up to a specific thickness (∼12.7 nm) and then decreases beyond that thickness. This trend is associated with the lightabsorption and carrier transport resistance of the small-molecular donor layer, both ofwhich strongly depend on the layer thickness. Experimental and calculated results showedthat the short-circuit current due to light absorption increased with the donorlayer thickness, while that due to current through the donor layer decreased with1/R. Since the total short-circuit current is the product of the light absorption current andcurrent through the donor layer, there is a trade-off, and the maximum powerconversion efficiency occurs at a specific organic donor layer thickness (e.g.∼12.7 nm in this experiment).