Role of Thin n-Type Metal-Oxide Interlayers in Inverted Organic Solar Cells

We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C-61-butyric acid methyl ester, sandwiched between an indium-tin-oxide/Al-doped zinc oxide (ZnO-Al) front, and tungsten oxide/aluminum back electrodes. The inverted solar cells convert photons to electrons at an external quantum efficiency (EQE) exceeding 70%. This is a 10-15% increase over EQEs of conventional solar cells. The increase in EQE is not fully explained by the difference in the optical transparency of electrodes, interference effects due to an optical spacer effect of the metal oxide electrode buffer layers, or variation in charge generation profile. We propose that a large additional splitting of excited states at ZnO-Al/polymer interface leads to the considerably large photocurrent yield in inverted cells. Our finding provides new insights into the benefits of n-type metal-oxide interlayers in bulk heterojunction solar cells, namely the splitting of excited states and conduction of free electrons simultaneously.