7.9% efficient vapor-deposited organic photovoltaic cells based on a simple bulk heterojunction

Abstract

Efficient organic photovoltaic cells (OPVs) based on vapor-deposited small molecule materials frequently utilize a planar-mixed heterojunction (PMHJ) architecture, in which a mixed layer of electron donating and accepting materials is sandwiched between neat, planar layers of the donor and acceptor. The neat layers contribute to photocurrent, help minimize the dark current and maximize the device open-circuit voltage. The use of organic semiconductors as the planar layers is not however without complication, as the low charge carrier mobility in these materials can hinder efficient charge collection. In this work, we examine OPVs where the neat donor layer between the anode and the mixture is replaced with a layer of MoO_x. An average device power conversion efficiency of (7.7 ± 0.3)% is realized under AM1.5G (100 mW cm⁻²) solar simulated illumination in a uniformly mixed heterojunction OPV based on the donor–acceptor pairing of 2-{[7-(4-N,N-ditolylaminophenylen-1-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile (DTDCPB)–C₇₀ without any adjacent donor or acceptor layers. Correcting for spectral mismatch, an average device power conversion efficiency of (7.9 ± 0.3)% is obtained, with a champion device efficiency of 8.2%. The use of MoO_xversus a neat donor layer is found to reduce the series resistance and facilitate charge collection from the mixed layer, leading to an increased fill factor and short-circuit current. To our knowledge, this result is among the highest single-cell efficiencies reported for vapor-deposited small molecule OPVs. We further find that mixed OPVs based on DTDCPB–C₇₀ exhibit a promising shelf lifetime, showing no degradation in efficiency after more than seven months. Thus, the removal of the neat planar organic layer can favorably affect device efficiency without potentially adversely impacting stability.