Direct Electrical Evidence of Plasmonic Near-Field Enhancement in Small Molecule Organic Solar Cells

We present a simple and versatile technique to introduce plasmonic silver nanoparticles into organic thin film devices by in situ vacuum deposition. Silver particles with 80 nm diameter at the back of small molecule organic solar cells increase the power conversion efficiency (PCE). Doped organic transport layers allow one to separate electrical and optical effects. By a systematic variation of the position of the silver particles within the solar cell stack, we can thus clearly distinguish a near-field photocurrent gain in the IR that decays to one-half on length scales of around 4 nm, and a less distance-dependent selective mirror effect for short wavelength, which allows one to optimize devices for different wavelengths simultaneously. Device optimization reveals that plasmonic increased absorption can be used to significantly reduce the thickness of the absorber layers and gain efficiency through improved transport properties. A plasmonic zinc phthalocyanine fullerene-C-60 solar cell that yields improved photocurrent, fill factor, and PCE of 2.6% includes one-half of the absorber material of an optimized reference device with PCE of 2.4%. The design priciples for plasmonic solar cells are general and were confirmed in thin devices containing zinc 1,8,15,22-tetrafluoro-phthalocyanine, improving the PCE from 2.7% to 3.4%.