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-C60 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%. © 2014 American Chemical Society

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-C60 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%. © 2014 American Chemical Society

Gold Nanoparticles Stabilized with Aromatic Thiols: Interaction at the Molecule-metal Interface and Ligands Arrangement in the Molecular Shell Investigated by SR-XPS and NEXAFS

Small gold nanoparticles capped with 4-trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepared with spherical shape and different mean sizes (5 to 8 nm). The functionalized gold nanoparticles (AuNPs-SEB) were deposited onto TiO2 substrates and the interaction at the molecule-gold interface, the molecular layer thickness as well as the ligands organization on the surface of Au nanospheres were investigated by means of Synchrotron Radiation induced X-ray Photoelectron Spectroscopy (SR-XPS) and angular dependent Near Edge X-ray Absorption Spectroscopy (NEXAFS) at the C K-edge. In order to obtain a better insight into the molecular shell features, the same measurements were also carried out on a self assembling monolayer of SEB anchored on a “flat” gold surface (Au/Si(111) wafer). The comparison between angular dependent NEXAFS spectra collected onto the self-assembling monolayer and AuNPs-SEB allowed for successfully probe the molecular arrangement of SEB molecules on the gold nanospheres surface. Furthermore, DFT calculations on the free SEB molecule as well as bonded to a small cluster of gold atoms were developed. The comparison with experimental results allowed to better understand the spectroscopic signatures in the experimental absorption spectra and to rationalize the molecular organization in SAM, NPs having a thin molecular shell, and NPs covered by a thick layer of ligands