Efficient Organic Photovoltaic Cells through Structural Modification of Chloroaluminum Phthalocyanine/Fullerene Heterojunctions

Abstract

The structure−function relationship of organic photovoltaic (OPV) cells based on the chloroaluminum phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction are explored. Optimization is achieved with the use of a molybdenum oxide (MoOx) and an underlying 3,4,9,10-perylene tetracarboxylic acid (PTCDA) interlayer at the hole extracting electrode, the latter acting as a structural template for the subsequent growth of the ClAlPc donor layer. OPV cells demonstrate power conversion efficiencies of 3.0% under simulated AM1.5G (air mass 1.5 global) illumination, with the short-circuit current (Jsc) showing an ∼25% improvement relative to a device without a templating layer. Results from X-ray diffraction and electronic absorption spectroscopy suggest an improved packing and crystallinity in the ClAlPc layer when deposited on the PTCDA template, which suggests an enhancement in charge transport through the film. External quantum efficiency measurements confirm an overall improvement in Jsc in the ClAlPc spectral region upon templating. The effect of the MoOx interlayer is to minimize losses in the open-circuit voltage and fill factor caused by significant band bending and pinning of the adjacent organic layer highest occupied molecular orbital levels to nonstoichiometric defect states in the near Fermi level region of MoOx. The results present an improved strategy for the development of higher-performance OPV cells based on small molecule heterojunctions.