Organic Thin-Film Solar Cells Using Electron-Donating Perylene Tetracarboxylic Acid Derivatives

Abstract

Various electron-donating amino groups were introduced into the perylene core of perylene tetracarboxylic acid derivatives (PTCs) to address the potential use in organic solar cells. Broad absorptions of the PTC solutions in the visible to near-infrared (NIR) region suggest that PTCs are promising light-harvesting molecules for solar cells. Electrochemical measurements reveal that the introduction of the electron-donating amino groups makes the energy level of the highest occupied molecular orbital (HOMO) shallow, imparting electron-rich characteristics to the PTCs. The electronic properties of the PTCs are studied on the basis of quantum chemical calculations. The results show that the variation of the amino groups has a significant influence on the HOMO level, while the lowest unoccupied molecular orbital (LUMO) level is relatively sensitive to the electronegativity of the PTC terminal atoms. The PTC thin films exhibit broad absorption bands in the visible to NIR region, as for the PTC solutions, and possess relatively shallow HOMO and LUMO energies that are higher or comparable to those of fullerene C60. These excellent properties encouraged us to employ amine-substituted PTCs as electron donors in a thin film solar cell with C60 as an n-type semiconductor. Photovoltaic devices with a structure of indium tin oxide (ITO)/PTCs/C60/bathocuproine (BCP)/Al were fabricated by spin-coating PTCs on an ITO electrode. The devices with perylene tetracarboxylic acid diimides (PTCDIs), bearing highly basic amino groups (Py-PTCDI, Me2N-PTCDI, and Ph2N-PTCDI), exhibit the higher power conversion efficiencies than those with carbazoyl PTCDI (Cz-PTCDI) and perylene tetracarboxylic acid dianhydrides (PTCDAs). The higher device performance originates from the efficient electron transfer from the PTCs to C60 as the results of the relatively shallow HOMO and LUMO levels of the PTCs bearing the highly basic amino groups. The dependence of the device performance on the PTC film thickness indicates that the introduction of diphenylamino groups on the perylene core suppresses the nonradiative decay of the exciton without decreasing the hole mobility. These results will provide important information for the molecular design of post PTC derivatives.