Bulk Heterojunction Solar Cells - Tuning of the HOMO and LUMO Energy Levels of Pyrrolic Squaraine Dyes

The optimization of organic photovoltaic (OPV) devices requires, amongst several factors mainly related to thin-film blend morphology, the capability to finely tune the characteristic energy levels of both the donor and the acceptor materials constituting the active layer. Squaraine compounds represent one of the most promising small molecule materials to be employed as the donor in the bulk heterojunction (BHJ) OPV configuration. In this study a series of arylhydrazone end-capped symmetric squaraine compounds bearing electron-withdrawing substituents on the aryl ring has been synthesized with the aim of increasing the device open circuit voltage by shifting the highest occupied molecular orbital (HOMO) energy level. These new semiconductors have been characterized by optical spectroscopy (UV/Vis absorption and IR transmittance) and electrochemical measurements (cyclic voltammetry and differential pulsed voltammetry). Our data enable the establishment of interesting structure-property relationships relating the nature, position and number of electron-withdrawing substituents to the position of the donor HOMO and lowest unocupied molecular orbital (LUMO) levels. The low energy shift effect on the HOMO level with respect to the parent unsubstituted squaraine can be as high as 0.3 eV. The effect is so strong that the derivative bearing the strongest acceptor no longer behaves as a donor semiconductor. Preliminary results on OPV device preparation and characterization are also discussed. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.