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Abstract: A series of porphyrins and N-confused porphyrins have been prepared in which the peripheral groups on the porphyrins are kept constant and the porphyrinoids differ only in their electronic nature. The materials have been blended into the active layer of MEH-PPV:PCBM bulk heterojunction solar cells and the performance of the cells is reported and discussed. All of the added porphyrinoids contribute to the photocurrent of the resultant solar cells and result in a broadening of the spectral response of the cells in accordance with the absorption spectra of the porphyrinoid. The efficiency of these devices is shown to correlate strongly with the ionisation potential (IP), and thus highest occupied molecular orbital (HOMO) level, of the added porphyrinoid. We argue that the relative energy of the HOMO levels of the porphyrinoids and the hole transporting polymer in these devices, coupled with the poor charge mobilities of N-confused porphyrins combine to generate porphyrinoid-based hole traps in these devices. This increases recombination within these devices, lowering both the devices'' charge densities and open circuit voltages and resulting in reduced cell efficiencies. We show that varying the porphyrinoid added, by metallation or N-alkylation of the N-confused porphyrin, allows us to systematically change the IP of the species and directly affect the power conversion efficiency of the resultant device. The implications of this work for optimising the performance of ternary blend bulk heterojunction solar cells are discussed. [Copyright &y& Elsevier]

Abstract: A series of three porphyrins varying only in the steric bulk of their peripheral groups have been synthesised and integrated into the active layer of bulk heterojunction solar cells. The porphyrins broaden the spectral response of the device and contribute to the total photocurrent generated. More importantly, the device characteristics change systematically with increasing steric bulk on the peripheral meso phenyl groups of the tetra-phenyl porphyrin. Optical, scanning probe and scanning transmission X-ray microscopy are used to demonstrate that the observed changes do not arise from morphological differences in film structure. Cyclic voltammetry, UV–vis spectroscopy and DFT calculations are used to establish that the porphyrin LUMO, HOMO and bandgap are independent of side group. We conclude that the variations in open-circuit voltage with side group are the result of the porphyrin acting as a bimolecular recombination centre, with an efficiency that is dependent on the side group type. The possibility of designing optimised macromolecules for OPV devices based on an understanding the effect of porphyrin steric bulk upon device performance is discussed. [Copyright &y& Elsevier]