A study of the factors influencing the performance of ternary MEH-PPV:porphyrin:PCBM heterojunction devices: Electronic effects in porphyrinoid ternary blend bulk heterojunction photovoltaic devices

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.