Organic photovoltaic cells show promise as a new technology for renewable energy. However one major problem with these devices is that the polymers typically employed absorb light in only a limited part of the solar spectrum (approximately 600nm and less). One way to further extend the spectral response of these devices is the addition of complimentary dye molecules. It has been shown that the absorption of light by the Q-bands of porphyrins incorporated into MEH-PPV/PCBM blends contributes to the total photocurrent generated by the device. Furthermore, N-confused porphyrins could be used to extend the absorption spectra of these devices even further, because they have Q-bands extending well beyond the region of a typical porphyrin. Despite this promise porphyrin aggregation within these ternary devices leads to disruption of the crucial morphology of the active layer and lowered device efficiency.
Porphyrin aggregation can be controlled by controlling the steric bulk of peripheral substituients. A series of tetraphenylporphyrins and N-confused tetraphenylporphyrins have been prepared in which the steric bulk of the peripheral substituents, and thus degree to which aggregation occurs, was varied. These have been used to manufacture a series of MEH-PPV/Porphyrin/PCBM ternary organic photovoltaic devices. The effect that these structural changes have on device performance will be presented.