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October 8th, 2009
David Lidzey and James Kingsley
Optimization of new and existing conjugated polymers for solar cells helps bring inexpensive renewable electricity closer to reality.
Over the last five years, many research groups3 have studied OPVs based on the polymer poly(3-hexylthiophene) (P3HT) and fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM). These materials can form an interpenetrating network of nanoscale domains. When incident light excites an electron-hole pair in the P3HT, the electron hops across to the PCBM because of its different electronic-energy levels. This produces separated charges that can be extracted at the device electrodes to generate electricity. Unfortunately, the low optical absorption of P3HT in the near-IR and poorly matched energy levels result in a fundamental limit to the power efficiency that can be obtained from P3HT/PCBM devices. Nevertheless, the amount of existing research on these materials and their commercial availability make them an excellent test system for OPV research.
Over the last year, we have invested significant effort into understanding and optimizing P3HT/PCBM thin-film device fabrication. Wide-ranging improvements have enabled us to produce consistently high-performing OPVs. In particular, we have optimized device efficiency by varying film thickness and exploring the effect of varying the relative ratio of P3HT and PCBM in the composite film. The P3HT/PCBM films are cast from solution, and so we looked at the type of solvent used for casting. We investigated different techniques to ‘grow' a nanoscale-network-like structure of efficient charge-transporting pathways within the film, such as thermal annealing at various temperatures, and the use of a plasticizing solvent vapor to promote crystallization of the P3HT molecules. The latter technique increases hole mobility within the film and also extends the device sensitivity to red light.
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