Donal Bradley

Abstract:

A model is proposed for the mechanism of charge recombination in polymer-fullerene blend photovoltaic films. The model explains recombination kinetics determined by transient optical spectroscopy and their dependence on light intensity, temperature and fullerene concentration. We show that under solar illumination conditions, charge recombination is limited by the activation of positive polarons out of deep traps, but carrier collection competes successfully with recombination in thin films. The model is supported by comparison with data on cells and independent measurements of charge transport.

Abstract:

Blends of poly(3-hexylthiophene) (P3HT) and poly(9,9′- dioctylfluorene-co-benzothiadiazole) (BT) electron donor and acceptor polymers were studied as organic solar cell materials. P3HT and BT exhibit relatively high hole and electron mobilities compared with other known conjugated polymers, an attractive feature for photocarrier collection. The two polymers were well mixed in solution and the resulting thin films were optically clear, indicating good compatibility on a submicron length. Almost perfect exciton transfer was observed from BT to P3HT. The external quantum and power conversion efficiencies of solar cells with blend films of 60 wt.% P3HT reached 2 % and 0.07 %, respectively. At low incident light intensity, all of the blend films as well as the pristine polymers showed monomolecular charge recombination.

Abstract:

We report studies of charge recombination and photovoltaic device performance in structures consisting of a fluorene-bithiophene co-polymer and nanocrystalline TiO2. Efficient photoinduced charge transfer is observed using a TiO2 film of high interfacial area while charge recombination is remarkably slow (∼ms). The influences of thickness, surface morphology, top contact material, light intensity and ionization potential of the polymer are studied. Quantum efficiencies of over 11% and monochromatic power conversion efficiencies of around 1 % are achieved.