The dependence of the internal quantum efficiency of P3HT:PCBM (Poly(3-hexylthiophene-2,5-diyl) :[6,6]-Phenyl C61 butyric acid methyl ester) solar cell on light intensity was measured over four orders of magnitude and for devices annealed for 4 and 10 min. We find that both trap assisted (Shockley-Read-Hall type) and bimolecular losses coexist, the relative magnitude of which is dependent on both the light intensity and the processing conditions. We suggest that the use of Langevin type charge recombination in conjunction with trap assisted recombination is not the best choice and show that the well-known exciton annihilation by charge polaron may better account for the bimolecular losses.
We show experimentally and theoretically enhancement of external quantum efficiency in the green-NIR spectrum for organic photovoltaic device, by the incorporation of patterned Au nano-disk arrays that extend from the front electrode into the active layer. Enhancement mechanisms and design rules are extracted by comprehensive simulations which match the experimental findings. The enhanced efficiency is shown to stem from field enhancement originating from both localized plasmonic resonances and periodic nano patch antennas configuration.
By examining poly(3-hexylthiophene)-fullerene
blend solar cells
with controlled variations of the active layer using subgap external
quantum efficiency combined with intensity dependent quantum efficiency
at short circuit, a direct correlation between trap-assisted recombination
parameters and the charge transfer state absorption strength is established.
Most significantly, by comparing devices with different polymer molecular
weight, regioregularity, C60 derivative chemistry, and
film drying speed, we find that samples with higher trap-assisted
recombination exhibit higher charge generation efficiency. Both findings
are in support of the general view that the charge transfer states
are involved in both processes. In addition, we find that the traps
are full at about 100 mW/cm2, which suggests, together
with the correlation with the charge transfer states, that the electrochemical
potential difference in the bulk exceeds the open circuit voltage
and that the loss of voltage is probably due to recombination associated
with charge extraction at the electrodes.
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