Recombination in Annealed and Nonannealed Polythiophene/Fullerene Solar Cells: Transient Photovoltage Studies versus Numerical Modeling

Transient photovoltage and differential charging have been used to measure the charge density and recombination rate in polymer solar cells consisting of regioregular poly(3-hexythiophene) (P3HT) blended with 1-(3-methoxycarbonyl)propyl-1-phenyl-6,6-methano fullerene (PCBM). Charge dynamics were found to be over an order of magnitude faster in nonannealed cells as compared to annealed cells. Numerical modeling demonstrated that physically reasonable changes in the domain size and phase segregation reproduced the change seen in the experimental results and thus suggests that measurements of recombination rate can be used as an indicator of the degree of intermixing of donor and acceptor in bulk heterojunction solar cells. Through calculation of the recombination flux at both open circuit and short circuit conditions, we determine that the increase in rate constant observed for the nonannealed devices is sufficient to explain the differences in device performance.