Langevin recombination and space-charge-perturbed current transients in regiorandom poly(3-hexylthiophene)

We have studied photogenerated charge carrier transport and recombination in thick films of regiorandom poly(3-hexylthiophene) by using the current mode time-of-flight technique. We measured the extracted charge as a function of light intensity and applied voltage, and found that for $\alpha d⪢1$ it saturates at a value that can be stored on the contacts $Q_e=C{U}_0$ as expected for diffusion controlled bimolecular recombination, i.e., Langevin recombination. As a consequence we can only measure space charge perturbed current transients. We measured a field-dependent hole mobility of the order of ${10}^{-5}{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$, which extrapolated to zero field determines the Langevin bimolecular recombination coefficient $4\times {10}^{-12}{\mathrm{cm}}^3∕\mathrm{s}$.