Comment on “Interface state recombination in organic solar cells”

In a recent paper, Street et al. [Phys. Rev. B 81, 205307 (2010)] propose first-order recombination due to interface states to be the dominant loss mechanism in organic bulk heterojunction solar cells, based on steady-state current-voltage characteristics. By applying macroscopic simulations, we found that under typical solar-cell conditions, monomolecular or bimolecular recombination cannot be inferred from the slope of the light-intensity-dependent photocurrent. In addition, we discuss the validity of calculating a mobility-lifetime product from steady-state measurements. We conclude that the experimental technique applied by Street et al. is not sufficient to unambiguously determine the loss mechanism.

Figure 1

(Color online) Simulation of the photocurrent vs voltage with bimolecular recombination $\left(\zeta =0.1\right)$ for different illumination densities (0.03 suns to 10 suns). (Top) The absolute photocurrent on a logarithmic scale. (Bottom) The photocurrent scaled to the generation rate. Here, it becomes clear that deviations from the shape of the characteristics are only seen above 1 sun, despite the recombination being bimolecular. (Inset) The short-circuit current and the photocurrent close to the quasiflat-band voltage, $J_{ph}\left(V_s-0.1\text{V}\right)$. An illumination of 1 sun corresponds to $G=6\times {10}^{27}{\text{m}}^{-3}{\text{s}}^{-1}$, as indicated by the vertical line. The thin line indicates the slope 1 behavior. Both current scale linearly with the generation rate up to at least one sun.

Figure 2

(Color online) Hecht Fits of the photocurrent-voltage curves with the three lowest light intensities from Fig. 1 according to Eq. (2). We found $\mu \tau /\left[d\left(d/2\right)\right]\approx 5$ to $6{\text{V}}^{-1}$ independent of the light intensity.