Intensity-dependent photocurrent generation at the anode in bulk-heterojunction solar cells

The efficiency that a solar cell can reach is ultimately limited by the number of photons absorbed in its active layer and the efficiency with which these photons are converted into electrical current. In this study we seek to quantify and separate the optical and electrical efficiencies in photovoltaic devices based on mixtures of 6,6]-phenyl C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (P3HT). The optical efficiency is determined by comparing the measured external quantum efficiency (EQE) and the maximum possible EQE (EQEmax) as determined by modeling the fractional dissipation of light in the active layer at a given wavelength and layer thickness (d_active). We determine, by examining the difference between EQE and EQEmax, that a significant contribution (up to 35%) of the total losses are electrical in nature. Comparison of the internal quantum efficiency (IQE) and the optical intensity distribution as a function of d_active shows photocurrent generation is anti-correlated to light intensity in the vicinity of the PEDOT:PSS/active layer interface. The magnitude of this effect is modeled using standard optical tools and a half Gaussian shaped reduced generation zone (RGZ) centered at this interface. Illumination-intensity (I₀) dependent measurements of the short-circuit-current density allows us to exclude vertical segregation and bi-molecular recombination as potential explanations for the RGZ. Examination of the work functions of P3HT and PEDOT:PSS gives evidence that in the devices positive charges build up at the interface due to permanent redox chemistry, leading to the formation of a dipolar layer with holes on the P3HT. The dependence of EQE on I₀ at low illumination intensities gives evidence for correlation of the charge build up and the size change of the reduced generation zone. We argue that the only physical phenomenon that explains a region-specific reduction in photocurrent generation and is consistent with a build up of positive charge at the interface is a reduced probability of exciton separation or quenching. In support of our findings, we show that reduction of the light intensity yields increased quantum efficiency consistent with our model. PACS  42.25.bs; 72.40.tw; 72.80.le