Light intensity dependence of open-circuit voltage and short-circuit current of polymer/fullerene solar cells

The open-circuit voltage (V_oc) of polymer/fullerene bulk heterojunction solar cells is investigated as a function of light intensity for different temperatures. The observed photogenerated current and V_oc are at variance with classical p-n junction-based models. The influence of light intensity and recombination strength on V_oc is consistently explained by a model based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of charge carriers. The light intensity dependence of the short-circuit current density (J_sc) is also addressed. A typical feature of polymer/fullerene based solar cells is that J_sc does not scale exactly linearly with light intensity (I). Instead, a power law relationship is found given by J _sc ∝ I^α, where α ranges from 0.9 to 1. In a number of reports this deviation from unity is attributed to the occurrence of bimolecular recombination. We demonstrate that the dependence of the photocurrent in bulk heterojunction solar cells is governed by the build-up of space charge in the device. The occurrence of space-charge stems from the difference in charge carrier mobility of electrons and holes. In blends of poly(3-hexyltniophene) and 6,6-phenyl C₆₁-butyric acid methyl ester this mobility difference can be tuned in between one and three orders of magnitude, depending on the annealing conditions. This allows us to experimentally verify the relation between space charge build-up and intensity dependence of J_sc. Model calculations confirm that bimolecular recombination leads only to a typical loss of 1% of all free charge carriers at J_sc for these devices. Therefore, bimolecular recombination plays only a minor role as compared to the effect of space charge in the intensity dependence of J_sc.