Published: September 01, 2011 r2011 American Chemical Society 2407 dx.doi.org/10.1021/jz201104d | J. Phys. Chem. Lett. 2011, 2, 2407–2411 LETTER pubs.acs.org/JPCL Analysis of the Relationship between Linearity of Corrected Photocurrent and the Order of Recombination in Organic Solar Cells George F. A. Dibb, †,‡,§ Thomas Kirchartz, †,§ Dan Credgington, †,‡ James R. Durrant, †,‡ and Jenny Nelson †, * ,§ † Centre for Plastic Electronics, ‡ Department of Chemistry, and § Department of Physics, Imperial College London, Exhibition Road, London, SW7 2AZ, United Kingdom b S Supporting Information O rganic photovoltaics (OPVs) present an opportunity for the low-cost manufacture of solar cells, with devices made from a polymer blended with a functionalized fullerene in a bulk heterojunction (BHJ) structure achieving power conversion effi- ciencies (PCEs) exceeding 7% in the literature. 1 There is there- fore considerable focus on developing methods by which the loss processes limiting the efficient generation and collection of charge carriers in this class of cells may be understood and minimized. In particular, “corrected photocurrent” analyses, based on consideration of the device photocurrent as a function of voltage after subtraction of the corresponding dark current, are widely used tools to analyze the function of such devices. 2À6 Such corrected photocurrents are typically observed to scale linearly with light intensity; this observation has been widely interpreted as indicating that the dominant loss processes limit- ing device performance scale linearly with charge carrier density in the device. In this Letter, we address the corrected photo- current analysis and the validity of its underlying assumptions. Additionally, we present a comparison of corrected photocurrent and transient optoelectronic analyses of the same device to demonstrate that the observation of such linear corrected photocurrents cannot be unambiguously employed to determine the order of the underlying loss pathways. Analyses of the efficiency of organic solar cells typically consider two main classes of loss process that reduce charge collection: recombination of photogenerated excitons or charge transfer states prior to separation (geminate recombination) and recombination between pairs of dissociated electron and hole polarons, where each is generated by a different absorption event (nongeminate recombination). This latter category encompasses so-called surface recombination at the electrode interface and recombination of separated charges in the active layer. In general, the current density J at any light intensity is determined by the balance of generation and recombination according to the con- tinuity equation À 1 e ∇ 3 J ¼ G À R ð1Þ where G is the generation rate of dissociated charges (and therefore may be reduced by geminate recombination), R is the nongeminate recombination rate, and e is the elementary charge. The corrected photocurrent analysis used to analyze these recombination processes 2,4À6 involves calculation of the corrected photocurrent J corr ðV Þ¼ J light ðV ÞÀ J dark ðV Þ ð2Þ corresponding to the difference between the current density J light generated by a solar cell under illumination and the current density J dark that flows in the dark at the same applied voltage, V. Several authors have found that J corr scales linearly with light intensity across a range of operating voltages in various polymer: fullerene systems. 4À6 It has been suggested that this observation implies that J corr is primarily determined either by a voltage- dependent geminate recombination pathway limiting G or by a Received: August 15, 2011 Accepted: September 1, 2011 ABSTRACT: We address the claim that the dependence of the “corrected photocurrent” (defined as the difference between the light and dark currents) upon light intensity can be used to determine the charge recombination mechanism in an organic solar cell. We analyze a poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) device using corrected photocurrent and transient photovoltage experiments and show that whereas the corrected photocurrent is linear in light intensity the charge recombination rate scales superlinearly with charge carrier density. We explain this apparent discrepancy by measuring the charge carrier densities at different applied voltages and light intensities. We show that it is only safe to infer a linear recombination mechanism from a linear dependence of corrected photocurrent on light intensity under the following special conditions: (i) the photogenerated charge carrier density is much larger than the dark carrier density and (ii) the photogenerated carrier density is proportional to the photogeneration rate. SECTION: Electron Transport, Optical and Electronic Devices, Hard Matter