PHYSICAL REVIEW B 83, 035205 (2011) Transient photoconductivity in polymer bulk heterojunction solar cells: Competition between sweep-out and recombination Sarah R. Cowan, 1 R. A. Street, 2 Shinuk Cho, 1,* and A. J. Heeger 1,† 1 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California 93106, USA 2 Palo Alto Research Center, Palo Alto, California 94304, USA (Received 16 July 2010; revised manuscript received 3 November 2010; published 18 January 2011) Transient photoconductivity measurements carried out on bulk heterojunction (BHJ) solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination. The transient photoconductance data imply the existence of a well-defined internal field; carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. At external voltages near open circuit where the internal field approaches zero, the photocurrent decays because of the recombination of photogenerated mobile carriers. Mobility and recombination lifetimes are evaluated for carriers in poly[3-hexylthiophene] (P3HT) : [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM) and poly[N-9 ′′ -hepta-decanyl-2,7-carbazole-alt-5,5-(4 ′ ,7 ′ -di-2-thienyl-2 ′ ,1 ′ ,3 ′ -benzothiadiazole)] (PCDTBT) : [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) solar cells. DOI: 10.1103/PhysRevB.83.035205 PACS number(s): 72.80.Le, 73.50.Pz, 73.50.Gr, 73.61.Wp I. INTRODUCTION In organic bulk heterojunction (BHJ) solar cells, ultrafast photoinduced charge transfer across the buried donor-acceptor interfaces creates mobile holes in the donor domains and mobile electrons in the acceptor domains, both of which are swept out and transported to the electrodes by the built-in internal electric field. Carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. The power conversion efficiency (PCE), η, of a solar cell is given by the well-known relation η = J sc V oc FF/P in , (1) where J sc is the short-circuit current, V oc is the open-circuit voltage, FF is the fill factor, and P in is the incident solar power. Recombination causes a reduction in J sc and FF and a corresponding loss of cell efficiency. 1–4 Thus, to obtain high PCE, carrier collection by sweep-out to the electrodes (characteristic time, τ sw ) and driven by the internal field must occur prior to carrier recombination within the cell (characteristic time, τ R ). Transient photoconductivity is the common technique to study the kinetics of sweep-out 5,6 and recombination 7–9 in low mobility materials. However, the technique is difficult to perform on organic solar cells. The cells are thin with an active layer thickness around 100 nm, and hence the resistor- capacitor (RC) time constant of the device limits the time resolution of transient measurements. Also, optical absorption and generation of electron-hole pairs occurs throughout the thin layer, making it difficult to distinguish the transient response of electrons and holes. While devices could be made thicker, the domain structure would probably be different from the actual solar cell. 10 The aim of this paper is to show that transient photocurrent measurements on optimized solar cells can resolve the sweep- out and recombination times, and to develop the analytical tools to extract the information. We show that in an optimized cell at high internal fields, charge sweep-out occurs much faster than recombination, corresponding to efficient extraction of charge at short circuit and reverse bias with minimal loss by recombination. At low internal voltages, the recombination lifetime is expected to be similar to the sweep-out time since the solar cell characteristics show that significant carrier loss occurs for voltages approaching the open-circuit condition. 11 For a typical mobility of 10 −4 cm 2 V −1 s −1 and a device of thickness 100 nm, the sweep-out time at an internal voltage of 0.3 V is about 2 μs. However, there are reports from other transient measurements of much longer response times, of order 100 μs to 1 ms from transient photovoltage measurements and those using charge extraction by linearly increasing voltage (CELIV). 8,12 We show that the measured recombination time in optimized solar cells is indeed of order a few μs. Characterization of the transient photoconductivity, therefore, provides insight into the physical mechanisms of the solar cells and may help to interpret the different transient experiments. Section II describes the relevant properties of the organic solar cells. Section III presents the data, develops the model to describe the transient response and analyzes the data based on the model. The conclusions are summarized in Sec. IV. II. PHOTOCONDUCTIVE PROPERTIES OF BULK HETEROJUNCTION CELLS The total solar cell current density J (V ) can be expressed as a sum of the dark and the photogenerated current: J (V ) = J D (V ) − edP C (V )G, (2) where J D (V ) is the dark current density, e is the electron charge, d is the BHJ film thickness, and V is the externally applied bias voltage. The photogenerated current can be expressed in terms of P C (V ), the normalized photocurrent, equivalent to the bias-dependent probability of collection of carriers at the electrodes prior to recombination. G is the effective generation rate per unit volume, including any loss resulting from optical excitations that do not reach the charge-separating interface. P C (V ) approaches unity in reverse 035205-1 1098-0121/2011/83(3)/035205(8) ©2011 American Physical Society