We analyse the current–voltage (J–V) characteristics of bulk heterojunction solar cells containing poly(3,4-ethylenedioxythiophene):polystyrenesulfonate [PEDOT:PSS]-coated indium tin oxide [ITO] as the anode, a 1 : 1 blend by weight of poly(3-hexylthiophene-2,5-diyl) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] as the active layer, and aluminium as the cathode. The J–V characteristics were recorded in the dark and under white-light illumination using a pulsed measurement scheme that ensured the two sets of measurements were obtained at equal temperatures. The dark current was subtracted from the photocurrent to obtain a corrected photocurrent Jph that excludes the photovoltage-induced injection current and is thus due solely to the flux of incident photons. The resultant Jph–V curves—which have a near-symmetric profile centred about the built-in potential VBI—provide critical information about the physical processes governing device operation. For the specific devices tested here, the open-circuit voltage VOC was 0.63 ± 0.002 V at an illumination level of ∼100 mW cm−2—some 50 ± 12 mV above the built-in potential of 0.58 ± 0.01 V. Hence, we find that VOC can exceed VBI by a considerable margin. A simple model is proposed to explain the shape of the Jph–V curves, and application of this model to the measured data indicates that one or both of the electrodes is preferentially selective towards its own carrier type—the anode to holes and/or the cathode to electrons. This ‘self-selectivity’—which results in a significant efficiency gain compared with devices that have non-selective electrodes—is consistent with vertical phase segregation in the active layer, yielding a P3HT-enriched region close to the anode and/or a PCBM-enriched region close to the cathode.
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