On the pseudo-symmetric current-voltage response of bulk heterojunction solar cells

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-C-61-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 J(ph) that excludes the photovoltage-induced injection current and is thus due solely to the flux of incident photons. The resultant J(ph)-V curves-which have a near-symmetric profile centred about the built-in potential V-BI-provide critical information about the physical processes governing device operation. For the specific devices tested here, the open-circuit voltage V-OC was 0.63 +/- 0.002 V at an illumination level of similar to 100 mW cm(-2)-some 50 +/- 12 mV above the built-in potential of 0.58 +/- 0.01 V. Hence, we find that V-OC can exceed V-BI by a considerable margin. A simple model is proposed to explain the shape of the J(ph)-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.