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

Abstract

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 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 ∼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.