Effect of voltage sweep direction on the performance evaluation of P3HT:PCBM solar cells

Investigations on the effect of direction of voltage sweeps, on the current density-voltage (J-V) characteristics in polymer bulk-heterojunction solar cells, based on the blend of poly(3-hexylthiophene) (P3HT) and phenyl [6,6] C-61 butyric acid methyl ester (PCBM), are reported with time. On the freshly prepared device, the direction of the voltage sweep did not have any effect; however, as the device started degrading, the change in direction of the voltage sweep resulted into different characteristics. Analysis beyond complete degradation, when all the photovoltaic parameters reduced to zero, revealed some interesting results. The J-V characteristics, measured with voltage sweep from -ve to +ve voltage, both in the dark and under illumination, were observed to pass through the second quadrant. On the other hand, with the change in the direction of voltage sweep, viz. from +ve to -ve voltage, the characteristics both in the dark and under illumination passed through the fourth quadrant. These results have been explained on the basis of polarization of the degraded active layer due to applied external voltage. This is an important effect and is observed to depend on the applied voltages during performance evaluation and becomes more prominent with time. This effect puts a question mark on the correctness of the method for calculation of the parameters of a degraded device. Studies on degradation of P3HT:PCBM solar cells showed that both the short circuit current density (J(sc)) and the power conversion efficiency () decay exponentially, whereas the open circuit voltage (V-oc) decays almost linearly with time. Copyright (c) 2012 John Wiley & Sons, Ltd.