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

Abstract

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] C61 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 (Jsc) and the power conversion efficiency (η) decay exponentially, whereas the open circuit voltage (Voc) decays almost linearly with time. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of degradation on electronic properties of polymer solar cells

“…However, the information about those based on P3HT is quite limited; however, P3HT forms charge transfer complexes with oxygen and forms some cation and anion redical species, which can be detected with electron spin resonance. Room temperature degradation of thiophene molecules can be represented by the scheme given below .…”

“…However, there may be various other effects as well which are not known today but may affect the performance of these cells. Very recently, we discovered one of such effects, caused by the formation of ionic entities on degradation . These issues make the modeling of degraded PSCs difficult.…”

“…Degradation curves are the reflection of different degradation processes happening inside the devices. Degradation in PSCs may be linear, exponential or a combination of different functions of time . To realize the stable solar cells, understanding of the processes responsible for different degradation mechanism, is of great importance.…”

“…Degradation in PSCs may be linear, exponential or a combination of different functions of time. [35,36] To realize the stable solar cells, understanding of the processes responsible for different degradation mechanism, is of great importance. Mapping and elucidation of different degradation processes will help us in realizing stable solar cells.…”

Enhanced regeneration of degraded polymer solar cells by thermal annealing

“…Degradation of PSCs is a complicated process and involves the interplay of several degradation mechanisms. 1,8,10 Indeed, degradation occurs throughout the device structure, from top to bottom electrode, and causes a loss in the electronic and optical properties of the devices. Currently, much effort is underway by a number of research groups in order to understand the relevant degradation mechanisms and hence develop appropriate mitigation strategies.…”