Degradation mechanism of P3HT:PCBM based bulk heterojunction solar cells.

We have studied the degradation mechanism of P3HT:PCBM-based bulk heterojunction solar cells by developing the structure ITO/PEDOT:PSS/P3HT:PCBM/Al comprising different acceptor materials, namely, PC60BM and PC70BM. Both the devices have nearly the same photoconversion efficiency (PCE) of ≈3.8% immediately after fabrication. The PCE is observed to decrease in both the devices with the passage of time. While the degradation rate is found to be higher in PC60BM based devices, its value is found to saturate after 100 minutes at a constant value of ≈2%. For PC70BM based cells, the devices show a sudden change in the degradation rate from low to high as the time progresses beyond ≈170 minutes. The PCE in these devices fell to a very low value after 250 minutes. This rapid fall in PCE is attributed to the formation of trap states in the energy levels of PC70BM molecules owing to their non-spherical structure.