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.