Stability and Reliability of PTB7:PC71BM and PTB7:PC61BM Inverted Organic Solar Cells: A Comparative Study

Blends of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) molecule with [6,6]-phenyl C
₇₁
-butyric acid methyl ester (PC71BM) and, sometimes, with [6,6]-phenyl C
₆₁
-butyric acid methyl ester (PC61BM) are popular for the fabrication of organic solar cell devices with efficiencies greater than 7% in many laboratories around the world. However, these devices have a poor lifetime, even in comparison with many other organic active layer solar cells. In this paper, PTB7:PC71BM and PTB7:PC61BM active layer devices fabricated in a stable process line have been systematically studied to understand the device degradation processes. The studies show that PTB7:PC61BM devices have a longer shelf-life in comparison with PTB7:PC71BM devices, with a median T80 (time for the efficiency to decrease to 80% of the pristine device) of 50 and 11 days, respectively. The lifetimes of both these devices when exposed to outdoor sunlight, however, are very low (median T80 of about 6 h). Time analyses of device photovoltaic parameters, dark current–voltage characteristics, and the spectra of external quantum efficiencies help get an insight into the reasons for device degradation. The shelf-life degradation process is dominated by trap formation and charge extraction limitations. Light-exposure degradation is dominated by lower light absorption because of material degradation in the device active layer. These results should help in the design of more stable and improved PTB7-based devices.