Thermally robust bulk heterojunction photocells based on PTB7:C70 composites

Abstract

Bulk heterojunction (BHJ) photocells using low energy-gap polymers such as 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) and modified fullerenes such as [6,6]-phenyl-C71-butyric acid methyl ester (C70-PCBM) show relatively high power conversion efficiency. It is known that the thermal annealing, which effectively improves the photovoltaic performance of BHJ photocells with semi-crystalline polymers such as poly (3-hexylthiophene), is not applicable to BHJ photocells with amorphous-type polymers such as PTB7. In this study, it is found that PTB7:C70-PCBM BHJ photocells degrade shortly at 80°C, which is within the range of the temperature shock cycling test condition specified in IEC 61646, the international standard for testing thin-film photovoltaic module. On the other hand, the photovoltaic performance of PTB7:C70 BHJ photocells composite has been found to significantly improve after the thermal annealing at 160°C. The results strikingly indicate not only that the lack of thermal robustness in PTB7:C70-PCBM BHJ photocells does not mainly arise from the nature of the polymer but the substituents of the fullerene used, but also that neat fullerenes should not be considered as just cheap and nasty substitutes of fullerene derivatives.