Abstract
Solar cells based on a glass-forming ternary blend of C60, C70 and PTB7 display a high power conversion efficiency above 5% up to an annealing temperature of 180 °C. The excellent thermal stability correlates with a high glass transition temperature of 224 °C.
Highlights
Organic photovoltaics receives tremendous interest as an alternative solar cell technology because of its compatibility with low-cost manufacturing through roll-to-roll coating and printing techniques
During the rst heating scan from 60 to 250 C we observe a continuous blue shi of Dlpeak with a clear change in slope d(Dlpeak)/dT around 224 Æ 1 C, which we identify as the glass transition temperature of the ternary blend (Fig. 2b)
We carried out plasmonic nanospectroscopy on neat PTB7 but did not observe any change in d(Dlpeak)/dT (ESI, Fig. S2†)
Summary
Organic photovoltaics receives tremendous interest as an alternative solar cell technology because of its compatibility with low-cost manufacturing through roll-to-roll coating and printing techniques. To prevent reorganisation of the nanostructure upon heating, it is necessary to select blends that are characterised by a high glass transition temperature Tg.[6] This concept was rst proposed by Yang et al and Bertho et al and is an accepted design criterion for thermally stable organic solar cells.[7,8,9] A nely mixed blend typically displays a single Tg and its nanostructure remains frozen in, as long as the blend remains far below this critical temperature As such the glass transition should be considered as a kinetic phenomenon that represents a nominal temperature below which relaxation of the donor polymer and diffusion of the fullerene acceptor are strongly slowed down but not prevented.[6]. We nd that photovoltaic devices based on PTB7 : C60 : C70 display excellent thermal stability with a glass transition temperature at Tg $ 224 C and a thermally stable photovoltaic performance up to $180 C
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