There is considerable interest in improving the performance of organic optoelectronic devices through processing techniques. Here, we study the effect of high-temperature annealing on the properties of the semiconducting polymer PTB7 and PTB7:fullerene blends, of interest as efficient organic photovoltaic (OPV)devices. Annealing to moderate temperature improves the PTB7 morphology and optoelectronic properties. High-temperature annealing also improves morphology but results in poorer optoelectronic properties. This is a result of side chain cleavage that creates by-products that act as trap states, increasing electronic disorder and decreasing mobility. We further observe changes to the PTB7 chemical structure after thermal cleavage that are similar to those following solar irradiation. This implies that side chain cleavage is an important mechanism in device photodegradation, which is a major “burn-in” loss mechanism in OPV. These results lend insight into side chain cleavage as a method of improving optoelectronic properties and suggest strategies for improvement in device photostability.
Bibliographical noteFunding Information:
This work was supported by the Office of Naval Research NDSEG fellowship (V.S.) and the Department of Energy SCGSR Program (L.J.P.). Work was partially supported by the Department of the Navy , Office of Naval Research Award No. N00014-14-1-0580 (S.D.O., M.F.T.). L.K.J., I.R., and I.D.W.S. were supported by the Engineering and Physical Sciences Research Council (grants EP/L017008/1 and EP/L012294/1 ). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy , Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We would like to thank Ging-ji Nathan Wong and Jeffery Tok for help with various experiments. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152. We would like to thank M.T. Sajjad and A. Ruseckas for assistance with the time-resolved PL measurements. We would also like to thank Chris Takacs for helpful discussions.
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