LAUSR

The morphological stability of an organic photovoltaic (OPV) device is greatly affected by the dynamics of donors and acceptors occurring near the device's high operational temperature. These dynamics can be quantified by the glass transition temperature (Tg ) of conjugated polymers. Because flexible side chains possess much faster dynamics, the cleavage of the flexible alkyl side chains will reduce chain dynamics, leading to a higher Tg . In this work, we systematically study the Tg for conjugated polymers with controlled sidechain cleavage. Isothermal annealing of polythiophenes featuring thermally-cleavable side chains at 140 °C, a temperature that is below the melting point of polymers, was found to remove more than 95% of alkyl sidechains in 24 hours, and raise the backbone Tg from 23 to 75 °C. Coarse grain molecular dynamics simulations were used to understand the Tg dependence on side chain cleavage. X-ray scattering indicates the relative degree of crystallization remains constant over the course of isothermal annealing. The effective conjugation length is not influenced by thermal cleavage; however, the density of chromophore is doubled after the complete removal of alkyl side chains. The combined effect of enhancing Tg and conserving crystalline structures during the thermal cleavage process could provide a pathway to improving the stability of optoelectronic properties in future OPV devices. This article is protected by copyright. All rights reserved.