An effective strategy for controlling the morphology of high-performance non-fullerene polymer solar cells without post-treatment: employing bare rigid aryl rings as lever arms in new asymmetric benzodithiopheneElectronic supplementary information (ESI) available. See DOI: 10.1039/c8ta07134a

Almost all high-performance non-fullerene polymer solar cells (NF-PSCs) require the optimization of the morphologies of active layers using additional complicated treatments, such as solvent additive processing and thermal annealing. There are still no guidelines currently available regarding controlling the morphologies of polymer donor and fused-ring acceptor blends from a structural point of view to avoid additional post-treatments. Here, we propose an efficient strategy to design polymers that can generate preferred morphologies and then be used to fabricate as-cast high-performance NF-PSCs. New asymmetric benzodithiophene (BDT) building blocks are employed to construct these polymers. The thienylthiol group with a long alkyl chain as one side substituent on BDT can modulate energy levels, broaden absorption spectra and improve solubility. For the key other side chain, bare rigid aryl rings (benzene, naphthalene and biphenyl) are used as lever arms to stir well the elongated acceptor (ITIC) phase and weaken entanglements among polymer chains during the spin-coating process, which enables favorable morphologies without any post-treatments. Finally, the polymer PBDTTAZ-NaPh yielded a power conversion efficiency (PCE) of 11.56%, which is a rare high value for as-cast NF-PSCs. Thus, it would be feasible to employ this proposed strategy to design high-performance NF-PSCs without post-treatment that meet the requirements for future industry-scale production. Weakening the entanglements between polymer chains enables the creation of high-performance non-fullerene solar cells without post-treatment.