Typically,conjugated polymers are composed of conjugated backbones and alkyl side chains.In this contribution,a cost-effective strategy of tailoring the length of alkyl side chain is utilized to design highperforming thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors PBDT-BiTPD(Cx) (x =48,52,56),in which x represents the alkyl side chain length in term of the total carbon number.A combination of light absorption,device,and morphology examinations make clear that the shorter alkyl side chains yield (i) higher crystallinity and more predominant face-on crystallite orientation in their neat and BHJ blend films,(ii) higher charge mobilities (6.7 × 10-4 cm2 V-1 s-1 for C48 vs.3.2 × 10-4 cm2 V-1 s-1 for C56),and negligible charge recombination,consequently,(iii) significantly improved fill-factor (FF) and short current (Jsc),while almost the same open circuit voltage (Voc) of ca.0.82 V in their corresponding BHJ devices.In parallel,as alkyl side chain lengths decrease from C56 to C48,power conversion efficiencies (PCEs) increased from 7.8％ for C56 to 11.1％ for C52,and further to 14.1％ for C48 in their BHJ solar cells made with a narrow bandgap non-fullerene acceptor Y6.This systematic study declares that shortening the side chain,if providing appropriate solubility in device solution processing solvents,is of essential significance for developing high-performing polymer donors and further improving device photovoltaic performance.