Abstract
A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2‐b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar‐cell devices of the high‐molecular‐weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.