Optimization and analysis of conjugated polymer side chains for high-performance organic photovoltaic cells (OPVs) reveal a critical relationship between the chemical structure of the side chains and photovoltaic properties of polymer-based bulk heterojunction OPVs. In particular, the impact of the alkyl side chain length on the pi-bridging (thienothiophene, TT) unit is considered by designing and synthesizing a series of benzodithiophene derivatives (BDT(T)) and thieno[3,2-b]thiophene-pi-bridged thieno[3,4-c]pyrrole-4,6(5H)-dione (ttTPD) alternating copolymers, PBDT(T)center dot(R-2)ttTPD, with alkyl chains of varying length on the TT unit. Using a combination of 2D X-ray diffraction, Raman spectroscopy, and electrical device characterization, it is elucidated in detail how these subtle changes to the chemical structure affect the molecular conformation, thin film molecular packing, blend film morphology, optoelectronic properties, and hence overall photo-voltaic performance. For copolymers employing both the alkoxy or alkylthienyl-substituted BDT motifs, it is found that octyl side chains on TT unit yield the maximum degree of molecular backbone coplanarity and result in the highest quality of molecular packing and optimized hole mobility. Inverted devices fabricated using this PBDTT-8ttTPD: polymer/[6,6]-phenyl-C-71-butylic acid methyl ester active layer show a maximum power conversion efficiency (PCE) of 8.7% with large area cells (0.64 cm(2)) maintaining a PCE of 7.5%.