Tuning the Molecular Weight of the Electron Accepting Polymer in All-Polymer Solar Cells: Impact on Morphology and Charge Generation

Molecular weight is an important factor determining the morphology and performance of all-polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide-based acceptor polymer are prepared with molecular weight varying from M_n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M_n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M_n = 167 kDa however is found to produce a drop in performance to 3% due to liquid–liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.