Efficient Phthalimide Copolymer‐Based Bulk Heterojunction Solar Cells: How the Processing Additive Influences Nanoscale Morphology and Photovoltaic Properties

The power conversion efficiency of poly(N‐(2‐ethylhexyl)‐3,6‐bis(4‐dodecyloxythiophen‐2‐yl)phthalimide) (PhBTEH)/fullerene bulk heterojunction solar cells improves from 0.43 to 4.1% by using a processing additive. The underlying mechanism for the almost 10‐fold enhancement in solar cell performance is found to be inhibition of fullerene intercalation into the polymer side chains and regulation of the relative crystallization/aggregation rates of the polymer and fullerene. An optimal interconnected two‐phase morphology with 15–20 nm domains is obtained when a processing additive is used compared with 100–300 nm domains without the additive. The results demonstrate that a processing additive provides an effective means of controlling both the fullerene intercalation in polymer/fullerene blends and the domain sizes of their phase‐separated nanoscale morphology.