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.