Controlling All-Polymer Solar Cell Morphology and Performance via Systematic Fluorination of Both Donor and Acceptor Polymers

The influence of the conjugated backbone fluorination on the blend film morphology and photovoltaic characteristics of all-polymer solar cells (all-PSCs) is systematically investigated. The fluorination effect analysis of both donor and acceptor polymers is enabled by implementing a random terpolymerization strategy which produces conjugated polymers with tunable fluorine-containing monomer loading. Experimental results reveal that systematic fluorination variation greatly influences both inter-chain interactions and solubility parameters, and ultimately the degree of phase separation and morphology evolution. Specifically, increasing fluorine content for both polymers reduces blend film domain sizes and enhances donor−acceptor polymer−polymer interfacial areas, affording increased short-circuit current densities (J_sc). Moreover, the greater temperature-dependent aggregation (TDA) of fluorinated polymers in the solution-phase minimizes disorder and intermixed feature proliferation accompanying increasing fluorination, which would otherwise promote charge-carrier recombination, reducing cell fill factors (FF). The optimized photoactive layers exhibit well-balanced exciton dissociation and charge transport characteristics, providing solar cells with a significant power conversion efficiency (PCE) enhancement versus devices with nonoptimal fluorination content. Overall, it is shown that proper and precise tuning of both donor and acceptor polymer is critical for optimizing all-PSC performance. Modification of a single component, on the other hand, may lead to local maximum PCE. This study provides a general synthetic methodology to predictably access conjugated polymer blends with desired fluorine-content and highlights the importance of optimizing both donor and acceptor components to achieve the full potential of all-PSCs devices.