Rational design of ternary blend organic solar cells based on block copolymer additives
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We utilize a combined computational and experimental approach to study the influence of block copolymer additives on the morphological and device characteristics of organic solar cells based on the conjugated polymer/fullerene bulk heterojunction morphology. Our study is motivated by the question whether such block copolymer additives can be utilized to influence the phase separation morphologies, interfacial properties, and, therefore, the device efficiencies of such organic photovoltaic devices. Towards this objective, we split our project into 3 parts: 1.) We utilize Single Chain in Mean Field simulations to investigate the influence of block copolymers on the morphological and interfacial characteristics of the polymer/fullerene blend. Based on these simulations, we identify a design rule for the formation of the equilibrium, cocontinuous donor/acceptor morphologies that are believed to be desirable for efficient charge collection in organic photovoltaics. We utilize this design rule to identify a large collection of blend formulations that give rise to bicontinuous phases, and identify which of these select blend formulations result from comparable volume mixtures of donors and acceptors, which typically yield high device efficiencies in organic photovoltaics. 2.) Based on the predictions from (1), in experiments, we design thermally-stable morphologies with nanoscale domain sizes and percolating donor/acceptor pathways. We demonstrate the manner in which the experimental results agree with the simulations and, hence, establish the validity of our simulation method for predicting phase behavior. 3.) We develop a kinetic Monte Carlo-based method to predict the device performance characteristics of arbitrary donor/acceptor morphologies and couple the morphology and device-level simulations in sequence to identify the blend formulations and resulting morphological features that give rise to the best device performance overall. We demonstrate that, by appropriately tuning the HOMO and LUMO energy levels of the block copolymer additive, an energy cascade can be exploited to further improve charge separation and device efficiencies. In total, our project constitutes a predictive framework for designing new additive-based organic photovoltaic blend formulations with optimized device properties.