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Solvent screening is a critical aspect of the nanomorphological control of low-cost, all-solution-processed bulk heterojunction organic photovoltaic cells. In order to reveal the correlations between solvent/solvent mixtures and the bulk heterojunction nanomorphologies during solutionprocessing, we constructed a multiscale, coarse-grained molecular simulation model for ternary solvent/P3HT/ PCBM mixtures to systematically investigate the nanomorphologies of P3HT/PCBM blends during solutionprocessing in solutions over a wide range of P3HT/PCBM solubilities and solvent evaporation rates experienced during spin-casting processes. The resulting bulk heterojunction layer morphologies of the dried films were in good agreement with available experimental results from as-spun films, which validates our coarse-grained model. Our simulations indicated that the bulk heterojunction morphologies formed in solution involve a complicated interplay among the affinities of the solvent, P3HT, and PCBM in the ternary system, as well as the solubilities of the donor and acceptor; in particular, the solubility of the lessmobile material (i.e., P3HT) can notably affect the film quality, compactness, and the degree of donor/acceptor domain fineness in the dried films. Therefore, the present study demonstrates that this multiscale molecular simulation model can be used to accurately investigate the morphological evolution of bulk heterojunction blends during solution-processing and can be readily applied to the modeling of other advanced all-solution-processed organic photovoltaic cells such as small-molecule bulk heterojunction organic photovoltaic cells.

The nanoscale mechanism determining the optimal electron donor/acceptor blending ratios is not yet clear. In this study, we used coarse-grained molecular simulations to simulate the thermal annealing process of poly-2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends to reveal the correlation between solubility of PCBM in electron donor materials and the optimal electron donor/acceptor blending ratio of the bulk heterojunction polymer solar cells. Substantial intercalation of PCBM into PBTTT is observed, and an interpenetrating network comprising two phasesPBTTT-plus-PCBM and pure PCBMis formed when the blending ratio is beyond 1:1. By comparing morphological properties of PBTTT:PCBM blends with those of the poly(3-hexylthiophene) (P3HT):PCBM blend, a blend which we investigated earlier, we reveal that, in addition to specific interfacial area and percolation probabilities, the solubility of PCBM in PBTTT also has significant effects in determining the optimal blending ratio. Due to PCBM intercalation into PBTTT, more PCBM must be inserted for the precipitation of pure PCBM domains for electron transport. Herein, we provide insight into the effects of PCBM solubility and the nanoscale mechanisms determining the optimal blending ratios and demonstrate how multiscale simulation can potentially aid the development of novel bulk heterojunction blends.

The nanomorphologies of the bulk heterojunction (BHJ) layer of polymer solar cells are extremely sensitive to the electrode materials and thermal annealing conditions. In this work, the correlations of electrode materials, thermal annealing sequences, and resultant BHJ nanomorphological details of P3HT:PCBM BHJ polymer solar cell are studied by a series of large-scale, coarse-grained (CG) molecular simulations of system comprised of PEDOT:PSS/ P3HT:PCBM/Al layers. Simulations are performed for various configurations of electrode materials as well as processing temperature. The complex CG molecular data are characterized using a novel extension of our graph-based framework to quantify morphology and establish a link between morphology and processing conditions. Our analysis indicates that vertical phase segregation of P3HT:PCBM blend strongly depends on the electrode material and thermal annealing schedule. A thin P3HT-rich film is formed on the top, regardless of bottom electrode material, when the BHJ layer is exposed to the free surface during thermal annealing. In addition, preferential segregation of P3HT chains and PCBM molecules toward PEDOT:PSS and Al electrodes, respectively, is observed. Detailed morphology analysis indicated that, surprisingly, vertical phase segregation does not affect the connectivity of donor/acceptor domains with respective electrodes. However, the formation of P3HT/PCBM depletion zones next to the P3HT/PCBM-rich zones can be a potential bottleneck for electron/hole transport due to increase in transport pathway length. Analysis in terms of fraction of intraand interchain charge transports revealed that processing schedule affects the average vertical orientation of polymer chains, which may be crucial for enhanced charge transport, nongeminate recombination, and charge collection. The present study establishes a more detailed link between processing and morphology by combining multiscale molecular simulation framework with an extensive morphology feature analysis, providing a quantitative means for process optimization.