Nanomorphology Evolution of Bulk Heterojunction Blend during Solution Processing

All-solution-processing is the ultimate goal for low-cost, roll-to-roll production of OPV devices. Solvents/solvent additives have significant effects on the BHJ nanomorphologies; therefore, comprehensive insights into the BHJ nanomorphology evolution during solution-processing are critical for solvent/solvent additive screening and nanomorphology control. In this work, the nanomorphology evolution of P3HT:PCBM blends during solution processing was revealed with molecular-level details from a series of in silico solvent evaporation experiments in solutions covering a wide range of P3HT/PCBM solubilities via a multiscale, coarse-grained (CG) molecular simulation framework we developed, see Fig. [*]a for the CG model we constructed for the ternary solvent/P3HT/PCBM systems. Then, we carried out a series of CGMD simulations of solvent evaporation processes and monitored the BHJ morphology evolution, see Fig. [*]b. The solvent evaporation rates in this work were compatible with those in spin-casting processes, and the P3HT:PCBM blend morphologies of dried films (Fig. [*]c) are in good agreements with experimental results from as-spun films, not only validating, but also demonstrating the capabilities of our CG model in simulating the nanomorphology of advanced all-solution-processed OPVs, which are usually fabricated under similar solvent evaporation rates without further thermal treatments. Our CGMD simulations revealed the correlations between solvent solubilities and BHJ morphologies. For solvents with poor P3HT/PCBM solubilities (e.g. toluene), large cavities were formed after solvent evaporation because P3HT/PCBM molecules cannot reorganize themselves into compact structures (see Fig. [*]c), thereby leading to BHJ films with poor quality. Therefore, the present study demonstrates that carrying out solution processing experiments in computer can be helpful for efficient solvent screening or revealing morphological details that are difficult to be extracted from current state-of-the-art experimental techniques for developing advanced all-solution-processed small molecule OPVs and organic light-emitting diodes. The results of this work have been published in the Journal of Physical Chemistry C in 2014.

Figure: P3HT:PCBM blend morphologies during solution-processing. (a) CG models of P3HT, PCBM, and solvent molecules. (b) BHJ nanomorphology evolution during solvent evaporation processes from CGMD simulations, the simulation cell size before solvent evaporation is 50[IMAGE png]50[IMAGE png]120 [IMAGE png]. (c) resultant dried BHJ film, the P3HT and PCBM molecules are colored in red and blue, respectively.
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barbarossapao 2015-09-16