(262b) Molecular Modeling of Nanoparticles and Conjugated Polymers During Synthesis of Photoactive Layers of Organic Photovoltaic Solar Cells

Molecular modeling of nanoparticles and conjugated polymers during synthesis of photoactive layers of organic photovoltaic solar cells

S.M. Mortuza (1), Corinna Cisneros (1), Mark Dela Cruz Bartolo (2) and Soumik Banerjee (1)

1. School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, 99164, USA

2. California State University San Marcos, San Marcos, CA 92096

Solar cells are widely considered as potential alternative energy devices that can meet the increasing global energy demand. In particular, organic photovoltaic (OPV) solar cells, which employ organic nanoparticles in conjugated polymers as photoactive layers for energy conversion, have generated significant scientific interest due to their ease of manufacturing, relative low costs and flexibility. However, the power conversion efficiency (PCE) of OPV solar cells is still limited to 10-12% in part due to recombination of excitons before they can reach a nanoparticle-polymer interface. Such losses can be minimized by synthesizing photoactive layers with specific morphologies that result in large contact surface area between polymer and nanoparticles in the photoactive layer achieved through solution-based synthesis process. The scientific literature lacks detailed study of the effect of synthesis parameters, such as choice of solvent, processing temperature and relative concentration of nanoparticles and polymers, on the morphology of the photoactive layer. In order to understand the effects of these synthesis parameters on the agglomeration of nanoparticle, we performed molecular dynamics (MD) simulations of binary systems that comprise [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a commonly used acceptor, and various solvents in our previous study. The study suggested that indane is a better choice of solvent than other commonly used solvents for fabricating the photoactive layer of OPV solar cells. A systematic study of the effect of temperature and concentration of PCBM on agglomerate formation of PCBM showed that the extent of agglomerate formation increases with increasing temperature and concentration.
In an effort to further determine the agglomeration of PCBM in the conjugated polymer, we simulated systems comprising PCBM in poly-hexylthiophene (P3HT) that mimic the structure of a realistic photoactive layer after spin coating. We evaluated the agglomeration behavior of PCBM by determining sizes of the largest clusters of PCBM in P3HT. To obtain further insight into the agglomerate structure of PCBMs, we evaluated the radial distribution functions (RDFs), which describe the coordination of one group of atoms or molecules, such as PCBMs or P3HTs, with the same or other group of atoms/molecules in the agglomerate. Our results indicate that the solubility of PCBM is greater in P3HT than in indane. Our simulations also demonstrate that PCBMs have a tendency to agglomerate in P3HT and the extent of agglomerate formation increases with increasing concentration of PCBM. We also analyzed the orientation of P3HTs to determine the extent of crystallinity. The results show that P3HTs have greater likelihood of parallel stacking in the system comprising PCBM and P3HT in weight ratio 1:2 than in the system that comprise 1:1 weight ratio of PCBM and P3HT. Figures 1 (a) and (b) show a magnified snapshot of a subsection of the simulation domain that illustrates the molecular
arrangement of PCBM and P3HT in systems comprising PCBM and P3HT in weight ratios 1:1 and 1:2 respectively at 310 K. The aggregation pattern of PCBM and P3HT in Figure 1(a) demonstrates a greater contact area of PCBM and polymer compared to that in Figure 1 (b). Overall, our results show that relatively low concentration of PCBM in P3HT leads to sparse networks for charge transfer. In contrast, greater concentration of PCBM in P3HT leads to formation of suitable networks for transporting holes to photoanode and electrons to photocathode respectively. Results presented in this study provide fundamental insight that can help in selecting favorable processing parameters during synthesis of photoactive layer for OPV applications.

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Figure 1: The molecular arrangement of PCBM and P3HT in systems of (a) 1:1 PCBM-P3HT and (b) 1:2 PCBM-P3HT at 300 K.