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Multiscale computational study of polymer solar cell active layers Open Access

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Other title
Subject/Keyword
Flory-Huggins
P3HT
PFTBT
polymer solar cell
molecular dynamics
dissipative particle dynamics
regioregularity
Type of item
Thesis
Degree grantor
University of Alberta
Author or creator
Garcia, Jan Ulric M.
Supervisor and department
Choi, Phillip (Chemical and Materials Engineering)
Examining committee member and department
Soares, Joao (Chemical and Materials Engineering)
Choi, Phillip (Chemical and Materials Engineering)
Elias, Anastasia (Chemical and Materials Engineering)
Department
Department of Chemical and Materials Engineering
Specialization
Chemical Engineering
Date accepted
2015-08-12T10:35:43Z
Graduation date
2015-11
Degree
Master of Science
Degree level
Master's
Abstract
Solar energy holds great potential in securing humanity’s energy future in a sustainable manner. Unfortunately, the costs of silicon photovoltaics continue to impede the use of solar energy. Polymer solar cells (PSCs) can make solar energy more affordable due to their lower production costs. However, PSC efficiencies remain too low to compete with silicon photovoltaics. One of the most important factors affecting PSC efficiencies is the phase morphology of the device active layers. To achieve the most efficient devices, a lamellar morphology is ideal. Using block copolymers in the active layer is a promising strategy to achieve the desired lamellar morphology. In this strategy, the polymer electron donor and the polymer electron acceptor are spliced covalently at one end. This unique structure allows the donor and acceptor to form lamellar phase domains with sizes around 10 nm. This strategy was shown to be effective with poly(3-hexylthiophene) (P3HT) as the donor and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2”-diyl) (PFTBT) as the acceptor. However, much optimization is still necessary to the design and processing of the active layer materials. One fundamental factor is P3HT regioregularity (RR). The percent RR is defined as the fraction of head-to-tail bonds in the polymer backbone. In terms of processing, device annealing temperature must also be optimized. In this dissertation, both factors were studied using multiscale computational modeling. Atomistic molecular dynamics (MD) simulations were used to calculate the Flory-Huggins interaction parameter (χ) between P3HT and PFTBT under different values of RR and temperature. The calculated χ values were then used as inputs for mesoscopic dissipative particle dynamics (DPD) simulations to predict the active layer phase morphologies of the P3HT-PFTBT system under different conditions. Through MD simulations, the average χ parameter values were estimated to be 3.3 for RR < 50%, 1.6 for RR=63%, and 0.9 for RR ≥ 90%. This χ-RR trend was attributed to the increased π-π stacking for lower RR values in the simulated amorphous phase. The cause for the RR-π-π stacking relationship remained unclear; the issue was not explored any further due to the time constraints of this dissertation. For temperatures from 373 K to 503 K, the χ parameter was found to follow a linear relationship with the reciprocal of the absolute temperature (1/T). The slope and intercept of the χ vs. (1/T) regression line were estimated to be 5370 K and -12.0, respectively. The optimal annealing temperature was 438 K. The temperature 503 K was found to be too high to maintain phase separation, i.e., extreme temperatures led to homogeneous mixing. Through DPD simulations, it was observed that systems with RR < 50% resulted in non-lamellar morphologies. The lamellar morphology was observed for RR values of at least 63%. Only slight improvements in the morphology were observed when RR was increased from 63% to 100%. Slight improvements to the morphology were also observed when temperature was increased from 373 K to 438 K. The simulations also showed that the lamellar morphology was only achievable with the diblock copolymer architecture. Simply mixing P3HT and PFTBT in a blend was not enough to achieve the lamellar morphology. Despite the qualitative utility of our method, much improvement can be made for future work. The equilibration of MD cells can be extended with better computing resources. High temperature equilibration can also be applied. The accuracy of DPD simulations can also be improved by considering anisotropic rod-rod interactions found in conjugated polymers such as those used in PSC applications.
Language
English
DOI
doi:10.7939/R31V5BK2D
Rights
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
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