Multiscale computational study of polymer solar cell active layers

  • Author / Creator
    Garcia, Jan Ulric M.
  • 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.

  • Subjects / Keywords
  • Graduation date
    Fall 2015
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.