Model-Guided Design and Optimization of Polymer-Electrolyte Dye Sensitized Solar Cells
Authors: Smolin Yuriy Y. , Kuba Austin G. , Lau Kenneth K.S. , Soroush Masoud 
Published: February 2020

Dye sensitized solar cells (DSSCs) are commonly regarded as a promising next generation solar cell technology because they have good performance at ~13% and can be made relatively inexpensively without intensive purification and fabrication steps. However, the liquid electrolyte of conventional DSSCs is prone to leakage and evaporation, which hinders DSSC applications, durability, and thermal stability. In addition, liquid electrolytes lead to significant electron recombination at the TiO2-electrolyte interface within the cells that limit DSSC performance. Novel polymer electrolyte DSSCs can overcome these disadvantages and can potentially enhance the cell I-V behavior. Experimental studies have demonstrated that polymer electrolytes are good alternatives to liquid electrolytes in DSSCs but efficiencies are less than half of liquid-electrolyte DSSCs. Furthermore, much of the current material research in this area has been based on scientific intuition and trial and error experimentation—a slow and inefficient process with many of the directions taken to address device challenges being out without theoretical guidance.

There have been relatively few studies that combine experimental and computational/theoretical approaches to better understand DSSC processes and optimize DSSC performance. Currently, there is a need for a better understanding of the effect of polymer electrolyte chemistry on cell performance. First-principles mathematical DSSC models can prove valuable in gaining this insight. By properly accounting for the physical and electrochemical processes occurring inside the cells, the models can elucidate competing effects within DSSC components, identify factors that affect the overall cell performance, and guide experimental work in designing and optimizing cell materials. In fact, many of the recent advances in the DSSC fields have relied on the computational design and screening of materials.

In our previous work [1], we theoretically and experimentally investigated the effects of polymer-electrolyte chemistry on the performance of polymer-electrolyte DSSCs. To gain a complete understanding of the photochemical processes inside the DSSCs, a first-principles macroscopic model was developed and integrated with experiments that utilized initiated chemical vapor deposition (iCVD) to synthesize and incorporate the polymer electrolytes within the mesoporous TiO2 photoanode. iCVD relies on the vapor delivery of monomer and initiator, which facilitates infiltration into the porous TiO2 substrate. By controlling the relative rates of diffusion and surface polymerization through iCVD process parameters, iCVD has the advantage of being able to conformally deposit thin films within the photoanode, which has 10-20 nm pores, high aspect ratios <1000, and a tortuous pore structure. Furthermore, iCVD overcomes the challenges of liquid deposition techniques such as liquid surface tension, poor wettability, and solute steric hindrance. This enables good interfacial contact at the TiO2 electrode and polymer electrolyte junction.

The model has been used to elucidate the effects of polymer chemistry on the cell performance, and to determine how and why the photochemical processes inside such solar cells are altered by poly(2‐hydroxyethyl methacrylate) (PHEMA), poly(glycidyl methacrylate) (PGMA), and poly(4-vinylpyridine) (P4VP). By varying the polymer electrolyte chemistry, the work show that DSSC cell characteristics, including open circuit voltage, short circuit current density, and fill factor can be tuned. Parameter estimation using the macroscopic model indicated that a shift in the conduction band of TiO2 and a suppression of the back electron transfer at the dye-TiO2-electrolyte interface are induced by the methyl ester of PGMA, the pyridine group of P4VP, and the hydrophilic pendant group of PHEMA. The model revealed that (a) a more favorable Lewis acid-base interaction between the polymer electrolyte and TiO2 electrode reduces charge recombination at the interface and thus leads to a higher short-circuit current, J
sc, and (b) a more preferential Li+ ion (present in the electrolyte) coordination with the polymer electrolyte leads to a higher TiO2 conduction band potential and in turn to a higher open-circuit voltage, V
oc.

Based on this understanding, a new polymer electrolyte, poly(1-vinylimidazole) (PVIZ) which has strong basic character and metal ion chelation, is selected as a highly strong candidate for producing DSSCs that would yield higher J
sc and V
oc simultaneously. Our prior studies of different polymer electrolytes yielded only an increase in J
sc or V
oc but not in both. To experimentally validate our model’s predictions, PVIZ has been conformally integrated into the mesoporous TiO2 photoanode via iCVD. Experimental results indicate that DSSCs incorporating PVIZ have 27% greater power conversion efficiency than liquid-electrolyte DSSCs, and have a higher J
sc and V
oc, confirming the model predictions. To our knowledge, this is the first demonstration of PVIZ as a polymer-electrolyte in DSSCs and highlights the importance of mathematical modeling in improving the solar technology.

[1] Y.Y. Smolin et al., Journal of Power Sources, 274 (2015) 156-164.