Redox mediation through integrating chain extenders in active ionomer polyurethane hard segments in CdS quantum dot sensitized solar cell

Photovoltaic conversion efficiency is expressed as a function of hole extracting capacity of redox active group of electrolyte structure. Hard segment functionalized polyurethane ionomers have been developed as gel polyelectrolyte for photovoltaic reaction at the interfaces of photoanode/electrolyte/counter electrode in quantum dot sensitized solar cell. 3-mercaptopropeonic acid functionalized Cd5 quantum dot is synthesized and grown over the surface of titanium nanooxide via spin coating technique to work as photosensitizer. The hard segments (urethane linkages) are designed as negatively charged, short chain and covalently linked as -CH2CH2CH2SO3- (pendant anion) moiety. Different chemical and electronic environment affect hole transfer capacity around redox active group. The gel structure is realized as a conductive host as well as single ion transport electrolyte. The oxidized QDs (hole) are scavenged with redox active group available on the surface of hard segments. The electrical conductivities and thermal stability are controlled with composition of redox active group and the nature of chain extenders in polyurethane chain. The energy gap of HOMO and LUMO energy levels varies with the composition and electronic environment around polar pendant group. Electrical conductivity increases with functionalization level (anion concentration) grafted over urethane linkages. Charge transport behavior is controlled by tuning energy levels at the interfaces of photoanode/electrolyte matrix. The potential barrier is reduced by tuning redox potential of valance band (QDs) and HOMO (redox active ionomer). Photovoltaic performances are studied by varying chain structure and hole conducting environment around functionalized urethane linkage in ionomer framework. The device FTO/TiO2/MPA-CdS/SGO/SPU/Pt exhibits maximum photovoltaic conversion efficiency of 1.16% with soft segment modified oxygenic rich composite ionomer [S (PU + CB)-PCL-EDA] gel polyelectrolyte. Thus, redox active side chains easily recharge the oxidized quantum dot and accelerate photovoltaic reversible reaction due to adsorption of conducting carbon black on ionomeric chain.

Automated summary

This study developed hard segment functionalized polyurethane ionomers as gel polyelectrolytes for quantum dot sensitized solar cells, achieving high photovoltaic conversion efficiency. The hole extracting capacity of the redox active group was controlled by adjusting the chemical and electronic environment. The effects of the structure of the hard segment on photovoltaic performance were thoroughly investigated, and it was found that the conductivity and thermal stability could be improved by functionalizing the polyurethane chain extenders and tuning the redox potentials. The experimental results demonstrate that high photovoltaic conversion efficiency can be achieved by selecting appropriate combinations of ionomers and hole conducting environments.