Rumbles-Reid Group Research Overview

Singlet fission (SF) has shown potential to overcome the Shockley-Queisser limit in single-junction solar cells and increase the maximum theoretical efficiency from 33% to 44%. In typical single-junction solar cells, the efficiency is limited in some part by thermalization losses. Singlet fission could circumvent this loss by generating two excitons from one high-energy photon. These excitons can either undergo charge or energy transfer at an interface to produce two charge carrier pairs.

For SF materials to be successfully incorporated into solar cell devices, not only does the SF process need to be efficient, but the subsequent triplets need to dissociate into charge carriers (or undergo energy transfer) in high yield. Organic solar cells that rely on singlet exciton dissociation have been shown to approach internal quantum efficiencies (IQE) near 100%. However, an IQE near 200% in SF materials has yet to be achieved. Triplet excitons in these materials are at approximately half the energy, are more localized, and exhibit different transport mechanisms than their singlet counterparts. Recent work has demonstrated that the driving force for exciton dissociation greatly influences charge transfer from both singlet and triplet states, and the lower energy of the triplet state requires electron acceptors with larger reduction potentials. However, it is not clear how charge transfer is affected by other qualities of SF-born triplets.

Our work is aimed at understanding charge transfer in SF materials in order to determine how triplet excitons can be selectively dissociated into charge carriers in high yields. Ultrafast transient absorption (TA) and time-resolved microwave conductivity (TRMC) are used to investigate the dynamics involved in charge transfer in crystalline thin films of a SF host sensitized with dilute concentrations of electron-acceptors. 

Photoredox catalysts are molecular systems that drive thermodynamically unfavorable reactions through the transfer of an excited state electron or hole. Many light harvesting organisms rely on photoredox catalysis to convert and store energy. In a photoredox catalysts, the charge separation before charge-transfer is crucial; the electron and hole separation must be long-lived for the charge-transfer to occur, and the electron-hole separation potential must be sufficiently large for the charges to undergo transfer. We believe understanding these charge distribution states are crucial in order to design new photoredox catalysts.

Using natures light harvesters as inspiration, our collaboration within the Bioinspired Light-escalated (BioLEC) Energy Research Frontier Center (EFRC) aims to engineer new artificial photoredox catalysts. A key component to synthesizing new photoredox catalysts is understanding how molecular structures affect the catalytic potential of photoredox catalysts. There is currently little known about how the charge density of the photoredox catalyst effects the photocatalytic cycle. Using time-resolved dielectric loss spectroscopy (TRDL) and other photophysical measurement techniques such as transient absorption (TA), we hope to provide useful information on how a photoredox catalysts’ electronic distribution influences its catalytic activity.