Spectroscopic Study of Charge-Transfer States in Organic Semiconductors

Abstract

To achieve net zero carbon emission required for a sustainable economy, global energy production requires a clean and reliable solution. Photovoltaic technology that directly converts sunlight into electricity has demonstrated its potential in contributing to a carbon free energy future. Among myriad solar technologies, photovoltaic cells based on organic semiconductors offer unique advantages of being light weight, flexible and low cost and have shown promising photovoltaic performance with efficiency climbing over 18%. In state-of-the-art organic solar cells, a mixture of polymer electron donor and electron acceptor molecules converts light energy to electrical energy. The rapid performance advancement from 11% to over 18% in recent years is largely achieved by the replacement of fullerene molecules with small molecules as electron acceptors, known as non-fullerene acceptors. These new materials not only unlock promising photovoltaic performance but more importantly pose new photophysical questions that challenge the research community’s original understanding of organic solar cells and suggest new design rules. Central to the photophysics of organic solar cells, as reviewed in Chapter 1, is the charge-transfer state formed between the electron donor molecular and the acceptor molecule. The work presented in this thesis focuses on understanding the properties of the charge-transfer state and its role in mediating energy loss in solar cells. Contrary to the traditional model in which significant driving energy is required to separate tightly bound electron-hole pair in the charge-transfer state, one surprising finding to the organic solar cell community is that the most efficient polymer/non-fullerene organic photovoltaics have negligible driving force for charge separation. Furthermore, compared to fullerene acceptors, non-fullerene acceptors have appreciable absorption, implying that charge generation via hole transfer from acceptor to donor could play an important role. In Chapter 2, via detailed time-resolved and steady state spectroscopic studies, we discover a slow yet efficient generation of the charge-transfer state and charge carriers via hole transfer using a model blend of polymer and non-fullerene acceptors. Our findings also allude to a new photophysical scheme in charge generation that was not observed in polymer/fullerene blends but important to efficient polymer/non-fullerene acceptor blends. Another remarkable property of many efficient polymer/non-fullerene blends is their high photoluminescence efficiency and consequently small non-radiative recombination loss, suggesting that “a great solar cell is also a great light emitting diode” also applies to organic solar cells and prompting research efforts on improving the luminescence efficiency of charge-transfer states. Based on Shockley-Queisser’s theoretical framework, an ideal solar cell should only suffer energy loss from radiative recombination as it is unavoidable, and that any non-radiative recombination is excess. In organic solar cells, however, due to molecular vibrations, non-radiative recombination loss contributes a significant amount to total energy loss. Current research efforts have shown that the non-radiative recombination loss follows an energy-gap law where higher gap materials have intrinsically lower loss. Moreover, photoluminescence yield of the charge-transfer state can be limited by that of the local exciton of the lower bandgap material when these states quantum mechanically mix. In Chapter 3, I combine spectroscopic methods and molecular dynamic calculations to examine in detail what molecular properties determine photoluminescence yield of the charge-transfer state and non-radiative recombination loss of the solar cell. After demonstrating an intrinsically emissive yet charge-generating small molecule blend, I show that due to wavefunction mixing between the charge-transfer state and the local exciton, both photoluminescence quantum yield and lifetime of the local exciton influences emission of the charge-transfer state. The latter is a new consideration for selecting materials for efficient organic photovoltaics and light emitting diodes. In Chapter 4, I propose and show current progress on a previously overlooked spectroscopy method directly detecting wavefunction mixing between the charge-transfer state and the local exciton of non-fullerene acceptor molecules. Our findings and proposal provide direction for molecular design and material selection to limit energy loss in organic solar cells.