Intra- and Intermolecular Charge Transfer Characteristics in Novel Non-Fullerene Acceptors

Abstract

Organic photovoltaic cells (OPVs) with Y-series non-fullerene acceptors (NFA) have recently reported high efficiencies closer to 20%. With the introduction of a non-fullerene acceptor called Y6 in 2019, organic solar cells were given a new life by igniting research interests among the scientific community. Moreover, the bulk heterojunction (BHJ) devices with Y6 and its derivatives as non-fullerene acceptors facilitated ultrafast charge transfer (CT) and exciton dissociation despite the negligible energy offsets at the donor/acceptor interface. It has been suggested that the high performance of Y6-containing donor-acceptor active layers is attributed to the efficient CT excitations in Y6 aggregates. Interestingly, Y6 molecules have been demonstrated to have distinctive molecular packing due to their molecular conformation, high molecular rigidity, and the absence of out-of-plane side chains. Therefore, much attention has been drawn to understanding the correlation between molecular packing and charge generation in Y6-based OPVs. Very recently, transient absorption spectroscopy (TAS) data have revealed the existence of Y6 inter-molecular excitations in both neat Y6 and blend Y6:PM6 films. However, it is still unclear if this strong CT character is correlated to electronic properties at the molecular level. Despite these superior performance metrics of Y6, whether it is solely contributed by its unique molecular packing or inherent in its molecular structure is still under debate. In particular, direct experimental verification of CT character in Y6 molecules remains elusive. Therefore, this research study aims to investigate the intra- and intermolecular charge transfer properties in novel non-fullerene acceptors such as Y6 and ITIC. The main objective is to develop experimental approaches to reveal the effect of molecular packing or aggregate formations on the charge transfer properties and bring insight into the structure-property-performance relationship of novel non-fullerene acceptors in organic photovoltaic cells.

This thesis is organized as follows: Chapter 1 introduces organic solar cells, the main objectives, and the scope of this research work. Chapter 2 deals with the fundamental concepts in organic photovoltaics, its history of development over the years, evolution of different materials and device architectures. In Chapter 3, we discuss in detail the materials used, different device fabrication methods, and characterization techniques for this research work. Various analytical methods used in this research study are also included in this chapter.

Chapter 4 will mainly discuss the investigation of charge-transfer characteristics of electronic excitations of isolated and aggregated Y6 and ITIC non-fullerene acceptor molecules using electroabsorption spectroscopy (EAS). We fabricated solid-solution thin films by dispersing NFA molecules in an insulating polymer and studied the corresponding CT properties. Interestingly, we found that the CT character already exists in isolated Y6 molecules but is strongly increased due to molecular aggregation. The molecular packing and electronic coupling keep increasing as the loading ratio of Y6 increases in the polymer, indicating the existence of long-range CT pathways. The strong CT character in Y6 is correlated to the excitonic parameters, polarizability (p), and dipole moment (μ), which are extracted from the second harmonic EA spectrum. It is also found that Y6 molecules have preferred molecular orientation from the first harmonic EA results.

In Chapter 5, we made efforts to do other characterization techniques such as GIWAXS, density functional theory (DFT) calculation, Franck-Condon (FC) fitting, and photoluminescence quantum yield (PLQY) measurements to further support our findings from EA measurements on Y6. It is found that the strong CT in packed Y6 molecules is not due to an increase in excited-state dipole moment, Δμ, as observed in other organic systems, but due to a reduced polarizability change, Δp. An exciton recombination model considering the overlap of the electron-hole wavefunctions is also proposed to elucidate the experimental findings. Interestingly, Δμ and Δp values deduced in different monomer and dimer configurations using DFT and four-state model calculations are consistent with our EA results. GIWAXS and Frank-Condon analysis helped to disseminate different aggregate and non-interacting molecular regions at different energy excitations of Y6.

In Chapter 6, we further expanded this research study to understand the effect of solvent additive (1,8- Diiodooctane - DIO), polymer (Polystyrene) and terminal groups in improving the intermolecular interactions of Y6. With increased concentration of DIO, Y6 shows enhanced molecular packing and charge transfer properties in optical absorption and electroabsorption spectra. We used Franck Condon analysis to determine the effect of DIO on aggregated and non-aggregated regions of Y6. Next, we fabricated solid-solution thin films of Y6 dispersed in polystyrene polymer where Y6 molecules tend to aggregate and exhibit strong CT character even at low loading ratios. Additionally, we tried the same dilution approach to probe the intra- and intermolecular charge transfer properties of other Y-series acceptors, such as BTP-S1 and BTP-S9. The weak CT character in isolated molecules has been increased with an increase in the NFA molecular loading ratio in PVK polymer. Quantitatively, the extracted Δμ and Δp values from the second harmonic EA spectrum confirm that the ratio between these parameters can be used to explain its CT characteristics. Interestingly, all these results further support the proposed exciton recombination model.

Chapter 7 summarizes all the experimental findings, which bring insights into the structure-property-performance correlation in Y6 non-fullerene acceptors. Overall, the findings of this research thesis offer a deeper understanding of the intra- and intermolecular interactions between NFA molecules that help in strong CT excitations, and the methodology to analyze the electroabsorption spectroscopy results to investigate the underlying electronic properties in organic photovoltaic materials. We also hope that this evidence of strong CT excitations in Y6 can be a significant leap into the development of next-generation single-component organic solar cells.

Date of Award	13 Jun 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sai Wing TSANG (Supervisor)