Photophysics of fused ring electron acceptors for photovoltaic applications

Abstract

Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures.

Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer.

Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices.

The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions.

Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that