The solar power conversion efficiencies of organic solar cells (OSC) have now increased up to 19%, closing the gap with inorganic and hybrid solar cells. The major breakthrough behind the rapid efficiency improvement is the development of non-fullerene acceptor molecules, replacing the traditional fullerene molecules as electron-accepting materials. Understanding the photophysical processes underlying these high-performance materials is crucial to OSC research. In this talk, I will present transient optical spectroscopy and structural analysis results on high-performance OSC blends based on state-of-the-art Y-type small molecule and polymeric acceptors. We find direct evidence that the interfacial D-A percolation plays a key role in suppressing interfacial charge recombination to enable efficient charge generation, and such morphology greatly improves the thermodynamic stability of the blend. Furthermore, we uncovered a new all-optical method for predicting the OSC performance of acceptor molecules, which will be a valuable tool for future material design and screening.
Two-dimensional (2D) metal halide perovskites have emerged as promising materials for next-generation solar cells and light-emitting diodes thanks to their outstanding optoelectronic properties, facile tunability, and superior stability over their 3D counterparts. However, the detailed structure-property relationship of 2D perovskites underlying their optoelectronic properties has remained unclear. In this project, we design and synthesize a variety of 2D perovskite single crystals, in both Ruddlesden-Popper and Dion-Jacobson phases. We then use a combination of steady-state and time-resolved optical spectroscopy methods to characterize the exciton properties and dynamics, specifically focusing on how they are affected by changes in structural properties. Our results will help us develop a fundamental understanding of 2D perovskites and enable rational material design and development.
Over the past six years, the solar power conversion efficiencies of organic solar cells (OSC) have greatly improved from 12% to 19%, closing the gap with inorganic and hybrid solar cells. The major breakthrough behind the rapid efficiency improvement is the development of non-fullerene acceptor molecules, replacing the traditional fullerene molecules as electron-accepting materials. Understanding the photophysical processes underlying these high-performance materials is crucial to OSC research. In this talk, I will present transient optical spectroscopy results on non-fullerene OSC blends with small interfacial energy offsets. By optically probing the time evolution of excited states, we show that free charges are generated via thermal activation of interfacial charge-transfer (CT) states on a hundred picosecond timescale. Reduced charge separation rate is observed at lower temperatures, leading to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OSC blends can achieve high photocurrent generation yields even at reduced temperatures given that the thermal dissociation rate of charges outcompetes the recombination rate.
Organic solar cells (OSCs) based on non-fullerene acceptors can achieve high charge generation yields despite near-zero donor-acceptor energy offsets to drive electron-hole separation. In this talk I will present experimental data to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer excitons (CTEs) that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron-hole encounters at later times, CTEs and emissive excitons are regenerated, thus setting up an equilibrium between excitons, CTEs and free charges. This endothermic charge separation process enables these systems to operate close to quasi-thermodynamic equilibrium conditions with no requirement for energy offsets to drive charge separation and achieve greatly suppressed non-radiative recombination.
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