Regulating Exciton Dynamics Facilitate Efficient Change Generation for High-performance Organic Photovoltaics

Abstract

Organic photovoltaics (OPVs) are currently approaching commercial viability thanks to the development of photoactive materials, which promote the realization of over 19% certified efficiency for single-junction cells. Even so, the power conversion efficiency (PCE) gap is still huge between inorganic/halide perovskite solar cells with OPVs, which is due to OPVs possessing intrinsic high exciton binding energy (E_b 0.3~0.5 eV) and suppressing efficient exciton dissociation as compared to other inorganic solar cells. Given this, efficient OPVs usually consist of a donor-acceptor blend, wherein the energetic separation between their frontier molecular orbitals provides a driving force to overcome the Coulomb interaction force to promote exciton dissociation. Therefore, to reach the interface, the exciton must have a long diffusion without the film thickness effect, promoting the bulk heterojunction developed by forming the nanoscale domains. Meanwhile, if these electrons and holes are not collected by the electrodes but encounter each other again at the donor/acceptor interface, they may recombine to form a low-energy spin-triplet exciton (T1). This irreversible process strongly limits the maximum achievable PCE of OPV. In this thesis, I will introduce my research efforts devoted to improving exciton diffusion length and minimizing the recombination loss, aiming to provide insights into high-performance OPVs.

Chapter II exhibits an exciting discovery of the relationship between photoactive materials and their photoluminescence quantum yield (PLQY), by studying the properties of the conjugated polymer D18 on both the material and device levels. Combining optical spectroscopy, X-ray diffraction, and simulation. The D18 exhibits stronger π-π interactions and interchain packing compared to classic donor polymers, as well as higher external photoluminescence quantum efficiency (~26%). The results of picosecond transient absorption spectroscopy and streak camera reveal the initial D18 excitons form delocalized intermediates, which decay radiatively with high efficiency in neat films. In single-component organic photovoltaic (OPV) cells based on D18, these intermediate excitations can be harvested with an internal quantum efficiency >30%, while in blends with Y6, they provide a pathway to free charge generation that partially bypasses performance-limiting charge-transfer states at the D18:Y6 interface. These findings not only explain why the current materials system with PCEs exceeding 20% are all based on the D18 series in single-junction OPVs but also provide novel insight into next-generation high-performance material design.

Chapter III highlighted that a device engineering strategy, planar-mixed heterojunction (PMHJ) structure, can be used to suppress the non-geminate recombination at donor-acceptor contacts, thus limiting the chance of exciton relaxation to suppress the lowest-energy triplet exciton (T1) formation in the same material system. This is totally different from the reported strategies, which are to seek materials with poor T1 characteristics to minimize the recombination loss. Especially for the material systems with enough exciton diffusion length, PMHJ exhibits great potential for high-performance OPVs. This finding provides a comprehensive understanding for future improvements in OPVs to reach their full promise.

In conclusion, through exploring the relationship between material property-device structure-work mechanisms, we have successfully elucidated how to intrinsically improve the OPV performance. These research findings provide important material design guidance and device reference for future further development of efficient OPV, pushing OPVs forward closer to the Shockley-Queisser limit.

Date of Award	2 Apr 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Alex JEN (Supervisor)