Theoretical Study On The Modulation Of Acceptor Excited States And Charge Related Processes In Non-Fullerene Organic Solar Cells

The development of renewable energy is the key to achieve the national " carbon peaking and carbon neutrality " strategic goals.Therefore,solar energy has attracted widespread attention as an inexhaustible,widely distributed,green,and harmless renewable energy source.Organic solar cells(OSCs)are attracting intensive attention due to their advantages of lightweightness,mechanical flexibility,semitransparency,and suitability for roll-to-roll printing processes.Based on the differences in photovoltaic layer acceptor materials,organic solar cells can be divided into two types:fullerene-based and non-fullerene-based.For fullerene acceptor materials,weak absorption in the visible region and difficulty in tuning the energy level lead to a maximum photovoltaic conversion efficiency(PCE)of only 12%for fullerene-based OSCs.Since 2015,non-fullerene acceptor(NFA)materials such as ITIC and Y6 have been introduced into the photovoltaic layer,substantially improving the device efficiency and enabling single-junction devices with PCE over 19%.Compared with fullerene acceptors,NFAs exhibit strong absorption in the visible to near-infrared wavelength range,tunable energy levels,a push-pull electronic structure,and various molecular aggregation modes,which make the excited state processes of NFA systems unique compared to those of fullerene systems.The clarification of these unique excited state processes and regulatory mechanisms will provide a more targeted strategy for optimizing NFA-based OSCs.Understanding the photoinduced charge process ofNFA molecules is a prerequisite for optimizing the photovoltaic process of OSCs.Although experimental studies have shown that the molecular properties of NFAs(including intramolecular electron-pull structure and intermolecular aggregation structure)have an effect on light absorption.However,an in-depth understanding of the photoinduced intra-and intermolecular charge transfer(intra-CT and inter-CT)dynamics in NFA molecules remains unclear.Especially.with the development of transient absorption(TA).some novel charge separation phenomena have been observed that are difficult to explain by conventional exciton images.For example.spontaneous charge separation can be free from the D/A interface in some NFA based heterosystems and take place even in NFA monomers.More interestingly,the corresponding spontaneous charge separation signal could not be observed in the NFA molecular solution compared to the NFA molecular film.This suggests that the aggregation effect of NFA molecules induces spontaneous charge separation in their excited states.However,the mechanism of molecular aggregation leading to spontaneous charge separation is still unclear.Furthermore,although charge separation can spontaneously take place in some NFA molecules,it is an open question why the device efficiency is far below the expected values when they are used as photovoltaic layers alone.This requires us to clarify the role of D/A interfaces in charge separation and charge recombination in typical NFA based heterogeneous systems.More importantly,from a macroscopic point of view,these excited state processes within the NFA system are not independent,and their competitive dynamics will significantly affect the upper limit of the device PCE.Therefore,in order to provide guidance for device optimization from a macroscopic perspective,it is necessary to build a complete picture of the excited state charge process based on an understanding of the microscopic dynamics of the NFA system.Based on this,this dissertation uses a tight-binding model combined with a nonad iabatic quantum dynamics approach and a phenomenological approach to investigate the charge processes involved in the NFA system at both the microscopic and macroscopic levels.The main research components are as follows:1.Non-fullerene acceptor photoexcitation dynamics and spontaneous charge separationBy separately applying a photoexcitation to a single NFA molecule and different molecular aggregates.we clarify the photoinduced electron transition details(including transition mode.peak position,and probability),the resulting intra-and inter-CT dynamics,and the modulating methods(such as the intra-molecular electron push-pull potential,the inter-molecular aggregation mode,and the photoexcitation intensity).Especially.we clarified that intra-CT and inter-CT effects are intrinsically coexistent in NFA molecules.by which the NFA molecular excited state can be spontaneously dissociated into free charges with a high efficiency at room temperature.These findings provide theoretical guidance for explaining the appearance of "polaron-like" TA signals in non-fullerene monomers and optimizing the design of photovoltaic systems.2.The role of donor/acceptor interfaces in non-fullerene-acceptor based organic solar cells:charge separation versus recombinationBased on the clarification of the excited state characteristics of NFA monomers,we further introduced the D/A interface to investigate the effects on charge separation and charge recombination.For charge separation,we demonstrate that the photoinduced spontaneous charge separation efficiency in typical NFA heterosystems can reach up to 67%,and the charge separation efficiency contributed by the D/A interface is only 25%.The more important role of D/A interfaces is to reduce the charge recombination rate,especially by optimizing the competition between radiative and non-radiative charge recombination and thus reducing the non-radiative energy loss.Systematic simulations demonstrate that there exists an optimal interfacial distance at which the non-radiative energy loss is minimal.This minimal non-radiative energy loss and optimal interfacial distance are also impacted by the interfacial energy offset.Hence,we propose that optimizing the interfacial distance combined with the actual interfacial energy offset of a given heterosystem is important to develop its best photovoltaic performance.3.Device performance tuning based on unique charge processes in non-fullerene systemsSince the excited state charge processes of the NFA system are not independent,but compete with each other.Therefore.combining the previous understanding of microscopic quantum dynamics,we construct the complete charge process dominated by the excited state of the NFA molecules.By developing multi-state dynamic equilibrium equations.we investigated the effect of NFA molecular aggregation structure on device performance at the macroscopic level.The results show that there exists an optimal J-aggregation ratio to maximize the PCE for the NFA system,and the aggregation ratio can be modulated by the device active layer thickness.This provides theoretical guidance for the optimization of photovoltaic layer morphology,especially for achieving high performance and large thickness in NFA-based OSCs.