Molecular Design And Theoretical Study Of Gradient Fused-ring Structure Non-fullerene Receptors

Due to the advantages of low-cost,easy to process and flexible,high-performance organic solar cells(OSCs)are one of the most reliable tools for China to achieve its carbon-neutral goal of equating carbon dioxide emission with carbon dioxide absorption.The low power conversion efficiency(PCE)of organic photovoltaic devices is a major bottleneck for the large-scale commercial application.Currently,efficient inorganic solar cell devices can achieve 20-40%of PCE,while organic solar cells are hardly to break through the PCE record of 15%before 2019.In the past two years,the PCE of OSCs has increased rapidly,and the highest PCE of single-junction OSC has reached 18%.These remarkable breakthroughs are attributed to the revolutionary ladder-type fuse ring non-fullerene acceptor(NFA)molecule Y6.Due to special ladder-type fuse ring core DA₂D and a new A₁-DA₂D-A₁(A₁/A₂:two different electron-accepting groups;D:electron-donating group)type of molecular framework,Y6 can maintain a strong light absorption in visible and near infrared region while the intramolecular electron push–pull(IEPP)effect can be furtherly improved.In this paper,a series of A₁-DA₂D-A₁ type NFAs based on Y6 is taken as the research object.The few key performance parameters as the acceptor materials of OSC are studied by first principle calculation,and the relationship between molecular modification and photovoltaic performance is analyzed in detail.This paper includes the following two parts:(1)Despite the recent exciting experimental progresses of Y6-based OSCs,the studies of molecular modifications of Y6,which may be the most effective and efficient way to improve PCE of OSC,are still few.In this work,a series of modified Y6 molecules has been systematically and rationally designed and modelled.Their physical and optical properties are predicted with reliable density functional theory(DFT)and time-dependent DFT calculations.We have computed following key parameters:1)dipole moments,affecting the molecular packing and morphology of an active layer;2)frontier molecular orbital(FMO)energy,important for open circuit voltage and energy level matching;3)UV–Vis absorption spectra,determining photon absorption,4)singlet-triplet energy gap(?)and exciton binding energy,critical for the voltage loss and charge recombination and for exciton separation correspondingly;5)charge(electron and hole)transfer/recombination(CT/CR)rate constants(6)/6)),indicating the efficiency of exciton separation;6)electron mobility(₀)),i.e.the speed of the carrier mobility in these NFAs.All these data are then applied to comprehensively make analyses to select the most potential candidates.The results show that proper extension of backbone by inserting thiophene rings with single bond connection is conducive to design high-efficient NFA.The most striking finding here is that one(named BTPTT-4F-2T)of the screened Y6 derivatives is superior to prototype Y6 in all aspects and may be next star high-performance NFA.(2)Since the high performance non-fullerene acceptor Y6 appears,many OSCs based on it and its derivatives have been fabricated and studied.Halogen substitution on end-group is a common and effective modification strategy in fuse-ring electron acceptor.Since Y6 was reported,related studies of halogen substitution on end-group have been published continuously.Recently,PM6/Y6-4Cl OSC has been made,and it shows slightly lower open circuit voltage(V_OC),higher short circuit density(J_SC),and smaller fill factor(FF)than prototype PM6/Y6 OSC.So far,this phenomenon is still not well understood.In the present work,the computed V_OCs and FFs,obtained with reasonable interfacial molecular modelling and reliable computational method,show exactly the same trends as experimental data.Furthermore,in PM6/Y6-4Cl,stronger simulated interfacial absorption spectrum,the smaller exciton binding energy of local excited state of Y6-4Cl monomer and charge transfer state at the interface,smaller singlet-triplet energy gap,the higher total oscillator strength of all CT states,larger interfacial electrostatic potential difference can reasonably explain the larger J_SC of PM6/Y6-4Cl OSC in experiment.Our results gain insights into the donor-acceptor interfacial properties of OSCs based on Y6and its derivatives,and a reliable computational method is also provided for future predictions of the similar type of OSCs.