Theoretical Study On The Charge Transfer Mechanism At Donor/Acceptor Interface In Organic Solar Cells For Non-fullerene Fused-ring Acceptors

In the last few decades,the persistent increase of the fossil fuel consumption and serious environmental problems prompt the exploration of new and clean renewable energy resources,especially for solar energy.Organic solar cells（OSCs）gain increasing attention owing to some advantages such as low cost,easy modification and mechanical flexibility compared with silicon solar cells.As a key component of OSC,active layer with electron donor and electron acceptor plays an important role in affecting the device performance.The electron donor materials of OSC have been developed abundantly,but the electron acceptor materials have been confined to fullerene derivatives.However,some drawbacks of fullerenes,such as relatively stable chemical structure,limited energy level and weak optical absorption,restrict its further development in OCSs.To address these issues,rapid progress in the development of non-fullerene（NF）acceptors has opened a new page for molecular design in OSCs.In this thesis,some theoretical simulations（density functional theory（DFT）,time-dependent DFT（TD-DFT）and molecular dynamic（MD）calculations）on NF fused-ring electron acceptor materials were carried out to search the high-efficacy acceptors for OSCs.Moreover,this work also deeply analyzes the charge transfer mechanism at the donor/acceptor（D/A）interface.The main studies are included in the following:1.Considering the low PCE of NFOSCs,it would require a system work to perform a deep theoretical comparative study on fullerenes and non-fullerene acceptors at the molecular level.Herein,a comparative study on a novel small acceptor molecule FENIDT and a classical acceptor molecule PC₆₁BM were presented based on DFT and TD-DFT method for searching the advantages and disadvantages on NF acceptors.Many properties,such as open circuit voltage(V_oc),energetic driving force（ΔE）,optical properties,transport properties,charge separation rate(k_CS)and charge recombination rate(k_CR)at the D/A interface,were evaluated and discussed.The calculated results indicated that FENIDT has advantage in V_oc,absorption spectrum,transport and interface parameters.Finally,according to the advantages of FENIDT,we designed a series of acceptor molecules and molecule 3will be a promising acceptor in NFOSC.2.Troisi indicates that multiple degenerates in the lowest unoccupied molecular orbital and low excited levels occupied in anions may affect the interface charge separation process.The three-dimensional imide derivative DBFI-T has the same property in the experimental report,and the theoretical calculation could provide a further proof.Then,molecules 1 and 2 were designed through replacing the central unit thiophene based on reported DBFI-T to screen suitable acceptor materials for NFOSC.DFT and TD-DFT calculations were performed to investigate the electronic structures,V_OC,ΔE,absorption spectrum,k_CS and k_CR.The charge transfer states were selected according to the frontier molecular orbital distributions and transition natures and the effect of low lying excited states was taken into account by the estimation of the k_CS.The results manifest that designed systems 1 and 2 exhibit excellent V_OC,sufficientΔE,suitable absorption spectra,large k_CS and small k_CR,suggesting that molecules 1 and 2 are promising acceptor materials for high-performance NFOSCs.3.The ITIC-based non-fullerene OSC shows outstanding performance compared with the fullerene OSC.We attempted to make a multiscale analysis and comparative study of interface characteristics including k_CS/k_CR and the relative position of charge transfer states to Frenkel exciton states by combining MD simulation and DFT calculation aiming at finding the critical factors determining the superior performance of ITIC for OSC.As a result,we believe that the reason which causes the superior performance of ITIC in OSCs is the better match between PBDB-T and ITIC and larger k_CS.