Green Solvent Processed High-performance Organic Solar Cells Achieved With Quaternary Strategies

Solution-processed organic solar cells（OSCs）have attracted a lot of attention because of their low-cost,light-weight,flexibility,semi-transparent and roll-to-roll printing properties.In the past several years,particularly binary devices based on the Y-series acceptors with excellent properties have achieved power conversion efficiencies of over 19%.However,organic semiconductor materials have relatively narrow absorption peaks,relatively large exciton binding energies and relatively high recombination losses.To further improve device performance,the solar spectrum coverage of the active layer is extended by introducing a guest component into the binary system consisting of a host material,increasing the absorption efficiency of the solar cell for near-infrared photons and improving the short-circuit current density(J_SC).This strategy can be used to increase the fill-factor（FF）by optimizing the structure of the guest components.In order to obtain a low band gap,however,it is often necessary to lower the LUMO energy level of the guest to achieve this and therefore often leads to a lowering of the open-circuit voltage(V_OC).In addition to the ternary strategies of“voltage-increased”and“current-increased”,the performance of the device can be improved by introducing a highly crystalline guest to form an alloy phase,which is often achieved with the increase of FF.Furthermore,the processed solvents for active layer materials are mainly chlorinated solvents such as chloroform,chlorobenzene,and o-dichlorobenzene,which are normally the“good”solvent for organic photovoltaic materials and benefit the formation of an efficient 3D network structure for charge separation and transport in the co-blended film.However,chlorinated solvents are usually toxic and highly evaporated,which is harmful to human health and environmental protection.Hence,in this thesis we have designed three quaternary OSC material systems with efficiencies exceeding 17%processed through a green solvent and combined with the quaternary strategy:1.Designing two phthalimide based ultra-wide band gap polymer donors e.g.Ph I-F and Ph I-Cl as guest acceptors,and constructed two quaternary OSC material systems,i.e.Ph I-F:PM6:L8-BO:BTP-e C9 and Ph I-Cl:PM6:L8-BO:BTP-e C9,respectively.Ph I-F and Ph I-Cl,with their absorption mainly in the short wavelength region,both have lower HOMO energy levels（-5.75 e V,-5.55 e V vs-5.54 e V）compared to PM6,and L8-BO has a higher LUMO energy level（-3.90 e V vs-4.09 e V）compared to BTP-e C9,which achieves an increase in open-circuit voltage.At the same time,the Ph I-based polymer donor has a similar chemical structure to PM6,which may be beneficial to ensure miscibility between the materials and achieve tuning of the co-blended film structure.Compared to the PM6:L8-BO and PM6:BTP-e C9 binary device,the quaternary devices processed with the green solvent toluene showed improved carrier mobilities(5.83/2.04×10^-4 cm²V^-1s^-1 and 5.75/2.01×10^-4 cm²V^-1s^-1)and reduced recombination losses,with efficiencies of 17.4%and 17.3%obtained for the Ph I-F and Ph I-Cl based quaternary devices,respectively.2.The quaternary system of Ph I-Cl:PM6:BTP-BO-4F:L8-BO was constructed by adding Ph I-Cl and L8-BO with PM6:BTP-BO-4F as the main binary system.Here,Ph I-Cl as the guest polymer has a lower HOMO energy level（-5.55 e V）and L8-BO as the guest acceptor has a higher LUMO energy level（-3.90 e V）.The V_OC was improved from 0.812 V to0.848 V,the J_SC from 24.44 m A/cm² to 25.26 m A/cm² and the FF from66.13%to 77.10%.To further enhance the short-circuit current,the PM6:BTP-BO-4F:L8-BO:BTP-e C9 quaternary system is designed with three acceptors having a similar conjugated main-chain structure,differing only in the side chains and end groups,which is designed to further improve the carrier mobilities of the device,obtained a higher PCE of17.45%and a higher J_SC of 26.61 m A/cm².