Fabrication And Mechanism Studies On High-Efficiency Ternary Organic Solar Cells

Organic solar cells(OSCs)are a promising candidate for green renewable energy resource due to their unique advantages,such as light weight,flexibility and semitransparent quality and solution processing,etc.In recent years,non-fullerene acceptor-based OSCs have made great progresses with rapidly increasing power conversion efficiencies(PCE).In addition to the development of novel donor and acceptor materials,the ternary blend strategy has been successfully used to improve the PCE of OSCs due to its advantages of broadening absorption spectrum,adjusting the morphology of active layer,etc.However,compared with the binary OSCs,there are more charge and energy transfer pathways in the ternary blend systems due to the changes of the blend morphology of the active layer and the interaction between the acceptor and acceptor molecules,which makes the whole photoelectric conversion process more complicated.All the above will increase the difficulty of the mechanical research and theoretical guidance of the ternary OSCs.In this thesis,we have mainly studied the influence of a third component(a second electron acceptor)on the optical properties,electrical properties,microstructure characteristics and photoelectric conversion process of the blend films in different ternary systems,and explored the potential approaches to enhance the PCE of OSCs with the help of ternary blend strategy.The main conclusions are as follows:1.The ternary OSCs based on the PBDB-T:ITIC:PC71BM blend have been fabricated and studied in detail.By optimizing the acceptor ratios between ITIC and PC71BM,the best photovoltaic performance is achieved in PBDB-T:ITIC:PC71BM(1:0.8:0.2)-based ternary solar cells.The PCE of the ternary organic solar cell is about 10.2%,which is much higher than 9.2%of the binary PBDB-T:ITIC(1:1)control device.Compared with the PBDB-T:ITIC(1:1)binary control device,the optimized PBDB-T:ITIC:PC71BM(1:0.8:0.2)ternary device demonstrates improved exciton separation efficiency and charge extraction efficiency,suppressed bimolecular recombination,and improved hole and electron carrier mobility.At the same time,the crystallinity of primary ITIC acceptor increases and the density of electron traps decreases.We have proposed that a small amount of PC71BM acceptors can promote the ITIC out of the blend domain due to its better compatibility with polymer PBDB-T donor,thus promoting to form pure ITIC phases.2.The ternary OSCs based on the PBDB-TF:IT-4F:O6T-4F blends have been fabricated and studied in detail.By optimizing the acceptor ratios between IT-4F and O6T-4F,the highest PCE was achieved in the PBDB-TF:IT-4F:O6T-4F(1:0.85:0.15)ternary solar cells.The PCE of the PBDB-TF:IT-4F:O6T-4F(1:0.85:0.15)ternary OSC is around 13.37%,which is much higher than 12.29%of the PBDB-TF:IT-4F(1:1)binary control device.A second acceptor O6T-4F can broaden the absorption spectrum due to its narrow bandgap,and thus enhance the Jsc of the resultant devices.It is found that a small amount of O6T-4F can promote the fluorescence quenching efficiency of the primary IT-4F acceptor.Moreover,the intermolecular interactions between O6T-4F and IT-4F help to optimize the interpenetrated networks between PBDB-TF and IT-4F,and thus improve the hole/electron carrier mobilities and inhibit bimolecular recombination.It is also found that there is a very rapid energy transfer from IT-4F to O6T-4F acceptors in the ternary blend film.Thus,the excitons formed on large IT-4F domains is transferred to O6T-4F,and then subjected to hole transfer at the PBDB-TF/O6T-4F interfaces,leading to improved exciton utilization of IT-4F.3.The ternary OSCs based on the PBDB-T:IT-4F:O6T-4F blends have been fabricated and studied in detail.An optimized ternary blend is PBDB-T:IT-4F:O6T-4F(1:0.8:0.2).The PCE of the optimized ternary OSCs is about 10.53%,which is higher than 9.48%of the PBDB-T:IT-4F(1:1)binary control device.In the optimized ternary device,the short-circuit current density(Jsc)of the device is increased since the narrow bandgap O6T-4F can broaden the active layer absorption into the near infrared region.It is shown that a small amount of O6T-4F can reduce the energy loss(Eloss)observed in binary PBDB-T:IT-4F system,and a higher VOC is achieved in the optimized PBDB-T:IT-4F:O6T-4F ternary blend OSCs.Morphology analysis shows that PBDB-T aggregation is more prominent in binary PBDB-T:IT-4F blend,and O6T-4F could inhibit the aggregation of PBDB-T in the ternary blend films.Transient absorption photophysical analysis shows that the increased O6T-4F content will accelerate the charge transfer process.At the same time,the more serious bimolecular recombination is existed in the ternary system,resulting in a poor FF.