Study On The Morphology Regulation And Photovoltaic Performance Of High-efficiency Ternary Organic Solar Cells

Organic solar cells（OSCs）are considered to be the most promising candidates for the realization of next-generation solar cell technologies due to their unique advantages,such as low cost,light weight,flexibility,and ease of processing.At present,the structure of OSCs is mainly based on the bulk-heterojunction（BHJ）structure of donor（D）and acceptor（A）,and the power conversion efficiency（PCE）of single junction devices can reach 18%through setting up new material structures,using additives,improving interfaces and optimizing morphology.However,due to the narrow absorption spectrum of organic materials,the absorption spectrum of the binary blend layer composed of one donor and one acceptor is difficult to match the solar spectrum,which limits the performance of OSCs devices.Therefore,ternary organic solar cells（TOSCs）not only retain the simple and easy preparation method of traditional two-component devices,but also have the advantages of expanding the light absorption range,adjusting the energy level matching of the D/A interface,optimizing the morphology,etc.It is an important way to obtain high PCE.However,there are still many problems in the research of TOSCs:（1）the interaction of the components of the ternary devices has a great impact on the performance of the device,reasonable screening of the third component to regulate the morphology of the active layer still requires in-depth research;（2）The compatibility of the active layer materials of ternary device with solvents is particularly important.The selection of solvents that can be commercialized and can improve device performance is the key to the development of organic solar cells;（3）Understanding The effect of the third component on the charge dynamics of the main system is also important for the ternary strategy to improve device performance.Based on the above issues,the following work has been carried out:（1）The fullerene acceptor material was introduced as the third component in the binary system of TBFCl50-BDD:BTP-4F.Since the lowest unoccupied molecular energy level（LUMO）of PC₇₁BM is higher than that of the non-fullerene acceptor BTP-4F,the open circuit voltage(V_OC)of the device is increased.In addition,PC₇₁BM can broaden the absorption spectrum based on TBFCl50-BDD:BTP-4F in the short-wave range,which facilitates the improvement of the short-circuit current-density(J_SC).Compared with the binary device,the best PCE of 13.15%was achieved for the ternary device based on TBFCl50-BDD:BTP-4F:PC₇₁BM,while V_OC,J_SC and Fill Factor（FF）were also improved to 0.860 V,24.64 m A cm^-2 and 62.04%,respectively.The physical and optical characterization of the device shows that bimolecular recombination and trap-assisted recombination are effectively suppressed.Moreover,surface tension calculations demonstrate that the miscibility of the binary blend can be improved by adding a third component,which facilitates charge transfer and collection.In addition,the grazing incidence wide-angle x-ray scattering（GIWAXS）and atomic force microscopy（AFM）studies show that the ternary active layers can form better face-on packing and the surface roughness is properly adjusted.Meanwhile,the grazing incidence small-angle x-ray scattering（GISAXS）shows that the introduction of PC₇₁BM can regulate the size of the phase domain to the acceptor and promote the charge transport of the active layer.（2）The green solvent o-xylene（o-XY）was used as solvent for both the donor and the acceptor,and the non-fullerene acceptor Y6-DT-4F as the third component was introduced into the PBB-F:IT-4F binary system to broaden the spectral absorption and optimize the morphology for efficient PCE.Among them,the third component Y6-DT-4F and IT-4F are compatible with each other and can form an"alloy acceptor",which can synergistically optimize the photon capture,carrier transport and collection capabilities of the ternary device.Meanwhile,Y6-DT-4F is highly crystalline,and as the third component introduced into the binary system can enhance crystallinity and facilitate charge transport.Therefore,the best performance based on the PBB-F:IT-4F:Y6-DT-4F device is PCE of 15.24%,J_SC of 23.68 m A cm^-2,V_OC of 0.896 V,and FF of 71.82%.The improvement in J_SCwas mainly related to the complementary uptake of PBB-F,IT-4F and Y6-DT-4F,while the improvement in FF was related to efficient and balanced charge transport and inhibition of trap-assisted recombination.More importantly,the GIWAXS results indicate that the crystallinity of the PBB-F:IT-4F binary blend layer can be tuned by adding the more crystalline Y6-DT-4F to achieve better phase separation.GISAXS shows that the alloys formed in the ternary blend films enlarge the size of acceptor domains for efficient charge transport.In addition,the electron transfer rate from the donor to the alloy acceptor is the fastest in the PBB-F:IT-4F:Y6-DT-4F blend film,and the fastest electron transfer rate in the blend film is beneficial to the improvement of the device performance.（3）The ternary device based on PM6:TBFCl50-BDD:IT-4F was prepared by selecting the polymer PM6,the terpolymer TBFCl50-BDD as the donors of the device and the non-fullerene small molecule IT-4F as the acceptor and using o-XY as its main solvent.Due to the similar molecular structures of PM6 and TBFCl50-BDD,the two polymers exhibited good miscibility.By introducing TBFCl50-BDD,the morphology of the ternary device was significantly improved,which facilitated the carrier transport.Also,the addition of 20 wt%TBFCl50-BDD enabled the devices to increase the hole and electron mobility and their ratio tended to 1.In addition,the introduction of the third component suppressed the trap-assisted recombination and bimolecular recombination,thus improving FF and J_SC.The optimized ternary device based on PM6:TBFCl50-BDD:IT-4F achieved an efficiency of 12.78%,which is 8.12%and 24.8%higher than the binary devices based on PM6:IT-4F and TBFCl50-BDD:IT-4F.