Controlling Active Layer Morphology Of Polymer Solar Cells Through Solvent Additive And Materials Side Chain Strategies

As a promising photovoltaic technology,polymer solar cells（PSCs）have attracted much attention of scientific community and industry due to their advantages including light weight,flexibility,large area-solution fabrication.With the emergence of a series of non-fullerene acceptor（NFA）materials,single junction binary PSCs have achieved high power conversion efficiency of more than 18%.The light absorbing active layer of PSCs mainly adopts bulk heterojunction structure,and controlling the active layer morphology has always been an important issue to explore for researchers.It is proved that the active layer with suitable phase size and continuous interpenetrating network nanostructure is the key factor for PSCs to achieve high power conversion efficiency（PCE）.Based on this,the paper focuses on controlling morphology of active layer through solvent additives and materials side chain strategies.In the first chapter,the development of PSCs and the methods of morphology control were described while the fabrication and performance characterization of PSCs were introduced in the second chapter.In the third chapter,three dihalogenated toluene solvent additives 2,4-difluorotoluene（DFT）,2,4-dichlorotoluene（DCT）and 2,4-dibromotoluene（DBT）were employed in the processing of all-polymer active layer based PBDB-T:N2200,in which chlorobenzene（CB）was used as the host solvent.It was found that the morphology of active layer processed by fluorinated DFT was close to that by pure solvent CB and the corresponding PCE of 8.04%was slightly higher than that of 7.9%of pure CB.However,with slightly increased phase separation,the surface roughness of the chlorinated DCT based active layer could be increased,and PCE was elevated to 8.54%.The brominated DBT could further enlarge phase separation of the active layer along with formation of some fibrous morphology,contributing to a PCE of 8.74%.In general,with the increase of halogen atom mass,the boiling point of the additive increases gradually,which makes DCT and DBT with higher boiling points significantly improve the overmixing problem of PBDB-T:N2200,thus optimizing the active layer morphology and device performance.In the fourth chapter,the photovoltaic performances of the all-polymer active layer consisting of N2200 and three polymer donors with the same backbone structure but different side chains were compared,respectively.These polymer donors are based on fluorobenzotriazole and benzodithiophene（BDT）modified with different side chains as main chain skeletons.It was found that relative to polymer donor J52 with BDT modified by the alkyl side chains,the other two polymer donor PBZ2Si-C6 and PBZ2Si-C7 comprising BDT modified by siloxane-terminated side chains displayed lower surface energies.The calculation results showed that Flory-Huggins interaction parameterχ_D,A between PBZ2Si-C7 with longer siloxane-terminated side chains and N2200 was the minimum,indicating that they had the best miscibility.Morphology tests showed that the domain uniformity of the three active layers was optimized according to the order of J52,PBZ2Si-C6,and PBZ2Si-C7,with increasing of hole mobility and improvement of charge transporting balance.In the all-polymer solar cells,the J52,PBZ2Si-C6,and PBZ2Si-C7 based active layers exhibited PCEs of 6.99%,8.05%,and8.54%,respectively.The results suggest that siloxane-terminated side chain would play a positive role in controlling active layer morphology and optimizing photovoltaic efficiency.In the fifth chapter,four active layers based on two polymer donors mentioned in the fourth chapter:J52 and PBZ-2Si bearing alkyl side chains and siloxane-terminated side chains,respectively,and two NFAs:i-IE-4F and i-IESi-4F comprising alkyl side chains and siloxane-terminated side chains,respectively,were investigated to explore that the effect of materials side chain strategies on morphology and photovoltaic performance.Relative to i-IE-4F,i-IESi-4F possesses lower surface energy.The Flory-Huggins interaction parametersχ_D,A of four active layers were calculated and the result showed that miscibility of blend film increases in the order of PBZ-2Si:i-IE-4F,J52:i-IE-4F,J52:i-IESi-4F,and PBZ-2Si:i-IESi-4F.The morphology tests confirmed that rough surfaces and large scales of phase separation existed in the former two active layers with i-IE-4F,which affected charge separation and transportation,leading to PCEs of less than 7.34%.However,the later two blend films of J52:i-IESi-4F and PBZ-2Si:i-IESi-4F with i-IESi-4F could suppress the formation of large phase domain,thus the corresponding device efficiencies of 12.67%and 14.54%were achieved,respectively.As an active layer composed of a polymer donor and a NFA both with siloxane-terminated side chains,PBZ-2Si:i-IESi-4F blend film achieved the optimal phase separation morphology,the lowest bimolecular recombination and excellent charge transport capability,which afforded a short circuit current of 22.55 m A/cm² and a fill factor of 74.03%.The results show that the side chain matching of donor and acceptor materials is a feasible method to optimize the active layer morphology and photovoltaic performance.