Morphology Adjustment And Stability Investigation Of High-performance Organic Solar Cells

As new-generation clean energy,organic photovoltaics have aroused extensive attention in recent years.Organic semiconductors have tunable energy levels,can be processed by large-scale solution printing,and construct flexible semi-transparent devices,which significantly reduce production energy consumption.Thus,organic photovoltaic has unique advantages for commercial applications,such as photovoltaic-building integration and wearable devices.Organic photovoltaics have developed rapidly in the past two decades with continuous breakthrough in power conversion efficiency,synergistically benefiting from material innovation,morphology adjustment and mechanism research.However,there are still only a few material systems that could exceed the efficiencies of 15%,and the development of morphology adjusting methods and mechanisms based on high-efficiency organic solar cells still lags behind,especially the stability research under light operation.Therefore,this dissertation explored the photovoltaic performances of imide benzotriazole（Tz BI）series polymer donors and Y6 series high-efficiency non-fullerene small molecule acceptors,in order to construct efficient organic solar cells with nice stability.The dissertation developed novel solid additive and bilayer device structure for morphology adjustment,and systematically investigated the influence of morphology evolutions and opto-electrical properties on device stability.The proposal of this dissertation hope to establish the relationship between material chemical structure,morphology,photovoltaic performances and stability.In chapter 2,Tz BI series polymer donors P2F-EHp,PFT-EHp and PTz BI-d F,in combination of Y6 series acceptors Y6-BO and Y6-DT,were utilized to explore the joint effects of fluorine substitution in donors,alkyl chain length in acceptors,and the function of liquid additives on blending morphology.Devices based on P2F-EHp:Y6 achieved outstanding efficiencies of 16%for the first time.PTz BI-d F with fluorine substitution on backbone thiopheneπbridge,had rigid framework and nice molecular packing,reached 16.8%in combination with Y6,and PTz BI-d F:PC₇₁BM:Y6 ternary device showed a efficiency exceeding17%.In addition,the organic solar cells constructed by Tz BI series donors and Y6 series acceptors showed excellent long-term light stability,proving its application potential to a certain extent.In chapter 3,aiming at overcoming the ratio sensitivity and lack of targeting of the above-mentioned liquid additives,as well as improving the morphology stability,the dissertation developed a novel small-molecule solid additive naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole（NT）for morphology adjustment based on PTz BI-d F:Y6-BO representative material system.Compared with traditional liquid additives,solid additives are not limited by film-forming kinetics,and can targeted regulate molecular stacking.NT can function in different material systems and solvents with high ratio tolerance,improving the plasticity in drying process as a compatibilizer,and promote the crystallization of host materials.Hence,NT can replace or simplify some complicated pre-or post-treatments,such as time-sensitive solvent-vapor annealing as well as batch-and content-sensitive liquid additives.In addition,NT can reduce the impact of material batch-to-batch differences.The organic solar cells based on presentative PTz BI-d F:Y6-BO system achieved a power conversion efficiency of 17.4%,and the devices maintained 85%of initial efficiency after 1200 h under light operation.In chapter 4,in order to minimize the influence of blending morphology on device stability,the dissertation investigated the possibility of overcoming incompatibility of donors and acceptors as well as improving morphology stability by constructing bilayer organic solar cells.M4-4F with nice solubility and high crystallinity,was utilized to construct bilayer device with wide-bandgap donor P2F-EHp via quasi-orthogonal solvents.Bilayer devices enabled P2F-EHp and M4-4F to retain optimized self-aggregation and crystallization properties,allowing more efficient charge transport channels and better vertical distribution,heading of island-like over aggregation in bulk-heterojunction caused by incompatibility of P2F-EHp and M4-4F.The efficiency increased from 12.7%of the bulk-heterojunction devices to 14.2%of bilayer devices,and the reduction of D/A interface in bilayer devices mitigated the unfavorable contact changes from donors and acceptors during aging process,which improved both light and heat stability.Although active layer morphology adjustment successfully improved device stability under heat and light,there are still about 15%of performances degradation in the organic photovoltaic devices,proving the existence of degradation factors beyond morphology evolution.In chapter 5,the dissertation systematically analyzed the light-induced device degradation based on the PTz BI-d F:Y6-BO system,and found that thermal annealing process at 110℃/10 min can partially restore the performances of the light-aging device.The reversible short-circuit current density and partially reversible fill factor were accompanied by the reversible field-dependent charge extraction ability as well as the reversible changes in the trap density of state distribution,and correlated with local charge carrier density and minute-scale charge accumulations,the degradation trend of which was similar with Shockley-Read-Hall recombination model.The irreversible open circuit voltage loss correlated with the broadening of the density of states,the enhancement of mono-molecular recombination and reduced charge carrier lifetime.The electron/hole mobility measured by SCLC method,bimolecular recombination and active layer morphology had no significant change before and after light aging.Therefore,the dissertation proposed a novel mechanism for light-induced device degradation,that long persistent radicals lead to the light-induced burn-in loss in organic photovoltaic devices.The speculation was further verified it by DFT calculations and electron paramagnetic resonance.The proposal of this theory makes up for the gap in light-induced degradation mechanism of organic photovoltaics,and promotes organic solar cells to move from laboratory research to practical application.