The Study On The Stability Of Active Layer Material Of Organic Solar Cell Under External Condition

As the third generation of photovoltaic technology,the unique merits regarding mechanical flexibility,rich color,semitransparency and building-integrated photovoltaics（BIPV）endows organic solar cells with great vision,regarded as a new generation of environment-friendly devices for photo-electric energy conversion.Benefitting from the innovative design of materials in recent years,striking progress has been made in power conversion efficiency with reported PCE exceeding 20%.The active layer material,which bearing photoelectric conversion,makes an important contribution to the progress.Correspondingly,the performance attenuation associated with the active layer has also become a critical cause that limits the industrialization of this brand-new photovoltaic technology.Indeed,one of the spotlight in OSC scientific challenges was cast on how to address the stability issues.Especially to significantly extend the operational lifetime of OSC under multiple factors,it is in urgent need of having a fundamental interpretation of various degradation mechanisms to propose effective strategies.In light of this discussion,the thesis carried out a series of studies,mainly focusing on the non-fullerene active layer materials and emphasizing the impact of environmental factors like irritation,water,oxygen and photooxidation on the molecular structure,photoelectric properties,film morphology and photovoltaic properties of devices.In the second chapter,the effect of water and oxygen exposure on efficiency and stability of OSC was first studied.It is found that PM6:Y6 mixture retains the initial photoelectric performance upon storage in indoor air,event exposure to the accelerated high-temperature water steam and oxygen environment.It is also demonstrated that the exposure of the active layer material to the water and oxygen does not affect the stability under light irritation.The above observations are verified in the PM6-Ir:Y6,PM6:N3and PTQ10:Y6 systems.Further results revealed the partial recovery of device performance（60%of initial value）after the substitution of the new cathode interface layer and top electrode,which account for the cause of degradation mainly stemming from hydrophilic interface layer and corresponding interface decay in the conventional device.In the third chapter,the sensitivity of the active layer materials based on non-fullerene system to photo-oxidation was studied.Upon illuminance in air,rapid performance degradation occurs in almost all non-fullerene systems.More than 50%loss of PCE in just 10 seconds was observed in J71:ITIC system.The destruction of ITIC acceptor from photooxidation was determined to be the main source of the rapid decline in device performance as elucidated by LBL configuration and the addition of photooxidized ITIC into the active layer.To this end,the photooxidation properties of thirty-three NFA were assayed,with the help of a systematic analysis of the correlation between the material structure and the photobleaching rate,deeper insights into the design strategy for improving the photooxidation stability of NFA by optimizing the material structure was acquired.Importantly,an effective and universal stabilizer,S6,was selected to significantly improve the photooxidation stability of NFA and its blends.In the fourth chapter,in the light of the photobleaching results in the previous chapter.A difference of 4 orders of magnitude photooxidation bleaching rate was observed in a-IDTBTR and l-IDTBTRh despite the tiny difference in conformation.There is no significant difference in intrinsic photobleaching rate in the solution state since the similar photo-oxygen sensitive groups in both two molecules.However,the discrepancy in their photooxidation stabilities begins to emerge along with the transformation from solution to film state and followed by thermal annealing.In contrast to the a-IDTBTR,the CB-processed l-IDTBTRh annealed film demonstrates superior resistance to light and oxygen.Meanwhile,from the AFM and GIWAXS evidence,crystal clusters,arising from the propensity of aggregation,appear in the l-IDTBTRh films,while the amorphous a-IDTBTRh has a predominant disorder structure.An increased density and compact molecule packing in l-IDTBTRh annealed film significantly reduces the permeability of oxygen and elevates resilience to light and oxygen.Moreover,the influence of various external environmental factors on photooxidative reactivity was discussed including light intensity,UV light,oxygen flow,temperature of film and active layer structure,all of which impose different stresses on the photooxidation of materials.In the fifth chapter,incorporating three structurally similar polymer acceptors PYTT-x（x=1-3）,only differing in the isomerized thiophene-fused ending-groups and location of the thiopheneπ-bridge attached to thiophene,construct all-polymer OSC with PBDB-T,the photostability of all-PSCs based on three isomeric polymer acceptors show a different decay pattern.The loss of FF dominates the decay of PBDB-T:PYTT-1 and PYTT-2 systems,while the open-circuit voltage(V_OC)and fill factor（FF）attenuated in the PBDB-T:PYTT-3 sample.It is noteworthy that three types of polymer systems demonstrate entirely different evolution behavior in recombination dynamics.The trap-assisted recombination was enhanced with time in PBDB-T:PYTT-1,but not affected in PBDB-T:PYTT-2.Especially,the transition of trap-assisted recombination to surface recombination as observed in PYTT-3 explain the V_OCloss.Further morphological characterization discloses a slight decline of molecular ordering in PBDB-T:PYTT-2 after light aging,verifying the better photostability.Although the molecular order in the other two systems remains constant,two major types of morphological degradation processes,namely,domain shrinkage and domain growth,are confirmed for PBDB-T:PYTT-1 and PBDB-T:PYTT-3,respectively.In the sixth chapter,the device performance and stability of non-fullerene organic solar cells in the outdoor were evaluated based on the self-built outdoor test system.It is found that,under similar irradiation intensities,the PCE obtained in outdoor is only86.5%of the recorded efficiency indoors with xenon lamp illuminance due to the reduction of V_OC and J_SC parameters.During the daytime,J_SC and P_max have a good agreement with solar irradiation intensity,while FF,R_s and R_sh show an opposite trend.Therefore,the best device performance is achieved at a light intensity of around 60m W/cm² at 10 a.m.and 4 p.m.It is also found that around 28%loss in PCE after two months of outdoor exposure,exhibiting significant loss of V_OC and FF but no degradation of J_SC.In addition,this study further indicates that with the gradual penetration of water and oxygen from the package edge into the device,the induced degradation of Y6 in the active layer takes the main responsibility for PCE reduction.By applying good encapsulation and nitrogen protection,the active layer material showed excellent tolerance to outdoor sunlight but confirmed the aging of buffer layer and corresponding interface.