Photovoltaic Property Regulation And Device Stability Of Benzodifuran Based Polymers

Organic solar cells（OSCs）offer several advantages.They are flexible and light,can be processed in solution,and produced in large-scale.With the development of recent years,the power conversion efficiencies（PCEs）of OSCs have been pushed to over 19%.However,there still many problems in OSCs for future commercialization.Compared with industrial silicon cells,OSCs have a lower PCE and are constrained by insufficient theoretical research,high cost,poor stability and high toxicity of the preparation process.Taking these problems as a starting point,this dissertation takes low-cost,renewable furan-based polymers as the polymer donor,and develops the research from the perspectives of efficiency reduction of energy loss(E_loss),environment-friendly cell development and stability respectively.A benzo[2,1,3]difuran and BDD based polymer,PBDF-BDD,as the polymer donor and a representative ring-fused NFA,IT-4F,as the electron acceptor in NF-OSCs,and applied two NSAs（1,8-diiodooctane（DIO）,1,8-dibromooctane（DBr O））and two ASAs（1-chloronaphthalene（CN）,diphenyl ether（DPE））as the representative SAs to systemically investigate the effect of different SAs on the photovoltaic performance,morphology,and their roles in V_oc and FF.The PBDF-BDD:IT-4F based deviecs exhibit a completely opposite trend of V_oc and FF were observed when processing with NSAs and ASAs.The addition of NSAs（DIO）significantly enhances the J-aggregation of IT-4F,conducive to charge separation and transfer,resulting in more balanced charge mobility and higher FF.However,the J aggregation will lead to the red-shift of absorption and resulted in the decrease of E_g^PV,moreover,the addition of NSA caueses the increase of energetic disorder and the decrease of E_CT,which lead to higher（?）E_rad and（?）E_non-rad.With the addition of NSA,the V_oc in PBDF-BDD:IT-4F device is gradually decreased over 0.1V.In contrast,the V_oc in PBDF-DBB:IT-4F device have negligible differences with the different ratios of ASAs,but the FF have a gradually increases attribute to the low charge carrier recombination.Herein,two benzo[1,2-b:4,5-b’]difuran（BDF）polymers（P-FT and P-FP）in combination with two electron acceptors（m-ITIC and Y6）in OSCs were employed to systemically investigate the impact of molecular orientation and interaction on energy loss especially the non-radiative combination loss(（?）E_non-rad)in OSCs.Polymers P-FT and P-FP have the same polymer backbone,but the different side chains will endow P-FT and P-FP with different molecular packing behaviors and intermolecular interactions with m-ITIC and Y6.The use of Y6 acceptor instead of ITIC provided a reduced energy loss in OSC,regardless of the donor used,and the energy loss was lower in OSC based on P-FP（0.575e V）,as compared to that based on P-FT.The Urbach energy analysis showed that P-FP:Y6 has the lowest energetic disorder,leading to the smallest（?）E_non-rad of 0.23e V.In addition,P-FT:Y6 blend film show face-on oriented packing,and the P-FT:Y6 based devices exhibited the outstanding EQE_max of 86.72%,this is the best case in BDF polymer based OSCs.We selected four BDF based polymers（P-Cl T、P-FT、P-P and P-FP）and two electron acceptors（m-ITIC and Y6）to examine the effect of water and oxygen on the stability of devices by controlling the oxygen content and humidity of the storage environment.The study on the ambient stabilities showed that these BDF polymers-based NF-OSCs possessed very promising ambient stabilities after around 800-1000 hours air aging even at the 70-80%humidity.The results also showed that the m-ITIC and Y6 devices possessed different efficiency loss trend provided with same polymer donor,which is related to the difference on the intermolecular interactions of BDF polymer and m-ITIC（or Y6）as well as their interactions with Mo O_x layer.The investigation for the underlying mechanism revealed that the active layer was insensitive to water and oxygen,and keep stable during long-term aging.The thin Mo O_x layer applied in these inverted NF-OSCs played a significant role in their ambient stabilities.The penetration of Mo O_x into active layer and their interactions during the air aging may strengthen the interface and form the favorable contact,and hence contributed to the increased performance at beginning where the efficiency loss affected by humidity was actually traded-off.This finding states that the interficial layer is highly of significance to the device stabilities and also partly to the device performance.Based on the excellent ambient stability,the ambient processabilities of these BDF polymers based NF-OSCs were also investigated.It found that the PCEs of the devices under ambient condition only had 0.3-2%loss compared to the devices from inert condition.In order to use environmentally friendly solvents for OSCs preparation,the ternary polymerization strategy was introduced into the efficient P-FP polymer to regulate their solubilities in halogen-free solvents.With the introduction of Th unit,the terpolymers P-FP-Th2 and P-FP-Th5 exhibit much better solubilities and a reduced intermolecular aggregation in the non-halogenated solvent o-XY.The power conversion efficiencies（PCEs）of these devices based on P-FP-Th2:IT-4F（o-XY）and P-FP-Th5:IT-4F（o-XY）reached up to 11.70%,which are comparable with that devices processed from the chlorinated and toxic chlorobenzene（CB）solvent.However,the alternating polymer P-FP based device only had a PCE of 3.32%while processing from o-XY solvent due to its limited solubility.Furthermore,the layer-by-layer devices via the sequential processing steps were also fabricated by dissolving P-FP-Th2（or P-FP-Th5）into o-XY and IT-4F into 2-methyltetrahydrofuran（2-THF）.The PCE of 11.19%was obtained from P-FP-Th5/IT-4F device,which were similar with the PCE of BHJ structure devices.