Effect Of Active Layer Microstructure On The Open-circuit Voltage Of Non-fullerene Organic Solar Cells

Organic solar cells（OSCs）have become a modern photovoltaic technology with great development potential due to unique advantages such as light weight,low cost,solution processing.The photoelectric conversion efficiency（PCE）of single junction OSCs has rapidly increased from less than 1%to more than 19%by constantly optimizing new organic materials and device processes.New breakthroughs have been made in the short-circuit current density(J_SC)and the filling factor（FF）due to the broadening absorption spectrum of materials and the high internal quantum efficiency（IQE）closing to 100%in the device.However,the open-circuit voltage(V_OC)of non-fullerene OSCs is still relatively low under various loss mechanisms,which has become the key to limit the further improvement of PCE.The morphology of active layer can greatly affect the performance of OSCs.At present,the research on the relationship between the morphology of active layer and the performance of devices mainly focuses on the influence of morphology on carrier separation and transport,further affecting the J_SC of devices.Theoretically,the microstructure of the active layer can also affect the energy level and the V_OC of devices considering the unique energy level characteristics of organic materials.However,there is a lack of relevant research on this relationship.Therefore,we have mainly focused on the influence of multi-scale microstructure characteristics such as aggregation structure,crystallization and phase separation of donor and acceptor molecules in the active layer film on the V_OC of the device besides the influence of active layer morphology on the J_SC of the device.In Chapter 2,in order to study the effect of the crystal morphology of the active layer on the V_OC of the device,we used the combined solvent strategy to control the film-forming dynamic process of PTB7-Th:ITIC active layer.The crystallization of ITIC is obviously increased and theπ-πstacking between PTB7-Th and ITIC molecules is also strengthened,resulting in the increased CT state energy level and contributing to the obvious increase of V_OCof the corresponding devices（0.71 V-0.86 V）.The relationship between‘the active layer crystallization-CT energy level-V_OC of the device’is preliminarily established in this work.In Chapter 3,in order to further study the response difference of the active layer morphology to additive and the influence mechanism on V_OC in different non-fullerene material systems,we have introduced 3v%DIO to regulate the crystallization and phase separation morphology of PTB7-Th:ITIC and PM6:Y6 blend films.It is revealed that the different changes of morphology are originated from the opposite changes of intermolecular interaction and interfacial tension between donor and acceptor by 3v%DIO in the two systems.Larger intermolecular interaction between donor and acceptor and higher crystallinity of acceptor can significantly improve the CT transport energy levels in the device,leading to the increased V_OCof the device.The influence mechanism of 3%DIO additives on the crystallization,phase separation morphology of active layer films and V_OC of non-fullerene OSCs is studied from the basic interaction between donor and acceptor molecules in this work.In Chapter 4,we further study the effect of molecular stacking structure of active layer films on the V_OCof devices based on DR3TSBDT:Y6 and PM6:Y6 systems.On the one hand,it is found that Y6 exhibits different miscibility between small molecule DR3TSBDT and polymer PM6.The interaction between Y6 and DR3TSBDT is relatively strong.The good miscibility of Y6 and DR3TSBDT can strongly affect the molecular conformations of each other.However,the molecular conformations of Y6 and PM6 are relatively weakly affected by each other.On the other hand,the crystal orientation and stacking structure of donor and acceptor molecules in the DR3TSBDT:Y6 and PM6:Y6 blending films are both effectively regulated by the combined annealing strategy.In Chapter 5,it is found that the trap-assisted recombination can be effectively suppressed,the photoinduced electron transfer,the exciton splitting and extraction efficiency can be increased in the DR3TSBDT:Y6 and PM6:Y6 OSCs by the combined annealing strategy,resulting in the significant improvement of the J_SC and FF.Therefore,the PCE of the DR3TSBDT:Y6 device is obviously increased from 1.14%to 12.33%,and the PCE of the PM6:Y6 device is increased from 15.37%to 16.60%.Combined with kasha model,it is found that the more compact J-aggregation structure of donor and accepter during annealing significantly reduces the energy level of the first excited singlet,resulting in a lower V_OC.This work has established the relationship between the change of J-aggregation structure of molecules and the V_OC of the device during annealing.