In-Situ Study On Controllable Morphology Evolution And Thermal Stability Of Organic Solar Cells By Grazing Incidence X-ray Scattering

Organic solar cells（OSCs）with the advantages of simple process,low cost,light weight,wide material source,flexibility has attracted worldwide attention.The active layer morphology of OSCs is crucial for the power conversion efficiencies（PCEs）and thermal stability of OSC devices.Although PCE over 18%has been achieved,there is still a gap between the PCEs of OSCs and the PCEs of conventional silicon-based solar cells.It is still necessary to optimize the morphology of active layer and enhance the PCE of OSCs by methods like adding third component,thermal annealing and adding solvent additive.In addition,the poor thermal stability of OSCs is one of the barriers to their commercialization,thus the active layer morphology stability must be enhanced to improve the thermal stability of OSCs.Hence,studying the mechanism of adjusting methods and thermal instability,and developing methods enhancing thermal stability for active layer morphology is of great significance.Comparing to typical morphology characterization techniques like atomic force microscope（AFM）like transmission electron microscopy（TEM）,synchrotron radiation grazing incidence X-ray wide angle scattering（GIWAXS）and grazing incidence small angle scattering（GISAXS）have the advantages of non-contact,non-destructive,which provide large-scale statistical information.More importantly,GIWAXS and GISAXS have the ability to in-situ investigate dynamic morphology evolution,which is of great significance for the researches of OSCs.In this paper,as the research object,the active layer morphology of OSC is manipulated by the approaches of adding third component,adding solvent additive,and using various heating rate before thermal aging.GIWAXS and GISAXS are used as the main methods of microstructural characterization.On this basis,the mechanism of the manipulating of active layer morphology and the structure-property relationship between active layer morphology and device performance are systematically studied.This work shows an example for studying active layer morphology of OSCs by GIWAXS and GISAXS techniques and achieves the enhancement of PCE and thermal stability of OSC devices,where three techniques of synchrotron radiation X-ray scattering are developed.Following research results are obtained.1.Two techniques of synchrotron radiation X-ray scattering are developed in the small angle X-ray scattering（SAXS）beamline（BL16B1）at Shanghai Synchrotron Radiation Facility（SSRF）.By developing a variable temperature and atmosphere experimental device for GIWAXS measurements,the technique of in-situ GIWAXS under 20℃-200℃and protective atmosphere or sample atmosphere is developed.By using the missing angle detector PILATUS 900K as the GIWAXS detector（or using the PILATUS 2M as the GISAXS detector）and using UV-Vis optical fiber spectrometer,the second-level time-resolved combination of grazing incidence X-ray scattering（GGIWAXS or GISAXS）and UV-vis in X-ray beamline is achieved,where the detecting ranges for GIWAXS,GISAXS and UV-Vis are 12-25 nm^-1,0.15-2.8 nm^-1,and 220 nm-1100 nm,respectively.A sample cell providing temperature and atmosphere conditions is developed,by which thermal annealing and solvent vapor annealing can be performed.Morphology evolution during these processes can be in-situ monitored by simultaneously using the sample cell and the combination technique.The development of these two techniques provides the experimental condition for the following studies on organic solar cells.2.As the third component,various weight ratios of PC₇₁BM is added to PTBTz-2/ITIC blend films to manipulate the active layer morphology and enhance the device PCE.When 35 w%PC₇₁BM is added,the device PCE achieves the highest value of 11.26%.Using the grazing incidence X-ray scattering experimental method developed in the first work,GIWAXS and GISAXS measurements on the morphology of blend films with different weight ratios of PC₇₁BM are performed to revealed the mechanism of PCE improvement.The added PC₇₁BM can enhance the crystallization of PTBTz-2in out-of-plane direction,reduce thep-pstacking distance and make donors aggregation more planar.Appropriate crystal size and phase separation and favorable bicontinuous network are formed in active layer,leading to the balance and enhancement of carrier mobility,which induces the improvement of device PCE.3.PTB7-Th/PC₇₁BM active layer morphology is manipulated by adding various volume ratios of DIO.By in-situ GIWAXS and GISAXS techniques,the mechanism of DIO on enhancing device PCE and harming device thermal stability is revealed.When3 v%DIO is added,the device PCE achieves the highest value of 9.17%.While the device thermal stability is decreased with the ratio of DIO increases.Using the grazing incidence X-ray scattering experimental method developed in the first work,in-situ GIWAXS and GISAXS were performed to reveal the mechanism of PCE improvement and stability degradation.It is demonstrated that PC₇₁BM molecules are partly dissolved by DIO,which can diffuse into donor domain and promote the crystallization of PTB7-Th,leading to enhanced crystallinity and crystal size.After adding 3 v%DIO,appropriate crystal size,aggregation size and phase separation scale are achieved,which provides abundant donor/acceptor interfaces and charge transport channels to improve device PCE.Simultaneously,the PC₇₁BM molecules dissolved by DIO facilitate the morphology evolution during thermal annealing.The reduced thermal stability of active layer morphology induces the degradation of device thermal stability.4.The strategy that using slow heating before thermal aging to enhance the thermal stability of OSCs is innovatively proposed,for which the mechanism is revealed.The non-fullerene system PM6/Y6 devices and fullerene system PTB7-Th/PC₇₁BM devices are heated to aging temperature with various heating rates.In-situ PCE investigations show that using slow heating rate can obviously improve the thermal stability of both two systems in the subsequent aging process.For PM6/Y6 system,the residual PCE of the device with the heating rate of 1℃/min after thermal aging at 80℃for 24 h is increased by 190.9%compared to the device with the heating rate of 20℃/min.Using the grazing incidence X-ray scattering experimental method developed in the first work,in-situ GIWAXS and GISAXS are performed,which shows that the crystal growth in device with slow heating rate is suppressed and active layer can maintain a relatively optimized morphology.It is attributed to the enough relax time for molecular chain with slow heating rate,leading to more local nucleation and enhance the competition between crystal nuclei to suppressed the crystal growth in active layer.