The Application Of Synchrotron Based Spectroscopies In The Energy Science Related Studies-Surface And Interface Structures In Polymer Solar Cells And Lithium-sulfur Cells

The increase of global energy consumption and it environmental impact have made sustainable and clean energy technologies highly desirable,especially in the transporation sector.For now,an efficiency energy transfer route is that the solar energy is transferred to electric energy by using polymer solar cell(PSC),subsequently the electric energy is stored and/or transferred to energy of motion by lithium-sulfur(Li/S)cells.The PSCs have attracted broad attentions in the industry and academia.However,the widely application of PSC is hampered by its low efficiency and bad stability.Therefore,it is crucial to improve these performances.In the PSCs,photon-induced excitations formed in the conjugated polymer layers when cells exposed to solar lights;following the excitations are sparated at the polymer/electron acceptors interfaces;and finally the electrons(holes)are collected at the cathode(anode)side.Therefore,the properties of polymer films,polymer:electron acceptors blend films,and the interfaces between cathode metals and polymer:electron acceptors blend films directly influence the device performances.Therefore,understanding the surface and interface chemistry of conjugated polymers with metal electrodes holds great promise for impacting the technology of polymer-based(opto-)electronic devices.Additional,Li/S cells have been attractively since they can deliver higher power and energy than any other lithium-ion cells owing to the high theoretical specific capacity of elemental S(1675 mA·h/g).However,there are technical challenges that preclude the widespread use of the Li/S cell,such as the formation of lithium polysulfides with high solubility in most organic solvent electrolytes.The diffusion of such species during the charge/discharge process leads to the loss of active material,a short cycle life of the sulfur electrode,low utilization of sulfur,and low Coulombic efficiency.To address these issues,sulfur is incorporated with conductive carbon materials.Although the cell performance has been significantly improved,an in-depth understanding of the function of the cathode material is still limited.Moreover,the changes on the cycled cathode materials are believed to be an esstianl factor for the capacity fade.As the surface of the cathode materials,which is the interface between cathode and electrolyte in the cells,can directly influence the insertion/extraction of Li ions to/from the cathode.Therefore,it is important to obtain a comprehensive picture on the whole process of a Li/S cell life cycle,including the cathode synthesis process and cycled cathodes.In this dissertation,we aim to gain new insights into the properties of surfaces and interfaces within the PSCs and Li/S cells.And,we want all the information obtaining from this dissertation can offer new strategy to explore and develop better devices performances of PSCs and Li/S cells.The mainly characterization tools we used in this dissertation are synchrotron based techniques,including synchrotron radiation photoemission spectroscopy(SRPES),near-edge X-ray absorption fine structure(NEXAFS)and X-ray emission spectroscopy(XES).Also,traditional lab based techniques including X-ray diffraction pattern(XRD),atomic force microscope(AFM),sum frequency generation(SFG)spectra,scanning electron microscope(SEM),Fourier transform infrared spectroscopy(FTIR)and Mg Ka X-ray photoelectron spectroscopy(XPS)are utilized as well.Specifically,the main achievements in this dissertation are listed following:(1)Poly(9,9-dioctylfluorene-co-bithiophene)(F8T2)have been widely applied in the polymer solar cell.We have presented a comprehensive study of the annealing effect on the film structures of poly(9,9-dioctylfluorene-co-bithiophene)(F8T2)by using XRD,AFM,SFG,NEXAFS,XES and SRPES.Angle-dependent NEXAFS measurements suggest that the fluorene rings and thiophene rings of the as-spun F8T2 film are tilted by 44.7° and 62.2°,respectively,with respect to the surface plane.These tilt angles get slightly changed after annealing.The appearance of a peak at 20=6.0° in XRD spectrum after the F8T2 film annealed at 130 ? indicates that the annealing can induce the crystallinity enhancement,which is in good agreement with the AFM and SFG results.The real space repeating distance of the annealed F8T2 film corresponds to that of extending octyl chains separation.Moreover,an electric dipole layer forms due to the annealing induced alignment of the side octyl chains towards the vacuum at the outmost surface.The surface diople induces both the highest occupied state(HOS)position and the core level peak shifting to higher binding energy for 0.38 eV.From the valence band spectra,the ionization potential(IP)and energy gap of as-spun F8T2 are estimated to be 5.51 eV and 2.34 eV,respectively.And annealing the F8T2 films at 130? almost has no influence on these values.(2)The F8T2 and Phenyl-C61-butyriic acid methyl ester(PCBM)are mixed with different weight ratios.The blend films before and after heat treatment are investigated by using SRPES,XAS and XES.The results lead to the clear interpretation of the annealing effect on the F8T2:PCBM blends.The F8T2 rich top layer has been verified by the C K-edge XAS,and the annealing induced migration of F8T2 to the surface is monitored by the C K-edge XAS and S 2p SRPES.By mixing with PCBM,the F8T2 S 2p peak shifts-0.5 eV to the lower binding energy.The peak red shifting is interpreted with the interface dipole between F8T2 and PCBM.According to the integer-charge transfer(ICT)model,the Ep+ value of F8T2 is estimated to be 3.89 eV.Also,the band gap value and band structures of the blend films are recorded by combining the C K-edge XAS/XES.The annealing effect is elucidated by comparing these information.(3)Interfacial structures of lithium and F8T2 are in-situ investigated by SRPES and XPS.The initial stage of interface formation has been clearly interpreted.Upon Li doping,significant blue shifts of C 1s and S 2p peaks have been monitored,which is due to the electron doping induced band bending effect.Meantime,additional peaks are observed in both C 1s and S 2p spectra,which are assigned to Li-C complex and Li2S deriving from the chemical reaction between Li and F8T2.Combining with the C 1s and S 2p XPS,the diffusion/reaction depths of Li in F8T2 are elucidated.With increasing Li does,new density states are detected between the HOS and Fermi level in the valence band spectra,indicating the formation of the gap states.Also,the 1.11 eV secondary cutoff shift indicates the downwards shifting of the vacuum level.Considering the 0.65 eV band bending value,the surface dipole is determined to be 0.44 eV.Finally,an energy level alignment diagram is obtained.(4)We have investigated the chemical bonding interaction of S in a CTAB(cetyltrimethylammonium bromide,CH3(CH2)15N +(CH3)3Br-)-modified sulfur-graphene oxide(S-GO)nanocomposite used as the cathode material for Li/S cells by S K-edge X-ray absorption spectroscopy(XAS).The results show that the introduction of CTAB to the S-GO nanocomposite and changes in the synthesis recipe including alteration of the S precursor ratios and the sequence of mixing of ingredients lead to the formation of different S species.CTAB modifies the cathode materials through bonding with Na2Sx in the precursor solution,which is subsequently converted to C-S bonds during the heat treatment at 155?.Moreover,GO bonds with CTAB and acts as the nucleation center for S precipitation.All these interactions among S,CTAB and GO help to immobilize the sulfur in the cathode,and may be responsible for the enhanced cell cycle life of CTAB-S-GO nanocomposite-based Li/S cells.(5)The degradation mechanism of the Li/S cell is investigated by monitoring the chemical environment and structures of the cycled cathode materials.The CTAB-GO-S nanocomposites have been used as cathode materials.The cell performances and the depth profiles information deriving from the simultaneously measured total-electron-yield and total-fluorescence-yield signals of S K-edge and C K-edge XAS are combining analyzed.The results show that for the long cycled Li/S cell,the loss of electrochemically active sulfur and the accumulation of a compact blocking insulating layer of unexpected sulfur byreaction products on the cathode surface during the charge/discharge processes make the capacity decay.Meanwhile,the missing of the conjugated structure of GO sheet is the other factor that should be accounted for.Moreover,the influence of the electrolyte on the cathode materials is also studied.The cell performance and XAS data are recorded on various cells with different LiNO3 concentrations in the electrolyte.The results indicate that higher LiNO3 concentration gives rise to better columbic efficiency by protecting the conjugated structure of the GO in the cathode.