Dye Sensitized Solar Cells And Perovskite Solar Cells:Fundamental Studies Employing Single Crystalline Model Systems

Solar cells is an effective,clean and renewable solution to meet the rapid increasing global energy consumption demand,because they can directly convert solar energy into electricity.As silicon based solar cells renew the record of power conversion efficiency(PCE)record year by year,researchers have invent a series of low-cost thin film photovoltaic material and devices to push their PCE to the Shockley-Queisser limit(33,7%).The most famous and important photovoltaic techniques among them are dye-sensitized solar cells(DSSCs)and the following perovskite solar cells(PSCs).The PCE of PSCs have increased rapidly from 3.8%to 22.1%since the 2009 and now it is comparable to the long-developed silicon solar cells.Although PSCs have realized a very attractive PCE,there are still a number of fundamental scientific issues,apart from manufacture technology,to be solved.For example,the phase composition and stability of perovskite absorber,nucleation and growth of perovskite during preparation,interfacial structure and charge transfer across the interface formed by perovskite and its adjacent charge selective materials.Insight into this important issues can help us understanding the high performance mechanism of perovskite solar cells and guide further cell performance improvement.In this work,we investigate the basic fundamental issues of DSSCs and PSCs by using single crystal photovoltaic materials as model systems.Raman spectroscopy and traditional electrochemical methods together with inormal photovoltaic techniques are used to study this systems.On the one hand,we study the interfacial structure and charge transfer of TiO2/dye molecules via surface enhanced Raman spectroscopy(SERS).On the other hand,we systematically investigate the intrinsic structure and internal interaction of perovskite materials by using perovskite single crystals.This interaction manner and strength is closely related to the stability of perovskite.Further,we study the intrinsic photovoltaic properties and nucleation-growth principles of perovskite materials to guide the fabrication of high performance perovskite solar cells.At the end of this thesis,we try to develop novel spectroscopic characterization methods to study perovskite solar cells under working conditions.In-situ photoluminescence,in-situ normal Raman and in-situ SERS was applied to investigate bulk properties of perovskite and TiO2/perovskite interface in practical device.The main contents and conclusions are outlined as following:1.We have developed the spectroscopic electrochemical investigation methods for semiconductor single crystal electrochemistry.For the first time,in-situ Raman spectroscopy have been used to study the important TiO2/dye interface in DSSCs based on TiO2 single crystal electrode.First of all,we have built a high-vacuum argon sputtering setup for cleaning treatment of TiO2 single crystalline surface.This setup was used to obtain semiconductor electrode with good bulk conductivity and clean surface.Then in the system of "rutile(110)/N719",we have obtained the in-situ Raman spectra of N719 molecules adsorbed on semiconductor single crystalline surface via shell-isolated nanoparticle enhanced Raman spectroscopy(SHINERS).These results have been compared with those obtain in the system of "P25/N719".Combined with semiconductor energy diagram extracted from Mott-schottky analysis,we suggest the structural model of TiO2/N719 interface and transfer charge pathway across this interface.Our results indicate that the binding of the SCN group of N719 to the TiO2 surface is the intrinsic nature of the TiO2/N719 interaction.The binding group in this interface are COO-,COOH and NCS.In-situ Raman spectra reveal that the interaction of NSC with TiO2 is stronger in P25/N719 system.Photoinduced charge transfer(PICT)occurs for both the P25 and rutile(110)TiO2 surfaces,and the potential to achieve PICT resonance depends on the band structure of the semiconductor.2.We have investigated the organic-inorganic interactions in organolead halide perovskites by Raman spectroscopy.By using CH3NH3PbI3 and CH3NH3Br3-xClx single crystals,we have studied the interaction manner between organic cation and the inorganic octahedral framework and its influence on stability and current-voltage hysteresis of perovskite solar cells.The Raman spectra of perovskites adopting MA as the A-site cation are compared with those adopting ammonic acids as cation,which can reveal the nature of the restricted rotation mode.High signal-to-noise ratio Raman spectra of perovskite material have been obtained by using a lower energy laser excitation of 1030 nm to suppress the fluorescence background.CASTEP calculations have been used to guide band assignment of Raman spectra.The restricted rotation mode of CH3-NH3+ is confirmed to be the C3 type along the C-N axis and this rotation have litter influence on the current-voltage hysteresis of perovskite solar cells.The interactions of the organic cation with the inorganic PbX3-framework are fulfilled mainly via the NH3+ end through N+-H…X hydrogen bonding.The strength of the interaction can be tuned by the composition of halides but is insensitive to the size of the organic cation.Moreover,doping of Cl with higher electronegativity strengthens the hydrogen bonding and improves the stability of perovskite.3.We have demonstrated how to use the law learned from growth of perovskite single crystals to guide the fabrication of high performance perovskite solar cells.In order to understand the cubic phase stabilization and crystallization kinetics in the most efficient perovskite with chemical composition of(FAPbI3)1-x(MAPbBr3)x(x = 0,0.05,0.10,0.20),we have investigated the single crystalline materials of this system.High quality perovskite single crystals with 1 cm size have been prepared by using inverse temperature crystallization method.We demonstrate that the best composition for a perfect ?-phase perovskite without segregation is x = 0.1-0.15,and such a mixed perovskite exhibits carrier lifetime as long as 11.0 ?s,which is over 20 times of that of FAPbI3 single crystal.Powder XRD,single crystal XRD and FT-IR results reveal that the incorporation of MA+ is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder.Moreover,we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of 8-phase.These findings benefit the understanding of a-phase stabilization behavior,and have led to fabrication of perovskite solar cells with highest efficiency of 19.9%via solvent management.4.We have tried to develop novel spectroscopic characterization methods to study perovskite solar cells under working conditions.Due to the all solid state configuration of perovskite solar cells,most characterization methods(such as scanning probe microscopy)can't be applied to investigate perovskite solar cells in situ.Further,practical cell working of perovskite solar cells require light illumination and external voltage,this makes in-situ techniques which can be used to study perovskite solar cells extremely rare.In this work,we use in-situ photoluminescence(PL),in-situ normal Raman and in-situ SERS w to investigate bulk properties of perovskite and TiO2/perovskite interface in practical device.The required spatial resolution can be achieved by confocal microscopic system.In-situ PL experiments reveal that the PL intensity and peak wavelength is voltage dependent.The PL intensity decreases and undergoes red shift as the cell voltage increases.The signal-to-noise ratio of the in-situ Raman spectra is poor due to the strong PL background.Fortunately,this situation can be improved by incorporation of Ag or Ag@TiO2 nanoparticle into the mesoporous layer of perovskite solar cells.The in-situ SERS by adopting this cell configuration may be promising for further investigation of TiO2/perovskite interface in perovskite solar cells.5.The further developing trends of perovskite solar cell are discussed as an outlook.The rapid performance promotion of perovskite solar cells can be attribute to four key breakthroughs during the development period since 2009.Considering the current situation of this field,we conclude that the stability and toxicity issues of perovskite solar cells are key problems to be solved in the future.