Interpretation Of The Electron Transport-recombination Models&the Equivalent-circuits For Dye-sensitized Solar Cells And Performance Optimization

Dye-sensitized solar cells （DSSCs） have been developed owing to the prospects of highperformance and low cost. The energy conversion procedure in DSSCs is affected by a seriesof electron processes such as generation, transport, accumulation and recombination.Therefore, electron transport-recombination processes in DSSCs should be investigated indetail to analyze the influencing factors of cell performance. Electrochemical impedancespectroscopy （EIS） and the single-diode model are two important methods to analyze theinfluencing factors of DSSCs’ current-voltage characterizations. Precise determination of themodel parameters as well as building a correlation between model parameters and internal cellprocesses by EIS interpretation will contribute to analyze limiting factors on cell performanceand explore specific strategies for cell optimization. A novel double-layered compound filmprepared by TiO₂mesocrystals with ordered structure and high light scattering property willalso help to further improve the performance of DSSCs.In the first part of this dissertation, a two-step method was proposed to preciselydetermine the parameters of the single-diode model for DSSCs. By using the graphic method,model parameters were extracted with proper precision from the linearized current-voltagecharacterizations and subsequently were set as the initial values for non-linear least squares（NLLS） fitting. Model parameters with high precision were obtained by the two-step fittingmethod. In the graphic fitting step, linearity deduced form the implicit equation of thesingle-diode model turned to express quite scattered plot. In this work, polynomial expressionwas applied to approximate experimental data to derive the linearity and the correspondingparameters. It was testified the error of the graphic fitting was about4%which was futurereduced by NLLS fitting to less than1%. The two-step method was also applied to analyzethe I-V characterizations extracted from references. It was shown parameter values fit by thetwo-step method are different form those reported in reference. The accuracy of the two-stepmethod was validated since the fitting method reported in some references was about5%.In the second part of this dissertation, the precisely determined parameters of thesingle-diode model were interpreted of their physical and electrochemical origins byanalyzing the EI spectra measured under three characteristic working conditions, the maximum-power point （MPP）, the open-circuit voltage point （OCVP）, and the short-circuitcurrent point （SCCP）. It was shown, the total recombination resistance, Rct, coincided with theparallel resistance, Rp. When cells are operated at OCVP and MPP, electrons recombinemainly at TiO₂/electrolyte interface and Rpis dominated by the resistance of the diode, Rdiode.Therefore, the process of electron charge-transfer at TiO₂/electrolyte interface is considered toperform as the diode element of the single-diode model. In the case of operated at SCCP,electrons turn to recombine on TCO/electrolyte interface and Rpis dominated by shuntresistance, Rsh. Therefore, the shunt resistance can be used to characterize the current leakagefrom uncovered layer of TCO to electrolyte. The ideality factor n was obtained from thelinearity of lnRctas a function of cell potential and agreed with those fits of the single-diodemodel. This indicates ideality factor is dominated by the characterizations of electroncharge-transfer on TiO₂/electrolyte interface. The series resistance, Rswas verified to beconstituted by four electron process. Subsequently, the model parameters were evaluated interms of their effects on I-V characteristics of DSSCs via a single-diode model simulation.This simulation demonstrated that series resistance Rsand ideality factor n play crucial rolesin limiting fill factor FF of DSSCs; Upon up-scaled DSSCs, Rsand Rshvalues will also affectthe short-circuit current Iscand the open-circuit voltage Voc, respectively. Finally, strategiesare proposed to optimize DSSCs based on the physical and electrochemical interpretations oflimiting factors of cell performance.In the third part of this dissertation, a novel kind of double-layered photoanode film wasprepared by nanoporous anatase TiO₂mesocrystals for the purpose to improve theperformance of DSSCs. EIS and the single-diode model were applied to investigate themechanisms of how electron transport-recombination processes influence on cell performance.In the first step, TiO₂nanocrystals were synthesized by controlling diethylenetriamine（DETA）/Ti molar ratio in hydrothermal system and DSSCs prepared by the synthesized TiO₂nanocrystals were investigated. It was shown, when more DETA was added in thehydrothermal system, parts of nanoparticles grow bigger leading to nonuniformity of TiO₂nanocrystals and aggravated electron recombination. Whereas, electron collection efficiencieswere improved attributing to faster electron transport in nanocrystalline films. However,further increasing DETA addition will result in decrease of dye adsorption and decline of cell performance. In the next step, one kind of TiO₂mesocrystals was synthesized with differentmorphologies by controlling tetrabutyl titanate （TBT）/acetic acid （HAc） volume ratio. TheTiO₂mesocrystals with highest surface area were used to prepare a layer of film covering ontop of a TiO₂nanocrystalline film to construct a novel double-layered photoanode structure.The DSSSs constructed by the double-layered photoanode film were characterized andcompared with cells based on a single-layered nanocrystalline film. It was shown, tightaggregation of nanocrystals in TiO₂mesocrystalline network resulted in low dye adsorption,whereas,3-D ordered structure in TiO₂mesocrystals contributed to high electron transportand electron lifetime. Therefore, when the thickness of nanocrystalline film and ofdouble-layered compound film is same of15μm, introducing a mesocrystalline film of4μmon top of a nanocrystalline film of11μm contributed to improve the efficiency from5.97%to6.36%.