Study On Dye-sensitized Solar Cells Based On Iodine And Cobalt Electrolytes

In1991, Michael Gr tzel reported dye-sensitized solar cells (DSCs), which hadan overall power conversion efficiency approaching7.1-7.9％under standard (GlobalAir mass1.5) illumination. DSCs have been considered to be a credible alternative toconventional inorganic silicon-based solar cells because of their ease of fabrication,high efficiency, and cost-effectiveness. DSCs have attracted tremendous attention andnumerous research attempts have been made to explore new photosensitizers duringthe past two decades. Most of the sensitizers have been based on iodine electronmediator. One of the main drawbacks with the iodide/triiodide redox couple is,however, the large driving force needed for the dye regeneration, which limits theimprovement of the efficiencies. Cobalt (II/III) redox shuttle has been developed to beenvironment-benign and to further lower the cost of DSCs. However, the efficiencymay not be improved while simply replacing the iodide/triiodide couple with cobaltelectrolytes in DSCs. So far, there are few investigations in the organic dyes based ondifferent electrodes, i.e., iodine and cobalt electrodes. To develop high-efficiencyorganic dyes, a comprehensive understanding on the structure-property relationship ofmetal-free organic photosensitizers and electrodes is necessary.Based on the above reasons, the research in this thesis includes the followingparts:(1) Four different types of organic dyes, M14,M24,XS51and XS52, aredesigned and synthesized for cobalt-based and iodine-based DSCs to improve thelight harvesting and to limit recombination. M14and M24contain truxene-basedtriarylamine parts, XS51contains four hexyloxyl groups on triarylamine, XS52includes an indoline part, and these parts serve as electron donors. In these fourorganic dyes, hydrocyanic acid serves as electron acceptor, and EDOT asπ-conjugated spacer, respectively. The dyes are synthesized throughN-Bromosuccinimide or Br2bromination, Suzuki coupling reaction, Vilsmeier-Haackreaction and Knoevenagel condensation. The structures of the result andintermediate compounds have been characterized by mass spectra (MS) and protonnuclear magnetic resonance (1H NMR) technology.(2) Employing the dyes of M14,M24, XS51and XS52, DSCs based on iodine and cobalt electrodes are prepared,including the preparation of TiO2thin-filmed electrode and counter electrode, sensitizers uptake, electrolyte injection and sealing process.(3) The photophysical andelectrochemical properties of the dyes, and the photovoltaic performance of the DSCsare investigated. Density function theory (DFT) and time-dependent DFT (TDDFT)are calculated. The relationship between the structures of the dyes and theperformance of DSCs are explored.Based on M14and M24, general impacts of the length of alkyl chains on theoptoelectronic features of DSCs employing the iodine and cobalt electrolytes areinvestigated. In conjunction with the iodine electrolyte, M14dye containing propylchains exhibits better Jscand thus PCE than M24featuring hexyl chains due to higheramounts of dyes adsorbed and comparable electron lifetime. In contrast, theperformances of M24sensitized cobalt cells are superior to those of M14benefitingfrom significantly improved electron lifetime. IMVS data have revealed that theelectron lifetime of the cobalt cells are strongly dependent upon the length of alkylchains, while the electron lifetime of the iodine cells are found to be only weaklydependent upon the length of alkyl chains. Interestingly, the charge density andelectron lifetime data have demonstrated that the dependence of photovoltaicperformance of cells on the TBP concentration is also correlated with the length ofalkyl chains. The dependence of conduction band shift and electron lifetime on theconcentration of4-tert-butylpyridine (TBP) in electrolyte is also investigated. TheJscof the M14sensitized cells decreased with increasing TBP concentration, while nosignificant change in Jscwas observed for M24. DFT and TDDFT results indicate thatthe electrons in the donor could be successively transferred to the acceptor part andthen injected into the conduction band of TiO2after being illuminated by the light.The short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc) and fillfactor (FF) of M14cells employing E2electrolyte are10.7mA cm-2,846mV and0.71, respectively, affording an overall power conversion efficiency (PCE) of6.42%.By comparison, the introduction of longer alkyl chains in M24significantly increasedVocfor about100mV, contributing to an evidently enhanced PCE of7.86%.According to our finding, we proposed that organic dyes with long alkyl chains indonor part should be preferred in the future development of organic sensitizers foriodine-free DSCs.Based on two organic dyes XS51and XS52derivated from triarylamine andindoline for dye-sensitized solar cells (DSCs), the effects of dye structure upon thephotophysical, electro-chemical characteristics and cell performance are investigated. The indoline dye XS52shows a red-shifted absorption and higher molar extinctioncoefficient than triarylamine dye. High short-circuit photocurrent density (Jsc) isobtained for XS52due to the strong electron-donoring capability of indoline unit.XS51with four hexyloxyl groups on triarylamine performs better steric hindrance andan improvement of photovoltage. The two hexyloxyl groups on indoline dye are notenough for retardation of charge recombination at the titania/electrolyte interface inDSCs, which leads to a little lower open-circuit voltage (Voc) in comparison withXS51. With cobalt electrolytes,5.88%of solar energy to power conversion efficiency(PCE) based on XS51is achieved with a short-circuit photocurrent density (Jsc) of10.5mA cm-2, an open-circuit voltage (Voc) of862mV, and a fill factor (FF) of0.65.The cells based on XS52show an increased Jscof11.5mA cm-2and a decreased Vocof830mV. The improvement of Jsccompensates for a slight drop in Voc, contributingto an enhanced PCE of6.58%under AM1.5-100mW cm2irradiation. Thephotovoltaic performances of the Co(II/III)tris(phen) redox couple are superior tothose of I-/I3-redox couple for thin-film DSCs sensitized by XS51-52, indicating thatrational design of sterically bulky organic dyes is needed for further development ofhigh-efficiency iodine-free devices.