Studies On Inhibiting Dye Aggregation And Its Effect On The Performance Of Dye-sensitized Solar Cells

Dye sensitized solar cells (DSCs) are one kind of solar cells which can convert light to electricity by means of harvesting the solar irradiation by the sensitizer anchored onto the semiconductor surface. DSCs have attracted considerable attention in scientific and industrial research. DSCs also offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. The photoelectric conversion efficiency and long-term stability of DSCs are two key issues for their practical applications. In this thesis, the performance and stability of aromatic amine-based cells were systematically studied through coadsorption of CDCA, as well as optimizing the inorganic semiconductor electrode and the electrolyte solution. The main contents and results are generalized as follows:In Chapter 1, the basic definition on the structure and principle for dye-sensitized solar cells, and the performance parameters of solar cells are introduced. On this basis, recent progress of high-efficiency sensitizers for dye-sensitized solar cells is reviewed. Then the research strategy of the thesis is presented.In chapter 2, the fabrication processes and measure methods for DSCs were described, with an emphasis on fabricating nano-scale titanium dioxide colloide and the electrode.In chapter 3, the effects of chenodeoxycholic acid (CDCA) in a dye solution as a co-adsorbent on the photovoltaic performance of DSCs based upon two organic dyes containing phenothiazine and triarylamine segments (P1 and P2) were investigated. It was found that the coadsorption of CDCA can hinder the formation of dye aggregates and improve electron injection and thus Jsc. When CDCA concentration is 10 mM, the photoelectric conversion efficiency of dye P1 and P2 reached the maximum,4.80% and 5.31%, respectively, improved by 33% and 25% in comparison with those without CDCA. Electrochemical impedance data indicate that the electron lifetime was improved by coadsorption of CDCA, accounting for the significant improvement of Voc.In chapter 4, the iminostibene and iminodibenzyl moieties were introduced into the organic sensitizer for the first time. The photophysical, electrochemical and photovoltaic properties of the iminostilbene dyes M1-M4 have been studied in detail. With the extension of the conjugated system, the absorption band red shifted and molar extinction coefficient increased. Dye M2 containing iminostibene and vinyl thiophene has the largest conjugated system and the lowest LUMO orbital energy. These characters are greatly favorable for improving the photoelectric conversion efficiency. Compared with the triphenylamine analogues, iminostilbene dyes showed much better photovoltaic performance. It indicated that iminostilbene unit was a kind of excellent electron donor. Further comparison revealed that the sensitizer with iminostibene as electron donor (M2 and M4) showed better performance than the iminodibenzyl derivatives (Ml and M3). In order to solve the problem of dye aggregation, chenodeoxycholic acid was used as a coabsorbent, concentration of 4-tert-butyl pyridine (TBP) in electrolyte was changed and various solvents, such as dichloromethane, ethanol, isopropanol, tert-butyl alcohol, toluene, methanol and tetrahydrofuran were applied into the dye bath. After a series of tests, the optimal conditions based on sensitizer M2 were determined:a isopropyl alcohol dye bath containing 5 mM CDCA, the liquid electrolyte with 1.0M of TBP. In the optimum conditions, the conversion efficiency based on dye M2 reached 5.96%(Jsc=13.28 mA cm-2, Voc=649mV, ff=0.69). Meanwhile, the quasi-solid DSCs performance and stability was studied, and its photoelectric conversion efficiency slightly lower than the liquid DSCs, but the stability increased substantially, the cells based on M1-M4 still retained>96% of its maximum efficiency after 1000 h accelerated tests under full sunlight soaking at 55℃.In chapter 5, in order to expand the absorption spectrum absorption, improve the efficiency of intramolecular charge transfer and the stability of DSCs, benzothiadiazole with strong electron-withdrawing ability was introduced into the iminostilbene dyes, constructing a new type of D-A-π-A sensitizer (S1-S4). The UV spectra and photovoltaic properties of sensitizers involving benzothiadiazole acetylene (S3 and S4) and benzothiadiazole dyes (S1 and S2) were investigated. When the molecular size of dye S3 and S4 increased, their absorption bands did not undergo redshift and the photoelectric conversion efficiency decreased. The effect of different conjugated bridge on the sensitizer’s performance was also investigated. The dye with phenyl (S2 and S4) as bridging unit showed better performance than thiophyl (S1 and S3). Although absorption spectrua were blue-shifted, the LUMO orbital energy level increased. In consequence, the electron injection efficiency, monochromatic photoelectric conversion efficiency and conversion efficiency was improved significantly. The photovoltaic results displayed that dye S2 had the best performance. The photoelectric conversion efficiency of S1-S4 without optimization were 5.16%,5.36%,1.39% and 2.86%, respectively. Great efforts on optimization of the DSCs devices have been made by adding co-adsorbent, changing the concentration of TBP in the electrolyte and optimizing the TiO2 electrode. Dye S2 exhibited impressive efficiencies of 6.71% and 4.91% with liquid electrolyte and solvent-free ionic electrolyte under AM 1.5 G respectively, and showed an excellent stability under light soaking at 55℃for 1000 h.In Chater 6, the photophysical, electrochemical, photovoltaic properties and stability of the benzo[c]thiophene sensitizer were systemically investigated and optimized. The absorption band red-shifted, when a large conjugated spacer was introduced into the sensitizer, meanwhile the aggregation of the dye was also formed. The efficiency was enhanced by using chenodeoxycholic acid (CDCA) as a co-adsorbent, optimizing the electrolyte concentration and electrode thickness to hinder the formation of dye aggregates. The conversion efficiency of dye X1 and X2 reached 4.97%(Jsc=13.44mA cm-2, Voc=592mV,ff=0.63) and 2.14% (Jsc=6.55mA cm-2, Voc=489 mV,ff= 0.67), which have been increased by 27.3% and 28.5%, respectively. The quasi-solid DSCs based on X1 still retained>95% of its maximum efficiency after 1000 h accelerated tests under full sunlight soaking at 55℃In chapter 7, the absorption spectra, electrochemical and photovoltaic properties of two cyanine dyes containing triphenylamine and benzothiadiazole moieties have been extensively investigated. Under irradiation of AM 1.5 G simulated solar light (100 mW cm-2), before optimization the IPCE maxima of the two cyanine dyes reached 57% and 78% and the conversion efficiency were 2.40% and 2.97% respectively. It suggested that the compound in which triphenylamine and benzothiadiazole moieties were connected by triple bond shows better charge transport properties. In order to decrease the H-aggregation and J-aggregation of cyanine dyes, the electrode was immersed in CDCA solution for a little time before sensitized by dyes. The photoelectric conversion efficiency of dyeⅠandⅡreached 2.59% and 3.28%, which have been improved by 7.9% and 10.4%, respectively.