Photovoltatic Performances Of Pure Organic D-A-π-A Sensitizers For Dye-sensitized Solar Cells With Quinoxaline As The Auxliary Acceptor

Owing to the high power conversion efficiency and low cost, dye-sensitized solar cells (DSSCs) have been regarded as one of the most prospective and potential photovoltaic devices. In a typical DSSC, the dye sensitizer plays key role in absorbing the sunlight and separating charges. As such important component, molecular structure of sensitizers should be elaborately tailored with the aim of broad absorption, suitable energy levels, controllable adsorption pattern and compact layer morphology as well as excellent stability. In this dissertation, we focused on D-A-π-A-featured sensitizers, in which an additional electron-withdrawing unit of quinoxaline is specifically incorporated into the π bridge as the additional electron-deficient acceptor, especially for broadening spectral response and optimizing energy levels.In the introduction, the working principle of DSSCs and their evaluation parameters is briefly described. The strategies for enhancing the photocurrent and photovoltage of DSSCs are also reviewed, and the molecular design and research contents of this thesis are outlined.Four novel quinoxaline-containing organic sensitizers (IQ1, IQ2, TQ1and TQ2) with D-A-π-A configuration were designed and studied. By theoretically investigating DFT and time-dependent DFT, the effects of auxiliary quinoxaline acceptor on the additional absorption band and electron transition were especially concerned about. The incorporated auxiliary quinoxaline unit with low band-gap can efficiently optimize energy levels and broaden absorption band with a new band which can result in an efficient light-harvesting for photovoltaic conversion, and conveniently tailor the solar cell performance with a facile structural modification on quinoxaline unit with long alkoxy groups. More importantly, the incorporated quinoxaline unit can essentially facilitate the electron transfer from the donor to the acceptor/anchor, resulting in a great improvement in both the electron distribution of donor unit and the photostability of synthetic intermediates and final sensitizers. The EIS Bode plots reveal that the replacement of methoxy group with octyloxy group increases the injection electron lifetime by2.4fold. IQ2and TQ2can perform well without any coadsorbent, successfully suppress the charge recombination and enhance the electron life time, resulting in a decreased dark current and enhanced Voc.Using a liquid-electrolyte, DSSC based on dye IQ2exhibited broad action spectrum and high optimizing efficiency (η=8.50%) with a high Voc of776mV.To date, only a few pure metal-free organic sensitizers based on an iodine electrolyte are capable of achieving9%in photovoltatic efficiency. For overcoming the photovoltaic bottleneck, an electron-withdrawing unit of2,3-diphenylquinoxaline as the additional acceptor was successfully incorporated to construct a novel D-A-π-A featured dye IQ4. Investigated theoretically by DFT and time-dependent DFT, the incorporated auxiliary quinoxaline unit with low band-gap can efficiently optimize energy levels and broaden absorption band, and the λmax of IQ4extend to529nm. The coadsorbent-free DSSC based on dye IQ4exhibits very promising conversion efficiency as high as9.24%, with a short circuit current density (Jsc) of17.55mA cm-2, open circuit voltage (Voc) of0.74V, fill factor (FF) of0.71under AM1.5illumination (100mW cm-2). IQ4-based DSSC devices with ionic liquid electrolyte can keep constant performance during1000h aging test under one sun at60℃. Due to the spatial restriction, the two phenyl groups grafted upon the additional electron-withdrawing quinoxaline is demonstrated as an efficient building block, not only improving its photo-and thermal-stability, but also being as a successful anti-aggregation functional unit.A series of sensitizers based on the auxiliary acceptor unit of quinoxaline, containing indoline as electron donor and cyanoacetic acid as acceptor/anchor, have been specifically developed as IQ6, IQ7and IQ8for high efficiency DSSCs. Inviting IQ4as the reference sensitizer, the general influence of different π-spacer in these D-A-π-A organic sensitizers on their absorption, energy levels and photovoltaic performances can be investigate carefully. It is found that, on one hand, the additional thienyl unit near the anchor group has no distinct obvious effect on the absorption spectra and energy levels. On the other hand, the additional thienyl group close to the donor group obviously red-shifts the absorption band and positively shifts the LUMO levels. Contrast with IQ4, the low charge collection efficiency (ΦCol) of IQ6, IQ7and IQ8result in their unsatisfactory IPCE plateaus. Moreover, although IQ6and IQ7possess the higher conduction band (ECB) than IQ4, the relatively faster charge recombination rate limits its Voc performance. After extending the π-linker on the basis of IQ4, the diffusion lengths of IQ6-IQ8becomes shorter, which dramatically decrease their photocurrent and impair their photovoltaic performances.Considering that the alkyl-functionalized cyclopentadithiophene (CPDT) segment as the conjugated bridge shows an extremely high molar absorption coefficient and high power conversion efficiency in D-π-A featured sensitizers. The CPDT unit was choosen as the alternative π-bridge based on IQ5to develop IQ21. Additionally, dye S3was introduced as the co-adsorbent in dye bath for inhibiting the recombination reaction. IQ5is used as the reference sensitizer to clearly illustrate the influence of both the π-spacer in quinoxaline-based organic sensitizers and the novel organic co-adsorbent upon the photovoltaic performances. It has shown that the utilization of CPDT unit can distinctly increase the Jsc, with a photovoltatic efficiency (9.03%). Moreover, the employment of novel co-adsorbent S3further improves the efficiency (10.41%) by suppressing the charge recombination rate and providing the enhanced light harvesting ability in the short wavelength region of IQ21.Two novel D-A-π-A dyes IQ13and IQ17were developed with pyrido[3,4-b]]pyrazine unit as the additional acceptor, indoline as the donor, cyanoacetic acid as the terminal acceptor and thiophene as the π-bridge. Due to its stronger electron-withdrawing character, the absorption spectra and IPCE of IQ13and IQ17is broader than that of the reference dye IQ4. Correspondingly, the IPCE extends to over800nm, which is broader than that of IQ4. However, IQ13and IQ17based DSSCs show the η of8.33%and8.76%, which is lower than that of IQ4(8.85%) under the same conditions, which might be attributed to the relatively faster charge recombination rate limits Voc performance IQ13and IQ17leading to the photovoltage loss over100mV than that of IQ4.