Photovoltaic Performances Of Pure Organic D-A-Ï€-A Sensitizers For Dye-sensitized Solar Cells

Dye-sensitized solar cell (DSSC) is a new photovoltaic technology with low cost, high efficiency and great potential of application. Sensitizer dyes in DSSCs are responsible for collecting photons and separating excitons. The nature of their spectra and energy levels largely determines the photoelectric conversion efficiency of DSSCs. Therefore, the development of high performance sensitizer dyes is an important subject in DSSCs research. In this thesis, we put our focus on the design and synthesis of broad absorption, high efficiency and highly stable pure organic sensitizers. We propose a novel D-A-π-A model to construct new dyes by introducing electron-withdrawing groups into the π-bridge of traditional D-π-A organic dyes as the additional acceptor. The influences of the additional acceptor on dye’s absorption, energy levels, stability and photovoltaic performance are intensively studied. The structure-performance relationships, including donor, acceptor,π-bridge and alkyl chains in D-A-π-A model are systematically investigated.The introduction section briefly describes the schematic structure, working principle of DSSCs and their evaluation parameters. The energy loss in DSSCs is analyzed from both quantum and energy level. The strategies for enhancing the photocurrent and photovoltage of DSSCs by molecular engineering on D-π-A configuration organic dyes are reviewed, and the molecular design and research contents of this thesis are outlined.Taking electron-withdrawing benzothiadiazole uint as the additional acceptor, four D-A-π-A dyes, namely WS-1-WS-4, are designed with different donor or π-bridge. By using a reference dye, the effect of the additional acceptor on molecular energy gap, absorption spectra, intramolecular charge-transfer and stability of indoline structure is focused. The donor effect of D-A-π-A model is investigated by comparing indoline and triphenylamine based dyes, and the π-bridge is optimized by structural screening between phenyl and thiophene. Dye WS-2with indoline as donor, thiophene as π-bridge shows the broadest absorption spectra among the four D-A-π-A dyes due to the stronger electron-donating donor and better molecular coplanarity from thiophene π-bridge. When applied in DSSCs, dye WS-2exhibits the broadest spectral response. After co-adsorption optimization, WS-2based device shows a high IPCE plateum at about80%in a broad range of400-700nm. The onset wavelength is extending to the long wavelength (>800nm). In combination with a liquid electrolyte, WS-2gives a high efficiency of8.7%with Jsc of17.7mA cm-2, Voc of650mV. Device based on a polymer ionic liquid electrolyte, WS-2 shows a6.6%efficiency and good long term stability during1000h light irradiation.A couple of new D-A-π-A dyes WS-6and WS-11were designed and synthesized by introducing long alkyl chains onto the thiophene bridge or inserting an additional thiophene unit between indoline and benzothiadiazole. The absorption and energy levels of WS-6is almost the same with those of WS-2, indicating that the alkyl chain on thiophene unit does not affect the electronic structure of the conjugation. However, it significantly increases the anti-aggregation ability of sensitizer dye. Under the same conditions without co-adsorbent, the η of WS-6based DSSC is higher than that of WS-2by45%, with a70mV higher Voc. Electrochemical impedance spectra study indicates that introduction of alkyl chains on WS-6increases the impedance of charger transfer from TiO2to electrolyte, and inhibits the unbeneficial charge recombination. Due to the additional thiophene unit between the indoline and benzothiadiazole, the HOMO level of WS-11increases by0.1eV, resulting in the smaller HOMO-LUMO gap and broader absorption spectra. Although dye WS-11based DSSCs shows broader spectral response with IPCE onset wavelength at around870nm, the IPCE value as well as cell efficiency is much lower than that of WS-6. The reason maybe the stronger electron donor moiety and upshifted HOMO level, resulting in insuffient dye regeneration driving force.A new D-A-π-A dye WS-9was developed based on the structure of WS-2by introducing one additional hexylthiophene between benzothiadiazole and cyanoacetic acid. The increased number of thiophene units in WS-9does not broaden its absorption spectra, but enlarges the spatial distance between internal acceptor and terminal acceptor, which may be benefit to suppress charge recombination between TiO2and oxidized dye. Compared to D-π-A reference dye LS-1, a new absorption band appears at around400nm in D-A-π-A model dyes. The electronic transitions of LS-1, WS-2and WS-9are analysed by time dependent density functional theory (TD-DFT). and the origin of the additional absorption band in D-A-n-A dyes is explained. The photo-stability of WS-9containing alkyl-chain is proven to be a little lower than that of WS-2in an AM1.5irradiation experiment. When employed in dye-sensitized solar cells (DSSCs). WS-9shows less dependence on dye bath and co-adsorbents. The optimized divices based on WS-9exhibit η above9%with Jsc greater than18.00mA cm-2and Voc around700mV, which is significantly better than WS-2. Ionic electrolyte based device shows high efficiency around7%and good stability under continuously AM1.5light irradiation for500h. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the injected electron lifetime and charge recombination resistance are largely increased in WS-9based DSSCs, indicating its optimized structure for supressing charge recombination processes.The effects of alkyl chain positions on dye absorption and energy levels are studied by designing an isomeride of dye WS-11, coded as WS-13. When the alkyl chain is close to the benzothiadiazole unit, the steric effect of alkyl chain makes aryl rings twisting, which is unbeneficial to molecular coplanarity and conjugation, resulting in a large blue-shift in the ICT absorption band and low molar extinction coefficient. A new D-A-π-A dye WS-14is obtained by replacing the cyanoacetic acid in WS-2by rhodanine acetic acid. Although the absorption spectrum of WS-14is broader than that of WS-2, its IPCE and power conversion efficiency is pretty low. A crooked configuration dye WS-15was developed with the same formula as that of WS-2. The extremely narrow and low absorption of WS-15shows that the nonlinear molecular structure is not preferable to intramolecular electronic transition.Two novel D-A-π-A dyes WS-5and WS-8were developed with benzotriazole unit as the additional acceptor, indoline as the donor, cyanoacetic acid as the terminal acceptor and thiophene as the π-bridge. An octyl chain was introduced on the benzotriazole unit in WS-5, instead with a methyl in WS-8. Due to the relative weak electron-withdrawing ability, the absorption spectra of WS-5and WS-8are narrower than that of their analogue dye WS-2. AM1.5light irradiation on dye loaded TiO2films indicates that the introduction of long alkyl chains on benzotriazole unit does not decrease the photostability of this kind of dyes. When applied in DSSCs, WS-5and WS-8could efficiently transfer photon into electron (IPCE>80%) in the range of400-580nm, corresponding a Jsc of13mA cm-2under standard measurements. The introduction of benzotriazole is extreamly benefitable to the photo voltage performance of DSSCs. The Voc value of methyl substituted WS-8is680mV, while boosted to780mV when octyl chain is introduced. The power conversion efficiency of benzotriazole dyes based DSSCs can reach8%.Two novel D-A-π-A dyes WS-24and WS-25were developed with benzoxadiazole unit as the additional acceptor, indoline as the donor, cyanoacetic acid as the terminal acceptor and furyl or thiophene as the π-bridge. Due to its stronger electron-withdrawing character, the absorption spectra and IPCE of WS-24and WS-25is broader than those of WS-2. However, the altitude of the IPCE plateau is around60%-70%, which is lower than that of WS-2. WS-24based DSSCs shows the η of7.7%, which is higher than that of WS-5under the same conditions. The reason may be that furyl in WS-24is less affinity to iodine in the electrolyte, resulting in less recombination loss and higher performance. Replacing the thiophene unit in WS-2with a furnyl leading to a new dye WS-19, which shows efficiency of9.36%with Jsc>18mA cm-2, Voc>700mV. A parallel comparison between different additional acceptor, including benthiadiazole, benzotriazole and benzoxadiazole units, with specific emphasis on spectral response and photovoltaic performance.Accordingly, D-A-π-A model provides a new strategy for developing broad absorption, high efficiency, and highly stable organic sensitizer dyes. It also provides a new method for tuning the spectra of organic sensitizers, modulating the electron-deficient degree of the additional acceptor. Acquiring broad spectral organic sensitizers need discreet molecular engineering, with a subtle balance between the spectral response width and the dynamic of electron transfer within DSSCs. More considerations are necessary for realizing real panchromatic organic sensitizers. The contradictory between spectra width and IPCE height as well as photovoltage remains challenges in future molecular design.