Study On Sensitizers With Hydroxyl As Anchoring Group And Arylamine As Hole Transport Materials For DSCs

Photosensitizer, as a key part in dye sensitized solar cells (DSCs) for getting higher efficiency and longer stability, harvests the sunlight to form excited state and achieves the process of electrons injection into the conduction band of nanosemiconductor TiO2. The resultant oxidized dye is regenerated by redox mediator in electrolyte. To decrease the cost of DSC, the metal-free organic dyes as the alternative to noble metal complex sensitizer have been a research topic of high priority and develop rapidly in recent years.Commonly, for most organic sensitizers in traditional DSCs (Type Ⅰ DSCs), electrons are injected into TiO2from the excited dyes by photoexcitation. As cyanoacrylic acid group was always introduced as acceptor and anchoring group, which has limited much more excellent dyes were developed. Furthermore, the introducing of vinyl group would reduce the stability of the DSCs by reaction between the dyes and electrolyte (I-/I3-). Type Ⅱ DSCs in which electrons are injected not only by photoexcitation of the dyes followed by electron injection from the excited dyes to TiO2but also by direct one-step electron injection from the dyes to TiO2by photoexcitation of the dye-to-TiO2charge-transfer bands. But Type Ⅱ DSCs have not been studied widely due to its low efficiency. In this paper we report four novel Type Ⅱ organic dyes LS201, LS202, LS203and LS204with Donor-π-Acceptor design in structure. Triphenylamine was introduced as a donor part in Type Ⅱ sensitizer, and catechol was instead by2-hydroxybenzonitrile as the electron acceptor/anchoring group. Electron injection can occur not only through the anchoring group (hydroxyl group) but also through the electron-withdrawing group (-CN) located close to the semiconductor surface. Compaired with the traditional organic D－π-A dyes with cyanoacrylic acid system, the electron donor,π-spacer and electron acceptors were linked by C-C single bonds without involvement of any vinyl group, which would hopefully increase the stability of DSCs. By using the strategy four novel dyes were successfully synthesized. The dye LS203giving the best solar energy conversion efficiency of3.4%, with Jsc=7.4mA·cm-2,Voc=0.67W,ff=0.69, the IPCE value reaches66.9%.In order to optimize the structure of Type Ⅱ organic dyes, LS211, LS213, LS221and LS223were synthesized with two different donors10-butyl-10H-phenothiazine and4-methoxy-N-(4-methoxyphenyl)-N-phenyl aniline were introduced in the molecular structure. Furthermore, two reference dyes RD-1and RD-2based on catechol were synthesized. In comparison with the two reference dyes which are typical type Ⅱ sensitizers, these new Type Ⅱ sensitizers with D-π-A structure exhibit better performance due to the stronger electronic push-pull effect. Among the four dyes, LS223gives the best solar energy conversion efficiency of3.6%, the maximum IPCE value reaches around74.9%.Although traditional DSCs based on liquid electrolyte have the best performance, such cells suffer from potential leakage problems associated with the corrosive and volatile nature of the liquid electrolyte and, thus, may be impractical for large-scale applications. In an effort to address these issues, solid-state hole transport materials (HTMs) have been the focus of many recent reports. To date, the best performance in solid-state DSCs is based on the HTM2,2’,7,7’-tetrakis (N,N-di-p-methoxy-phenylamine)9,9’-spirofluorene (Spiro-OMeTAD). However, the solubility of Spiro-OMeTAD is quite low, and the cation of Spiro-OMeTAD has absorption in visible region which might compete with the sensitizers to absorb light. In this paper, four compounds TFA-2, TFA-4, FCF-6and CFC-6were synthesized, expect that these compounds can be used as HTMs in solid-state DSCs. Finally, TFA-2and TFA-4were selected to be HTMs, and in company with the sensitizer TC305, the solid-state DSCs were fabricated. The devices based on TFA-2and TFA-4reached the efficiencies of1.20%(Voc=0.99V, Jsc=2.30mA·cm-2,ff=0.53) and0.72%(Voc=0.91V, Jsc=2.03mA·cm-2, ff=0.39) respectively. The maximum IPCE value reached35.7%and30.4%.