Journal article

Investigation of the photoinduced electron injection processes for p-type triphenylamine-sensitized solar cells

We have carried out theor. investigation aiming at modeling the assessment of mechanisms of photoinduced processes in a recent p-type org. metal-free dye derived from the triphenylamine (P-1) structure. In the P-1 system, one uses the triphenylamine moiety as the electron donor, malononitrile as the electron acceptor, and a thiophene that plays the role of the conjugated chain. Basically, the difference between the P-1 dye and the common org. dyes used in the n-type DSSC field is the anchoring group located on the electron donor group. In a first step, DFT and TDDFT approaches have been exploited to calc. the key parameters controlling both the intramol. charge transfer (ICT) and hole transfer rate consts. in the Gurney-Gerischer-Marcus (GGM) formalism, for either a solvent-controlled adiabatic or a nonadiabatic electron transfer. These are: (i) the electronic coupling; (ii) the reorganization energies; and (iii) the variation of the Gibbs energy. The gathered results are in agreement with the exptl. trends. (i) The vertical ICT excited states energy has been calcd. at 2.67 eV, in perfect line with the expt. (2.65 eV). (ii) Two mechanisms can be conceived for the hole transfer and regeneration process. The first deals with the redn. of dye mol. at the excited state followed by an electron transfer from the reduced dye to the oxidized regenerator. The second implies a redox reaction between the excited dye and the oxidized regenerator, followed by an electron transfer from the cathode to the oxidized dye. (iii) Our theor. investigation suggests that the first mechanism is dominant. Secondly, we propose structural modifications improving the TPA-based DSSCs hole transfer efficiency and we show that an addnl. -CN graft on the malononitrile unit combined to the functionalization of the TPA moieties by -OMe groups (to give P-1b) should significantly improve the key parameters related to the electron injection. Indeed, for P-1b, we have noticed an increase of both the RLHE factor (0.907) and the injection driving force (-0.33 eV). This dye is therefore expected to be a very promising mol. in the p-type DSSC field.


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