N,S,P-hybrid donor-π-acceptor organic dyes for dye-sensitized solar cell: Synthesis, optical properties, and photovoltaic performances

New donor-π-acceptor organic (D-π-A) dyes composed of triarylamine, oligothiophene, and phosphole subunits were prepared, and their optical and photovoltaic properties were investigated. The regioselective α-lithiation of the thiophene ring of 2-(thiophen-2-yl)phospholes bearing an ester group, followed by treatment with tributyltin chloride afforded 2-(5-(tributylstannyl)thiophen-2-yl)phosphole derivatives, which underwent Stille coupling with 5′-(p-(diarylamino)phenyl)-5-bromo-2,2′-bithiophene to give triarylamine-terthiophene-phosphole hybrid π systems bearing the terminal ester group. The alkaline hydrolysis of the ester group yielded the target dyes, bearing the carboxylic acid anchoring group. The UV-vis absorption spectra of the new N,S,P-hybrid dyes displayed broad and intense π-π∗ transitions with two absorptionmaxima in the visible region. Density functional theory (DFT) calculations of two dye models revealed that each highest occupied molecular orbital (HOMO) resides on the triarylamine-oligothiophene π network, whereas each lowest unoccupied molecular orbital (LUMO) is basically located on the phosphole subunit. In addition, the time-dependent DFT calculations of the models showed that the lowest energy bands of these hybrid dyes are mainly consisted of the HOMO-to-LUMO+1 and HOMO-to-LUMO transitions with the large intramolecular charge-transfer character. The N,S,P-hybrid-dye-sensitized TiO2 cells exhibitedmoderate power conversion efficiencies of up to 5.6%. The present findings corroborate the potential utility of the phosphole skeletons as the acceptor components in the D-π-A sensitizers.