Abstract
A molecular design is presented for tailoring the energy levels in D-π-A organic dyes through fluorination of their acceptor units, which is aimed at achieving efficient dye-sensitized solar cells (DSSCs). This is achieved by exploiting the chemical structure of common D-π-A organic dyes and incorporating one or two fluorine atoms at the ortho-positions of the cyanoacetic acid as additional acceptor units. As the number of incorporated fluorine atoms increases, the LUMO energy level of the organic dye is gradually lowered due to the electron-withdrawing effect of fluorine, which ultimately results in a gradual reduction of the HOMO-LUMO energy gap and an improvement in the spectral response. Systematic investigation of the effects of incorporating fluorine on the photovoltaic properties of DSSCs reveals an upshift in the conduction-band potential of the TiO2 electrode during impedance analysis; however, the incorporation of fluorine also results in an increased electron recombination rate, leading to a decrease in the open-circuit voltage (Voc). Despite this limitation, the conversion efficiency is gradually enhanced as the number of incorporated fluorine atoms is increased, which is attributed to the highly improved spectral response and photocurrent.
Highlights
A molecular design is presented for tailoring the energy levels in donor-p bridge-acceptor (D-p-A) organic dyes through fluorination of their acceptor units, which is aimed at achieving efficient dye-sensitized solar cells (DSSCs)
This was made possible by incorporating one or two fluorine atoms at the ortho-positions of the cyanoacetic acid in the organic dye to act as an additional acceptor unit and reduce the HOMOLUMO energy gap
Subsequent cyclic voltammograms (CVs) analyses confirmed that the lowest unoccupied molecular orbital (LUMO) energy level is gradually reduced as the electron-withdrawing power of the acceptor part of the dye molecule is enhanced by the incorporation of fluorine
Summary
A molecular design is presented for tailoring the energy levels in D-p-A organic dyes through fluorination of their acceptor units, which is aimed at achieving efficient dye-sensitized solar cells (DSSCs). The use of ruthenium polypyridine complexes as a photosensitizing dye has allowed conversion efficiencies of more than 11% to be achieved, which results from the broad absorption spectra created by metal-to-ligand charge transfer (MLCT) and favorable photovoltaic properties[5,6,9,10,11] Their widespread application has been limited by the scarcity and high cost of Ru, problems associated with isomerization or decomposition during the purification process, and a low molar extinction coefficient[12,13]. We discuss the gradual change in the optical and photovoltaic properties of the synthesized organic dyes as the number of incorporated fluorine atoms increases, and explore the effect this has on the energetic and kinetic characteristics of the photoanode in DSSCs
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