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Tailoring of energy levels in D-π-A organic dyes via fluorination of acceptor units for efficient dye-sensitized solar cells.

Lee MW, Kim JY, Son HJ, Kim JY, Kim B, Kim H, Lee DK, Kim K, Lee DH, Ko MJ - Sci Rep (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: 1] Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Korea [2] Department of Chemistry, Sogang University, Seoul, 121-742, Korea.

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.

No MeSH data available.


Related in: MedlinePlus

Optical properties of M series dyes.(a) UV-vis absorption spectra in THF. (b) UV-vis absorption spectra on 2-μm-thick TiO2 films.
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f2: Optical properties of M series dyes.(a) UV-vis absorption spectra in THF. (b) UV-vis absorption spectra on 2-μm-thick TiO2 films.

Mentions: The UV-vis absorption spectra of the M series dyes in THF are shown in Fig. 2a, and their corresponding optical data is listed in Table 1. Generally speaking, D-π-A organic dyes exhibit two major absorption bands, namely the intramolecular charge transfer (ICT) band in the visible region and the local π–π* absorption band in the UV region2829. The M series dyes, however, also exhibit two distinct absorption bands in the UV (360–370 nm) and visible region (410–430 nm), with ICT absorption peaks (λmax) observed at 415, 422 and 428 nm for M5, M6 and M7, respectively. This clearly demonstrates that in the case of organic dyes with a D-π-A-A configuration (i.e., M6 and M7), the absorption peak and onset are red-shifted in comparison to the absorption behavior of the more common D-π-A configuration (M5). Furthermore, even the only difference in chemical structure between M6 and M5 is the presence of one additional electron-withdrawing unit, i.e., fluorine at the ortho-position of the cyanoacetic acid (Fig. 1), this is nevertheless sufficient to induce a 7 nm red-shift in the ICT absorption peak. What is more, the two fluorine atoms incorporated at ortho-positions in M7 create an even greater red-shift of 13 nm. This gradual increase in the red-shift of the absorption peak with the number of incorporated fluorine atoms can be attributed to a lowering of the LUMO energy level due to the electron-withdrawing effect of fluorine22; and as a result, the HOMO-LUMO energy gap (E0-0) is also reduced from 2.57 eV in M5 to 2.5 and 2.30 eV in M6 and M7, respectively. Moreover, as shown in Table 1, a difluoro-substituted dye (M7) exhibits not only the most extended absorption range, but also has an enhanced molar extinction coefficient (ε) when compared to a monofluoro-substituted dye (M6).


Tailoring of energy levels in D-π-A organic dyes via fluorination of acceptor units for efficient dye-sensitized solar cells.

Lee MW, Kim JY, Son HJ, Kim JY, Kim B, Kim H, Lee DK, Kim K, Lee DH, Ko MJ - Sci Rep (2015)

Optical properties of M series dyes.(a) UV-vis absorption spectra in THF. (b) UV-vis absorption spectra on 2-μm-thick TiO2 films.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4296309&req=5

f2: Optical properties of M series dyes.(a) UV-vis absorption spectra in THF. (b) UV-vis absorption spectra on 2-μm-thick TiO2 films.
Mentions: The UV-vis absorption spectra of the M series dyes in THF are shown in Fig. 2a, and their corresponding optical data is listed in Table 1. Generally speaking, D-π-A organic dyes exhibit two major absorption bands, namely the intramolecular charge transfer (ICT) band in the visible region and the local π–π* absorption band in the UV region2829. The M series dyes, however, also exhibit two distinct absorption bands in the UV (360–370 nm) and visible region (410–430 nm), with ICT absorption peaks (λmax) observed at 415, 422 and 428 nm for M5, M6 and M7, respectively. This clearly demonstrates that in the case of organic dyes with a D-π-A-A configuration (i.e., M6 and M7), the absorption peak and onset are red-shifted in comparison to the absorption behavior of the more common D-π-A configuration (M5). Furthermore, even the only difference in chemical structure between M6 and M5 is the presence of one additional electron-withdrawing unit, i.e., fluorine at the ortho-position of the cyanoacetic acid (Fig. 1), this is nevertheless sufficient to induce a 7 nm red-shift in the ICT absorption peak. What is more, the two fluorine atoms incorporated at ortho-positions in M7 create an even greater red-shift of 13 nm. This gradual increase in the red-shift of the absorption peak with the number of incorporated fluorine atoms can be attributed to a lowering of the LUMO energy level due to the electron-withdrawing effect of fluorine22; and as a result, the HOMO-LUMO energy gap (E0-0) is also reduced from 2.57 eV in M5 to 2.5 and 2.30 eV in M6 and M7, respectively. Moreover, as shown in Table 1, a difluoro-substituted dye (M7) exhibits not only the most extended absorption range, but also has an enhanced molar extinction coefficient (ε) when compared to a monofluoro-substituted dye (M6).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: 1] Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Korea [2] Department of Chemistry, Sogang University, Seoul, 121-742, Korea.

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.

No MeSH data available.


Related in: MedlinePlus