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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.


Chemical structure of M series dyes.
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f1: Chemical structure of M series dyes.

Mentions: In light of the limitations of existing organic dyes, we propose a simple molecular design to tailor the energy levels in D-π-A organic dyes through the fluorination of their acceptor units, with the aim of creating more efficient dye-sensitizers for solar cells. Through this, we demonstrate that the LUMO energy level of the D-π-A organic sensitizer can be gradually lowered by increasing the number of fluorine atoms introduced as additional acceptor units due to their electron-withdrawing ability. Using the chemical structure of a common D-π-A organic dye (M5) as a basis, monofluoro- and difluoro-substituted organic dyes are synthesized using a simple method involving commercially available fluorobenzene derivatives, and are designated as M6 and M7, respectively (Fig. 1). The incorporated fluorine is intended to act as an additional acceptor unit to promote electron transfer to the anchoring group by means of its electron-withdrawing effect, which should result in a reduced HOMO-LUMO energy gap22. As a result, the synthesized D-π-A-A dyes exhibit a significantly improved spectral response that ensures a higher conversion efficiency than can be achieved with unmodified D-π-A organic dyes. Although there have been several studies in the past pertaining to the fluorination of organic dyes for DSSCs22232425, these all introduced fluorine at a single position within the molecular structure of the organic dye. Thus, to the best of our knowledge, this is the first demonstration that both the spectral response and conversion efficiency can be improved by increasing the incorporation of fluorine (i.e., monofluoro- and difluoro-substitution). We therefore herein 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.


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)

Chemical structure of M series dyes.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Chemical structure of M series dyes.
Mentions: In light of the limitations of existing organic dyes, we propose a simple molecular design to tailor the energy levels in D-π-A organic dyes through the fluorination of their acceptor units, with the aim of creating more efficient dye-sensitizers for solar cells. Through this, we demonstrate that the LUMO energy level of the D-π-A organic sensitizer can be gradually lowered by increasing the number of fluorine atoms introduced as additional acceptor units due to their electron-withdrawing ability. Using the chemical structure of a common D-π-A organic dye (M5) as a basis, monofluoro- and difluoro-substituted organic dyes are synthesized using a simple method involving commercially available fluorobenzene derivatives, and are designated as M6 and M7, respectively (Fig. 1). The incorporated fluorine is intended to act as an additional acceptor unit to promote electron transfer to the anchoring group by means of its electron-withdrawing effect, which should result in a reduced HOMO-LUMO energy gap22. As a result, the synthesized D-π-A-A dyes exhibit a significantly improved spectral response that ensures a higher conversion efficiency than can be achieved with unmodified D-π-A organic dyes. Although there have been several studies in the past pertaining to the fluorination of organic dyes for DSSCs22232425, these all introduced fluorine at a single position within the molecular structure of the organic dye. Thus, to the best of our knowledge, this is the first demonstration that both the spectral response and conversion efficiency can be improved by increasing the incorporation of fluorine (i.e., monofluoro- and difluoro-substitution). We therefore herein 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.

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.