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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

Calculated frontier molecular orbitals and experimental energy-level diagrams of the M series dyes.
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f3: Calculated frontier molecular orbitals and experimental energy-level diagrams of the M series dyes.

Mentions: To compare the electronic structures of the M series dyes, the HOMO and LUMO electron density surfaces for their frontier molecular orbitals were analyzed by density functional theory (DFT) using the hybrid B3LYP32 function with a 6-31G*3334 basis set. As shown in Fig. 3, electrons are uniformly delocalized from the donor to the π-bridge at the HOMO level in all of the investigated dyes, which is favorable for an efficient electron transfer1535. Meanwhile, at the LUMO level, the excited electron is shifted toward the cyanoacrylic acid acceptor unit and its adjacent benzene moiety. Since the M series has the same molecular structure with respect to the donor part and π-bridge, the electron density distribution at the HOMO level should be similar for each dye. Indeed, the calculated HOMO energy levels (EHOMO) are very similar at –4.95, –5.00 and –5.03 eV (versus a vacuum) for M5, M6 and M7, respectively. At the LUMO level, on the other hand, the electron density distribution varies slightly due to the additional acceptor unit. The LUMO of M6 has a greater contribution from the cyanoacrylic acid acceptor unit than is seen in M5, while the LUMO of M7 exhibits a further increase in the contribution from this acceptor unit. This trend indicates that the electron-withdrawing effect of fluorine leads to a more effective charge separation between the donor and acceptor part22. Consequently, the calculated LUMO energy level (ELUMO) gradually decreases with an increasing number of incorporated fluorine atoms, giving a value of –2.65, –2.74 and –2.80 eV (versus vacuum) for M5, M6 and M7, respectively. This trend in the calculated EHOMO and ELUMO values matches well with the experimental data obtained from cyclic voltammograms (CVs) and UV-vis spectra listed in Table 1. For example, the EHOMO values determined from the oxidation potential of each dye were all very similar at 1.10, 1.13 and 1.09 V (versus NHE) for M5, M6 and M7, respectively. Similar agreement is also evident in the ELUMO values estimated from the 0-0 transition energy (E0-0) of –1.47, –1.38 and –1.21 V (versus NHE) for M5, M6 and M7, respectively. As predicted by the DFT calculations, the magnitude of ELUMO gradually lowers as the electron-withdrawing power of the acceptor part of the dye molecule is enhanced by additional acceptor units. This means that compared to the conduction-band potential (Ecb) of TiO2 (–0.28 V versus NHE)36, each dye has a sufficiently high ELUMO to meet the minimum energy difference (~0.4 eV) needed for effective electron injection from a dye to the conduction band of TiO237.


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)

Calculated frontier molecular orbitals and experimental energy-level diagrams of the M series dyes.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Calculated frontier molecular orbitals and experimental energy-level diagrams of the M series dyes.
Mentions: To compare the electronic structures of the M series dyes, the HOMO and LUMO electron density surfaces for their frontier molecular orbitals were analyzed by density functional theory (DFT) using the hybrid B3LYP32 function with a 6-31G*3334 basis set. As shown in Fig. 3, electrons are uniformly delocalized from the donor to the π-bridge at the HOMO level in all of the investigated dyes, which is favorable for an efficient electron transfer1535. Meanwhile, at the LUMO level, the excited electron is shifted toward the cyanoacrylic acid acceptor unit and its adjacent benzene moiety. Since the M series has the same molecular structure with respect to the donor part and π-bridge, the electron density distribution at the HOMO level should be similar for each dye. Indeed, the calculated HOMO energy levels (EHOMO) are very similar at –4.95, –5.00 and –5.03 eV (versus a vacuum) for M5, M6 and M7, respectively. At the LUMO level, on the other hand, the electron density distribution varies slightly due to the additional acceptor unit. The LUMO of M6 has a greater contribution from the cyanoacrylic acid acceptor unit than is seen in M5, while the LUMO of M7 exhibits a further increase in the contribution from this acceptor unit. This trend indicates that the electron-withdrawing effect of fluorine leads to a more effective charge separation between the donor and acceptor part22. Consequently, the calculated LUMO energy level (ELUMO) gradually decreases with an increasing number of incorporated fluorine atoms, giving a value of –2.65, –2.74 and –2.80 eV (versus vacuum) for M5, M6 and M7, respectively. This trend in the calculated EHOMO and ELUMO values matches well with the experimental data obtained from cyclic voltammograms (CVs) and UV-vis spectra listed in Table 1. For example, the EHOMO values determined from the oxidation potential of each dye were all very similar at 1.10, 1.13 and 1.09 V (versus NHE) for M5, M6 and M7, respectively. Similar agreement is also evident in the ELUMO values estimated from the 0-0 transition energy (E0-0) of –1.47, –1.38 and –1.21 V (versus NHE) for M5, M6 and M7, respectively. As predicted by the DFT calculations, the magnitude of ELUMO gradually lowers as the electron-withdrawing power of the acceptor part of the dye molecule is enhanced by additional acceptor units. This means that compared to the conduction-band potential (Ecb) of TiO2 (–0.28 V versus NHE)36, each dye has a sufficiently high ELUMO to meet the minimum energy difference (~0.4 eV) needed for effective electron injection from a dye to the conduction band of TiO237.

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