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Investigation into Photoconductivity in Single CNF/TiO(2)-Dye Core-Shell Nanowire Devices.

Li Z, Rochford C, Javier Baca F, Liu J, Li J, Wu J - Nanoscale Res Lett (2010)

Bottom Line: A vertically aligned carbon nanofiber array coated with anatase TiO(2) (CNF/TiO(2)) is an attractive possible replacement for the sintered TiO(2) nanoparticle network in the original dye-sensitized solar cell (DSSC) design due to the potential for improved charge transport and reduced charge recombination.Although the reported efficiency of 1.1% in these modified DSSC's is encouraging, the limiting factors must be identified before a higher efficiency can be obtained.This work employs a single nanowire approach to investigate the charge transport in individual CNF/TiO(2) core-shell nanowires with adsorbed N719 dye molecules in dark and under illumination.

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ABSTRACT
A vertically aligned carbon nanofiber array coated with anatase TiO(2) (CNF/TiO(2)) is an attractive possible replacement for the sintered TiO(2) nanoparticle network in the original dye-sensitized solar cell (DSSC) design due to the potential for improved charge transport and reduced charge recombination. Although the reported efficiency of 1.1% in these modified DSSC's is encouraging, the limiting factors must be identified before a higher efficiency can be obtained. This work employs a single nanowire approach to investigate the charge transport in individual CNF/TiO(2) core-shell nanowires with adsorbed N719 dye molecules in dark and under illumination. The results shed light on the role of charge traps and dye adsorption on the (photo) conductivity of nanocrystalline TiO(2) CNF's as related to dye-sensitized solar cell performance.

No MeSH data available.


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Normalized photo-induced current decay of Sample d2 a without dye and b with dye. Inset shows the same data on a linear scale
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Figure 4: Normalized photo-induced current decay of Sample d2 a without dye and b with dye. Inset shows the same data on a linear scale

Mentions: While the previously mentioned I–V measurements were made under steady state conditions, the samples in fact exhibit a transient response to either the introduction or removal of incident light. The presence of transient photoconductivity is well documented in nanocrystalline TiO2 thin films [24,25,33], but is interesting to observe in a single NW. In this measurement, the samples were first exposed to one sun illumination at a constant bias of 100 mV. The high surface-to-volume ratio and thus high density of electron traps due to hydroxylated surface Ti sites [34] prevents achievement of steady state current until all traps are filled and equilibrium is reached between trapping and de-trapping events. It was observed (not shown) that after dye attachment, the time required to reach steady state current was reduced by approximately 75%. This suggests that the dye molecules may passivate many of the hydroxylated surface Ti sites and greatly reduce the number of electron traps. However, since nearly 1 min is still required to reach steady state current after dye molecules are attached, it is likely that many potential dye adsorption sites remain and the dye loading is not optimized. Once a steady state current was achieved, the incident light was removed and the photo-induced current decay was recorded. Figure 4 shows the normalized photo-induced current decay profile in log–log and linear scale (inset) for samples without and with attached dye molecules. The mechanism responsible for the observed slow current decay may be described as follows. As previously mentioned, under illumination, nonequilibrium holes become trapped in deep traps due to oxygen vacancies leading to an excess electron density equal to the trapped hole density [35]. These traps are assumed to be concentrated in certain regions due to inhomogeneities such as grain boundaries. This leads to local electric fields that spatially separate charge carriers and require electrons to overcome a potential barrier in order to recombine [21]. As time goes on, the separation between the quasi Fermi levels increases causing the recombination time to increase along with it. As can be seen in Fig. 4, the decrease in current is immediate and initially very fast followed by a much slower leveling off as the probability for the electron capture by tunneling through surface and inter-grain potential barriers decreases, which is consistent with previous work done on nanocrystalline TiO2 thin films [25]. It can be clearly seen that the photoconductivity decay is more rapid when the dye is present. This is consistent with the above observations that the dye attachment passivates some hole traps on the surface. As the number of hole traps decreases, the recombination time also decreases [25].


Investigation into Photoconductivity in Single CNF/TiO(2)-Dye Core-Shell Nanowire Devices.

Li Z, Rochford C, Javier Baca F, Liu J, Li J, Wu J - Nanoscale Res Lett (2010)

Normalized photo-induced current decay of Sample d2 a without dye and b with dye. Inset shows the same data on a linear scale
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2926887&req=5

Figure 4: Normalized photo-induced current decay of Sample d2 a without dye and b with dye. Inset shows the same data on a linear scale
Mentions: While the previously mentioned I–V measurements were made under steady state conditions, the samples in fact exhibit a transient response to either the introduction or removal of incident light. The presence of transient photoconductivity is well documented in nanocrystalline TiO2 thin films [24,25,33], but is interesting to observe in a single NW. In this measurement, the samples were first exposed to one sun illumination at a constant bias of 100 mV. The high surface-to-volume ratio and thus high density of electron traps due to hydroxylated surface Ti sites [34] prevents achievement of steady state current until all traps are filled and equilibrium is reached between trapping and de-trapping events. It was observed (not shown) that after dye attachment, the time required to reach steady state current was reduced by approximately 75%. This suggests that the dye molecules may passivate many of the hydroxylated surface Ti sites and greatly reduce the number of electron traps. However, since nearly 1 min is still required to reach steady state current after dye molecules are attached, it is likely that many potential dye adsorption sites remain and the dye loading is not optimized. Once a steady state current was achieved, the incident light was removed and the photo-induced current decay was recorded. Figure 4 shows the normalized photo-induced current decay profile in log–log and linear scale (inset) for samples without and with attached dye molecules. The mechanism responsible for the observed slow current decay may be described as follows. As previously mentioned, under illumination, nonequilibrium holes become trapped in deep traps due to oxygen vacancies leading to an excess electron density equal to the trapped hole density [35]. These traps are assumed to be concentrated in certain regions due to inhomogeneities such as grain boundaries. This leads to local electric fields that spatially separate charge carriers and require electrons to overcome a potential barrier in order to recombine [21]. As time goes on, the separation between the quasi Fermi levels increases causing the recombination time to increase along with it. As can be seen in Fig. 4, the decrease in current is immediate and initially very fast followed by a much slower leveling off as the probability for the electron capture by tunneling through surface and inter-grain potential barriers decreases, which is consistent with previous work done on nanocrystalline TiO2 thin films [25]. It can be clearly seen that the photoconductivity decay is more rapid when the dye is present. This is consistent with the above observations that the dye attachment passivates some hole traps on the surface. As the number of hole traps decreases, the recombination time also decreases [25].

Bottom Line: A vertically aligned carbon nanofiber array coated with anatase TiO(2) (CNF/TiO(2)) is an attractive possible replacement for the sintered TiO(2) nanoparticle network in the original dye-sensitized solar cell (DSSC) design due to the potential for improved charge transport and reduced charge recombination.Although the reported efficiency of 1.1% in these modified DSSC's is encouraging, the limiting factors must be identified before a higher efficiency can be obtained.This work employs a single nanowire approach to investigate the charge transport in individual CNF/TiO(2) core-shell nanowires with adsorbed N719 dye molecules in dark and under illumination.

View Article: PubMed Central - HTML - PubMed

ABSTRACT
A vertically aligned carbon nanofiber array coated with anatase TiO(2) (CNF/TiO(2)) is an attractive possible replacement for the sintered TiO(2) nanoparticle network in the original dye-sensitized solar cell (DSSC) design due to the potential for improved charge transport and reduced charge recombination. Although the reported efficiency of 1.1% in these modified DSSC's is encouraging, the limiting factors must be identified before a higher efficiency can be obtained. This work employs a single nanowire approach to investigate the charge transport in individual CNF/TiO(2) core-shell nanowires with adsorbed N719 dye molecules in dark and under illumination. The results shed light on the role of charge traps and dye adsorption on the (photo) conductivity of nanocrystalline TiO(2) CNF's as related to dye-sensitized solar cell performance.

No MeSH data available.


Related in: MedlinePlus