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Analytical model for the photocurrent-voltage characteristics of bilayer MEH-PPV/TiO2 photovoltaic devices.

Chen C, Wu F, Geng H, Shen W, Wang M - Nanoscale Res Lett (2011)

Bottom Line: An analytical model is developed for the photocurrent-voltage characteristics of the bilayer polymer/TiO2 photovoltaic cells.Bilayer polymer/TiO2 cells consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and TiO2, with different thicknesses of the polymer and TiO2 films, were prepared for experimental purposes.The experimental results for the prepared bilayer MEH-PPV/TiO2 cells under different conditions are satisfactorily fitted to the model.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, PR China. mtwang@ipp.ac.cn.

ABSTRACT
The photocurrent in bilayer polymer photovoltaic cells is dominated by the exciton dissociation efficiency at donor/acceptor interface. An analytical model is developed for the photocurrent-voltage characteristics of the bilayer polymer/TiO2 photovoltaic cells. The model gives an analytical expression for the exciton dissociation efficiency at the interface, and explains the dependence of the photocurrent of the devices on the internal electric field, the polymer and TiO2 layer thicknesses. Bilayer polymer/TiO2 cells consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and TiO2, with different thicknesses of the polymer and TiO2 films, were prepared for experimental purposes. The experimental results for the prepared bilayer MEH-PPV/TiO2 cells under different conditions are satisfactorily fitted to the model. Results show that increasing TiO2 or the polymer layer in thickness will reduce the exciton dissociation efficiency in the device and further the photocurrent. It is found that the photocurrent is determined by the competition between the exciton dissociation and charge recombination at the donor/acceptor interface, and the increase in photocurrent under a higher incident light intensity is due to the increased exciton density rather than the increase in the exciton dissociation efficiency.

No MeSH data available.


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The measured and fitted photocurrent-voltage curves for ITO/TiO2/MEH-PPV/Au devices. (a-c) Panels are for the devices with different TiO2 and MEH-PPV layer thicknesses measured under the same illumination intensity; while (c, d) panels are used to show the influence of illumination intensity on the same device. The incident intensity was 15.85 mW/cm2 (a-c), 3.0 mW/cm2 (d) and 9.6 mW/cm2 (e). The k0/v0 values obtained by fitting the experimental data to Equation 15 are marked on the respective panels.
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Figure 4: The measured and fitted photocurrent-voltage curves for ITO/TiO2/MEH-PPV/Au devices. (a-c) Panels are for the devices with different TiO2 and MEH-PPV layer thicknesses measured under the same illumination intensity; while (c, d) panels are used to show the influence of illumination intensity on the same device. The incident intensity was 15.85 mW/cm2 (a-c), 3.0 mW/cm2 (d) and 9.6 mW/cm2 (e). The k0/v0 values obtained by fitting the experimental data to Equation 15 are marked on the respective panels.

Mentions: In order to calculate the electric fields Ep and En, the accumulated charge density at the D/A interface is assumed to be a constant and Q=1.0 × 10-4 C/m2 [33]. We find that Q has a weak influence on the calculated results by our model, for which the reason may be that the internal electric field in the devices is only slightly modified due to the band bending created by the accumulation of the charge carriers at the D/A interface [24]. Therefore, it is reasonable that we simply assume Q is a constant. In spite of the parameters εp = 4ε0 which is comparable to εp = 3ε0 [45], εn = 55ε0 [49], αp(λ = 470 nm) = 105 cm-1, and Lp = 15 nm [12,13,50], there are still three parameters (i.e., λ, k0/v0, and β) needed to obtain Jph by Equation 15. Our calculated data revealed that the shape of Jph-V curve is strongly dependent on the values of λ, but less dependent on the values of k0/v0 and β. Therefore, the parameter λ can be first obtained by curve fitting taking the order of magnitude of 10-5 for k0/v0 and that of 10-3 for β; then, the values of k0/v0 and β can be obtained by the best fit. In our model, we take λ = 3 and β is a constant with a value of 5 × 10-3 V-2. Finally, the ratio k0/v0 is the only adjustable fit parameter in fitting the experimental photocurrent. Since k0 and v0 are zero-field recombination rate constants, the ratio k0/v0 is independent of the electric field. However, the ratio k0/v0 depends on the used materials or the geometry of the devices [48] such as the TiO2 (polymer) film thickness as shown in Figure 4.


Analytical model for the photocurrent-voltage characteristics of bilayer MEH-PPV/TiO2 photovoltaic devices.

Chen C, Wu F, Geng H, Shen W, Wang M - Nanoscale Res Lett (2011)

The measured and fitted photocurrent-voltage curves for ITO/TiO2/MEH-PPV/Au devices. (a-c) Panels are for the devices with different TiO2 and MEH-PPV layer thicknesses measured under the same illumination intensity; while (c, d) panels are used to show the influence of illumination intensity on the same device. The incident intensity was 15.85 mW/cm2 (a-c), 3.0 mW/cm2 (d) and 9.6 mW/cm2 (e). The k0/v0 values obtained by fitting the experimental data to Equation 15 are marked on the respective panels.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211440&req=5

Figure 4: The measured and fitted photocurrent-voltage curves for ITO/TiO2/MEH-PPV/Au devices. (a-c) Panels are for the devices with different TiO2 and MEH-PPV layer thicknesses measured under the same illumination intensity; while (c, d) panels are used to show the influence of illumination intensity on the same device. The incident intensity was 15.85 mW/cm2 (a-c), 3.0 mW/cm2 (d) and 9.6 mW/cm2 (e). The k0/v0 values obtained by fitting the experimental data to Equation 15 are marked on the respective panels.
Mentions: In order to calculate the electric fields Ep and En, the accumulated charge density at the D/A interface is assumed to be a constant and Q=1.0 × 10-4 C/m2 [33]. We find that Q has a weak influence on the calculated results by our model, for which the reason may be that the internal electric field in the devices is only slightly modified due to the band bending created by the accumulation of the charge carriers at the D/A interface [24]. Therefore, it is reasonable that we simply assume Q is a constant. In spite of the parameters εp = 4ε0 which is comparable to εp = 3ε0 [45], εn = 55ε0 [49], αp(λ = 470 nm) = 105 cm-1, and Lp = 15 nm [12,13,50], there are still three parameters (i.e., λ, k0/v0, and β) needed to obtain Jph by Equation 15. Our calculated data revealed that the shape of Jph-V curve is strongly dependent on the values of λ, but less dependent on the values of k0/v0 and β. Therefore, the parameter λ can be first obtained by curve fitting taking the order of magnitude of 10-5 for k0/v0 and that of 10-3 for β; then, the values of k0/v0 and β can be obtained by the best fit. In our model, we take λ = 3 and β is a constant with a value of 5 × 10-3 V-2. Finally, the ratio k0/v0 is the only adjustable fit parameter in fitting the experimental photocurrent. Since k0 and v0 are zero-field recombination rate constants, the ratio k0/v0 is independent of the electric field. However, the ratio k0/v0 depends on the used materials or the geometry of the devices [48] such as the TiO2 (polymer) film thickness as shown in Figure 4.

Bottom Line: An analytical model is developed for the photocurrent-voltage characteristics of the bilayer polymer/TiO2 photovoltaic cells.Bilayer polymer/TiO2 cells consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and TiO2, with different thicknesses of the polymer and TiO2 films, were prepared for experimental purposes.The experimental results for the prepared bilayer MEH-PPV/TiO2 cells under different conditions are satisfactorily fitted to the model.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, PR China. mtwang@ipp.ac.cn.

ABSTRACT
The photocurrent in bilayer polymer photovoltaic cells is dominated by the exciton dissociation efficiency at donor/acceptor interface. An analytical model is developed for the photocurrent-voltage characteristics of the bilayer polymer/TiO2 photovoltaic cells. The model gives an analytical expression for the exciton dissociation efficiency at the interface, and explains the dependence of the photocurrent of the devices on the internal electric field, the polymer and TiO2 layer thicknesses. Bilayer polymer/TiO2 cells consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and TiO2, with different thicknesses of the polymer and TiO2 films, were prepared for experimental purposes. The experimental results for the prepared bilayer MEH-PPV/TiO2 cells under different conditions are satisfactorily fitted to the model. Results show that increasing TiO2 or the polymer layer in thickness will reduce the exciton dissociation efficiency in the device and further the photocurrent. It is found that the photocurrent is determined by the competition between the exciton dissociation and charge recombination at the donor/acceptor interface, and the increase in photocurrent under a higher incident light intensity is due to the increased exciton density rather than the increase in the exciton dissociation efficiency.

No MeSH data available.


Related in: MedlinePlus