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Efficient Performance of Electrostatic Spray-Deposited TiO 2 Blocking Layers in Dye-Sensitized Solar Cells after Swift Heavy Ion Beam Irradiation

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ABSTRACT

A compact TiO2 layer (~1.1 μm) prepared by electrostatic spray deposition (ESD) and swift heavy ion beam (SHI) irradiation using oxygen ions onto a fluorinated tin oxide (FTO) conducting substrate showed enhancement of photovoltaic performance in dye-sensitized solar cells (DSSCs). The short circuit current density (Jsc = 12.2 mA cm-2) of DSSCs was found to increase significantly when an ESD technique was applied for fabrication of the TiO2 blocking layer, compared to a conventional spin-coated layer (Jsc = 8.9 mA cm-2). When SHI irradiation of oxygen ions of fluence 1 × 1013 ions/cm2 was carried out on the ESD TiO2, it was found that the energy conversion efficiency improved mainly due to the increase in open circuit voltage of DSSCs. This increased energy conversion efficiency seems to be associated with improved electronic energy transfer by increasing the densification of the blocking layer and improving the adhesion between the blocking layer and the FTO substrate. The adhesion results from instantaneous local melting of the TiO2 particles. An increase in the electron transport from the blocking layer may also retard the electron recombination process due to the oxidized species present in the electrolyte. These findings from novel treatments using ESD and SHI irradiation techniques may provide a new tool to improve the photovoltaic performance of DSSCs.

No MeSH data available.


Cross-sectional FE-SEM images of a bare FTO substrate, b pristine TiO2/FTO, and c O2 ion-irradiated TiO2/FTO. The thickness of the pristine and irradiated TiO2 was about 1.1 and 0.67 μm, respectively. (Inset: images in 100 nm scale.)
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Figure 4: Cross-sectional FE-SEM images of a bare FTO substrate, b pristine TiO2/FTO, and c O2 ion-irradiated TiO2/FTO. The thickness of the pristine and irradiated TiO2 was about 1.1 and 0.67 μm, respectively. (Inset: images in 100 nm scale.)

Mentions: Surface morphologies of the pristine and the SHI-irradiated TiO2 films are presented in Figure 3. The electrosprayed TiO2 films reveal an aggregation pattern, and the spherical particles form an interconnected porous framework of nano-sized building blocks (Figure 3). The observed nano-aggregated particles may be ascribed to the existence of a Coulumbic force lower than the stretching force resulting from weak repulsion between adjacent spray droplets. Under SHI irradiation, these nano-aggregated TiO2 particles melted and solidified on the FTO substrate and consequently formed a rather flat, nonporous structure with the FTO layer (see Figure 3). This results in a compact interface at FTO/TiO2 for both blocking electron recombination and increasing electronic transport. The fragmentation of the aggregated particles into smaller grains under SHI irradiation can be explained by a thermal spike model. If a large amount of energy is deposited by the projectile ions to the electronic subsystem of the target material, this energy can be shared among electrons by electron–electron coupling and later transferred quickly to the surrounding lattice through electron–phonon coupling. Thus, a sudden temperature rise on the time scale of 10-12 s along the ion track resulted in a molten state. The subsequent heat transfer to the surrounding lattice results in resolidification of this molten liquid phase. If this cooling rate slows to a critical value, nucleation of crystalline phases can be expected along the ion trajectory [28,29]. Therefore, we speculate that the surface of the TiO2 particles may undergo an ion-beam-induced molten state in a short duration of time (10-12 s). These molten state particles were attached with FTO substrate, enhancing the inter-particle connectivity (compact) to improve the conductivity of the film. The measured conductivity of the pristine and the SHI-irradiated TiO2 films found to be 2.31 × 10-2 and 1.2 Scm-1, respectively, indicating large improvement in the electron conductivity. Cross-sectional SEM images of the pristine and the SHI-irradiated TiO2 films are illustrated in Figure 4. Figure 4b suggests that the pristine ESD TiO2 layer has nano-aggregates and an inhomogeneous interface (contact) with the FTO layer, mostly due to the removal of polymer templates from ESD coating during sintering treatment. The observed inhomogeneous TiO2/FTO interface in the pristine sample was further compressed by SHI irradiation using O2 ions. This interface modification was confirmed by Figure 4c, showing that the TiO2 particles adhered well to the FTO layer. The thickness of the pristine film, ~1.1 μm, was reduced to ~0.67 μm after O ion irradiation. This is ascribed to the compact nature of TiO2 film formed by SHI irradiation. It is noteworthy to mention that improving the compact nature of the TiO2 blocking layer upon SHI irradiation can facilitate electron transport and also reduce electron recombination back to the electrolyte.


Efficient Performance of Electrostatic Spray-Deposited TiO 2 Blocking Layers in Dye-Sensitized Solar Cells after Swift Heavy Ion Beam Irradiation
Cross-sectional FE-SEM images of a bare FTO substrate, b pristine TiO2/FTO, and c O2 ion-irradiated TiO2/FTO. The thickness of the pristine and irradiated TiO2 was about 1.1 and 0.67 μm, respectively. (Inset: images in 100 nm scale.)
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Related In: Results  -  Collection

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Figure 4: Cross-sectional FE-SEM images of a bare FTO substrate, b pristine TiO2/FTO, and c O2 ion-irradiated TiO2/FTO. The thickness of the pristine and irradiated TiO2 was about 1.1 and 0.67 μm, respectively. (Inset: images in 100 nm scale.)
Mentions: Surface morphologies of the pristine and the SHI-irradiated TiO2 films are presented in Figure 3. The electrosprayed TiO2 films reveal an aggregation pattern, and the spherical particles form an interconnected porous framework of nano-sized building blocks (Figure 3). The observed nano-aggregated particles may be ascribed to the existence of a Coulumbic force lower than the stretching force resulting from weak repulsion between adjacent spray droplets. Under SHI irradiation, these nano-aggregated TiO2 particles melted and solidified on the FTO substrate and consequently formed a rather flat, nonporous structure with the FTO layer (see Figure 3). This results in a compact interface at FTO/TiO2 for both blocking electron recombination and increasing electronic transport. The fragmentation of the aggregated particles into smaller grains under SHI irradiation can be explained by a thermal spike model. If a large amount of energy is deposited by the projectile ions to the electronic subsystem of the target material, this energy can be shared among electrons by electron–electron coupling and later transferred quickly to the surrounding lattice through electron–phonon coupling. Thus, a sudden temperature rise on the time scale of 10-12 s along the ion track resulted in a molten state. The subsequent heat transfer to the surrounding lattice results in resolidification of this molten liquid phase. If this cooling rate slows to a critical value, nucleation of crystalline phases can be expected along the ion trajectory [28,29]. Therefore, we speculate that the surface of the TiO2 particles may undergo an ion-beam-induced molten state in a short duration of time (10-12 s). These molten state particles were attached with FTO substrate, enhancing the inter-particle connectivity (compact) to improve the conductivity of the film. The measured conductivity of the pristine and the SHI-irradiated TiO2 films found to be 2.31 × 10-2 and 1.2 Scm-1, respectively, indicating large improvement in the electron conductivity. Cross-sectional SEM images of the pristine and the SHI-irradiated TiO2 films are illustrated in Figure 4. Figure 4b suggests that the pristine ESD TiO2 layer has nano-aggregates and an inhomogeneous interface (contact) with the FTO layer, mostly due to the removal of polymer templates from ESD coating during sintering treatment. The observed inhomogeneous TiO2/FTO interface in the pristine sample was further compressed by SHI irradiation using O2 ions. This interface modification was confirmed by Figure 4c, showing that the TiO2 particles adhered well to the FTO layer. The thickness of the pristine film, ~1.1 μm, was reduced to ~0.67 μm after O ion irradiation. This is ascribed to the compact nature of TiO2 film formed by SHI irradiation. It is noteworthy to mention that improving the compact nature of the TiO2 blocking layer upon SHI irradiation can facilitate electron transport and also reduce electron recombination back to the electrolyte.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

A compact TiO2 layer (~1.1 μm) prepared by electrostatic spray deposition (ESD) and swift heavy ion beam (SHI) irradiation using oxygen ions onto a fluorinated tin oxide (FTO) conducting substrate showed enhancement of photovoltaic performance in dye-sensitized solar cells (DSSCs). The short circuit current density (Jsc = 12.2 mA cm-2) of DSSCs was found to increase significantly when an ESD technique was applied for fabrication of the TiO2 blocking layer, compared to a conventional spin-coated layer (Jsc = 8.9 mA cm-2). When SHI irradiation of oxygen ions of fluence 1 × 1013 ions/cm2 was carried out on the ESD TiO2, it was found that the energy conversion efficiency improved mainly due to the increase in open circuit voltage of DSSCs. This increased energy conversion efficiency seems to be associated with improved electronic energy transfer by increasing the densification of the blocking layer and improving the adhesion between the blocking layer and the FTO substrate. The adhesion results from instantaneous local melting of the TiO2 particles. An increase in the electron transport from the blocking layer may also retard the electron recombination process due to the oxidized species present in the electrolyte. These findings from novel treatments using ESD and SHI irradiation techniques may provide a new tool to improve the photovoltaic performance of DSSCs.

No MeSH data available.