WO3 passivation layer-coated nanostructured TiO2: An efficient defect engineered photoelectrode for dye sensitized solar cell

Arya Babu; Arya Vasanth; Shantikumar Nair; Mariyappan Shanmugam

doi:10.1088/1674-4926/42/5/052701

Abstract

Major loss factors for photo-generated electrons due to the presence of surface defects in titanium dioxide (TiO₂) were controlled by RF-sputtered tungsten trioxide (WO₃) passivation. X-ray photoelectron spectroscopy assured the coating of WO₃ on the TiO₂ nanoparticle layer by showing Ti 2p, W 4f and O 1s characteristic peaks and were further confirmed by X-ray diffraction studies. The coating of WO₃ on the TiO₂ nanoparticle layer did not affect dye adsorption significantly. Dye sensitized solar cells (DSSCs) fabricated using WO₃-coated TiO₂ showed an enhancement of ~10% compared to DSSCs fabricated using pristine TiO₂-based photo-electrodes. It is attributed to the WO₃ passivation on TiO₂ that creates an energy barrier which favored photo-electron injection by tunneling but blocked reverse electron recombination pathways towards holes available in highest occupied molecular orbital of the dye molecules. It was further evidenced that there is an optimum thickness (duration of coating) of WO₃ to improve the DSSC performance and longer duration of WO₃ suppressed photo-electron injection from dye to TiO₂ as inferred from the detrimental effect in short circuit current density values. RF-sputtering yields pinhole-free, highly uniform and conformal coating of WO₃ onto any area of interest, which can be considered for an effective surface passivation for nanostructured photovoltaic devices.

1. Introduction

The most important technological arrival of nanotechnology has endorsed multifarious nanostructured functional materials for energy conversion applications^[1-4]. Nanostructured functional materials have been showing superior characteristics in terms of structural, optical, electrical and mechanical perspectives^[5-8]. Size-dependent properties of nanomaterials and their consequences on various applications have been realized as a potential possible approach to improve performance of resulting products including photovoltaics, sensors, batteries and light-emitting diodes^{[2, 9-11]}. However, it has been observed that nanostructures, such as particles, fibers and planar materials, exhibit surface sensitive characteristics due to the presence of chemically unsaturated defect states which are established through the modified surface-to-volume ratio^[12-16]. Such remarkable changes over the surface of functional materials lead to unavoidable degeneracies, including surface states which can have energy distribution throughout the bandgap of resulting materials^[17-21]. Surface states in a material plays a critical role on charge transport and recombination as they are electronically active mostly in the vicinity of band-edges^{[22, 23]}. However, nanostructured functional materials cannot be avoided in energy conversion devices as they show dominant optical and electrical transport characteristics which are highly preferred specifically for light-matter interaction^[24-26]. Thus, alternate measures are required to employ such attractive materials for energy-related applications and those measures are expected to control the detrimental effects of surface defects on charge transport to minimize the recombination loss. A surface with active unsaturated dangling bonds is highly sensitive to charge transport in solar cells and thus it is an essential factor to passivate/modify using appropriate methods^[27-30].

A variety of metal oxides and dielectrics were used in DSSCs to control the recombination at TiO₂/dye/electrolyte interfaces^[30-38]. Thickness, dielectric properties, surface coverage and electronic quality of the passivation layers have been studied and reported to be considered as dominant factors which determine the ability to control the defects^[39-44]. Wet-chemical methods such as hydrothermal process, physical vapor deposition techniques, including RF/DC sputtering, electron beam/thermal evaporation and chemical vapor deposition, were widely used to apply passivation on functional materials such as TiO₂ in DSSCs^[45-49]. Atomic layer deposition has been identified as a superior technique to passivate nanostructured defective material as it often yields ultra-thin (few atomic layers) high-quality dielectrics which can be conformally coated onto any randomly distributed nanostructures^{[50, 51]}. The highly conformal surface coverage results in efficient passivation which controls the detrimental role of defects on charge transport in DSSCs^[52-54].

The present work identified tungsten oxide (WO₃) as a possible wide bandgap material which can be used for effective surface passivation on TiO₂ to modify the surface to achieve better control over defects. Results showed the possibility of improving DSSC performance by coating WO₃ via RF-sputtering on TiO₂ in which defects are most dominating in terms of interfacial recombination.

2. Experimental section

Commercially available fluorine-doped tin oxide (FTO) glass substrates were used for DSSC fabrication process. The FTO-coated glass substrates were cleaned by sonicating them in soap solution for 15 min, then in DI water and finally with ethanol for 15 min to remove all sort of contaminations. The cleaned FTO-coated glass substrates were kept in an oven for drying at 72 ˚C. Colloidal TiO₂ paste was prepared by mixing the required amount of TiO₂ powder (Sigma Aldrich, 99.9% purity) and polyethylene glycolate (PEG). Both were mixed and ground thoroughly using a mortar and pestle until a required form of colloidal paste was obtained. TiO₂ colloidal paste was doctor-bladed onto FTO-coated glass substrates using adhesive tape as a shadow-mask, which were removed after making the TiO₂ films. Colloidal TiO₂ films coating the FTO samples were then annealed at 450 ˚C for 3 h with a ramp rate of 3 ˚C per minute.

WO₃ thin film was coated on TiO₂ nanoparticle films using the RF sputtering technique. Samples were fixed on the substrate holder using temperature-sustainable, vacuum-compatible Kapton tape. They were placed in the RF-sputter chamber in which tungsten was used as target with oxygen as a bombarding process gas. The oxygen gas flow rate was maintained at 70 sccm with a FR-power of 25 W. To assess the effect of thickness of WO₃ surface passivation on TiO₂, the present study varied the thickness values of WO₃ to 15, 22, and 30 nm on TiO₂. This thickness variation was achieved by varying the duration of WO₃ coating on TiO₂ for 7, 10, and 15 min. The WO₃-coated TiO₂ nanoparticle layer on FTO was removed from the RF-sputter chamber and annealed at 120 ˚C.

In the present work, two categories of DSSCs were fabricated using (i) pristine TiO₂ and (ii) WO₃-coated TiO₂ to validate the effect of WO₃ passivation on TiO₂ nanoparticle layer in DSSC performance. Ruthenium-based N719 dye was prepared using a mixture of 50 mL of tert-butyl alcohol and 50 mL of acetonitrile. Then, 0.5 mM of N719 dye (59.43 mg for 100 mL) was added and stirred for 30 min to make the required homogeneous N719 dye. The pristine TiO₂ and WO_3-coated TiO₂ samples prepared on FTO-coated glass substrates were immersed in dye solution to sensitize. DSSCs were fabricated using N719-adsorbed pristine TiO₂ and WO₃-coated TiO₂ as photo-anodes to compare the effect of WO₃ passivation. Hole-transporting iodide/tri-iodide was used as an electrolyte along with a platinum counter electrode for the DSSCs. The only difference between the two categories of DSSCs was the WO₃ passivation on the TiO₂ nanoparticle layer.

Surface morphology of the pristine and WO₃-coated TiO₂ samples were characterized using a JSN 7610S Plus JEOL-JSM-6490-LA scanning electron microscope (FESEM). X-ray photoelectron spectroscopy (XPS) used a Kratos Analytical unit. X-ray diffraction (XRD) patterns of the WO₃ coated samples were obtained using PANalytical X’Pert PRO X-Ray Diffractometer. UV–visible optical spectra of the samples were obtained in a Perkin Elmer Lambda-750 UV–visible spectrometer. The fabricated DSSCs were characterized under 1-Sun illumination (AM 1.5 G) in a Newport Oriel Class A solar simulator to measure required J–V characteristics using a Keithley-2420 digital source meter.

3. Results and discussion

Figs. 1(a) and 1(b) show surface morphology of the TiO₂ nanoparticle layer coated on the FTO substrate for DSSC fabrication. Macroscopic clustered TiO₂ was observed at the low (1000×) magnification in Fig. 1(a), and a further magnified image exhibited smaller size TiO₂ particles agglomerated on the macroscopic clusters as shown in Fig. 1(b) which was obtained at the magnification of 40 000×. While the lower magnified image exhibited a macroscopic cluster film, the higher one showed the nanoparticles agglomerated as clusters. Figs. 1(c) and 1(d) were obtained from the TiO₂ nanoparticle layer after coating the WO₃ passivation layer at 15 and 30 nm, respectively. It is evident that the coating of thin WO₃ on the TiO₂ nanoparticle layer did not change the morphology significantly in terms of surface morphology. The WO₃ coating was performed by RF-sputtering, which is a gas-phase plasma assisted physical vapor deposition process to achieve uniform coating of functional material. The surface morphology images shown in Figs. 1(c) and 1(d) clearly addressed the pristine nature of the TiO₂ nanoparticle layer was not modified as the RF-sputtering process uniformly covered the TiO₂ nanoparticles and the coating did not block the porosity of the TiO_2, as can be seen in Figs. 1(a) and 1(b). In general, the surface passivation coating is applied on nanostructured functional materials in which defects are playing a dominated role on charge transport characteristics and deteriorates the performance of the resulting solar cells. Thus, it is essential to coat the surface passivation layer but if it is too thick or not uniform then the surface passivation layer itself might block the charge transport and thus recombination would dominate. In the present study, the surface morphology images prove that WO₃ did not affect TiO₂ morphology as it was thin enough and uniform.

Figure 1.Surface morphology of pristine TiO₂ at the magnifications of (a) 1000× and (b) 40 000× showing macroscopic clusters and agglomerated particles respectively, (c, d) WO₃ coated TiO₂ nanoparticles layer showing randomly distributed TiO₂ nanoparticles and the porous nature.

Fig. 2 shows results from XPS and XRD studies performed on the WO_3-coated TiO₂ nanoparticle layer. Fig. 2(a) shows a high-resolution XPS scan obtained from the sample showing Ti 2p peaks. Specifically, Ti 2p_3/2 and Ti 2p_1/2 were 457.4 and 463.1 eV respectively, showing the spin-orbital splitting of photoelectrons. The binding energy difference between two peaks was 5.7 eV. Fig. 2(b) shows a high-resolution XPS scan of tungsten (W) which belongs to the WO₃ passivation layer on the TiO₂ nanoparticle layer. The spin-orbital splitting of photoelectrons exhibited W 4f_7/2 and W 4f_5/2 at the binding energy values of 34.4 and 36.3 eV respectively. The energy difference between the two peaks was observed to be 1.9 eV. The high-resolution XPS scan showing O 1s spectrum obtained from the sample is shown in Fig. 2(c). The peak occurred at 528.7 eV. All these characteristic peaks, identified with respect to their binding energy values, confirmed the WO₃ coating on the TiO₂ nanoparticle layer. Fig. 2(d) shows XRD pattern of the WO₃-coated TiO₂ nanoparticle layer. The XRD peak appeared at 2θ of 25.8° represents (101) which corresponds to the anatase TiO₂ phase and this was confirmed from JCPDS-84-1286. The TiO₂ is considered to be phase pure as it did not show the rutile phase at 2θ value of 27.36°. Further the XRD peaks at 2θ values of 38.3° (004), 48.5° (200) and 55.6° (211) also assert the TiO₂ nanoparticle layer. The indices (105), (133) and (220) at 2θ values of 54.4°, 63.1°, 75.5° represent the WO₃ thin film coating on the TiO₂ nanoparticle layer. Thus, the XRD pattern obtained from the WO₃ passivation thin film-coated TiO₂ nanoparticle layer also confirmed both the functional materials with respective to their characteristic peaks.

Figure 2.(Color online) (a–c) High resolution XPS scans showing Ti 2p, W 4f and O 1s peaks from the WO₃ coated TiO₂ sample. (d) XRD pattern obtained from the same sample.

Fig. 3(a) shows photographs of the pristine TiO₂ nanoparticle layer along with 15, 22, 30 nm WO₃-coated TiO₂. It was very obvious that WO₃ passivation was thin enough, which did not change the TiO₂ much in terms of the color. However, it was very clear from the picture that WO₃ was coated on the surface of the TiO₂ nanoparticle film. In this study, WO₃ was coated on TiO₂ to modify/suppress the defects which are actively participating in the charge transport process in DSSCs. Thus, it was a genuine expectation that WO₃ should not affect the dye adsorption on the surface of TiO₂. The photograph shown in Fig. 3(b) shows the pristine TiO₂ nanoparticle layer along with 15, 22, and 30 nm WO₃-coated TiO₂ after the dye loading process. It provided evidence that coating of WO₃ on the TiO₂ nanoparticle layer did not affect the dye loading significantly. In general, a surface passivation layer is expected to control the defects in semiconductors to improve the charge transport characteristics to yield high performance photovoltaic devices. If the dye loading into the TiO₂ nanoparticle layer is impeded by WO₃, then exciton generation will be reduced significantly which, in turn, would result in lower photo-current and overall performance. Fig. 3(c) shows UV–visible optical absorption spectroscopic measurements performed on the pristine TiO₂ nanoparticle layer in comparison with TiO₂ samples coated with the 15, 22, and 30 nm WO₃ surface passivation layer. As shown in the Fig. 3(c), the pristine and WO₃-coated TiO₂ did not show any significant absorption in the spectral window of 400–800 nm in which a major portion of visible energy photons are available. Thus, passivating TiO₂ by WO₃ will not impede visible spectral photons to reach dye molecules to generate excitons. Thus, WO₃ coating on TiO₂ can be categorized as an advantage for DSSC operation in terms of defect passivation but has no disadvantage in terms of absorption loss due to the optimum band structure characteristics. In DSSCs, incident photons enter through a transparent electrode, FTO, and travel through the TiO₂ nanoparticle layer to reach organic dye to generate excitons. Passivation layers are coated on TiO₂ in DSSCs to control defects. If the passivation layer scatters or disperses photons between TiO₂ and dye, it will lead to major optical loss which would affect the photo-current generation. Thus, it is important to confirm the passivation layer does not interact with incident photons in the visible spectral range. Further, UV–visible spectra of WO₃-coated TiO₂ samples exhibited that absorption at the high energy spectral region was significantly suppressed after WO₃ coating. WO₃ exhibits an optical bandgap in the range of 2.6–3 eV. When pristine TiO₂ showed absorption in the high energy range, WO₃ coating suppressed the exciton generation at this specific range due to the heterostructure alignment in the band-structure^[55]. It could be attributed to the thermalization effect which also contributed to the increment in the absorption of photons in the longer wavelength regions. This effect was observed in all three TiO₂ samples with WO₃ coating.

Figure 3.(Color online) Digital photographic images of (a) TiO₂ nanoparticle layer coated on FTO covered glass substrates showing the deposition of WO₃ surface passivation layer with different thickness values. (b) Photographs of the samples after N719 dye coating confirming the WO₃ did not affect the loading much. (c) UV–visible absorption spectra of the samples.

Fig. 4(a) shows illuminated J–V characteristics of the DSSCs which used pristine TiO₂ as a photo-anode along with the WO₃ passivation layer-coated TiO₂. The illuminated J–V characteristics of the DSSCs showed that coating of WO₃ on the TiO₂ nanoparticle played a significant role in terms of all photovoltaic parameters including open circuit voltage (V_OC), short circuit current density (J_SC), fill factor (FF) and efficiency (η). All the key photovoltaic parameters measured under AM 1.5 illumination are listed in Table 1 along with standard deviation in η. The reference DSSC, which used a pristine TiO₂-based photo-anode (no WO₃ coating), resulted in V_OC, J_SC, FF and η of 745 mV, 9.3 mA/cm², 59.0%, and 4.1% respectively. The DSSC, which used 22 nm coating of WO₃ on TiO₂ as a photo-anode, resulted in V_OC, J_SC, FF and η of 712 mV, 11.1 mA/cm², 57.2%, and 4.5%, respectively. This is approximately 10% enhancement in η due to the increment in J_SC. As can be seen in Table 1, further increment in the duration of WO₃ coating on TiO₂ affected photovoltaic parameters significantly. It can be directly attributed to the reduction in J_SC by creating a thicker energy barrier between the TiO₂ and dye where the charge carrier injection occurred. It is well known that if the energy barrier thickness increases, then tunneling probability would decrease. Thus, an optimum thickness of the passivation layer is required to passivate the defects. In the present study, it was found that 10 min WO₃ coating on TiO₂ performed better than other DSSCs.

Figure 4.(Color online) (a) J–V characteristics of the DSSC with and without WO₃ passivation on TiO₂. (b) Comparison of P_MAX of DSSCs, dark current characteristics of DSSCs in (c) linear and (d)semi-log scale showing the saturation current values.

Table Infomation Is Not Enable

Fig. 4(b) shows power density–voltage characteristics of the reference DSSCs along with DSSCs used WO₃ passivation showing the maximum power point (P_MAX) values. It was noticed that DSSC used 10 min of WO₃ passivation on TiO₂ resulted in 4.5 mW/cm² while the reference DSSC exhibited 4 mW/cm². This is one of the parameters along with the increment in J_SC improved the ~10% enhancement in η for the DSSC with 22 nm WO₃ passivation on TiO₂. The DSSC with 15 min of WO₃ passivation showed 3.8 mW/cm². It clearly indicated that increasing thickness of WO₃ affected the P_MAX and thus the detrimental effect on photovoltaic performance. Fig. 4(c) shows dark current–voltage characteristics of the DSSCs with a semi-log plot of the same in the higher applied range of 0.7 to 1 V. It is possible to assess the electronic quality of interfaces in DSSCs through dark saturation current (I_O) which can be obtained from the general I_D = I_Oexp(qV/kT–1), where I_D and I_O are current density and saturation current respectively under dark conditions. The parameters in equation q, V, k and T are electronic charge, applied bias to the DSSCs, Boltzmann constant and temperature at which the DSSCs work respectively. It is well known that DSSCs with good quality interfaces exhibit higher I_O values at higher applied bias regions, lower I_O values at lower and medium forward bias regions. Increasing I_O in the low and medium forward bias regions can directly be attributed to the defect dominated charge transport. Thus, the parameter (I_O) needs to be suppressed to obtain a DSSC with improved photovoltaic performance. Inset in Fig. 4(c) clearly showed the DSSC with 10 min WO₃ coating resulted in higher I_O value compared with other DSSCs. Fig. 4(d) shows semi-log plots of dark current characteristics of the reference DSSC along with DSSC used WO₃ passivation. We have chosen the applied bias region close to 0.3 V to consider the I_O values for the DSSCs. It is evident that DSSC with 10 min of WO₃ passivation on TiO₂ exhibited 1 × 10^–7 A, which is 20 times lower than the I_O values showed by reference DSSC which was 2 × 10^–6 A. In this calculation, 0.3 V applied bias has been considered to extract the I_O values for all DSSCs for the reason that a linear dark current increment was observed in the semi-log plot as shown in Fig. 4(d). If the dark current is highly linear in the log-scale, it is a perfect exponential diode characteristic feature. Thus, the applied bias of 0.3 V has been used to extract I_O values.

Figs. 5(a) and 5(b) show the energy level diagram representing FTO/TiO₂/dye/electrolyte and FTO/TiO₂/WO₃/Dye/electrolyte interfaces respectively. The only difference between the two categories of DSSCs examined in the present work was WO₃ passivation on TiO₂. Thus, the variation in photovoltaic performance is directly attributed to the ability of WO₃ to passivate defects in TiO₂. Photo-generated electrons in the lowest unoccupied molecular orbital (LUMO) of the dye need to be injected into the conduction band edge of TiO₂ (E_C) in which they are all transported to reach the FTO electrode by diffusion as represented by process (1). Meanwhile, the holes from the highest occupied molecular orbital (HOMO) of the dye will be regenerated by the electrolyte as shown in process (2). These two categories are electron and hole transport, which eventually contribute to photo-current in DSSCs. However, the photo-electrons in the bulk of TiO₂ nanoparticle layer can be trapped by defects present in the bandgap of TiO₂ as illustrated by process (3). If the trap states, which are close to the trapped electron, can trap a hole from the HOMO of the dye as shown in process (4), then electron-hole recombination will occur at the bandgap of TiO₂ mediated by defects. Fig. 5(b) shows the possible changes that can occur at the electron–hole transport and recombination kinetics after coating the WO₃ surface passivation layer on TiO₂. The photo-generated electron injection from the LUMO of the dye will be injected through the WO₃ surface passivation layer by tunneling as the thickness is facilitating such a transport process. Photo-generated electrons in the TiO₂ further can be trapped by defects but the probability of hole trapping, process (4), is rather difficult as the WO₃ will not allow the trap states in TiO₂ to interact with holes at the HOMO of the dye. Thus, hole trapping will not occur at the TiO₂/dye interface after coating the passivation layer. The photo-generated electron will not be lost without having a hole in the bulk vicinity of TiO₂. The probability of recombination loss is expected to be suppressed by coating a thin WO₃ film on TiO₂. This situation will lead to improved DSSC performance.

Figure 5.(Color online) Schematic illustration of (a) FTO/TiO₂/dye/electrolyte and (b) FTO/TiO₂/WO₃/Dye/electrolyte interfaces in DSSCs showing electron–hole transport and recombination mediated by defects in TiO₂.

4. Conclusion

A dominant recombination pathway at the TiO₂/dye interface was significantly suppressed by passivating the TiO₂ surface using RF-sputtered WO₃ film. RF-sputtering is a versatile technique to coat WO₃ surface passivation on the TiO₂ nanoparticle layer without blocking the porosity. DSSC with WO₃ passivation exhibited ~10% enhancement in performance. Results from DSSCs confirmed that there is an optimum thickness of WO_3, at which the electron-hole transport was facilitated while the recombination probability was reduced.

Acknowledgements

We thank the Amrita Centre for Nanosciences for providing us with the PVD cluster equipment to coat various functional thin films for the energy conversion technology development.

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