Improved photovoltaic performance of dye-sensitized solar cells using dual post treatment based on TiCl 4 and urea solution

Solar power generation owns promising merits of sustainability, pollution-free, reproducible and unlimited resources. Dye sensitized solar cell (DSSC) invented by Micheal Grätzel and co-workers at 1991, has attracted intense attentions in photovoltaics field owing to its superiority such as flexibleness, lightweight and low-cost [1–3], expected to be a potential candidate device for alternating traditional inorganic solar cells. As so far, DSSC has made record power conversion efficiencies (PCE) of 14% [4–6] thanks to the unceasing effort of researchers in the field of chemistry, physics and materials science. One way to improve the PCE is to explore varies of novel materials for the main components of DSSC, namely metal-oxide photoanode [7–9], redox shuttle electrolyte couples [10, 11] and catalysing photocathode [12]. Another strategy to optimize the performance of the solar cell is a post-treatment of the photoanode layer, mainly represented by titanium dioxide (TiO2) film. Titanium tetrachloride (TiCl4) has been used as an effective post-treating reagent of TiO2 electrode for high-efficiency DSSC [13–15]. After the TiCl4 treatment, an additional layer of TiO2 is formed on the TiO2 nanoparticles, which will optimize the DSSC performance through several functions of increasing surface area, improving electron transport, light scattering and so on. The hydrolysis reaction occurring during the TiCl4 treatment to constitute a new TiO2 layer influenced mainly by the concentration of the TiCl4 aqueous solution and an optimized value of 10–50 mM is commonly used. In recent years, graphite carbon nitride (g-C3N4) has been also applied to optimize TiO2 electrode due to its high thermal tolerance and chemical stability, easy-preparation and relative negative conduction band that could inhibit charge-recombination [16–18]. Composite or post-treatment of urea materials to the TiO2 electrode have been used to form a cover of g-C3N4 thin film on the surface of TiO2 electrode and an opportune quantity of urea material is desirable for optimizing the device performance. However, very few study has been reported on combination of multi-treatment with different materials for TiO2 electrode in order to improve the cells performances up till now. In this paper, a dual post treatment based on TiCl4 and urea solution was implemented on different layer of TiO2 electrode, also the TiO2 particle


INTRODUCTION
Solar power generation owns promising merits of sustainability, pollution-free, reproducible and unlimited resources. Dye sensitized solar cell (DSSC) invented by Micheal Grätzel and co-workers at 1991, has attracted intense attentions in photovoltaics field owing to its superiority such as flexibleness, lightweight and low-cost [1][2][3], expected to be a potential candidate device for alternating traditional inorganic solar cells. As so far, DSSC has made record power conversion efficiencies (PCE) of 14% [4][5][6] thanks to the unceasing effort of researchers in the field of chemistry, physics and materials science. One way to improve the PCE is to explore varies of novel materials for the main components of DSSC, namely metal-oxide photoanode [7][8][9], redox shuttle electrolyte couples [10,11] and catalysing photocathode [12].
Another strategy to optimize the performance of the solar cell is a post-treatment of the photoanode layer, mainly represented by titanium dioxide (TiO 2 ) film. Titanium tetrachloride (TiCl 4 ) has been used as an effective post-treating reagent of TiO 2 electrode for high-efficiency DSSC [13][14][15]. After the TiCl 4 treatment, an additional layer of TiO 2 is formed on the TiO 2 nanoparticles, which will optimize the DSSC performance through several functions of increasing surface area, improving electron transport, light scattering and so on. The hydrolysis reaction occurring during the TiCl 4 treatment to constitute a new TiO 2 layer influenced mainly by the concentration of the TiCl 4 aqueous solution and an optimized value of 10-50 mM is commonly used. In recent years, graphite carbon nitride (g-C 3 N 4 ) has been also applied to optimize TiO 2 electrode due to its high thermal tolerance and chemical stability, easy-preparation and relative negative conduction band that could inhibit charge-recombination [16][17][18]. Composite or post-treatment of urea materials to the TiO 2 electrode have been used to form a cover of g-C 3 N 4 thin film on the surface of TiO 2 electrode and an opportune quantity of urea material is desirable for optimizing the device performance. However, very few study has been reported on combination of multi-treatment with different materials for TiO 2 electrode in order to improve the cells performances up till now. In this paper, a dual post treatment based on TiCl 4 and urea solution was implemented on different layer of TiO 2 electrode, also the TiO 2 particle This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Micro & Nano Letters published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology morphology, dye-adsorption amount on TiO 2 electrode and the photovoltaic performance of the DSSC based on those electrodes were investigated systematically to understand the relationship between device structure and properties. The TiCl 4 treatment at the second and forth TiO 2 layer were supposed to retain the connectivity of the TiO 2 particles in different layers, aiming to improve the electron transfer efficiency and inhibit the charge recombination [19].

Materials and instrumental analysis
Solvents used in this experiments were repurified from appropriate reagents. All reagents were purchased from Shanghai Macklin, Shanghai Aladdin Bio-Chem Technology and Sigma-Aldrich. Perkin-Elmer Lambda 35 UV-visible spectrometer was used to record the light absorption at room temperature. The phase identification was performed using X-ray diffraction (XRD, Cu Kα radiation, Philips PW1830). The 2θ scans were taken between 20 • and 80 • with steps of 0.05 • , with 2s counting time per angular value. The nanocrystalline morphology was characterized using a field-emission scanning electron microscope (SEM, Hitachi SU-8010), and the microstructure was characterized using a transmission electron microscope (TEM, JEM 210F).

General procedure for preparing TiO 2 films
Fluorine doped Tin oxide (FTO) glass substrates were cleaned in an order of detergent solution, ultra-pure H 2 O and ethyl alcohol (EtOH) within an ultrasonic bath, rinsed with H 2 O and EtOH, and followed by N 2 flow for drying. Screen-printing technique was used to prepare nanocrystalline films on the substrate with TiO 2 paste purchased from Dalian HeptaChroma Solar Tech Co., Ltd., coded as DHS-TPP3, consisting of 20 nm nanoparticles. The substrates with printed films were slowly sintered from room temperature to 500 • C and retain for 15 min in a muffle furnace. TiCl 4 treatment was implemented with a TiCl 4 aqueous solution of 40 mM at 70 • C for 30 min and then SCHEME 1 Schematic illustration of preparation of the TiO 2 films rinsed with H 2 O and EtOH, serially, followed by sintering again at 450 • C for 30 min. Urea treatment is performed by dipping the photoanode films into the urea solution with a concentration of 1 g/mL at 50 • C for 30 s, followed by calcination in a muffle furnace at 500 • C for 1 h immediately after being pulled out from the urea solution.

Preparation of TiO 2 films with different surface treatment
The post-treatments were prepared as shown in Scheme 1. The film thickness of 6 layers TiO 2 film was measured to be 12-14 μm.
Electrode 2T: Sintered two layers of TiO 2 films on FTO substrate was processed with TiCl 4 treatment and then printed with another four layers of TiO 2 films, followed by calcination again.
Electrode 2TU: Sintered two layers of TiO 2 films on FTO substrate was processed with TiCl 4 treatment and urea treatment in sequence, followed by print of another four layers of TiO 2 films, and calcination again.
Electrode 4T: Sintered four layers of TiO 2 films on FTO substrate was processed with TiCl 4 treatment and then printed with another two layers of TiO 2 films, followed by calcination again.
Electrode 4TU: Sintered four layers of TiO 2 films on FTO substrate was processed with TiCl 4 treatment and urea treatment in sequence, followed by print of another two layers of TiO 2 films, and calcination again.
Electrode 6T: Sintered six layers of TiO 2 films on FTO substrate was processed with TiCl 4 treatment, followed by calcination.
Electrode 6TU: Sintered six layers of TiO 2 films on FTO substrate was processed with TiCl 4 treatment and urea treatment in sequence, followed by calcination.

Fabrication of DSSC
Prepared TiO 2 electrodes were activated under 80 • C for 30 min before using, and then immersed into a standard N719 dye EtOH solution with concentration 0.3 mM of for 24 h. Ptcounter electrodes were prepared with screen-printing technique on FTO glass substrates using one layer of Pt nanoclusters paste that purchased from Dalian HeptaChroma Solar Tech Co., Ltd., coded as DHS-PtSP. After the screen-print, the counter electrodes were sintered at 400 • C for 30 min. As prepared Pt-counter electrode and dye-sensitized TiO 2 electrode were sealed as sandwich structure cells using a hot-melt Surlyn film with thickness of 25 μm as a spacer between the electrodes. A drop of the electrolyte solution [0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI) + 0.1 M LiI + 0.2 M I 2 + 0.5 M 4-tert-butylpyridine in acetonitrile] was driven into the cell space through the hole pre-drilled on the counter electrode via vacuum pump. Finally, a thin normal glass slide was used to seal the hole with hot-melt surlyn film.

Characterization and measurements of DSSC
The fabricated DSSC were illuminated via an aperture area of 0.03 cm 2 using a lightproof black mask to eliminate light diffusions on the apparent surface area of the TiO 2 film, which was ca. 0.36 cm 2 . DSSC photovoltaic performance was characterized by photocurrent-voltage (I-V) and incident photon-tocurrent conversion efficiency (IPCE) measurements. I-V characteristics were measured by an AAA grade solar simulator (XES-70S1, SAN-EI Electric, Japan) at 100 mW cm −2 under a simulated AM 1.5G sunlight. The IPCE measurements were carried out with ZAHNER CIMPS photo-electrochemical workstation in the wavelength range from 380 to 800 nm. In both case, a standard silicon photovoltaic cell (AK-200, Konica Minolta Inc.) was used to calibrate the intensity of the incident light before experiments.

Morphology and phase analysis
The size and morphology of the TiO 2 particles scraped from the electrodes were investigated through TEM and HRTEM as shown in Figure 1. Without urea treatment, the TEM images of the TiO 2 particles present an approximately average particle size of 25 nm owing a clear edge (Figure 1(a,b)). After treated with urea solution and sintering, there is a thin layer with thickness of 1-2 nm covering the TiO 2 particles as displayed in Figure 1(c,d). These results of g-C 3 N 4 polymeric formation on the surface of TiO 2 particles are in accordance with the reference papers [16,17]. The crystallinity and the morphology of the prepared electrodes were also studied by XRD and SEM methods. Due to the low concentration and weak crystallization, no obvious differences were found between electrodes with and without urea treatment. Figure 2 Figure 3, the TiO 2 particle amounts generated by TiCl 4 treatment was too little to modify the thickness of the electrode, the width and appearance of the electrode 2T, 4T and 6T in cross-sectional view remain the same.

Dye adsorption
In order to clarify the influence of the different TiO 2 surface treatment on the dye adsorption amount, the UV-vis absorption spectra of the dye TiO 2 -desorption solution (0.1 mol L −1 NaOH, THF/H 2 O = 1:1) were measured and shown in Figure 4. The adsorption capacity T (mol/cm 2 ) can be calculated by the following formula, and the related data was presented in Table 1.  where A is the absorbance, ε is the molar absorption coefficient (L/mol⋅cm), b is the colorimetric plate width (1 cm), c is the concentration of the dye solution (mol/L), n is the amount of substances in the sample solution, v is the volume of the sample solution (L), S is the TiO 2 electrode area (0.423 cm 2 ). The characteristic absorption peak value of the dye N719 at 500 nm in TiO 2 -desorption solution was chosen as the absorbance A, which corresponds to the ε value of the first absorption peak at the long wavelength side in N719 ethanol solution. The TiCl 4 treatment results in an apparent increase in dye adsorption due to more specific binding sites generated on TiO 2 surface. The dye loading amount augmented by 6.95% from electrode 2T to 4T and further increased by 11% from electrode 4T to 6T, leading to a maximum value of 1.11 × 10 −7 mol/cm 2 . It implies that the binding sites created by TiCl 4 treatment mainly functioned on outer layer of TiO 2 films to adsorb more dye molecules, which will contribute for the better light-harvest and consequently improved photocurrent. While the urea treatment leads to coat a thin layer of g-C 3 N 4 outside TiO 2 nanoparticles surface as shown in Figure 1. As comparing the dye adsorption amount before and after urea treatment, it can be FIGURE 4 UV-vis absorption spectra of the TiO 2 -desorption solutions drawn that g-C 3 N 4 coating largely influence the dye loading at the outer layer of TiO 2 film as well (dye amount changes, 2T to 2TU: +4.28%; 4T to 4TU: −1.50%; 6T to 6TU: −24.10%), whereas the urea treatment at the second and forth layer of TiO 2 film affected the dye adsorption very little.

Photocurrent-voltage characteristics
I-V characterization of the DSSCs based on those electrodes were measured in accordance with the condition mentioned experiment Section 2.5. Figure 5 illustrates I-V curves plotted for the DSSCs and the data is listed in Table 2.
From the results it can be seen obviously that both open circuit voltage V oc and short circuit current J sc were improved gradually according to the TiCl 4 treatment on electrode 2T to 6T (V oc : 2T to 4T: +4.4%; 4T to 6T: +2.3%; J sc : 2T to 4T: +27.5%; 4T to 6T: +19.7%), leading to the overall photovoltaic conversion efficiency enhancement of 29.9% from 2T to 4T and 24.9% from 4T to 6T. The variation tendency of J sc are in good agreement with the changing tendency of dye adsorption amount.   The increase in V oc probably resulted from small rise of the quasi-Fermi level of nanocrystalline TiO 2 due to the surfacestate density [15]. Additional urea treatments on electrode 2T, 4T and 6T, result in further enhanced J sc and V oc , leading to PCE of 6.36%, 8.61% and 8.94%. IPCE spectral response of the cells based on the electrodes are displayed in Figure 6, well correlating with the change of their J sc . In order to reveal the details of improved J sc and V oc , electrochemical impedance spectroscopy (EIS) was carried out to clarify the interfacial charge transfer and recombination pro-cesses for the DSSC. Impedance was performed with AC amplitude of 10 mV at frequency range from 10 -1 Hz to 10 5 Hz in the dark. The equivalent circuit was represented in Figure 7, where R s stand for the series resistance; C pt and R pt represented the interface capacitance and charge transfer resistance at the Pt/electrolyte interface, respectively. Figures 8 and 9 shows the Nyquist and Bode plot of the EIS for the DSSC based on the electrodes and related data were summarized in Table 3. There are two semicircles with different size were observed in the Nyquist plots, identified as charge transfer resistance R Pt at the Pt/electrolyte interface and electron transport resistance R ct at the TiO 2 /dye/electrolyte interface, respectively. As the same Pt counter electrode and electrolyte components were applied in the cells, the smaller semicircles representing the R Pt are quite similar to each other. The radiuses of the R Pt semicircles increased after the TiO 2 electrodes were further treated with urea solution, which accord with the V oc values changes of the DSSCs. In the EIS-Bode phase plot, the following equation is  = 1∕ ( Along with the urea treatment on TiO 2 electrode, the ƒ peak in lower frequency range decreased and thus the τ e was enhanced accordingly, which were in good accordance with the situation of V oc increase in I-V curves. These results disclosed that the formation of g-C 3 N 4 on the TiO 2 particle surface can effectively suppress the injected electrons recombination and prolong the electron life time, on account of the more negative conduction band of g-C 3 N 4 (−1.12 eV) than that of TiO 2 (−0.29 eV) restraining the electron shift from TiO 2 to triiodide in the electrolyte. It resulted in the increase of electron concentration and hence the photocurrent value J sc as well [17]. Interestingly, the photovoltage and photocurrent of electrode 4TU increased greater extent than those of 2TU and 6TU, probably in respect that not only the modification of interface contact is more effective when the g-C 3 N 4 is constituted in the middle layer of the TiO 2 electrode, but also there is less disrupt of the TiO 2 particle connection, inducing faster electron transport from TiO 2 to FTO substrate [16,17].

CONCLUSIONS
To enhance the photovoltaic performance of DSSC, a dual post treatment based on TiCl 4 and urea solution was applied on different layers of TiO 2 electrode. TiCl 4 treatment on the sixth layer of TiO 2 film could provide more binding site for dyeadsorption than that on the second and fourth layer, leading to increased dye molecule amount and thus improved J sc . and V oc . On the other hand, g-C 3 N 4 formation from urea treatment on the sixth layer decreased the dye loading, resulting in less extent of V oc and J sc increase, but still the DSSC presented the highest PCE of 8.94%. Urea treatment in the fourth TiO 2 layer produce more interface contact without interruption of the connection among TiO 2 particles that makes for highest rise in V oc and J sc . These results empirically demonstrated that dual post-treatment of TiCl 4 and urea solution could improve the DSSC performance. Further surface modifications and device optimizations for improving energy conversion efficiency and long-time stability are in progress.