Performance-Enhancing Sulfur-Doped TiO₂ Photoanodes for Perovskite Solar Cells

Abstract

High-performance electron transport layer (ETL) anode generally needs to form a uniform dense layer with suitable conduction band position and good electron transport properties. The TiO₂ photoanode is primarily applied as the ETL because it is low-cost, has diverse thin-film preparation methods and has good chemical stability. However, pure TiO₂ is not an ideal ETL because it lacks several important criteria, such as low conductivity and conduction band mismatch with compositional-tailored perovskite. Thus, TiO₂ is an inefficient photo-anode or ETL for high-performance perovskite devices. In this study, sulfur as dopant in the TiO₂ photo-anode thin film is used to fabricate solid-state planar perovskite solar cells in relatively high humidity (40–50%). The deposited S-doped thin film improves the power conversion efficiency (PCE) of the device to 6.0%, with the un-doped TiO₂ producing a PCE of 5.1% in the best device. Improvement in PCE is due to lower recombination and higher photocurrent density, resulting in 18% increase in PCE (5.1–6.0%).

1. Introduction

Perovskite solar cells (PSCs) have been attracting great attention in the past decade due to their rise in power conversion efficiencies (PCE), with certified efficiencies now greater that 25% [1,2,3,4,5]. A typical PSC consists of multiple layers of solid thin films, which include an electron transport layer (ETL), perovskite absorber layer and hole transport layer (HTL). These layers are aligned in the heterojunction according to its distinct device configurations. Regardless of the device configurations, an optimised ETL can potentially improve the overall behaviour of the PSCs in terms of the efficiency, hysteresis management and reproducibility of the device [6,7]. The functions of an ETL are closely related to its photoelectrochemical and structural properties by effectively facilitating electron collection and transfer from the photon-absorbing perovskite thin film. Good indications of a high-performance ETL or photo-anode include a thin, dense and suitable conduction band and good electron transfer. These characteristics also aim to minimise the interfacial recombination, facilitate electron movement and manage charge accumulation. The TiO₂ photo-anode is primarily applied as an ETL because it is low-cost, has numerous thin film preparation methods and has suitable chemical stability [8,9,10]. However, pure TiO₂ is not an ideal ETL because it lacks several important criteria such as low conductivity and conduction band mismatch with compositional-tailored perovskite (e.g., triple-cation perovskite system). Hence, there is room for improvement in the performance of TiO₂ as the ETL [1].

Metal doping of TiO₂ has become a practical technique to modify the band position and improve the thin film conductivity to fabricate photo-anodes with high charge extraction capacity. Dopants, such as yttria, niobium, magnesium and tin, have been proven to enhance the overall PSC efficiency due to the enriched layer conductivity and high electron mobility [11,12,13,14]. Although many dopants have been used to modify the TiO₂ photoanode for PSCs, the application of sulfur as dopant for these cells has not been reported yet. S-doped TiO₂ has been demonstrated to enhance photocurrent, light absorbance and photocatalytic activity and modify the band gap energy in other applications, such as coatings and photo-catalysis [15,16]. These properties could improve the characteristics of the photo-anode thin films for perovskite solar devices.

In this study, sulfur was used for the first time as dopant in the TiO₂ photo-anode thin film in solid-state planar heterojunction solar cells. Methylammonium lead iodide (CH₃NH₃PbI₃) was used as the light absorber and spiro-OMeTAD as HTL. The effects of charge extraction and blocking of holes from the TiO₂ mesoporous layer was eliminated by choosing a planar PSC architecture. A sol-gel solution containing sulfur and titanium salts was spin coated onto the FTO glass to form the S-doped TiO₂ photo-anode thin film. The doping concentration of sulfur is tuned by controlling the sulfur salt content in the sol-gel solution.

2. Materials and Methods

Titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol), thiourea (≥99.0%), absolute ethanol (99.95%), acetylacetone (≥99%), lead iodide (PbI₂, 99%), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, 99%), 4-tert-butylpyridine (tBP, 96%), N,N-dimethylformamide (DMF, 99.8%), 2,20,7,70-tetrakis-(N,N-dip-methoxyphenylamine)-9,90-spirobifluorene (spiro-OMeTAD, SHT-263, Solarpur), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine) cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209 Co(III) TFSI, 98%) and anhydrous acetonitrile (99.8%) were obtained from Sigma Aldrich and used as received. Prior to the deposition of photoanode layer, FTO-coated glass (FTO, TEC15, 15 Ω/sq, Greatcell Solar Materials) was first etched to design the desired pattern of electrode using zinc powder and 3M HCL. Next, the FTO substrate is cleaned using ultrasonic bath for 10 min, in the following order: soap (2% Hellmanex in water), acetone then propanol (99.5%), to remove the organic residue on the surface of FTO-glass substrate. In this modified sol-gel method, S-doped TiO₂ photoanode is prepared by first mixing 1.2 mL acetyl acetone, 7 mL TTIP, 35 mL absolute ethanol and calculated amounts of thiourea as sulfur source. To evaluate to effect of sulfur doping concentration, the amount of thiourea is prepared with 5%, 10% and 15% mol thiourea compared to TTIP. The precursor solution was then magnetically stirred for 180 min to produce a clear and homogeneous solution. A blank precursor solution was also prepared without the thiourea as control. The deposition of photoanode was carried out using spin coating method. 3 layers of TiO₂ photoanode was spin-coated at 3000 rpm with drying process at 120 °C for 10 min after each layer deposition. Afterwards, the TiO₂ photoanode was sintered at 450 °C to complete the deposition. The nomination of all sample relates to the sulfur doping content of the initial precursor solution.

To investigate the optical properties of the photoanode thin film, room temperature absorbance measurement (300 to 700 nm) is recorded using a UV-Vis spectrophotometer (Lambda 35, PerkinElmer). The XRD diffractogram of the photoanode thin film were analysed using X-ray diffractometer model Bruker D8 Advance (2θ, 10–70°). The FTIR spectrum of TiO₂ photoanodes were evaluated using FTIR spectrometer (Shimadzu—840 s). Fluorescence spectroscopy was used to assess the charge recombination behaviours of TiO₂ photoanodes. The fluorescence spectroscopy analysis was carried out using a Perkin Elmer Luminescence spectrometer (LS 55) with a thin film holder accessory, at room temperature and 300 nm as excitation wavelength. Morphological study of the TiO₂ photoanodes were carried out using field-emission scanning electron microscopy (FESEM) were analysed using FE-SEM SUPRA VP55 equipped with an energy dispersive X-ray (EDX) spectroscopy for elemental detection and mapping. The photoelectrochemical (PEC) properties of the TiO₂ photoanodes were determined via linear sweep voltammetry (LSV) using an electrochemical workstation (Metrohm Autolab), simulated AM 1.5 at a calibrated intensity 100 mWcm⁻² at NTP conditions. TiO₂ samples as the working electrode is immersed in Na₂SO₄ solution (0.5 M), Pt as counter electrode and Ag/AgCl as the reference electrode. The scanning rate is kept at 20 mVs⁻¹ (−0.4 to 1.4 V vs. Ag/AgCl).

The perovskite solar devices were prepared by depositing the perovskite absorber layer, hole transporting layer and silver contact, consecutively. The perovskite absorber layer is deposited using a 2-step method where in the first layer, 100 µL of PbI₂ solution consisting of PbI₂ powder (507 mg) in DMF (1 mL) and tBP (100 µL) is spin coated on the TiO₂ photoanode at 3000 rpm for 30 s then annealed at 100°C for 10 min. Previous study have emphasised the addition of tBP to enhance the hydrophobicity of PbI₂ and the deposited perovskite absorber [17]. Then, 250 µL of MAI solution consisting of 35 mg MAI powder in 1 mL isopropanol at 4000 rpm for 30 s then annealed at 100 °C for 30 min. After allowing the temperature to cool down to room temperature, hole transport layer solution is then spin coated at 4000 rpm for 30 s. The hole transport layer solution consisted of spiro-OMeTAD powder (72.3 mg), chlorobenzene (1 mL), 4-tert butylpyridine (28.8 μL), Li-TFSI solution (17.5 μL, 520 mg/mL in acetonitrile) and FK209 solution (29.0 μL, 520 mg/mL in acetonitrile). All of the perovskite solar devices layer solutions were prepared and spin coated in RH40-50%. To finish the perovskite solar devices, Ag counter electrode was thermal evaporated onto the hole transport layer under high vacuum atmosphere with 0.07 cm² active area. The photocurrent-voltage (I-V) characteristics were measured by applying a reverse scan at a rate of 0.1 V/s in a Keithley 2400 source meter under AM 1.5 G solar illumination. The average values of PSC devices were measured over 10 samples.

3. Results

3.1. UV-Visible Spectroscopy

Given the boiling temperature of elemental sulfur at 444.6 °C, evaluating the behaviour of the S-doped TiO₂ thin film towards the change in sintering temperature is interesting. Figure 1a shows the UV-vis absorption spectra of various TiO₂ thin films sintered at 450 °C and 500 °C. The sintering temperatures were selected based on the temperature required to produce highly-crystalline TiO₂ thin films, that is, at least 450 °C [18]. 550 °C is not suitable due to the formation of rutile TiO₂ at this point, and this type of TiO₂ is less photo-active than anatase TiO₂ [19]. Firstly, all TiO₂ samples exhibit the typical absorbance behaviour of TiO₂ materials, in which the absorbance is low at higher wavelength and gradually increases at the UV region. Compared with the un-doped TiO₂ photo-anode, the S-doped TiO₂ displayed a small red shift of the absorption edge due to the new energy levels within the band gap caused by the doping of S. This phenomenon has been reported in a previous study [20,21]. Based on a previous study, the mechanism of TiO₂ band gap reduction could be attributed to the interaction between the S dopant (d and p orbitals) with the TiO₂ energy levels [22]. The bandgaps were reduced from 3.56 eV (un-doped TiO₂ photoanode) to 3.41 eV (10% S-doped TiO₂ photoanode) as estimated from the Tauc plots in Figure 1b. When the sintering temperature was increased to 500 °C, the absorption spectrum of the S-doped TiO₂ photoanode was reduced. This phenomenon suggested the S dopant has diffused out of the thin film, which was also found in a previous study [15]. Thus, to produce S-doped TiO₂ with anatase phase, 450 °C was selected as the sintering temperature.

The different doping levels of S were investigated using UV-vis spectroscopy. Figure 1b shows the absorption response of all S-doped TiO₂ photo-anodes with various concentrations of S. With 5% S dopant, the absorption bands were improved compared with those of the un-doped TiO₂ photo-anode. According to the plots, 10% S-doped TiO₂ showed the highest light absorbance. Thus, 10% was selected as the optimum concentration for sulfur doping.

3.2. X-ray Diffraction (XRD) Analysis

Changes in the crystalline phase of the S-TiO₂ photo-anode thin film were analysed using X-ray diffraction. The X-ray diffractograms of the un-doped and S-doped TiO₂ photo-anode thin film sintered at 450 °C are shown in Figure 2. Each sample had defined TiO₂ diffraction peaks, which were correlated with the characteristics of nano-crystalline anatase TiO₂ peaks with several FTO diffraction peaks. According to a previous study, the characteristic peaks of anatase TiO₂ are found at the angles of 27.0°, 38.2°, 55.1° and 62.0° corresponding to the (1 0 1), (1 0 4), (1 0 5) and (2 0 4) planes [23]. Rutile-phase TiO₂ was not found in the sintered TiO₂ photo-anode thin-film sample, which could be related to the sintering temperature of 450 °C. The sintering temperature is an important factor in producing the S-doped TiO₂ photo-anode thin film because a higher temperature could lead to the formation of un-desired, less photo-active rutile phase and could reduce the incorporation of S (boiling point: 444.6 °C) in the thin-film lattice [15]. The XRD diffractogram of the S-doped TiO₂ was compared with that of pure TiO₂, and the two diffraction patterns were almost similar with no S characteristic peak. This observation indicated that the S ions had entered the lattice structure of TiO₂ or the concentration of the S compounds were lower than the detection limit of the equipment, as observed in a previous study [24]. In addition to the diffraction pattern, the data in the XRD pattern could be used to calculate the average particle size by using the Scherrer equation as follows:

$${\mathrm{D}}_{\mathrm{hkl}}=\frac{0.89\mathsf{\lambda }}{\mathsf{\beta } \cos \mathsf{\theta }}$$

(1)

where λ is the wavelength of the Cu Kα laser used (0.1541 nm), θ is the diffraction angle of the peak and β refers to the full width of the peak measured at half maximum intensity (FWHM) [25]. Thus, the average particle sizes of both samples were calculated according to Equation (1), resulting in 23.8 nm and 18.0 nm for the un-doped and S-doped TiO₂ photo-anode, respectively. Thus, S-doping had a direct effect on the average particle size of the sintered TiO₂ photo-anode. A previous study has reported that doping via substitution of a lattice atom with a dopant would reduce the grain size compared with the un-doped sample, as observed in this study [26].

3.3. Fourier-Transform Infrared (FTIR) Spectroscopy

The FTIR spectrum can be an effective tool to detect the functional groups in the TiO₂ thin films. Figure 3a shows the FTIR spectra of the S-doped and un-doped TiO₂ thin films. The broad absorption peak in the region between 1650 cm⁻¹ and 3450 cm⁻¹ could be associated to the O-H vibrations of the absorbed water molecules on the TiO₂ thin film surface (Figure 3a) [24,27]. Most importantly, the drop in the intensity in this region for the S-doped TiO₂ thin film would probably be attributed to the hydrophobicity of S. Close inspection of Figure 3b shows that the S distinct peak was absent in the un-doped TiO₂ sample but found in the S-doped TiO₂ sample. In a previous study, the distinct peak at 1086 cm⁻¹ and 1242cm⁻¹ corresponded to the characteristic of bidentate ${\mathrm{SO}}_4^{2-}$ co-ordination with metals, such as Ti⁴⁺, and the formation of Ti-O-S bonds, respectively [22]. The presence of thiourea was negligible due to the absence of the C=O bond absorption bands. Thus, the FTIR spectra had proven the formation of S-doped TiO₂ thin film by using the precursor solution and sintering at 450 °C to form the Ti-O-S bonds.

3.4. Fluorescence Spectroscopy

Fluorescence emission spectroscopy is important to examine the ability of charge carrier trapping, migration and transfer and study the behaviour of electron/hole pairs in the semiconductor particles, such as TiO₂ [28]_. To elucidate the effects of S-doping on the recombination of the photo-generated electron/hole pairs produced in the TiO₂ thin film, the fluorescence spectra were examined for the S-doped and un-doped TiO₂. Figure 4 illustrates the intensity of the fluorescence emission from the electron/hole recombination in the sample. The S-doped TiO₂ thin film emits lower fluorescence intensity, which strongly suggested a lower radiative charge recombination and thus higher photo-catalytic efficiency [28]. Additionally, the produced fluorescence spectra showed similar patterns of peaks with only distinct intensities, which suggests that the addition of S in the TiO₂ lattice did not change the TiO₂ photo-catalytic mechanism.

3.5. Field Emission Scanning Electron Microscope (FESEM) Analysis

The effect of sulfur doping on the microstructure of the TiO₂ thin film photo-anode was assessed using the FESEM micrograph. Figure 5a,b indicate the FESEM morphological images of the un-doped and S-doped TiO₂ photo-anode thin films sintered at 450 °C, respectively. High-magnification images of the TiO₂ thin film revealed the nano-crystallisation of the deposited thin film, as suggested in the XRD diffractograms in Figure 2. Additionally, the thin film showed a dense and uniform morphology of homogenous granular and spherical grain. As mentioned in a previous study, the incorporation of S in the TiO₂ lattice would not contribute to a major change in the morphology of the thin film [22]. The EDX elemental spectra were recorded for the un-doped and S-doped TiO₂ thin films to observe the elements, as shown in Figure 5c,d, respectively. The EDX spectra for the S-doped TiO₂ thin film featured a minor peak for S at approximately 2.3 keV [24]. Additionally, the Ti peaks were observed at approximately 0.2 keV and a strong peak at 4.5 keV, which were attributed to the surface and bulk TiO₂, respectively [29]. Figure 5e,f show the EDX elemental mapping of Ti, O and S in the un-doped and S-doped TiO₂ photo-anode thin films, respectively. The mapping images suggested the highly distributed behaviour of S ions throughout the S-doped TiO₂ thin film and the absence of S ions in the un-doped TiO₂ sample. Additionally, the presence of carbon in the S-doped TiO₂ photo-anode thin film most likely due to the usage of thiourea in the preparation of the thin film, as also described in previous study [30].

3.6. Linear Sweep Voltammetry (LSV) and Photocurrent–Voltage (J–V) Curve Studies

The photo-current response of the undoped and S-doped TiO₂ ETLs fabricated by spin-coating and sintered at 450 °C were evaluated using linear sweep voltammetry. The measurements were performed in the dark and under visible light-irradiated environment. The samples were soaked in 0.5 M Na₂SO₄ solution at a scan rate of 20 mV/s, and the results are displayed in Figure 6a. At 1.0 V, the photo-current densities of the un-doped and S-doped samples were 0.0611 mA/cm² and 0.0695 mA/cm² (13.75% increase), respectively. Hence, the incorporation of S in the TiO₂ lattice increased the number of charge carriers (electron/hole pairs) in the photo-catalytic reaction of the thin film [23]. A previous study has reported similar responses, in which an increase in the photo-current to a certain extent was observed when the S content was increased, and a higher loading would be detrimental towards the photo-current readings due to the charge recombination of the electron-hole pair [31]. Thus, the incorporation of S as dopant could serve as a beneficial component to increase the photo-current in the thin film.

Perovskite solar devices with different photo-anodes were fabricated in high-humidity environment as described in the experimental procedure. Figure 6b (inset) shows the schematic diagram of the fabricated planar structured PSC in a relatively high humidity (40–50%). The photocurrent–voltage (J–V) characteristic curves were observed under 100 mW cm⁻² (1 sun) illumination. Figure 6b shows the J–V curve of the heterojunction perovskite solar devices based on the FTO/S-doped TiO₂ or TiO₂/CH₃NH₃PbI₃/spiro-MeOTAD/Ag for the best device. The un-doped TiO₂ sample (reference device) produced a short-circuit density (JSC) of 13.1 mA cm⁻², open circuit voltage of 0.982mV, fill factor of 39.88% and PCE of 5.1%. By contrast, the S-doped perovskite device was improved with device performance of J_SC of 13.9 mA cm⁻², open circuit voltage of 0.997 mV, fill factor of 43.18% and PCE of 6.0%. The values of best device and device average over 10 made samples are summarised in

Table 1. Photovoltaic parameters derived from J-V measurements for perovskite solar cells device using S-doped TiO₂ and undoped TiO₂ with a 0.07 cm² active area.

		Jsc (mA/cm2)	VOC (V)	FF (%)	PCE (%)	Rct (Ω)
TiO2	Best	13.30	0.98	39.88	5.10	18.72
	Average	13.18 ± 0.08	0.97 ± 0.01	38.70 ± 0.77	4.98 ± 0.13	20.68 ± 1.54
S-doped TiO2	Best	13.9	0.997	43.18	6.0	14.26
	Average	13.65 ± 0.17	0.99 ± 0.003	42.19 ± 0.83	5.76 ± 0.19	14.68 ± 1.05

. The device equipped with S-doped TiO₂ had better J_SC, FF and PCE compared with the un-doped TiO₂, but the V_OC was relatively the same. The improvement in the J_SC of the S-doped perovskite device was attributed to the increase in the electron collection capacity of the charge carrier and photo-current density, as described by Figure 6a, as reported in a previous study [1]. Additionally, Table 1 shows that the charge transfer resistance (R_CT) in the S-doped TiO₂ perovskite device was lower than that of the un-doped TiO₂ device. The lower resistance could facilitate higher electron transfer in the perovskite device, resulting in the higher overall PCE.

4. Conclusions

In summary, the application of S-doped TiO₂ photo-anode as the ETL for planar PSC had been demonstrated. The optimized sintering temperature was 450 °C. The optimized sulfur dopant was 10% because a higher loading induced a drop in the absorbance capacity of the photo-anode. The S as dopant was proved to be present in the TiO₂ photo-anode layer after the deposition of S containing the precursor solution. The S-doped TiO₂ photo-anode was superior to the un-doped TiO₂ photo-anode in terms of higher absorbance, photo-catalytic activity and photo-current density (13.75% increase). Perovskite solar devices with S-doped TiO₂ also showed better efficiency (18% increase) fabricated under relative humidity of 40–50%. The increase in the electron collection capacity of the carrier charge and photo-current density enhanced the PCE at 6.0%. Additional studies on the relation between hydrophobicity of ETL and stability of the PSC device should be performed. This work may facilitate the tailoring of the TiO₂ photo-anode to match the energy levels of an ETL with the perovskite absorber layer.