Low-Temperature Ionic Layer Adsorption and Reaction Grown Anatase TiO2 Nanocrystalline Films for Efficient Perovskite Solar Cell and Gas Sensor Applications

A low-temperature (90 °C) and directly grown anatase titanium dioxide (TiO2) nanocrystalline film using successive ionic layer adsorption and reaction (SILAR) for perovskite solar cell and gas sensor applications. TiO2 nanocrystalline electron transfer layer (ETL) improves the power conversion efficiency (PCE) of perovskite solar cells due to faster charge transport kinetics as well as slower charge recombination process. The optimized TiO2 nanocrystalline ETL (15 L) demonstrates as high as ~10% PCE with a short circuit current density of 18.0 mA/cm2, open circuit voltage of 0.81 V and fill factor of 66.3% in perovskite solar cells. Furthermore, room-temperature ammonia sensing characteristics of TiO2 nanocrystalline film (25 L) were demonstrated for various concentration levels of ammonia in dry air conditions. A high room-temperature response of 80% was achieved at 100 ppm of ammonia with rapid response and recovery signatures of 30 and 85 s, and nearly fifteen days stability, respectively. The response of the sensor to other gases such as formaldehyde, petrol, ethanol acetone, and ammonia etc, indicated a high selectivity towards volatile organic compounds of ammonia gas. The room temperature operation, with high selectivity, repeatability and fast transition times, suggests potentially useful in flexible and cost-effective production in optoelectrochemical device technology.

SCieNTifiC RepORTs | (2018) 8:11016 | DOI: 10.1038/s41598-018-29363-0 methods involve either long deposition times or complicated processing steps, with lack of clarity in the roles of some of the synthetic steps in these growth processes. The valuable works motivated us to handle this challenge in the synthetic route and applications potential, such as the use of successive ionic layer adsorption and reaction (SILAR) technique. In spite of its simplicity, SILAR has a number of advantages: unlike physical technique, SILAR does not require high quality target and/or substrate nor it does require vacuum at any stage; the deposition rate and the thickness of the film can be easily controlled over a wide range by changing the deposition cycles; there are virtually no restrictions on substrate material, dimensions or its surface profile; moreover, it is convenient for large area direct deposition, which helps to enhance interconnection and adherence of film with superior properties 19 .
In this work, we unveil SILAR-mediated binder-free chemical synthesis of phase pure anatase TiO 2 films directly grown on flourine-tin-oxide (FTO) and soda-lime glass (SLG) substrates for perovskite solar cell and chemical gas sensor applications. By tuning the SILAR cycles, anatase TiO 2 nanocrystalline films grown at low-temperature (90 °C) on FTO were compatible with a sequentially deposited and full coverage perovskite layer 20 . As a result, the perovskite solar cells based on this TiO 2 ETL demonstrate a PCE of ~10%, which is the one of the best figures for directly grown TiO 2 ETL-based perovskite solar cells prepared at such low temperatures. Secondly, this anatase TiO 2 nanocrystalline films on SLG substrate were employed as a sensor material to investigate selectivity for various volatile oragnic compunds (VOCs) such as ammonia, petrol, formaldehyde, ethanol, and acetone. The transient response, repeatability and response time for ammonia sensing were also determined as function of temperature and gas concentration. Finally, the stability of TiO 2 film sensors with potential commercial viability is demonstrated.

Results and Discussion
Surface morphology and elemental analysis studies. The FE-SEM image shown in Fig. 1a demonstrates the spherical surface morphology of 15 L, optimized (discussed later), SILAR TiO 2 . Inset of Fig. 1a shows the cross-sectional view of TiO 2 ETL which is relatively uniform of ~149 nm in thickness. The elemental surface composition analysis carried out by EDX (Fig. 1b) confirms stoichiometric quantities of Ti and O in for TiO 2 . The lattice parameters were measured from the HR-TEM images and the positions of the main diffraction peaks in the SAED patterns (Fig. 1c,d). The, 0.36 nm distance between two adjacent lattice planes was in accordance with the (101) reflection plane of anatase TiO 2 (JCPDS card no. 21-1272) 21  TiO 2 was more stable than the other facets 22 , and our HR-TEM images also demonstrated the strongest ring pattern for (101) plane than others in SAED spectrum. Moreover, the diffraction bright and sharp ring pattern confirms nanocrystalline nature of SILAR-mediated TiO 2 film. The XRD pattern of SILAR-mediated TiO 2 film on SLG substrate, which avoids FTO reflection contributions, is shown in Fig. 2a where well-defined reflections confirmed tetragonal crystal structure of anatase TiO 2 (JCPDS card no. 21-1272) 23 . No other impurity/phase peak was noticed, indicating high phase purity of the product. The various structural parameters such as grain-size, dislocation densities, and texture coefficient values, determined from XRD pattern of TiO 2 have tabulated in Table S1  † (Electronic Supporting Information, ESI). Furthermore, the dislocation density and texture coefficient values investigated from every diffraction peak are displayed in Table S1  † . From Table S1 † , the maximum texture coefficient of 2.011 was associated with (101) plane, which is also the preferred growth direction. The Williamson and Hall (W-H) plot (Fig. 2a inset) and the peak broadening method were utilized to estimate the average crystallite size as well as the micro-strain in the TiO 2 particles. The positive values of slope and micro-strain (ε = 12.6 × 10 −3 ) revealed an existence of tensile strain in TiO 2 film. The average crystallite-size measured from the intercept value of W-H plot was 7 nm (±1 nm) which was in close agreement with the average value (7.2 nm) obtained from Scherrer's formula and HR-TEM analysis i.e. 8 (±2) nm. The micro-strain was assigned to a relative change in interplanar distance which shifted diffraction peak positions, the difference in crystallite-size values (measured by W-H plot and Scherrer's formula) and dislocation densities in TiO 2 crystals 24 . The Raman spectrum of anatase TiO 2 film (Fig. 2b) confirmed tetragonal space group D 4th (I41/amd) having six Raman active modes (1A 1g + 2B 1g + 3E g ). The Raman peaks located at 143 (E g ), 199 (Eg), 396 (B 1g ), 514 (A 1g ), and 636 cm −1 (E g ) were assigned to the anatase TiO 2 25 . In addition, as-presented in Fig. 2c,d, XPS analysis was used to confirm Ti 4+ at TiO 2 film surface. The XPS spectrum of TiO 2 (Fig. S1  † ) showed Ti, O and C core level photoemissions, and there was no trace of any other impurity peak. The carbon peak at 284.8 eV was assigned to residual carbon in the sample, and adventitious carbon. The core-level emissions at binding energies of 458.9 and 464.6 eV are of Ti 2 p3/2 and Ti 2 p1/2 , respectively 26 . The peak with a binding energy of 530 eV is from O1 s , confirming presence of oxygen anion in the lattice (Ti-O-Ti) 27 . The BET-derived values of 121 m 2 /g specific surface-area and 6.54 nm average pore-size were confirm the mesoporous nature of the TiO 2 material. A H1-type hysteresis loop supported for mesoporosity in anatase TiO 2 film (obtained using scratched film powder) 28 . The adsorption-desorption isotherm and pore-size distribution plots for TiO 2 are shown in Fig. S2 † and Table S2 † . Since nanoparticles in TiO 2 film were mesoporous in nature, with a high surface area and small pore-size, and uniformly distributed across the substrate surface, we investigated their potential for perovskite solar cell as well as efficient and selective gas sensor applications.  Fig. 3a-c. The high binding energy cut-off (E cut-off ) value of the TiO 2 nanostructured film was determined to be 8.40 eV. The highest occupied molecular orbital (HOMO) energy onset value, the binding energy onset (E onset ), relative to the Fermi level of Au (set at 0 eV), was measured to be −5.23 eV. The valence band minimum (VBM) was calculated according to eq. 1: where, h is Planck's constant and ν is the frequency of vibration of light, corresponding to −7.58 eV for TiO 2 . The optical band gap energy (Eg) of the TiO 2 film was estimated using eq. 2: where, A is the proportionality constant and α is the absorption coefficient. Fig. S3 † presents UV-vis spectrum of TiO 2 film as an ETL where the E g value was obtained by extrapolating the linear portion of the curve near the onset of the absorption edge to the energy axis (inset of Fig. S3  † ). The E g value was determined to be 3.47 eV for the TiO 2 , which was close to the reported value 29 . Using the optical band gap, the conduction band minimum (CBM) energy level was also evaluated using eq. 3: The calculated CBM level for TiO 2 was −4.0 eV. To obtain a clear understanding of the electron transfer pathway through the ETL, an energy band diagram of the FTO/TiO 2 ETL/perovskite/spiro-OMeTAD/gold solar cell was proposed based on their appropriate band levels.
Photovoltaic performance. The UV-vis absorption spectrum of the perovskite absorber layer deposited on TiO 2 ETL was obtained (Fig. 4a). The perovskite layer on TiO 2 ETL demonstrates a wide absorbance in the visible region (centered at ~750 nm). The nanocrystallinity, smaller particle-size, mesoporous signature and higher surface area of the prepared SILAR-based anatase TiO 2 film increased the overall perovskite adsorption, which was corroborated from the enhanced UV-Vis absorption. The performance of the perovskite solar cell was tested under illumination of simulated AM1.5 G simulated solar light (100 mW cm −2 ), and electronic parameters are listed in Table 1. The TiO 2 ETL exhibited perovskite solar cell performance with an open circuit voltage (V OC ) of 0.81 V, short-circuit current density (J SC ) of 18.0 mA cm −2 , and a fill factor (ff ) of 66.3% corresponding to a PCE (ƞ%) of 9.7% (Fig. 4b). To optimize the anatase TiO 2 ETL thickness for best perovskite solar cell performance, the  Table S3 † , respectively. For the 10 L SILAR deposition, the FTO/TiO 2 ETL/perovskite/spiro-OMeTAD/gold device exhibited 8.5% PEC. Whereas, the device assembled with 20 L SILAR deposition had a PCE of 7.0%, and the best performance i.e. 9.7% was achieved for device fabricated for 15 L SILAR deposition at 8.7 mV s −1 scan rate in reverse scan direction. The comparative study of our TiO 2 ETL with different synthesis methods for perovskite solar cell application was demonstrated in Table S4 †30-35 , signifying potentiality of proposed chemical method followed developed phase-pure TiO 2 nanostructure in perovskite-based solar cells, which can be applied in developing other metal oxide nanostructured thin films too. To further verify the cell performance, EQE spectra of TiO 2 ETL was obtained and presented in Fig. 4c. The TiO 2 ETL device exhibited higher EQE value at 520 nm (~84.1%). Impedance spectroscopy in Fig. 4d (Nyquist plot) for the device conducted under 1 sun illumination at an applied bias voltage of 0.2 V, revealed two distinct semicircles (one in the high-frequency range and another in the low-frequency range) in the measured frequency range of 0.1 Hz to 100 kHz. The series resistance (R S ) is related to the wiring and FTO substrate measured in the high-frequency region. R 1 is the interface resistance of the counter electrode within the equivalent circuit representation, and CPE 1 is its capacitance. In the Nyquist plot, the main arc is responsible for the TiO 2 ETL/perovskite/spiro-OMeTAD interfaces as expressed by a combination of the interface recombination resistance (R 2 ) and chemical capacitance (CPE 2 ) 36 . The PL measurement confirms the photo-induced charge transfer and charge recombination loss in optolectrochemical devices 37 . Figure 5a presents the PL spectra of the glass/perovskite and glass/TiO 2 /perovskite layer configurations. The PL intensity of the glass/TiO 2 /perovskite film was less than that of the glass/perovskite film, indicating that the charge transfer effectively occurs prior to non-radiative carrier recombination at the interface, and this improved the electron extraction rate from the perovskite absorber   (Fig. S6a-c † ) whose data is summarized in Table S7 † . A very little hysteresis was obtained at slow scan rate i.e. 8.7 mV s −1 as there was a light difference between the PCE values obtained during the reverse (9.7%) and forward (9.1%) scans, suggesting chemical stability and mechanical robustness of developed ETL layer (Fig. S6c  † ).

Gas sensor activities.
It is well-known that the gas sensing mechanism is based on the alteration in the resistance of the sensing material, which is influenced by the adsorption-desorption process of target gas molecules via charge transfer processes. The resistance of the TiO 2 film was stabilized before exposure to various VOCs viz. ammonia, petrol, formaldehyde, ethanol, and acetone to a maximum of 100 ppm. Sensitivity, operating temperature and transient gas response study of the anatase TiO 2 nanocrystalline SLG substrate supported film (25 L) was envisaged for sensitivity study for various gases as for gas sensor thick film is essential. Above this condition, films started peeling off from the substrate surface due to its weak mechanical adhesion. The sensitivity study of TiO 2 sensor for 100 ppm gas concentration at room-temperature is shown in Fig. 6a. The TiO 2 nanocrystalline film showed better sensitivity (80%) towards ammonia over other target gases. The ammonia gas response of the TiO 2 sensor as function of operating temperature was studied (Fig. 6b). The response was recorded maximum at room-temperature (300 K) whereas, it decreased with at higher temperatures. The maximum ammonia response at room-temperature could be because of high number of ambient oxygens as well as target gas molecules adsorption on the TiO 2 sensor surface; whereas decreased adsorption process for higher operating temperatures deteriorated the ammonia detection response 38 . The transient ammonia response of anatase TiO 2 nanocrystalline film sensor is shown in Fig. 6c. As the ammonia exposed in testing chamber, the diffusing ammonia molecules adsorb on the TiO 2 film sensor surface over time. After specific time, sensor reached to its equilibrium (saturation level), where ammonia response remained constant. Furthermore, the sensing system was opened to external atmosphere after saturation. The ammonia molecules were desorbed and hence, the ammonia response decreased. With time, TiO 2 sensor achieved its original state of resistance. The response and recovery time values of TiO 2 film sensor for ammonia sensing were 30 and 85 s, respectively, signifying the use of TiO 2 material as potential room-temperature ammonia sensor. The dynamic repeatability study of TiO 2 sensor was studied and is shown in Fig. 6d. The 100 ppm ammonia gas was tested for several times at room-temperature in order to observe sensor. It was observed that TiO 2 sensor shows approximately same ammonia response over a few cycles, signifying an effective and repeatable contribution of sensor adsorption/desorption sites. The ammonia response of the TiO 2 sensor was studied for various concentrations at the room-temperature (Fig. 6e). The lowest ammonia concentration detected response at 10 ppm was 2.5%. The ammonia response increased with concentration. The ammonia response of TiO 2 film sensor increased until the sensor surface available for adsorption of target ammonia molecules. The highest ammonia response for 1000 ppm concentration was 98%. The comparison of reported sensor performances of different materials used for ammonia detection is provided in theTable S5 † and also the sensor performances of previously reported TiO 2 sensor materials for different gases are given in the Table S6 †39-48 . For higher ammonia concentration above 1000 ppm, the gas response gets was saturated, which we assume is due to maximal coverage for the TiO 2 surface by ammonia molecules to the point where limited resistance changes are detected. The stability plot of TiO 2 film for ammonia sensing is shown in Fig. 6f. The response stability of TiO 2 sensor towards 100 ppm concentrations of ammonia was studied for 15 days and observed to be approximately constant. Sensing mechanism. The gas-sensing mechanism of sensor involves the adsorption of gas molecules on the metal oxide film surface and charge redistribution between the surface and the adsorbed molecules, which eventually produces changes in the electronic structure and conductivity of sensor material 49 . The ammonia sensing mechanism on TiO 2 film sensor followed two stages i.e. oxidation and reduction. In oxidation process, air oxygen species oxidizes the TiO 2 sensor surface by capturing conduction electron offering depletion region underneath TiO 2 sensor surface and consequently conductional channel shrinks 50 . The width of depletion region formed depends on number of air oxygen species adsorbed and number of conduction electrons available in TiO 2 sensor at the measurement temperature 51 . These interconnected space-charge depleted TiO 2 nanoparticles within the sensor film offer additional resistance to that of the grain boundary resistance, because of an increased Schottky barrier potential (qV b ) in air 52 . In the second stage, ammonia gas absorption reduces the TiO 2 sensor surface by donating electrons via chemisorbed anion species according to 38 :  In this reduction process, the overall depletion width of TiO 2 reduces by gaining electrons from oxygen ion species and increasing overall conductivity and reducing the Schottky barrier height. Consequently, the inter-particle conduction pathway width is increased rendering the entire film more conductive.

Conclusion
In summary, mesoporous anatase TiO 2 nanocrystalline films were directly growth on FTO and soda-lime glass substrates by using a simple SILAR method at low-temperature i.e. 90 °C. The morphological, structural, optical and electrical properties of TiO 2 nanocrystalline films are consistent across the substrate allowing ETL layers to be grown for perovskite solar cells. The best performing perovskite solar cell was constructed using a 15-layer SILAR grown TiO 2 ETL achieved a PCE of ~9.7%, with J SC of 18.05 mA/cm 2 , V OC of 0.81 V and ff of 66.3%. The considerable 84% room-temperature sensor response of anatase TiO 2 film sensor towards ammonia among various volatile gases demonstrates the film's capability for resistance-based sensing. A wide range of gas concentrations detected by TiO 2 film sensor at room-temperature confirmed the 2.5% response as lowest ammonia detection limit at 10 ppm whereas, at 1000 ppm it was 98%. The fast response and recovery time values of present sensor for ammonia sensing were recorded as 30 s and 85 s, and nearly fifteen days stability, respectively. As-prepared anatase TiO 2 nanocrystalline film-based ammonia sensor demonstrated good repeatability, high selectivity, stability, and fast transition times, signifying its potentiality as room-temperature VOCs based gas sensor. Furthermore, the use of this novel process may in future find great potential in large-scale fabrication of perovskite solar cell and gas sensor models in practical applications.
Preparation of SILAR TiO 2 . Synthesis of anatase TiO 2 nanocrystalline films on two substrates viz. FTO and glass was carried out by low-temperature SILAR chemical deposition. Briefly, 0.1 M TiCl 4 was prepared in 50 ml de-ionized water as the Ti4+ source (eq. 1). In another beaker, 0.1 M (50 ml) K 2 S 2 O 8 was prepared (eq. 2) and kept at 90 °C constant temperature and used as sulphate/oxide source (eq. 3). The substrate (either FTO or glass) was dipped in 0.1 M TiCl 4 solution for 20 s and then the same substrate was dipped in de-ionized water for 10 s to remove remaining loosely bonded ions. A similar procedure was performed in 0.1 M K 2 S 2 O8 solution at 90 °C using the same substrate, representing one cycle (eq. 4). After about ten such cycles, a whitish film started to appear on both substrates. The dipping cycles were continued until twenty on FTO substrate and twenty-five on the glass substrate were coated, to form uniform and relatively thick whitish TiO 2 films. The deposited films were rinsed with de-ionized water and air calcined at 450 °C temperature for 1 h to obtain anatase TiO 2 nanocrystalline films (eq. 5), whose chemical reaction mechanism is proposed below which further were characterized and employed for perovskite solar cell and chemical gas sensing applications. in isopropanol (15 mg per ml) for 20 s, rinsed with isopropanol, and dried by nitrogen gas. A volume of 60 μl spiro-OMeTAD solution was spin-coated on the perovskite/TiO 2 layer at 3000 rpm for 30 s. The solution was prepared by dissolving 72 mg spiro-OMeTAD in 1 ml of chlorobenzene, to which 28.8 μl of 4-tert-butyl pyridine and 14.4 μl of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg Li-TFSI in 1 ml of acetonitrile, Sigma-Aldrich, 99.8%) were added. Finally, 70 nm of gold was deposited at ~10 −6 bar via thermal evaporation on the spiro-OMeTAD coated film for electrical contacts.
Gas sensing measurement details. For the gas response measurement, changes in electrical resistance of TiO 2 film sensors during the interaction of target gases were recorded and the response (S) of sensor was calculated according to: a g a where, R a and R g are the resistances of TiO 2 film sensor in presence of air and target gas, respectively. The desired concentration of target gas was obtained by the static liquid gas distribution method, which was calculated by the following formula 27 : 22 4 TV 273MV 1000 (13) where, C (ppm) is the desired target gas concentration; ρ (58.08 g mol −1 ) is the density of the liquid, V′ is the volume of liquid acetone (µL), T (298 K) is testing temperature, M is the molecular weight of acetone (0.788 g cm −3 ), and V is the volume of testing chamber (0.250 L).
Characterization tools. Surface morphology and elemental composition of TiO 2 films were analyzed using field-emission scanning electron microscopy (FE-SEM, JSM-7001F, JEOL Ltd., Tokyo, Japan) digital images and energy dispersive X-ray (EDX) pattern, respectively. The high resolution transmission electron microscopy (HR-TEM, Titan 80-300, FEI, Hillsboro, OR, USA) and selected area electron diffraction (SAED) images were recorded using a FEI TECNAI G2 20 S-TWIN equipped with a LaB6 cathode and a GATAN MS794 PCCD camera. These micrographs were obtained at an acceleration voltage of 200 kV. The powder of TiO 2 film sample was suspended in ethanolic solution separately and dropped onto a Formvar/carbon, 200 mesh copper grids before HR-TEM measurements. The phase of TiO 2 film was confirmed from X-ray diffraction (XRD) pattern (XRD-6000, Shimadzu, Japan) with Cu-Kα radiation (λ = 0.1542 nm). Diffraction pattern was recorded from 20° to 70° (2θ) with a 2° min −1 scan speed. The phase analysis was additionally performed using a Raman scattering spectrum scanned on Renishaw Invia Raman Microscope. The laser radiation (λ = 532 nm) was focused on the TiO 2 film surface. The spectrum was measured from 100 to 1000 cm −1 . The surface-sensitive quantitative spectroscopic technique of X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) spectra of the TiO 2 nanostructures were acquired with PHI 5000 Versa Probe (Ulvac-PHI) under high vacuum conditions (6.8 × 10 −8 pa) using a monochromatic Al Kα X-ray source (1486.6 eV). The data was collected from a spot size of 100 × 100 μm 2 . The carbon 1 s photoemission line (284.6 eV) was used for internal calibration as a reference. The optical properties of the TiO 2 nanostructured films were measured at room-temperature using a UV-vis spectrophotometer (V-530, Jasco). The photovoltaic performances and current-voltage (JV) characteristics of perovskite solar cells, for various synthesis conditions, were determined using a solar simulator (Sol3A Class AAA, Oriel Instruments) and a Keithley 2400 source measurement unit. The AM1.5 G simulated solar light (100 mW cm −2 ) light intensity level was calibrated using a standard Si reference cell certified by the Newport Corporation. All the devices were measured in a light-tight sample holder, with an active area of 0.06 cm 2 for each cell fixed using an aperture mask. The external quantum efficiency (EQE) was recorded using a quantum efficiency measurement system (QEX10, PV Measurements, Inc.) as a function of wavelength from 300 to 850 nm. The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy spectra of perovskite-coated on both the glass and glass/TiO 2 were also performed using a micro-photoluminescence measurement system (IK350IR-G, 325-nm He-Cd laser). The gas sensing properties of TiO 2 films gown directly on soda-lime glass were measured in a stainless-steel cylindrical chamber with 250 ml volume capacity. Resistivity in air and target gases were measured using a two-electrode arrangement, with silver paint metal contacts of 10 mm × 10 mm area, using a Keithley 6514 electrometer as a function of time and gas concertation. In order to control temperature, a small heater equipped with a thermocouple was used.