Development of spray pyrolysis-synthesised Bi2O3 thin films for photocatalytic applications.

Photocatalysis is a green and cost-effective approach to environmental remediation. While TiO2 is considered one of the benchmark photocatalysts, alternative materials such as Bi2O3 have recently attracted increasing scientific attention as prospective visible light photocatalysts. This study aimed to develop a strategy for Bi2O3 thin film deposition via ultrasonic spray pyrolysis and systematically study process variables for the deposition of ß-Bi2O3 thin films for photocatalytic applications. To achieve the aim, the precursor solution concentration as well as deposition and annealing temperature were optimised. The structural, optical, morphological, chemical and wettability properties of the obtained Bi2O3 thin films were investigated with respect to the effect on the photocatalytic oxidation of 10 ppm methyl orange (MO). The highest photocatalytic activity (48% in 5 h) under UV-A was recorded for the ß-Bi2O3 film deposited using 0.1 M precursor solution at 300 °C and heat-treated for 1 h in air at 350 °C. Deposition at 300 °C resulted in an amorphous film structure, whereas annealing at 350 °C led to the formation of the ß-Bi2O3 phase with the dominant facet orientation (220). These results show the suitability of spray pyrolysis for the deposition of Bi2O3 thin films with promising results for MO dye degradation, expanding the range of suitable photocatalytic materials.

Fluorene- and fluorenone-based molecules as electron-transporting SAMs for photovoltaic devices.

New semiconductors containing fluorene or fluorenone central fragments along with phosphonic acid anchoring groups were synthesized and investigated as electron transporting materials for possible application in photovoltaic devices. These derivatives demonstrate good thermal stability and suitable electrochemical properties for effective electron transport from perovskite, Sb2S3 and Sb2Se3 absorber layers. Self-assembled fluorene and fluorenone electron-transporting materials have shown improved substrate wettability, indicating bond formation between monolayer-forming compounds and the ITO, TiO2, Sb2S3, or Sb2Se3 surface. Additionally, investigated materials have compatible energetic band alignment and can passivate perovskite interface defects, which makes them interesting candidates for application in the n-i-p structure perovskite solar cell.

4.9% Efficient Sb2S3 Solar Cells from Semitransparent Absorbers with Fluorene-Based Thiophene-Terminated Hole Conductors.

Fluorene-based hole transport materials (HTMs) with terminating thiophene units are explored, for the first time, for antimony sulfide (Sb2S3) solar cells. These HTMs possess largely simplified synthesis processes and high yields compared to the conventional expensive hole conductors making them reasonably economical. The thiophene unit-linked HTMs have been successfully demonstrated in ultrasonic spray-deposited Sb2S3 solar cells resulting in efficiencies in the range of 4.7-4.9% with an average visible transmittance (AVT) of 30-33% (400-800 nm) for the cell stack without metal contact, while the cells fabricated using conventional P3HT have yielded an efficiency of 4.7% with an AVT of 26%. The study puts forward cost-effective and transparent HTMs that avoid a post-coating activation at elevated temperatures like P3HT, devoid of parasitic absorption losses in the visible region and are demonstrated to be well aligned for the band edges of Sb2S3 thereby ascertaining their suitability for Sb2S3 solar cells and are potential candidates for semitransparent applications.

A post-deposition annealing approach for organic residue control in TiO2 and its impact on Sb2Se3/TiO2 device performance.

We report a systematic investigation on the influence of two-step post-deposition treatments (PDTs) on TiO2 buffer layers deposited by ultrasonic spray pyrolysis (USP) for emerging Sb2Se3 photovoltaics. Air annealing is a typical method for recrystallizing chemically deposited TiO2 films. However, organic residues (such as carbon species) from a precursor solution based on titanium tetraisopropoxide and acetylacetone may still remain on the TiO2 surface, therefore requiring an additional annealing step. We demonstrate that vacuum annealing can be a suitable technological approach to decrease the concentration of carbon species in TiO2 films. Vacuum annealing was performed at temperatures at 160-450 °C prior to the 450 °C air annealing step. It was found that vacuum annealing at 160 °C followed by subsequent air annealing led to better device performance. This was explained by achieving an optimal balance between the removal of carbon content during vacuum annealing and the active recrystallization of TiO2 during air annealing. The decrease of carbon concentration by employing the two-step approach was supported by changes in the lattice parameters of TiO2 and proven by X-ray photoelectron spectroscopy (XPS). The given study provides experimental evidence on how nanoscale carbon species in the TiO2 heterojunction partner layer of a Sb2Se3 solar cell can affect the device's performance. By this approach, we generate complementary insights on how the quality of the main interface has an impact and can take a key role despite the optimized Sb2Se3 grain structure and orientation.

Optimization of the Sb2S3 Shell Thickness in ZnO Nanowire-Based Extremely Thin Absorber Solar Cells.

Extremely thin absorber (ETA) solar cells made of ZnO/TiO2/Sb2S3 core-shell nanowire heterostructures, using P3HT as the hole-transporting material (HTM), are of high interest to surpass solar cell efficiencies of their planar counterpart at lower material cost. However, no dimensional optimization has been addressed in detail, as it raises material and technological critical issues. In this study, the thickness of the Sb2S3 shell grown by chemical spray pyrolysis is tuned from a couple of nanometers to several tens of nanometers, while switching from a partially to a fully crystallized shell. The Sb2S3 shell is highly pure, and the unwanted Sb2O3 phase was not formed. The low end of the thickness is limited by challenges in the crystallization of the Sb2S3 shell, as it is amorphous at nanoscale dimensions, resulting in the low optical absorption of visible photons. In contrast, the high end of the thickness is limited by the increased density of defects in the bulk of the Sb2S3 shell, degrading charge carrier dynamics, and by the incomplete immersion of the P3HT in the structure, resulting in the poor hole collection. The best ETA solar cell with a short-circuit current density of 12.1 mA/cm2, an open-circuit voltage of 502 mV, and a photovoltaic conversion efficiency of 2.83% is obtained for an intermediate thickness of the Sb2S3 shell. These findings highlight that the incorporation of both the absorber shell and HTM in the core-shell heterostructures relies on the spacing between individual nanowires. They further elaborate the intricate nature of the dimensional optimization of an ETA cell, as it requires a fine-balanced holistic approach to correlate all the dimensions of all the components in the heterostructures.

Semitransparent Sb2S3 thin film solar cells by ultrasonic spray pyrolysis for use in solar windows.

The integration of photovoltaic (PV) solar energy in zero-energy buildings requires durable and efficient solar windows composed of lightweight and semitransparent thin film solar cells. Inorganic materials with a high optical absorption coefficient, such as Sb2S3 (>105 cm-1 at 450 nm), offer semitransparency, appreciable efficiency, and long-term durability at low cost. Oxide-free throughout the Sb2S3 layer thickness, as confirmed by combined studies of energy dispersive X-ray spectroscopy and synchrotron soft X-ray emission spectroscopy, semitransparent Sb2S3 thin films can be rapidly grown in air by the area-scalable ultrasonic spray pyrolysis method. Integrated into a ITO/TiO2/Sb2S3/P3HT/Au solar cell, a power conversion efficiency (PCE) of 5.5% at air mass 1.5 global (AM1.5G) is achieved, which is a record among spray-deposited Sb2S3 solar cells. An average visible transparency (AVT) of 26% of the back-contact-less ITO/TiO2/Sb2S3 solar cell stack in the wavelength range of 380-740 nm is attained by tuning the Sb2S3 absorber thickness to 100 nm. In scale-up from mm2 to cm2 areas, the Sb2S3 hybrid solar cells show a decrease in efficiency of only 3.2% for an 88 mm2 Sb2S3 solar cell, which retains 70% relative efficiency after one year of non-encapsulated storage. A cell with a PCE of 3.9% at 1 sun shows a PCE of 7.4% at 0.1 sun, attesting to the applicability of these solar cells for light harvesting under cloud cover.

Influence of Post-UV/Ozone Treatment of Ultrasonic-Sprayed Zirconium Oxide Dielectric Films for a Low-Temperature Oxide Thin Film Transistor.

Solution-processed metal oxides require a great deal of thermal budget in order to achieve the desired film properties. Here, we show that the deposition temperature of sprayed zirconium oxide (ZrOx) thin film can be lowered by exposing the film surface to an ultraviolet (UV) ozone treatment at room temperature. Atomic force microscopy reveals a smooth and uniform film with the root mean square roughness reduced from ~ 0.63 nm (UVO-O) to ~ 0.28 nm (UVO-120) in the UV-ozone treated ZrOx films. X-ray photoelectron spectroscopy analysis indicates the formation of a Zr-O network on the surface film, and oxygen vacancy is reduced in the ZrOx lattice by increasing the UV-ozone treatment time. The leakage current density in Al/ZrOx/p-Si structure was reduced by three orders of magnitude by increasing the UV-ozone exposure time, while the capacitance was in the range 290-266 nF/cm2, corresponding to a relative permittivity (k) in the range 5.8-6.6 at 1 kHz. An indium gallium zinc oxide (IGZO)-based thin film transistor, employing a UV-treated ZrOx gate dielectric deposited at 200 °C, exhibits negligible hysteresis, an Ion/Ioff ratio of 104, a saturation mobility of 8.4 cm2 V-1S-1, a subthreshold slope of 0.21 V.dec-1, and a Von of 0.02 V. These results demonstrate the potentiality of low-temperature sprayed amorphous ZrOx to be applied as a dielectric in flexible and low-power-consumption oxide electronics.

Effect of the Titanium Isopropoxide:Acetylacetone Molar Ratio on the Photocatalytic Activity of TiO2 Thin Films.

TiO2 thin films with different titanium isopropoxide (TTIP):acetylacetone (AcacH) molar ratios in solution were prepared by the chemical spray pyrolysis method. The TTIP:AcacH molar ratio in spray solution varied from 1:3 to 1:20. TiO2 films were deposited onto the glass substrates at 350 °C and heat-treated at 500 °C. The morphology, structure, surface chemical composition, and photocatalytic activity of the obtained TiO2 films were investigated. TiO2 films showed a transparency of ca 80% in the visible spectral region and a band gap of ca 3.4 eV irrespective of the TTIP:AcacH molar ratio in the spray solution. TiO2 films consist of the anatase crystalline phase with a mean crystallite size in the range of 30-40 nm. Self-cleaning properties of the films were estimated using the stearic acid (SA) test. A thin layer of 8.8-mM SA solution was spin-coated onto the TiO2 film. The degradation rate of SA as a function of irradiation time was monitored by Fourier-transform infrared spectroscopy (FTIR). An increase in the TTIP:AcacH molar ratio from 1:4 to 1:8 resulted in a ten-fold increase in the photodegradation reaction rate constant (from 0.02 to the 0.2 min-1) under ultraviolet light and in a four-fold increase under visible light.

TiO2 thin films by ultrasonic spray pyrolysis as photocatalytic material for air purification.

In this study, we showed that the TiO2 thin films deposited onto window glass are practicable for air purification and self-cleaning applications. TiO2 films were deposited onto window glass by ultrasonic spray pyrolysis method. Different deposition temperatures were used in the range of 250-450°C. The structural, morphological, optical properties and surface chemical composition were investigated to understand probable factors affecting photocatalytic performance and wettability of the TiO2 thin films. The TiO2 thin films were smooth, compacted and adhered adequately on the substrate with a thickness in the range of 100-240 nm. X-ray diffraction patterns revealed that all the TiO2 thin films consisted of anatase phase structure with the mean crystallite size in the range of 13-35 nm. The optical measurements showed that the deposited films were highly transparent (approx. 85%). The wettability test results showed that the TiO2 thin films sprayed at 350°C and 450°C and annealed at 500°C for 1 h were superhydrophilic. The photocatalytic activity of the films was tested for the degradation of methyl tert-butyl ether (MTBE) in multi-section plug-flow reactor. The TiO2 film deposited at 350°C exhibited the highest amount of conversion of MTBE, approximately 80%.

Uniform Sb2S3 optical coatings by chemical spray method.

Antimony sulfide (Sb2S3), an environmentally benign material, has been prepared by various deposition methods for use as a solar absorber due to its direct band gap of ≈1.7 eV and high absorption coefficient in the visible light spectrum (1.8 × 105 cm-1 at 450 nm). Rapid, scalable, economically viable and controllable in-air growth of continuous, uniform, polycrystalline Sb2S3 absorber layers has not yet been accomplished. This could be achieved with chemical spray pyrolysis, a robust chemical method for deposition of thin films. We applied a two-stage process to produce continuous Sb2S3 optical coatings with uniform thickness. First, amorphous Sb2S3 layers, likely forming by 3D Volmer-Weber island growth through a molten phase reaction between SbCl3 and SC(NH2)2, were deposited in air on a glass/ITO/TiO2 substrate by ultrasonic spraying of methanolic Sb/S 1:3 molar ratio solution at 200-210 °C. Second, we produced polycrystalline uniform films of Sb2S3 (E g 1.8 eV) with a post-deposition thermal treatment of amorphous Sb2S3 layers in vacuum at 170 °C, <4 × 10-6 Torr for 5 minutes. The effects of the deposition temperature, the precursor molar ratio and the thermal treatment temperature on the Sb2S3 layers were investigated using Raman spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and UV-vis-NIR spectroscopy. We demonstrated that Sb2S3 optical coatings with controllable structure, morphology and optical properties can be deposited by ultrasonic spray pyrolysis in air by tuning of the deposition temperature, the Sb/S precursor molar ratio in the spray solution, and the post-deposition treatment temperature.

Sb2S3 grown by ultrasonic spray pyrolysis and its application in a hybrid solar cell.

Chemical spray pyrolysis (CSP) is a fast wet-chemical deposition method in which an aerosol is guided by carrier gas onto a hot substrate where the decomposition of the precursor chemicals occurs. The aerosol is produced using an ultrasonic oscillator in a bath of precursor solution and guided by compressed air. The use of the ultrasonic CSP resulted in the growth of homogeneous and well-adhered layers that consist of submicron crystals of single-phase Sb2S3 with a bandgap of 1.6 eV if an abundance of sulfur source is present in the precursor solution (SbCl3/SC(NH2)2 = 1:6) sprayed onto the substrate at 250 °C in air. Solar cells with glass-ITO-TiO2-Sb2S3-P3HT-Au structure and an active area of 1 cm2 had an open circuit voltage of 630 mV, short circuit current density of 5 mA/cm2, a fill factor of 42% and a conversion efficiency of 1.3%. Conversion efficiencies up to 1.9% were obtained from solar cells with smaller areas.

Low-cost plasmonic solar cells prepared by chemical spray pyrolysis.

Solar cells consisting of an extremely thin In2S3/CuInS2 buffer/absorber layer uniformly covering planar ZnO were prepared entirely by chemical spray pyrolysis. Au nanoparticles (Au-NPs) were formed via thermal decomposition of a gold(III) chloride trihydrate (HAuCl4·3H2O) precursor by spraying 2 mmol/L of the aqueous precursor solution onto a substrate held at 260 °C. Current-voltage scans and external quantum efficiency spectra were used to evaluate the solar cell performance. This work investigates the effect of the location of the Au-NP layer deposition (front side vs rear side) in the solar cell and the effect of varying the volume (2.5-10 mL) of the sprayed Au precursor solution. A 63% increase (from 4.6 to 7.5 mA/cm(2)) of the short-circuit current density was observed when 2.5 mL of the precursor solution was deposited onto the rear side of the solar cell.

Modification of light absorption in thin CuInS2 films by sprayed Au nanoparticles.

The chemical spray pyrolysis method was used to deposit CuInS2 (CIS) thin films and Au nanoparticles (NPs) in two configurations: glass/Au-NP layer covered with CuInS2 film (Au-NP/CIS) and glass/CuInS2 films covered with Au-NP layer (CIS/Au-NP). According to X-ray diffraction (XRD), the spray of 2 mM HAuCl4 aqueous solution with a volume of 2.5 to 15 ml onto a glass substrate at 340°C results in metallic Au nanoparticles with a similar mean crystallite size in the range of 30 - 38 nm. The mean crystallite sizes remain in the range of 15 - 20 nm when grown onto a CIS film. The prepared films show plasmonic light absorption with increasing intensity in the spectral range of 500- 800 nm when increasing the volume of HAuCl4 solution sprayed. When compared to bare CIS on glass, the absorptance was increased ca. 4.5 times in the case of glass/Au-NP/CIS and ca. 3 times in the case of glass/CIS/Au-NP configuration. The glass/Au-NP/CIS configuration had an advantage since Au-NP could be embedded without chemically damaging the CIS.