Poole-Frenkel effect in nanocrystalline SnO2:F thin films prepared by a sol-gel dip-coating technique

Abstract

The electrical transport properties of fluorine-doped tin oxide thin films prepared by a sol-gel dip-coating technique have been studied in detail. X-ray diffraction patterns confirm the crystalline nature of the films having a grain size in the nanometer range. The fluorine concentration in the films was varied from 1.62 to 12% and was measured by energy dispersive X-ray analysis. It was observed that the current-voltage (I -V) characteristics of the films were non-linear in nature, which could be explained by the Poole-Frenkel model of thermionic emission. With an increase in temperature, the non-linearity becomes more and more pronounced. The presence of adsorbed oxygen and fluorine atoms at grain boundaries is assumed to be the cause of this effect. These atoms produce defect levels, which trap electrons and create a potential barrier across the grain boundaries. In the presence of an external field, the barrier height is attenuated, resulting in the thermionic emission of electrons from the trapped level to the conduction band. The trapped potentials (φ t ) are calculated for different doping concentrations in the films. These values range from 589 to 703 meV. The maximum room temperature conductivity of the films was found to be of the order of 1 Ω -1 cm -1 . Activation energies (E a ), calculated from Arrhenius plots, range from 85 to 130 meV for different fluorine concentrations in the films. may be of use in the production of films with improved electrical characteristics. Previously, Scalvi et al.[12] reported the Poole-Frenkel effect in antimony-doped tin oxide (ATO) thin films derived from the SGDC route. However, to our knowledge, no communication on this subject for FTO films has been reported so far. ExperimentalSnCl 2 ⋅ 2H 2 O (99.95%), HF (40%) and isopropyl alcohol (99.8%) were taken as starting materials. Details of the deposition conditions are described elsewhere [9]. Several solutions were made by varying the amount of HF taken in the starting solutions, which resulted in the fluorine concentration in the films ranging from 1.62 to 12%. The resulting solutions were mixed with isopropyl alcohol, stirred for 1 h at a temperature of ~70-75 °C and then aged for 2 h to get the required sol. Ultrasonically cleaned glass substrates were dipped into and then withdrawn vertically from the solution very slowly at a rate of 8 cm/min for 10 to 15 times and then dried at ~80 -100 °C to form the gel. Thus the films were deposited layer by layer to achieve uniformity. Thereafter the films were annealed at ~300-325 °C in air for half an hour to oxidize all unreacted Sn bonds to form the required FTO films.The structural properties of the films were studied by X-ray diffraction (XRD; Philips PW 1730/PW 1710, using CuK α line). Fluorine concentrations in the films were measured by energy dispersive X-ray analysis (EDX; Leica S-440, Oxford ISIS), which could detect elements from boron (5) to uranium (92). Electrical characteristics of the films were studied by the standard four-probe method using a Keithley mu...

Highly efficient electrochemical degradation of perfluorooctanoic acid (PFOA) by F-doped Ti/SnO2 electrode

“…S6), which could affect the mass transfer of target molecule and Rct at the electrode-solution interface [30,32]. Unlike many organic fluorine materials with highly hydrophobic surfaces [35], Ti/SnO 2 -F electrode has an obvious hydrophilic property with 8.9 º of contact angle, even lower than 19.4 º of Ti/SnO 2 -Sb electrode.…”

“…In addition, only one arc can be observed on the Nyquist plot, which suggests that the electrooxidation and OER appear to a simple electrode process [31]. Generally, the OEP level of electrode is affected by two factors: the resistance of anode material and the resistance to charge transfer (Rct) at the electrode-solution interface [30,32]. Four-point probing for the above two electrodes was performed for getting the sheet resistances and specific resistivities (Table S1).…”

Preparation, characterization and electrical properties of fluorine-doped tin dioxide nanocrystals

“…Correlated with the XRD studies, there is also a significant negative correlation between the grain size and the sheet resistance, which can be attributed to grain boundary scattering. For higher fluorine content, the sheet resistance increased, this fact can be attributed to the excess fluorine atoms which lie in the grain boundary regions forming Sn-F bonds and cannot act favorably for the growth of SnO 2 nanocrystals [41].…”

Recent developments in TiO2 as n- and p-type transparent semiconductors: synthesis, modification, properties, and energy-related applications

“…The fabrication process should also have flexibility in tuning the TSO properties for improved device performance. ZnO, TiO 2 (Nb-doped TiO 2 -TNO), and doped SnO 2 are widely used as indium-free TSOs for low-e windows and other applications [17][18][19][20][21][22][23][41][42][43][44][45][46]. These materials are compared in Table 1.…”

“…These materials offered the best performance in terms of conductivity, transmissivity, excellent environmental stability, reproducibility, and surface morphology [12, 13,21,22]. Nowadays, TSOs are a major component in organic light-emitting diodes (OLEDs), copper indium gallium diselenide (CIGS) solar cells, dyesensitized solar cells (DSSC), and blue GaN-based LEDs [4-8, 10, 16, 17], in which other well-known TSOs are also widely used, including SnO 2 :Sb, ZnO:In/Al/F/B/Ga, In 2 O 3 :/F/Sb/Pb, and Cd 2 SnO 4 .…”