An atmospheric pressure-based electrospraying route to fabricate the multi-applications bilayer (AZO/ITO) TCO films

Abstract

A bilayer film composed of an upper-layer of aluminum-doped ZnO (AZO) film and a lower-layer of tin-doped indium oxide (ITO) film (designated as AZO/ITO) has been successfully fabricated using an atmospheric pressure-based electrospraying method and studied as an excellent transparent electrode, efficient field-emitter and the dye-sensitized solar-cell (DSSC) electrodes in order to develop a cost-effective alternative fabrication method. The morphological, optical and electrical properties were examined for the single layer of an ITO, an AZO and the bilayer AZO/ITO film. The composition of the films was confirmed with the help of energy-dispersive spectroscopy (EDS) and X-ray photospectroscopy (XPS). The bilayer AZO/ITO film presented an enhanced optical transparency of 95% with improved electrical performance, exhibited a resistivity of 9.3 × 10⁻⁵ Ω cm and a mobility of 49.5 cm² V⁻¹ s⁻¹ compared to single layer TCO films that might be useful for optoelectronic devices. Moreover, the lowest turn-on field of ∼3.8 V μm⁻¹, and highest emission current density of 3.9 mA cm⁻² was obtained for bilayer AZO/ITO film under the field of 7.6 V μm⁻¹, and the highest field enhancement factor (β) was estimated to be 3315 ± 2, compared with AZO and ITO single layer films. Furthermore, films exhibit good long period emission current stability with a variation of less than 4% during 30 h under a field of 7.6 V μm⁻¹. For DSSCs, the photoanode composed of AZO film only showed a power conversion efficiency (PCE) as low as 0.57% with a short-circuit photocurrent density (J_SC) of 2.5 mA cm⁻² and an open-circuit voltage (V_OC) of 425 mV. The photoanode with ITO film only exhibited higher PCE (0.84%) because of higher J_SC (4.0 mA cm⁻²), whereas V_OC (375 mV) was lower than that of AZO film. Compared to AZO and ITO single layer films, the bilayer structured AZO/ITO film demonstrated much higher PCE (2.4%) because of both higher J_SC (7.4 mA cm⁻²) and V_OC (600 mV). Addition of an AZO layer over the ITO layer significantly improved the electron transport and lifetime compared to the ITO only film. One order of magnitude slower charge recombination rate for the bilayer film than for the ITO film was found to be a major factor for the improved efficiency. The fabricated cells show highly durable cell performances, even after 40 days under atmospheric conditions. The proposed atmospheric pressure-based electrospraying film fabrication method was well suitable for the deposition of bilayer AZO/ITO films which found multi-applications as an excellent transparent electrode, efficient field-emitter and improved DSSC when compared with other vacuum-based methods.