Effect of calcination environments and plasma treatment on structural, optical and electrical properties of FTO transparent thin films

Madhav Kafle,Leela Pradhan Joshi,Gobinda Gyawali,Bhim P Kafle,Deepak P Subedi,Armila Rajbhandari,Soo W Lee,Ramesh K Kapadi,Rajendra Adhikari

doi:10.1063/1.4991403

Abstract

The dependence of the structural, optical and electrical properties of the FTO thin films on the film thickness (276 nm - 546 nm), calcination environment, and low temperature plasma treatment were examined. The FTO thin films, prepared by spray pyrolysis, were calcinated under air followed by either further heat treatment under N2 gas or treatment in low temperature atmospheric plasma. The samples before and after calcination under N2, and plasma treatment will be represented by Sair, SN2 and SPl, respectively, hereafter. The thin films were characterized by measuring the XRD spectra, SEM images, optical transmittance and reflectance, and sheet resistance of the films before and after calcination in N2 environment or plasma treatment. The presence of sharp and narrow multiple peaks in XRD spectra hint us that the films were highly crystalline (polycrystalline). The samples Sair with the thickness of 471 nm showed as high as 92 % transmittance in the visible range. Moreover, from the tauc plot, the optical bandgap Eg values of the Sair found to be noticeably lower than that of the samples SN2. Very surprisingly, the electrical sheet resistance (Rsh) found to decrease following the trend as Rshair &gt; RshN2 &gt; RshPl. The samples exposed to plasma found to possess the lowest RshPl (for film with thickness 546 nm, the RshPl was 17 Ω/sq.).

Highlights

The results from our experimental measurements on the dependence of the structural, surface, optical and electrical properties of the fluorine doped tin oxide (FTO) thin films on the deposition time, calcination environments and plasma treatment are summarized
As mentioned in the previous section, all of the samples were calcinated under air at 500 oC followed by further heat treatment under N2 except the spectrum at the top most, which was produced by the sample calcinated under hot air oven followed treatment under low temperature plasma
We investigated the effect of variation of deposition time, calcination environments and plasma treatment on optical properties of FTO films over the radiation wavelength range 200 nm - 1100 nm, with 2 nm wavelength interval, at room temperature

Summary

Introduction

Almost all of the thin film based solar cell technologies (such as dye sensitized solar cells (DSSC) and perovskite solar cells (PSC)) and light emitting diode (LED) technologies employ transparent conducing oxides (TCOs) for the purpose of both the optical access and a low-resistance electrical connection.[1,2,3,4,5,6,7,8,9] Typically, for the DSSCs and PSCs, TCOs such as indium doped tin oxide (ITO) and fluorine doped tin oxide (FTO) are widely used as the front contact and work as an anode to extract separated charge carriers from the absorbing region.[3,4,7,8,9] In these systems, rapid and facile injection of carriers into a TCO with sufficient mobility is critical. The choice of FTO for perovskite solar cell technology is driven by additional considerations, including work function, band alignment, carrier mobility, mechanical stability, materials compatibility and processing.[1,10] the advantages of SnO2 are that it is inexpensive both in terms of raw materials and processing, because it can be deposited using chemical methods such as spray pyrolysis

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Conclusion