The Thermomechanical Properties of Thermally Evaporated Bismuth Triiodide Thin Films

Abstract

Bismuth triiodide (BiI₃) has been studied in recent years with the aim of developing lead-free semiconductors for photovoltaics. It has also appeared in X-ray detectors due to the high density of the Bismuth element. This material is attractive as an active layer in solar cells, or may be feasible for conversion into perovskite-like material (MA₃Bi₂I₉), being also suitable for photovoltaic applications. In this study, we report on the thermomechanical properties (stress, hardness, coefficient of thermal expansion, and biaxial and reduced Young's moduli) of BiI₃ thin films deposited by thermal evaporation. The stress was determined as a function of temperature, adopting the thermally induced bending technique, which allowed us to extract the coefficient of thermal expansion (31 × 10^-6 °C^-1) and Young's biaxial modulus (19.6 GPa) for the films. Nanohardness (~0.76 GPa) and a reduced Young's modulus of 27.1 GPa were determined through nanoindentation measurements.

Figure 1

(a) X-ray diffraction pattern of bismuth triiodide thin films deposited on Z-cut quartz, 211 Precision glass, 7059 Corning glass, 〈111〉 silicon and fused silica (normalized at (003) peak). (b) Tauc’s plot (black) and photoluminescence spectra (red) of BiI₃ thin films (the extrapolation of the curve to (αhν)^1/2 = 0 and the center of the photoluminescence emission gives an estimated band gap of 1.71 eV and 1.74 eV, respectively). (c) Refractive index and extinction coefficient as a function of photon energy (the inset is a photo of a sample −25 mm × 5 mm in size). (d,e) Scanning electron microscope images of top view and cross section of BiI₃ films, respectively and (f) AFM image.

Figure 2

(a) Typical stress versus temperature curves of BiI₃ deposited on fused silica, 7059 Corning glass, 211 Precision glass and Z-cut quartz substrates; (b) derivatives of stress, extracted from (a), with respect to temperature (dσ/dT) of BiI₃ versus CTE (or α) of the substrates. The data are average of measurements performed in five films deposited in each substrate at different runs. The CTE and biaxial Young’s modulus of BiI₃ were extracted from the indicated fitting.

Figure 3

Typical nanoindentation curve of a 2.2 μm-thick BiI₃ film deposited on 211 Precision glass.

Figure 4

Maximum penetration depth dependence of hardness (a) and reduced Young’s modulus (b), obtained by nanoindentation for 2.2 μm thick BiI₃ films deposited on 211 Precision glass, 7059 Corning glass, Z-cut quartz, 〈111〉 silicon and fused silica.

Figure 5

Scheme for the apparatus used in the thermally induced bending technique, to determine the curvature dependence of the film-substrate composite as a function of temperature.