Optical Constants Determination of Thermally-Evaporated Undoped Lead Iodide Films from Transmission Spectra Using the PUMA Method

0.7-μm thick lead iodide (PbI2) films thermally-evaporated on glass substrates held at different temperatures 𝑇s (35−195 ℃) are studied. Typical observed X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs of such PbI2 films prepared at high substrate temperatures 𝑇s ( 100 ℃) were found to be crystalline with hexagonal 2H-polytypic structure with the c-axis perpendicular to the surface. The room-temperature normal-incidence transmittance 𝑇exp(𝜆) of the PbI2 films has been measured as a function of spectral wavelength 𝜆 in the range 300−1100 nm and was used to retrieve the spectral dependence of their optical constants 𝑛(𝜆) and 𝜅(𝜆) using the Pointwise Unconstrained Minimization Approach (PUMA) method. The energy variation of the absorption coefficient 𝛼(𝜆)(=4π𝜅(𝜆)𝜆⁄) of the PbI2 films in the region of strong optical absorption was analyzed using various interband transition models and was found to be reasonably described by an approximate power-law relation 𝛼ℎ𝑣∝(ℎ𝑣−𝐸)𝑚, with 𝑚=2 and 𝐸gopt≅2.2 eV(±2%) (Tauc interband dielectric model, where 𝐸gopt is the optical bandgap energy). But, it is more remarkable for 𝑚=12⁄ and 𝐸g≅2.45 eV(±2%) (direct interband transition model) over a broader spectral range. For 𝑇s 100 ℃ and in the transparent and weak absorption regions, the PUMA-retrieved 𝑛(𝜆)−𝜆 data of the PbI2 films was found to fit the Wemple-DiDomenico (WDD) dispersion formula, with bandgap energy parameter 𝐸o≅3.9 eV≅2 𝐸gopt, single-oscillator energy strength 𝐸d≅19 eV and static index of refraction 𝑛o≅2.5. Analysis of the data in the absorption tail to Urbach formula yielded an Urbach-tail parameter ΓU that decreased with increasing substrate temperature to a value around 75 meV at the high substrate temperature side. These results indicate that using film growth temperatures beyond 100 ℃ leads to an enhancement in the crystallinity of the PbI2 films and reduces band tailing.