Optical modelling data for room temperature optical properties of organic–inorganic lead halide perovskites

The optical properties of perovskites at ambient temperatures are important both to the design of optimised solar cells as well as in other areas such as the refinement of electronic band structure calculations. Limited previous information on the optical modelling has been published. The experimental fitting parameters for optical constants of CH3NH3PbI3−xClx and CH3NH3PbI3 perovskite films are reported at 297 K as determined by detailed analysis of reflectance and transmittance data. The data in this study is related to the research article “Room temperature optical properties of organic–inorganic lead halide perovskites” in Solar Energy Materials & Solar Cells [1].


Subject area
Physics More specific subject area

Photovoltaics
Type of data Table,  Parameters for optical model of each material were presented.

Data, experimental design, materials and methods
The fabrication details of CH 3 NH 3 PbI 3 thin film at room temperature are presented with surface morphology characterised by Atomic Force Microscopy. The optical properties of each individual layer were investigated by variable angle spectroscopic ellipsometry and spectrophotometry. In order to determine the optical properties of CH 3 NH 3 PbI 3 thin film, the optical properties from any substrate components were studied first. Different optical models were built to deduce the optical parameters of each layer.

Material preparation
The CH 3 NH 3 PbI 3 film was deposited onto a borosilicate glass substrate coated with 45 nm thick compact TiO 2 layer using a relatively standard sequential solution processing technique as previously reported [2].

Cleaning of borosilicate glass substrate
Borosilicate glass substrates (2.5 Â 2.5 mm 2 ) were cleaned in 2% Hallmanex detergent, acetone and Isopropanol in ultrasonic bath for 10 min in each cleaning agent followed by oxygen plasma treatment for 10 min.

TiO 2 compact layer deposition
Compact TiO 2 layer was deposited by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol at 2500 rpm for 60 s on glass followed by anneal at 500 1C for 30 min.

Perovskite film fabrication
The CH 3 NH 3 PbI 3 film was fabricated using sequential solution processing technique as previously reported [2] on borosilicate glass substrate coated with 45 nm compact TiO 2 layer. Methylammonium iodide (MAI) was synthesised following a previously reported method [3] by reacting 24 mL of 0.20 mol methylamine (33 wt% in absolute ethanol, Aldrich), 10 mL of 0.04 mol hydroiodic acid (57 wt % in water, Aldrich) and 100 mL ethanol in a 250 mL round bottom flask at 0 1C for 2 h with stirring. After the reaction, the precipitate was recovered by a vacuum evaporator at 60 1C and then dissolved in ethanol followed by sedimentation in diethyl ether until the white MAI powder appears. The final product was collected and dried at 60 1C in an oven and dehydrated in a vacuum chamber. PbI 2 was purchased from Sigma-Aldrich and dissolved as received in N, N-dimethylformamidthe (DMF) with a concentration of 462 mg/ml under stirring at 70 1C. Perovskite was deposited by spin-coating PbI 2 solution at 2000 rpm for 60 s, followed by annealing at 70 1C for 30 min after cooling down, the film was dipped into MAI solution dissolved in a 2-propanol (10 mg/ml) for 20 s and then annealed at 100 1C for 10 min.

Surface roughness characterisation
The surface morphology was characterised by Atomic Force Microscopy (AFM). The top view of glass, TiO 2 compact layer and CH 3 NH 3 PbI 3 thin film AFM images are shown in Fig. 1. The surface roughness of glass and TiO 2 compact layer is negligible compared with CH 3 NH 3 PbI 3 thin film. The median surface roughness of CH 3 NH 3 PbI 3 thin film is 50 nm and has been considered in the optical modelling, included as error bars representing measurement uncertainty.

Optical measurement and modelling
Ellipsometry was carried out using a J.A. Woollam M-2000 Spectroscopic Ellipsometer in the wavelength range of 370-1690 nm. All ellipsometry data in this study are collected from three incident angles 451, 501 and 551. The reflection (R) and transmission (T) measurements were carried out using a Varian Cary UV-vis-NIR spectrophotometer at normal incidence. Ellipsometry data and T were obtained for borosilicate glass and TiO 2 on glass in the 0.8-3 eV. R for the borosilicate glass and TiO 2 /Glass are not required, as the ellipsometry has been carried out in reflection mode and T in the 1-4 eV range provides complementary information due to the different path length the light travels in the sample. WVASE s is used to model the optical properties of the individual borosilicate glass and TiO 2 layer based on the ellipsometry and transmission data for these layers [4]. The parameters are then fed into the model for the multi-layer stack to develop the optical model for the perovskite.

Glass substrates
Cauchy model and two Gaussian dispersions (see Table 1 for parameters) were used to model the absorption of the 2.8 mm thick borosilicate glass (to be later used for the CH 3 NH 3 PbI 3 film modelling) and the second glass substrate (to be later used for the CH 3 NH 3 PbI 3 À x Cl x film modelling) in the transparent region and above 3 eV respectively. Fig. 2(a); (b); and (c) shows the experimental and modelled amplitude component Ψ; phase difference Δ; transmission T of the borosilicate glass and the second glass substrate (except phase difference Δ for glass not shown as it is zero for the whole wavelength range) respectively which are used to determine the real (ε 1 ) and imaginary (ε 2 ) parts of its dielectric constants, see Fig. 2(d). A surface SiO 2 layer on the bottom of 2.8 mm thick borosilicate glass has been modelled to be 106 nm thick using optical constants from Palik [5].

TiO 2 compact layer on glass
Tauc-Lorentz dispersion (see Table 2 for parameters) has been used to model the TiO 2 thin film by combining Tauc bandedge with Lorentz broadening function [6]. The fittings and corresponding optical constants are shown in Fig. 3. Fig. 3(a); (b); and (c) shows the experimental and modelled amplitude component Ψ; phase difference Δ; transmission T of the TiO 2 on borosilicate glass respectively which are used to determine the real (ε 1 ) and imaginary (ε 2 ) parts of its dielectric constants, see Fig. 3(d). The TiO 2 film thickness has been determined to be 44 nm.

Perovskite
The optical properties and films thicknesses of borosilicate glass and TiO 2 layer were extracted and were fixed in the simulation of CH 3 NH 3 PbI 3 perovskite properties. Two Psemi-Triangle (PSTRI) oscillators were used to describe the electronic transitions at absorption peaks, and Gaussian oscillators were used for the other regions. While for vapour-deposited CH 3 NH 3 PbI 3 À x Cl x perovskite Table 1 Parameters for Optical Models of ε 2 for borosilicate glass and glass. A, E and B represent the amplitude, centre energy and broadening of Gaussian oscillator. A n and B n are parameters in Cauchy dispersion for refractive index n.

Oscillators (borosilicate glass)
A (eV 2 ) film on glass by Wehrenfennig, optical properties of glass (measured and modelled in previous section) with a thickness of 1.7 mm [7] were used to model the CH 3 NH 3 PbI 3 À x Cl x perovskite properties. The R and T were digitised from literature between 0.8-2.5 eV, and two PSTRI oscillators were able to reproduce the experimental results in this range without the use of other oscillators at higher energy. Tables 3 and 4 list the parameters used for modelling ε 2 . A, E and B represent the amplitude, centre energy and broadening of each oscillator respectively. WL and WR stand for the endpoint positions relative to centre energy position, while AL and AR are the relative magnitudes of the left and right control points compared to amplitude (A). Fig. 4 shows the experimental and modelled amplitude component Ψ; phase difference Δ; transmission T of the CH 3 NH 3 PbI 3 perovskite which are used to determine the optical constants as a supplementary method. The CH 3 NH 3 PbI 3 perovskite film thickness has been determined to be 173 nm and 40 nm surface roughness with 89% void. The optical constants n and k deduced from this modelling are compared with other data sets in Fig. 5(a) and (b) respectively. (c) transmission T; (d) real (ε 1 ) and imaginary (ε 2 ) parts of dielectric constants of borosilicate glass and glass substrates (except phase difference Δ for glass not shown as it is zero for the whole wavelength range).

Acknowledgement
This work was supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) (Grant no. ARENA 1-SRI001). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government.