Crystal structure, stability, and optoelectronic properties of the organic-inorganic wide-band-gap perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$: Candidate for transparent conductor applications

Structural stability, electronic structure, and optical properties of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ hybrid perovskite are examined from theory as well as experiment. Solution-processed thin films of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ exhibited a high transparency in the wavelength range of 400–825 nm (1.5–3.1 eV for which the photon current density is highest in the solar spectrum) which essentially justifies a high band gap of 4 eV obtained by theoretical estimation. Also, the x-ray diffraction patterns of the thin films match well with the $\left\{00l\right\}$ peaks of the simulated pattern obtained from the relaxed unit cell of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$, crystallizing in the $I4/mcm$ space group, with lattice parameters, $a=9.30$ Å, $c=13.94$ Å. Atom projected density of state and band structure calculations reveal the conduction and valence band edges to be comprised primarily of barium $d$ orbitals and iodine $p$ orbitals, respectively. The larger band gap of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ compared to ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ can be attributed to the lower electronegativity coupled with the lack of $d$ orbitals in the valence band of ${\mathrm{Ba}}^{2+}$. A more detailed analysis reveals the excellent chemical and mechanical stability of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ against humidity, unlike its lead halide counterpart, which degrades under such conditions. We propose La to be a suitable dopant to make this compound a promising candidate for transparent conductor applications, especially for all perovskite solar cells. This claim is supported by our calculated results on charge concentration, effective mass, and vacancy formation energies.

Figure 1

XRD patterns of the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ films obtained on glass substrates from solution held under ambient conditions for (a) 7 days, (b) 14 days, (c) 21 days, and (d) 25 days. The bottom pattern is the simulated XRD pattern from our ab initio calculated structure of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ in the $I4/mcm$ space group. Only peaks corresponding to $\left\{00l\right\}$ planes are shown in the simulated pattern.

Figure 2

(a) Relaxed structure of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$. The C-N dipoles in the organic molecule, as highlighted in (a), are relaxed in three different orientations, i.e., when they are aligned (b) $\perp$ (c) along [111] and (d) [001] directions.

Figure 3

Band structure and total density of states (DOS) for ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ at relaxed lattice parameters. The valence bands near $E_F$ are majorly contributed by iodine $p$ orbitals (see inset for atom projected DOS)m while those near conduction bands are composed of Ba $d$ orbitals.

Figure 4

Transmission and reflectance spectra of films obtained on glass substrates via spin coating of the precursor solution after waiting for 25 days. A transmittance of $90\%$ and more is observed in the demarcated wavelength range of 400–825 nm for the film.

Figure 5

(a) Total and atom projected density of states for ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{Ba}}_{1-x}{\mathrm{La}}_x{\mathrm{I}}_3$ ($x=0\%,3.125\%,6.25\%$, and $12.5\%$) at relaxed lattice parameters. (b) Vacancy formation energy vs $E_F$ for single vacancy defect at Ba $\left(v_{\text{Ba}}\right)$ and I $\left(v_{\text{I}}\right)$ sites in parent and $3.125\%$ La-doped $\left({\mathrm{La}}_{\text{Ba}}\right)$ compound. Black lines indicate the result for a doped compound without any vacancy. Each defect is simulated in three charged states ($q=-1,0,+1)$.