Theoretical Study of Abnormal Thermal Expansion of CuSCN and Effect on Electronic Structure

Abstract

CuSCN, as a new type of inorganic hole-transporting semiconductor with a wide bandgap (>3.4 eV), is attracting much attention in the fabrication of perovskite solar cells. In this article, by using first-principles density functional theory (DFT) and the quasi-harmonic approximation (QHA) approach, we have studied lattice dynamics and abnormal thermal expansion of the system, including α- and β-CuSCN phases. The influence of the abnormal thermal expansion of the lattice on the electronic structure, especially on the bandgap of the system, was explored and discussed. We found that due to the shearing modes and the three acoustic modes along the direction of the c-axis, the α- and β-CuSCN show a negative thermal expansion (NTE) in the direction of the c-axis. The torsion modes of the Cu–N–C–S atomic chains in the α-CuSCN may lead to an NTE in the directions of the a, b-axes of the α-phase. As a result, our theoretical results demonstrated that the α-CuSCN exhibits an anisotropic bulk NTE. While the β-CuSCN displays a strong uniaxial negative thermal expansion in the direction of the c-axis, in the directions of the a, b-axes, it exhibits positive thermal expansion. Our DFT calculations also predicted that the α-CuSCN has a direct bandgap, which increases slightly with increasing temperature. However, the β-CuSCN has an indirect bandgap at low temperature, which converts to a direct bandgap near the temperature of 375 K due to the strong positive expansion in the ab plane of the phase. Our work revealed the mechanisms of the abnormal thermal expansion of the two phases and a strong coupling between the anisotropic thermal expansion and the electronic structures of the system.

Influence of A/B Element Substitution on Negative Thermal Expansion in AB(CN)6 (A = Al, Ga, In; B = Co, Fe, Mn, Cr, V, Ti): A Density Functional Theory Study

“…Interestingly, it should be noted that some transverse acoustic (TA) phonon modes exhibit large negative Grüneisen parameters which also contribute greatly to the NTE of the systems. In our previous study, we found that the acoustic modes also play an important role in the NTE of the material such as Zn 2 GeO 4 , Ni 2 W­(CN) 8 , and CuSCN. , Our findings are helpful for tuning the NTE of materials.…”

DFT Computational Approach to Interfacial Recombination in MXene-Assisted Perovskite Solar Cells with β-CuSCN HTL

“…The phonon spectra of the β-CuSCN are divided into four regions: two low-frequency regions (below 9 THZ), one intermediate-frequency region (12–12.9 THZ and 22.2–22.8 THZ), and one high-frequency region (63.9–65.4 THZ). A frequency gap of 0.32 THz around 5 THz for β-CuSCN, which reduces the number of scattering channels, elongates the phonon relaxation time, and increases lattice thermal conductivity . It can be attributed to sizable atomic mass, which decreases phonon frequency as the same as seen for MX 2 (M = Mo, W; X = S, Se, Te) .…”

“…The three acoustic phonon branches include the in-plane longitudinal acoustic (LA), transverse acoustic (TA), and out-of-plane transverse acoustic (ZA) modes, with the ZA mode having a quadratic relation at the point while the other two have linear relations. The optical phonon branches contain in-plane and longitudinal out-of-plane transverse optical modes with symmetric and antisymmetric vibrational orientations. , …”

Pressure-Dependent Structural and Band Gap Tuning of Semiconductor Copper(I) Thiocyanate (CuSCN)

“…The preliminary indirect band gap was 2.18 eV of β-CuSCN at 1 atm (Figure S3a), which is smaller than the experimental results. 40,41 As a result of the presence of artificial selfinteractions, this underestimation of the band gap is a wellknown problem in DFT calculations. As shown in Figure S4, the calculated band gap continuously decreases as pressure increases, and the decrease rate of the β phase band gap with increasing pressure is faster than that of the α phase.…”