Symmetry Breaking Induced Anisotropic Carrier Transport and Remarkable Thermoelectric Performance in Mixed Halide Perovskites CsPb(I_1–xBr_x)₃

Abstract

We present a combination of first-principles calculations and the Boltzmann transport theory to understand the carrier transport and thermoelectric performance of mixed halide perovskite alloys CsPb­(I1–x Br x )3 with different Br compositions. Our computational results correlate the conduction band splitting in CsPb­(I1–x Br x )3 to the significant anisotropy in their carrier transport properties, such as effective masses and deformation potential constants. Such band splitting originates from the symmetry-broken crystal structures of CsPb­(I1–x Br x )3 polymorphs: with residue stresses/strains in asymmetric CsPb­(I1–x Br x )3, nondegenerate orbitals reconstruct the conduction band and reduce the Pb-halide antibonding character along certain directions. While the Seebeck coefficient (S) and the relaxation time-normalized electrical conductivity (σ/τ) show weak directional anisotropy, the carrier relaxation time (τ) is highly direction-dependent. The reconstruction of the conduction band finally leads to significantly anisotropic and enhanced thermoelectric power factors (PF = S 2σ) in CsPb­(I1–x Br x )3 compared to those in pure CsPbI3 and CsPbBr3, showing anomalous nonlinear alloy behavior. A delicate balance between S 2σ and combined measurement of the carrier effective mass and deformation potential constant, m*E DP, is confirmed. The lattice thermal conductivities of CsPb­(I1–x Br x )3 are significantly suppressed compared to those of their pure counterparts due to strong mass disordering and strain fields upon halogen substitution. As a result, symmetry breaking in CsPb­(I1–x Br x )3 leads to anisotropy in carrier transport, high PF, and scattered phonon transport (ultralow thermal conductivity), concurrently contributing to their promising thermoelectric figures of merit (ZT) up to 1.7 at room temperature. The principles behind the asymmetry-induced factors would serve as new design concepts to tailor the thermoelectric properties of alloys, mixtures, superlattices, and low-dimensional materials.

Synergistic Regulation Effect of Nitrate and Calcium Ions for Highly Luminescent and Robust α‐CsPbI3 Perovskite

“…The stronger conductivity of the two α phase CsPbI 3 NCs is due to the small carrier effective mass, eliminated the defects, and enhanced short‐range lattice order deriving from enhanced lattice symmetry after the modification of Ca(NO 3 ) 2 . [ 43 ]…”

“…The stronger conductivity of the two α phase CsPbI 3 NCs is due to the small carrier effective mass, eliminated the defects, and enhanced short-range lattice order deriving from enhanced lattice symmetry after the modification of Ca(NO 3 ) 2 . [43] Most importantly, the stability of the pristine γ-CsPbI 3 NCs and 0.30 mmol Ca(NO 3 ) 2 modified α-CsPbI 3 NCs was investigated. As comparison, sample-R1, and sample-R2 was further explored.…”

Strain Engineering for Tailored Carrier Transport and Thermoelectric Performance in Mixed Halide Perovskites CsPb(I1–xBrx)3

“…The phase diagram in Figure d reveals that, at 300 K, the whole range of compositions are stable against phase separation with the critical temperature being ∼276 K. Considering the entropy effect, from Figure a–c, we can see that the highest entropy and the lowest Helmholtz free energy at higher temperatures are achieved at the 3/6 composition, and the lowest internal energy belongs to the third polymorph at this composition (3/6-III) CsPbI 3/2 Br 3/2 . Therefore, we chose the (3/6-III) CsPbI 3/2 Br 3/2 polymorph structure as an example to investigate the strain effect of mixed halide perovskites, and CsPbI 3 is chosen as the representative pure perovskite owing to the better thermoelectric performance than that of CsPbBr 3 according to our previous study . The optimized configurations of pure halide perovskite CsPbI 3 and the representative polymorphic (3/6-III) CsPbI 3/2 Br 3/2 structure are shown in Figure e,f.…”

“…On the other hand, unstrained metal halide perovskites have already shown potentials in thermoelectric applications because of their ultralow thermal conductivities (typically <1 W m –1 K –1 ) and excellent carrier transport properties . Through the Seebeck or the Peltier effect, they are suitable for applications in waste heat recycling, space power generation, and solid-state cooling. − To enhance the thermoelectric performance, , the band edges and carrier transport properties can be fine-tuned using various strategies, such as carrier concentration optimization, lattice defects introduction, and band structure engineering. − Moreover, the coupling between the thermoelectric properties and the internal strain field is quite unique in mixed halide perovskites because of the heterogeneous thermal expansion coefficients, lattice mismatches between the two different single-halide crystals, and microdomains/grains with residual stresses resulting from either phase de-mixing or epitaxial growth on a substrate as in their single-halide counterparts …”

Interface properties of CsPbBr$_3$ /CsPbI$_3$ perovskite heterostructure for solar cell

“…Hembram) ORCID(s): 0000-0003-1278-7974 (J. Kumar) For instance, several studies have been carried out with change in alkali metal [28][29][30], cation [6,[31][32][33][34], halide [35][36][37][38][39] to capture those scenarios with different experiment and theory. Technologically, the desired compositions with defined structures are achieved by simple chemical process, where micro to nano sized samples are formed in the localised scale [32,33,37,38,40].…”