Band versus Polaron: Charge Transport in Antimony Chalcogenides

Abstract

Antimony sulfide (Sb 2 S 3 ) and selenide (Sb 2 Se 3 ) are emerging earth-abundant absorbers for photovoltaic applications. Solar cell performance depends strongly on charge-carrier transport properties, but these remain poorly understood in Sb 2 X 3 (X = S, Se). Here we report band-like transport in Sb 2 X 3 , determined by investigating the electron–lattice interaction and theoretical limits of carrier mobility using first-principles density functional theory and Boltzmann transport calculations. We demonstrate that transport in Sb 2 X 3 is governed by large polarons with moderate Fröhlich coupling constants (α ≈ 2), large polaron radii (extending over several unit cells), and high carrier mobility (an isotropic average of >10 cm 2 V –1 s –1 for both electrons and holes). The room-temperature mobility is intrinsically limited by scattering from polar phonon modes and is further reduced in highly defective samples. Our study confirms that the performance of Sb 2 X 3 solar cells is not limited by intrinsic self-trapping.

Strong electron-phonon coupling and quasi-1D polaron in Sb$_2$S$_3$

“…The system then delocalizes slightly again along the centre of the [Sb 4 S 6 ] chains in the supercell(Figure 4d), The degeneracy of the many 1D channels in the electron polaron is broken with thermal disorder as seen in the MD simulation. We would like to point out that the multiple hoppings mediated by a series of localized and delocalized states of the carrier has been observed in 1D organic semiconductors [50], with the transport mechanism explained successfully using the transient localization theory as somewhere in between of the band and hopping regime [17,51].…”

“…Experimentally, the importance of electron-phonon coupling in Sb 2 S 3 has been studied by Chong and co-workers, who observed coherent phonon generation in pump-probe experiments, and assigned it to the B 3g longitudinal optical phonon mode. Theoretical calculations have found the presence of large polaron radii extending over several unit cells and moderate Fröhlich coupling constants for antimony dichacolgenide systems [17].…”

Identifying the ground state structures of point defects in solids

“…CdTe and GaAs are tetrahedral semiconductors that adopt the zincblende structure (space group F43m). Sb 2 Se 3 and Sb 2 S 3 are layered materials (space group Pnma), composed of one-dimensional [Sb 4 X 6 ] n ribbons with covalent metal-chalcogen bonds within each ribbon and van der Waals interactions between ribbons [54][55][56][57][58][59][60] . Beyond these covalentlybonded crystals, we also studied four metal oxides (In 2 O 3 , ZnO, CeO 2 , TiO 2 ).…”

Concluding remarks: emerging inorganic materials in thin-film photovoltaics

“…So it might be that you have particular structural dimensionality, but then the bond strength, dielectric functions, effective masses each have their own anisotropy 20 . But one point we thought would be of direct relevance to this community is the carrier mobility, which shows that the conduction pathways in Sb 2 X 3 can be classified as 2D or 3D depending on the composition and carrier type 21 .…”

“…20 But one point we thought would be of direct relevance to this community is the carrier mobility, which shows that the conduction pathways in Sb 2 X 3 can be classified as 2D or 3D depending on the composition and carrier type. 21…”