Photoinduced charge separation, intrinsic self-doping and device applications of heterojunction materials: Mixed-halide MX solids

Abstract

Halogen-bridged mixed-valence transition metal linear chain complexes (or MX chains, M: Pt, Pd, Ni; X: Cl, Br, I) are highly anisotropic, quasi-one-dimensional (Q1D) materials that are related to conducting polymers, mixed stack charge-transfer salts, and oxide superconductors in terms of their low dimensionality and competing electron-phonon and electron-electron interactions. Each set of counterion and equatorial ligand that holds the MX chains in a three-dimensional array acts as a different “template” for the apparently soft PtX chain geometry by determining the M-M distance. Recently we prepared single crystals of mixed-halide MX x X′ 1− x materials which consist of segments of pure MX and MX′ producing junctions between the distinct halide segments. We have extensively studied the following three systems: (1) PtCl x Br 1− x , (2) PtCl x I 1− x and (3) PtBr x I 1− x for a variety of concentrations x . For the PtCl x Br 1− x system we obtained direct spectroscopic evidence for charge separation from resonance Raman (RR) experiments on doped and photoexcited single crystals: electron polarons prefer to locate on the PtBr segment while the hole polarons are trapped within PtCl segments. Theoretical calculations based on a two-band Peierls-Hubbard model demonstrate strong hybridization of PtCl and PtBr electronic bands as the driving force for separation. In contrast to PtCl x Br 1− x , both the PtCl x Br 1− x and the PtCl x Br 1− x systems exhibit qualitatively different behavior, such as intrinsic “self doping”. Important photovoltaic device applications of these novel electronic materials are discussed.