Mobile Ion-Driven Modulation of Electronic Conductivity Explains Long-Timescale Electrical Response in Lead Iodide Perovskite Thick Pellets

The favorable optoelectronic properties of metal halide perovskites have been used for X- and γ-ray detection, solar energy, and optoelectronics. Large electronic mobility, reduced recombination losses of the electron-hole pairs, and high sensitivity upon ionizing irradiation have fostered great attention on technological realizations. Nevertheless, the recognized mixed ionic-electronic transport properties of hybrid perovskites possess severe limitations as far as long-timescale instabilities and degradation issues are faced. Several effects are attributed to the presence of mobile ions such as shielding of the internal electrical field upon biasing and chemical interaction between intrinsic moving defects and electrode materials. Ion-originated modulations of electronic properties constitute an essential peace of knowledge to further progress into the halide perovskite device physics and operating modes. Here, ionic current and electronic impedance of lead methylammonium iodide perovskite thick pellets are independently monitored, showing self-consistent patterns. Our findings point to a coupling of ionic and electronic properties as a dynamic doping effect caused by moving ions that act as mobile dopants. The electronic doping profile changes within the bulk as a function of the actual ion inner distribution, then producing a specific time dependence in the electronic conductivity that reproduces time patterns of the type ∝t, a clear fingerprint of diffusive transport. Values for the iodine-related defect diffusivity in the range of D_ion ∼ 10^-8 cm² s^-1, which corresponds to ionic mobilities of about μ_ion ∼ 10^-6 cm² V^-1 s^-1, are encountered. Technological realizations based on thick perovskite layers would benefit from this fundamental information, as far as long-timescale current stabilization is concerned.

CuAlSe2 Inclusions Trigger Dynamic Cu+ Ion Depletion from the Cu2Se Matrix Enabling High Thermoelectric Performance

Atomic-scale incorporation of CuAlSe₂ inclusions within the Cu₂Se matrix, achieved through a solid-state transformation of CuSe₂ template precursor using elemental Cu and Al, enables a unique temperature-dependent dynamic doping of the Cu₂Se matrix. The CuAlSe₂ inclusions, due to their ability to accommodate a large fraction of excess metal atoms within their crystal lattice, serve as a "reservoir" for Cu ions diffusing away from the Cu₂Se matrix. Such unidirectional diffusion of Cu ions from the Cu₂Se matrix to the CuAlSe₂ inclusion leads to the formation, near the CuAlSe₂/Cu₂Se interface, of a high density of Cu-deficient β-Cu_2-δSe nanoparticles within the α-Cu₂Se matrix and the formation of Cu-rich Cu_1+yAlSe₂ nanoparticles with the CuAlSe₂ inclusions. This gives rise to a large enhancement in carrier concentration and electrical conductivity at elevated temperatures. Furthermore, the nanostructuring near the CuAlSe₂/Cu₂Se interface, as well as the extensive atomic disorder in the Cu₂Se and CuAlSe₂ phases, significantly increases phonon scattering, leading to suppressed lattice thermal conductivity. Consequently, a significant improvement in ZT is observed for selected Cu₂Se/CuAlSe₂ composites. This work demonstrates the use of in situ-formed interactive secondary phases in a semiconducting matrix as an elegant alternative approach for further improvement of the performance of leading thermoelectric materials.