Terahertz photonics

Abstract:

Measuring terahertz (THz) conductivity on an ultrafast time scale is an excellent way to observe charge-carrier dynamics in semiconductors as a function of time after photoexcitation. However, a conductivity measurement alone cannot separate the effects of charge-carrier recombination from effective mass changes as charges cool and experience different regions of the electronic band structure. Here we present a form of time-resolved magneto-THz spectroscopy which allows us to measure cyclotron effective mass on a picosecond time scale. We demonstrate this technique by observing electron cooling in the technologically-significant narrow-bandgap semiconductor indium antimonide (InSb). A significant reduction of electron effective mass from 0.032 me to 0.017 me is observed in the first 200 ps after injecting hot electrons. Measurement of electron effective mass in InSb as a function of photo-injected electron density agrees well with conduction band non-parabolicity predictions from ab initio calculations of the quasiparticle band structure.

Abstract:

Lead-free silver-bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu₂AgBiI₆ using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu₂AgBiI₆ are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm² V^-1 s^-1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu₂AgBiI₆ as an attractive lead-free material for photovoltaic applications.

Abstract:

Metal halide perovskites have fascinated the research community over the past decade, and demonstrated unprecedented success in optoelectronics. In particular, perovskite single crystals have emerged as promising candidates for ionization radiation detection, due to the excellent opto-electronic properties. However, most of the reported crystals are grown in organic solvents and require high temperature. In this work, we develop a low-temperature crystallization strategy to grow CsPbBr₃ perovskite single crystals in water. Then, we carefully investigate the structure and optoelectronic properties of the crystals obtained, and compare them with CsPbBr₃ crystals grown in dimethyl sulfoxide. Interestingly, the water grown crystals exhibit a distinct crystal habit, superior charge transport properties and better stability in air. We also fabricate X-ray detectors based on the CsPbBr₃ crystals, and systematically characterize their device performance. The crystals grown in water demonstrate great potential for X-ray imaging with enhanced performance metrics.

Abstract:

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu₂AgBiI₆: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10⁵ cm^-1 near the absorption onset, several times that of CH₃NH₃PbI₃. Solution-processed Cu₂AgBiI₆ thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm² V^-1 s^-1), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI₃. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs₂AgBiI₆ thin films have not been synthesized to date, Cu₂AgBiI₆ is a valuable example of a stable Ag⁺/Bi³⁺ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.