Kai Shum

Synthesis and characterization of CsSnI3 thin films

We report on the synthesis and characterization of CsSnI3 perovskite semiconductor thin films deposited on inexpensive substrates such as glass and ceramics. These films contained polycrystalline domains with typical size of 300 nm. It is confirmed experimentally that CsSnI3 compound in its black phase is a direct band-gap semiconductor, consistent with the calculated band structure from the first principles. The band gap is determined to be ∼1.3 eV at Γ point at room temperature.

Schottky solar cells based on CsSnI3 thin-films

We describe a Schottky solar cell based on the perovskite semiconductor CsSnI3 thin-film. The cell consists of a simple layer structure of indium-tin-oxide/CsSnI3/Au/Ti on glass substrate. The measured power conversion efficiency is 0.9%, which is limited by the series and shunt resistance. The influence of light intensity on open-circuit voltage and short-circuit current supports the Schottky solar cell model. Additionally, the spectrally resolved short-circuit current was measured, confirming the unintentionally doped CsSnI3 is of p-type characteristics. The CsSnI3 thin-film was synthesized by alternately depositing layers of SnCl2 and CsI on glass substrate followed by a thermal annealing process.

Energy barrier at the N719-dye/CsSnI3 interface for photogenerated holes in dye-sensitized solar cells

This report is to address the question if black γ-polymorph of cesium tin tri-iodide (B-γ-CsSnI3) can be used as a solid-state hole-transport material in the conventional DSSCs with the N719 dye to replace the liquid electrolyte as reported by I. Chung et al. on Nature 485, 486, (2012). Here we demonstrate rigorously that B-γ-CsSnI3 is not energetically possible to collect photogenerated holes because of the large energy barrier at the interface of N719/B-γ-CsSnI3. Therefore, it cannot serve as a hole-transporter for the conventional DSSCs although it is a good hole-conducting material. A solution-based method was employed to synthesize the B-γ-CsSnI3 polycrystalline thin-films used for this work. These thin-films were then characterized by X-ray diffraction, Hall measurements, optical reflection, and photoluminescence (PL). Particularly, spatially resolved PL intensity images were taken after B-γ-CsSnI3 was incorporated in the DSSC structure to insure the material integrity. The means of ultraviolet photoemission spectroscopy (UPS) was used to reveal why B-γ-CsSnI3 could not act as the substitute of liquid electrolyte in the conventional DSSCs. For the completeness, other two related compounds, one is the yellow polymorph of CsSnI3 and other is Cs2SnI6 with tetravalent tin instead of double-valent tin in CsSnI3 were also investigated by UPS.

Temperature dependence of the band gap of perovskite semiconductor compound CsSnI3

The temperature dependence of the bandgap of perovskite semiconductor compound CsSnI3 is determined by measuring excitonic emission at low photoexcitation in a temperature range from 9 to 300 K. The bandgap increases linearly as the lattice temperature increases with a linear coefficient of 0.35 meV K−1. This behavior is distinctly different than that in most of tetrahedral semiconductors. First-principles simulation is employed to predict the bandgap change with the rigid change of lattice parameters under a quasi-harmonic approximation. It is justified that the thermal contribution dominates to the bandgap variation with temperature, while the direct contribution of electron-phonon interaction is conjectured to be negligible likely due to the unusual large electron effective mass for this material.The temperature dependence of the bandgap of perovskite semiconductor compound CsSnI3 is determined by measuring excitonic emission at low photoexcitation in a temperature range from 9 to 300 K. The bandgap increases linearly as the lattice temperature increases with a linear coefficient of 0.35 meV K−1. This behavior is distinctly different than that in most of tetrahedral semiconductors. First-principles simulation is employed to predict the bandgap change with the rigid change of lattice parameters under a quasi-harmonic approximation. It is justified that the thermal contribution dominates to the bandgap variation with temperature, while the direct contribution of electron-phonon interaction is conjectured to be negligible likely due to the unusual large electron effective mass for this material.

Electron‐hole recombination lifetimes in a quasi‐zero‐dimensional electron system in CdSxSe1−x

The recombination lifetimes for the radial and angular quantum number conserved 1S–1S and 1P–1P transitions from three‐dimensionally confined electrons in CdSxSe1−x were measured by time‐resolved photoluminescence (PL). The assignment of the observed transitions was supported by calculations of eigen energy levels and squared matrix element ratio for these transitions as well as well‐resolved PL peaks arising from 1S–1S and 1P–1P transitions.

Barrier potential design criteria in multiple-quantum-well-based solar-cell structures

The barrier potential design criteria in multiple‐quantum‐well (MQW) ‐based solar‐cell structures is reported for the purpose of achieving maximum efficiency. The time‐dependent short‐circuit current density at the collector side of various MQW solar‐cell structures under resonant condition was numerically calculated using the time‐dependent Schrodinger equation. The energy efficiency of solar cells based on InAs/GayIn1−yAs and GaAs/AlxGa1−xAs MQW structures were compared when carriers are excited at a particular solar‐energy band. Using InAs/GayIn1−yAs MQW structures it is found that a maximum energy efficiency can be achieved if the structure is designed with barrier potential of about 450 meV. The efficiency is found to decline linearly as the barrier potential increases for GaAs/AlxGa1−xAs MQW‐structure‐based solar cells.

Clock delivery using laminated polymer fibre circuits

We demonstrate a thin-cladding polymer-fibre-based optical clock distribution circuit for board-level optical clock delivery applications. The robust yet flexible 1-to-64 node distribution scheme offers, at 30 cm distribution length, 9.45 GHz bandwidth, 23 ps optical skews, 3.5 dB power distribution uniformity and an overall excess loss of 6.2 dB.

Dislocation-related photoluminescence peak shift due to atomic interdiffusion in SiGe/Si

Low-temperature photoluminescence (PL) spectroscopy was used to study electronic states associated with threading dislocations (D lines) in strain-relaxed Si1−xGex layers. The structures investigated were grown by ultrahigh vacuum chemical vapor deposition at 550 °C and consist of a Si(001) substrate, followed by a stepwise graded buffer layer, followed by a thick uniform composition Si1−xGex layer. The PL peak positions of the four D lines after isochronal annealing at temperatures between 600 and 800 °C were measured. We show that the large energy shift of the D1 line is due to a change in the local band gap energy at the dislocation core due to strain-driven diffusion of Ge atoms away from the dislocation core with an activation energy Ea, which varies with Ge mole fraction x.

Piezospectroscopy of GaAs and GaAs/GaAlAs single quantum wells grown on (001) Si substrates

The effects of large external stress (S) along [100] on the optical features associated with biaxially strained bulk GaAs and two GaAs/GaAlAs single quantum wells (SQWs) grown on (001) Si have been observed using photoreflectance at 300 K. This stress configuration makes it possible to externally alter the light (LH)‐ and heavy (HH)‐hole splitting in both the bulk material and the SQWs. In a SQW of width 200 A, the ground state was continuously tuned from LH to HH. In the bulk material, a stress‐induced anticrossing of the LH and HH features of the fundamental gap was determined with an interesting polarization effect.

ZnO thin films synthesized by chemical vapor deposition

This paper describes the experimental results of our recent attempt to synthesize device quality ZnO thin films on silicon and sapphire substrates by means of chemical vapor deposition. The surface features and crystal quality of these films are studied by scanning electron microscope and optical spectroscopy, respectively. Although it was not successful to deposit crystalline thin film on silicon substrate, high quality thin films on sapphire substrates were synthesized.