Terahertz photonics

Abstract:

We report an analysis method that combines microphotoluminescence mapping and lifetime mapping data of single semiconductor nanowires to extract the doping concentration, nonradiative lifetime, and internal quantum efficiency along the length of the nanowires. Using this method, the doping concentration of single Si-doped wurtzite InP nanowires are mapped out and confirmed by the electrical measurements of single nanowire devices. Our method has important implication for single nanowire detectors and LEDs and nanowire solar cells applications.

Abstract:

One-dimensional semiconductors such as nanowires and nanotubes are attractive materials for incorporation in photovoltaic devices as they potentially offer short percolation pathways to charge-collecting contacts. We report the observation of free-electron lifetimes in TiO2 nanotubes of the order of tens of picoseconds. These lifetimes are surprisingly short compared to those determined in films of TiO2 nanoparticles. Samples of ordered nanotube arrays with several different tube wall thicknesses were fabricated by anodization and have been investigated by means of optical-pump-terahertz-probe (OPTP) spectroscopy, which allows measurement of transient photoinduced conductivity with picosecond resolution. Our results indicate a two-stage decay of the photoexcited electron population. We attribute the faster component to temporary immobilization of charge in shallow trap states, from which electrons can detrap again by thermal excitation. The slower component most likely reflects irreversible trapping in states deeper below the conduction band edge. Free-electron lifetimes associated with shallow trapping appear to be independent of the tube wall thickness and have very similar values for electrons directly photoexcited in the material and for those injected from an attached photoexcited dye. These results suggest that trap states are not predominantly located at the surface of the tubes. The effective THz charge-carrier mobility in the TiO2 nanotubes is determined (0.1-0.4 cm²/(Vs)) and found to be within the same range as carrier mobilities reported for TiO2 nanoparticles. Implications for the relative performance of these nanostructures in dye-sensitized solar cells are discussed.

Abstract:

The performance of perovskite solar cells has been progressing over the past few years and efficiency is likely to continue to increase. However, a negative aspect for the integration of perovskite solar cells in the built environment is that the color gamut available in these materials is very limited and does not cover the green-to-blue region of the visible spectrum, which has been a big selling point for organic photovoltaics. Here, we integrate a porous photonic crystal (PC) scaffold within the photoactive layer of an opaque perovskite solar cell following a bottom-up approach employing inexpensive and scalable liquid processing techniques. The photovoltaic devices presented herein show high efficiency with tunable color across the visible spectrum. This now imbues the perovskite solar cells with highly desirable properties for cladding in the built environment and encourages design of sustainable colorful buildings and iridescent electric vehicles as future power generation sources.

Abstract:

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Abstract:

High-efficiency perovskite solar cells typically employ an organic–inorganic metal halide perovskite material as light absorber and charge transporter, sandwiched between a p-type electron-blocking organic hole-transporting layer and an n-type hole-blocking electron collection titania compact layer. Some device configurations also include a thin mesoporous layer of TiO2 or Al2O3 which is infiltrated and capped with the perovskite absorber. Herein, we demonstrate that it is possible to fabricate planar and mesoporous perovskite solar cells devoid of an electron selective hole-blocking titania compact layer, which momentarily exhibit power conversion efficiencies (PCEs) of over 13%. This performance is however not sustained and is related to the previously observed anomalous hysteresis in perovskite solar cells. The “compact layer-free” meso-superstructured perovskite devices yield a stabilised PCE of only 2.7% while the compact layer-free planar heterojunction devices display no measurable steady state power output when devoid of an electron selective contact. In contrast, devices including the titania compact layer exhibit stabilised efficiency close to that derived from the current voltage measurements. We propose that under forward bias the perovskite diode becomes polarised, providing a beneficial field, allowing accumulation of positive and negative space charge near the contacts, which enables more efficient charge extraction. This provides the required built-in potential and selective charge extraction at each contact to temporarily enable efficient operation of the perovskite solar cells even in the absence of charge selective n- and p-type contact layers. The polarisation of the material is consistent with long range migration and accumulation of ionic species within the perovskite to the regions near the contacts. When the external field is reduced under working conditions, the ions can slowly diffuse away from the contacts redistributing throughout the film, reducing the field asymmetry and the effectiveness of the operation of the solar cells. We note that in light of recent publications showing high efficiency in devices devoid of charge selective contacts, this work reaffirms the absolute necessity to measure and report the stabilised power output under load when characterizing perovskite solar cells.