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Register a new userSingle nanowire solar cells beyond the Shockley-Queisser limit
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Light management is of great importance to photovoltaic cells, as it determines the fraction of incident light entering the device. An optimal pn-junction combined with an optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At one sun illumination a short circuit current of 180 mA/cm^2 is obtained, which is more than one order of magnitude higher than what would be predicted from Lambert-Beer law. The enhanced light absorption is shown to be due to a light concentrating property of the standing nanowire as shown by photocurrent maps of the device. The results imply new limits for the maximum efficiency obtainable with III-V based nanowire solar cells under one sun illumination.
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The Shockley-Queisser (SQ) limit, introduced by W. Shockley and H. J. Queisser in 1961, is the most well-established fundamental efficiency limit for single-junction photovoltaic solar cells. For widely-studied semiconductors such as Si, GaAs and lead-halide perovskite, the SQ limits under standard solar illumination (1-sun) are 32.7%, 32.5% and 31% for bandgaps of 1.12 eV, 1.43 eV and 1.55 eV, respectively. Here, we propose that the fundamental efficiency limits for single-junction solar cells may be surpassed via photon confinement, substantially raising the theoretical limits to 49%, 45.2% and 42.1% for Si, GaAs and methylammonium lead iodide (MAPbI3) perovskite cells under 1-sun. Such enhancement is possible through the containment of luminescent photons within the solar cell, allowing the suppression of both non-radiative and radiative recombination losses, which were considered inevitable in the classical SQ model. Importantly, restricting photon emission from the solar cells raises the open-circuit voltage (VOC) to values approaching the semiconductor bandgaps, surpassing the theoretical VOC values predicted by the SQ model. The fill factors of the cells are expected to increase substantially, resulting in current-voltage characteristics with very-high squareness for ideal diode operation. Our work introduces a new framework for improving solar cell performance beyond the conventional limits.
We demonstrate controlled pumping of Cooper pairs down to the level of a single pair per cycle, using an rf-driven Cooper-pair sluice. We also investigate the breakdown of the adiabatic dynamics in two different ways. By transferring many Cooper pairs at a time, we observe a crossover between pure Cooper-pair and mixed Cooper-pair-quasiparticle transport. By tuning the Josephson coupling that governs Cooper-pair tunneling, we characterize Landau-Zener transitions in our device. Our data are quantitatively accounted for by a simple model including decoherence effects.
We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from $lambda$ = 900 nm to $lambda$ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for energies Et > 0.8eV the current energy relation is linear, similar to what was observed in NbN nanowires, whereas the current-energy relation deviates from linear behaviour for total energies below 0.8eV.
Phase-locked ultrashort pulses in the rich terahertz (THz) spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temperature superconductivity, and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional THz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved THz spectroscopy (20 - 50 THz) to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM). We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast ($<$50 fs) formation of a local carrier depletion layer.
We review principles and trends in the use of semiconductor nanowires (NWs) as gain media for stimulated emission and lasing. Semiconductor nanowires have recently been widely studied for use in integrated optoelectronic devices, such as LEDs, solar cells, and transistors. Intensive research has also been conducted on the use of nanowires for sub-wavelength laser systems that take advantage of their quasi-one-dimensional nature, flexibility in material choice and combination, and intrinsic optoelectronic properties. First, we provide an overview on using quasi-one-dimensional nanowire systems to realize sub-wavelength lasers with efficient, directional, and low-threshold emission. We then describe the state-of-the-art for nanowire lasers in terms of materials, geometry, and wavelength tunability. Next, we present the basics of lasing in semiconductor nanowires, define the key parameters for stimulated emission, and introduce the properties of nanowires. We then review advanced nanowire laser designs from the literature. Finally, we present interesting perspectives for low-threshold nanoscale light sources and optical interconnects. We intend to illustrate the potential of nanolasers in many applications, such as nanophotonic devices that integrate electronics and photonics for next-generation optoelectronic devices. For instance, these building blocks for nanoscale photonics can be used for data storage and biomedical applications when coupled to on-chip characterization tools. These nanoscale monochromatic laser light sources promise breakthroughs in nanophotonics, as they can operate at room temperature, potentially be electrically driven, and yield a better understanding of intrinsic nanomaterial properties and surface state effects in low-dimensional semiconductor systems.
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