501 research outputs found
An electronic ratchet is required in nanostructured intermediate band solar cells
We investigate in this letter the intrinsic properties that have limited the
efficiency of nanostructured intermediate band solar cells. Those devices take
advantage of intra-band transitions, which occur on narrow energy width, and
present low radiative recombination efficiency. We derive the minimum
requirements in terms of those two characteristics to achieve efficiencies in
excess of the Shockley-Queisser limit, and show that compatible nanostructures
are challenging to obtain. Especially, we evidence that currently
experimentally considered materials cannot overcome the best single junction
cells. In order to solve those issues, we consider devices including an
electronic ratchet mechanism. Firstly, such devices are shown to be much less
sensitive on the limitations of the nanostructures characteristics, so that
requirements for high efficiencies can be met. Secondly, we show that quantum
well devices present advantages over their quantum dots counterparts, although
they have attracted much less interest so far
Absorption coefficient and non-equilibrium generalized Planck's law for improved hot carrier photoluminescence spectroscopy
International audienceThe generalized Planck's law describes the light emitted by a blackbody. In the past this law has been generalized to semiconductors. Whereas Planck's orginial formulation roots on a same temperature between the body and the emitted photon gas, the generalized expression for semiconductors has been used to describe electron-hole plasmas in non-equilibrium with the lattice. Here we show experimentally how to determine different electron and hole temperatures in non-equilibrium with the pho-ton gas. Since the absorption coefficient varies with the carrier density and is part of the generalize Planck's law, we particularly emphasize the importance of the absorption coefficient in the analysis of hot carrier photoluminescence spectra. Index Terms-hot carrier solar cells, hot carrier photolumines-cence, non-equilibrium generalized Planck's law, non-equilibrium electron and hole distribution
Experimental evidence of hot carriers solar cell operation in multi-quantum wells heterostructures
International audienceWe investigated a semiconductor heterostructure based on InGaAsP multi quantum wells (QWs) using optical characterizations and demonstrate its potential to work as a hot carrier cell absorber. By analyzing photoluminescence spectra, the quasi Fermi level splitting Dl and the carrier temperature are quantitatively measured as a function of the excitation power. Moreover, both thermodynamics values are measured at the QWs and the barrier emission energy. High values of Dl are found for both transition, and high carrier temperature values in the QWs. Remarkably, the quasi Fermi level splitting measured at the barrier energy exceeds the absorption threshold of the QWs. This indicates a working condition beyond the classical Shockley-Queisser limit
Generalized Reciprocity Relations in Solar Cells with Voltage-Dependent Carrier Collection: Application to p-i-n Junction Devices
Two reciprocity theorems are important for fundamental understanding of the
solar cell operation and applications to device evaluation: (1) the
carrier-transport reciprocity connecting the dark-carrier injection with the
short-circuit photocarrier collection and (2) the optoelectronic reciprocity
connecting the electroluminescence with the photovoltaic quantum efficiency at
short circuit. These theorems, however, fail in devices with thick depletion
regions such as p-i-n junction solar cells. By properly linearizing the
carrier-transport equation in such devices, we report that the dark-carrier
injection is related to the photocarrier collection efficiency at the operating
voltage, not at short circuit as suggested in the original theorem. This leads
to the general form of the optoelectronic reciprocity relation connecting the
electroluminescence with the voltage-dependent quantum efficiency, providing a
correct interpretation of the optoelectronic properties of p-i-n junction
devices. We also discuss the validity of the well-known relation between the
open-circuit voltage and the external luminescence efficiency. The impact of
illumination intensity and device parameters on the validity of the reciprocity
theorems is quantitatively investigated
Ultrathin mono-resonant nano photovoltaic device for broadband solar conversion
International audienceNano-resonators can be used in photovoltaics to drastically improve the ability of the device to absorb light and generate photo-carriers, therefore enabling a reduction of the absorber volume. Conventionally, the harvest of the spectrally broad solar spectrum is achieved via the tedious engineering of multiple optical resonances. In this paper, we propose a breakthrough approach, which consists in reducing the solar spectral range with a spectral conversion layer to match only one resonance that can then be easily designed. We use a Maxwell solver and a ray-tracing code to optimize the nano-resonator and its spectral converter. We show that 66.2% optical efficiency can be theoretically achieved in less than 40 nm mean thick absorber while leading to device design enabling collection of photo-generated carriers
Thermoelectrical Field Effects in Low Dimensional Structure Solar Cells
Taking into account the temperature gradients in solar cells, it is shown
that their efficiency can be increased beyond the Shockley-Queisser limit (J.
Appl. Phys. 32 (1961) 510). The driving force for this gain is the temperature
gradient between this region and its surroundings.
A quantitative theory is given. Though the effect is found to be weak in
conventional solar cells, it is argued that it can be substantially increased
by proper choice of materials and design of the device. In particular, it is
shown that the insertion of a quantum well can enhance the efficiency beyond
one of the single gap cell, due to the presence of temperature jumps at the
heterojunctions.Comment: Published in Special issue Physica E 14 (1-2) on Nanostructures in
Photovoltaic
- …