A nondestructive insight into properties of nanoscale Si-layered system buried within a crystalline wafer is possible due adequate numerical analysis of dielectric functions and optical parameters. The investigation and development were made on an example of the heavily doped and/or highly excited Si:P. The comparison of predicted and measured performances was made on high efficiency bi-facial silicon solar cells.
In a series of previous articles [1], we have described phenomena that can be grouped under an integrating term as Giant Photoconversion. This designation covers discoveries and innovations in the field of silicon photovoltaics obtained mainly at the nanoscale. In this paper, we describe how to modulate the crystal lattice of the silicon wafer by burying in its interior a nanolayer with a specific crystallinity. Theoretical background has been recently proved by using mass production machines usually in service in the micro electronic industry. The new phenomena appear in a buried nanolayer of a silicon metamaterial having a specific crystalline phase and called SEG-Matter (Secondary Electron Generation – Matter). We have conceived a design and related manufacturing of new devices for high efficiency solar cells using our experimental results. The technology can be seen as a relatively simple development of the conventional c-Si cell manufacturing process completed by an amorphizing ion implantation and related thermal treatment. Both can be integrated in a production line. An original protocol has been developed first by a laboratory production on small dimension cells (a square 2 cm) in the Photonics Systems Laboratory of the Strasbourg University, then pursuit on 4-inch c-Si wafers in the LAAS CNRS in Toulouse and finally on standard c-Si wafers of the SEGTON AdT Company. The proof of concept of such solution have been recently done on commercial-sized wafers of crystalline silicon having a square 6-inch format. We obtained an increase in PV efficiency of about 2%.
A nanoscale layer of amorphized silicon is obtained by implantations with silicon ions through a P-doped FZ-silicon wafer material few nanometers below the wafer surface. After a controlled annealing, the amorphized silicon material is sandwiched between two layers of recrystallized silicon. Defects remain at the interface c-Si/a-Si/c-Si. Photoluminescence at very low temperature is experimented to determine the energy levels generated by this design. TEM pictures show that some nanocrystalline elements are located close to the interface surrounded by a-Si. However, the photoluminescence spectra do not present any signal of luminescence from them. This could be due to random sizes of nanocrystals. Then, a scan from energies below the silicon bandgap has been realized at 8 K. The spectrum is composed of multiple narrow peaks close to the conduction band and a broadband from 0.78 eV to 1.05 eV. In order to determine the origin of these signals, spectra of three distinct peaks were collected at different temperatures from 8 K to 120 K. The broadband collapses more rapidly by increasing the temperature than the narrow lines and theirs maxima of intensity differ.
Nanoscale Si-layered systems represent an attractive way to enlarge optical and electrical functions in Si optoelectronic,
photonic and PV technology. Physical interactions transform the initial Si material to a new Si-based metamaterial. The
device architecture also plays a role in specific nonlinear features. The newly observed behavior requires a better insight
into understanding the mechanisms determining the macroscopic performance. We report here some specific electrical
properties resulting from the complexity of the electron transport in different test structures designed and manufactured
by us. One of the most important parameters concerns state of the device surface. Measurements have been carried out in
different conditions of illumination (spectral composition, intensity, with/without optical bias), acquisition mode
(duration of acquisition) and device polarization mode (photodiode, photovoltaic). Time-resolved current collection with
stabilized voltages, as well as time-resolved voltage variation under stabilized currents, both made under light excitation,
allowed observation of extremely long time constants.
Layered semiconductor structures like delta-dopings and buried amorphizations, where modified optoelectronic features
result simultaneously from material composition and from device design, can considerably widen optoelectronic
applications of conventional materials. Multi-interface novel devices (MINDs) based on a nanoscale Si-layered system
buried within the heavily P-doped Si wafer have an unusual reflection, absorption and internal light propagation, which
can be dominated by a dense free-carrier gas confined within a surface potential well.
First, a model of optical functions of the heavily doped Si:P using experimental data published previously for extremely
heavily P-doped Si using the Transition Matrix Approach (TMA) to simulate the electromagnetic optical response and
field propagation has been constructed. The dielectric function combines oscillation functions and a dense free-carrier
gas (Lorentz-Drude approach) and can take into account an inhomogeneous P-doping distribution. Next, an optical
model of the real multi-interface device, based on electron microscopy data, has been constructed. A simplified sequence
of buried, optically active interfaces and corresponding layers (with transformed material and refraction indexes) is
possible due to a planar geometry. Finally, we compare our simulated and experimental reflectivity. In this way we could
determine particularly difficult-to-measure parameters. The method presented could be useful for device characterization
during the fabrication.
Analytic representation of optical function is necessary for the device and structural modeling. Nowadays optoelectronic
devices consist of complex materials combined together, therefore accurate representation of each part is even more
important and results in prediction of overall structure properties. The complete model for amorphous and crystalline
Si:P dielectric function is presented. Range of accuracy, known problems and model parameters are studied and
described. New interesting features of Si:P dielectric functions are discussed. The influence of dopants and free-carriers
is taken into account and studied separately and their overlap is also analyzed. The influence of Drude damping time on
the optical response of heavily doped Si:P is studied. All results are then compared with experimental data.
KEYWORDS: Metamaterials, Scanning electron microscopy, Reflectivity, Silicon, Photovoltaics, Amorphous silicon, Crystals, Electron beams, Optical properties, Chemical species
A new class of ordered structures that exhibit exceptional properties not readily observed before in nature or in the
laboratory is called metamaterials. Their properties arise from qualitatively new response functions that are not observed
in the constituent materials and result from the inclusion of artificially fabricated, intrinsic and extrinsic, lowdimensional
components. Low-dimensional or nanostructured Si materials as, for example, nanoscale Si-layered systems
combined with an active interface with its crystalline defects show new PV behavior never observed before in nature and
in engineering. To observe such nanolayers buried within a Si single-crystal one has to conserve a local strain that plays
an important role in the metamaterial formation. To do this, one uses techniques based on X-ray spectroscopy or more
recently proposed SEM and EDS images of just cleaved edges. The microscopy results of layered structures have been
compared with those obtained from reflectivity simulations from our code based on Lorentz-Drude theory and
experimental reflectivity measured in integrated hemispheres. An excellent agreement can be observed.
One of the most challenging topics of today's research and development concerns efficiency of light-to-electricity
conversion, preferably on Si-derived devices. The best of the possible/imaginable solutions has to allow overcoming the
indirect Si bandgap constraints. This aim becomes realizable by transforming the hard photon-matter interaction into a
soft photon-electron-electron interaction with additional new low-energy mechanisms to allow a multistage conversion
cycle. Such effects have been observed by us within new Si-derived metamaterials obtained by multiple transformations,
leading to a nanoscale Si-layered system. In such systems, a giant photoconversion could be observed for the first time
due to hot electron interactions with active interfaces and conditioned crystalline defects transformed into Si
metamaterial units, called tectons. Today it seems to be the best way to overcome conversion shortages of the bulk, thinfilm
or any other Si-based devices. We present in this work a background and three experimental demonstrations of giant
photoconversion.
A complete model for the heavily doped and/or highly excited Si:P dielectric function is
presented in the limit of the free-carrier gas approach. New interesting features of Si:P dielectric
functions are presented and discussed. The influence of dopants and free-carriers is taken into
account independently and their common features usually assumed in the literature, are analysed.
The influence of Drude damping time on the optical response of heavily doped Si:P is studied. All
results are compared with experimental data.
Nanostructured Si devices based on a nanoscale Si-layered system may constitute an
interesting system for enlarging optical and electrical functions in Si optoelectronic technology.
Strong enough physical interactions transform the initial material, without changing its chemical
composition, leading to a Si metamaterial. We report here some specific electrical properties which
illustrate the complexity of the electron transport in test structures. I-V measurements on samples
differentiated exclusively by their surface features are given for PV and photodiode modes as well as
time-resolved current collection under stabilized voltage. The measurements have been carried out
over a large range of solar light excitation intensities.
Investigation of some light-matter interactions in Multi-Interface Novel Devices (MIND)
containing a nanoscale Si-layered system have led to a method for predicting free-carrier density
dependent nonlinear optical properties as a function of doping, light excitation intensity and carrier
injection. The approach is based on the well-known t-matrix approximation. A simplified a few-layer
optical model has been constructed that will reproduce the main features/parameters of real systems.
The degree of model and simulation self-consistency is discussed using basic physical functions and
published experimental data.
Near perfect agreement between the simulated model and the corresponding experimental results
has been obtained. In this way, the simulation allowed us to determine the main origins/components of
the strong optical nonlinearity characteristic of one of the most specific MIND behaviours.
There are several ways to nanostructure Si. Some of them, e.g. nanoscale Si-layerd systems buried
within the n+ layer of a crystalline Si can provide an initial material with unpredicted optoelectronic
behavior. Such a transformation leads to a PV Si metamaterial, whose optoelectronic properties arise from
qualitatively new response functions that are (i) not observed in the constituent materials and (ii) result
from the inclusion of artificially fabricated, intrinsic and extrinsic, low-dimensional components. We show
that an extremely strong c-Si:P absorptance, determined by the free-carrier population, is larger than can
result from conventional conversion. The density of the population confined within the surface layer
delimited by the nanoscale Si-layerd system increases by injection of additional carriers from a
nanostratum (transformed up to a Si-metamaterial) lying just behind the top c-Si:P-layer.
Progress in PV conversion efficiency requires optoelectronic breakthroughs. Completing one-step PV
conversion by additional new low-energy mechanisms is one of the most important challenges of modern
photovoltaics. Si is a basic PV material which cannot be efficient enough in its bulk or thin-film form
because of its indirect bandgap. One way has been indicated by multi-interface solar cell on single-crystal Si
combining nanostructured Si materials and device improvements leading to a PV metamaterial conditioned
simultaneously both electrically and mechanically by the built-in electrical and local stress fields. This allows
a two membrane-like conversion cycle where a nanoscale Si-layered system plays a specific role due to an
active interface and crystalline defects. We demonstrate a step-like collection efficiency curve proving new
low-energy (0.3 eV) carrier generation and multiplication has been measured under solar intensities on
dedicated test structures.
The optoelectronics and photonics properties of silicon are fundamentally influenced by the density of carriers
present near the sample surface. One way of generating very large densities of such carriers is to confined them in a Si
monolayer made by implantation in an amorphous Si (Si-a) nanolayer followed by a suitable thermal treatment. In that
way, one can photo-generate a two dimensional (2D) plasma which modifies the complex refractive index of the
nanolayer.
Our work describes the modification of the reflection and absorption induced by incident light in the same devices
when varying their electronic passivation. The different studied samples contain a buried amorphous substructure and
are distinguishable exclusively by the thickness of the SiO2 covering layer.
The measurements of the different samples show large differences in their absorption coefficients. The absorption
coefficient α measured at 450 nm for a Si sample with a thin passivation layer is α~4.0x104 cm-1 that is nearly 5 times
lower than a sample which is processed with a complementary passivation α~1.8x105 cm-1. In both cases average flux
of photons (1015 s-1 cm-2) is the same. This result confirms the role of the free-carrier population, induced by the
incident light, which is confined near the surface. In the conventional modeling of the absorption only the surface
recombination rate governed by the thickness of the SiO2 covering layers has to be taken into account. In the present
work, we show that the carrier confinement also plays an important role. Such results are very interesting in the context
of optoelectronics and photonics silicon nano-structured devices.
The nowadays quite indispensable enhancement of PV conversion efficiency cannot be obtained without new
mechanisms. The most useful of these mechanisms have to appear in the front face of the device, i.e. in the emitter, so
as to allow exploitation of the energetic photons of the solar spectrum. Such an improvement can be realized through a
multistage PV conversion starting by primary generation (photon absorption) followed by secondary generations (hot
carrier collisions with low-energy generation centers). This cascade-like process is possible, for example, in
multiinterface devices containing several emitter strata. Some of these strata assume the primary free-carrier generation
while others do the secondary free-carrier generation.
In this work we report investigations of new mechanisms based on I(V) curves measured on test samples with
different multiinterface architectures, electronic passivations, front grids, collecting electrodes and so on. The
measurements have been performed under a variable intensity incident light beam conserving always its spectral (solar)
composition, except for analogous measurement cycle without a UV component. The same beam intensities with a
filtered UV component complemented these investigations. The measurements have been compared with those of a
weak excitation from a typical halogen lamp (relatively stable flux without a UV component). The test structures show
a clear improvement of the PV conversion in the UV range induced by impact ionization within the superficial
nanostratum.
Completing one-step PV conversion by additional new low-energy mechanisms is one of the most important
challenges of modern photovoltaics. Si is a basic PV material which is not efficient enough to convert the light into
electricity in its bulk or thin-film form because of its indirect bandgap. Progress in conversion efficiency requires
breakthroughs. One way has been indicated by low-dimensional or nanostructured Si materials such as, for example,
nanoscale Si-layered systems combined with an active interface with its crystalline defects.
We have demonstrated low-energy carrier multiplication experimentally under attenuated solar excitation in
nanostructured Si in both the optical and electronic approaches. This paper gives results, among others, concerning: 1) a
simulation based on previously determined parameters and 2) experiments using a reference cell. In the simulation, the
nanolayer thicknesses are 5 nm for a-Si (estimation based on TEM images) and 20 nm for (estimation based on
EELS data). To be simple and directly useful in the future development, the project has been limited to comparative
measurements of the short-circuit currents of our test cells relative to the new generation photovoltaics. The ratio of
short-circuit currents shows steps which allow estimating a characteristic energy Eδ = 0.274 eV, previously determined
by us from spectral response and modeling. The effect is particularly visible under weak incident beams.
Thanks to these investigations, the fabrication of a very highly efficient Si solar cell becomes more realistic. The
results suggest scientific and technological prospectives.
Solar photon energy can be better used when totally transformed on collectable free-carriers. The conversion of one
energetic photon could result in more than one free-carrier pair if a low-energy mechanism is involved. Such PV
conversion represents a multistage nonlinear process and requires especially dedicated low-energy centers. A cascade-like
progression is induced by the primary/fundamental/interband absorption. As shown by us previously, the
corresponding structure can be realized, for example, with nanostructured Si.
The experimental devices convert 400 nm photons into collectable primary and secondary free-carriers. The excess
carriers can be drawn out into the external electrical circuit even in a multiinterface architecture containing a carrier
collection limit. The superficial effect seems to be totally independent of the presence or not of a buried amorphized
layer. This is the first simple experimental evidence for low-energy generation. The performance is inversely
proportional to the incident light intensity.
The thermodynamic limit of conventional photovoltaic conversion is lower than 30%, while in the case of the
mechanism reported here, it can be propelled above 60%. An optimization of the effect by a suitable conditioning and
annealing should be possible, opening the way to different applications, especially in the areas of nanophotovoltaics and
very high efficiency solar cells.
A nonlinearly of the photovoltaic conversion which depends on the light excitation has been observed on multi-interface Si devices provided with a nanoscale layered system. The effect has been visualized on the carrier collection efficiency. An analysis and simulation of the spatial collection components has been done. In the experimental part, two approaches are reported: the collection dependency on the equipment used and on the light excitation. First, the same test cell has been measured using two types of equipment in three different laboratories, respectively: 1) filters with light focalization and 2) monochromator. Next, the collection has been measured with different intensities of probing flux without any optical bias. The difference of the excitations used in the filter apparatus with a focused spot (having after focusing the near-solar intensity) can be roughly estimated as five orders of magnitude. We conclude that the nonlinearity of the photovoltaic conversion depends on the density of free carriers confined inside the surface zone due to a carrier collection limit. This limit appears with a nanoscale Si-layered system at the upper crystalline/amorphous interface. The same system introduces new carrier generation centers within the crystalline Si. These centers are at the origin of a low energy carrier multiplication which strength the nonlinearity. As shown in the paper the nanoscale Si transformations lead, for example, to a considerable infrared collection improvement. This suggests that Si solar cells with very high efficiency should be realizable using this way.
Multi-interface novel devices (MIND) exhibit a dramatically low UV- and blue-spectrum photovoltaic (PV) performance. A paradox could even be observed, the better the electronic passivation the poorer the PV performance. The paradox appears under relatively low excitations in comparison with intense laser fluxes usually at its origin. The effect can be explained by solar light induced opacity, which reduces considerably or even totally the photon penetration into deeper layers, from which exclusively the photocarrier collection is possible. This opacity results from a feedback occasioned by the free-carrier absorption: better surface passivation, higher free-carrier density, stronger surface dead zone absorptance. The total energy of the incident short wavelength beam can be absorbed before a carrier collection limit buried in the emitter. This limit acts simultaneously on the electronic performance, blocking free-carriers, and on the optical performance, being at the origin of an enhancement of the surface absorptance. As a consequence, a thin surface zone dominates the optical functions of MIND cells through the free-carrier gas confined inside it. In this work we report specific effects concerning the solar-light induced opacity in MIND cells. The investigation allows modification of the free-carrier confinement using different device architectures. The main characterization methods were reflectivity and spectral response with a varying incident beam. The results prove the domination of the free-carrier optical functions on the MIND PV conversion.
KEYWORDS: Silicon, Solar cells, Photovoltaics, Luminescence, Nanocrystals, Interfaces, Lamps, Ultraviolet radiation, Quenching (fluorescence), Solar energy
A surprising photovoltaic (PV) conversion at 400 nm has been observed in nanoscale Si-layered systems (ns-Si-ls) during spectral response measurements. In conventional solar cells the UV and blue PV conversion may be poor because of the surface recombination within a thin superficial layer. In multi-interface novel devices (MIND) containing ns-Si-ls this conversion is always negligible within an even thicker surface dead zone from which practically no free-carriers can be collected. So the measured 400 nm band PV conversion in MIND cells is totally inconsistent with usually observed effects.
Another CE paradox concerns its inversely proportional variation versus incident flux intensity, lower the intensity higher the CE, which value can even exceed unity. This new effect is also localized at the superficial nanostratum and originates from postimplantation defects and nanostructures formed during the implantation process. A similar low energy free-carrier generation has been observed recently in MIND cells with buried ns-Si-ls having a relatively very thin superficial stratum because of an excellent electronic passivation. No available publication mentions such an effect despite extensive investigations on the subject of structural and optical properties of Si nanoparticles, Si nanolayers, new Si-based materials such as semiconductor silicides and the luminescence-center doped Si materials.
In this work, the carrier collection properties of the superficial Si nanostratum are reported and discussed in detail in relation to incident flux intensity. An additional low energy generation was observed experimentally. The effect could have capital importance for a breakthrough in the PV conversion efficiency in Si solar cells with nanotransformations.
The conversion efficiency of solar energy into electrical energy by solar cells is limited by the conversion thermodynamics at about 30%. This efficiency can be improved if the incident solar radiation is absorbed not only in the basic but also in a graft-modified material of reduced volume (as a nanostructure). In this case the new material can form a substructure giving a complementary absorption which cannot be obtained in the classical way by an exclusively material modification. A new design of very- and ultra-high efficient solar cells has been conceived over the last years in which ion beam processing can be extensively employed. Our previous concepts, modeling and simulations have demonstrated that a considerable increase in silicon single-crystal Back Surface Field solar cell performance is possible. This mode of operation leads to a multilayer material superposition and a device design known as a multiinterface novel device. In this paper, a review of experimental results obtained on simplified multiinterface solar cells is presented. Measurements of optical absorption and carrier photogeneration carried out on structures modified by impurity implantation have confirmed the fundamental improvements by widened absorption and IR photogeneration which result in one of the possible Si material modifications. The observed results are encouraging. The increase in absorption (up to (lambda) <EQ 3200 nm) is the best yet measured. The IR photocurrent was first observed in a single-crystal Si device (the upper wavelength limit (lambda) <EQ 2500 nm was imposed by the measuring equipment). These results have been completed with other structural, optical and electronic measurements.
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