For the case of Silicon solarcell, increasing its ef ﬁciency while keeping a low cost process is one of the goals of the Si-PV industry to continuously decrease the cost of the power generation. This is the industrial condition to remain a major player in the provision of power generation solutions in the forthcoming years. The different loss paths in a Sisolarcell are well identi ﬁed and many developments have been performed to solve them. Among them, the thermalization mechanism that is the consequence of the mismatch between the solar spectrum energy (UV region) and the solarcell band gap energy (1.1 eV of the Sisolarcell) can be overcome. Such an objective can be achieved by using frequencyconversionlayers so-called Down Conversion (DC) [ 1 – 4 ] or Down Shifting (DS) [ 1 , 5 ] layers. Such layers have been developed in order to convert one UV incident photon into two IR ones (DC case) or one UV incident photon into one visible one (DS case) that can be absorbed by the Sicell. To reach this objective many systems using a couple of trivalent ions such as Pr 3+ -Yb 3+ , Tb 3+ -Yb 3+ , Ce 3+ -Yb 3+ for DC process or one trivalent ions such Pr 3+ , Tb 3+ for the DS conversion have been studied. Unfortu- nately, the major drawbacks of these layers are the use of a non Si-compatible process due to the nature of the host matrix as well as the low absorption cross section of the rare earth ions that limits their excitability in the solar spectrum range [ 5 – 9 ]. To overcome this problem, a Si-PV compatible host matrix containing sensitizers has been developed. The presence of sensitizers allows to ef ﬁciently excite rare earth ions. Moreover, to keep a low cost process for a future develop- ment in the SI-PV industry, the host matrix developed should have good anti-reflective properties.
L. Dumont * , P. Benzo * , J. Cardin * , C. Labbé * , I-S Yu Ϯ , F. Gourbilleau *
* CIMAP CNRS/CEA/ENSICAEN/UCBN, 6 Boulevard Maréchal Juin, 14050 Caen Cedex 4, France
Ϯ Department of Materials Science and Engineering, National Dong Hwa University, Da Hsueh Rd, Shoufeng, Hualien 97401,Taiwan
L. Dumont et al, SiNx: Tb 3+ –Yb 3+ , an efficient down-conversion layer compatible with a silicon solarcell process, , Solar Energy Materials and Solar Cells 145, 84-92, 2016
F. Gourbilleau et al., FrequencyConversionLayersforSiSolarCellEfficiencyImprovement in Frontiers in Electronic Technologies, Springer, Singapore, 85-91, 2017 L. Dumont et al, Down-shifting Si-based layer forSisolar applications, Solar Energy, Materials and Solar Cells 169, 132-144, 2017
L. Dumont et al., First down converter multilayers integration in an industrial Sisolarcell process, Progress in Photovoltaics: Research and App., 27, 2, 152-162, 2019 Ing Song Yu, et al, Monolithic crystalline silicon solar cells with SiNx layers doped with Tb 3+ and Yb 3+ rare-earth ions, Journal of Rare Earths, 37,5, 515-519, 2019
We report on the eﬃciency improvement of Cu(InGa)Se 2 (CIGS) based solar cells obtained upon
coating the cell with a Yb-doped SnO x layer. This layer is deposited by reactive sputtering and serves as a photon down-shifting converter. The direct excitation of the SnO x host matrix with UV photons leads to a strong emission of near infrared photons from the Yb 3+ ions suggesting an eﬃcient energy transfer from SnO x to the Yb 3+ ions. The deposition the Yb:SnO x ﬁlms at higher temperatures results in an enhancement of the PL emission as well as in an improvement of the transport properties. The optimized ﬁlms exhibit a transmittance around 80 % in the visible region, a resistivity of 6 ×10 −3 Ωcm and a mobility as high as 50.1 cm 2 /Vs. Such SnO x layers doped with 1.3 at. % of Yb were deposited at 100 ◦ C on conventional CIGS based solar cells to replace the standard ZnO n-type conductive layer. The solar cells performances are noticeably improved. This is witnessed by a net gain of 10% of the external quantum eﬃciency (EQE) at 360 nm. The short-circuit current (J SC ) increased by about 0.56 mA/cm 2 while the ﬁll factor reaches 64.4 %. As an overall result, the best solarcell exhibited a remarkable enhancement in eﬃciency of about 0.6 %. This improvement of the photovoltaic eﬃciency by a simple substitution of i-ZnO by Yb:SnO x for CIGS cells oﬀers possible application to other solar cells. These results are encouraging towards enhancing the eﬃciency of solar cells at low cost which will contribute to the large deployment of clean energy.
component and the increase of J sc with increasing base thicknesses contribute to the V oc improvement. The slight
differences in the resistances and FF (see Table III ) are attributed to variations in the process and native oxide on the surface formed during chemical etching.
An intrinsic AlGaAs layer was then introduced between the p-AlGaAs base and the n-InGaP emitter with two different motivations. First, the intrinsic AlGaAs should prevent the diffusion of Be doping atoms from reaching the n-type layer during growth . Second, an intrinsic layer is expected to lower the concentration of ionized impurities and thus to increase the carrier diffusion length by decreasing the defect center concentration induced by dopants  . Solar cells with different p-doped and intrinsic AlGaAs thicknesses are compared in Table III . Nevertheless, only minor differences can be seen in efficiencies of solar cells with similar total thicknesses (p-type + intrinsic, see B.4 vs B.5 in Table III ). We observe a slight increase in the EQE at short wavelength for B.6 compared to B.7, which is believed to be correlated to the thicker intrinsic layer of 650 nm preventing Be diffusion in the InGaP emitter. From dark and one-sun J-V measurements, no clear conclusion can be drawn on the impact of the intrinsic layer thickness ( Table III and Fig 5(b) ). However, PL measurements exhibit a two-fold radiative intensity enhancement from the B.7 structure with a thinner intrinsic layer (20 nm) and a thicker p- doped layer (1000 nm as compared to 350 nm in B.6) (see Fig 5(c) ). Further optimization of this thickness is needed. In the following, optimized solarcell structures are made of a 100 nm-thick intrinsic layer, slightly thicker than the 20 nm of B.7, in combination with 900 nm-thick and 1900 nm-thick doped base layers.
Abstract— the influence of the absorber layer (i-layer)
properties on the amorphous solar cells parameters has been an object of research since the 1980s. In this study, a numerical simulation was carried out to study the influence of the intrinsic layer by using a novel technique based on graded band gap for amorphous single junction solar cells. In this context, we use the software called AMPS-1D. The optimized properties of the different layers of a-Si:H solarcell, especially intrinsic layer, were suggested to obtain the maximum conversionefficiency. Indeed, the use of intrinsic multilayer can control the spectral overlap by employing band-gap grading which the potential initial conversionefficiency of single-junction solarcell reach to 11.52%.
Tb 3+ -Yb 3+ co-doped SiN x down-conversionlayers compatible with silicon Photovoltaic Technology
were prepared by reactive magnetron co-sputtering. Efficient sensitization of Tb 3+ ions through a SiN x
host matrix and cooperative energy transfer between Tb 3+ and Yb 3+ ions were evidenced as driving mechanisms of the down-conversion process. In this paper, the film composition and microstructure are investigated alongside their optical properties, with the aim of maximizing the rare earth ions incorporation and emission efficiency. An optimized layer achieving the highest Yb 3+ emission intensity was obtained by reactive magnetron co-sputtering in a nitride rich atmosphere for 1.2 W/cm² and 0.15 W/cm² power density applied on the Tb and Yb targets, respectively. It was determined that depositing at 200 °C and annealing at 850 °C leads to comparable Yb 3+ emission intensity than depositing at 500 °C and annealing at 600 °C, which is promising for applications toward silicon solar cells.
1.1. Brief description of the impact ionization phenomenon
Concepts for improving the efficiency of solar cells are the preoccupation of many scientists. Both in research laboratories and in manufacturing, improvement of efficiency is a high priority. The solutions of this improvement could provide on reducing losses such as those by thermalization and the non-absorption of low energy photons . Peter Würfel  showed that, the theoretical upper limit of maximum solarcellefficiency is approximately 0.86. Various methods have been proposed in order to improve the efficiency of solarcell, such as the solar thermal conversion method [1-3], the method of Tandem cells [1,2,4], the method of concentrator cell [1,2,5], the method of two step excitation in three levels system [1,2], and the method of impact ionization [1,2,6,7] on which the current study focuses.
[8,9] and down/up-conversionlayers [8,10–14] . Those frequency conversions are usually achieved by using rare earth-doped matrices. The up-conversion layer concerns the photons transparent for the Si-SC and thus is placed at the bottom of the cell. The down-conversion or down-shifting layers are deposited on top of the solar cells to improve the use of the incoming photons with an energy higher than the Si bandgap. In a down-conversion (DC) layer, one UV photon is trans- formed in two IR photons having an energy just above the SC bandgap. Thus the number of photons used by the cell increases while the thermalization decreases. Whereas a down-shifting (DS) layer will absorb one UV photon and reemit a single photon with an energy above the SC bandgap. Thus the down-shifted photons wavelengths better match the spectral photo-absorptivity response of the Si-SC. In addition the thermalization process will take place in the DS layer instead of the SC. Thus by thermally isolating the DS layer from the SC, with transparent materials, the thermalization could be partially decoupled from the cell which reduces the entropy ﬂow and may increase the cell e ﬃciency as described by Landsberg eﬃciency  . A DS layer is composed of a matrix containing one rare earth ion whereas in a down-conversion layer, two or more rare earth ions are incorpo- rated in the matrix. Many matrices and rare earth ions have been
ﬁlm (2 0 0) oriented is the most adapted host matrix to sensitise 1 at.% Nd 3+ ions for an emission around
1064 nm making such Nd-doped layers interesting for photon conversion by down shifting process.
The best single crystal silicon solarcell efﬁciency reaches a maximum of 24.7%  close to the limit of 31% calculated by Shockley and Queisser  . One of the origins of this limited efﬁ- ciency is related to the spectral mismatch: photons with energy smaller than the band-gap are not absorbed (sub-band-gap trans- mission) and a large part of the energy of photons with energy lar- ger than the band gap is lost as heat (thermalization losses). Photon management is one of the third generation photovoltaic principles which can lead to the improvement of silicon solarcell yield  . High energy photon might be split in one or two photons with a smaller energy. Each of these photons can subsequently be absorbed by the solarcell and generate an electron–hole pair. These processes, known as down-shifting (DS) for one emitted photon or down-conversion (DC) for two emitted photons, are ben- eﬁcial forsolar cells with a smaller band-gap where thermalization losses are the major loss factor. Lanthanides ions are ﬁtted to DS or DC purpose as their atomic energy levels allow efﬁcient spectral
Keywords: photovoltaics; ordered bulk heterojunctions; solution processing; light scattering; surface traps; electrochemical anodization; solvothermal synthesis; metal oxide; TiO 2 ; ZnO
The increasing global demand for energy has spurred research efforts to find new and improved sources of cheap, environmentally neutral, renewable energy. Inorganic solar cells based on materials such as crystalline silicon, cadmium telluride, or copper indium germanium selenide (CIGS) constitute mature technologies that exhibit a relatively high power conversionefficiency (PCE) of around 12%–20% in deployed modules [ 1 ] and thus dominate commercially available photovoltaic technologies. However, the relatively long energy payback times of inorganic solar cells [ 2 , 3 ] have partially impeded their pace to widespread deployment, and thus alternative approaches are being explored. Organic photovoltaics [ 4 ], dye-sensitized solar cells [ 5 ], halide perovskite solar cells [ 6 ], and quantum-dot solar cells [ 7 ] are examples of next generation solution-processable solarcell technologies that have emerged as lower cost, lower energy payback time alternatives to replace conventional solar cells [ 8 , 9 ]. Among these technologies, halide perovskite solar cells (HPSCs) are currently the topic of intense scientific and engineering interest due to their facile synthesis, use of earth-abundant constituent elements and high device performance [ 10 ]. A major breakthrough occurred in 2012 when
I.3.3.2. The sensitizing matrix
Richards shows that experimental external quantum yields are lower than the theory because of the poor absorption cross section of RE ions . Indeed, most of the rare earth ions have a small absorption cross-section (around 10 -20 cm² ) which decreases the efficiency of the system. In order to increase the absorption, a sensitizer has to be used. Such sensitizer has an absorption cross- section higher than the rare earth ions and will absorb the photons before transferring their energy to the ions. Two kinds of sensitizer have been described by Dexter. The first kind is a dopant that may be another rare earth that have higher absorption cross-section such as the Ce 3+ (10 -18 cm -1 ).They also have broader absorption and emission cross-section enabling an excitation and an energy transfer over a broader spectral range. They are then used in three rare earths systems [108,109]. The second is the matrix itself that have an absorption coefficient of around 10 3 cm -1 (at 0.8 eV) . In this case the matrix should be chosen for its bandgap that must have an energy higher than the absorption energy of the selected rare earth ions. In that way, the matrix will absorb the photons in a wide range of energy and transmits easily the energy to the surrounding rare earth ions (at the RE energy of absorption). It is important to note that the ideal matrix should absorb the photons with an energy higher than twice the energy of the bandgap of the SC but be transparent for all the photons with a lower energy. Indeed, absorption of photons below this limit will decrease the cellefficiency under its usual efficiency without the DC layer .
to photoelectrode-oxide structure strategy (Gao et al., 2014; Ko et al., 2011; Lai et al., 2010; Lu et al., 2014; Pan et al., 2009; Suh et al., 2007; Wang et al., 2010; Xu et al., 2010; Zhang et al., 2008, 2007; Zhao et al., 2013).
One-dimensional-like structures (Figure 6.1), such as nanowires (NW), nanorods (NR), nanotubes (NT) or nanobelts (NB) can facilitate the electron transfer (Gao et al., 2014; Lu et al., 2014; Martinson et al., 2007; Xu et al., 2010). This structure accommodates direct conduction pathway for a rapid electrons collection. Moreover, they have lower trap density that induces a faster electron transport. Electron diffusion coefficient in ZnO NW is reported about 0.05-0.5 cm 2 s -1 , which is much larger than in nanoparticle ZnO layers under DSSC operation condition (10 -5 -10 -3 cm 2 s -1 ) (Magne et al., 2013; Zhang et al., 2009). The limited performance of ZnO nanowires is due to their insufficient surface area that provokes low dye adsorption. The other one-dimensional-like structures such as NT or NR can become an alternative. Different from NW, these structures have cavity and pores that provide a larger surface area for dye adsorption. Xi et.al have compared the cell performance of ZnO-NT and ZnO-NW. They prepared ZnO NT using hydrothermal reaction and they observed there is significant improvement in short circuit current (J sc ), that increase from 0.12 mA.cm -2 (ZnO- NW) to 0.41 mA.cm -2 (ZnO-NT). The J sc improvement was clearly due to the increase in surface area that provided more sites for dye adsorption (Xi et al., 2012).
Fig. 2.21 – a) Phase diagram of InGaN ternary by Stringfellow , b) phase diagram of InGaN ternary by Burton , c) phase diagram of InAlN ternary by Burton .
since it is a combination of two other mismatches.
This phenomenon starts the main problem of In containing nitrides growth. We want to reduce the temperature to avoid indium desorption and surface roughening, but if we decrease the temperature too much, we make the indium form droplets which play the role of sink for InN formation or at least indium-rich phases in InGaN, InAlN or InGaAlN bulk material. Moreover, there are different scenari that could occur to form these indium-rich areas. It depends on the temperature and so the mobility either of adatoms or atoms directly embedded in the material. First, at the highest temperatures, where phase separation occurs (depending on material and growth techniques), it is supposed that InN phases could be formed by coalescence of In droplets on the surface during the growth. Second, a direct precipitation of InN from the ternary or quaternary bulk lattice by nucleation followed by a crystal growth mechanism could occur. This requires long distance diffusion and thus an enough high temperature and growth time. Third, a mechanism called spinodal decomposition can be highlighted. Here, the diffusion of atoms is possible on long and shorter range and thus working at lower temperatures. This last mechanism does not require a nucleation to form precipitates, and this typically works when the indium content is high enough to initiate the condensation of an unstable alloy. 
In spite of favorable energy levels properties, the integration of Bi-based catalysts onto semiconducting silicon electrodes for the CO 2 photoelectrochemical conversion has been only reported very recently in two studies. [22,35] In the first one, the catalytic coating was deposited onto oxide-free Si by a Bi 3+ -assisted chemical etching method from a bismuth salt solution containing hydrogen fluoride.  Although such a method offers some benefits in terms of simplicity, controlled morphology and thickness, the most efficient reported photocathode for CO 2 -to- formate conversion yielded relatively low cathodic photocurrent densities of ca. 10 mA cm -2 at -1.0 V vs Reversible Hydrogen Electrode (RHE) under simulated sunlight. Higher photocurrent densities of ca. 17 mA-cm -2 were measured in the Gong et al.’s work from Si photocathodes modified with spin-coated Bi 2 O 3 nanotubes.  Such a catalytic material was deposited onto Si from a mixture containing Bi 2 O 3 nanotubes (NTs) and a
conduction band of BFCO to SRO (since ϕSRO>ϕBFCO) until the Fermi levels equalize, thus forming a depletion region in BFCO with an upward band bending (top panel of Figure 49(b)). This contact is a Schottky-type and is highly resistive. In contrast, the contact region formed at the interface between BFCO and ITO is Ohmic and has a low resistance, inducing a downward band bending since ϕBFCO>ϕITO (top panel of Figure 49(b)). The region of this contact does not affect the conduction behavior under an applied voltage. The middle and bottom panel of Figure 49(b) show the modification of energy band alignments in the presence of the FE polarization (P) for the two possible orientations. A positive voltage applied to the SRO bottom electrode results in an electric field with opposite direction to the bottom barrier field and in the same direction as the top one. When the BFCO films are poled, the energy band diagram is modified as shown in the middle panel of Figure 49(b) for “upward” polarization, while the modified energy band alignment for “downward” polarization is illustrated in the bottom panel of Figure 49(b). The origin of this modification is thought to be the modulation of the energy band induced by polarization : The positively charged holes and the negatively charged electrons will move under the electric depolarization field, thus accumulate near the ITO and the SRO interfaces, respectively, when polarization is “downward” (and in opposite directions for “upward” polarization). These charge accumulations near interfaces further contribute to the shift of the energy levels, resulting in a reduction or an increase of the barrier heights. This demonstrates the possibility to design a photodiode that can be controlled/switched by electric poling.
carbon, and between carbons particles provides electronic pathways for electrical current to flow between reaction sites and the current collector. The proton conducting polymer interfaces with incoming gases, with water, with the catalyst and its support, and with the membrane. The proton conducting poly- mer facilitates the transport of protons necessary for ORR and HOR. The proton conducting polymer thus plays a key role in the electrochemistry of the CL—it transfers protons from inside the anode CL to the CL/proton exchange membrane (PEM) interface and from the membrane/CL interface to inside the cathode CL; it must allow reactant gases to access reaction sites and facilitate the transport of water by being both permeable and pore-forming. The polymer introduced in the CL typically possesses a high ionic conductivity, virtually no electronic conductivity, and a fine-tuned permeability and wettability. It must be chemically and interfacially compatible with the polymer constituting the PEM, and as in the case of PEMs, must be stable towards oxidation, reduction and attack by radicals, perhaps more so because of its proximity to reaction sites.
FIG. 1. Schematics of the annealing-free fabrication process forSi 3 N 4 nonlinear photonics (a)-(f).
Critically, under such a low deposition rate which is 40 % lower than that of standard LPCVD silicon nitride reported in  and 30 % lower than the deposition rate mentioned in , the thermal activation energy enables silicon and nitrogen to dispose at the silicon nitride film surface via atomic surface migration phenomena, while compelling hydrogen to escape the film. Furthermore, between the two deposition stages, the wafer is rotated by 45° in order to distribute the uniaxial strain along the overall film thickness, thus avoiding film cracks upon subsequent subtractive patterning. Each deposition run is carried out at 780 °C with post-deposition cooling to around 630 °C for 20 minutes. Controlled ramp-ups and -downs from/to 780 °C at 10 °C/minute to/from 630 °C are used prior to each deposition which is carried out under a 112 mTorr pressure using NH 3 (200 sccm) and SiH 2 Cl 2 (80 sccm) precursor gases, thus providing a ratio between the two precursor gases
For each of the sample sets all further deposition parameters were kept constant in order to change as few material properties as possible except for the local Ga and In composition. In order to investigate if other material properties have changed we conducted X-ray fluorescence, X-ray diffraction and scanning electron miscroscopy measurements on these absorber layers. For the first set of samples, the material characterization results have been presented in sec. 4.5. The integral composition varied only marginally. We have observed that the X-ray diffraction peaks were influenced by the varying lattice constant due to the different Ga gradients through- out the absorber layer. Furthermore the mean grain size increases for increasing substrate temperature / more flat Ga-gradient. With the applied measurement techniques no further particular difference was observed. The material character- isation results for the second set of samples can be found in Appendix C. SEM images show no distinctive difference between the grain size and grain structure of the different absorber layers. The preferred orientation does not vary significantly as can be seen in the XRD diagram. A detailed peak analysis of the (112) peak showed that the peak-width and position correlates with the variation of the lattice parameter due to the Ga gradient.