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Silicon Nanostructures for Photovoltaics

Silicon Nanostructures for Photovoltaics

carriers created by the energetic photons of the solar spectrum give rise to the emission of two photons having a lower energy. Unfortunately, detrimental effects such as Auger recombination and/or intraband relaxation reduce drastically the efficiency of the process and consequently do not favor an increase in the solar cell yield. To enhance the absorption range of the solar spectrum, the cell structure has been adapted, giving rise to the tandem cell structure. This consists in stacking layers of different materials absorbing different parts of the solar spectrum. The use of amorphous silicon (a-Si) in a multi-junction solar cell/tandem cell has been the most popular choice over the years. Different junction devices with appropriately graded bandgaps can be placed in a stack to form a multi-junction device. The top junction absorbs the higher-energy photons and transmits the lower-energy photons to be absorbed by the bottom junctions. In the 1980s, remarkable advancements were made in the study of amorphous Si (a-Si)- based structures like silicon carbide, silicon nitride, etc. It has been demonstrated that the multi-junction tandem structures of a-Si such as a-SiC/a-Si heterojunctions as well as a-Si/poly-Si and a-Si/Ge alloys 40 could result in a stable multi-junction with reduced light-
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Biotemplated Silica and Silicon Materials as Building Blocks for Micro- to Nanostructures

Biotemplated Silica and Silicon Materials as Building Blocks for Micro- to Nanostructures

ABSTRACT: Silicon is essential in several energy-related devices, including solar cells, batteries and some electrochemical systems. These devices often rely on micro- or nanostructures to function efficiently, and require patterning of metallic surfaces. Currently, constructing silicon features at the micro- and nanoscale requires top-down energy-intensive processes, such as e-beam lithography, chemical etching or anodization. While it is difficult to form silicon in aqueous solution, its oxide, silica, can easily be synthesized using sol-gel chemistry and nucleated onto templates with diverse shapes to create porous or continuous architectures. Here, we demonstrate that novel silica nanostructures can be synthesized via biomineralization, and that they can be reduced to silicon using magnesiothermal reduction. We selected three biotemplates to create silica structures with various aspect ratios and length scales. First, we use diatomaceous earth as a model silica material to optimize our process, and we also biomineralize silica onto two micro- organisms, the high aspect ratio M13 bacteriophage, and the helical Spirulina major algae. During our process, the shape of the materials is preserved, resulting in silicon nanowires, nano-porous networks, spirals and other micro- and nano-structures with high surface area. Our method provides an alternative for the creation of silicon nanostructures, using pre-formed silica synthesized in solution. The process could be extended to a broader range of microorganisms and metal oxides for the rational design of on-demand micro- and nanostructured metals. In addition, we show that the intrinsic composition of the biotemplates as well as their growth medium can introduce impurities that could potentially be used as dopants in the final silicon structures, and that could allow for tuning the composition of n-doped or p-doped biotemplated silicon for use as semiconducting building blocks.
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Fast light-emitting silicon-germanium nanostructures for optical interconnects

Fast light-emitting silicon-germanium nanostructures for optical interconnects

By the 1990s, a different form of SiGe NS, namely the three- dimensional (3D) self-assembled system produced by the Stranski-Krastanov growth mode in lattice mismatched materials, had been demonstrated [5]. It has been shown that dislocation-free SiGe growth can be achieved using a higher temperature (!600 o C), and that the non-planar geometry is mainly responsible for the significant increase of the SiGe critical layer thickness [5]. Compared to two-dimensional (2D) Si/SiGe NSs, the PL and electroluminescence (EL) quantum efficiency in 3D Si/SiGe NSs is higher (up to ~1%), especially for T > 50 K [6]. Despite many successful demonstrations of PL and EL in the spectral range of 1.3–1.6 µm, which is important for optical fiber communications, the proposed further development of 3D Si/SiGe based light emitters was discouraged by several studies indicating a type II energy band alignment at Si/SiGe heterointerfaces [6], where the spatial
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Self-assembled silicon-germanium nanostructures for CMOS compatible light emitters

Self-assembled silicon-germanium nanostructures for CMOS compatible light emitters

The same model also explains the experimentally ob- served strong decrease in carrier radiative lifetime (~100 times) as the detection photon energy increases from 0.77 to 0.89 eV (Fig. 3). Under a low level of excitation inten- sity, holes are localized within the Ge-rich core of SiGe clusters, and the hole wave function does not penetrate into the Si barriers. Thus, in this quasi-type II energy band alignment, electron and hole wave functions do not overlap, causing a long carrier radiative lifetime. Under high level excitation, holes occupying the excited energy states in small size SiGe clusters, as well as holes leaking into the SiGe wetting layer with a lower Ge concentration, have their wave function significantly extended into the Si bar- riers. Therefore, a stronger overlap between electron and hole wave functions is responsible for a shorter radiative lifetime, i.e., a higher PL quantum efficiency at greater photon energy. Thus, this explanation is consistent with both, the decrease of the PL decay time at greater photon energy and the PL spectral blue shift as excitation intensity increases.
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Silicon Nanowires for Photovoltaics : from the Material to the Device

Silicon Nanowires for Photovoltaics : from the Material to the Device

Keywords: " Solar cells", " Silicon Nanowires (SiNWs)", "Heterojunction with Intrinsic Thin layer (HIT)", "Hybrid", "Metal Assisted Chemical Etching (MACE)", "Agglomeration", "Optical Properties". Titre: Nanofils de silicium pour le solaire: du matériau à la cellule photovoltaïque Résumé: Les cellules solaires à base de nanofils de silicium offrent une alternative intéressante pour la réalisation de panneaux photovoltaïques à haut rendement et à faible coût. Elles bénéficient notamment des excellentes propriétés optiques des nanofils qui forment une surface à très faible réflectivité tout en piégeant efficacement la lumière. Dans cette thèse, nous utilisons et améliorons une méthode de gravure chimique peu coûteuse et industrialisable pour la fabrication de forêts de nanofils de silicium. En adaptant la mouillabilité du substrat et des nanofils, nous avons remédié au problème d'agglomération inhérent à cette méthode lorsqu’on veut obtenir des forêts denses et désordonnées de nanofils. En combinant cette méthode de gravure chimique à la lithographie assistée par nanosphères, nous avons pu fabriquer des réseaux ordonnés de nanofils avec un contrôle précis des propriétés géométriques (diametre des nanofils et distance entre eux). Les propriétés optiques de ces réseaux ont été étudiées théoriquement et expérimentalement afin d'identifier les configurations optimales. Nous avons ensuite fabriqué des cellules solaires à partir de ces différents types de nanofils et deux types de structures. Le premier type, des cellules solaires HIT (Hétérojonction avec couche mince Intrinsèque) à base de nanofils de silicium, a été fabriqué par RF-PECVD. L'optimisation des conditions de dépôt plasma nous a permis d'obtenir des cellules solaires hautement performantes: rendements de 12,9% et facteurs de forme au-delà de 80%. Le second type, des cellules solaires hybrides, est basé sur la combinaison d'une couche organique et des nanofils de silicium. La caractérisation des cellules fabriquées montre des rendements prometteurs. Enfin, nous présentons des résultats préliminaires pour transférer ces concepts à une technologie couches minces.
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Controlled nucleation and growth process for large grained polycrystalline silicon photovoltaics

Controlled nucleation and growth process for large grained polycrystalline silicon photovoltaics

In overview,. from this and other works it seems that a relatively high anneal temperature is required to grow a useable large-grained silicon film by this process[r]

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Structural and electronic properties of self-assembled nanostructures on silicon surfaces

Structural and electronic properties of self-assembled nanostructures on silicon surfaces

in agreement with the lower intensity of the lobes associated with adatom b and c in the STM image of Fig. 3.10b). Each pentamer saturates five of the surrounding dangling bonds [133]. By intro- ducing two pentamers per Si(331)-(12×1) unit cell, the number of dangling bonds has been reduced from 36 to 26. Some of the remaining dangling bonds are satu- rated by simple adatoms as in the case of the Si(110) surface. In the STM image in Fig. 3.10b) the additional protrusion linking two successive pentamers may indicate the location of a first adatom. After placing these adatoms there are a total of 26 − 2 × 3=14 restatoms per unit cell (the factor of two is due to the glide plane symmetry). Stekolnikov et al. [128] have noted that it is energetically more favor- able to leave some restatoms unsaturated than to introduce the maximum number of adatoms into the model. This allows further energy minimization by electron transfer from the adatom to the restatom as in the Si(111)-(7×7) reconstruction. It is clear that at this point total energy calculations are required in order to deter- mine the optimum adatom arrangement and the relaxed coordinates of each atom. Going back to the high resolution STM image in Fig. 3.10b), we see for instance that an individual pentagon does not exhibit a mirror symmetry along the [¯1¯16] direction indicating a distortion of the pentagon in order to reduce internal strain. It is also interesting to note that on the Si(110) surfaces, pentamers always occur in twin pairs rotated by 180 o with respect to each other, whereas on Si(331) all pentamers point into the same direction and are further apart from each other. Comparing the transition temperature of 810 o C [24] for Si(331)-(12×1) with 730 o C [146] for Si(110)- (16×2), it appears that it is actually the Si(331) which is the more favorable surface for pentamer formation (for comparison the transition temperature of 870 o C [95] for the Si(111)-(7×7) reconstruction is slightly higher). Here again, total energy calculations allow to quantify the trade-off between surface dangling bond reduction and induced surface stress and to analyze the details of the bonding configuration. In summary, by combining our LEED and STM data and by comparing similari- ties and differences between the Si(331)-(12×1) and Si(110)-(16×2) reconstruction, we have derived a complete structural model for the Si(331) surface containing the pentamer as an essential ingredient. Thus besides adatoms, dimers and tetramers, pentamers emerge as a universal building block for silicon surface reconstructions.
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Microcrystalline silicon deposited from SiF4/H2/Ar plasmas and its application to photovoltaics

Microcrystalline silicon deposited from SiF4/H2/Ar plasmas and its application to photovoltaics

d. Particles behave as dust, and reach the TEM grid thanks to convection (when plasma is off or simply when the chamber vacuum is broken for unloading the substrate). For the aggregates, only hypotheses (c) and (d) can be considered because such large particles are inevitably negatively charged in the plasma and the plasma sheath above the substrate acts as a barrier. Precisely, by being trapped within the plasma, these particles have time to grow to become aggregates of 100 nm. Concerning nanoparticles, if hypothesis (b) is true, another 100 s will be sufficient to completely cover the surface, because, based on the plasma conditions, the deposition rate is 2 Å/s (see Figure 23 at 50 W) and the typical distance between two seeds in Figure 32 is roughly 40 nm. Therefore during 100 s thanks to lateral growth, seeds will percolate and form a uniform and dense film. It is the reason why we discard the second hypothesis (b). Finally, both (a) and (c) predict that nanoparticles seen in Figure 32 are coming from the plasma.
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Aligned three-dimensional prismlike magnesium nanostructures realized onto silicon substrate

Aligned three-dimensional prismlike magnesium nanostructures realized onto silicon substrate

The structure and composition of the as-prepared 3D Mg nanostructures are identified by XRD and EDX 共inset兲, as shown in Fig. 2 . To remove the effect of the well-crystallized single-crystal silicon substrate, Mg nanostructures are manu- ally scratched from the substrate with a sharp knife for XRD and EDX characterizations. The main elements seen in the EDX spectrum are O, Mg, and Si. Si comes from the scratched silicon substrate and O comes from MgO that is formed at the surface of the Mg nanostructures when, taking out of the vacuum deposition chamber, they are let in contact with atmospheric oxygen. However, MgO is not detected by XRD, which is mainly due to its very small thickness at the Mg surface. This is confirmed by the subsequent HRTEM characterization.
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Sillicon photonics based on monolithic integration of III-V nanostructures on silicon

Sillicon photonics based on monolithic integration of III-V nanostructures on silicon

1.6 Conclusion The dawn of the photonic age has come with the potential integration of photonic devices on silicon. Although light emission has been enabled on Si by using germanium, erbium doping, Si nanocrystal or coupling of the two later and Raman excitation, the use of III-V compound materials reveals more benefits because of their better optical and electronic properties: direct band gap, high mobility, facilitation of light emission and absorption. Hybrid silicon lasers based on InP lasers bonded on silicon platform shown their advantages in short term operation and a proof of concept for the silicon photonic routine, but long term utilization, low cost fabrication, reduced time consuming and high density integration require monolithic integration on silicon substrate. Gallium phosphide (GaP) is the best candidate for dislocation free growth of pseudomorphic heteroepitaxial GaP/Si because it is quasi lattice-matched with silicon. The inclusion of ~2.2 % of nitrogen allows the exact lattice-matching with silicon. The growth of GaP on Si substrate has to deal with the problems of growing a general polar semiconductor compound on a non-polar elemental one: the difference in lattice constant could reveal misfit dislocation, the difference in thermal expansion coefficient could generate tensile strain when cooling down from growth temperature to room temperature, the formation of anti-phase boundaries, stacking faults and microtwins which act as charge trapping and are detrimental for optical and electrical properties of the devices. In this context, one of the main challenges for the successful photonics integration is the improvement of the crystal quality of the material. Another challenge is to find an efficient light emitter on silicon substrate, in the pseudomorphic approach.
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Shape-controlled ZnO Nanostructures for Gas Sensing Applications

Shape-controlled ZnO Nanostructures for Gas Sensing Applications

miniaturized gas sensors substrates [8], presented on Fig. 1, by an ink-jet method (Microdrop AG) [9]. Fig. 1. Miniaturized gas sensor substrate (a), silicon platform before (b) and after (c) sensitive layer deposition 2.4. Gas test set-up Gas tests were performed using a PC controlled setup composed of different gas bottles connected to mass flow controllers (QualiFlow) commanded by an Agilent Data Acquisition/Switch Unit 34970A. Sensors were placed in a specially designed measurement cell containing also the humidity and temperature sensors driven by another Agilent 34970A. The integrated sensor heater was commanded by a HP6642A tension controller. The NI 6035E electronic card established the connection between computer and measurements. Freshly prepared sensors were initially conditioned by a progressive in situ heating of the sensitive layers up to 500°C. Afterwards, all sensors were exposed to different reactive gases under 50% relative humidity and a total gas flow rate of 1L/min. The reported tests were performed at three different temperatures (500°C, 400°C and 340°C).
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15.7% Efficient 10-μm-Thick Crystalline Silicon Solar Cells Using Periodic Nanostructures

15.7% Efficient 10-μm-Thick Crystalline Silicon Solar Cells Using Periodic Nanostructures

The 10-μm-thick crystalline silicon photovoltaics with peak efficiency of 15.7% that we present in this article incorporate our design for a two-dimensional inverted nanopyramid surface texture and rear metallic reflector light-trapping structure that has been previously shown to possess excellent anti-reflection and long-wavelength absorption capabilities. [10,11] The device provides near-Lambertian absorption across the bulk of the spectrum from 500- 1100 nm. Peak short-circuit current measures 34.5 mA cm -2 , marking a substantial improvement over previous devices in this thickness range. Through an analysis of current loss mechanisms, we quantify the sources of current loss in the device to focus attention on specific areas of improvement. Much of the potential for additional current gain is dependent on reducing parasitic absorption losses in the back aluminum reflector and nitride anti- reflection coating (ARC), while improvements to the device design that increase open-circuit voltage (V OC ) could yield device efficiencies competitive with current commercial crystalline
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Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials develop- ment of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early- stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I–V curve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.
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Multiscale in modelling and validation for solar photovoltaics

Multiscale in modelling and validation for solar photovoltaics

simple design, with an n-type front emitter on a p-type c – Si wafer along with an Al back surface field, significant improvements have been achieved introducing passiv- ation schemes leading to the so-called PERC (Passivated Emitter and Rear Cell) and PERL (Passivated Emitter, Rear Locally-diffused) cells with a record ef ficiency of 24.7% (that has been reevaluated to 25%) [ 10 ] obtained on a small size cell (few cm 2 ). This record has been unbeaten for more than 15 years, but it has been broken several times recently using various new concepts, namely the interdigitated back contact cell (IBC) architecture [ 11 ], and, very recently, the so-called TopCon concept, tunnel-oxide passivated rear contact and high-quality top surface passivation [ 12 ]. Another very powerful concept is the silicon heterojunction (SHJ) combining crystalline silicon with very thin layers of hydrogenated amorphous silicon (a-Si:H). Doped a-Si:H, p-type and n-type, is used to produce the front emitter junction and the back surface field, respectively, on an n-type crystalline silicon absorber. A very thin undoped a-Si:H (so-called intrinsic) is inserted between the doped a-Si:H layers and the c –Si wafer to achieve outstanding surface passivation. Double side contacted SHJ solar cells have demonstrated ef ficiencies of 25.1% on 160 mm thick c–Si [ 13 ], and record open circuit voltages of 750 mV on 100 mm thick c– Si [ 14 ]. Finally, the present record ef ficiencies are held by a technology combining the interdigitated back contact structure with the SHJ concept. A value of 26.3% has been published in [ 15 ], while a record value of 26.6% has been recorded in the best research-cell ef ficiency chart from the US National Renewable Energy Laboratory (NREL) [ 16 ].
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Technology platform for the fabrication of titanium nanostructures

Technology platform for the fabrication of titanium nanostructures

The titanium removal rate (RR) as well as the silicon dioxide RR depend on many CMP parameters such as the head and platen rotation speeds, the polishing pad and sub- pad materials, and the amount of slurry. For the fabrication of the devices one wants to remove the silicon dioxide at the same rate as the titanium in order to be able to decrease the metal thickness down to few nanometers (step 5 to 6 of Fig. 4 ). The easiest way to adapt both material removal rates (MRRs) is to modify the slurry chemistry and/or the nano- particle density. The original slurry used is a commercial solution from Allied High Tech Products, Inc. with 50 nm silica nanoparticles diluted in a basic solution. Figure 5
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Synthesis and applications of difluorobenzothiadiazole based conjugated polymers for organic photovoltaics

Synthesis and applications of difluorobenzothiadiazole based conjugated polymers for organic photovoltaics

cm 2 ). 1. Introduction Compared with inorganic silicon based solar cells, organic solar cells show huge commercial potentials for flexible and large area applications, because the devices could be manufactured by existing low-cost industrial technology, such as ink-jet and roll to roll printing. Another important advantage of organic solar cells is the diversity of the molecular structures in organic chemistry, where numerous combinations of various building blocks are available. So the properties of the resulting materials, such as solubility, charge mobility and energy levels for photovoltaic applications can be easily tuned. 1
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Heteroepitaxial growth of gold nanostructures on silicon by galvanic displacement

Heteroepitaxial growth of gold nanostructures on silicon by galvanic displacement

To visualize the gold nano- particle⫺silicon interfaces of Au/ Si(111) and Au/Si(100) and con- firm heteroepitaxy, cross- sectional high resolution TEM (HRTEM) investigations were car- ried out. Figure 4a shows cross- sectional HRTEM image for a gold nanocrystallite on Si(111) that reveals the coincidence of four gold lattice fringes with three silicon lattice fringes, as marked by the yellow and pink lines, respectively. In addition, the top epitaxial gold planes are clearly parallel to the direction of those of the underlying silicon substrate. Similar results were ob- served for Au/Si(100) as shown in Figure 4b. In the case of gold on Si(100) (Figure 4b), the gold⫺silicon in- terface displays a significant degree of heterogeneityO some areas have very clear coincident gold and silicon lat- tices, whereas others appear less ordered (Supporting Information). In the inset of Figure 4b, one ill-defined re- gion is highlighted, the exact composition of which is as- of-yet unknown (vide infra).
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Photoluminescence fatigue in three-dimensional silicon/silicon-germanium nanostructures

Photoluminescence fatigue in three-dimensional silicon/silicon-germanium nanostructures

New Jersey 07102, USA 2 Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario K1A 0R6, Canada (Received 17 December 2011; accepted 22 February 2012; published online 26 March 2012) We find fatigue of low temperature photoluminescence (PL) in Si/SiGe three-dimensional island morphology nanostructures under continuous excitation. Initially, the PL intensity slowly decreases by less than 15%, and after 10 min it decreases rapidly by more than 80%. After the PL intensity stabilizes, a complete recovery requires heating the sample to nearly room temperature. We propose that accumulation of charge within SiGe islands is responsible for the enhancement of Auger recombination and hence the observed PL fatigue. V C 2012 American Institute of Physics.
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Design and optimization of silicon nanostructures

Design and optimization of silicon nanostructures

Figure 3: A verage reflectance as a function of the ellipse parameters of the parabola-shaped structure. The period of the nanostructures plays an important role in the reduction of the reflectance. Practically, the period of the nanostructures can be determined by adjusting the diameter of the nanomask used during the fabrication. Therefore, it is necessary to optimize the period before the fabrication step. Figures 4.a and 4.b show the variation of the average reflectance and the short- circuit current density, respectively, as function of the period. As can be seen, the optimal periods of the pillar, truncated cone, cone and parabola-shaped structures are around 750 nm, 500 nm, 400 nm and 350 nm, respectively. As could be expected, the optimal period depends on the structure shape. A small period is adequate for nanocone and nanoparabloid shapes because their peaks are sharp. With these optimal periods, the AR structures act as an effective medium for the longer wavelengths, as a photonic crystal for wavelengths comparable to the period and as a grating diffraction for the smaller wavelengths.
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Multijunction photovoltaics: integrating III–V semiconductor heterostructures on silicon

Multijunction photovoltaics: integrating III–V semiconductor heterostructures on silicon

We have developed promising building blocks for GaAsPN/silicon-based dual-junction solar cells. The tandem GaAsPN/silicon double-junction solar cell will be electrically connected with a tunnel junction (TJ), which connects successive p-n junctions made up of a p-type semiconductor and an n-type semiconductor. One of the main issues in developing the dual-junction solar cell is obtaining an efficient TJ. We have modeled this and found high theoretical current densities for both GaP/silicon and silicon/silicon TJs with experimentally attained GaP alloy doping levels and considering an n-doped silicon bottom absorber. 4 Modeling the top-PIN-junction GaAsPN absorber with a ‘tight-binding’ calculation crossed with critical thickness modeling showed that a GaAsPN alloy (composition 9% arsenide and 4% nitride) is promising with an expected bandgap energy of 1.81eV and a critical thickness that allows pseudomorphic growth of a 1µm- thick absorber.5 To assess this material independently of defects potentially generated at the GaP/silicon interface, we grew a lattice-matched 100nm-thick GaAsPN alloy on the 001 face of a gallium phosphide (GaP) substrate. After a post-growth annealing step, this alloy displays strong absorption around 1.8–1.9eV, and efficient photoluminescence at room temperature suitable for targeted solar cell top junction development.
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