Efficiency Of Silicon-based Solar Cells Research Articles

Effect of Al2O3/SiO2 ratio on the structure and luminescence properties of Eu3+/Yb3+ co-doped Na2O–CaO–SiO2–Al2O3–ZnO–F glasses

The Eu3+/Yb3+ co-doped Na2O–CaO–SiO2–Al2O3–ZnO–F glasses were prepared. Various Al2O3/SiO2 ratios were used and their impact on the structure and optical properties of the glasses was examined. The results indicate that as the content of Al2O3 increases, the propensity for precipitation in the glass intensifies, while its thermal stability diminishes. The transmission spectra reveal that the incorporation of Al2O3 causes a shift towards longer wavelengths in the UV cut-off point of the glasses, which can be attributed to the alteration of the glass's network structure. Consequently, there is an increase in the amount of non-bridging oxygens present, ultimately resulting in a reduction of the optical bandgap. The analysis of luminescent properties suggests that as the Al2O3/SiO2 ratio increases, the luminescence intensity of the Eu3+ ion also increases but that of the Yb3+ ion decreases. The reason for this is that the Al2O3 added to the glass disrupts the symmetry of the surrounding environment where Eu3+ ions exist. As a result, the likelihood of electric dipole transition for these ions is enhanced. Simultaneously, it disrupts the glass network and increases the distance between rare earth ions, leading to a reduction in the luminescence intensity of Yb3+ ions. In addition, the energy transfer process of Eu3+→Yb3+ has potential application prospects in improving the efficiency of silicon-based solar cells by converting ultraviolet light into infrared light.

The Shockley–Queisser limit and the conversion efficiency of silicon-based solar cells

According to some estimates, every hour, Sun provides planet Earth with more energy than humankind consumes in a whole year. Part of this energy has been essential to ensure living conditions (warmth and food production, for instance) and, more recently, to generate electricity by means of atmospheric (aeolian) and/or geographical (hydropower) sources. In addition to these, the direct (photovoltaic PV) conversion of solar radiation into electricity represents a very elegant method of power generation that causes minimum (or no) environmental disturbance. As a result, numerous efforts have been dedicated to further advance the achievement of clean and sustainable electricity, as supplied by the PV science and technology. Within this scenario, the association of PV with the silicon (Si) semiconductor industry played a crucial role–either by introducing new scientific insights or by promoting successive costs reductions in the field of renewable energy conversion. Yet, the performance of these so-called (either crystalline or amorphous) Si-based solar cells was always a matter of concern. In fact, the subject gained attention with the seminal work by Walter Shockley and Hans Queisser that, in 1961, proposed a model according to which the maximum efficiency of a solar cell is determined by both the solar spectrum and the properties of the semiconductor material. Since then, the work by Shockley and Queisser (also known as the Shockley–Queisser limit) has experienced some improvements and became a reference in the field. Motivated by these facts, along with the main scientific–technological achievements they provided, the Shockley–Queisser limit and the conversion efficiency of the Si-based solar cells along the last 60 years form the basis of this paper.

Fluoroborophosphate glasses doped with Cr3+, Nd3+ and Yb3+as efficient light converters for silicon based solar cells

In this work, we present the results of our study on a fluoroborophosphate glass system doped with different amounts of Cr3+, Yb3+ and Nd3+ ions to evaluate its performance for the conception of luminescent solar concentrator (LSC) to enhance the efficiency of silicon-based solar cells. The obtained results evidenced the presence of an energy transfer from Cr3+ to Yb3+, which becomes more efficient by introducing Nd3+ and results in the enhancement of the emission of Yb3+ at a wavelength ∼980 nm well matched to the silicon band gap. The energy transfer from Cr3+ to Yb3+ was observed under different excitation wavelengths although it was more efficient for wavelengths shorter than 514 nm. From this study, we may suggest that this fluoroborophosphate glass tri-doped with Cr3+, Nd3+,Yb3+ may be considered as a convenient material for the conception of LSCs able to harvest visible light and give efficient NIR emission at nearly 1000 nm required to enhance the efficiency of silicon based solar cells.

Energy transfer study in GdVO4: Bi3+, Yb3+ obtained by microwave-assisted hydrothermal method

Influence of dopants on the structure, morphology and luminescence properties, and in particular the energy transfer mechanism of GdVO4 co-doped with Bi3+ and Yb3+ ions is discussed. Submicro- and microcrystals were prepared by the microwave-assisted hydrothermal method. Phase purity, size and structure of obtained samples were characterized by the X-ray powder diffraction and the transmission electron microscopy. Luminescence properties were investigated by analyzing photoluminescence and excitation spectra and decay time curves. Intense yellow-green luminescence from Bi3+ ions in the range of 400–800 nm and the near-infrared emission from Yb3+ ions about 1000 nm upon an indirect excitation via the (O2−–V5+) charge transfer state at 266 nm and via the (Bi3+–V5+) charge transfer state at 330 nm was recorded. No concentration quenching effect was observed in the samples doped with up to 7 mol% of Yb3+ ions. The near-ultraviolet sensitized near-infrared emission has been explained by the energy transfer from the GdVO4 host and Bi3+ ions to Yb3+ ions. The energy transfer processes between the host and dopants ions have been characterized in detail. The impact of phonon-assisted processes on the near infrared Yb3+ emission has been investigated. The obtained spectroscopic characteristics prove that GdVO4 co-doped with Bi3+ and Yb3+ ions is a promising candidate for use as the phosphor in luminescent concentrators in photovoltaic applications, which can increase the efficiency of silicon-based solar cells.

Tetracene Ultrathin Film Growth on Hydrogen-Passivated Silicon.

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), an SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost upright-standing molecules. The molecular arrangement is very similar to the Tc bulk phase, with only a slightly higher average angle between the conjugated molecular plane normal and the surface normal (α) around 77°. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as well as for (sub)mono- to several-monolayer coverages. Annealing to room temperature, however, changes the film structure toward a smaller α of about 63°. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wave functions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

Strongly enhanced upconversion in trivalent erbium ions by tailored gold nanostructures: Toward high-efficient silicon-based photovoltaics

Upconversion of sub-band-gap photons constitutes a promising way for improving the efficiency of silicon-based solar cells beyond the Shockley-Queisser limit. 1500nm to 980nm upconversion by trivalent erbium ions is well-suited for this purpose, but the small absorption cross section hinders real-world applications. We employ tailored gold nanostructures to vastly improve the upconversion efficiency in erbium-doped TiO2 thin films. The nanostructures are found using topology optimization and parameter optimization and fabricated by electron beam lithography. In qualitative agreement with a theoretical model, the samples show substantial electric-field enhancements inside the upconverting films for excitation at 1500nm for both s- and p-polarization under a wide range of incidence angles and excitation intensities. An unprecedented upconversion enhancement of 913±51 is observed at 1.7Wcm−2. We derive a semi-empirical expression for the photonically enhanced upconversion efficiency, valid for all excitation intensities. This allows us to determine the upconversion properties needed to achieve significant improvements in real-world solar-cell devices through photonic-enhanced upconversion.

Near-infrared quantum cutting via energy transfer in Bi3+, Yb3+ co-doped Lu2GeO5 down-converting phosphor

In recent years, down-converting phosphors have grown rapidly and been still growing for their potential to significantly improve the photoelectric conversion efficiency of silicon-based solar cells. However, the discovery of new optical materials with optimized properties (in terms of emission intensity, chemical and thermal stability and quantum yield) would still be of the utmost importance. Herein, we report a novel down-converting phosphor Lu2GeO5: Bi3+, Yb3+. Our research results show that the energy transfer efficiency from Bi3+ to Yb3+ in Lu2GeO5 is as high as 65%. Via such an energy transfer from Bi3+ to Yb3+, an absorbed ultraviolet photon gives rise to almost two emitted infrared photons. The theoretical quantum yield of Lu2GeO5: Bi3+, Yb3+ system is calculated to reach up to 165%. The mechanism of the energy transfer from Bi3+ to Yb3+ is proved to be an electric dipole-dipole interaction. Meanwhile, Lu1.87GeO5: 0.03Bi3+, 0.10 Yb3+ phosphor exhibits an excellent resistance against thermal quenching. The present investigation provides people a new down-converting phosphor Bi3+, Yb3+ co-doped Lu2GeO5 with near-infrared quantum cutting to achieve high photoelectric conversion efficiency of silicon-based solar cells.

On the efficient Te4+→Yb3+ cooperative energy transfer mechanism in tellurite glasses: A potential material for luminescent solar concentrators

Te4+/Yb3+ co-doped 80%TeO220%Li2O glasses (amounts in mol%) were prepared by the conventional melt-quenching method in an ambient atmosphere, in order to investigate their optical characteristics for application in solar cells. An efficient near-infrared down-conversion mechanism was observed in these samples, involving the emission of two near infrared photons (at around 978 nm) after the absorption of one photon in the ultraviolet-blue region. This effect occurred by means of a process of cooperative energy transfer (CET) from Te4+ to Yb3+ ions. The CET efficiency (ηCET) was calculated from radiative transitions (lifetime) and also by a new method, developed in this work, based on rate equations quantifying the non-radiative transitions (thermal effect) involved in the system. The two methods showed very good agreement, with ηCET between 82 and 100% obtained for the sample prepared using a higher Yb3+ concentration (4 mol%). The results suggested that these tellurite materials could have potential applications in improving the efficiency of silicon-based solar cells.

Efficient near ultraviolet to near infrared downconversion photoluminescence of La2GeO5: Bi3+, Nd3+ phosphor for silicon-based solar cells

Recently, downconversion materials have attracted considerable research due to large promotion of the efficiency of silicon-based solar cells through the absorption of solar radiation via quantum cutting. However, due to lack of ideal downconversion material, it is emergent to seek novel downconversion phosphors with large ultraviolet absorption and high quantum efficiency. Herein, we reported the photoluminescence performance and dynamics of a downconversion phosphor La2GeO5: Bi3+, Nd3+. This phosphor presents a strong ultraviolet absorption and emits a series of intense near infrared emissions by quantum cutting. The emission energy from the Nd3+ matches the energy gap of silicon-based solar cells which can improve the solar cell efficiency.Energy transfer from Bi3+ to Nd3+ in La2GeO5 occurs, proving that energy transfer mechanism of Bi3+→Nd3+ is an electric dipole-dipole (DMSO‑d6) interaction. The energy transfer efficiency of Bi3+→Nd3+ and theoretical quantum efficiency for La2GeO5: Bi3+, Nd3+ are calculated to be as high as 91% and 191%, respectively. La1.92GeO5:0.03Bi3+, 0.05Nd3+ phosphor presents a promising thermal behavior with an activation energy of 0.152 eV. Thus, the development of near-infrared downconversion Bi3+, Nd3+ co-doped phosphors might open up a new approach to achieve high efficiency silicon solar cells by means of quantum cutting.

Transparent hole-collecting and buffer layers for heterojunction solar cells based on n-type-doped silicon

Hole-collecting and buffer layers more transparent than amorphous silicon layers are regarded as one of the possible improvements needed in amorphous/crystalline silicon heterojunctions to help reach the limit efficiency of silicon-based solar cells. Recent studies have demonstrated the suitability of sub-stoichiometric molybdenum oxide (MoOx) as hole-extracting layer in heterojunction solar cells based on n-type crystalline silicon. However, up to now MoOx has mainly been used in combination with amorphous silicon, so limiting its potential as a transparent emitter. This work examines a hole-selective contact based on the combination of a passivating hydrogenated amorphous silicon oxide (a-SiOx:H) and MoOx. The use of these two high-band gap materials (1.9 and 3.5 eV, respectively) should lead to best cell performances if compared to heterojunction devices based only on amorphous silicon. Solar cells were produced and optimized in terms of layers thicknesses. The widely reported problem of s-shape in the light current–voltage characteristics, due to undesired barrier formation against photo-carrier collection, was addressed and solved. The temperature response of the devices was studied and the cell stability under thermal treatment up to 130 °C was demonstrated.

Enhancing the Efficiency of Silicon-Based Solar Cells by the Piezo-Phototronic Effect.

Although there are numerous approaches for fabricating solar cells, the silicon-based photovoltaics are still the most widely used in industry and around the world. A small increase in the efficiency of silicon-based solar cells has a huge economic impact and practical importance. We fabricate a silicon-based nanoheterostructure (p+-Si/p-Si/n+-Si (and n-Si)/n-ZnO nanowire (NW) array) photovoltaic device and demonstrate the enhanced device performance through significantly enhanced light absorption by NW array and effective charge carrier separation by the piezo-phototronic effect. The strain-induced piezoelectric polarization charges created at n-doped Si-ZnO interfaces can effectively modulate the corresponding band structure and electron gas trapped in the n+-Si/n-ZnO NW nanoheterostructure and thus enhance the transport process of local charge carriers. The efficiency of the solar cell was improved from 8.97% to 9.51% by simply applying a static compress strain. This study indicates that the piezo-phototronic effect can enhance the performance of a large-scale silicon-based solar cell, with great potential for industrial applications.

Synthesis of Metal Oxide-Coated Conductive Metal Powders and Their Application to Front Electrodes for Solar Cells

Recently, improvement in the conversion efficiency of silicon-based solar cells has been achieved by decreasing emitter doping concentration, because the lightly doped emitter can effectively prevent the recombination of electrons and holes generated by solar light irradiation. This type of emitter is very thin due to the low doping concentration, thus conductive materials (i.e., silver) used for front electrodes can easily penetrate the emitter during a firing process because of their large diffusivity in silicon. This results in junction leakage currents which might reduce cell efficiencies. In this study, -coated Ag powders were synthesized by an ultrasonic spray pyrolysis method and applied to the conductive materials of the front electrode to control the junction leakage current. The shell obstructs the Ag diffusion into the emitter during the firing process. The powder is spherical with a core-shell structure and the thickness of the shell is tens of nanometers. Solar cells were fabricated using pure Ag powders or the -coated Ag powder as front electrode materials, and the conversion efficiency and junction leakage current were compared to investigate the role of the shell during the firing processes.

Near-Infrared Quantum Cutting Platform in Thermally Stable Phosphate Phosphors for Solar Cells

This study investigated the photoluminescent properties of Tb(3+)-Yb(3+)-, Ce(3+)-Tb(3+)-Yb(3+)-, and Eu(2+)-Yb(3+)-doped KSrPO4. The samples were prepared by a solid-state reaction with various doping concentrations. Emission at near-infrared range was focused on the application of luminescent solar concentrator for solar cells. Quantum cutting (QC) energy transfer was confirmed by the lifetimes of the donor. Near-infrared QC involved emission of Yb(3+) ions was achieved by excitation of Ce(3+), Tb(3+), and Eu(2+) ions, where the energy transfer processes occurred from Ce(3+) to Tb(3+) to Yb(3+), Tb(3+) to Yb(3+), and Eu(2+) to Yb(3+), respectively. In addition, the concentration quenching effect of Yb(3+) ions was avoided by low doping concentrations. The overall quantum efficiencies were calculated, and the maximum efficiency reaches 139%. The energy diagrams for divalent and trivalent rare-earth ions in KSrPO4 host lattice were analyzed. Results of this study demonstrate that heat-stable phosphate phosphors are promising candidates for increasing the efficiency of silicon-based solar cells.

Optimizing surface plasmon resonance effects on finger electrodes to enhance the efficiency of silicon-based solar cells

In this paper we demonstrate a technique for improving the conversion efficiency in conventional silicon solar cells by using surface plasmon resonance (SPR) effects to harvest incident light energy over metal finger electrodes. According to three-dimensional finite-difference time-domain (3D-FDTD) analysis, incident light covering a broad bandwidth of the solar spectrum can be transmitted through metallic hole array structures. Although the light absorption region beneath the metal finger electrodes cannot generate a photocurrent, in this study, we employ the extraordinary transmission (EOT) phenomenon, due to SPR effects, to dramatically increase the degree of light harvesting below the metal electrodes and, thereby, improve the efficiency of the entire solar cell. Experimental data reveal that the excess photocurrent density was approximately 190% of the normal current density of a standard solar cell. Therefore, the negative effect of covering the absorption area with opaque metal finger electrodes can be minimized or eliminated completely by taking advantage of the SPR effect of the metal electrodes.

Enhanced cooperative near-infrared quantum cutting in Pr3+-Yb3+ co-doped phosphate glass

Pr3+ and Yb3+ co-doped phosphate glasses are prepared to study their optical properties. Excitation and emission spectra and decay curves are used to characterize their luminescence. We demonstrate that upon excitation of Pr3+ ion with one high energy photon at 470 nm, two near-infrared (NIR) photons are emitted at 950 1100 nm (Yb3+:2F5/2→2F7/2) through an efficient cooperative energy transfer (CET) from Pr3+ to Yb3+. The maximum energy transfer efficiency (ETE) and the corresponding quantum efficiency approach up to 90.17% and 190.17%, respectively. The glass materials might find potential application for improving the efficiency of silicon-based solar cells.

Rare-earth doped optical spectral conversion materials with their applications

Based on the research work carried out in our laboratory in recent years, we focus on three categories of rare-earth doped spectral conversion materials, i.e., red phosphor for white-LED lighting, near-infrared downconversion materials and upconversion nanosized biological fluorescent labeling materials. Our work on the red phosphor for white-LED has been mainly focused on Eu3+ doped materials, and the energy transfer from sensitizers to Eu3+ ions is investigated in order to enhance the efficiency under the excitation of near-ultraviolet light. For the purpose of enhancing the efficiency of silicon-based solar cells, we have carried out the studies on near-infrared downconversion materials based on stepwise and cooperative energy transfer mechanisms and have clarified the downconversion mechanisms in NaYF4:Ho3+, Yb3+ and YVO4:Yb3+ materials. Biological fluorescent labeling is another important application of luminescent materials, which has some stringent requirements on the materials, such as the nanoparticle size, water-solubility, non-toxicity and luminescent efficiency. In our recent work, core/shell NaYF4:Yb3+, Er3+(Tm3+)/NaYF4 UCNPs have been successfully synthesized and transferred to aqueous phase by coating a cross-linked PMAO polymer. The cross-linked core/shell nanoparticles are very stable for several days, and are successfully internalized into human prostate cancer cells, which substantiates the utility of these nanoparticles in biolabeling applications. To sum up, due to significant existing and potential applications in biological and chemical sensing, new energy resources, energy saving and environmental protection, rare-earth doped spectral conversion materials have been attracting extensive attention. The future of rare-earth doped optical spectral conversion materials would be bright.

Broadband downconversion based near-infrared quantum cutting via cooperative energy transfer in YNbO4:Bi3+, Yb3+ phosphor

YNbO4:Bi3+, Yb3+ phosphor was prepared to study the quantum cutting process of converting one ultraviolet photon into two near-infrared photons. An intense near-infrared emission of Yb3+:2F5/2 → 2F7/2 around 1 μm was observed under the ultraviolet excitation belonging to the broadband absorption of the [NbO4]3- group and the Bi3+ ion. The photoluminescence spectra and decay lifetime measurements indicate efficient energy transfer from Bi3+ to Yb3+ ions, which is attributed to be of a cooperative energy transfer mechanism. The YNbO4:Bi3+, Yb3+ phosphor with optimized doping concentration may be applicable in improving the efficiency of silicon-based solar cells.

Efficient ultraviolet-blue to near-infrared downconversion in Bi–Dy–Yb-doped zeolites

Ultraviolet-blue to near-infrared (NIR) downconversion is investigated for the Dy3+–Yb3+ couple in zeolites by steady-state and time-resolved photoluminescence (PL) spectra, and PL excitation spectra. Upon excitation of the 4F9/2 level of Dy3+, NIR quantum cutting could occur through a two-step energy transfer from one Dy3+ ion to two neighbouring Yb3+ ions via an intermediate level. The energy transfer efficiency from the 4F9/2 level is estimated to be 42%, and the intrinsic PL quantum efficiency of Yb3+ emission reaches 54%. The findings may have potential application in enhancing the energy efficiency of silicon-based solar cells.

Photoluminescence of porous silicon stain etched and doped with erbium and ytterbium

A novel low cost process has been developed for application in porous silicon-based photodetectors and solar cells, where conventional doping processes are not affordable because of the high processing cost and technical difficulties. Ytterbium and erbium (Yb 3+–Er 3+) ions were introduced into luminescent porous silicon stain etched by thermal diffusion. Doping profiles were evaluated by energy-dispersive spectroscopy analysis. The visible and near-infrared photoluminescence of Yb 3+–Er 3+ co-doped stain-etched porous silicon layers is observed and evaluated under 980 nm pumping. Up-conversion processes that could improve the efficiency of silicon-based solar cells are detected.