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Reported is the photoluminescence enhancement due to surface plasmon from the metallic nanoparticles that are linked to the surface of a GaAs capped InAs quantum dots. In this study, spherical silver (Ag) nanoparticles are investigated where the different densities of Ag nanoparticles are deposited on four InAs/GaAs quantum dot samples. The PL enhancement due to Ag nanoparticles has been observed to be improved with increasing nanoparticle density. The photoluminescence enhancement is interpreted in terms of enhanced scattering from the surface plasmon excited in the Ag nanoparticles.
The use of plasmonic nanoparticles as light scattering elements for light trapping in solar cells is studied. From theoretical considerations it follows that Ag particles with a diameter on the order of 100 nm possess ideal light scattering properties. It is demonstrated that these particles can be fabricated using the selective aerosol deposition technique. Because this newly developed technique provides excellent control over critical parameters such as particle size and surface coverage it is a valuable tool for optimizing plasmonic solar cells. The initial experiments show that embedding Ag particles with a diameter of 180 nm into amorphous silicon solar cells enhances the current output.
Low dimensional structures like quantum dots (QDs) offers the the ability to tune the absorption properties of standard semiconductor materials. However, QDs are relatively weak light absorbers and hence may benefit significantly from coupling with plasmonic modes in nearby metal structures. In the case of a Si QD absorber layer for photovoltaic applications, enhanced absorption would lead to improved power conversion efficiency. Silver metal nanoparticles (MNPs) were deposited on Si QD structures using the self-assembly method of evaporation and annealing. Room temperature photoluminescence (PL) measurements were used to study the surface plasmon (SP) enhanced emission from the samples. The results were compared to conventional metal back reflectors. Enhanced surface plasmon coupled emission (SPCE) from Si QDs in the vicinity of silver metal nanoparticles (MNPs) is observed with a good correlation between the enhancement and the resonance excitation. Quenching was observed from the same emitter layers placed in close proximity to thin flat silver reflector layers, indicating the importance of the spacer layer between a metal layer and the quantum dots in optimising enhancement. The results have implications for the design of SP-enhanced QD solar cells.
ZnO nanorods were grown homogenously and vertically on ITO using electrochemical techniques. The physical properties of the nanorods were characterized using SEM and optical absorption. The electrical conductivity, deduced using STM at different tip heights, and was found to be 20 Ω-cm with a carrier concentration of 3x1015 cm-3.The results show that electrochemically grown ZnO nanorods have electrical properties suitable for use in electronic devices such as solar cells and transistors. A-Si:H p-i-n solar cells were then deposited after the fabrication on the ZnO on ITO-coated substrates. The results show that the textured solar cell performance was 30% higher than the planar solar cell.
In the development of novel materials for enhanced photovoltaic (PV) performance, it is critical to have quantitative knowledge of the initial performance, as well as the performance of these materials over the required 25-year lifetime of the PV system. Lifetime and degradation science (L&DS) allows for the development of new metrology and metrics, coupled to degradation mechanisms and rates. Induced absorbance to dose (IAD), a new metric being developed for solar radiation durability studies of solar and environmentally exposed photovoltaic materials, is defined as the rate of photodarkening or photobleaching of a material as a function of total absorbed solar radiation dose. In a reliability engineering framework, these quantitative degradation rates can be determined at various solar irradiances making possible real time and accelerated testing. The potential to predict power losses in a photovoltaic system over time caused by the accumulation of this kind of degradation can be calculated for real time applications or extrapolated for accelerated exposure conditions. Three formulations of poly (methyl methacrylate) (PMMA) used for mirror augmented PV systems were analyzed for the changes in IAD after accelerated testing.
Comparative studies have been carried out on the performance of the photovoltaic devices with dissimilar shapes of the InN nanostructures fabricated on p-Si (100). The devices fabricated with the nanodots show a superior performance compared to the devices fabricated with the nanorods. The discussions have been carried out on the superior junction property, larger effective junction area and inherent random pyramidal topographical texture of the cell fabricated with nanodots. Such single junction devices exhibit a promising fill factor and external quantum efficiency of 38% and 27%, respectively, under concentrated AM1.5 illumination.
The effect of silver nanoparticles showing localised plasmonic resonances on the efficiency of thin film silicon solar cells is studied. Silver (Ag) nanodiscs were deposited on the surface of silicon cells grown on highly doped silicon substrates, through hole-mask colloidal lithography, which is a low-cost and bottom-up technique. The cells have no back reflector in order to exclusively study the effect of the front surface on their properties. Cells with nanoparticles were compared with both bare silicon cells and cells with an antireflection coating. We optically observe a resonance showing an absorption increase controllable by the disc radius. We also see an increase in efficiency with respect to bare cells, but we see a decrease in efficiency with respect to cells with an antireflection coating due to losses at wavelengths below the plasmon resonance. As the material properties are not notably affected by the particles deposition, the loss mechanism is an important absorption in the nanoparticles. We confirm this by numerical simulations.
A plasmonic back reflector has been fabricated for light-trapping application in thin film Si photovoltaic devices. The back reflector comprises of a 2D array of self-organized Ag NPs separated from a planar Ag mirror by a ZnO layer deposited by atomic-layer deposition. The diffuse reflectance and parasitic absorption losses can be modulated by varying the ZnO thickness. A maximum diffuse reflectance peak value of 30% at 950 nm, with a bandwidth of 400nm, is observed for ∼100 nm diameter NPs at a distance of 50 nm from the Ag mirror. Finite-difference time-domain simulations of a 100nm Ag sphere near a mirror were used to understand the experimentally observed trends in diffuse reflectance and parasitic absorption, with distance from the mirror. Particles very close to the mirror can couple to delocalized surface plasmons or exhibit Fano resonance effects, thereby increasing parasitic absorption. Particles situated away from the mirror are influenced by driving-field effects due to the interaction of incident and reflected photons, which modulate the scattering cross-section.
A compact, single element concentrator comprising a near linear array of prisms has been designed to simultaneously split and concentrate the solar spectrum. Laterally aligned solar cells with different bandgaps are devised to be fabricated on a common Si substrate, with each cell absorbing a different spectral band optimized for highest overall power conversion efficiency. Epitaxial Ge on Si is used as a low cost virtual substrate for III-V materials growth. Assuming no optical loss for the prism concentrator, no shadowing and perfect carrier collection for the solar cells, simulations show that 39% efficiency can be achieved for a parallel four-junction (4PJ) InGaP-GaAs-Si-Ge cell under 200X concentration, and higher efficiency is possible with more junctions.
The drive to reduce the thickness of solar cells is putting ever greater demands on light-trapping techniques. Techniques are required to improve absorption of light within the semiconductor, while not adversely affecting the electrical properties of the device. Conventional diffraction gratings can scatter visible and near-infrared photons into large angles, which get trapped in the silicon layer by total internal reflection. However, diffraction gratings typically have large feature sizes and so increase the overall surface area of a solar cell compared to the planar case. A periodic arrangement of metal nanoparticles acts as a diffraction grating, but an over-coated semiconductor will have a similar surface area to a planar layer due a combination of a low particle height and low surface coverage.
Random arrays of identical metal nanoparticles feature Lorentzian scattering peaks that can be tuned by modifying the size and shape of the particle. Periodic arrays have much more complicated scattering peaks, due to the enhancement and suppression of scattering at different wavelengths caused by the constructive and destructive interference between each nanoparticle. In effect the scattering spectrum of the individual nanoparticle is modified by the diffractive orders of the array, and so both parameters must be optimized together.
We have studied periodic arrays of metal nanoparticles fabricated using electron-beam lithography, and characterised their reflectance properties. The optical properties of the fabricated arrays were found to be in good agreement with finite-difference time-domain (FDTD) simulations. Au and Al nanoparticles are found to have a strong scattering effect and Al nanoparticles are also shown to exhibit an anti-reflection effect in combination with scattering. This work is focused on verifying that FDTD simulations can accurately model metal nanoparticle arrays and then extending the simulations to determine the previously unknown transmittance characteristics of metal nanoparticle arrays on silicon.
Thin film silicon solar cells are an attractive option for the production of sustainable energy but their low response at long wavelengths requires additional measures for absorption enhancement. The most successful concepts are based on light scattering interface textures whose understanding is greatly facilitated by considering a superposition of periodic textures that diffract the light into oblique angles, ideally beyond the critical angle of total internal reflection. Because the thickness of the active layers is on the same scale as the wavelength, interference of diffracted waves gives rise to resonance phenomena. We discuss the absorption enhancement in terms of a perturbation approach using the modal structure of a corresponding device with flat interfaces.
Surface plasmon enhanced InAs/GaAs quantum dot solar cells are reported. Light trapping by metallic nanostructures offers the potential to realize high efficient quantum dot based intermediate band solar cells. Both Au and Ag nanoparticles spherical metal nanoparticles are synthesized by the salt reduction method. The large area coupling of metal nanoparticles and quantum dot solar cell surface is carried out by using 1,3-propanedithiol as linker molecules. The conversion efficiency of the solar cells has been increased from 9.5% to 11.6% after deposition of Au nanoparticles and from 9.5 to 10.9% after incorporating Ag nanoparticles. The conversion efficiency enhancement is mainly as a result of improved photocurrent due to enhanced forward scattering from the plasmonic nanostructures.
The Lambertian limit represents a benchmark for the enhancement of the effective path length in solar cells, which is important as soon as the absorption length exceeds the absorber thickness. In previous publications it has been shown that either extremely thick or extremely thin solar cells can be driven close to this limit by exploiting up to date photon management. In this contribution we show that the Lambertian limit can also be achieved with thin-film solar cells based on amorphous silicon for practically relevant absorber thicknesses. Departing from superstrates, which are currently incorporated into state-of-the-art thin-film solar cells, we show that their topology has simply to be downscaled to typical feature sizes of about 100 nm in order to achieve this goal. By systematically studying the impact of the modulation height and the lateral feature sizes of the incorporated textures and of the absorber thickness we are able to deduce intuitive guidelines how to approach the Lambertian limit in randomly textured thin-film solar cells.
We report on our systematic study of light trapping effects using Ag/ZnO BRs for nc-Si:H solar cells. The texture of Ag and ZnO was optimized to achieve enhancement in photocurrent. The light trapping effect on photocurrent enhancement in solar cells was carefully investigated. Comparing to single-junction solar cells deposited on flat stainless steel substrates, the gain in Jsc by using Ag/ZnO BRs is 57% for nc-Si:H solar cells. This gain in Jsc is much higher than what has been achieved by advanced light trapping approaches using photonic structures or plasmonic light trapping reported in the literature. We achieved a Jsc of 29-30 mA/cm2 in a nc-Si:H single-junction solar cell with an intrinsic layer thickness of ∼2.5 μm. We compared the quantum efficiency of single-junction cells to the classical limit of fully randomized scattering and found that there is a 6-7 mA/cm2 difference between the measured Jsc and the classical limit, in which 3-4 mA/cm2 is in the long wavelength region. However, by taking into consideration the losses from reflection of the top contact, absorption in the doped layers, and imperfect reflection in the BRs, the difference disappears. This implies we have reached the practical limit if the scattering from randomly textured substrates is the only mechanism of light trapping. Therefore, we believe future research for improving photocurrent should be directed toward reducing (i) reflection loss by the top contact, the absorption in ZnO and at the Ag/ZnO interface, and (ii) p layer absorption.
Phosphorus (P) doped ultra thin n+-layer is formed on crystalline silicon (c-Si) at low substrate temperatures of 80 – 350 °C using radicals generated by the catalytic reaction of phosphine (PH3) with a tungsten catalyzer heated at 1300 °C. The sheet carrier concentration obtained by Hall effect is in the range between 3×1012cm-2 and 8×1012cm-2. The distribution of P atoms obtained by secondary ion mass spectrometry (SIMS) indicates that P atoms locate within the depth of 4 nm from surface and the profile has almost the same distribution independent of any doping conditions such as substrate temperature or radical exposure time. The sheet carrier concentration is 1.15 – 2.12% of the amount of P atoms incorporated through the radical doping. The ratio of activated donors increases with substrate temperature during the radical doping, suggesting that P-related species bonded on the c-Si surface require thermal energy for their activation. Using the n+-layer formed by radical doping, the reduction of surface recombination velocity for n-type c-Si wafer is attempted. The effective minority carrier lifetime of the n-type c-Si sample covered with 6-nm-thick intrinsic amorphous Si (i-a-Si) layers on both side increases from 32 μs to1576 μs by the radical doping of P atoms to n-type c-Si surface, suggesting that the radical doping can be utilized for the formation of passivation layers on a-Si/ n-c-Si hetero-interface.
A thin metal film with nano-apertures, called “nano-mesh electrode,” generates near-field lights near the electrode. We investigated carrier excitations in semiconductors by the near-field light. Finite-difference time-domain (FDTD) method revealed that when the infrared light irradiates the Au nano-mesh electrode on Ge, near-field lights are generated and absorbed in the surface region of the Ge. In order to measure the photocurrent involved by near-filed lights, we fabricated a Schottky cell and applied a Au nano-mesh electrode on the n-type Ge. The efficiency of the Schottky cell with the Au nano-mesh electrode improved in infrared region compared to plain the Au-film Schottky cell. The agreement between theoretical simulations and experiments indicates that near-field lights enhance the carrier excitation in the semiconductor.
This work investigates a novel method to enhance light trapping within polycrystalline silicon (poly-Si) thin films for photovoltaic applications. The method combines the use of hydrogen ion implantation for creation of surface textures in poly-Si thin films and the deposition of silver nanostructures on the textured surface. Poly-Si thin films were prepared by solid phase crystallization of amorphous silicon (a-Si) layer deposited on a SiO2/Si substrate. The a-Si was annealed at various temperatures 600 -1050 °C for 48 hours to grow grains of different size in p-Si, as confirmed by x-ray diffraction (XRD) measurements. These samples were then implanted with 20-keV hydrogen ions to a dose of 1017/cm2, and some with an additional implant with 90-keV argon ions to a dose 5×1015 /cm2. Following implantation, these samples were annealed in an Ar ambient at different temperatures. Surface blistering effects were observed using an optical microscope. Optical specular reflection measurements in the spectral range 400-1100 nm indicated that the reflectance of the samples with higher blistering had decreased remarkably from 40% to 10%. Lastly, the poly-Si samples with various textures were deposited with silver thin film followed by annealing in nitrogen ambient for forming Ag nanostructures on textured poly-Si surfaces. Scanning Electron Microscope (SEM) was used to image the surface structures. The formation of Ag nanoparticles on the poly-Si surface, with textures created by implantation followed by low-temperature annealing (e.g., 400 °C), can significantly reduce light reflection as opposed to the case with Ag nanoparticles formed on an un-textured, poly-Si surface.