[Show abstract][Hide abstract] ABSTRACT: The structural and optical properties of erbium-doped silicon-rich silica samples containing different Si concentrations are studied. Intense photoluminescence (PL) from luminescence centers (LCs) and silicon nanoclusters (Si NCs), which evolves with annealing temperatures, is obtained. By modulating the silicon concentrations in samples, the main sensitizers of Er(3+) ions can be tuned from Si NCs to LCs. Optimum Er(3+) PL, with an enhancement of more than two, is obtained in the samples with a medium Si concentration, where the sensitization from Si NCs and LCs coexists.
Full-text · Article · Sep 2014 · Nanoscale Research Letters
[Show abstract][Hide abstract] ABSTRACT: Low-cost upgraded metallurgical grade silicon (UMG-Si) with inherent boron (B) and phosphorus (P) compensation is a novel material for photovoltaic application. This paper presents the impact of solar irradiance intensity and temperature on the performance of compensated crystalline silicon solar cells. For the same rated output power, compensated crystalline silicon solar cells generate less electricity than the reference silicon solar cells at low irradiance intensity, owing to the strong injection dependence of the carrier lifetime due to high concentration of B-O complexes in compensated silicon. However, at high temperature, compensated crystalline silicon solar cells generate more electricity than the reference silicon solar cells, which mainly originates from the lower temperature-variation of the minority electron mobility in compensated silicon. It suggests that compensated silicon solar cells will be more appropriate for high irradiation application, which often contains high irradiance intensity and high temperature. These results are of great significance for understanding the actual outdoor performance of the solar cells based on the UMG-Si and their application in the photovoltaic (PV) industry.
No preview · Article · Sep 2014 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: The determination of boron and phosphorus ionization energies in compensated silicon is very important for assessing the ionization level of dopants and their interaction with each other. In this paper, we achieved the boron and phosphorus ionization energies in compensated silicon by temperature-dependent luminescence for the first time. The results show that the boron and phosphorus ionization energies in heavily-compensated silicon have the same values as those in non-compensated silicon. This strongly suggests that both boron and phosphorus impurities with a concentration of ≤ 10 17 cm −3 should act as isolated acceptors and donors, but do not form complexes in silicon.
[Show abstract][Hide abstract] ABSTRACT: Low-cost upgraded metallurgical grade silicon (UMG-Si) with inherent boron (B) and phosphorus (P) compensation is a novel material for photovoltaic application. In this study, we demonstrate the negative impact of the light-induced degradation (LID) on the efficiency of p-type UMG-Si solar cells. By a following illumination at elevated temperatures, the LID effect in the compensated silicon solar cells can be fully deactivated, and the minority carrier diffusion length is recovered to the original level. The conversion efficiency of the compensated silicon solar cells is increased by a value of more than 3% absolutely compared to the degraded state and is quite stable under the following illumination at room temperature. It is shown that both the boron-oxygen defect density and deactivation energy mainly depend on the total boron concentration rather than the net doping concentration, which strongly suggests that boron is directly involved in the generation and deactivation of boron-oxygen defect at the solar cell level. These results are of significance for understanding the LID effect of the solar cells based on low cost UMG-Si.
No preview · Article · Oct 2013 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: The energy transfer mechanism between luminescent centers (LCs) and Er3+ in erbium-doped silicon-rich oxide (SROEr) films prepared by electron beam evaporation is investigated. Intense photoluminescence of the LCs (weak oxygen bonds, neutral oxygen vacancies, and Si=O states) within the active matrixes is obtained. Fast energy transfer from Si=O states to Er3+ takes advantage in the SROEr film and enhances the light emission from Er3+. The introduction of Si nanoclusters, which induces the Si=O states and facilitates the photon absorption of the Si=O states, is essential to obtain intense photoluminescence from both Si=O states and Er3+.
Full-text · Article · Aug 2013 · Nanoscale Research Letters
[Show abstract][Hide abstract] ABSTRACT: The dissociation of iron-boron pairs (FeB) in Czochralski silicon under strong illumination was investigated. It is found that the dissociation process shows a double exponential dependence on time. The first fast process is suggested to be caused by a positive Fe in FeB capturing two electrons and diffusion triggered by the electron-phonon interactions, while the second slow one would involve the capturing of one electron followed by temperature dependent dissociation with an activation energy of (0.21 ± 0.03) eV. The results are important for understanding and controlling the behavior of FeB in concentrator solar cells.
[Show abstract][Hide abstract] ABSTRACT: The size-dependent coupling between localized surface plasmons (LSPs) and excitons within a silicon nitride (SiN<sub>x</sub>) matrix is investigated. A strong correlation between the photoluminescence (PL) enhancement and this resonance coupling is observed. From the analysis of the relationship between the dipolar resonance peaks of Ag nanostructures with various sizes and those of PL enhancement, we ascribe the enhancement of PL from the SiN<sub>x</sub> matrix by the addition of Ag nanostructures mainly to the LSP resonance coupling.
[Show abstract][Hide abstract] ABSTRACT: N-type compensated silicon has attracted much attention in recent years, due to its low cost and large tolerance to most impurities. In this paper we have investigated the photovoltaic properties of n-type compensated silicon solar cells with the Al-alloyed emitter. It is found that the open-circuit voltage is strongly influenced by the Al-alloyed emitter depth and the shallow depth might cause shunt paths. The short-circuit current is mainly limited by the moderate minority carrier diffusion length in n-type compensated silicon, which originates from the large net doping electron concentration and the great amount of total dopants. Combined with the experimental information, the simulation suggests that the short-circuit current can be strongly improved by using thinner wafers and therefore the conversion efficiency could get significantly improved. These results are of interest for understanding the potential of n-type compensated silicon in the fabrication of high-efficiency and low-cost solar cells.
No preview · Article · Jun 2013 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: The electroluminescence (EL) wavelength tailoring of silicon-rich silicon nitride (SiNx)-based light-emitting devices (LEDs) is achieved by the modulation of the dimensions of Ag nanoparticles. Two EL peaks are observed in our SiNx-based LEDs, both of which are red shifted with the increasing sizes of Ag nanoparticles. A reasonable explanation on this shift is proposed from the calculation of the local electric field surrounding Ag nanoparticles based on a simple model. This red shift of the two EL peaks as well as the evolution of their relative intensities is mainly originated from its weakening electric field enhancement with the increase of the size of Ag nanoparticles. Our work provides an alternative approach toward the fabrication of SiNx-based LEDs with tunable EL wavelengths.
No preview · Article · May 2013 · IEEE Journal of Selected Topics in Quantum Electronics
[Show abstract][Hide abstract] ABSTRACT: The external quantum efficiency as well as its degeneration in silicon nitride light-emitting devices is significantly improved by the addition of metal nanostructures. The origin of this efficiency droop phenomenon is investigated in detail via the analysis of the dominant process contributing to the light output power and the carrier injection conditions for the devices with and without metal nanostructures. We attribute this droop mainly to the nonradiative Auger recombination as the introduction of metal nanostructures would suppress the Auger process but make the carrier overflow more serious due to the enhanced local electrical field by localized surface plasmon resonance.
No preview · Article · Feb 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: A maximal enhancement of ~6.5 times of the external quantum efficiency (EQE) for SiNx-based light-emitting devices (LEDs) is achieved by magnetron sputtering a silver nanostructures layer onto the active matrix. The enhancement of EQE is affected by the dimension and morphology of silver nanostructures, which can be controlled by the sputtering time and the post treatment of rapid thermal annealing. The optimal size of silver nanostructures is about 100 nm in diameter by comparing the integrated electroluminescence intensity under the same input power. The optimization of EQE for SiNx-based LEDs is discussed by considering the contributions of the enhancement of light-extraction efficiency induced by the surface roughening of the front electrode, internal quantum efficiency due to the coupling between excitons and localized surface plasmons, and carrier injection efficiency. Our work may provide an alternative approach for the fabrication of Si-based light sources with promising luminescence efficiency.
[Show abstract][Hide abstract] ABSTRACT: A series of silicon-rich oxide (SRO) and erbium-doped SRO (SROEr) films imbedded with structural tunable silicon nanoclusters (Si NCs) have been fabricated using sputtering followed by post-annealing. The coalescence of Si NCs is found in the films with large Si excess. The energy transfer rate between Si NCs and Er3+ is enhanced, but the luminescence efficiencies of both Si NCs and Er3+ are reduced by the coalescent microstructures. Optimization of the microstructures of Si NCs is performed, and the preferential optical performance for both Si NCs and Er3+ could be achieved when Si NCs were separated in microstructures.
Full-text · Article · Jan 2013 · Nanoscale Research Letters
[Show abstract][Hide abstract] ABSTRACT: A multilayer structure of ITO/SiNx/Ag/p/p<sup>+</sup>-Si/Au was fabricated to improve the extraction of the orange-yellow electroluminescence from SiNx-based light-emitting devices (LEDs), and an about 5 times enhancement of external quantum efficiency (EQE) was obtained. This improved light-extraction is mainly originated from the increase of root-mean-square roughness of ITO electrode and reflectivity at longer wavelength via the addition of elongated Ag nanostructures. For the structure with the dipolar resonance peak of Ag nanostructures far from the emission wavelength of SiNx matrix, the increased surface roughness of ITO electrode has a dominant effect on the improvement of the light-extraction. Moreover, the decrease of on-series resistance by the addition of Ag nanostructures due to its enhanced local electrical fields also has a benignant contribution to the improved EQE. Our work may provide a promising approach to improve the EQE of LEDs, which is not limited to SiNx matrix.
[Show abstract][Hide abstract] ABSTRACT: Germanium (Ge)-doped crystalline silicon has attracted much attention in recent years, due to its promising properties for meeting the increasing requirements for photovoltaic applications. This paper has reviewed our recent results on Ge-doped crystalline silicon and corresponding solar cells. It includes that Ge doping improves the fracture strength of crystalline silicon, and suppresses the Boron–Oxygen (B–O) defects responsible for the light induced degradation (LID) of carrier lifetime. Ge doping in crystalline silicon will not only benefit for reduction of breakage during the cell fabrication processes, but also improve the solar cell efficiency and the power output of corresponding modules under sunlight illumination.
No preview · Article · Jan 2013 · Journal of Crystal Growth
[Show abstract][Hide abstract] ABSTRACT: The coupling between localized surface plasmons (LSPs) within silver nanostructures and excitons in a silicon-rich silicon nitride (SiN
) matrix has been demonstrated via the Purcell effect. A simple model is employed for the estimation of the Purcell factor as well as the average position of excitons within a luminescence matrix. The estimated average position of the excitons is located at approximately 40 nm beneath the top surface of the SiN
films. The approaches for further improving the optoelectrical properties of the luminescence matrix are anticipated based on the model we adopted. The optimization of the thickness of the luminescence matrix as well as the size and shape of metal nanostructures may be the alternative approaches. Besides, the application of multilayers with the luminescence matrix inserted between barrier layers (we defined it as confined structures here) may be also an available choice. Our work may provide a deep comprehension on the coupling between LSPs and excitons, which is not limited to a certain luminescence material but with unconfined structures.
Full-text · Article · Dec 2012 · Nanoscale Research Letters
[Show abstract][Hide abstract] ABSTRACT: We have investigated the influence of the compensation level on the performance of p-type crystalline silicon solar cells by combining theoretical calculations and experiments. With the compensation level increasing, the fraction of ionized boron (B) atoms increases. The hole mobility is significantly reduced due to the increased ionized impurities scattering, while the electron mobility almost keeps constant due to the inverse contributions of ionized impurities scattering and electron–hole scattering. The carrier lifetime in compensated silicon, mainly dominated by high concentration of dopants, could get improved. Correspondingly, the minority carrier diffusion length increases, causing a higher short-circuit current. However, the open-circuit voltage first decreases and then increases with the increase of the compensation level, owing to the competition effect of the reduced net doping concentration and the increased excess carrier concentration. All these factors could finally result in a higher efficiency solar cell. These results suggest that intentional dopant compensation might be utilized for the improvement of solar cell efficiency, which is especially interesting for those based on highly-doped silicon feedstock.
No preview · Article · Dec 2012 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: The modulation effect of microstructures on photoluminescence (PL) properties of silicon nanoclusters (Si NCs) in silicon-rich oxide (SRO) matrix prepared by electron-beam evaporation (EBE) and magnetron sputtering (MS) is investigated. A loose and porous microstructure is obtained from the SRO film prepared by EBE, while compact microstructure is acquired from that prepared by MS. The Si NCs with high density and good quality are formed in the SRO film prepared by EBE, and preferable PL performance of Si NCs is achieved in the EBE film with loose and porous microstructure. In contrast, the density and quality of Si NCs in the compact SRO film are suppressed and the PL properties are deteriorated due to the volumetric mismatching during the precipitation procedure of Si NCs. Optimization of the microstructures in SRO films should be made to reduce the volumetric mismatching and improve the PL properties of Si NCs.
No preview · Article · Dec 2012 · Applied Physics A
[Show abstract][Hide abstract] ABSTRACT: Two obvious Gauss peaks are observed in SiNx-based light-emitting devices with silver nanoparticles deposited onto the luminous layer, both of which are blue shifted with the increase of injected current. The origin of these two peaks is discussed by means of the changes of their positions, relative intensities, and full width at half maximum. We attribute the blue-shift of both electroluminescence peaks to the improvement of carrier injection as carriers can be injected into higher energy levels along their corresponding band tails, which is also confirmed by the changes of the transport mechanism.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the effect of boron (B) and phosphorus (P) compensation on the performance of Czochralski (CZ) silicon solar cells. Both majority and minority carrier mobilities are significantly reduced by the dopant compensation. Correspondingly, the minority carrier diffusion length becomes smaller. The compensated silicon solar cells show weaker spectral responses and therefore lower short-circuit current. However, the open-circuit voltage can be influenced by the net doping concentration in our silicon wafers. A higher open-circuit voltage could be obtained from the compensated solar cells due to its larger net doping concentration. As a result, the compensated solar cells could have the same efficiency as the conventional ones. It suggests that the dopant compensation in silicon with a doping level of ≤1017 cm−3 is not a serious issue for the improvement of solar cell efficiency. The results are of significance for the upgraded metallurgical grade silicon (UMG-Si) application in the fabrication of high efficiency and low-cost solar cells.
No preview · Article · Jun 2012 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: Based on an ITO/Ag/SiNx/p+-Si/Al structure, a significant enhancement of the external quantum efficiency was achieved compared with the device without Ag island film. Analysis showed that the increase of the light-extraction efficiency resulted from the surface roughening of ITO electrode has a main contribution to this enhancement. The increase of the internal quantum efficiency induced by the enhancement of spontaneous emission rate and the carrier injection level also has an instructive contribution. Our results demonstrate that localized surface plasmons enhanced SiNx-based light emitting devices show great promise for the development of efficiency Si-based optical device.
No preview · Article · Jan 2012 · Applied Physics Letters