Guobin Jia

Silicon Nanowire Solar Cells With Radial p-n Heterojunction on Crystalline Silicon Thin Films: Light Trapping Properties

We present a concept for a core-shell silicon nanowire thin-film solar cell showing strong light trapping. Nanowires are wet chemically etched into a several micrometer-thick laser-crystallized silicon thin film on glass. The nanowires are equipped with an a-Si heteroemitter deposited as a shell around the nanowires by plasma-enhanced chemical vapor deposition to achieve a radial p-n heterojunction. The space between the nanowires is filled with ZnO:Al, acting as a transparent contact. Our core-shell nanowire solar cells reached an efficiency of 8.8%. The main emphasis of this study is on the optical properties of the nanowire solar cell system.

Cathodoluminescence investigation of silicon nanowires fabricated by thermal evaporation of SiO

Silicon nanowire samples fabricated by thermal evaporation of SiO powder were investigated by Cathodoluminescence. Three main bands were found at low temperatures, namely, peak 1 at about 620–650 nm (2.0–1.91 eV), peak 2 at 920 nm (1.35 eV), and peak 3 at 1280 nm (0.97 eV). An additional broad band (peak 4) in the infrared region with its maximum at ∼1570 nm (0.79 eV) appears at room temperature. The origins of the emission bands are discussed.

Regular Dislocation Networks in Si. Part II: Luminescence

Regular dislocation networks formed as a result of the direct bonding of Cz-Si wafers with oxide remnants on the pre-bonding surfaces were investigated. Besides the dislocation network, oxide precipitates were detected at the bonding interface. The precipitate density across the network was ~5×1010 cm-2, except small irregularly distributed circular areas, several mm in diameter, where the density was remarkably lower (<5×108 cm-2). The dislocation network structure was not affected by the change in the precipitate density. Photoluminescence spectroscopy (PL) and light beam induced current (LBIC) mapping were applied for characterization of such dislocation networks. For the locations with high precipitate density, PL signal from dislocations and that from the band-to-band transitions were enhanced. On the other hand, the LBIC results indicated that oxide precipitates are active recombination centers and thus should suppress the observed radiative transitions. The controversy can be explained in the assumption that the D-band PL signal increases due to scattering of excitation light by the precipitates and due to related expansion of the excitation area of the dislocation network. The light reflection from the precipitate layer also enhances the detected band-to-band PL signal. The shape of PL spectra from the samples in the range of photon energies 0.75 – 1.15 eV was not influenced by the oxide precipitates.

Photoluminescence study on defects in multicrystalline silicon

We report on spatially resolved luminescence measurements on ribbon-grown silicon samples. It is found that the band-edge luminescence shows anomalous temperature behavior, namely an increase in the radiation intensity with temperature. Phosphorous diffusion gettering is found to enhance this effect. The anomalous temperature behavior is attributed to nonradiative recombination governed by shallow traps. A shift in the phonon replica of the band edge luminescence peak has been observed and associated with tensile stress.

A miniaturised autonomous sensor based on nanowire materials platform: the SiNAPS mote

A micro-power energy harvesting system based on core(crystalline Si)-shell(amorphous Si) nanowire solar cells together with a nanowire-modified CMOS sensing platform have been developed to be used in a dust-sized autonomous chemical sensor node. The mote (SiNAPS) is augmented by low-power electronics for power management and sensor interfacing, on a chip area of 0.25mm2. Direct charging of the target battery (e.g., NiMH microbattery) is achieved with end-to-end efficiencies up to 90% at AM1.5 illumination and 80% under 100 times reduced intensity. This requires matching the voltages of the photovoltaic module and the battery circumventing maximum power point tracking. Single solar cells show efficiencies up to 10% under AM1.5 illumination and open circuit voltages, Voc, of 450-500mV. To match the battery’s voltage the miniaturised solar cells (~1mm2 area) are connected in series via wire bonding. The chemical sensor platform (mm2 area) is set up to detect hydrogen gas concentration in the low ppm range and over a broad temperature range using a low power sensing interface circuit. Using Telran TZ1053 radio to send one sample measurement of both temperature and H2 concentration every 15 seconds, the average and active power consumption for the SiNAPS mote are less than 350nW and 2.1 μW respectively. Low-power miniaturised chemical sensors of liquid analytes through microfluidic delivery to silicon nanowires are also presented. These components demonstrate the potential of further miniaturization and application of sensor nodes beyond the typical physical sensors, and are enabled by the nanowire materials platform.

1.5 μm Emission from a Silicon MOS-LED Based on a Dislocation Network

A novel Si MOS-LED is demonstrated, which is fully compatible with Si technology. It is based on a dislocation network fabricated by wafer direct bonding. Light emission at 1.5 μm was observed when the network was near the Si/SiO2 interface close to/inside the accumulation layer induced by the gate voltage

DEFECT CHARACTERIZATION OF POLY-Ge AND VFG-GROWN Ge MATERIAL

Germanium is an attractive model system for studying the crystallization mechanism and optimization of the growth processes in photovoltaics. In comparison to Si it has a lower melting point and that is why its usage is cost effective. The main aim of our work was to verify the similarities in the growth related defect formation between Ge and Si. We apply standard Si characterization methods to poly and VGF-grown n-type Ge. Room temperature and 80 K EBIC measurements were done to reveal the defect structure. Photoluminescence spectra were used to characterize the optical properties as for instance the Ge band-to-band or defect originated transitions. Additionally, photoluminescence and cathodoluminescence maps were preformed to reveal the defect distribution/activity, too, by using the direct Ge band-to-band transition.

Investigations on the recombination activity of grain boundaries in MC silicon

This paper focuses on the influence of the effective intra-grain minority charge carrier diffusion length and surface recombination velocity at grain boundaries on solar cell parameters. Both can be extracted in principle from Light- and Electron Beam Induced Current measurements (LBIC and EBIC). Multicrystalline floatzone (mc FZ) silicon with different grain sizes was processed to solar cells with and without hydrogenation step, followed by LBIC and EBIC characterization. A theoretical model is used which can be applied to measured LBIC or EBIC profiles in order to obtain values for the effective intra-grain diffusion length and the recombination velocity at grain boundaries. Efficiencies reached on the processed solar cells (up to 16.0%) are the highest reported so far for material with such a small grain size, and the positive effect of hydrogenation can clearly be seen. The obtained results are very useful for other cost effective small grained mc silicon materials.

Stark effect at dislocations in silicon for modulation of a 1.5 μm light emitter

A MOS-LED and a p-n LED emitting based on the dislocation-related luminescence (DRL) at 1.5 micron were already demonstrated by the authors. Here we report recent observation of the Stark effect for the DRL in Si. Namely, a red/blue-shift of the DRL peak positions was observed in electro- and photo-luminescence when the electric field in the pn-LED was increased/lowered. Fitting the experimental data yields a strong characteristic coefficient of 0.0186 meV/(kV/cm)2. This effect may allow realization of a novel Si-based emitter and modulator combined in a single device.

Fabrication of self-assembled spherical Gold Particles by pulsed UV Laser Treatment

We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.