Dengyuan Song

Investigation of lateral parameter variations of Al-doped zinc oxide films prepared on glass substrates by rf magnetron sputtering

Sputter-deposited Al-doped zinc oxide (ZnO : Al) is an interesting transparent conductive oxide (TCO) material for application in electronic devices and thin-film solar cells. A phenomenon in the planar magnetron sputtering of the ZnO : Al films that is not well investigated as yet are the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material, whereby the lateral distribution of the film properties depends strongly on the sputtering parameters. In this work, the lateral distributions of the electrical, optical, and surface structure properties of the ZnO : Al films prepared by the rf magnetron sputtering on glass substrates are investigated across a distance of 64 mm using the four-point probe technique, optical transmission and reflection measurements, X-ray diffraction, and scanning electron microscopy. We find that the lateral variations of the parameters of the ZnO : Al films prepared by the rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. Hence, it seems that the sputter-deposited ZnO : Al is a promising TCO material for large-area thin-film solar cells.

Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells

We report work progress on the growth of Si quantum dots in different matrices for future photovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantum dots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PL decay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength.

Large open-circuit voltage improvement by rapid thermal annealing of evaporated solid-phase-crystallized thin-film silicon solar cells on glass

In this letter, we investigate the impact of rapid thermal annealing (RTA) on thin-film polycrystalline silicon (pc-Si) solar cells on glass made by evaporation of amorphous silicon (a-Si) and subsequent solid-phase crystallization (SPC). These devices have the potential to deliver low-cost photovoltaic electricity and are named EVA cells (SPC of EVAporated a-Si). The RTA is used to perform a high-temperature (>700°C) process for point defect annealing and dopant activation. RTA processes have predominantly been developed for wafer-based devices yet also have great potential for low-temperature devices such as thin-film pc-Si on glass solar cells. Parameter variations are performed on EVA solar cells to determine optimum values for point defect removal and dopant activation while minimizing dopant diffusion causing junction smearing. The 1-Sun open-circuit voltage, Voc, of the as-crystallized pc-Si devices is rather modest (135mV). However, after RTA and subsequent hydrogen passivation in a rf PECVD plasm...

Structural characterization of annealed Si1−xCx/SiC multilayers targeting formation of Si nanocrystals in a SiC matrix

Amorphous Si1−xCx/SiC multilayer films were prepared by alternating deposition of Si-rich Si1−xCx and near-stoichiometric SiC layers by using magnetron sputtering. The as-deposited films were annealed at different temperatures (Ta) from 800 to 1100 °C. The influence of Ta and Si content in the Si-rich layer on the layered structural stability and on the formation of Si and/or SiC nanocrystals (NCs) is investigated by a variety of analytical techniques, including x-ray reflectivity (XRR), x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectrometry (FTIR). XRR showed that Si1−xCx/SiC multilayers annealed at temperatures of up to 800 °C retain their layered structure. XRD revealed that Si NCs were formed in samples with a high Si content in the Si-rich layer for Ta≥800 °C. At annealing temperatures of 900 °C or greater, the formation of Si NCs was accompanied by the formation of β-SiC NCs. Additionally, the formation of Si and SiC NCs was c...

Structure and characteristics of ZnO:Al/n-Si heterojunctions prepared by magnetron sputtering

Abstract Aluminium-doped zinc oxide (ZnO:Al) films were deposited by RF magnetron sputtering on single-crystal n-Si substrates to form ZnO:Al/n-Si heterojunctions. The structure of the ZnO:Al films was analysed by X-ray diffraction spectroscopy and scanning electron microscopy. The structural properties of the ZnO:Al/n-Si junction were studied by transmission electron microscopy. The electrical junction properties were characterised by current–voltage (I–V) and capacitance–voltage (C–V) methods. The conductance measured shows a room-temperature turn-on voltage of 0.2–0.4 V. The ideality factor and the junction built-in potential deduced from I–V and C–V plots are 1.62 and 0.56 V at room temperature, respectively. Temperature-dependent dark forward current vs. voltage measurements suggest that trap-assisted multistep tunnelling is the dominant carrier transport mechanism in this heterojunction.

Fabrication and characterization of Si nanocrystals in SiC matrix produced by magnetron cosputtering

Si-rich amorphous silicon carbide thin films were prepared by magnetron cosputtering and were subsequently annealed to form Si nanocrystals embedded in a SiC matrix. A sputter target consisted of a patterned Si wafer on top of a carbon target. The ratio of carbon to silicon in deposited films was adjusted by means of a different silicon wafer open area. X-ray photoelectron spectroscopy spectra show that various compositions were obtained by changing the sputtered area ratio of carbon to silicon target. Analysis of atomic force microscopy shows that surface roughness increases significantly after annealing. Transmission electron microscopy reveals that Si nanocrystals do not form at temperatures less than 800°C, while they are clearly established, with sizes ranging from 3to7nm, as the temperature is at 1100°C. IR spectra show that increase in annealing temperature for the Si-rich Si1−xCx (x<0.5) films favors the formation of Si–C bonds and increase of the short-range order. Optical studies show a blueshif...

Fabrication and electrical characteristics of Si nanocrystal/c-Si heterojunctions

Heterojunctions (HJs) were fabricated from p-type Si nanocrystals (Si NCs) embedded in a SiC matrix on an n-type crystalline Si substrate. Transmission electron microscopy revealed that Si NCs are clearly established, with sizes in the range of 3–5nm. The HJ diodes showed a good rectification ratio of 1.0×104 at ±1.0V at 298K. The ideality factor, junction built-in potential, and open-circuit voltage are ∼1.24, 0.72V, and 0.48V, respectively. Measurement of temperature-dependent I-V curves in forward conduction suggests that, in the medium voltage range, junction interface recombination can be described as the dominant current transport mechanism.

Recent advances in polycrystalline silicon thin-film solar cells on glass at UNSW

Polycrystalline Si (pc-Si) thin-film solar cells on glass are a very promising approach for lowering the cost of photovoltaic solar electricity. This paper reports on the status of three distinctly different pc-Si thin-film solar cells on glass under development at the University of New South Wales (UNSW). The cells are termed EVA, ALICE and ALICIA, are less than 3 microns thick, and are made by vacuum evaporation, a fast and inexpensive Si deposition method. EVA cells are made on non-seeded glass, whereas ALICE and ALICIA are both made on a thin large-grained pc-Si seed layer formed on glass by metal-induced crystallisation. All three solar cells seem to be capable of voltages of over 500 mV and, owing to their potentially inexpensive and scalable fabrication process, have significant industrial appeal.

Monolithically Integrated Polycrystalline Silicon Thin-Film Mini-Modules on Glass

A new concept for the fabrication of polycrystalline silicon (poly-Si) thin-film photovoltaic modules on glass is presented. The concept is based on the formation of individual cells with interdigitated contacts, followed by cell interconnection using stencil printing. The metallisation of the individual thin-film poly-Si solar cells is realised with the self-aligning maskless photolithography (SAMPL) method. The effects of post-fabrication treatments (baking and etching) and Si sidewall profile on the performance of individual cells are investigated. The monolithic series interconnection of neighbouring cells is achieved by printing silver paste to connect the air-side contact of one cell with the glass-side contact of the adjacent solar cell. The limitations on efficiency are analysed by Suns-Voc measurements. First experimental results for a thin-film mini-module with three individual cells are presented, showing that this new method is a promising technology for the fabrication of poly-Si on glass thin-film solar modules

Structural characterization of solid phase crystallized Si 0.5 Ge 0.5 films for photovoltaic applications

Intrinsic Si0.5Ge0.5 thin films were deposited on Si wafer substrates by RF sputtering in various substrate temperatures, film thickness and Ar (or Ar-H2) sputtering atmosphere for photovoltaic applications. The as-deposited films were annealed at different temperatures from 850°C to 1000°C. Effects of deposition parameter and annealing temperature on structural properties of the samples were studied by x-ray diffraction and Raman measurements. Thick sample (≫1µm) shows large crystallized grain size in low annealing temperature region (≪ 950°C), while thin sample (∼600 nm) has a better crystallinity at high annealing temperature region (≫ 950°C). The samples deposited at 250°C exhibits improved grain size in high annealing temperature compared with the ones deposited at 100 °C and 400 °C. The introduction of H2 gas during sputtering can effectively improve the crystallization quality in low annealing temperature region, but have no obvious effect on high temperature annealing samples. Raman spectra reveal the Si-Si bond can only be well formed as well as Ge precipitation can be observed for high temperature annealing samples.