Yidan Huang

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.

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...

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.

Time-resolved and time-integrated photoluminescence analysis of state filling and quantum confinement of silicon quantum dots

In this paper we report studies of the optical properties of silicon quantum dot structures. From time-resolved and time-integrated photoluminescence measurements we investigate the state-filling effect and carrier lifetime, and discuss the parabolic confinement of quantum dot structures and the large energy splitting between quantum dot levels. The photoluminescence intensities for different quantum dot levels decay with a stretched exponential function and the decay times are in the range 2–100μs depending on the observation wavelength and the dot size.

Rapid thermal annealed Molybdenum back contact for Cu2ZnSnS4 thin film solar cells

In this work, an industrially viable manufacturing process—rapid thermal annealing (RTA) of Molybdenum back contact is proposed and investigated to improve the performance of sputtered Cu2ZnSnS4 (CZTS) solar cells. The RTA process was found to facilitate Na diffusion from soda lime glass to Mo as well as CZTS and improve the crystallinity of the Mo film. Consequently, the surface morphology of the subsequently deposited CZTS absorbers is improved, which results in significant enhancement of open circuit voltage, short-circuit current density, fill factor, and conversion efficiency.

Electroluminescence from Si nanocrystal/c-Si heterojunction light-emitting diodes

Silicon nanocrystals have shown attractive properties for photonic and photovoltaic applications. We demonstrate all-Si light-emitting diodes based on boron-doped Si nanocrystal/c-Si p-n heterojunction structure, which show electroluminescence in the visible/infrared regions. The electroluminescencespectra of these diodes can be modified by changing the quantum confining barriers from SiO2 to Si3N4. Our results are an important demonstration of electroluminescence from boron-doped Si nanocrystals—a wide band gap absorber material for third generation photovoltaics.

Heteroepitaxial growth of Cu2ZnSnS4 thin film on sapphire substrate by radio frequency magnetron sputtering

The heteroepitaxy of tetragonal Cu2ZnSnS4 (CZTS) thin films on hexagonal sapphire (0001) single crystal substrates is successfully obtained by radio frequency magnetron sputtering. The sputtered CZTS film has a mirror-like smooth surface with a root mean square roughness of about 5.44 nm. X-ray θ-2θ scans confirm that CZTS film is (112) oriented on sapphire with an out of plane arrangement of CZTS (112) ‖ sapphire (0001). X-ray Phi scan further illustrates an in plane ordering of CZTS [201¯] ‖ sapphire [21¯1¯0]. The high resolution transmission electron microscopy image of the interface region clearly shows that the CZTS thin film epitaxially grows on the sapphire (0001) substrate. The band gap of the film is found to be approximately 1.51 eV.

Epitaxial Cu2ZnSnS4 thin film on Si (111) 4° substrate

To explore the possibility of Cu2ZnSnS4 (CZTS)/Si based tandem solar cells, the heteroepitaxy of tetragonal Cu2ZnSnS4 thin films on single crystalline cubic Si (111) wafers with 4° miscut is obtained by molecular beam epitaxy. The X-ray θ-2θ scan and selected area diffraction patterns of the CZTS thin films and Si substrates, and the high resolution transmission electron microscopy image of the CZTS/Si interface region demonstrate that the CZTS thin films are epitaxially grown on the Si substrates. A CZTS/Si P-N junction is formed and shows photovoltaic responses, indicating the promising application of epitaxial CZTS thin films on Si.

Conductivity of self-organized silicon quantum dots embedded in silicon dioxide

Silicon quantum dots (SiQDs) embedded in silicon dioxide are being investigated as a means of engineering a wide band gap semiconductor for potential application in silicon based tandem solar cells. The conductivity of the self-organized silicon dots embedded in the oxide is an important parameter in characterizing the electronic transport mechanisms. We present in this paper our initial results on measurement of the resistivity as a function of temperature. In order to reduce contact resistance aluminium contacts are annealed to induce spiking through upper layers of oxide and thus producing a large contact surface area. Samples with various initial silicon rich concentrations are compared. Activation energies for various tentative conduction mechanisms are calculated from this data and possible conduction models presented.