Shinsuke Miyajima

ZnO Films with Very High Haze Value for Use as Front Transparent Conductive Oxide Films in Thin-Film Silicon Solar Cells

We successfully increased the haze value of zinc oxide (ZnO) films fabricated using metal–organic chemical vapor deposition (MOCVD) by conducting glass-substrate etching before film deposition. It was found that with increasing the glass treatment time, the surface morphology of ZnO films changed from conventional pyramid-like single texture to greater cauliflower-like multi texture. Further, the rms roughness and the haze value of the films increased remarkably. Using ZnO films with a high haze value as front transparent conductive oxide (TCO) films in hydrogenated microcrystalline silicon (µc-Si:H) solar cells, we improved the quantum efficiency of these cells particularly in the long-wavelength region.

High Quality Aluminum Oxide Passivation Layer for Crystalline Silicon Solar Cells Deposited by Parallel-Plate Plasma-Enhanced Chemical Vapor Deposition

We investigated hydrogenated aluminum oxide (a-Al1-xOx:H) as a high quality rear surface passivation layer of crystalline silicon solar cells. The a-Al1-xOx:H films were deposited by plasma-enhanced chemical vapor deposition (PECVD) using a mixture of trimethylaluminum (TMA), carbon dioxide (CO2), and hydrogen (H2) at a low substrate temperature of about 200 °C. The ratio of CO2 to TMA during deposition and thermal annealing after the film deposition are the key factors in achieving high quality passivation. A 28-nm-thick a-Al1-xOx:H film deposited by PECVD showed a low surface recombination velocity of about 10 cm/s.

Preparation of Nanocrystalline Silicon in Amorphous Silicon Carbide Matrix

We have successfully prepared silicon quantum dots/amorphous silicon carbide multilayers by the thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon-rich hydrogenated amorphous silicon carbide (a-Si1-xCx) multilayers. Raman scattering spectroscopy and transmission electron microscopy (TEM) revealed that silicon quantum dots were formed in only a-Si1-xCx layers. We also found that the size of silicon quantum dots can be controlled by the thickness of a-Si1-xCx layers.

Photoluminescence from Silicon Quantum Dots in Si Quantum Dots/Amorphous SiC Superlattice

We prepared size-controlled silicon quantum dots superlattices (Si-QDSLs) by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon-rich hydrogenated amorphous silicon carbide (a-Si1+xC:H) multilayers. Transmission electron microscope (TEM) observation revealed that silicon quantum dots were formed in only a-Si1+xC:H layers. The size of silicon quantum dots can be controlled by the thickness of the a-Si1+xC:H layers. It was found that hydrogen plasma treatment (HPT) significantly enhanced the photoluminescence of the Si-QDSLs. The luminescence peaks shifted to shorter wavelength with decreasing the diameter of the silicon quantum dots in the Si-QDSL.

Characterization of Undoped, N- and P-Type Hydrogenated Nanocrystalline Silicon Carbide Films Deposited by Hot-Wire Chemical Vapor Deposition at Low Temperatures

Undoped, n- and p-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) films were successfully deposited on glass and silicon substrates at a low substrate temperature of about 300 °C by hot-wire chemical vapor deposition. The structural, optical, and electrical properties of the films were investigated by X-ray diffraction (XRD), Fourier transform infrared absorption (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry, photothermal deflection spectroscopy (PDS), and conductivity measurements. The XRD and FTIR measurements revealed a clear correlation between the average grain size and width of the SiC stretching mode vibration of the films. The dark conductivity of the films was increased from 5.8×10-11 to 6.2×10-6 S/cm with increasing the grain size from 6.4 to 16.6 nm. The detailed analysis of the dark conductivity indicates that the Fermi level position is affected by the grain size of the films. Spectroscopic ellipsometry measurements showed that the dielectric functions of the films are strongly affected by the grain size. The n- and p-type nc-3C–SiC:H films were also successfully deposited using phosphine, hexamethyldisilazane, and dimethylaluminumhydride as dopants. For the n- and p-type films, the dark conductivities of 5.32×10-0 and 7.67×10-4 S/cm were achieved, respectively. The optical absorption spectra of the doped films indicate that p-type doping significantly affects absorption coefficients above the bandgap of nc-3C–SiC:H compared with n-type doping. For the n-type films, the absorption coefficients below the bandgap are affected by free carrier absorption as well as by localized states within the bandgap.

High efficiency protocrystalline silicon/microcrystalline silicon tandem cell with zinc oxide intermediate layer

The authors develop a hydrogenated protocrystalline silicon (pc-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) double-junction solar cell structure employing a boron-doped zinc oxide (ZnO:B) intermediate layer. Highly stable intrinsic pc-Si:H and μc-Si:H absorbers are prepared by a 60MHz very-high-frequency plasma-enhanced chemical vapor deposition technique. Degenerate ZnO:B intermediate and back reflectors are deposited via a metal organic chemical vapor deposition technique. Because the ZnO:B intermediate layer reduces the potential thickness for the pc-Si:H absorber in the top cell, this double-juncion structure is a promising candidate to fabricate highly stable Si-based thin-film solar cells. Consequently, the high conversion efficiency of 12.0% is achieved.

Optimization of Amorphous Silicon Oxide Buffer Layer for High-Efficiency p-Type Hydrogenated Microcrystalline Silicon Oxide/n-Type Crystalline Silicon Heterojunction Solar Cells

Intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) films deposited by very high frequency plasma-enhanced chemical vapor deposition (60 MHz VHF-PECVD) at a low substrate temperature of approximately 200 °C were used as a front buffer layer in p-type hydrogenated microcrystalline silicon oxide/n-type crystalline silicon (p-µc-SiO:H/n-c-Si) heterojunction solar cells. We found that the oxygen concentration in the i-a-SiO:H buffer layer strongly affected the solar cell performance and that the short wavelength response in quantum efficiency (QE) was improved by oxygen addition. Employing a p-µc-SiO:H/i-a-SiO:H/n-Si [Czochralski (CZ), 200 µm, (100)]/i-a-Si:H/n-a-Si:H/Ag/Al configuration, we achieved an efficiency of 17.9% with Voc of 671 mV.

High-quality nanocrystalline cubic silicon carbide emitter for crystalline silicon heterojunction solar cells

We developed a highly transparent n-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) emitter for crystalline silicon (c-Si) heterojunction solar cells. A low emitter saturation current density (J0e) of 1.4×101 fA/cm2 was obtained under optimal deposition conditions. A c-Si heterojunction solar cell fabricated on a p-type c-Si wafer without texturing showed an active area efficiency of 17.9% [open-circuit voltage (Voc)=0.668 V, short-circuit current density (Jsc)=36.7 mA/cm2, fill factor=0.731]. The high Jsc value is associated with excellent quantum efficiencies at short wavelengths (<500 nm).

Fabrication of microcrystalline cubic silicon carbide/crystalline silicon heterojunction solar cell by hot wire chemical vapor deposition

The n-type microcrystalline cubic silicon carbide (µc-3C–SiC:H) films were deposited by hot wire chemical vapor deposition (HWCVD) at a low substrate temperature (~300 °C). Heterojunction silicon based photovoltaic devices were fabricated by depositing wide band gap n-type µc-3C–SiC thin films on p-type Si wafers, whose thickness and resistivity were 200 µm and 1–10 Ω cm, respectively. The silicon wafers were textured using alkaline etchant prior to the device fabrication. The photovoltaic parameters of a typical device were found to be Voc=560 mV, Jsc=35.0 mA/cm2, fill factor (F.F.) = 0.724, and η=14.20%. Numerical analysis was performed using automat for simulation of hetero structures (AFORS-HET), a one-dimensional device simulators to determine the probable cause of the changes in device parameters before and after the ageing of the filament.

Optimization of p-Type Hydrogenated Microcrystalline Silicon Oxide Window Layer for High-Efficiency Crystalline Silicon Heterojunction Solar Cells

Wide-gap, high-conductivity p-type hydrogenated microcrystalline silicon oxide (p-µc-SiO:H) films deposited by very high frequency plasma-enhanced chemical vapor deposition (60 MHz VHF-PECVD) at a low substrate temperature of approximately [200 °C] were used as window layers in n-type crystalline silicon (n-c-Si) heterojunction (HJ) solar cells. We investigated the effect of p-µc-SiO:H window layer thickness on HJ solar cells by changing deposition time and silane (SiH4) flow rate. The effects of carbon dioxide flow rate on the p-µc-SiO:H window and i-a-SiO:H buffer layer were also studied. Employing a p-µc-SiO:H/i-a-SiO:H/n-c-Si [Czochralski (CZ), 200 µm, (100)]/i-a-Si:H/n-a-Si:H configuration, we achieved an efficiency of 17.8% (active area efficiency) with an open-circuit voltage (Voc) of 665 mV. The solar cells showed a spectral response of about 0.83 at a wavelength of 400 nm, which is higher than that of conventional HJ solar cells with amorphous silicon window layers.