Hajime Shirai

Highly efficient crystalline silicon/Zonyl fluorosurfactant-treated organic heterojunction solar cells

We demonstrate a highly efficient hybrid crystalline silicon (c-Si) based photovoltaic devices with hole-transporting transparent conductive poly-(3,4-ethlenedioxythiophene):poly(styrenesufonic acid) (PEDOT:PSS) films, incorporating a Zonyl fluorosurfactant as an additive, compared to non additive devices. The usage of a 0.1% Zonly treated PEDOT:PSS improved the adhesion of precursor solution on hydrophobic c-Si wafer without any oxidation process. The average power conversion efficiency η value was 10.8%-11.3%, which was superior to those of non-treated devices. Consequently, c-Si/Zonyl-treated PEDOT:PSS heterojunction devices exhibited the highest η of 11.34%. The Zonyl-treated soluble PEDOT:PSS composite is promising as a hole-transporting transparent conducting layer for c-Si/organic photovoltaic applications.

Role of Hydrogen Plasma during Growth of Hydrogenated Microcrystalline Silicon : In Situ UV-Visible and Infrared Ellipsometry Study

We have applied in situ UV-visible and infrared phase-modulated ellipsometry to investigate the role of hydrogen plasma during the growth of hydrogenated microcrystalline silicon (μc-Si:H) by plasma-enhanced chemical vapor deposition (PECVD). The results of the deposition of μc-Si:H from the SiH 2 highly diluted in H 2 , layer-by-layer (LbL) technique and post-hydrogenation experiments showed that the 3-dimensional cross-linking and relaxation of a Si network near the growing surface were essential for the formation of microcrystalline silicon. The major role of hydrogen plasma is the creation of the free volumes on the growing surface due to the inhomogeneous etching of the Si network and the promotion of the cross-linking reactions

Optical emission spectroscopy study toward high rate growth of microcrystalline silicon

Abstract A systematic optical emission spectroscopy (OES) study was carried out to enhance the deposition rate of microcrystalline silicon (μc-Si:H) with conventional r.f. plasma-enhanced chemical vapor deposition (r.f. PECVD). Among the various plasma parameters, the combination of total pressure, r.f. power, electrode distance and cathode heating was effective to promote the deposition rate without deteriorating the film crystallinity. Strong correlations among the OES intensity, SiH, intensity ratio, I H α / I Si* , deposition rate and Raman intensity ratio, I μc-Si / I a-Si were confirmed in the case of r.f. SiH 4 and H 2 PECVD. A relatively high deposition rate was achieved of ∼5 A/s in the μc-Si:H film growth by optimizing the deposition parameters. The effects of higher pressure, higher r.f. power, inter electrode distance and cathode heating (SiH 4 gas heating) are demonstrated in the growth of μc-Si:H from strong H 2 -diluted SiH 4 by a conventional r.f. glow discharge.

Synthesis of silicon nanocones using rf microplasma at atmospheric pressure

We report the synthesis of silicon nanocones using the rf microplasma discharge at atmospheric pressure. The products formed underneath the tube electrode on Fe-coated crystalline silicon were constituted mainly of silicon and silicon oxide despite the use of a methane-argon mixture. Carbon nanotubes and silicon nanowires were also formed around the silicon nanocones. The number density and average size of silicon nanocones increased with the plasma exposure time accompanied by the enlargement of their surface distribution. The growth mechanism of silicon nanocones is discussed in terms of the catalytic growth via diffusion of silicon with nanocrystalline Si particle through FeSix nanoclusters, and enhanced Si oxidation by the plasma heating.

Fast Deposition of Microcrystalline Silicon Using High-Density SiH4 Microwave Plasma

A novel microwave discharge utilizing a spokewise antenna was applied for the fast deposition of hydrogenated microcrystalline silicon (µc-Si:H) film from SiH4 and Ar without the H2 dilution method. Systematic deposition studies were employed with total pressure, H2 dilution ratio and flow rate of SiH4, Fr[SiH4], as variables, combined with optical emission spectroscopy (OES) and Langmuir probe characterizations. It was found that the deposition rate exhibits a maximum at 40–50 mTorr at the axial distance of 10 cm from the quartz glass plate and the film crystallinity strongly depend on the total pressure. Correlation among OES signal intensity, SiH, the intensity ratio, IHα /ISi* , deposition rate and film crystallinity were demonstrated. By combining the SiH4 depletion and lower pressure conditions, a high deposition rate of 40 A/s was achieved in µc-Si:H growth with high crystallinity and photosensitivity from SiH4 and Ar plasma.

Magneto-Seebeck coefficient of a bismuth microwire arrayin a magnetic field

The enhancement of the magneto-Seebeck coefficient of a bismuth microwire array under a magnetic field is measured at temperatures from 45to295K. The measured magneto-Seebeck coefficient exhibits a peak at a certain magnitude of magnetic field, with the peak shifting to higher magnetic fields and becoming broader with increasing temperature. The results show that the magneto-Seebeck coefficient can be improved by approximately 20% by applying an appropriate external magnetic field and temperature. The Boltzmann equation with a relaxation-time approximation is solved numerically to determine the magnetic field and temperature dependences of the magneto-Seebeck coefficient for the bismuth microwire array. The experimental results are compared with calculations, and the two sets of results are shown to be in very good agreement, clarifying the mechanisms contributing to the magneto-Seebeck coefficient for bismuth. The wire array structure is thus suitable for enhancing the thermoelectric properties of materi...

Surface Morphology and Crystallite Size during Growth of Hydrogenated Microcrystalline Silicon by Plasma-Enhanced Chemical Vapor Deposition

The correlation between the surface morphology and the crystallite size during growth of hydrogenated microcrystalline silicon (µ c-Si:H) has been discussed using spectroscopic UV-visible phase-modulated ellipsometry, Raman spectroscopy and atomic force microscope (AFM). Through the deposition study of µ c-Si:H on Corning 7059 glass and Cr substrates from SiH4 highly diluted in H2 and layer-by-layer (LbL) technique by plasma-enhanced chemical-vapor deposition (PECVD), we found that the surface roughness during µ c-Si:H growth strongly influenced the creation of the crystallite phase and the relaxation of Si network. The ion bombardment induces surface roughness and decreases the crystallinity in µ c-Si:H growth. The LbL technique using H2 plasma promotes the nucleation and coalescence processes in the early stage of µ c-Si:H growth.

Optical anisotropy in solvent-modified poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) and its effect on the photovoltaic performance of crystalline silicon/organic heterojunction solar cells

An investigation was carried out into the effect of uniaxial optical anisotropy in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) on the photovoltaic performance of crystalline Si/PEDOT:PSS heterojunction solar cells fabricated by spin coating using either a methanol (MeOH) solvent alone or using MeOH and ethylene glycol (EG) as cosolvents. Spectroscopic ellipsometry revealed that the extraordinary index of refraction increased by the use of the cosolvents. In contrast, the ordinary index of refraction indicated metallic properties and was almost independent of the concentration of MeOH or EG. The highest conductivity was found for a (PEDOT:PSS):(MeOH):(EG) weight ratio of 1:1:0.1, and this sample exhibited a relatively high power conversion efficiency of 11.23%. These findings suggest that the increase in the extraordinary index of refraction leads to an enhancement of the hole mobility in PEDOT:PSS, resulting in improved photovoltaic performance.

Efficient Crystalline Si/Poly(ethylene dioxythiophene):Poly(styrene sulfonate):Graphene Oxide Composite Heterojunction Solar Cells

Efficient crystalline silicon (c-Si) heterojunction solar cells with conductive poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and graphene oxide (GO) composite are demonstrated using a structure of Ag/PEDOT:PSS/PEDOT:PSS:GO composite/c-Si (100)(ρ: 3–5 Ωcm)/Al. The power-conversion efficiency η increased to 10.7% under illumination of AM1.5 100 mW/cm2 simulated solar light by adjusting the PEDOT:PSS and GO mixing concentration ratio. The GO addition to conductive PEDOT:PSS suppressed electron recombination and/or promoted the hole current at the anode. The soluble PEDOT:PSS:GO composite is promising as a hole-transporting transparent conducting layer for c-Si photovoltaic applications.

Electronic transport properties of a bismuth microwire array in a magnetic field

The magneto-Seebeck coefficient and magneto-resistivity of a polycrystalline bismuth microwire array were measured under magnetic fields of 0 to 2 Tesla and at temperatures of 50 to 300 K. To avoid the influence of contact resistance between the wire array and the electrodes, bulk bismuth was used for the electrodes. In the absence of a magnetic field, the Seebeck coefficient and resistivity were -76 /spl mu/V/K and 1.8 /spl mu//spl Omega/m at 300 K, respectively. The magneto-Seebeck coefficient for the wire array increased with the application of an external magnetic field, attributable to the precise control of impurities and carrier scattering process in the fabrication of the wire array. The phonon drag effect was observed below 100 K, with a corresponding increase in the magneto-Seebeck coefficient under high magnetic fields. However, the magneto-resistivity was also raised under higher magnetic fields, detracting from the thermoelectric properties. Through analysis of the power factor, the optimum magnetic field was determined for each temperature, revealing a trend for the optimum magnetic field to increase with temperature. The power factor was improved by a maximum factor of 1.12, achieved at 200 K and 0.25 Tesla. Further improvements appear to be possible by eliminating the bulk bismuth employed for the electrodes.