Toshihiko Toyama

Correlation between microstructure and photovoltaic performance of polycrystalline silicon thin film solar cells

Intrinsic polycrystalline silicon (poly-Si) thin films have been prepared by plasma enhanced chemical vapor deposition at a very high excitation frequency (100 MHz) for the development of p–i–n junction solar cells. The correlation between poly-Si microstructure and photovoltaic performance of poly-Si solar cells has been investigated as a function of deposition parameters such as deposition pressure, SiH4 flow rate and SiH4 concentration. It is found that the poly-Si microstructure including crystalline volume fraction, Xc, and crystallographic orientation is closely related to the atomic hydrogen flux during film growth, which is evidenced by plasma diagnostics in the in situ measurement of optical emission spectroscopy. Basically, with an increase in Xc, open circuit voltage tends to decrease, while poly-Si with Xc > 50% and (220) preferential orientation is essential for high short circuit current. Based on these results, the role of poly-Si microstructure in the photovoltaic performance of poly-Si solar cells is discussed.

Blue light emission from a-C:H by thin film electroluminescence structure cell

Abstract A series of experimental trials to realize the flat panel display devices using plasma CVD produced a-C:H has been reported. Fabrication technology and basic properties of the active material a-C:H are briefly introduced. Then, technical data both on the injection type and the intrinsic type EL devices are presented. The developed devices show a luminance of 30cd/m2 for the blue color emission, and 40cd/m2 for white light so far. Possibilities of performance improvements will be discussed comparing with our recent data on a-SiC LED.

Amorphous-SiC thin-film p-i-n light-emitting diode using amorphous-SiN hot-carrier tunneling injection layers

An approach to improve the luminosity of hydrogenated amorphous silicon carbide (a-SiC:H)-based thin-film visible light-emitting diodes is discussed. High bandgap near-stoichiometric hydrogenated amorphous silicon nitride (a-SiN:H) is utilized as a hot-carrier tunneling injection layer. An improvement of both carrier injection efficiency and luminosity is observed. Technical data on the new approach for carrier injection and on the recombination mechanism are presented. Preliminary results are also presented on the photoluminescence and electroluminescence properties of a-SiC:H/a-SiN:H multilayers. >

Carrier injection mechanism in an a-SiC p-i-n junction thin-film LED

A systematic study has been done on the carrier injection mechanism and electroluminescent properties of an amorphous silicon-carbide p-i-n junction thin-film light-emitting diode (a-SiC TFLED). The analysis of the junction characteristics reveals that the main contribution to the junction current comes from electrons injected by tunneling from the n-layer through the i-n interface notch barrier, while the electroluminescent property of the TFLED is determined by the injection process of holes. This process also takes place by tunneling, in this case from the p-layer through the p-i interface notch barrier. On the basis of the results of the analysis, a method to improve the LED performance using a hot-carrier-tunneling injector structure is proposed. With this structure, the brightness of the TFLED is increased by more than one order of magnitude to about 20 cd/m/sup 2/, with an injection current density of 600 mA/cm/sup 2/. >

Structural and luminescence properties of nanostructured ZnS:Mn

We have studied structural and luminescence properties of nanostructured (NS-) ZnS:Mn which has potential applications in thin-film electroluminescence (TFEL) devices. As a NS-ZnS:Mn system, a ZnS:Mn/Si3N4 multilayer having thicknesses of 2.5 nm for ZnS and 0.6 nm for Si3N4 was prepared by a conventional rf-magnetron sputtering method. Grazing incidence x-ray reflectometry and x-ray diffractometry show that ZnS:Mn nanocrystals were formed between the amorphous Si3N4 layers. Photoluminescence intensity associated with the Mn2+ transitions per total thickness of the ZnS:Mn layers is increased in NS-ZnS:Mn in comparison with that of the ZnS:Mn thin film, indicating the effects due to quantum confinement. The TFEL device with NS-ZnS:Mn as an emission layer exhibits a reddish-orange broad band emission with the maximum luminance of 2.8 cd/m2 under the 1-kHz sinusoidal wave operation at a voltage of 20.5 V0−p.

Improvement of carrier injection efficiency in a-SiC p-i-n LED using highly-conductive wide-gap p, n type a-SiC prepared by ECR CVD

Abstract Highly conductive and wide band gap p- and n- type a-SiC have been prepared by ECR (Electron Cyclotron Resonance) plasma CVD. By utilizing these wide-gap materials as carrier injector layers in a-SiC p-i-n junction LED, the EL intensity is increased by more than one order of magnitude with increasing the energy gap of the injector layers, and at the same time the EL spectra shift towards shorter wavelength. These improvements are attributed to the increase in the carrier injection efficiency.

Influence of CdS window layer on 2-μm thick CdS/CdTe thin film solar cells

Influence of the CdS window layer on the PV performances of 2-μm thick CdS/CdTe solar cells has been studied as a function of the CdS thickness, d C d S . With a reduction of d C d S from 114 to 95 nm, J S C increases due to an increase in blue response. While, at d c d S <85 nm, the conversion efficiency largely decreases due to a decrease in V O C and FF. The deterioration of the crystallinity of CdTe due to a decrease in the sulfur composition x of the CdTe 1 - x S x mixed-crystal layer is concluded to be the most possible mechanism for the large decreases in V O C and FF.

Highly conductive p-type microcrystalline SiC:H prepared by ECR plasma CVD

Highly conductive p-type microcrystalline SiC:H films have been prepared by ECR (electron cyclotron resonance) plasma CVD. The material with an optical energy gap of 2.25 eV exhibits a dark conductivity as high as 10 S cm-1 which is more than seven orders of magnitude higher than that of amorphous SiC:H prepared by conventional RF plasma CVD. The optimal energy gap can be controlled in the range from 2.0 to 2.8 eV, while retaining excellent conductivity. Utilizing this material as a wide-gap heterojunction contact in amorphous silicon solar cell, a conversion efficiency of 12.0% has been obtained with a large open circuit voltage.

Solution-processed ZnO nanocrystals in thin-film light-emitting diodes for printed electronics

Thin-film light-emitting diodes(LEDs) containing solution-processed ZnO nanocrystals (NCs) were prepared as printed electronics. The electroluminescent (EL)properties of thin-film LEDs were investigated along with the structural and photoluminescence(PL)properties of the ZnO NCs. Scanning electron microscope and x-ray diffraction studies revealed that the crystal sizes D were ranged from 5–11 nm, and can be controlled by varying growth time t G in the Zn 2 + / OH − solution at 40 ° C . The time evolution of D was analyzed using Lifshitz–Slyozov–Wagner theory, showing that growth is limited by diffusion. The results of PL studies indicated that increases in the peak energies in the ultraviolet (UV) region could be attributed to the quantum-size effects on the exciton emission in the NCs with a small D , the ZnO surfaces became sufficiently passivated as D increases. Printed layers containing well-passivated ZnO NCs with different D of 8–11 nm were used as emission layers in thin-film LEDs together with pentacene hole transport layers. The current-voltage characteristics were analyzed using the trapped-charge-limited current mechanism. EL spectral measurements revealed the presence of weak UV emission that increased slightly as D decreased.

Microstructural dependence of electron and hole transport in low-temperature-grown polycrystalline-silicon thin-film solar cells

Carrier transport properties of undoped polycrystalline silicon (poly-Si) thin films prepared by SiH4–H2 plasma at low temperature have been investigated. The ac-conductivity measurement technique has been applied to poly-Si i layers with an n-i-n junction structure in order to characterize the electron conductivity along the growth direction. Furthermore, the hole conductivity has been measured with p-i-p junction structures. The temperature dependence of ac conductivity reveals that poly-Si films with relatively low crystalline volume fraction (Xc∼50%) exhibit intrinsic character, while the poly-Si films with high Xc (>50%) exhibit n-type character with activation energies less than 0.15 eV. Based on these results, the relationship among microstructure, carrier transport, and photovoltaic performance of poly-Si solar cells is discussed.