Kaori Miyazaki

Concept and performance of a field-effect amorphous silicon solar cell

We propose a novel solar cell structure based on the concept of forming p or n window layers by the field effect instead of impurity doping, and we verify its performance experimentally. The device, a field-effect amorphous silicon solar cell (FESC), was designed with the aid of a device simulator and fabricated by a plasma chemical vapour deposition system. We have verified that the output current of the FESC was amplified by the field-effect bias application to the gate electrode. The fundamental properties of this new type of amorphous silicon solar cell are demonstrated for the first time.

Characteristics of field effect a-Si:H solar cells

Abstract Two dimensional device simulation has revealed advantages of a solar cell which utilizes an inversion layer induced by the field effect instead of a heavily doped window layer. The cell performance improvements have been predicted not only in the short circuit current due to the increased quantum efficiency for light with short wavelengths but also in the open circuit voltage and the fill factor by the use of metals with larger workfunctions. The conversion efficiency has been estimated to increase to 50% by the use of the field effect solar cell.

An ab initio molecular-orbital study on hydrogen-abstraction reactions at the growing surface of hydrogenated amorphous silicon

Energy profiles have been evaluated by an ab initio molecular-orbital method for hydrogen-abstraction reactions from surface model compounds of growing hydrogenated amorphous silicon (a-Si:H) by a SiH3 radical, a presumed main precursor to a-Si:H, as well as by a hydrogen radical which should coexist in the silane plasma chemical vapor deposition. The activation energies calculated for these two reactions decrease as the cluster size of the film surface model SinH2n+2 increases from n=1 to n=4 to converge for n⩾4. This trend is in parallel with the variation of atomic charge delocalization. Both activation energies (0.22 and 0.28 eV, respectively) for the largest model, Si7H16, were low enough to induce the hydrogen abstractions from the surface to form dangling bonds, which spontaneously react with SiH3 radicals to form Si–Si bond. From thus produced H3Si–Si≡surface, hydrogen can be eliminated with SiH3 (or H) to reproduce a dangling bond. The initial step of the a-Si:H film growth is deduced by the calc...

Device simulation and fabrication of field effect solar cells

The performance of a novel hydrogenated amorphous silicon (a-Si : H) solar cell which utilizes the field effect solar cell (FESC) has been investigated both theoretically and experimentally. The theoretical analysis has been done for bothp- andn-channel FESCs by employing a two-dimensional device simulator which is based on current continuity and Poisson equations. The calculated performance is compared with that of conventional (p-i-n)a-Si : H solar cells. The calculation demonstrated that bothn-channel andp-channel FESCs could improve the conversion efficiency by as much as 50%.In order to check the reliability of simulation, the transport properties of intrinsica-Si : H film and thin film transistor (TFT) have also been calculated and compared with the experimentally obtained characteristics. Experimental verification of TFT and FESC has been attempted by using MgO anda-SiN : H as dielectric layer materials. Preliminary results are presented.

A novela-Si : H solar cell designed by two-dimensional device simulation

The performance of a novel hydrogenated amorphous silicon (a-Si : H) solar cell that utilizes the field effect (field effect solar cell (FESC)) has been investigated theoretically. The analysis has been done for bothp- andn-channel FESCs, employing a two-dimensional device simulator which is based on current continuity and Poisson equations. The calculation predicted that bothn-channel andp-channel FESCs could offer improved conversion efficiency as compared with the conventional-dopedp-i-n solar cell with equal qualityi (intrinsic)-layer. The observed improvement can be mainly attributed to the quantum-efficiency increase for high-energy photons. Cell parameters that may affect the energy conversion efficiency of FESCs have also been evaluated with the simulator.

Combinatorial Fabrication Process for a-Si:H Thin Film Transistors

A combinatorial approach is proposed and demonstrated for the parallel fabrication of a-Si:H, alloy and a-Si:H based devices, by employing simple masking schemes in conventional plasma-enhanced chemical vapor deposition (PECVD). The results are presented for a-Si:H thin film transistors. A (7×7) combinatorial device library was deposited on a (indium tin oxide/glass) substrate with the thicknesses of a-SiN:H and a-Si:H as combinatorial variables along the X and Y axes, respectively. Different a-Si:H TFTs in the library were evaluated to yield electrical performance with on-to-off current ratios exceeding 104 and threshold voltages from 0.3 to 4.5 V. Combinatorial PECVD offers an efficient and low cost means of studying the a-Si:H device performance and optimization.

Quantum chemical studies on initial surface process ina-Si: H plasma CVD

The surface morphology ofa-Si: H is strongly dependent on substrate material and temperature, especially in the initial stage of deposition. Furthermore, some surface treatments could induce a drastic change in the initial growth mode. For example, the hydrogen plasma treatment of highly oriented pyrolytic graphite (HOPG) surface prior toa-Si: H deposition changed the growth mode from inhomogeneous to homogeneous one. In order to elucidate this surface chemical process, molecular orbital calculations were performed and compared with the experimental observation of the surface by AFM and STM. The calculation verified hydrogen addition tosp2 carbons on HOPG to facilitate the bonding of SiH3 to neighbouring carbons, which correspond to the nucleation or pinning of precursors as the origin of homogeneous growth ofa-Si: H film.