Kyung Hoon Jun

Stable protocrystalline silicon and unstable microcrystalline silicon at the onset of a microcrystalline regime

We investigated the light-soaking behaviors and the thermal annealing kinetics of amorphous silicon-based solar cells incorporating hydrogen-diluted films as i-layers deposited at several hydrogen dilution ratios. From the investigation, we confirmed that the protocrystalline silicon was most stable against light soaking, and also that the film deposited at the onset of the microcrystalline regime, which were known to have the most competent device quality and stability, was less stable. Charged dangling bonds defects caused by inhomogeneous microstructure of the onset of the microcrystalline regime, was suggested as one of reasons for the instability at the onset regime.

Enhanced vertical photo-sensitivity in μc-Si:H/a-Si:H superlattices

Abstract The microcrystalline silicon/amorphous silicon (μc-Si:H/a-Si:H) superlattice showed an enhanced vertical photo-sensitivity (photo-conductivity/dark-conductivity), whereas it reserved a lateral photo-sensitivity nearly unchanged. The film was fabricated by alternating the mixing of SiH4 and H2 in a photo-chemical vapor deposition system. The fact that a high vertical photo-sensitivity and an obvious crystalline volume fraction can be obtained at the same time distinguishes the μc-Si:H/a-Si:H superlattice from the bulk μc-Si:H. The change of the vertical dark-conductivity with the sublayer thickness was explained by the change of the a-Si:H sublayer’s electrical conduction property. We think that the thin a-Si:H sublayers play an important role of perturbing a columnar structure of the μc-Si:H.

Simulation of the depolarization effect in porous silicon.

We describe a radiative transfer (RT) equation for the simulation of optical scattering effects in a nanostructured semiconductor for spectroscopic ellipsometry (SE). As an example, we chose porous silicon (PS), whose pores are considered to act as light scatterers. We examined the effects of pore radius, slab thickness, and incident angle. The volume scattering effect in the internal morphology of the PS generates incoherent light, leading to depolarization. By simulating the four Stokes parameters through the RT equation, we could theoretically assess the degree of polarization that is essential for SE measurements of some nanostructured semiconductors.

Optical evidence of amorphous-network change in the initial-growth stage a-Si :H

We investigated the optical characteristics of thin (<22 nm) hydrogenated amorphous silicon (a-Si:H) films by spectroscopic ellipsometry (SE). For comparison, we prepared H2 diluted as well as undiluted a-Si:H samples on c-Si substrates. As the thickness decreases, the peak positions of dielectric functions (er,ei) shifted to the higher-energy sides both in the H2 diluted and the undiluted a-Si:H films. In addition, a noticeable difference of growth behavior between the H2 diluted and the undiluted a-Si:H films was observed. To preclude effects caused by the incorporated hydrogens, the samples were annealed for sufficient dehydrogenation, which was verified by Fourier transform infrared spectroscopy (FTIR). Even after this dehydrogenation, there still remained a difference between the H2 diluted and the undiluted samples, also, the evolution of the (er,ei) peak positions remained in the same relation as before the annealing. A simple chemical-alloy effect between silicon and hydrogen cannot explain these phenomena which may be a result of changes in the amorphous network. It is presumed that the structure of the amorphous network in the initial-growth stage is different from that of the amorphous network and the structure difference is enhanced by H2 addition.

Simulation of depolarization effect by a rough surface for spectroscopic ellipsometry

We employed the integral equation method (IEM) to simulate optical scattering by a randomly rough surface for spectroscopic ellipsometry. An explicit Mueller-matrix expression of the IEM for single scattering by moderately small surface roughness makes it possible to calculate the depolarization effect. The IEM allows a relatively rigorous assessment of the surface-scattering effect in a wide spectral range.

Optimization of p-a-SiC:H/p-nc-SiC:H double layer structure for a high efficiency a-Si:H based solar cell

We optimized a p-a-SiC:H/p-nc-SiC:H double p-layer structure to achieve high performances for a-Si:H based solar cells. We could reduce remarkably the recombination in the buffer layer and at the p/buffer/i interfaces by employing a lightly boron-doped (/spl sim/1000 ppm) p-nc-SiC:H buffer material with a high conductivity, low defect density, and low absorption, resulting in a good buffering effect of improving V/sub oc/ and J/sub sc/ values. As a result, we could achieve the 11.2 % initial-efficiency for a-Si:H solar cell and 10.4 % for a-Si:H multilayer solar cell without using any back reflector. Since the a-Si:H multilayer solar cell shows the excellent light-soaked behavior, a 8.98 % stabilized-efficiency (degradation ratio: 13.4 %) is obtained.

Characteristics of intrinsic protocrystalline silicon films prepared by photo-CVD method

We investigated the light-soaked behaviors of hydrogen diluted amorphous silicon films and solar cells using the films as active layers. The films were fabricated at various hydrogen dilution ratios by using a mercury-sensitized photo-chemical vapor deposition system. We found a moderate hydrogen dilution ratio where the stable phase protocrystalline silicon, a transient material prepared just before onset of microcrystalline regime, can be obtained. We observed that the films prepared at the onset of microcrystalline regime is unstable in spite of the facts that the films have: i) more blue-shifted amorphous TO phonon bands; ii) nano-sized crystalline silicon grains; and iii) lower broadening parameters (C) in comparison with protocrystalline silicon. We claim two new facts from our experiments: i) the relatively high stability of the protocystalline silicon is attributed to the densest amorphous silicon network structure; ii) the decreased stability at the onset of microcrystallization is due to clustered hydrogen bonds accompanied by high atomic hydrogen content.

Light scattering effect by rough surface of the solar cell material

The light scattering effect by rough surface of amorphous silicon was examined by several methods. We use the newly adopted Integral equation method and compare it with the first order approximation or high frequency approximation (geometric optics solution) of Kirchhoff method. The surface of amorphous silicon was assumed as Gaussian rough surface. The enhancement of light absorption by surface scattering can be predicted more accurately by the method suggested here.

Low degradation and high annealing effects of amorphous silicon multilayer processed through alternate hydrogen dilution

Alternately hydrogen diluted a-Si:H multilayer is shown to be a promising concept for the fabrication of stable a-Si:H solar cells or other a-Si:H based devices. The alternately hydrogen diluted amorphous silicon multilayers were obtained by toggling both the H/sub 2//SiH/sub 4/ dilution ratio and the total flow rate of the gases under continuous UV light irradiation into the reaction chamber of a photo-chemical vapor deposition system. The authors applied these multilayers as the active layer of p-i-n type thin film solar cells. They report on the light-soaking and the annealing behavior of the solar cells. The multilayer solar cell has exceptionally high recovery rate at low temperatures, which makes the solar cell degradation behavior highly sensitive to the cell temperature during degradation.