Strain gradient theory with couple stress for crystalline solids

A new phenomenological strain gradient theory for crystalline solid is proposed. It fits within the framework of general couple stress theory and involves a single material length scale Ics. In the present theory three rotational degrees of freedom omega (i) are introduced, which denote part of the material angular displacement theta (i) and are induced accompanying the plastic deformation. omega (i) has no direct dependence upon u(i) while theta = (1 /2) curl u. The strain energy density omega is assumed to consist of two parts: one is a function of the strain tensor epsilon (ij) and the curvature tensor chi (ij), where chi (ij) = omega (i,j); the other is a function of the relative rotation tensor alpha (ij). alpha (ij) = e(ijk) (omega (k) - theta (k)) plays the role of elastic rotation reason The anti-symmetric part of Cauchy stress tau (ij) is only the function of alpha (ij) and alpha (ij) has no effect on the symmetric part of Cauchy stress sigma (ij) and the couple stress m(ij). A minimum potential principle is developed for the strain gradient deformation theory. In the limit of vanishing l(cs), it reduces to the conventional counterparts: J(2) deformation theory. Equilibrium equations, constitutive relations and boundary conditions are given in detail. For simplicity, the elastic relation between the anti-symmetric part of Cauchy stress tau (ij), and alpha (ij) is established and only one elastic constant exists between the two tensors. Combining the same hardening law as that used in previously by other groups, the present theory is used to investigate two typical examples, i.e., thin metallic wire torsion and ultra-thin metallic beam bend, the analytical results agree well with the experiment results. While considering the, stretching gradient, a new hardening law is presented and used to analyze the two typical problems. The flow theory version of the present theory is also given.

Peculiarity of constant photocurrent method for silicon films with mixed amorphous-nanocrystalline structure

The results of conductivity, photoconductivity and constant photocurrent method absorption measurements by DC and AC methods in hydrogenated silicon films with mixed amorphous-nanocrystalline structure are presented. A series of diphasic silicon films was deposited by very high frequency plasma enhanced chemical vapor deposition technique, using different hydrogen dilution ratios of silane. The increase of hydrogen dilution ratio results in five orders of magnitude increase of conductivity and a sharp increase of grain volume fraction. The comparison of the absorption spectra obtained by DC and AC methods showed that they are similar for silicon films with the predominantly amorphous structure and films with high grain volume fraction. However we found a dramatic discrepancy between the absorption spectra obtained by DC and AC constant photocurrent methods in silicon films deposited in the regime of the structure transition from amorphous to nanocrystalline state. AC constant photocurrent method gives higher absorption coefficient than DC constant photocurrent method in the photon energy range of 1.2-1.7 eV. This result indicates the possibility of crystalline grains contribution to absorption spectra measured by AC constant photocurrent method in silicon films with intermediate crystalline grain volume fraction. (c) 2008 Published by Elsevier B.V.

The diphasic nc-Si/a-Si : H thin film with improved medium-range order

A series of silicon film samples were prepared by plasma enhanced chemical vapor deposition (PECVD) near the threshold from amorphous to nanocrystalline state by adjusting the plasma parameters and properly increasing the reactions between the hydrogen plasma and the growing surface. The microstucture of the films was studied by micro-Raman and Fourier transform infrared (FTIR) spectroscopy. The influences of the hydrogen dilution ratio of silane (R-H = [H-2]/[SiH4]) and the substrate temperature (T-s) on the microstructural and photoelectronic properties of silicon films were investigated in detail. With the increase of RH from 10 to 100, a notable improvement in the medium-range order (MRO) of the films was observed, and then the phase transition from amorphous to nanocrystalline phase occurred, which lead to the formation of diatomic hydrogen complex, H-2* and their congeries. With the increase of T-s from 150 to 275 degreesC, both the short-range order and the medium range order of the silicon films are obviously improved. The photoconductivity spectra and the light induced changes of the films show that the diphasic nc-Si/a-Si:H films with fine medium-range order present a broader light spectral response range in the longer wavelength and a lower degradation upon illumination than conventional a-Si:H films. (C) 2004 Elsevier B.V. All rights reserved.

Nanostructure in the p-layer and its impacts on amorphous silicon solar cells

The open circuit voltage (V-oc) of n-i-p type hydrogenated amorphous silicon (a-Si:H) solar cells has been examined by means of experimental and numerical modeling. The i- and p-layer limitations on V-oc are separated and the emphasis is to identify the impact of different kinds of p-layers. Hydrogenated protocrystalline, nanocrystalline and microcrystalline silicon p-layers were prepared and characterized using Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), optical transmittance and activation energy of dark-conductivity. The n-i-p a-Si:H solar cells incorporated with these p-layers were comparatively investigated, which demonstrated a wide variation of V-oc from 1.042 V to 0.369 V, under identical i- and n-layer conditions. It is found that the nanocrystalline silicon (nc-Si:H) p-layer with a certain nanocrystalline volume fraction leads to a higher V-oc. The optimum p-layer material for n-i-p type a-Si:H solar cells is not found at the onset of the transition between the amorphous to mixed phases, nor is it associated with a microcrystalline material with a large grain size and a high volume fraction of crystalline phase. (c) 2006 Elsevier B.V. All rights reserved.

Hydrogenated p-type nanocrystalline silicon in amorphous silicon solar cells

A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature. The optoelectrical and structural properties of this novel material have been investigated by Raman and UV-VIS transmission spectroscopy measurements indicating that these films are composed of nanocrystallites embedded in amorphous SiHx matrix and with a widened bandgap. The observed downshift of the optical phonon Raman spectra (514.4 cm(-1)) from crystalline Si peak (521 cm(-1)) and the widening of the bandgap indicate a quantum confinement effect from the Si nanocrystallites. By using this kind of p-layer, a-Si:H solar cells on bare stainless steel foil in nip sequence have been successfully prepared with a V c of 0.90 V, a fill factor of 0.70 and an efficiency of 9.0%, respectively. (c) 2006 Elsevier B.V. All rights reserved.

Light-induced changes in diphasic nanocrystalline silicon films and solar cells

A series of diphasic nanocrystalline silicon films and solar cells was prepared using different hydrogen dilution ratios of silane by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). It was observed that after light soaking the open circuit voltage (V-oc) of the diphasic solar cells increased, while that of amorphous silicon solar cells decreased. Raman scattering spectroscopy was performed on the series of diphasic silicon films before and after light soaking. It was found that after light soaking the nanostruclures in the diphasic nanocrystalline silicon films were changed. Both the grain sizes and grain volume fraction reduced, while the grain boundary components increased. These results provide experimental evidence for the conjecture that the light-induced increase in V-oc of the diphasic nanocrystalline solar cells might be induced by the changes in the nanostructure of the intrinsic layer. (c) 2006 Elsevier B.V. All rights reserved.

Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/1T0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 mu m)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm(2). (c) 2006 Elsevier B.V. All rights reserved.

Nanostructure in the p-layer and its impacts on amorphous silicon solar cells

The open circuit voltage (V-oc) of n-i-p type hydrogenated amorphous silicon (a-Si:H) solar cells has been examined by means of experimental and numerical modeling. The i- and p-layer limitations on V-oc are separated and the emphasis is to identify the impact of different kinds of p-layers. Hydrogenated protocrystalline, nanocrystalline and microcrystalline silicon p-layers were prepared and characterized using Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), optical transmittance and activation energy of dark-conductivity. The n-i-p a-Si:H solar cells incorporated with these p-layers were comparatively investigated, which demonstrated a wide variation of V-oc from 1.042 V to 0.369 V, under identical i- and n-layer conditions. It is found that the nanocrystalline silicon (nc-Si:H) p-layer with a certain nanocrystalline volume fraction leads to a higher V-oc. The optimum p-layer material for n-i-p type a-Si:H solar cells is not found at the onset of the transition between the amorphous to mixed phases, nor is it associated with a microcrystalline material with a large grain size and a high volume fraction of crystalline phase. (c) 2006 Elsevier B.V. All rights reserved.

Hydrogenated p-type nanocrystalline silicon in amorphous silicon solar cells

A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature. The optoelectrical and structural properties of this novel material have been investigated by Raman and UV-VIS transmission spectroscopy measurements indicating that these films are composed of nanocrystallites embedded in amorphous SiHx matrix and with a widened bandgap. The observed downshift of the optical phonon Raman spectra (514.4 cm(-1)) from crystalline Si peak (521 cm(-1)) and the widening of the bandgap indicate a quantum confinement effect from the Si nanocrystallites. By using this kind of p-layer, a-Si:H solar cells on bare stainless steel foil in nip sequence have been successfully prepared with a V c of 0.90 V, a fill factor of 0.70 and an efficiency of 9.0%, respectively. (c) 2006 Elsevier B.V. All rights reserved.

Light-induced changes in diphasic nanocrystalline silicon films and solar cells

A series of diphasic nanocrystalline silicon films and solar cells was prepared using different hydrogen dilution ratios of silane by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). It was observed that after light soaking the open circuit voltage (V-oc) of the diphasic solar cells increased, while that of amorphous silicon solar cells decreased. Raman scattering spectroscopy was performed on the series of diphasic silicon films before and after light soaking. It was found that after light soaking the nanostruclures in the diphasic nanocrystalline silicon films were changed. Both the grain sizes and grain volume fraction reduced, while the grain boundary components increased. These results provide experimental evidence for the conjecture that the light-induced increase in V-oc of the diphasic nanocrystalline solar cells might be induced by the changes in the nanostructure of the intrinsic layer. (c) 2006 Elsevier B.V. All rights reserved.

Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/1T0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 mu m)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm(2). (c) 2006 Elsevier B.V. All rights reserved.