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.

Amorphous silicon nanoparticles in compound films grown on cold substrates for high-efficiency photoluminescence

Silicon nanoparticles have been fabricated in both oxide and nitride matrices by using plasma-enhanced chemical vapour deposition, for which a low substrate temperature down to 50 degreesC turns out to be most favourable. High-rate deposition onto such a cold substrate results in the formation of nanoscaled silicon particles, which have revealed an amorphous nature under transmission electron microscope (TEM) examination. The particle size can be readily controlled below 3.0 nm, and the number density amounts to over 10(12) cm(-2), as calculated from the TEM micrographs. Strong photoluminescence in the whole visible light range has been observed in the as-deposited Si-in-SiOx and Si-in-SiNx thin films. Without altering the size or structure of the particles, a post-annealing at 300 degreesC for 2 min raised the photoluminescence efficiency to a level comparable to the achievements with nanocrystalline Si-in-SiO2 samples prepared at high temperature. This low-temperature procedure for fabricating light-emitting silicon structures opens up the possibility of manufacturing integrated silicon-based optoelectronics.

Study on Er3+ emission from the erbium-doped hydrogenated amorphous silicon suboxide film

The erbium-doped hydrogenated amorphous silicon suboxide films containing amorphous silicon clusters were prepared. The samples exhibited photoluminescence peaks at around 750 nm and 1.54 mum, which could be assigned to the electron-hole recombination in amorphous silicon clusters and the intra-4f transition in Er3+, respectively. Correlations between the intensities of these two photoluminescence peaks and oxidation and dehydrogenation of the films during annealing were studied. It was found that the oxidation is triggered by dehydrogenation of the films even at low annealing temperatures, which decisively changes the intensities of the two photoluminescence peaks. On the other hand, the increase of Er content in the erbium-doped hydrogenated amorphous silicon suboxide film will enhance Er3+ emission at 1.54 mum, while quench amorphous silicon cluster emission at 750 nm, such a competitive relationship, was also observed in the erbium-doped silicon nanocrystals embedded in SiO2 matrix. Moreover, we found that Er3+ emission is not sensitive to whether silicon clusters are crystalline or amorphous. The amorphous silicon clusters can be as sensitizer on Er3+ emission as that of silicon nanocrystals. (C) 2003 American Institute of Physics.

Microstructure characterization of transition films from amorphous to nanocrocrystalline silicon

Hydrogenated silicon (Si:H) films near the threshold of crystallinity were prepared by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a wide range of hydrogen dilution R-H = [H-2]/[SiH4] values of 2-100. The effects of H dilution R-H on the structural properties of the films were investigated using micro-Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. The obtained Raman spectra show that the H dilution leads to improvements in the short-range order and the medium-range order of the amorphous network and then to the morphological transition from amorphous to crystalline states. The onset of this transition locates between R-H = 30 and 40 in our case, and with further increasing R-H from 40 to 100, the nanocrystalline volume fraction increases from similar to23% to 43%, and correspondingly the crystallite size enlarges from similar to2.8 to 4.4 nm. The FTIR spectra exhibit that with R-H increasing, the relative intensities of both the SiH stretching mode component at 2100 cm(-1) and wagging mode component at 620 cm(-1) increase in the same manner. We assert that these variations in IR spectra should be associated with the formation of paracrystalline structures in the low H dilution films and nanocrystalline structures in the high H dilution films. (C) 2003 Elsevier Science B.V. All rights reserved.

Role of amorphous silicon domains on Er3+ emission in the Er-doped hydrogenated amorphous silicon suboxide film

An investigation on the correlation between amorphous Si (a-Si) domains and Er3+ emission in the Er-doped hydrogenated amorphous silicon suboxide (a-Si:O:H) film is presented. On one hand, a-Si domains provide sufficient carriers for Er3+ carrier-mediated excitation which has been proved to be the highest excitation path for Er3+ ion; on the other hand, hydrogen diffusion from a-Si domains to amorphous silicon oxide (a-SiOx) matrix during annealing has been found and this possibly decreases the number of nonradiative centres around Er3+ ions. This study provides a better understanding of the role of a-Si domains on Er3+ emission in a-Si:O:H films.

Structural characterization of stable amorphous silicon films

A kind of hydrogenated diphasic, silicon films has been prepared by a new regime of plasma enhanced chemical vapor deposition (PECVD) in the region adjacent to the phase transition from amorphous to crystalline state. The photoelectronic and microstructural properties of the films have been investigated by the constant photocurrent method (CPM), Raman scattering and nuclear magnetic resonance (NMR). Our experimental results and corresponding analyses showed that the diphasic films, incorporated with a subtle boron compensation, could gain both the fine photosensitivity and high stability, provided the crystalline fraction (f) was controlled in the range of 0 < f < 0.3. When compared with the conventional hydrogenated amorphous silicon (a-Si:H), the diphasic films are more ordered and robust in the microstructure, and have a less clustered phase in the Si-H bond configurations. (C) 2002 Elsevier Science Ltd. All rights reserved.

Violet/blue emission from hydrogenated amorphous carbon films deposited from energetic CH3+ ions and ion bombardment

Considering the complexity of the general plasma techniques, pure single CH3+ ion beams were selected for the deposition of hydrogenated amorphous (a) carbon films with various ion energies and temperatures. Photoluminescence (PL) measurements have been performed on the films and violet/blue emission has been observed. The violet/blue emission is attributed to the small size distribution of sp(2) clusters and is related to the intrinsic properties of CH3 terminals, which lead to a very high barrier for the photoexcited electrons. Ion bombardment plays an important role in the PL behavior. This would provide further insight into the growth dynamics of a-C:H films. (C) 2002 American Institute of Physics.

Gap states of hydrogenated amorphous silicon near and above the threshold of microcrystallinity with subtle boron compensation

The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability. (C) 2001 American Institute of Physics.

Hydrogenated amorphous silicon films with significantly improved stability

A new regime of plasma-enhanced chemical-vapor deposition (PECVD), referred to as "uninterrupted growth/annealing" method, has been proposed for preparation of high-quality hydrogenated amorphous silicon (a-Si:H) films. By using this regime, the deposition process no longer needs to be interrupted, as done in the chemical annealing or layer by layer deposition, while the growing surface is continuously subjected to an enhanced annealing treatment with atomic hydrogen created in the hydrogen-diluted reactant gas mixture at a relatively high plasma power. The intensity of the hydrogen plasma treatment is controlled at such a level that the deposition conditions of the resultant films approach the threshold for microcrystal formation. In addition, a low level of B-compensation is used to adjust the position of the Fermi level close to the midgap. Under these conditions, we find that the stability and optoelectronic properties of a-Si:H films have been significantly improved. (C) 2001 Elsevier Science B.V. All rights reserved.

Photoluminescence and microstructure of the Erbium-Doped hydrogenated amorphous SiOx(0 < x < 2)

The hydrogenated amorphous SiOx films (a-SiOx:H) with various oxygen contents have been prepared using plasma enhanced chemical vapor deposition technique. The films were implanted with erbium and annealed by rapid thermal annealing. An intense photoluminescence (PL) of Er at 1.54 mum has been observed at 77 K and at room temperature. The PL intensity depends strongly on both the oxygen content of the film and the rapid thermal annealing temperature and reaches its maximum if the ratio of O/Si in the film is approximately equal to 1.0 at 77 K and to 1.76 at room temperature. The microstructure of the film also has strong influences on the PL intensity. The PL intensity at 250 K is slightly more than a half of that at 15 K. It means that the temperature quenching effect of the PL intensity is very weak.

Photoluminescence of erbium-doped hydrogenated amorphous SiOx(0 < x < 2)

Hydrogenated amorphous SiOx films are fabricated via plasma enhanced chemical vapor deposition technique. After erbium implantation and rapid thermal annealing, photoluminescence (PL) are measured at 77 K and room temperature (RT), respectively. We observed the strong PL at 1.54 mu m at RT. The 1.54 mu m PL intensity changes with the variation of concentration of oxygen. The most intense PL at 77 K in a-SiOx:H (Er) corresponds to O/Si = 1.0 and at RT to O/Si = 1.76. Based on our results, we propose that Er ions contributed to PL come from O-rich region in the film. Er ions in Si-rich region have no relation with FL. Temperature dependence of the intensity of the 1.54 mu m line of the Er3+ transition displays a very weak temperature quenching in Er-doped hydrogenated amorphous Si. The PL intensity at 250 K is a little more one half of that at 15 K.

Role of surface defect states in visible luminescence from oxidized hydrogenated amorphous Si hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in amorphous silicon dioxide matrix was fabricated by oxidizing hydrogenated amorphous Si/hydrogenated amorphous Ge (a-Si:H/a-Ge:H) multilayers. The structures before and after oxidation were systematically investigated. The orange-green light emission was observed at room temperature and the luminescence peak was located at 2.2 eV. The size dependence in the photoluminescence peak energy was not observed and the luminescence intensity was increased gradually with oxidation time. The origin for this visible light emission is discussed. In contrast to the simple quantum effect model, the surface defect states of nanocrystalline Ge are believed to play an important role in radiative recombination process. (C) 1999 American Institute of Physics. [S0003-6951(99)02425-0].