Ellipsometry characterization of a-Si:H layers for thin-film solar cells

In order to determine microscopic structures of hydrogenated amorphous silicon (a-Si:H) layers incorporated in a-Si:H-based thin-film solar cells, the spectroscopic ellipsometry (SE) analysis of a-Si:H layers prepared by plasma-enhanced chemical vapor deposition has been performed. In particular, we have characterized the a-Si:H layers by applying a new dielectric function model that allows the evaluation of the SiH₂ bond densities in a-Si:H networks. This model is based on our finding that the a-Si:H dielectric functions in the visible/ultraviolet region vary systematically with the formation of SiH₂-clustered microvoids. We have applied this model to estimate the SiH₂ content in a-Si:H layers fabricated on glass substrates, on which the characterization of the SiH₂ bonding is generally difficult. The validity of the SE analysis has been confirmed from the direct characterization of the SiH_n local structures using infrared ellipsometry.     

Advanced deposition phase diagrams for guiding Si:H-based multijunction solar cells

Phase diagrams have been established to describe very high frequency (vhf) plasma-enhanced chemical vapor deposition (PECVD) of intrinsic hydrogenated silicon (Si:H) and silicon–germanium alloy (Si_1?xGe_x:H) thin films on crystalline Si substrates that have been over-deposited with n-type amorphous Si:H (a-Si:H). The Si:H and Si_1?xGe_x:H films are prepared under conditions used for the top and middle i-layers of high efficiency triple-junction a-Si:H-based n–i–p solar cells. Identical n/i cell structures were co-deposited in this study on textured (stainless steel)/Ag/ZnO which serve as substrate/back-reflectors in order to relate the phase diagrams to the performance parameters of single-junction solar cells. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si_1?xGe_x:H solar cells are obtained when the i-layers are prepared under previously-described maximal H₂ dilution conditions.     

Characterization of HF-PECVD a-Si:H thin film solar cells by using OES studies

Hydrogenated amorphous silicon (a-Si:H) films show considerable potential for the fabrication of thin film solar cells. In this study, the a-Si:H thin films have been deposited in a parallel-plate radio frequency (RF) plasma reactor fed with pure SiH₄. The plasma diagnostics were performed simultaneously during the a-Si:H solar cell deposition process using an optical emission spectrometer (OES) in order to study their correlations with growth rate and microstructure of the films. During the deposition, the emitting species (SiH*, Si*, H*) was analyzed. The effect of RF power on the emission intensities of excited SiH, Si and H on the film growth rate has been investigated. The OES analysis revealed a chemisorption-based deposition model of the growth mechanism. Finally, the a-Si:H thin film solar cell with an efficiency of 7.6% has been obtained.     

Influence of sputtering conditions on H content and SiH bonding in aSi: H alloys

The influence of sputtering conditions on H concentration and SiH bonding has been determined for the case of diode reactive sputtering of Si in ArH mixtures, and a simple model for SiH reaction kinetics has been developed. H content and SiH bonding can be varied and controlled over wide ranges by appropriate selection of sputtering conditions. SiH reaction kinetics are found to be governed primarily by deposition rate (reaction time), H partial pressure (H availability) and possibiy substrate temperature (mobility of Si and H on the surface of the growing film). H concentration increases with decreasing deposition rate and increasing H partial pressure. SiH bonding increases with increasing deposition rate and decreasing H partial pressure. Polymerization of H and Si (SiH₂, SiH₃) occurs for lowest deposition rates and highest H partial pressures. Polymerization reactions are enhanced in films deposited at very high target power densities. The high power density causes substrate temperature to be higher than expected, and the enhanced polymerization may be due to increased Si and H surface mobility at the elevated temperatures.     

Heat treatment of amorphous silicon p-i-n solar cells with high-pressure H2O vapor

We report improvement in characteristics of hydrogenated amorphous silicon (a-Si:H ) p-i-n structured solar cells by high-pressure H₂O vapor heat treatment. a-Si:H p-i-n solar cells were formed on glass substrates coated with textured SnO₂ layer. P-, i-, and n-type a-Si:H layers were subsequently formed by plasma enhanced chemical vapor deposition. Finally an indium-tin-oxide layer was coated on the n-type a-Si:H surface. Heat treatment at 210 °C with 2 × 10⁵ Pa H₂O vapor for 1 h was applied to the a-Si:H p-i-n solar cells. Electrical characteristics were measured when samples were kept in dark and illuminated with light of AM 1.5 at 100 mW/cm². The heat treatment with H₂O vapor increased fill factor (FF) and the conversion efficiency from 0.54 and 7.7% (initial) to 0.57 and 8.4%, respectively. Marked improvement in solar cell characteristics was also observed in the case of a poor a-Si:H p-i-n solar cell. FF and the conversion efficiency were increased from 0.29 and 3.2% (initial) to 0.56 and 7.7%, respectively.     

High quality amorphous Si solar cells for large area mass production Micromorph tandem cells

This paper reports on the development of an amorphous silicon cell used in the top cell of Micromorph® tandem solar modules produced in the pilot line of Oerlikon Solar in Trübbach — Switzerland. Tuning of the process parameters used for PECVD deposition of the absorber layers such as process pressure, RF power density, SiH₄/H₂ ratio, and substrate temperature can result in significant improvement in the material quality of the absorber layer and therefore in the performance and light induced degradation of the a-Si cell. We have measured the single layer properties of different absorber layers by infrared spectroscopy and have found a strong correlation between both the microstructure factor R and the H-content bonded to Si and the stabilized efficiency or relative degradation of the a-Si cells containing the corresponding absorber layers. A combination of absorber layers with superior material quality, adapted p-doped and buffer layers and ZnO front and back contacts with enhanced light trapping have achieved record values for the conversion efficiency of industrial thin a-Si single junction cells and modules. Our results show initial efficiencies on test cells prepared on 1.4 m² substrates of over 11%, an active area efficiency of 10.5% for a champion 1.4 m² a-Si single junction module and an 8.7% stabilized conversion efficiency for an industrial 1.4 m² a-Si single junction champion module.     

Influence of surface treatments on crystalline germanium heterojunction solar cell characteristics

Hydrogenated amorphous Si (a-Si:H) has been applied to crystalline germanium (c-Ge) heterojunction solar cells and the influence of the surface treatments applied before a-Si:H deposition process has been studied. We found that PH₃ exposure treatment after surface oxide removal by annealing is effective to improve c-Ge heterojunction solar cell performance. The conversion efficiency of the c-Ge heterojunction solar cell applied PH₃ exposure treatment was up to 5.29% and the solar cell had better temperature coefficient than the c-Ge homojunction solar cell. These results suggest that the c-Ge substrate surface after oxide removal by annealing is covered with negatively charged dangling bonds, and the phosphorus adsorbed onto the c-Ge surface provides electron as a donor and corrects the band bending induced by negatively charged dangling bonds.     

Relative importance of hydrogen atom flux and ion bombardment to the growth of μc-Si:H thin films

An investigation of the relative importance of H atoms and ions to the transition from amorphous to microcrystalline silicon growth was performed by applying in situ plasma diagnostics and a 2D simulator of SiH₄/H₂ discharges. The growth transition was achieved by reducing the % SiH₄ in the SiH₄/H₂ discharges while keeping all the other plasma parameters constant. The distribution of the main species in the discharge space, as well as the flux of H atoms and ions per monolayer and the energy transferred by each to the growing film surface, was estimated from the simulation results. H atoms flux was found to be much higher compared to ions but the total amount of energy transferred from both H atoms and ions was found to be much lower than the activation energy required for crystallization of stable a-Si:H films with low H-content. These results support the theory that in the present conditions the formation of microcrystals proceeds via the initial growth of an unstable a-Si:H with high H content, which reduces significantly the energy barrier for crystallization.     

Effect of the hydrogen content on optical band gap in a-Si:H:N

The effect of the hydrogen content on the optical band gap in glow discharge a-Si:H:N produced from SiH₄ and N₂ mixtures is demonstrated. The hydrogen content in the film can be well controlled by making a non-plasma-excited region above the substrate surface during deposition. When the concentration of hydrogen is high, a large amount of nitrogen atoms included in the film does not cause a widening of the optical band gap. The effects of the hydrogen content on the infrared absorption peak wavelength relevant to SiN lattice vibration and the optical band gap are discussed.     

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Properties of microcrystalline P doped Si:H films

Microcrystalline (μc) P (phosphorous) doped Si:H films have been prepared on several substrates by glow discharge decomposition in a mixture of H₂, PH₃ and SiH₄ under various deposition conditions. The P content in deposited μc-films strongly depended on the deposition conditions. Their electrical properties were not much affected by the volume fraction of crystalline phase but were rather determined by the P content. The incorporation mechanism of P atoms into Si:H film is discussed.     

Optical properties and chemical bonding characteristics of amorphous SiNX:H thin films grown by the plasma enhanced chemical vapor deposition method

Amorphous silicon nitride (SiN_X:H) thin films grown by the plasma enhanced chemical vapor deposition (PECVD) method are presently the most important antireflection coatings for crystalline silicon solar cells. In this work, we investigated the optical properties and chemical bonding characteristics of the amorphous SiN_X:H thin films deposited by PECVD. Silane (SiH₄) and ammonia (NH₃) were used as the reactive precursors. The dependence of the growth rate and refractive index of the SiN_X:H thin films on the SiH₄/NH₃ gas flow ratio was studied. The chemical bonding characteristics and the surface morphologies of the SiN_X:H thin films were studied using the Fourier transform infrared spectroscopy and atomic force microscopy, respectively. We also investigated the effect of rapid thermal processing on the optical properties and surface morphologies of the SiN_X:H thin films. It was found that the rapid thermal processing resulted in a decrease in the thickness, increase in the refractive index, and coarser surfaces for the SiN_X:H thin films.     

The interpretation of the electric and optical properties of a-Si:H films produced by rf glow discharge through dark conductivity,photoconductivity and pulse controlled capacitance-voltage measurements

This paper deals with the interpretation of transport properties of amorphous silicon hydrogenated films (a-Si:H) through dark conductivity, photoconductivity and pulse controlled capacitance-voltage measurements. a-Si:H films were produced by rf glow discharge coupled either inductively or capacitively to a 3% SiH₄/Ar mixture at different crossed electromagnetic static fields. The data concerned with the dark activation energy, photoactivation energy, variation of the density of localized states and photosensitivity, (σ_ph/σ_d)_25°C, of a-Si:H films can account for their optoelectronic properties which are strongly dependent on the deposition parameters. We also observed that crossed electromagnetic static fields applied during film formation influences hydrogen incorporation in a different manner than previously proposed.     

Light-induced changes in hydrogenated amorphous silicon solar cells deposited at the edge of crystallinity

A series of hydrogenated amorphous silicon (a-Si:H) films were deposited in the transition region from amorphous to nanocrystalline phases by changing hydrogen dilution ratio R, deposition gas pressure, and RF power. Single junction a-Si:H solar cells were made using these materials as the intrinsic layers in the structure of n–i–p type on ZnO/Ag/stainless steel substrates. Light-induced degradations in the photovoltaic parameters were characterized on these cells after 1 Sun solar illumination for 150 h. The stabilized efficiencies were compared in conjunction with the structures in the intrinsic layers, which were revealed by high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrometry (FTIR). It was found that the solar cells incorporated protocrystalline intrinsic layer as the i-layer give a better initial efficiency, while solar cells made from nanostructured i-layers have a better stability of ～7% degradation against light soaking, as a result, both have nearly the same final stabilized efficiency. The best device stabilized efficiency reaches ～10.2% (0.25 cm², AM1.5G) for the intrinsic layer deposited at a high pressure of 2 Torr.     

The effects of deposition parameters on a-Si:H films fabricated by microwave glow discharge techniques

Hydrogenated amorphous silicon (a-Si:H) films have been fabricated by a novel method of microwave glow-discharge deposition from SiH₄ and H₂, operating at 2.45 GHz. The properties of the deposited films are dependent upon the confinement of the microwave plasma by a magnetic field, and upon the orientation of the substrates with respect to the electric field. The quality of these materials is comparable to that of films deposited in conventional radio-frequency glow-discharge systems.     

Thermodynamic analysis of gas phase chemistry in hot wire chemical vapor deposition of a-Si:H and μc-Si:H

Gas phase reactions amongst filament-generated radicals play a crucial role in growth and properties of films deposited by hot wire chemical vapor deposition (HWCVD) technology. Gas phase species of interest are SiH₄, H₂, Si, H, SiH₃, SiH₂ and SiH. Partial pressures of these species for different sets of deposition conditions have been determined from the standard Gibbs free energy data. Equilibrium concentrations of the film forming precursors have been determined. The effect of the various process parameters on the equilibrium concentration of the precursors has been studied. H, Si and SiH are found to be the dominant species in gas phase above a filament temperature of 2300 K. However SiH₃ and SiH₂ concentration peaks are between 1900 and 2300 K, of the filament temperature.     

Optical absorption edge in structure-inhomogeneous a-Si:H-based alloys

In this paper we measure microstructure and optical absorption edge of a-Si:H and silicon-rich a-SiN_r:H films prepared at deposition rates ∼0.8 nm/s by radio frequency plasma enhanced chemical vapor deposition method from hydrogen diluted SiH₄ and SiH₄ + NH₃ mixtures, respectively. Microstructure of films was studied by atomic force microscopy and infrared spectroscopy. Both a-Si:H and a-SiN_r:H films are inhomogeneous on a scale of ∼50 nm and contain Si-rich islands with hydrogen (in a-Si:H) or hydrogen and nitrogen (in a-SiN_r:H) collected at their boundaries. It was found that different atomic configurations of N and H determined from IR data should be attributed to such islands and their boundaries. It was established that the optical gap is determined by the concentration of hydrogen (in a-Si:H) or nitrogen (in a-SiN_r:H) in the islands while it is insensitive to variations of content of these alloy atoms at island boundaries. These results are interpreted in terms of a quantum well model modified to take into account structure of alloy atoms.     

Deposition of phosphorus doped a-Si:H and μc-Si:H using a novel linear RF source

A roll-to-roll PECVD system for thin film silicon solar cells on steel foil has been developed by ECN in collaboration with Roth and Rau AG. It combines MW–PECVD for fast deposition of intrinsic Si and novel linear RF sources, which apply very mild deposition conditions, for the growth of doped Si layers. The RF and MW sources can be easily scaled up to deposition widths of up to 150 cm. Here, we report on n-type doping, achieved by RF–PECVD from a H₂/SiH₄/PH₃ mixture in the reaction chamber. The best n-type a-Si:H layers showed E_act = 0.27 eV and σ_d = 2.7 × 10^?3 S/cm. Also thin layers down to 20 nm were of device quality and were deposited at a rate of 0.4 Å/s. Furthermore, n-type μc-Si:H layers with thicknesses of 150 nm, with E_act = 0.034 eV and σ_d = 2 S/cm were grown. Good quality n-type μc-Si:H layers can be made for layer thicknesses down to 50 nm at a rate of 0.15 Å/s. To conclude, the novel RF source is well-suited for the growth of n-doped a-Si:H and μc-Si:H layers for roll-to-roll solar cell production.     

Mechanisms of the growth of nanocrystalline Si:H films deposited by PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were deposited using plasma-enhanced chemical vapor deposition from a SiF₄/SiH₄/H₂ gas mixtures. Properties were examined of nc-Si:H films produced by decreasing the deposition temperature (T_d) under two different hydrogen dilution ([H₂]) conditions. For these films, the X-ray diffraction, the Raman scattering, the Fourier transform infrared absorption and the stress were investigated. Our results show that the decrease in T_d has significant effects in the decrease of the average grain size (〈δ〉), the crystalline volume fraction (ρ) values, and an increase in the density of SiH-related bonds (N_SiH) values. On the contrary, increases in [H₂] decreased the 〈δ〉 and the N_SiH, while the ρ were increased. Our experiments also confirmed that the increase in ρ corresponds with the decrease in N_SiH. In view of these results, it may be concluded that the use of both low T_d and high [H₂] conditions might lead to growth of nc-Si:H films with small grains and high crystallinity. In this context, the surface processes (such as diffusion and etching) for the growth of nc-Si:H films were extensively discussed in this current work.     

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175 °C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N₂O), hydrogen (H₂), silane (SiH₄), and diborane (B₂H₆) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71 eV to 2.0 eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7 × 10^− 5 S/cm to 5.1 × 10^− 7 S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3 eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (V_oc) = 800 mV, short circuit current density (J_sc) = 16.3 mA/cm², fill-factor (FF) = 72%, and photovoltaic conversion efficiency (η) = 9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. J_sc, FF and V_oc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.