Effect of preparation conditions on the properties of glow-discharge intrinsic amorphous silicon

The influence of the preparation conditions (process pressure, substrate temperature, RF-power density and deposition time/thickness) on the optical and electrical properties of intrinsic hydrogenated amorphous silicon (a-Si:H) has been investigated with the aim of optimising such films to be used as absorbent layers of a-Si:H-based p-i-n solar cells. Highly photosensitive films have been obtained at high growth rates (6.2 Å s⁻¹) in the depletion regime using a high process pressure (1000 mTorr), a moderate substrate temperature (250°C) and a relatively high RF-power density (35.2 mW cm⁻²). These films have excellent properties for the application in question.     

Toward stabilized 10% efficiency of large-area (>5000 cm) a-Si/a-SiGe tandem solar cells using high-rate deposition

This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10⁶ for a-Si:H films has been maintained up to the deposition rate of 15 Å/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm² a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.     

I-V characteristics of a-Si-c-Si hetero-junction diodes made by hot wire CVD

p-Type hydrogenated amorphous silicon (a-Si:H) was deposited on n-type crystalline silicon (c-Si) substrates to obtain hetero-junction diodes. Additionally, a thin intrinsic a-Si:H layer was inserted between both the p-type film and the n-type substrate to study its passivation effect on the c-Si surface. The amorphous films were obtained by the hot wire chemical vapor deposition (HWCVD) technique, using a tungsten filament and silane (SiH₄), hydrogen (H₂) and diborane (B₂H₆) gases, where the deposition parameters such as gas flow, substrate temperature and filament temperature were varied. Optical band gap, deposition rate and conductivity were measured for all the films. We studied the influence of the quality of the amorphous films upon the performance of the hetero-junction diodes. In particular, the diode ideality factor (n) and the saturation current density (J₀) were determined by measuring the current-voltage characteristics in dark conditions. It is shown that the presence of the intrinsic layer is fundamental for making good diodes, since devices made without this film cause the diodes to have high saturation current density and ideality factor (J₀>10×10⁻⁶ A/cm², n>4) as compared to diodes with a good intrinsic layer (J₀=5×10⁻⁹ A/cm², n=1.39). The results obtained are encouraging, but the quality of the intrinsic films still should be improved for applying them to HIT solar cells.     

Structural and optical properties of textured ZnO:Al films on glass substrates prepared by in-line rf magnetron sputtering

Textured ZnO:Al films with excellent light scattering properties as a front electrode of silicon thin film solar cells were prepared on glass substrates by an in-line rf magnetron sputtering, followed by a wet-etching process to modify the surface morphologies of the films. Deposition parameters and wet etching conditions of the films were controlled precisely to obtain the optimized surface features. All as-deposited films show a strong preferred orientation in the [0 0 1] direction under our experimental conditions. The microstructure of the films was significantly affected by working pressure and film compactness was reduced with increasing working pressure, while the effect of a substrate temperature on the microstructure is less pronounced. A low resistivity of 4.25×10⁻⁴ Ω cm and high optical transmittance of above 80% in a visible range were obtained in the films deposited at 1.5 mTorr and 100 °C. After wet etching process, the surface morphologies of the films were changed dramatically depending on the microstructure and film compactness of the initial films. By controlling the surface feature, the haze factor and angular resolved distribution of the textured ZnO:Al films were improved remarkably when compared with commercial SnO₂:F films. The textured ZnO:Al and SnO₂:F films were applied as substrates for a silicon thin film solar cells with tandem structure of a-Si:H/μc-Si:H. Compared with the solar cells with the SnO₂:F films, a significant enhancement in the short-circuit current density of the μc-Si:H bottom cell was achieved, which is due to the improved light scattering on the highly textured ZnO:Al film surfaces in the long wavelength above 600 nm.     

Fabrication of stable hydrogenated amorphous silicon from SiH2Cl2 by ECR-hydrogen-plasma

Relatively stable high bandgap hydrogenated amorphous silicon (a-Si:H) films were prepared by the microwave electron cyclotron resonance (ECR) hydrogen plasma CVD method using SiH₂Cl₂ source gas. The substrate position relative to the position of the generation of reactive species affects the structure of these films. The films prepared under optimized condition showed rather high bandgap, 1.83 eV. However, the defect density was low, 3 × 10¹⁵ cm⁻³, and the photosensitivity was greater than 7 orders of magnitudes. The defect density was found to saturate at relatively low values ( 3 × 10¹⁶ cm⁻³) independent of the illumination intensity.     

Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering

Polycrystalline WO₃ thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O₂ gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO₃, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm⁻² at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO₃ films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm.     

Low temperature deposition of high open-circuit voltage (>1.0 V) p-i-n type amorphous silicon solar cells

P-i-n type hydrogenated amorphous silicon (a-Si:H) solar cells were deposited by the radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process at a low substrate temperature of 125 °C, which is compatible with low-cost poly (ethylene terephthalate) (PET) plastic substrates. Wide band gap (E_opt>1.88 eV) intrinsic a-Si:H films were achieved before the onset of the microcrystalline regime by changing the hydrogen dilution ratios. On the other hand, the structural, optical and electrical properties of p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) window layers have been optimized at 125 °C. High quality p-a-SiC:H film with high optical band gap (E₀₄=2.02 eV) and high conductivity (σ_d=1.0×10⁻⁷ S/cm) was deposited at ‘low-power regime’ under low silane flow rates and high H₂ dilution conditions. With the combination of wide band gap p-a-SiC:H window layers and intrinsic a-Si:H layers, a high V_oc of 1.01 V (efficiency=5.51%, FF=0.72, J_sc=7.58 mA/cm²) was obtained for single junction a-Si:H p-i-n solar cell at a low temperature of 125 °C. Finally, flexible a-Si:H solar cell on PET substrate with efficiency of 4.60% (V_oc=0.98 V, FF=0.69, J_sc=6.82 mA/cm²) was obtained.     

Local aluminum-silicon contacts by layer selective laser ablation

We demonstrate damage free selective laser ablation of silicon nitride from a silicon nitride/amorphous silicon double layer. This approach allows local contact formation to passivated silicon. Thereby the remaining amorphous silicon dissolves in evaporated aluminum by annealing. This technique is especially useful for contacting thin emitters since it avoids any damage to the silicon substrate. We demonstrate a local contact resistivity of 0.8±0.3 mΩ cm² on a phosphorous diffused emitter with a peak doping density of 2×10²⁰ cm⁻³. Laser treated as well as non-treated areas show the same carrier lifetime of 2000 μs on 100 Ω cm mono-crystalline silicon, proving the selective ablation.     

Phosphorus-doped glass proton exchange membranes for low temperature direct methanol fuel cells

Phosphorus-doped silicon dioxide thin films were used as ion exchange membranes in low temperature proton exchange membrane fuel cells. Phosphorus-doped silicon dioxide glass (PSG) was deposited via plasma-enhanced chemical vapor deposition (PECVD). The plasma deposition of PSG films allows for low temperature fabrication that is compatible with current microelectronic industrial processing. SiH₄, PH₃ and N₂O were used as the reactant gases. The effect of plasma deposition parameters, substrate temperature, RF power, and chamber pressure, on the ionic conductivity of the PSG films is elucidated. PSG conductivities as high as 2.54 × 10⁻⁴ S cm⁻¹ were realized, which is 250 times higher than the conductivity of pure SiO₂ films (1 × 10⁻⁶ S cm⁻¹) under identical deposition conditions. The higher conductivity films were deposited at low temperature, moderate pressure, limited reactant gas flow rate, and high RF power.     

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH₄) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s⁻¹ and photosensitivity 10⁶ were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak (Γ_TO) and deviation in bond angle (Δθ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.     

Influence of silane flow on structural, optical and electrical properties of a-Si:H thin films deposited by hot wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of hydrogenated amorphous silicon (a-Si:H) films deposited from pure silane (SiH₄) using hot wire chemical vapor deposition (HW-CVD) technique are systematically studied as a function of silane flow rate between 5 and 30 sccm. We found that the properties are greatly affected by the silane flow rate over the range we studied. The device quality a-Si:H films with a photosensitivity >10⁵ were deposited by HW-CVD at a deposition rate >10 Å s⁻¹ using low silane flow rate. However, a-Si:H films deposited at higher silane flow rate and/or higher deposition rates show degradation in their structural and electrical properties. The FTIR studies indicate that the hydrogen bonding in a-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H₂) and (Si–H₂)_n complexes when films were deposited at higher silane flow rate. The hydrogen content in the a-Si:H films increases with increase in silane flow rate and was found to be less than 10 at.%. The Raman spectra show increase in disorder and the Rayleigh scattering with increase in silane flow rate. The optical band gap also shows an increasing trend with silane flow rate. Therefore, only the hydrogen content cannot be accounted for the increase in the optical band gap. We think that the increase in the optical band gap may be due to the increase in the voids. These voids reduce the effective density of material and increase the average Si–Si distance, which is responsible for the increase in the band gap. Silane flow rate of 5 sccm, appears to be an optimum flow rate for the growth of mono-hydrogen (Si–H) bonded species having low hydrogen content (4.25 at%) in a-Si:H films at high deposition rate (12.5 Å s⁻¹), high photosensitivity (10⁵) and small structural disorder.     

Improved quality a-SiC:H films prepared by photo chemical vapour decomposition of silane and acetylene

Hydrogenated amorphous and microcrystalline silicon carbon alloy films have been grown by photo-CVD using C₂H₂ as a source gas of carbon. The hydrogenated amorphous silicon carbon (a-SiC:H) film with a band gap of ~2.0 eV prepared at a very low hydrogen (LD) concentration exhibits better photo-electronic properties compared to that at high hydrogen dilution (HD) having a similar optical gap. Notwithstanding a high deposition rate, the high photosensitivity ( 10⁶), the low density of the defect states ( 6 × 10¹⁶cm⁻³) and the Urbach energy parameter (72 meV) for the a-SiC:H film prepared at low hydrogen dilution and pressure are impressive. On the other hand, low pressure along with high hydrogen dilution have been found to be conducive to microcrystalline silicon carbon alloy (μc-Si:H) formation. Interestingly, crystallites are of silicon while carbon remains in the amorphous and grain boundary regions.     

Effect of deposition parameters on the properties of hydrogenated amorphous silicon films prepared by photochemical vapour deposition

Hydrogenated amorphous silicon (a-Si:H) films were prepared by the process of mercury-sensitized photochemical vapour deposition. Substrate temperature, hydrogen dilution and chamber pressure were varied to optimize the rate of deposition and optoelectronic properties of these films. The properties as well as the growth rate depend sensitively on the substrate temperature. At optimum deposition conditions an a-Si:H film has been obtained with dark conductivity and photoconductivity ∼10⁻¹⁰ S cm⁻¹ and ∼10⁻⁴ S cm⁻¹, respectively, along with an optical band gap of 1.75 eV. The properties of the film are comparable to those obtained for films prepared by the glow discharge deposition technique.     

Reduction of the phosphorous cross-contamination in n-i-p solar cells prepared in a single-chamber PECVD reactor

A new approach to reduce phosphorous contamination in the intrinsic layer during the deposition of amorphous silicon (a-Si:H) n-i-p solar cells prepared in single-chamber reactors is presented. This novel process consists of a hydrogen etching plasma performed after the n-layer deposition, which prevents a recycling of phosphorous from the reactor walls when exposed to a hydrogen-rich plasma during the subsequent i-layer deposition. The implemented process reduces the phosphorous cross-contamination in the i-layer, as corroborated by secondary ion mass spectroscopy measurements. Furthermore, the end of the etching process can be easily monitored by measuring the DC bias voltage at the powered electrode. By applying this process, we were able to improve the fill factor from 70% up to 75%, without degradation in the other parameters of the cell, neither in the initial nor in the stabilized state. Finally, by implementing this process in a-Si:H/a-Si:H tandem solar cells we obtained an initial efficiency of 10.3% (V_oc=1.76 V, FF=74.5%, J_sc=7.8 mA cm⁻²); light soaking test resulted in a stabilized efficiency of 8.5%.     

Performance of a miniaturized silicon reformer-PrOx-fuel cell system

A fuel cell made with silicon is operated with hydrogen supplied by a reformer and a preferential oxidation (PrOx) reactor those are also made with silicon. The performance and durability of the fuel cell is analyzed and tested, then compared with the results obtained with pure hydrogen. Three components of the system are made using silicon technologies and micro electro-mechanical system (MEMS) technology. The commercial Cu-ZnO-Al₂O₃ catalyst for the reformer and the Pt-Al₂O₃ catalyst for the PrOx reactor are coated by means of a fill-and-dry method. A conventional membrane electrode assembly composed of a 0.375 mg cm⁻² PtRu/C catalyst for the anode, a 0.4 mg cm⁻² Pt/C catalyst for the cathode, and a Nafion™ 112 membrane is introduced to the fuel cell. The reformer gives a 27 cm³ min⁻¹ gas production rate with 3177 ppm CO concentration at a 1 cm³ h⁻¹ methanol feed rate and the PrOx reactor shows almost 100% CO conversion under the experimental conditions. Fuel cells operated with this fuel-processing system produce 230 mW cm⁻² at 0.6 V, which is similar to that obtained with pure hydrogen.     

A preliminary study of a miniature planar 6-cell PEMFC stack combined with a small hydrogen storage canister

The fabrication and performance evaluation of a miniature 6-cell PEMFC stack based on Micro-Electronic-Mechanical-System (MEMS) technology is presented in this paper. The stack with a planar configuration consists of 6-cells in serial interconnection by spot welding one cell anode with another cell cathode. Each cell was made by sandwiching a membrane-electrode-assembly (MEA) between two flow field plates fabricated by a classical MEMS wet etching method using silicon wafer as the original material. The plates were made electrically conductive by sputtering a Ti/Pt/Au composite metal layer on their surfaces. The 6-cells lie in the same plane with a fuel buffer/distributor as their support, which was fabricated by the MEMS silicon–glass bonding technology. A small hydrogen storage canister was used as fuel source. Operating on dry H₂ at a 40 ml min⁻¹ flow rate and air-breathing conditions at room temperature and atmospheric pressure, the linear polarization experiment gave a measured peak power of 0.9 W at 250 mA cm⁻² for the stack and average power density of 104 mW cm⁻² for each cell. The results suggested that the stack has reasonable performance benefiting from an even fuel supply. But its performance tended to deteriorate with power increase, which became obvious at 600 mW. This suggests that the stack may need some power assistance, from say supercapacitors to maintain its stability when operated at higher power.     

Drastic reduction in surface recombination velocity of crystalline silicon by surface treatment using catalytically-generated radicals

A method of reducing surface recombination velocities (SRVs) of crystalline silicon (c-Si) wafers is presented for the case when c-Si surfaces are passivated by amorphous Si (a-Si) thin films. It is demonstrated that the surface treatment of c-Si wafers prior to a-Si deposition, using hydrogen atoms generated by catalytic reaction of hydrogen gas with heated catalyzer, lowers SRV effectively, and that SRV is drastically reduced to less than 3 cm/s when a small amount of doping impurity is added during atomic hydrogen treatment.     

Design,fabrication and performance analysis of a 200 W PEM fuel cell short stack

A PEM fuel cell short stack of 200 W capacity, with an active area of 100 cm² has been designed and fabricated in-house. The status of unit cell performance was 0.55 W cm⁻². Based on the unit cell technology, a short stack has been developed. The proper design of uniform flow distribution, cooling plate and compressed end plate were important to achieve the best performance of the short stack. The performance of four cells stack was analyzed in static and dynamic modes. In the static mode of polarization curve, the stack has peak power density of 0.55 W cm⁻² (220 W) at 0.5 V per cell, when the voltage was scanning from low to high voltage (1.5–3.5 V), and resulted in minimum water flooding inside the stack. In this study a series of dynamic loadings were tested to simulate the vehicle acceleration. The fuel cell performances respond to dynamic loading influenced by the hydrogen/air stoichiometric, back pressure, and dynamic-loading time. It was needed high hydrogen stoichiometric and back pressure to maintain high dynamic performance. In the long-time stable power testing, the stack was difficult to maintain at high performance, due to the water flooding at high output power. An adjusting cathode back-pressure method for purging water was proposed to prevent the water flooding at flow channels and maintain the stable output power at 170 W (0.42 W cm⁻²).     

Plasma-induced TCO texture of ZnO:Ga back contacts on silicon thin film solar cells

This paper considers texturing of ZnO:Ga (GZO) films used as back contacts in amorphous silicon (a-Si) thin film solar cells. GZO thin films are first prepared by conventional methods. The as-deposited GZO surface properties are modified so that their use as back contacts on a-Si solar cells is enhanced. Texturing is performed by simple dry plasma etching in a CVD process chamber,at power=100 W, substrate temperature=190 °C (temperature is held at 190 °C because thin film solar cells are damaged above 200 °C), pressure=400 Pa and process gas H₂ flow=700 sccm. Conventional a-Si solar cells are fabricated with and without GZO back contact surface treatment. Comparison of the with/without texturing GZO films shows that plasma etching increases optical scattering reflectance and reflection haze. SEM and TEM are used to evaluate the morphological treatment-induced changes in the films. Comparison of the a-Si solar cells with/without texturing shows that the plasma treatment increases both the short-circuit current density and fill factor. Consequently, a-Si solar cell efficiency is relatively improved by 4.6%.     

Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of ∼2.8×10⁻⁴ Ω cm was achieved for a film thickness of 1100 nm (sheet resistance ∼2.5 Ω/□), with a Hall mobility of 18 cm²/V s and a carrier concentration of 1.3×10²¹ cm⁻³. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed.