Measurements of effective optical reflectivity using a conventional flatbed scanner—Fast assessment of optical layer properties

A conventional flatbed scanner equipped with an additional diffusor is used for rapid measurements of an important figure of merit for optical surfaces, the effective reflectivity of devices such as solar cells. The application of the technique to multicrystalline silicon wafers with light-trapping structures obtained by electrochemical etching is shown. The use of this method for rapid quality control in production environments as well as in the lab is envisaged: even with a non-optimized diffusor, a spatial resolution of 0.1×0.1 mm² can be achieved, with an accuracy of the reflectivity measurements of 1% and data-acquisition times around 10 s per wafer.     

Deposition pressure effects on material structure and performance of micromorph tandem solar cells

Tandem solar cells represent an elegant way of overcoming the efficiency limits of single-junction solar cells and reducing the light-induced degradation of amorphous silicon films. Stacked structures consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell allow a good utilization of the solar spectrum due to the band gap values of the two materials. These devices, firstly introduced by the IMT research group, were designated as “micromorph” tandem solar cells. To better exploit this concept, it is important to tune parameters like the band gaps and the short-circuit currents.In this work, we have realized micromorph tandem solar cells on Asahi U-type TCO-covered glass substrates. The intrinsic layer of both the amorphous top cell and the microcrystalline bottom cell is grown by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at 100 MHz at low substrate temperature (150 °C). Finally, a ZnO reflector and a metal contact complete the structure. No intermediate optical mirror between the two cells is used at this stage. Undiluted a-Si:H, with reduced band gap when compared to H₂-diluted amorphous silicon, is used as absorber layer in the top cell. As for the bottom cell, the high-pressure–high-power regime (up to 267 Pa–80 W) has been explored aiming at growing high-quality microcrystalline silicon at large deposition rates. The effect of the structural composition of the microcrystalline absorber layer on the current–voltage characteristic and spectral response of tandem devices has been investigated. An efficiency of 11.3% has been obtained with short-circuit current densities around 13 mA/cm², open-circuit voltages 1.34 V and fill factors 66%.     

Microcrystalline-phase p-type a-Si:O:H windows prepared by Cat-CVD

Different amounts of oxygen, boron-doped hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) deposition were carried out using catalytic chemical-vapor deposition (Cat-CVD) process. Pure silane (SiH₄), hydrogen (H₂), oxygen (O₂), and diluted diborane (B₂H₆) gases were used at the deposition pressure of 0.1–0.5 Torr. The tungsten catalyst temperature (T_fil) was varied from 1700 to 2100 °C. Sample transmittance measurement shows an optical-band gap (E_gopt) variation from 1.45 to 2.1 eV X-ray diffraction (XRD) spectra have revealed silicon microcrystalline phases for the samples prepared at the temperature greater than T_fil1900 °C. For the used silicon oxide deposition conditions, no strong tungsten filament degradation was observed after a number of sample preparations.     

Low-temperature sintering behaviors of nanosized Ce0.8Gd0.2O1.9 powder synthesized by co-precipitation combined with supercritical drying

Nanocrystalline Ce_0.8Gd_0.2O_1.9 (GDC20) powder was synthesized by ammonia co-precipitation combined with supercritical ethanol drying route, followed by characterizations with TG/DSC, XRD, BET, HR-TEM, and FESEM techniques. After calcination at 600 °C, the powder has a high specific surface area of 146.5 m² g⁻¹ and an average crystal size of 5 nm without hard-agglomeration. The nano-GDC powder showed excellent sinterability, where by pressureless sintering at 900 °C for 4 h, the relative density of more than 98% and average grain size of 84 nm have been attained, which is attributed to the powder's ultrafine nanocrystal size, weak agglomeration and high homogeneity. Investigations indicate that in the initial sintering stage, grain growth behavior is mainly controlled by volume diffusion mechanism with an activation energy of 5.4 eV.     

High-rate microcrystalline silicon deposition for p–i–n junction solar cells

We have developed a high-rate plasma process based on high-pressure and silane-depletion glow discharge for highly efficient microcrystalline silicon (μc-Si:H) p–i–n junction solar cells. Under high-rate conditions (2–3 nm/s), we find that the deposition pressure becomes the dominant parameter in determining solar-cell performance. With increasing deposition pressure from 4 to 7–9 Torr, short-circuit current increases by 50% due to a remarkable improvement in quantum efficiencies at the visible and near infrared. As a result, the maximum efficiency of 9.13% has been achieved at an i-layer deposition rate of 2.3 nm/s. We attribute the improved performance of high-pressure-grown μc-Si:H solar cells to the structural evolution toward denser grain arrangement that prevents post-oxidation of grain boundaries.     

Comparison of the properties of porous silicon films with different back contacts (Ag, Al) for possible photovoltaic applications

In this work, porous silicon (PS) films were prepared by anodization on polished substrates of (1 0 0) Si for a fixed current density (I_d)20 mA cm⁻² and for a fixed anodization time of 30 min using different screen-printed (SP) back contacts, namely Ag and Al. The properties of PS formed using Ag as the back contact were found to be superior compared to the corresponding film using Al as the back contact. The PS formed with Ag-back contact exhibits higher porosity, negligible photoluminescence (PL) decay, better adherence to the substrate and smooth surface morphology compared to that formed with Al as the back contact for the same current density and time of anodization. Fourier transform infrared (FTIR) studies indicated significant presence of Si–O related features at 1050–1150 cm⁻¹ for PS films formed with Al as back contact, which could be responsible for traps and interface (PS–Si) defect densities as compared to corresponding PS films with Ag as the back contact. Measurements of capacitance–voltage (C–V) and current–voltage (I–V) were used for the investigation of the electrical properties of PS films with different back contacts. The frequency-dependent C–V characteristics were analysed to understand the effects of interface states and traps on the properties of PS films. The results have been analysed in terms of eutectic temperature and back surface field (BSF) across the metal–silicon interface.     

Degradation studies of transparent conducting oxide: a substrate for microcrystalline silicon thin film solar cells

Aluminium doped ZnO films have been developed by RF-magnetron sputtering at 350 °C substrate temperature on glass substrate and commercially available SnO₂-coated glass substrate. The developed ZnO and SnO₂/ZnO films can be used as the substrates of microcrystalline silicon based solar cell. The electrical, optical properties and surface morphologies of ZnO film and SnO₂/ZnO bi-layer films have been investigated and they are compared with the commercially available SnO_2-coated glass substrate. The resistivities of ZnO and SnO₂ films are comparable (10⁻⁴ Ω-cm). Surface morphologies of different transparent conducting oxide coated substrates before and after H-plasma exposure were studied by scanning electron microscopy. The optical transmission of ZnO, SnO₂/ZnO and SnO₂ films are comparable and varies from 85 to 90% in the visible region. The optical transmission reduces drastically to less than 20% in SnO₂ films and for ZnO film it remains almost unchanged after H-plasma exposure. For SnO₂/ZnO film transmission decreases slightly but remains considerably high (80%). The performance of microcrystalline silicon solar cells fabricated on different transparent conducting oxides as substrates (ZnO/glass, SnO₂/glass and ZnO/SnO₂/glass double layer) is investigated in detail.     

A note on fast protonic solid state electrochromic device: NiOx/Ta2O5/WO3−x

In the present investigation, the electrochromic properties of a fast protonic solid state device: NiO_x/Ta₂O₅/WO_3−x prepared at room temperature (300 K) is reported. The non-stoichiometric tungsten oxide thin film is prepared by reactive DC magnetron sputtering technique on ITO coated glass; the oxides of tantalum (300 nm) and nickel (100 nm) are prepared by electron beam evaporation. This proton device has a coloration efficiency of 82.4 cm²/C and coloration and bleaching time of 6 and 5 s, respectively, and a transmittance variation of 60%. The work function of WO_3−x thin films by Kelvin probe in uncolored and colored states are 4.73 and 4.30 eV, respectively.     

Structural, electrical and photovoltaic characterization of Si nanocrystals embedded SiC matrix and Si nanocrystals/c-Si heterojunction devices

Thin films of Si nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix (Si-NC:SiC) were prepared by alternating deposition of Si-rich silicon carbide (Si_1−xC_x) and near-stoichiometric SiC mutilayers (Si_1−xC_x/SiC) using magnetron cosputtering followed by a post-deposition anneal. Transmission electron microscopy and Raman spectroscopy revealed that the Si NCs were clearly established, with sizes in the range of 3–5 nm. Optical studies showed an increase in the optical band gap after annealing from 1.4 eV (as-deposited) to 2.0 eV (annealed at 1100 °C). P-type Si-NC:SiC/n-type crystalline silicon (c-Si) heterojunction (HJ) devices were fabricated and their electrical and photovoltaic properties were characterized. The diode showed a good rectification ratio of 1.0×10⁴ at the bias voltage of ±1.0 V at 298 K. The diode ideality factor and junction built-in potential deduced from current–voltage and capacitance–voltage plots are 1.24 and 0.72 V, respectively. Illuminated I–V properties showed that the 1-sun open-circuit voltage, short-circuit current density and fill factor of a typical HJ solar cell were 463 mV, 19 mA/cm² and 53%, respectively. The external quantum efficiency and internal quantum efficiency showed a higher blue response than that of a conventional c-Si homojunction solar cell. Factors limiting the cell's performance are discussed.     

Electrical and optical properties of highly conducting CdO:F thin film deposited by sol–gel dip coating technique

Highly conducting fluorine-doped cadmium oxide (CdO:F) thin films were deposited by sol–gel dip coating technique on glass and Si substrates. F concentration in the films was varied from 2.0% to 13.8% as determined from energy dispersive X-ray analysis. X-ray diffraction pattern showed that the films were polycrystalline in nature. The optimum F concentration for obtaining maximum conductivity was found to be 9.7%. The corresponding electrical conductivity was found to be 1.088×10⁴ S/cm and mobility 60.41 V/cm². Analysis of UV-VIS-NIR spectrum of the film with F concentration 9.7% showed a direct band gap energy of 2.3 eV.     

Energy analysis of silicon solar cell modules based on an optical model for arbitrary layers

An optical model for arbitrary layers is developed and a one-dimensional steady-state thermal model is applied to analyze the energy balance of silicon solar cell modules. Experimental measurements show that simulations are in good agreement, with a maximum relative error of 8.43%. The wind speed v_wind, ambient temperature T_amb and irradiance G are three main factors influencing the temperature of a photovoltaic panel. Over the course of a day the electrical output is reduced by the module temperature to only 32.5% of the rated value. Optical studies reveal that before 8:00 hours and after 16:00 hours, significant incident energy is lost by reflection because of the large angle of incidence θ_in, while at other times of day optical losses are nearly the same due to only small changes of transmission for θ_in < 45°. In addition, some optical losses result from the mismatched refractive indexes of encapsulating materials, especially at the ethylene-vinyl-acetate (EVA)/anti-reflection coating (ARC) and the ARC/Si interfaces. The uses of SiO₂ and TiO₂ as ARC materials for un-encapsulated and encapsulated Si solar cells are investigated by simulation. Comparing the results indicates that TiO₂ as ARC reduces the reflective optical loss within λ = 0.4–1.1 μm after encapsulation, while SiO₂ as ARC increases the loss by 5%. Energy allotment analysis shows that from 9:00 to 15:00, the reflective and transmissive optical losses are relatively steady at 26% and 13% of the incident energy, while the convective and radiative heat losses account for a further 30% and 24%, respectively. Thus, only 7% of incident energy is converted to electrical power.     

Solid-state dye-sensitized photocell based on pentacene as a hole collector

A solid-state dye-sensitized photovoltaic cell consisting of vacuum deposited pentacene onto ruthenium dye-coated TiO₂ electrode doped with iodine was fabricated. Cell delivers a short-circuit current of 3.6 mA cm⁻² and an open-circuit voltage of 415 mV at 100 mW cm^–2 (1.5 air mass). The efficiency and the fill factors of the above cell are 0.8% and 0.5%, respectively. Studies of the photocurrent action spectra showed that the dye is mainly responsible for this photocurrent generation. Preliminary results under extended illumination suggested that “long term” stability of the cell is promising.     

The growth kinetics and optical confinement studies of porous Si for application in terrestrial Si solar cells as antireflection coating

The mesoporous porous-silicon (PS) layers were grown on 1 0 0, 1 1 0, and 1 1 1 oriented wafers at constant current density of 20 mA cm⁻². The pore sizes and surface morphologies were measured by atomic force and scanning electron microscopes. The thickness x of the PS formed and the refractive index were measured by an ellipsometer as a function of time duration t (in min) of anodization. The x vs. t data were fitted into a power law x=at^c where c is a dimensionless constant and growth kinetics was established. The growth is practically independent of orientation. This is due the reason that the growth rate is controlled largely by the availability of holes which exchange their charge with oxidizing species and desirably large concentrations of holes were available at current density of 20 mA cm⁻². For a similar reason the growth of PS layer on the front surface of the n⁺ region of n⁺–p solar cells could also be done at current density of 20 mA cm⁻² nearly at the same rate. A large concentration of holes could be injected from p region into the n⁺ region because the positive contact was made on the p side and thus the junction was forward biased. The PS ARC of thickness 70 nm showed increase 26% in the short circuit current density J_sc and 24% in efficiency of the cells. However, the improvement in the values of the open circuit voltage V_oc were lower than the expected value indicating that the PS layers had enhanced recombination of minority carriers at the front surface or in the front emitter region immediately below the PS layer.     

Microcrystallisation in Si:H: the effect of gas pressure in Ar-diluted SiH4 plasma

Using noble gas argon as a diluent of SiH₄ in RF glow discharge, undoped μc-Si:H thin films have been developed at a low power density of 30 mW/cm². It has been found that the gas pressure is a critical factor for the growth of μc-Si:H films. Undoped μc-Si:H films having σ_D10⁻⁶ S/cm and ΔE<0.57 eV have been obtained at and above a critical pressure of 0.8 Torr. When the RF power density is increased to 90 mW/cm², a more crystalline as well as highly conducting (σ_D10⁻⁴ S/cm) μc-Si:H film has been achieved at a deposition rate of 30 Å/min, which is much higher than that attained from H₂-diluted SiH₄ plasma, by conventional approach. The crystallinity of the films has been identified by the sharp Raman peak at 520 cm⁻¹ and a large number of micrograins in the TEM micrographs. The metastable state of Ar, denoted as Ar^*, plays the crucial role in inducing microcrystallisation by transferring its de-excitation energy at the surface of the growing film. A mechanism has been proposed to explain the dependence of the formation of μc-Si:H film on the working gas pressure in the plasma.     

Influence of hydrogen dilution on structural, electrical and optical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PE-CVD)

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited from pure silane (SiH₄) and hydrogen (H₂) gas mixture by conventional plasma enhanced chemical vapour deposition (PE-CVD) method at low temperature (200 °C) using high rf power. The structural, optical and electrical properties of these films are carefully and systematically investigated as a function of hydrogen dilution of silane (R). Characterization of these films with low angle X-ray diffraction and Raman spectroscopy revealed that the crystallite size in the films tends to decrease and at same time the volume fraction of crystallites increases with increase in R. The Fourier transform infrared (FTIR) spectroscopic analysis showed at low values of R, the hydrogen is predominantly incorporated in the nc-Si:H films in the mono-hydrogen (SiH) bonding configuration. However, with increasing R the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (SiH) to di-hydrogen (SiH₂) and (SiH₂)_n complexes. The hydrogen content in the nc-Si:H films decreases with increase in R and was found less than 10 at% over the entire studied range of R. On the other hand, the Tauc's optical band gap remains as high as 2 eV or much higher. The quantum size effect may responsible for higher band gap in nc-Si:H films. A correlation between electrical and structural properties has been found. For optimized deposition conditions, nc-Si:H films with crystallite size 7.67 nm having good degree of crystallinity (84% ) and high band gap (2.25 eV) were obtained with a low hydrogen content (6.5 at%). However, for these optimized conditions, the deposition rate was quite small (1.6 Å/s).     

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B₂H₆ gas (diborane) phase ratio (Variation in B₂H₆ gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm)⁻¹) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).     

Layered GdBa0.5Sr0.5Co2O5+δ as a cathode for intermediate-temperature solid oxide fuel cells

The layered GdBa_0.5Sr_0.5Co₂O_5+δ (GBSC) perovskite oxides are synthesized by Pechini method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The single cell of NiO–SDC (Sm_0.2Ce_0.8O_1.9)/SDC (20 μm)/GBSC (10 μm) is operated from 550 to 700 °C fed with humidified H₂ as fuel and the static air as oxidant. An open circuit voltage of 0.8 V and a maximum power density of 725 mW cm⁻² are achieved at 700 °C. The interfacial polarization resistance is as low as 0.88, 0.29, 0.13 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. The ratio of polarization resistance to total cell resistance decreases with the increase in the operating temperature, from 60% at 550 °C to 21% at 700 °C, respectively. The experimental results indicate that GBSC is a promising cathode material for IT-SOFCs.     

Effectiveness of anisotropic etching of silicon in aqueous alkaline solutions

Optical effectiveness of anisotropic etching of (1 0 0) silicon in inorganic alkaline solution has been studied from the view point of its application in commercial silicon solar cells. The damage caused by ID saw or wire saw during slicing of the wafer is required to be removed for fabrication of solar cells. The etch rates for removal of the surface damages for boron doped Czochralski wafers of 1–2 Ω cm resistivity in 20% NaOH solution at 80°C was measured and was found to be 1.4 μm/min. After the damage removal, texturisation was obtained in 2% NaOH solution buffered with isopropyl alcohol at 80°C. An optical effectiveness parameter f_eff,λ was defined and its value was estimated from the study of reflectivity and topography of the wafers textured for different durations of time. The kinetics of anisotropic etching was studied which indicated that growth of pyramids begins at preferential sites which may arise due to crystalline defects or wetting. Silicon solar cells have been realized by standard process involving phosphorous diffusion and vacuum evaporated front and back contacts. The value of optical effectiveness parameter is found to have a direct correlation with the improvement in short circuit current density of the textured cells.     

Purification and characterization of [Fe]-hydrogenase from high yielding hydrogen-producing strain, Enterobacter cloacae IIT-BT08 (MTCC 5373)

Fe-hydrogenase from Enterobacter cloacae IIT-BT08 was purified 1284 fold with specific activity of 335 μmol H₂/min/mg protein for hydrogen evolution using reduced methyl viologen as an electron-donor at 25 °C. The molecular weight of the monomeric enzyme was determined to be 51 kDa by MALDI-ToF mass spectrometry. The PI of the enzyme was 5.6 displaying its acidic nature. The optimal temperature and pH for hydrogen evolution was 37 °C and 7–7.2 respectively. The affinity constant, K_m for reduced methyl viologen was 0.57 ± 0.03 mM and that of reduced ferredoxin was 0.72 ± 0.04 μM. The enzyme contained 11.47 gm-atom Fe/mol of Fe-hydrogenase. Electron paramagnetic resonance analysis ascertained the existence of iron molecules as [4Fe–4S] clusters. The internal amino acid sequences of trypsin digested peptides of hydrogenase as determined by ESI MS/MS Q-ToF showed 80-87% identities with the respective sequences of Clostridium sp. and Trichomonas sp. hydrogenase.     

Thermal imaging of solid oxide fuel cell anode processes

A Si-charge-coupled device (CCD), camera-based, near-infrared imaging system is demonstrated on Ni/yttria-stabilized zirconia (YSZ) fragments and the anodes of working solid oxide fuel cells (SOFCs). NiO reduction to Ni by H₂ and carbon deposition lead to the fragment cooling by 5 ± 2 °C and 16 ± 1 °C, respectively. When air is flowed over the fragments, the temperature rises 24 ± 1 °C as carbon and Ni are oxidized. In an operational SOFC, the decrease in temperature with carbon deposition is only 4.0 ± 0.1 °C as the process is moderated by the presence of oxides and water. Electrochemical oxidation of carbon deposits results in a ΔT of +2.2 ± 0.2 °C, demonstrating that electrochemical oxidation is less vigorous than atmospheric oxidation. While the high temperatures of SOFCs are challenging in many respects, they facilitate thermal imaging because their emission overlaps the spectral response of inexpensive Si-CCD cameras. Using Si-CCD cameras has advantages in terms of cost, resolution, and convenience compared to mid-infrared thermal cameras. High spatial (0.1 mm) and temperature (0.1 °C) resolutions are achieved in this system. This approach provides a convenient and effective analytical technique for investigating the effects of anode chemistry in operating SOFCs.