Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

Theory of amorphous silicon solar cell (b): a five layer analytical model

A theoretical analysis of recombination kinetics and space charge distribution in amorphous silicon is carried out with a view to bring out the underlying physics. A uniform excitation with a flat quasi-Fermi level and a constant np product has been used as a probe to estimate the relative importance of various parameters. Recombination rates have been calculated for various ratios of capture rates for Coulomb attractive and neutral traps. In practice a large ratio of capture rates exists and for this case two peaks of recombination maxima are found to lie in the space charge regions corresponding to transitions at the energy level E₁ (for D⁺–D⁰ transition) at the p–i edge and for E₂ energy level (corresponding to D⁰–D⁻ transition) at the i–n interface. A two independent level model therefore holds to a good approximation. The dangling bond density is found to determine both the space charge distribution and the recombination rate. Based on space charge density distribution i-layer can be divided in the five parts. The two recombination rate peaks are found to exist at the p–i and i–n space charge transitions respectively. This enables us to develop a simple model for the i-layer of the p–i–n diode.     

Optimized resistivity of p-type Si substrate for HIT solar cell with Al back surface field by computer simulation

For HIT (heterojunction with intrinsic thin-layer) solar cell with Al back surface field on p-type Si substrate, the impacts of substrate resistivity on the solar cell performance were investigated by utilizing AFORS-HET software as a numerical computer simulation tool. The results show that the optimized substrate resistivity (R_op) to obtain the maximal solar cell efficiency is relative to the bulk defect density, such as oxygen defect density (D_od), in the substrate and the interface defect density (D_it) on the interface of amorphous/crystalline Si heterojunction. The larger D_od or D_it is, the higher R_op is. The effect of D_it is more obvious. R_op is about 0.5 Ω cm for D_it = 1.0 × 10¹¹/cm², but is higher than 1.0 Ω cm for D_it = 1.0 × 10¹²/cm². In order to obtain very excellent solar cell performance, Si substrate, with the resistivity of 0.5 Ω cm, D_od lower than 1.0 × 10¹⁰/cm³, and D_it lower than 1.0 × 10¹¹/cm², is preferred, which is different to the traditional opinion that 1.0 Ω cm resistivity is the best.     

An EQCM study for a novel aromatic poly(amine-imide) electrochromic thin film

A new aromatic poly(amine-imide) electrochromic thin film synthesized with N,N-bis(4-aminophenyl)-N′,N′-diphenyl-1,4-phenylenediamine and 3,3′,4,4′-benzo-phenonetetra carboxylic dianhydride, abbreviated as poly(PD-BCD), was studied. The poly(PD-BCD) thin-film electrode has been characterized by cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM). As the polymer chain acquires positive charge during the oxidation of poly(PD-BCD) to its radical cation state or dication state, the anions would insert into the polymer matrix in order to neutralize the charge. However, when the electrodes were cycled in electrolytes containing different cations, including 0.1 M LiClO₄/acetonitrile (ACN), 0.1 M NaClO₄/ACN and 0.1 M TBAClO₄/ACN, the experimental results revealed two mechanisms for the redox reaction. A plot of mass change (Δm) vs. accumulated charge (Q) gave a slope, from which the electrochromic mechanism can be extracted. The slopes of Δm–Q obtained from the CV–EQCM measurements in three electrolytes were different for the first redox stage, but the slopes were almost the same for the second redox stage. This means that, in addition to the involvement of anions, cations also play an important role in the first redox stage, however, the role of the cations is less in the second stage. Moreover, two reaction mechanisms for the two reaction stages of poly(PD-BCD) are proposed in this study.     

Step potential analysis of cobalt oxide-based electrochromic devices

The characterization of electrochemical behavior of electrochromic intercalation device based on cobalt oxide thin film was carried out using the step potential excitation method. A method based on generating plots of current density as a function of passed charge has been applied for characterization of electrochromic cobalt oxide thin films using an aqueous KOH electrolyte. The device resistance and the intercalation capacity of the material are calculated. Dynamic built-in potential estimated from step potential experiment and plots of the built-in potential as function of the passed charge, V_bi(ΔQ), are generated for intercalation process. The intercalation efficiency curve is obtained to confirm the nature of energy distribution of intercalation sites in electrochromic cobalt oxide.     

Performance evaluation of solar dryer system for optimal operation: mathematical modelling and experimental validation

An analytical moisture diffusion model which considers the influence of external resistance to mass transfer is developed. The methodology to determine constant and variable moisture diffusion coefficients, D_eff is proposed. A laboratory model of mixed-mode solar dryer is constructed to perform 16 experiments for different performance dependent variables under simulated indoor conditions. The potatoes (Solanum tuberosum) of Kufri Safed variety have been chosen as the test food product. The range of variables investigated is absorbed thermal energy (150–750 W/m²); air mass flow rate (0.009–0.022 kg/s); loading density (1.08–4.33 kg/m²) and sample thickness (5–18 mm). The efficiency results have been analysed to identify the value of each process variable leading to optimal operation of dryer. The study reveals that dryer with sample thickness of 8 mm and loading density of 4.33 kg/m² can operate optimally for absorbed energy of 450 W/m² and air mass flow rate of 0.017 kg/s.     

Estimates of monthly mean daily diffuse irradiation in the Southeastern United States

The objective of the present work was to identify correlation(s) that may be used to obtain estimates of the monthly mean daily diffuse irradiation on a horizontal surface, Q_D, at stations in the Southeastern United States. For this purpose, two sets of correlations (totaling 14 expressions in all) were tested for their statistical efficiency in making such estimates, using long-term measured and calculated relevant solar data (including long-term measured records for Q_D) for one such station: Memphis, Tennessee. It is shown that two of these correlations are very efficient and thus are recommended for the aforementioned purpose. Estimates of Q_D at five other stations within about 10° latitude of Memphis are obtained, using the recommended correlations.     

A three-dimensional numerical simulation of impinging jet arrays on a moving plate

The structure of the flow field and its effect on the heat transfer characteristics of a jet array system impinging on a moving heated plate are investigated numerically for Reynolds numbers between 100 and 400 and for steady state conditions. An array consisting of 24 square jets (3 rows × 8 columns) impinging on a moving heated flat surface is considered as a representative pattern.The simulations have been carried out for jet-to-jet spacing in the range 2D–5D and for nozzle exit to plate distance of 0.25D, where D is the jet width. The velocity ratios of the moving heated plat to the jet velocity (R_m = u_p/u_j) used are in the range 0.25–1.0. The obtained results were compared with published data for the case of fixed heated plate (R_m = 0.0). The results show that the streamwise profile of the Nusselt number exhibit strong periodic oscillations, spatially. The amplitude of the periodic oscillations of the Nusselt number is attenuated as one proceeds in the downstream direction. For such small nozzle-to-plate spacing used, the results show that the ratio R_m has no effect on the oscillations of Nusselt number.     

Investigation of natural convection via heatlines for Rayleigh–Bénard heating in porous enclosures with a curved top and bottom walls

The current study deals with the heatline-based analysis of natural convection in porous cavities with the curved top and bottom walls involving the Rayleigh–Bénard heating. The streamline cells are weak, and the wall-to-wall heatlines are observed for all the cases at the low Da_m involving two test cases, Pr_m?=?0.015 and 7.2. At the high Da_m, the convective force takes the command, and multiple heatline cells are observed for all the concave (except for high wall concavity) and convex cases. The directions of the streamlines (for all Da_m) and heatlines (at the high Da_m) are exactly opposite for the concave and convex cases. The case 3 (concave) is the efficient case based on the largest heat transfer rate for Pr_m?=?0.015 involving all Da_m and for Pr_m?=?7.2 involving the low Da_m. At Pr_m?=?7.2 and high Da_m, the case 1 (concave or convex) may be the efficient cases compared with the cases involving high wall curvatures.     

An approach to the design of solar coil collector

A design procedure using the concept of heat exchanger “effectiveness and transfer unit” has been developed and applied to a Solar Coil Collector fabricated and tested earlier. For laminar flow the coil wall temperature T_c is obtained from the modified expression ). Where T_b = bulk liquid temperature; Q = wall heat flux; h_t = heat transfer coefficient in straight tubes; D_t = tube diameter; D_c = coil diameter; and p = a number. The calculated results are checked with the actual dimensions of the collector and are found in agreement.     

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q(λ_m) of the test cell at λ_m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q(λ_m) calculation. The ratio of the calculated to measured Q(λ_m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q(λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.     

Thermodynamic assessment of solid oxide fuel cell system integrated with bioethanol purification unit

A solid oxide fuel cell system integrated with a distillation column (SOFC–DIS) has been proposed in this article. The integrated SOFC system consists of a distillation column, an EtOH/H₂O heater, an air heater, an anode preheater, a reformer, an SOFC stack and an afterburner. Bioethanol with 5 mol% ethanol was purified in a distillation column to obtain a desired concentration necessary for SOFC operation. The SOFC stack was operated under isothermal conditions. The heat generated from the stack and the afterburner was supplied to the reformer and three heaters. The net remaining heat from the SOFC system (Q_SOFC,Net) was then provided to the reboiler of the distillation column. The effects of fuel utilization and operating voltage on the net energy (Q_Net), which equals Q_SOFC,Net minus the distillation energy (Q_D), were examined. It was found that the system could become more energy sufficient when operating at lower fuel utilization or lower voltage but at the expense of less electricity produced. Moreover, it was found that there were some operating conditions, which yielded Q_Net of zero. At this point, the integrated system provides the maximum electrical power without requiring an additional heat source. The effects of ethanol concentration and ethanol recovery on the electrical performance at zero Q_Net for different fuel utilizations were investigated. With the appropriate operating conditions (e.g. C_EtOH = 41%, U_f = 80% and EtOH recovery = 80%), the overall electrical efficiency and power density are 33.3% (LHV) and 0.32 W cm⁻², respectively.     

Photoconductivity and defect levels in ZnxCd1−xSe with (x=0.5, 0.55) crystals

Gold surface barriers on Zn_xCd_1−xSe alloys have been investigated for composition with x=(0.5, 0.55). The electrical characteristics were studied as a function of air annealing. The common feature of all the Schottky devices was the reduction of reverse bias leakage current after heating in air. Typical measurements of capacitance as a function of bias voltage can provide information on the charge density and diffusion potential. The barrier height was found to increase after air annealing at 200°C for 2 min. The spectral response of the photocurrent and photocapacitance associated with these device layers enable a donor level at 0.13 eV and acceptor levels at 1.08, 1.3 and 1.45 eV below the bottom of the conduction band to be identified. The results are discussed in terms of the effects of oxygen absorption.     

Two-dimensional resolution of minority carrier diffusion constants in different silicon materials

Hall measurements are a common method to determine the majority charge carrier diffusion constant. But the diffusion constant of the minority carriers D_n, the more interesting parameter in photovoltaics, is rather hard to detect. In this paper we introduce a method to determine D_n locally resolved and mapped in two dimensions. For that purpose the local diffusion length L_diff, which can be calculated from LBIC (laser beam induced current) measurements, has been combined with the local bulk lifetime τ_b received by μ-PCD (microwave-detected photo conductance decay) measurements. We evaluated the diffusion constants of the minority charge carriers D_n for different p-type silicon materials with a resolution of 100 μm. The measurements were carried out on solar cells before and after remote plasma hydrogen passivation in order to get an impression of the diffusion constant dependency on hydrogen incorporation.     

Influence of electrode on electrical conduction of ZincPhthalocyanine (ZnPc) thin films

The electrical conduction properties of ZincPhthalocyanine (ZnPc) thin films have been studied using copper, silver and aluminium electrodes. The sandwich structures were prepared by the thermal evaporation method. The I–V characteristics were investigated to identify the dominant charge transport mechanism in the films. Among all possible mechanisms, it was observed that the data fits well to the SCLC type of conduction in the Al/ZnPc/Al and Schottky type of conduction prevails in the Ag/ZnPc/Ag and Cu/ZnPc/Cu devices. The trap levels and its dependence of structure have been studied and results are discussed. The charge transport phenomenon in the ZnPc films seems to depend highly on the electrode material and temperature. The carrier mobility increases with increasing temperature whereas the density of trapped holes decreases with increasing temperature. The barrier height also decreases with increase in temperature. The influence of the temperature on the electrical parameters such as saturation current density (J_s), barrier height (Φ_b), density of states in the valence band edge N_d (m⁻³), the position of the Fermi level E_F (eV), ionized acceptor atom density N_e (m⁻³), activation energy ΔΦ (eV), mobilities of hole (μ₀) and the concentration of free holes in the valence band (n₀) have been discussed in detail.     

The use of the self-calibration method for accurate determination of silicon solar cell internal quantum efficiency

The application of the self-calibration method as a means of increasing the accuracy of spectral response, SR, and internal quantum efficiency, Q(λ), measurements is discussed. The principle of the method is the precise calculation of Q(λ_m) of a test cell for light at λ_m≈0.8 μm, where the response is weakly dependent on the emitter and base parameters. The ratio of the calculated and measured Q(λ_m) values gives the corresponding factor for shifting the experimental spectral response curve. The sequence of calculations is described, and an algorithm of the necessary operations for a computer is developed. Several examples of the use of the self-calibration method for correction of SR measurements of solar cells with low shunt resistance demonstrate its very high effectiveness. The corrected Q(λ) values follow the respective actual data with very high accuracy even when the measured SR is decreased by factor 2–3 due to low shunt resistance of the solar cell.     

Nanostructure Cu2ZnSnS4 thin film prepared by sol–gel for optoelectronic applications

Thin film of Cu₂ZnSnS₄ (CZTS) has been successfully deposited by sol–gel technique on n-type silicon and glass substrates to fabricate a heterojunction photodiode. The structural properties of the film were investigated by atomic force microscopy. The AFM image of the Cu₂ZnSnS₄ film reveals that the film is a nanostructure material formed from nanoparticles with the particle size of 50–90 nm. The optical band gap, E_g of the Cu₂ZnSnS₄ film was found to be 1.48 eV and the obtained optical band gap suggests that CZTS is very suitable for photovoltaic and optoelectronic applications. The current–voltage characteristics of the Al/n-Si/Cu₂ZnSnS₄/Al diode exhibit a good rectification behavior with ideality factor of 2.84 and barrier height of 0.738 eV. The interface states of the diode were analyzed by series resistance and conductance-voltage methods. The presence of interface states in series resistance–voltage plots was confirmed by the illumination. The interface state density D_it for the diode was found to be 3.63 × 10¹² eV⁻¹ cm⁻². The obtained results indicate that the Al/n-Si/Cu₂ZnSnS₄/Al diode is a photosensor based on controlling of interface states by illumination.     

Temperature dependence of the optical band gap and refractive index of poly(ethylene terepthalate) oligomer–DDQ complex thin film

The temperature dependence of the optical band gap and refractive index dispersion of thin film of poly(ethylene terepthalate) oligomer–DDQ charge transfer complex has been investigated. The absorption edge shifts to the lower energy as consequence of the thermal annealing on film and the fundamental absorption edge corresponds to a direct energy gap. The temperature coefficient of the optical band gap for the film was found as dE_g/dT = − 3.15 × 10⁻³ eV/K. The temperature dependence of the refractive index has also been investigated and it is observed that the refractive index changes by annealing temperatures.     

Optimization of surface charge transfer processes on rutile TiO2 nanorods photoanodes for water splitting

Electrochemical impedance spectroscopy (EIS) has been performed to investigate the photocatalytic properties of titanium dioxide nanorods in a photoelectrochemical water splitting system. A two-channel transmission line model has been proposed to interpret the frequency response of the main charge transfer processes that occur at nanorod/electrolyte and platinum/electrolyte interfaces. EIS was then employed to determine that the dramatic effect of the annealing treatment on the photocurrent density had its origin on a poor charge transfer at the titania/electrolyte interface. X-ray photoelectron spectroscopy and thermogravimetry measurements have been used to prove the relevance of the presence of chlorine coming from the synthesis process of TiO₂ nanorods.     

Vortex structure behind highly heated two cylinders in parallel arrangements

Vortex structures behind twin, highly heated cylinders in parallel arrangements have been investigated experimentally. The experiments were conducted under the following conditions: cylinder diameter, D=4mm; mean flow velocity, U_∞ = 1.0 m/s; Reynolds number, Re=250; cylinder clearance, S/D=0.5 to 1.4; and cylinder heat flux, q=0 to 72.6kW/m². For S/D>1.2, the Karman vortex street is formed alternately behind each cylinder divided on the slit flow. The slit flow velocity increases with a decrease in S/D and decreases with increasing heat flux. For S/D < 1.2, the wake vortexes become asymmetric having small and large scale vortexes divided by the slit flow. In the small scale vortexes, the symmetric counter‐rotating twin vortexes are formed just behind the cylinders. In the large scale vortexes, the generated vortexes have a similar structure to a Karman vortex even though the Strouhal number is approximately half of the ordinary single cylinder vortex. For isothermal conditions, the transition phenomena from symmetric to asymmetric wake structures are observed in the range of 0.9 < S/D <1.2. In addition, the asymmetric vortexes are irregularly switched up and down in the case of isothermal conditions. In the highly heated condition, the switching phenomena and the transition phenomena could not be observed and the small scale vortexes always formed behind the upper cylinder. The critical S/D increases approximately 30% in the heated condition (q=72.6kW/m²). As a result, the increased local kinematic viscosity and S/D play a key role for the vortex structure and formation behind arrangements of two parallel cylinders. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20244