Parametric simulation of the back-surface field effect in the silicon solar cell

Recombination of minority carriers in the solar cell is a major contributing factor in the loss of quantum efficiency and cell power. While the surface recombination is dealt with by depositing a passivation layer of SiO₂ or SiN_x, the bulk recombination is minimized by use of nearly defect-free monocrystalline substrate. In addition, the back-surface field (BSF) effect has been very useful in aiding the separation of free electrons and holes in the bulk. In this study, the key BSF parameters and their effect on the performance of a typical p-type front-lit Si solar cell are investigated by use of Medici, a 2-dimensional device simulator. Of the parameters, the doping concentration of the BSF layer is found to be most significant. That is, for a p-type substrate of 1 × 10¹⁴ cm⁻³ acceptor concentration, the optimum doping concentration of the BSF layer is 1 × 10¹⁸ cm⁻³ or more, and the maximum cell power can be increased by 24%, i.e., 25.4 mW cm⁻² vs. 20.5 mW cm⁻², by using a BSF layer with optimum doping. With regards to the BSF layer thickness, the impact is less. That is, the maximum cell power is about 11% higher at 100 μm than at 5 μm, which translates to an increase of 1.2% μm⁻¹. In practice, therefore, it would be better to rely on the control of the doping concentration than the thickness in maximizing the BSF effect in real Si solar cells.     

Lithium inserted ZnSnN2 thin films for solar absorber: n to p-type conversion

ZnSnN₂ is a non-toxic and earth-abundant photoabsorber material for flexible photovoltaic devices because of its excellent optoelectronic behavior. However, theoretical studies show that the alkaline-earth metallic (Li, Na, K, Rb, Cs, and Fr) dopants in ZnSnN₂, particularly lithium (Li), display shallow-acceptor behavior and improve the performance of ZnSnN₂ semiconductors. Orthorhombic phase structure with (002) preferred orientation was observed for Li-doped films and the lattice parameters agree well with reported standards. Secondary ion mass spectroscopy (SIMS) analysis revealed the incorporation of Li in Li:ZnSnN₂ films. XPS, the density of states, and Born effective charge analysis revealed the chemical bonding states of Li–ZnSnN₂. In contrast to the pristine n-type ZnSnN₂, Li:ZnSnN₂ thin films showed conductivity with p-type hole concentrations varying between 1.14 × 10²⁰–9.47 × 10¹⁹ cm^?3 and the highest mobility of 20.03 cm²V^?1s^?1. Therefore, we obtained p-type conductivity by substituting an organolithium reagent (C?H?Li) on the Zn site, which highlights that Li:ZnSnN₂ can be effectively used as the photoanode layer for next-generation thin-film solar cell devices.     

The effect of polyvinyl alcohol (PVA) on the optical,electrical and solid state properties of copper zinc tin sulfide (CZTS) deposited by sensitive spray pyrolysis (SSP)

Recently, the use of CZTS as the basis for other generation of low cost thin films solar cells has stimulated further researches. Its excellent p-type absorber nature, relatively high absorption coefficient and ideal energy band-gap of 1.5eV motivated these efforts. Additionally, CZTS consist of earth-abundant, cheap and non-toxic elements with very low manufacturing cost. Initially, copper indium gallium selenide (CIGS) solar cell device emerged but suffered limitations in further development because of rare indium and gallium in the device structure therefore, CZTS is recently preferred as an alternative to CIGS commercial solar cell absorber layer. In this work, solution mixture of CZTS and PVA was deposited on a substrate at temperature of 150 °C. Sensitive spray pyrolysis was used to grow the thin films where calculated amount of the precursor mixture was allowed to fall and be deposited on a heated substrate to form CZTS/PVA thin films. Subsequently, the thin film samples were annealed at a temperature of 200^oCfor 1 h to achieving pure crystalline thin film formation. SEM, XRD analysis, Optical, Solid State properties and Raman analysis were studied. The XRD analysis showed that the thin films fell into the pure kesterite structure of CZTS. Results show that produced thin films exhibited higher absorption coefficient and optical conductivity than pure CZTS, 10⁶ m^?1 and 10¹⁴(S^?1) against 10⁴cm^?1 and 10¹²(S^?1) respectively. The band-gap is between 1.53eV and 1.73eV. Using a PVA concentration of 0.05 M yielded highest absorbance and optical conductivity with lowest real dielectric constant and transmittance. These improved optical, electrical and solid state properties suitably qualify these thin films as absorber layer material for solar cell applications.     

A highly uniform CuO@SiO2 porous sphere with improved electrochemical sensing performance for the accurate determination of vanillin in food samples

A porous sphere of CuO@SiO₂ was obtained by simple calcination of the copper silicate (CuSiO₃) sphere. The formation of the porous sphere was studied in detail with the support of various physical characterization techniques. The CuO@SiO₂ was coated on an electrode surface where it demonstrates high catalytic activity for the electro-oxidation of vanillin in phosphate buffer solution (PBS; pH 7.0). The modified electrode has good surface adsorption (1.861 × 10^?10 mol cm^?2) and rate constant (2.866 × 10⁵ cm³ mol^?1 s^?1) characteristics for vanillin detection. The CuO@SiO₂-modified surface exhibited good linear range (0.05 μM–1.2 μM and 6.2 to 111.2 μM), detection limit (53 nM), sensitivity (2.88 μA μM^?1 cm^?2), selectivity, stability, and reproducibility. The CuO@SiO₂-modified electrode was further examined for the determination of vanillin in real samples including biscuits and chocolates with satisfactory recoveries.     

High sensitivity Schottky junction diode based on monolithically grown aligned polypyrrole nanofibers: Broad range detection of m-dihydroxybenzene

Aligned p-type polypyrrole (PPy) nanofibers (NFs) thin film was grown on n-type silicon (100) substrate by an electrochemical technique to fabricate Schottky junction diode for the efficient detection of m-dihydroxybenzene chemical. The highly dense and well aligned PPy NFs with the average diameter (∼150–200 nm) were grown on n-type Si substrate. The formation of aligned PPy NFs was confirmed by elucidating the structural, compositional and the optical properties. The electrochemical behavior of the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode was evaluated by cyclovoltametry (CV) and current (I)-voltage (V) measurements with the variation of m-dihydroxybenzene concentration in the phosphate buffer solution (PBS). The fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode exhibited the rectifying behavior of I–V curve with the addition of m-dihydroxybenzene chemical, while a weak rectifying I–V behavior was observed without m-dihydroxybenzene chemical. This non-linear I–V behavior suggested the formation of Schottky barrier at the interface of Pt layer and p-aligned PPy NFs/n-silicon thin film layer. By analyzing the I–V characteristics, the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode displayed reasonably high sensitivity ∼23.67 μAmM⁻¹cm⁻², good detection limit of ∼1.51 mM with correlation coefficient (R) of ∼0.9966 and short response time (10 s).     

A new photoanode architecture of dye sensitized solar cell based on ZnO nanotetrapods with no need for calcination

We introduce a photoanode architecture in dye sensitized solar cell comprising building blocks of ZnO nanotetrapods with a mean arm diameter of 40 nm and arm lengths of 500–800 nm. This photoanode features a decent roughness factor up to 400, good network forming ability and limited electron-hopping interjunctions. Even without calcination, a power conversion efficiency up to 3.27% (under 100 mW cm^?2) has been achieved at a film thickness of 31.2 μm. The avoidance of the calcination step is an outstanding feat for flexible solar cells. We have also employed impedance spectroscopy to interpret the solar cell performance features.     

Sputtering of bismuth thin films under MeV Cu heavy ion irradiation: Experimental data and inelastic thermal spike model interpretation.

The sputtering of bismuth (Bi/Si) thin films deposited onto silicon substrates and irradiated by swift Cu^q+ heavy ions (q = +4 to +7) was investigated by varying both the ion energy over the 10 to 26‐MeV range and the ion fluence ϕ from 5.1 × 10¹³ cm⁻² to 3.4 × 10¹⁵ cm⁻². The sputtering yields were determined experimentally via the Rutherford backscattering spectrometry technique using a 2‐MeV He⁺ ion beam. The measured sputtering yields versus Cu⁷⁺ ion fluence for a fixed incident energy of 26 MeV exhibit a significant depression at very low ϕ‐values flowed by a steady‐state regime above ~1.6 × 10¹⁴ cm⁻², similarly to those previously pointed out for Bi thin films irradiated by MeV heavy ions. By fixing the incident ion fluence to a mean value of ~2.6 × 10¹⁵ cm⁻² in the upper part of the yield saturation regime, the measured sputtering yield data versus ion energy were found to increase with increasing the electronic stopping power in the Bi target material. Their comparison to theoretical predicted models is discussed. A good agreement is observed between the measured sputtering yields and the predicted ones when considering the contribution of 2 competitive processes of nuclear and electronic energy losses via, respectively, the SRIM simulation code and the inelastic thermal spike model using refined parameters of the ion slowing down with reduced thermophysical proprieties of the Bi thin films.     

Electrochemical solar cells with two photoactive electrodes

One of the major goals of solar energy utilization is the photolysis of water. Energetic requirements make electrolysis cells driven by two-photon processes more efficient in the solar spectrum. We report a new design of this type: n-type SnO₂ made light-sensitive by dye Victoria Blue B (VBB) on the photoanode and meso-tetraphenylporphyrin (TPP) on the photocathode. Characteristics are photopotential more than 1 V and photocurrent 100 μA (while light irradiation 100 mW cm^?2), and quantum efficiency greater than 1%. Of special interest is the mechanism that takes place at the pigmented SnO₂ electrodes. The VBB layer is considered as an n-type and TPP as a p-type organic semiconductor. The results are discussed in terms of a Schottky barrier model.     

Effect of sintering temperature and thermoelectric power studies of the system MgFe2−xCrxO4

Mixed metal oxides showing the spinel structure exhibit interesting structural and electrical properties. Substances with specific compositions in the system MgFe_2?xCr_xO₄ were synthesized by the simple co-precipitation method and have been investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM) to study the effect of temperature on the size of particles and grains. The infrared spectrum shows, two strong bands around 600 and 500 cm^?1. An elemental composition of one of the samples, MgFeCrO₄ was found by energy dispersive X-ray spectroscopy (EDS). The thermoelectric power measurements carried out from room temperature to 500 °C, show both n-type and p-type behavior.     

Performance of plasma sputtered fuel cell electrodes with ultra-low Pt loadings

Ultra-low Pt content PEMFC electrodes have been manufactured using magnetron co-sputtering of carbon and platinum on a commercial E-Tek^® uncatalyzed gas diffusion layer in plasma fuel cell deposition devices. Pt loadings of 0.16 and 0.01 mg cm^?2 have been realized. The Pt catalyst is dispersed as small clusters with size less than 2 nm over a depth of 500 nm. PEMFC test with symmetric electrodes loaded with 10 μg cm^?2 led to maximum reproducible power densities as high as 0.4 and 0.17 W cm^?2 with Nafion^®212 and Nafion^®115 membranes, respectively.     

Ultrahigh sensitivity SIMS analysis of oxygen in silicon

We report on the detection of very low oxygen concentration in silicon by a secondary‐ion mass spectrometry (SIMS) method. Using a magnetic IMS 6F Cameca SIMS spectrometer and applying a very high primary Cs⁺ ion flux, prolonged presputtering, extensive vacuum chamber baking, titanium sublimation pump, and an LN trap, we have reached a detection limit of ~2 × 10¹⁵ O atoms/cm³ in chemical vapor deposition epitaxial Si films. This value appears to be at least 10 times lower than in any published or unpublished source known to the authors, including the reference sensitivities listed by the instrument manufacturer. Most likely, the key improvement that has allowed us to drive the detection limit to 10¹⁵ at/cm³ is the use of an ion pump in the analysis chamber. The working pressure in our analysis chamber is ~10⁻¹⁰ Thorr, ie, 1 decade lower than that the commercially equipped with a turbo pump. This paper demonstrates optimized analytical conditions for the oxygen measurements in Si, as a function of depth: (i) Very shallow profiles are practically impossible to measure accurately because of native oxide at the surface. (ii) Shallow‐to‐medium range profiles, up to ~20 μm, are the most amenable to SIMS measurements. (iii) Medium‐to‐deep (~20‐50 μm) range is required to follow interdiffusion and segregation in epitaxial layers when the oxygen‐free layer is grown on a CZ Si substrate. (iv) Extremely deep profiles, up to full thickness of the wafer, definitely necessitate beveling.     

Achievement of highly conductive p-type transparent NdCuOS film with Cu deficiency and effective doping

Future advanced invisible or transparent electronics necessitate the need to overcome the well-known challenge in achieving high-performance p-type transparent semiconducting oxides (TSOs). Here, we report our success in achieving an outstanding p-type TSO thin film NdCuOS, which is the best performing p-type TSO reported to date based on the figure of merit (FoM) according to the best of our knowledge. In this work, we designed a novel chemical solution method to prepare the highly performing NdCuOS films with different doping elements. Our success in using a chemical solution method to grow semiconducting NdCuOS demonstrates that highly conductive oxychalcogenide films are possible to be prepared by a solution method. Such a solution method is facile, economically efficient, and scalable. Among our NdCuOS films with different dopants, we find that Mg-doped NdCuOS film demonstrates a very high p-type conductivity of 52.1 S cm^?1 and optical transmittance of 54.3% with a huge FoM value of 1706 μS. This surpassed all the other p-type films reported so far in terms of FoM. Strong photoluminescence peaks at 3.0 eV are observed for our films, indicating their great potential applications for UV or blue light LED and other devices. The science behind such a successful achievement of high-performance p-type NdCuOS film is analyzed and discussed. A transparent p-n diode with very low leakage current (9.12  μA at ?3 V) and turn-on voltage (1.1 V) is successfully fabricated, and it demonstrates a good device performance.     

Experimental investigation of first and second laws in a direct absorption solar collector using hybrid Fe3O4/SiO2 nanofluid

In this study, energy and entropy analysis of a residential-type direct absorption solar collector using hybrid Fe₃O₄/SiO₂ nanofluid is evaluated experimentally. The hybrid nanofluid samples are prepared in the different volume ratios of Fe₃O₄/SiO₂ (25:75, 50:50 and 75:25) and different volume fractions (500 ppm, 1000 ppm and 2000 ppm). The appropriate nanofluid samples for using as the working fluid of the collector are chosen based on the results of stability and optical properties of nanofluid. Then, outdoor thermal performance of collector is investigated using the experimental setup based on EN12975-2. Measurement of nanofluid optical properties using the spectrophotometry method shows that the extinction coefficient of 2000 ppm hybrid Fe₃O₄/SiO₂ nanofluid is on average 10 cm^?1 higher than that of the base fluid. Results of energy analysis display that the collector efficiency is increased by mass flow rate and volume fraction of nanofluid asymptotically. The asymptotic value is about 83% for 2000 ppm hybrid Fe₃O₄/SiO₂ nanofluid. The findings indicate that the variation of exergy efficiency of a direct absorption solar collector with the volume fraction and mass flow rate is similar to energy efficiency. The enhancement of exergy efficiency is 66.4% for mass flow rates of 0.0225 kg s^?1 by increasing the volume fraction from 0 to 2000 ppm. It is also observed that dimensionless entropy generation number is decreased by nanofluid volume fraction and by mass flow rate. The lowest entropy generation number is obtained in the mass flow rate of 0.0225 kg s^?1 and the volume fraction of 2000 ppm. The variation of Bejan number by volume fraction shows that the contribution of pressure drop in entropy generation is insignificant.

     

Towards sustainable materials for solar energy conversion: Preparation and photoelectrochemical characterization of Cu2ZnSnS4

The feasibility of a new fabrication route for films of the attractive solar absorber Cu₂ZnSnS₄ (CZTS) has been studied, consisting of electrodeposition of metallic precursors followed by annealing in sulfur vapour. Photoelectrochemical measurements using a Eu³⁺ contact have been used to establish that the polycrystalline CZTS films are p-type with doping densities in the range (0.5–5) × 10¹⁶ cm⁻³ and band gaps of 1.49 ± 0.01 eV, making them suitable for terrestrial solar energy conversion. It has been shown that a somewhat Cu-poor composition favours good optoelectronic properties.     

Design and simulation of neutron radiography system based on 241Am–Be source

Neutron imaging is extended rapidly as a means of non-destructive testing (NDT) of materials. Various effective parameters on the image quality are needed to be studied for neutron radiography system with good resolution. In the present study a portable system of neutron radiography has been designed using ²⁴¹Am–Be neutron source. The effective collimator parameters were calculated to obtain relatively pure, collimated and uniform neutron beam. All simulations were carried out in two stages using MCNPX Monte Carlo code. In the first stage, different collimator configurations were investigated and the appropriate design was selected based on maximum intensity and uniformity of neutron flux at the image plane in the outlet of collimator. Then, the overall system including source, collimator and sample was simulated for achieving radiographic images of standard samples. Normalized thermal neutron fluence of 2.61×10^?5 cm^?2 per source particle with n/γ ratio of 1.92×10⁵ cm^?2 μSv^?1 could be obtained at beam port of the designed collimator. Quality of images was assessed for two standard samples, using radiographic imaging capability in MCNPX. The collimated neutron beam in the designed system could be useful in a transportable exposure module for neutron radiography application.     

Functionalized organic dyes containing a phenanthroimidazole donor for dye-sensitized solar cell applications

Five functionalized organic dyes (H6-10) containing a phenanthroimidazole unit as an electron donor were synthesized and characterized for use in dye-sensitized solar cell (DSSC) applications. Under standard global AM 1.5 solar conditions, the DSSCs based on dye H6 displayed the best performance, with an incident photon-to-current conversion efficiency (IPCE) exceeding 70% at wavelengths of 400–530 nm, a short-circuit photocurrent density of 10.98 mA cm^?2, an open-circuit voltage of 0.68 V, a fill factor of 0.69, and an overall conversion efficiency of 5.12%. This efficiency is ～94% of that for JK2 cells (5.46%) and ～72% of that for N719 cells (7.07%) under the same conditions.     

P-type silicon nanowire-based nano-floating gate memory with Au nanoparticles embedded in Al2O3 gate layers

P-type Si nanowire (NW)-based nano-floating gate memory (NFGM) with Au nanoparticles (NPs) embedded in Al₂O₃ gate layers is characterized in this study. The electrical characteristics of a representative p-type Si NW-based NFGM exhibit a counterclockwise hysteresis loop indicating the trapping and detrapping of electrons in the Au NP nodes of the NFGM device. The threshold voltage shift of the device is 5.4 V and the device has good retention over a lapse of time of 5 × 10⁴ s. On the other hand, the p-type Si NW-based top-gate device without any Au NPs does not exhibit any significant threshold voltage shift. This observation reveals that the memory behavior of the p-type Si NW-based NFGM is due to the trapping and detrapping of charge carriers in the Au NPs.     

Evolution of microstructure and crack pattern in NiO thin films under 200 MeV Au ion irradiation

NiO thin films grown on Si (100) substrate by electron beam evaporation method and sintered at 700 °C were irradiated with 200 MeV Au¹⁵⁺ ions. The fcc structure of the sintered films was retained up to the highest fluence (1×10¹³ ions cm^?2) of irradiation. However the microstructure of the pristine film underwent a considerable modification with increasing ion fluence. 200 MeV Au ion irradiation led to compressive stress generation in NiO medium. The diameter of the stressed region created by 200 MeV Au ions along the ion path was estimated from the variation of stress with ion fluence and found to be ～11.6 nm. The film surface started cracking when irradiated at and above the fluence of 3×10¹² ions cm^?2. Ratio of the fractal dimension of the cracked surface obtained at 200 MeV and 120 MeV (Mallick et al., 2010a) Au ions was compared with the ratio of the radii of ion tracks calculated based on Coulomb explosion and thermal spike models. This comparison indicated applicability of thermal spike model for crack formation.     

Effect of 120 MeV Au9+ ion irradiation on the structure and surface morphology of ZnO/NiO heterojunction

ZnO/NiO thin films, each of thickness 100 nm, were deposited on Si(100) substrate by pulsed laser deposition method. The resulting heterojunction, ZnO/NiO/Si, was irradiated by 120 MeV Au⁹⁺ ions and characterized by grazing incidence X‐ray diffraction (GIXRD), Raman spectroscopy, and atomic force microscopy (AFM). The GIXRD confirmed the presence of both NiO and ZnO in the samples. Ion irradiation induced suppression of crystalline nature, and the recrystallization of the same occurred at the fluence of 1 × 10¹³ ions cm⁻². The occurrence of most intense band at 302 cm⁻¹ in Raman spectra corresponds to the symmetric stretching vibration of ZnO. The linear shift of stretching mode of ZnO with ion fluence could be associated with the effect of compressive stress in the material. AFM analysis of the films indicated that the rms roughness increased when the film is irradiated at a fluence of 1 × 10¹² ions cm⁻². Beyond this fluence, the value of roughness decreased up to fluence of 1 × 10¹³ ions cm⁻² and increased thereafter. To see the effect of the stress of buffer layer on the surface layer, we calculated the stress for NiO layer with ion fluence form the lattice parameter. Comparing the stress of buffer layer with roughness of surface layer at the given fluence, we can say that the compressive stress in the buffer layer could possibly control the roughness of the surface layer.     

Silaindacenodithiophene‐Based Fused‐Ring Non‐Fullerene Electron Acceptor for Efficient Polymer Solar Cells

In this work, a new A‐D‐A type nonfullerene small molecular acceptor SiIDT‐IC, with a fused‐ring silaindacenodithiophene (SiIDT) as D unit and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (INCN) as the end A unit, was design and synthesized. The SiIDT‐IC film shows absorption peak and edge at 695 and 733 nm, respectively. The HOMO and LUMO of SiIDT‐IC are of −5.47 and −3.78 eV, respectively. Compared with carbon‐bridging, the Si‐bridging can result in an upper‐lying LUMO level of an acceptor, which is benefit to achieve a higher open‐circuit voltage in polymer solar cells (PSCs). Complementary absorption and suitable energy level alignment between SiIDT‐IC and wide bandgap polymer donor PBDB‐T were found. For the PBDB‐T:SiIDT‐IC based inverted PSCs, a D/A ratio of 1: 1 was optimal to achieve a power conversion efficiency (PCE) of 7.27%. With thermal annealing (TA) of the blend film, a higher PCE of 8.16% could be realized due to increasing of both short‐circuit current density and fill factor. After the TA treatment, hole and electron mobilities were elevated to 3.42 × 10⁻⁴ and 1.02 × 10⁻⁴ cm²·V⁻¹·s⁻¹, respectively. The results suggest that the SiIDT, a Si‐bridged fused ring, is a valuable D unit to construct efficient nonfullerene acceptors for PSCs.