A novel Al and Y codoped ZnO/n-Si heterojunction solar cells fabricated by pulsed laser deposition

Al and Y codoped ZnO (AZOY) transparent conducting oxide (TCO) thin films were first deposited on n-Si substrates by pulsed laser deposition (PLD) to form AZOY/n-Si heterojunction solar cells. However, the properties of the AZOY emitter layers are critical to the performance of AZOY/n-Si heterojunction solar cells. To estimate the properties of AZOY thin films, films deposited on glass substrates with various substrate temperatures (T_s) were analyzed. Based on the experimental results, optimal electrical properties (resistivity of 2.8 ± 0.14 × 10^?4 Ω cm, carrier mobility of 27.5 ± 0.55 cm²/Vs, and carrier concentration of 8.0 ± 0.24 × 10²⁰ cm^?3) of the AZOY thin films can be achieved at a T_s of 400 °C, and a high optical transmittance of AZOY is estimated to be >80% (with glass substrate) in the visible region under the same T_s. For the AZOY/n-Si heterojunction solar cells, the AZOY thin films acted not only as an emitter layer material, but also as an anti-reflected coating thin film. Thus, a notably high short-circuit current density (J_sc) of 31.51 ± 0.186 mA/cm² was achieved for the AZOY/n-Si heterojunction solar cells. Under an AM1.5 illumination condition, the conversion efficiency of the cells is estimated at only approximately 4% (a very low open-circuit voltage (V_oc) of 0.24 ± 0.001 V and a fill factor (FF) of 0.51 ± 0.011) without any optimization of the device structure.     

Polymer impinged CdS/CuInSe2 solar cell

In the present paper we report, effect of conjugated polymer (polyaniline) impinging in nanostructured CdS/CuInSe₂ heterojunction thin film solar cell. The heterojunction architecture for the solar cell is achieved by sandwiching the conjugated conducting polymer in n and p type of wide band gap semiconducting material by multilayer chemical deposition methods onto the ITO coated glass substrate at room temperature. The obtained multilayer thin film heterojunction of ITO/CdS/Polymer/CuInSe₂/Ag has been characterized for structural, compositional, optical and solar cell characteristics by illuminating it to 100 mW/cm² intensity light source. The X-ray diffraction pattern (XRD) confirms formation of CdS/CuInSe₂ phase while on polymer impinging the crystallite size observed to be increased from 13 to 19 nm. The compositional analysis by energy dispersive X-ray spectra (EDAX) supports presence of expected elements in the heterojunction. The energy band gap calculated from absorbance spectra shows significant shift in its value from polymer and CdS/CuInSe₂ band gap. I–V analysis shows increase in conversion efficiency from 0.26 in CdS/CuInSe₂ to 0.55% in CdS/Polymer/CuInSe₂ heterojunction upon illumination.     

Twenty-two percent efficiency HIT solar cell

We have achieved the world's highest solar cell conversion efficiency of 22.3% (V_oc: 0.725 V, I_sc: 3.909 A, FF: 0.791, total area: 100.5 cm², confirmed by AIST) by using a heterojunction with intrinsic thin layer (HIT) structure. This is the world's first practical-size (>100 cm²) silicon solar cell that exceeds a conversion efficiency of 22% as a confirmed value. This high efficiency has been achieved mainly due to improvements in a-Si:H/c-Si hetero-interface properties and optical confinement.The excellent a-Si:H/c-Si hetero-interface of the HIT structure enables a high V_oc of over 0.720 V and results in better temperature properties. In order to reduce the power-generating cost, we are now investigating numerous technologies to further improve the conversion efficiency, especially the V_oc, of HIT solar cells, with the aim of achieving 23% efficiency in the laboratory by 2010.     

Fabrication of nc-Si/c-Si solar cells using hot-wire chemical vapor deposition and laser annealing

In this paper, we present the performance of Si heterojunction solar cells prepared by hot-wire chemical vapor deposition and laser annealing. Under high hydrogen-dilution-ratio conditions, the crystallinity of the phosphorous-doped emitter layers was greatly improved due to hydrogen-induced crystallization. The grain boundary defects of the nano-crystalline emitter layer were further promoted using a laser (355 nm) crystallization technique. It was found that both the short-circuit current density and fill factor of the Si heterojunction solar cells were mainly dependent on the energy density of the laser beam. An efficiency of 14.2% is achieved for the n-nc-Si/p-c-Si heterojunction solar cell under a laser irradiation density of 382 mW/cm².     

Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

n-Type silicon quantum dots and p-type crystalline silicon heteroface solar cells

Heteroface devices have been realized by depositing phosphorus-doped silicon (Si) quantum dots (QDs) (n-type) on a p-type crystalline silicon substrate. To compare the quantum confinement effect, different sizes (3, 4, 5, and 8±1 nm) of Si QD were fabricated, whose optical energy bandgaps are in the ranges of 1.3–1.65 eV. The electrical and photovoltaic properties of heterojunction devices were characterized by illuminated and dark I–V measurements, C–V measurements, and spectral response measurements. The diodes showed a good rectification ratio of 5×10⁶ for 4 nm Si QDs at the bias voltage of ±1.0 V at 298 K. The ideality factor and junction built-in potential deduced from current–voltage (I–V) and capacitance–voltage (C–V) plots are 1.86 and 0.847 V for 3 nm QD device, respectively. From the illuminated IV characteristics, the open circuit voltages were 556, 540, 512, and 470 mV for mean QD diameters 3, 4, 5, and 8±1 nm, respectively. Temperature-dependant dark I–V measurements suggest that the carrier transport in the devices is controlled by recombination in the space-charge region. This study indicates the silicon QDs can be good candidates for all-silicon tandem solar cells.     

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency

The p-type a-Si:H/n-type c-Si (P⁺ a-Si:H/N⁺ c-Si) heterojunction was simulated for developing solar cells with high conversion efficiency and low cost. The characteristic of such cells with different work function of transparent conductive oxide (TCO) were calculated. The energy band structure, quantum efficiency and electric field are analyzed in detail to understand the mechanism of the heterojunction cell. Our results show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work function, and the TCO work function should be large enough in order to achieve high conversion efficiency of P⁺ a-Si:H/N⁺ c-Si solar cells. With the optimized parameters set, the P⁺ a-Si:H/N⁺ c-Si solar cell reaches a high efficiency (η) up to 21.849% (FF: 0.866, V_OC: 0.861 V, J_SC: 29.32 mA/cm²).     

Optimization of lead- and cadmium-free front contact silver paste formulation to achieve high fill factors for industrial screen-printed Si solar cells

Screen-printed n⁺–p–p⁺ solar cells were fabricated on Cz single crystalline Si material, with a 45 Ω/sq emitter and PECVD SiNx antireflective coating with a thickness of 700 Å, using different Ag pastes and commercial leaded reference paste (CN33-462, Ferro Corp.). Ag and Al contacts were co-fired using a mass-production line equipped with mesh belt conveyer furnace systems (Centrotherm thermal solution GmbH & Co. KG). The average results for single crystalline Si solar cells (156 cm²) are: I_sc=5.043 A, V_oc=0.621 V, R_s=0.0087 Ω, R_sh=15.3 Ω, FF=0.773, and E_ff=16.45%. R_sh and fill factor values of fabricated cells were slightly higher when compared with the commercial leaded Ag paste, although cells were fabricated by metallizing the lead-free silver pastes. For the lead-free Ag paste used in this study, the line pattern continuity is retained with improved edge definition in sharp contrast to that of reference Ag paste. Average value of R_s was also equivalent approximately to that of the leaded Ag paste.     

Degradation modeling of InGaP/GaAs/Ge triple-junction solar cells irradiated with various-energy protons

Degradation modeling of InGaP/GaAs/Ge triple-junction (3J) solar cells subjected to proton irradiation is performed with the use of a one-dimensional optical device simulator, PC1D. By fitting the external quantum efficiencies of 3J solar cells degraded by 30 keV, 150 keV, 3 MeV, or 10 MeV protons, the short-circuit currents (I_SC) and open-circuit voltages (V_OC) are simulated. The damage coefficients of minority carrier diffusion length (K_L) and the carrier removal rate of base carrier concentration (R_C) of each sub-cell are also estimated. The values of I_SC and V_OC obtained from the calculations show good agreement with experimental values at an accuracy of 5%. These results confirm that the degradation modeling method developed in this study is effective for the lifetime prediction of 3J solar cells.     

Hybrid solar cells of layer-by-layer thin films with a polymer/fullerene bulk heterojunction

A hybrid organic solar cell was fabricated by integrating a layer-by-layer (LbL) ultrathin polymer film with a polymer/fullerene bulk heterojunction film. The short-circuit current density (J_SC) was significantly improved by a factor of five compared to that observed for LbL-based solar cells. The open-circuit voltage (V_OC) remained at >0.7 V, which is higher than that reported for bulk heterojunction solar cells. These findings suggest that the LbL thin film can not only enhance J_SC but also tune V_OC. An optimized hybrid polymer solar cell showed the power conversion efficiency to be as high as ～1%.     

CdS1−yTey thin films: Production and bandgap investigation

In CdS/CdTe solar cells it is necessary to determine the efficiency limitations related with the intermixing at the interface between the CdS window layer and CdTe absorber layer. So understanding the properties of the solid solution (CdS_xTe_1?x on the CdTe side which is CdTe rich and CdS_1?yTe_y on the CdS side which is CdS rich) that is always formed in this region is essential. We produced thin films of CdS_1?yTe_y solid solution-which is CdS rich – by first producing CdS:In thin films on glass substrates by the spray pyrolysis technique and then annealing the films in nitrogen atmosphere at 400 °C in the presence of Te vapor. We are the first who produce this solid solution by this simple and low cost method. The composition and morphology of the films were determined by energy dispersive X-ray detection (EDAX) measurements and scanning electron microscopy (SEM) observations respectively. Eight values of y in the range 0 ? y ? 0.2845 were obtained. The transmittance was measured and used to investigate the optical bandgap energy by using the second derivative of the absorbance. It is found that the films show a single hexagonal phase for y ? 0.0852 and then a mixed (hexagonal and cubic) phase for 0.0997 ? y ? 0.2845. Bandgap energies in the range 2.259 ? E_g ? 2.528 eV were obtained. Urbach tailing in the bandgap was also investigated.     

Maximum-power-point tracking control of solar heating system

The present study developed a maximum-power point tracking control (MPPT) technology for solar heating system to minimize the pumping power consumption at an optimal heat collection. The net solar energy gain Q_net (=Q_s ? W_p/η_e) was experimentally found to be the cost function for MPPT with maximum point. The feedback tracking control system was developed to track the optimal Q_net (denoted Q_max). A tracking filter which was derived from the thermal analytical model of the solar heating system was used to determine the instantaneous tracking target Q_max(t). The system transfer-function model of solar heating system was also derived experimentally using a step response test and used in the design of tracking feedback control system. The PI controller was designed for a tracking target Q_max(t) with a quadratic time function. The MPPT control system was implemented using a microprocessor-based controller and the test results show good tracking performance with small tracking errors. It is seen that the average mass flow rate for the specific test periods in five different days is between 18.1 and 22.9 kg/min with average pumping power between 77 and 140 W, which is greatly reduced as compared to the standard flow rate at 31 kg/min and pumping power 450 W which is based on the flow rate 0.02 kg/s m² defined in the ANSI/ASHRAE 93-1986 Standard and the total collector area 25.9 m². The average net solar heat collected Q_net is between 8.62 and 14.1 kW depending on weather condition. The MPPT control of solar heating system has been verified to be able to minimize the pumping energy consumption with optimal solar heat collection.     

Organic thin-film solar cell employing a novel electron-donor material

A highly efficient organic thin-film solar cell based on a heterojunction structure employing a novel electron-donor (ED) material, tetraphenyldibenzoperiflanthene (DBP), has been demonstrated for the first time. An organic photovoltaic (OPV) cell with 0.033-cm² active area, comprising DBP as an ED layer, fullerene C₆₀ as an electron-acceptor (EA) layer, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as an exciton-blocking (EB) layer, has exhibited an open-circuit voltage (V_oc) of 0.92 V, a short-circuit current density (J_sc) of 6.3 mA/cm² and a conversion efficiency of 3.6% at 100-mW/cm² simulated AM1.5G sunlight. Meanwhile, those of a conventional cell employing copper phthalocyanine (CuPc) for an ED layer are 0.51 V, 4.3 mA/cm², and 1.4%, respectively. The high V_oc and J_sc of the DBP-based cell is attributed to the DBP's highest occupied molecular orbital (HOMO) level 5.5 eV and the effective light absorption, respectively.     

Synthesis and decomposition reactions of metal amides in metal–N–H hydrogen storage system

The synthesis and decomposition properties of some metal amides M(NH₂)_x such as LiNH₂, NaNH₂, Mg(NH₂)₂ and Ca(NH₂)₂ were investigated, which play important roles for designing a new family of metal–N–H hydrogen storage systems. Both the gas chromatographic examination and X-ray diffraction measurement indicated that the reaction between alkali or alkaline earth metal hydride MH_x (such as LiH, NaH, MgH₂ and CaH₂) and gaseous NH₃ could quickly proceed at room temperature by ball milling and the corresponding metal amides were easily synthesized in high quality. The kinetics of these kind of reactions is faster in the order of NaH > LiH > CaH₂ > MgH₂, which is consistent with the inverse order of electronegativity of those metals, i.e. Na < Li = Ca < Mg. The thermal decomposition properties indicated that both Mg(NH₂)₂ and Ca(NH₂)₂ decomposed and emitted NH₃ at lower temperature than LiNH₂.     

Optoelectronic properties of chemically deposited Bi2S3 thin films and the photovoltaic performance of Bi2S3/P3OT solar cells

Thin films of bismuth sulfide (Bi₂S₃), prepared on conductive tin-doped indium oxide (ITO)-glass substrates by chemical deposition showed a variation of optical band gap with thickness: 1.8 eV for a 50 nm film (deposited at 40 °C for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi₂S₃ compound is 5.3 eV, as estimated from the ionization energy and electron affinity of elemental Bi and S, and thus the electron affinity of chemically deposited Bi₂S₃ film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi₂S₃/P3OT/Au structure, Bi₂S₃ was established as an electron acceptor. To produce ITO/Bi₂S₃/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi₂S₃ film and a gold film was thermally evaporated. Under 100 mW/cm² tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi₂S₃ film with thickness of 50 nm has the highest open circuit voltage (V_oc) of 440 mV and short-circuit current density (J_sc) of 0.022 mA/cm². The addition of a CdS thin film (90 nm) between ITO and B₂S₃ would increase V_oc to 480 mV, and J_sc to 0.035 mA/cm². The role of surface morphology and optoelectronic properties of the Bi₂S₃ film in the photovoltaic performance of the junction is discussed.     

Design,preparation, and durability of TiO2/SiO2 and ZrO2/SiO2 double-layer antireflective coatings in crystalline silicon solar modules

This paper reports the use of a combination of numerical calculations and experimental work to establish the optimum photovoltaic transmittance (T_pv) and durability of the quarter wave, the quarter-half wave, and the non-quarter wave double-layer TiO₂–SiO₂ and ZrO₂–SiO₂ antireflective coatings (ARCs) on solar glass towards practical photovoltaic applications. Numerical calculations based on 4 × 4 propagation matrix method indicated that the non-quarter wave double-layer ARCs exhibited higher T_pv values than those of the quarter wave and the quarter-half wave ARCs. Such calculated values are in good agreement with the experimental T_pv values. For examples, the T_pv values for the non-quarter wave double-layer TiO₂–SiO₂ and ZrO₂–SiO₂ ARCs prepared by sol–gel reached 94.4 ± 0.1% and 94.3 ± 0.1%, respectively. In terms of the coating durability, the non-quarter wave double-layer coatings with a dense and thicker TiO₂ or ZrO₂ barrier layer on solar glass exhibited less than 1% reduction in T_pv after 96 h highly-accelerated temperature and humidity stress test (HAST), as compared with the standard single-layer porous SiO₂ used in industry which tested in the same HAST conditions to be greater than (15.4%) after 48 h. Single crystalline Si modules encapsulated by the non-quarter wave TiO₂–SiO₂ or ZrO₂–SiO₂ AR-coated glass are more durable, with only less than 10% degradation in efficiency after 48 h HAST, as compared with Si modules encapsulated by single-layer porous SiO₂ AR-coated glass which have signification loss in efficiency (circa. 21.8%).     

Optimisation of NaOH texturisation process of silicon wafers for heterojunction solar-cells applications

The formation of a pyramidal structure on the surface of 〈1 0 0〉-oriented monocrystalline-silicon wafers is an effective and well known method to reduce reflection losses from the front surface of both silicon solar cells and silicon-heterojunction solar cells (SHJs). The consequence of this texturisation is an important optical gain, with a subsequent increase of the short-circuit current density (J_sc) and thus of the conversion efficiency of the devices. On the other hand, silicon-heterojunction solar cells are critically affected by the surface quality of the c-Si substrates, so the right combination of optimum texturisation- and cleaning steps previous to emitter (a-Si:H) deposition are indispensable in the fabrication process. The main goal of this work has been to perform a systematic and comprehensive analysis aimed at optimising the texturisation process based on the use of alkali solutions of NaOH with de-ionised water (DIW) and isopropyl alcohol (IPA) in different types of monocrystalline-silicon wafers for silicon-heterojunction solar-cell (a-Si:H/c-Si) applications. Three types of 〈1 0 0〉 silicon substrates have been used: polished float-zone (FZ) wafers and rough- (as-cut) and polished Czochralski (CZ) wafers. The texturisation process has been evaluated from images obtained by Scanning Electron Microscopy (SEM) and from hemispherical-reflectance spectra. Different etching times, temperatures and NaOH concentrations of the solutions as well as cleaning treatments of the wafers prior to the texturisation process have been analysed. Results show different conditions of the optimum texturisation process for each type of silicon wafers. An effective texturisation of FZ and CZ substrates has been achieved. Finally, SHJ solar cells have been obtained from FZ and CZ silicon wafers textured by the chemical processes optimised in this work.     

Ebeam fabrication of silicon nanodome photovoltaic devices without metal catalyst contamination

In this paper, the fabrication of silicon nanodome solar cells on crystalline wafers is reported. Crystalline silicon was patterned by ebeam lithography to define the silicon nano pillars with diameter of 100 nm, 1 μm and 5 μm. Unlike conventional bottom up growth of silicon nanowire from gold (Au), our method is free from contaminant. Consequently, it is a valuable method to fully evaluate the effect of nanostructures on solar cell performances. The fabricated devices were characterized through scanning electron microscopy, absorption measurements, illuminated solar cell I–V characteristics and monochromatic incident photon-to-electron conversion efficiency.     

Wide-optical bandgap with improved conductivity p-μc-Si:Ox:H films prepared by Cat-CVD

Boron-doped hydrogenated microcrystalline silicon oxide (p-μc-Si:O_x:H) films have been deposited using catalytic chemical vapor deposition (Cat-CVD). The single-coiled tungsten catalyst temperature (T_fil) was varied from 1850 to 2100 °C and films were deposited on glass substrates at the temperatures (T_sub) of 100–300 °C. Different catalyst-to-substrate distances of 3–5 cm and deposition pressures from 0.1 to 0.6 Torr were considered.Optical and electrical characterizations have been made for the deposited samples. The sample transmittance measurement shows an optical-bandgap (Eg_opt) variation from 1.74 to 2.10 eV as a function of the catalyst and substrate temperatures. One of the best window materials was obtained at T_sub=100 °C and T_fil=2050 °C, with Eg_opt=2.10 eV, dark conductivity of 3.0×10^?3 S cm^?1 and 0.3 nm s^?1 deposition rate.     

Formation and consumption of NO in H2 + O2 + N2 flames doped with NO or NH3 at atmospheric pressure

Flat premixed burner-stabilized H₂ + O₂ + N₂ flames, neat or doped with 300–1000 ppm of NO or NH₃, were studied experimentally using molecular-beam mass-spectrometry and simulated numerically. Spatial profiles of temperature and concentrations of stable species, H₂, O₂, H₂O, NO, NH₃, and of H and OH radicals obtained at atmospheric pressure in lean (? = 0.47), near-stoichiometric (? = 1.1) and rich (? = 2.0) flames are reported. Good agreement between measured and calculated structure of lean and near-stoichiometric flames was found. Significant discrepancy between simulated and measured profiles of NO concentration was observed in the rich flames. Sensitivity and reaction path analyses revealed reactions responsible for the discrepancy. Modification to the model was proposed to improve an overall agreement with the experiment.