Carrier transport in high-efficiency ZnO/SiO2/Si solar cells

Carrier transport in ZnO/SiO₂/n-Si solar cell has been theoretically analyzed with a consideration that the photo-carrier transport from silicon to ZnO layer through the barrier is dominated by quantum mechanical tunneling process of minority carrier. It was found that the highest efficiency of the cell could be achieved at SiO₂ layer thickness of around 20 Å. The efficiency of the cells decreases as the surface states density Q_ss becomes higher. Moreover, the efficiency increases as the electron concentration of ZnO layer is increased due to the decrease of work function of ZnO. It was also found that the lower transmittance of the high carrier concentration ZnO due to the free-carrier absorption at infrared wavelength region does not give any significant effect to the cell performance. The efficiency of higher than 25% is achievable by optimizing the involved device parameters.     

The path towards a high-performance solution-processed kesterite solar cell

Despite the promise of thin-film Cu(In,Ga)(S,Se)₂ (CIGSSe) chalcopyrite and CdTe photovoltaic technologies with respect to reducing cost per watt of solar energy conversion, these approaches rely on elements that are either costly and/or rare in the earth's crust (e.g., In, Ga, Te) or that present toxicity issues (e.g., Cd), thereby potentially limiting these technologies in terms of future cost reduction and production growth. In order to develop a photovoltaic technology that is truly compatible with terawatt deployment, it is desirable to consider material systems that employ less toxic and lower cost elements, while maintaining the advantages of the chalcopyrite and CdTe materials with respect to appropriate direct band gap tunability over the solar spectrum, high device performance (e.g., >10% power conversion efficiency) and compatibility with low-cost manufacturing. In this review, the development of kesterite-based Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells, in which the indium and gallium from CIGSSe are replaced by the readily available elements zinc and tin, will be reviewed. While vacuum-deposited devices have enabled optimization within the compositional phase space and yielded selenium-free CZTS device efficiencies of as high as 6.8%, more recently a liquid-based approach has been described that has enabled deposition of CZTSSe devices with power conversion efficiency of 9.7%, bringing the kesterite-based technology into a range of potential commercial interest. Electrical characterization studies on these high-performance CZTSSe cells reveal some of the key loss mechanisms (e.g., dominant interface recombination, high series resistance and low minority carrier lifetime) that limit the cell performance. Further elucidation of these mechanisms, as well as building an understanding of long-term device stability, are required to help propel this relatively new technology forward.     

Analysis of the effect of hydrogen-radical annealing for SiO2 passivation

The effect of hydrogen-radical annealing for SiO₂ passivation was examined. The annealing effect was analyzed by measuring effective lifetime and C-V characteristics and was compared with the effects of forming gas annealing (FGA) and hydrogen RF plasma annealing. The effect of hydrogen-radical annealing is much higher than those of FGA and hydrogen RF plasma annealing. It was also confirmed that both changes of surface recombination velocity and interface state density showed the same tendency. Furthermore, the investigations of hydrogen-radical density showed that by using microwave afterglow method, hydrogen-radicals could be generated much more than that by RF plasma. Accordingly, more interface trap density could be terminated and surface recombination velocity was effectively decreased by using microwave afterglow method.     

Improvement in dye-sensitized solar cells employing TiO2 electrodes coated with Al2O3 by reactive direct current magnetron sputtering

A novel surface modification method was carried out by reactive dc magnetron sputtering to fabricate TiO₂ electrodes coated with Al₂O₃ for improving the performance of dye-sensitized solar cells (DSSCs). The Al₂O₃-coated TiO₂ electrodes had been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis spectrophotometer, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The study results revealed that the modification to TiO₂ increases dye absorption amount, reduces trap sites on TiO₂, and suppresses interfacial recombination. The impact of sputtering time on photoelectric performance of DSSCs was investigated. Sputtering Al₂O₃ for 4 min on 5-μm thick TiO₂ greatly improves all cell parameters, resulting in enhancing the conversion efficiency from 3.93% to 5.91%. Further increasing sputtering time decreases conversion efficiency.     

The formation of CuInSe2 thin-film solar cell absorbers by laser annealing of electrodeposited precursors

The formation of the compound semiconductor CuInSe₂ by laser annealing of electroplated precursor films in inert gas atmosphere represents an entire non-vacuum production process of thin-film solar cell absorbers. Besides this technological aspect, the impact of extreme annealing rates on structural properties of the resulting semiconductor is interesting from a fundamental research point of view. For this reason, we compared absorbers processed by laser annealing with absorbers annealed with moderate heating rates in the range of 1 K/s by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). All absorbers processed with laser or furnace annealing consist of crystalline CuInSe₂ in the chalcopyrite crystal structure with a high degree of cation disorder. We show that laser annealing does not lead to unintentional selenium loss during the semiconductor formation process.     

A feasibility study of integrally colored Al–Si as a solar selective absorber

The solar selective properties of integrally colored Al–Si alloy (11.6 wt% Si) have been investigated. Optical measurements showed a continuous decrease of reflectance, i.e. an increase of absorptance, with increasing film thickness. A maximum solar absorption of 0.85 was achieved for Si–Al₂O₃ coatings thicker than 13 μm but such thick aluminum oxide coatings have very high thermal emittance.The reflectance of the Si–Al₂O₃ coated aluminum could be understood from a four flux radiative transfer theory. Using this theory the optical performance of the coating as a solar absorber was modeled for different size and volume fractions of silicon particles and coating thicknesses. A solar absorptance of just 0.90 can be achieved from a 10 μm thick coating of about 0.3 volume fraction of silicon. For thinner coatings (1 μm) the solar absorptance was only 0.70 for the same volume fraction.     

Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell

In this paper, novel TiO₂ submicro-rings were synthesized via potentiostatic anodization of titanium powder coated on transparent conducting oxide glass. The TiO₂ submicro-rings film was characterized by SEM, XPS and 3D optical profiling. Accordingly, a possible growth mechanism of submicro-rings was discussed. The TiO₂ submicro-rings based dye-sensitized solar cell (DSSC) with the film thickness of ca. 3.1 μm was assembled and a conversion efficiency of 1.36% was achieved under AM 1.5 illumination. The photoelectron transport properties of TiO₂ submicro-rings based DSSC were also discussed according to the electron impedance spectroscopy.     

Characteristics of indium zinc oxide thin films prepared by direct current magnetron sputtering for flexible solar cells

The In₂O₃-ZnO (IZO) thin films were prepared on polyethylene terephthalate substrate at room temperature by direct current (dc) magnetron sputtering. The properties of IZO thin films were studied in terms of O₂ concentration and deposition parameters. As the O₂ concentration in O₂/Ar gas increased, the transmittances of the films were increased up to 90% and the resistivities were decreased. The systematic variation of process parameters including dc power, gas pressure and target-to-substrate distance was performed to examine the properties of the deposited films. It was disclosed that there was an optimum O₂ concentration for high transmittance and low resistivity. With decrease in dc power and gas pressure and increase in target-to-substrate distance, the IZO films with high transmittance and low resistivity were obtained. The observation of the IZO films by atomic force microscopy indicated that the microstructure and surface morphology of the films were responsible for the transmittance. It was demonstrated that IZO films with a resistivity of 5.1×10⁻⁴ Ω cm and an optical transmission of 90% in the visible spectrum could be prepared at room temperature on flexible substrates.     

Mix-solvent-thermal method for the synthesis of anatase nanocrystalline titanium dioxide used in dye-sensitized solar cell

Nanocrystalline TiO₂ with almost pure anatase form has been synthesized through the Mix-solvent-thermal method (MST) by using TiCl₄ as the starting material. The mean size of the synthesized TiO₂ is 10 nm with narrow distribution. High-performance dye-sensitized solar cell with nanocrystalline TiO₂ electrode formed from MST was achieved. Its I_sc and V_oc values reached 21.62 mA/cm² and 727.9 mV, respectively, and the photovoltaic conversion efficiency reached 9.13%, i.e. 7.5% higher than those of solar cells with TiO₂ made from the traditional precursor of titanium alkoxides. To our knowledge they are the highest values obtained from the solar cells with nanocrystalline TiO₂ electrode formed from the hydrolysis of TiCl₄.     

Issues on the chalcopyrite/defect-chalcopyrite junction model for high-efficiency Cu(In,Ga)Se2 solar cells

Considering the chalcopyrite/defect-chalcopyrite junction model for Cu(In_1−xGa_x)Se₂-based devices and our previously reported findings for the Cu(In_1−xGa_x)₃Se₅ defect chalcopyrites, we have postulated that uniform high-Ga-content photovoltaic structures (with x > 0.35) do not yield acceptable device performance due to the electrical and structural differences between both types of materials (chalcopyrite and defect-chalcopyrite).In this contribution, the structural properties of the surface region of Ga containing absorber materials have been studied by grazing incidence X-ray diffraction. We find that there are significant differences between surface and bulk. A structural model is proposed for the growth of the chalcopyrite/defect-chalcopyrite junction relative to its Ga content. And we demonstrate that closely lattice matched high-Ga-content structures (x > 0.35) can produce solar cells withv acceptable performances. The high-voltage and low-current electrical outputs from high Ga structures are very desirable in module fabrication because overall resistive losses can be substantially reduced.     

Stability of a-Si:H solar cells deposited by Ar-treatment or by ECR techniques

Stability against light soaking was studied for amorphous silicon (a-Si:H) solar cells using three different i-layers; (a) device-quality a-Si:H (standard a-Si:H) with bandgap of 1.75 eV, (b) narrow bandgap (1.55 eV) a-Si:H fabricated by Ar* chemical annealing and (c) a-Si:H(Cl) fabricated from SiH₂Cl₂. Both the narrow bandgap a-Si:H and the a-Si:H(Cl) solar cells showed much improved stability than that of the standard a-Si:H solar cells: e.g., fill factor of the narrow bandgap a-Si:H cell only slightly decreased from 56% to 53%, while that of the standard a-Si:H cell degraded from 62% to 51%. In addition, mobility–lifetime products of the a-Si:H(Cl) cell also exhibited improved stability than that of the standard a-Si:H solar cell.     

Characterisation and application of new carboxylic acid-functionalised ruthenium complexes as dye-sensitisers for solar cells

A series of ruthenium complexes with and without TiO₂ anchoring carboxylic acid groups have been synthesised and characterised using nuclear magnetic resonance (NMR), UV–vis and luminescence. These complexes were adsorbed on thin films of the wide band-gap semiconductor anatase and were tested as photosensitisers under standard conditions. I/V characteristics of such devices revealed superior performance of the non-symmetric complexes [4′-(4-bromophenyl)-[2,2′; 6′,2″] terpyridine]Ru(II) [4′-(4-bromophenyl)-[2,2′; 6′,2″] terpyridine-4,4″-dicarboxylic acid] and [2,2′; 6′,2″] terpyridine Ru(II) [2,2′; 6′,2″] terpyridine-4′-carboxylic acid with a maximum output power 0.016 mW cm⁻² under illumination at 100 mW cm⁻² AM1.5 and efficiencies 3 times higher than the symmetric complexes.     

Impact of surface roughness on the electrical parameters of industrial high efficiency NaOH-NaOCl textured multicrystalline silicon solar cell

Sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) solution (1:1 ratio by volume) based texturization process at 80-82 °C is an easy, low cost and comparatively new and convenient option for fabrication of any multicrystalline silicon (mC-Si) solar cell. In the present study atomic force microscope is used to observe the intragrain surface in a miniscule area (3 μm × 3 μm) of NaOH-NaOCl textured surface by two and three dimensional analysis, roughness analysis and section analysis. The r.m.s value of the surface parameter of 7.0 nm ascertains the smoothness of the textured surface and further the surface reflectivity is minimized to 4-6% in the 500-1000 nm wavelength range by a proper silicon nitride anti-reflection coating. Comparing with the standard HF-HNO₃-CH₃COOH acid textured cell, the NaOH-NaOCl textured cell shows a comparatively lower value of series resistance of 7.17 mΩ, higher value of shunt resistance of 18.4 Ω to yield a fill factor of 0.766 leading to more than 15% cell efficiency in the industrial cell processing line. This AFM study yields different surface roughness parameters for the NaOH-NaOCl textured wafers which can be used as a reference standard for optimized texturing.     

Synthesis and characterization of β-FeSi2 grown by thermal annealing of Fe/Si bilayers for photovoltaic applications

In this article we have reported the synthesis and characterization of FeSi₂ thin films. The Fe/Si thin films were obtained by electron beam evaporation. Thermal annealing was carried out at 650°C for 1 h. The formation of the β-FeSi₂ layers were characterized by the X-ray diffraction method and found to be polycrystalline in nature. The structural parameters were evaluated from the XRD pattern. The possible optical transition in these films is found to be direct and allowed.     

Thermal decomposition of (NH4)2SO4 in presence of Mn3O4

The main objective of this work is to develop a hybrid water-splitting cycle that employs the photon component of sunlight for production of H₂ and its thermal (i.e. IR) component for generating oxygen. In this paper, (NH₄)₂SO₄ thermal decomposition in the presence of Mn₃O₄, as an oxygen evolving step, was systematically investigated using thermogravimetric/differential thermal analyses (TG/DTA), temperature programmed desorption (TPD) coupled with a mass spectrometer (MS), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) techniques. Furthermore, thermolysis of ammonium sulfate, (NH₄)₂SO₄, in the presence of Mn₃O₄ was also investigated by conducting flow reactor experiments. The experimental results obtained indicate that at 200-450 °C, (NH₄)₂SO₄ decomposes forming NH₃ and H₂O and sulfur trioxide that in the presence of manganese oxide react to form manganese sulfate, MnSO₄. At still higher temperatures (800∼900 °C), MnSO₄ further decomposed forming SO₂ and O₂.     

Influence of Alq3/Au cathode on stability and efficiency of a layered organic solar cell in air

A relatively long lifetime organic solar cell, containing a thin tris-8-hydroxy-quinolato aluminum (Alq₃) layer under an Au cathode, is described. Half-lifetime of the cell in the darkness is over 7 weeks and 30% of the initial power conversion efficiency (η) is obtained after 18 weeks. This represents a substantial increase in lifetime compared to that of the unencapsulated Al-cathode cells. Efficiency is enhanced by 60 times compared to a cell that does not contain Alq₃, being 0.60%. The proposed role of Alq₃/Au cathode in this cell is discussed in detail.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

Preparation of La0.6Sr0.4Co0.2Fe0.8O3 nanoceramic cathode powders for solid oxide fuel cell (SOFC) application

The La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) nanoceramic powders were prepared by sol–gel process using nitrate based chemicals for SOFC applications since these powders are considered as more promising cathode materials for SOFC. The citric acid was used as a chelating agent and ethylene glycol as a dispersant. The powders were calcined at 650 °C/6 h, 900 °C/3 h and 1150 °C/2 h in air using Thermolyne 47900 furnace. These powders were characterized by employing SEM/EDS, XRD, porosimetry and TGA/DTA techniques.     

Biindene-C60 adducts for the application as acceptor in polymer solar cells with higher open-circuit-voltage

Two C₆₀ derivatives, biindene-C₆₀ monoadduct (BC₆₀MA) and biindene-C₆₀ bisadduct (BC₆₀BA), were synthesized by an one-pot reaction of 1, 1′-biindene and C₆₀, for the application as acceptor materials in Polymer Solar Cells (PSCs). The two C₆₀ derivatives possess good solubility in toluene and o-dichlorobenzene, and the solubility of BC₆₀BA is even better than that of BC₆₀MA. The electrochemical properties and the LUMO energy levels of the two fullerene derivatives were investigated by cyclic voltammetry. The LUMO energy levels of BC₆₀MA and BC₆₀BA were 0.06 and 0.17 eV higher (up-shifted) than that of PCBM, respectively. The PSCs based on P3HT as donor and BC₆₀MA or BC₆₀BA as acceptor exhibited higher V_oc of 0.68 and 0.82 V, respectively, which is benefited from the higher LUMO energy levels of the C₆₀ derivatives. The power conversion efficiency of the PSC based on P3HT/BC₆₀MA was 2.21% after annealing at 140 °C for 5 min.     

Strategy to improve the performance of dye-sensitized solar cells: Interface engineering principle

As next generation solar cells, dye-sensitized TiO₂ solar cells attract many scientists’ attention throughout the world. Although currently there are no commercially available products on the market, construction of large modules and long-term stability tests have been carried out by many companies and laboratories worldwide; commercialized DSC products may be appearing in the near future. Improving DSC performance and long-term stability is a great challenge not only to the academic research but also for industrial applications. Here the interface molecular engineering principle is proposed as the main strategy to meet this challenge in view of recent progress in this domain.