InGaN photocell significant efficiency enhancement on Si – an influence of interlayer physical properties

Nearly similar molar ratio of In and Ga in indium gallium nitride (InGaN) /Si photocells prefers to match InGaN conduction level energy to Si valance energy band for ohmic contact between two cells. At high temperature fabrication process, InGaN–Si interface shows highly defecting prone. Considering those tussles, InGaN‐based/Si‐based double‐junction tandem solar cell was designed and fabricated. In_0.4Ga_0.6 N cell was fabricated on Si photocell by implementing AlN/SiO₂/Si₃N₄ interlayers. Interlayer influence on quantum efficiency of InGaN cell was studied under ideal irradiance AM1.5 solar spectrum at 300°K. Because of insertion of interlayers between InGaN and Si; the gradual efficiency enhancement with respect to the overlayer h‐GaN (a = 3.183 nm) plane lattice was found to 8.3%, 5.9% and 5.1% for AlN (a = 3.11 nm), for SiO₂ (a = 4.9 nm) and for Si₃N₄ (a = 7.76 nm), respectively. AlN was found to be an excellent and SiO₂ as preferable interlayer compared with Si₃N₄. Coherence (in‐plane lattice matching) of nano‐interlayer appears to reduce photonic electro‐migration hurdle between InGaN and Si; therefore, progressive enrichment of efficiency was realized. Copyright © 2016 John Wiley & Sons, Ltd.     

Efficient and robust visible light photocatalytic H2 production based on CdSe quantum dots sensitized titania

In this work, cadmium selenide quantum dots (CdSe QDs) with different sizes were synthesized and employed as visible light sensitizers of titania, in comparison with other organic molecules based sensitizers, including the well-known ruthenium complex sensitizer, tris(4,4-dicarboxy-2,2-bipyridyl)ruthenium(II) chloride, phenolic-formaldehyde resin and poly (4-vinylphenol). The different sensitizers are linked to titania via different molecular linkages through self-assemble processes. CdSe QDs adsorbed onto titania via stabilization ligand (mercaptopropionic acid) are more stable and efficient in terms of photocatalytic H₂ generation and photocurrent generation. The CdSe QDs with a diameter of 2.5 nm exhibits a strong absorption peak centred at 500 nm (CdSe500) and shows the best photocatalytic performance than other QDs with larger size and organic sensitizers. The turnover number of CdSe500 QDs for H₂ generation reaches ca. 9000 after 96 h reaction, with a 0.6% quantum yield under irradiation at 450 nm (light intensity = 35 mW/cm²). During the initial 3.0 h reaction, the turnover numbers of different types of sensitizers are estimated about 4.3, 52.5, 323.2 and 16.5 for phenolic-formaldehyde resin, poly (4-vinylphenol), CdSe500 QDs and ruthenium complex, respectively. These results highlights the advantages of utilizing CdSe QDs as stable visible light sensitizers for solar energy conversion.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

Simulation of an efficient silicon heterostructure solar cell concept featuring molybdenum oxide carrier‐selective contact

Transition metal oxides/silicon heterocontact solar cells are the subject of intense research efforts owing to their simpler processing steps and reduced parasitic absorption as compared with the traditional silicon heterostructure counterparts. Recently, molybdenum oxide (MoO_x, x < 3) has emerged as an integral transition metal oxide for crystalline silicon (cSi)‐based solar cell based on carrier‐selective contacts (CSCs). In this paper, we physically modelled the CSC‐based cSi solar cell featuring MoO_x/intrinsic a‐Si:H/n‐type cSi/intrinsic a‐Si:H/n⁺‐type a‐Si:H for the first time using Silvaco technology computer‐aided design simulator. To analyse the optical and electrical properties of the proposed solar cell, several technological parameters such as work function and thickness of MoO_x contact layer, intrinsic a‐Si:H band gap, interface recombination, series resistance, and temperature coefficient have been evaluated. It has been shown that higher work function of MoO _x induces the formation of a favourable Schottky barrier height as well as an inversion at the front interface, stimulating least resistive path for holes. Utilising thinner MoO_x layer implies reduced tunnelling of minority charge carriers, thus enabling the device to numerically attain 25.33% efficiency. With an optimised interface recombination velocity and reduced parasitic absorption, the proposed device exhibited higher V_oc of 752 mV, J_sc of 38.8 mA/cm², fill‐factor of 79.0%, and an efficiency of 25.6%, which can be termed as the harbinger for industrial production of next‐generation efficient solar cell technology.     

Improved efficiency of Al0.36Ga0.64As solar cells with a ppnn structure

Improvement of efficiency of Al_0.36Ga_0.64As solar cells is advanced in two aspects of minority-carrier lifetime: reduction of majority-carrier concentration in the emitter and base layers, and reduction of deep levels in the back-surface-field (BSF) layer. A pp⁻n⁻n structure is proposed to optimize the use of the effect of reduced majority-carrier concentration, and its effectiveness verified in a preparatory experiment on Al_0.3Ga_0.7As solar cells. A very poor photoluminescence (PL) decay time (below 0.3 ns) of a BSF layer heavily doped with Si becomes 14-fold longer when Se is applied to the dopant instead of Si, resulting in an improvement of the external quantum efficiency near the absorption edge. These two aspects of this study lead to the realization of 16.6% efficiency under 1-sun, AM 1.5 global conditions with an Al_0.36Ga_0.64As solar cell.     

Study of carbon-based hole-conductor-free perovskite solar cells

In this paper, the perovskite solar cells (PSCs) with the structure of FTO glass/compact TiO₂/mesoporous TiO₂/CH₃NH₃PbI₃/carbon electrode were designed and fabricated. The CH₃NH₃PbI₃ absorption layers were prepared using one-step solvent-engineering method. The effects of volume ratios of dimethyl sulfoxide (DMSO) to N, N-dimethylformamide (DMF) and molar ratios of PbI₂ to CH₃NH₃I (MAI) on structure and morphology of CH₃NH₃PbI₃ absorption layers, as well as photovoltaic performance of PSCs were studied. The results show that the PSCs based on a specific ratio of PbI₂:MAI = 1:1.4 and DMSO:DMF = 1:3 exhibited an optimal photovoltaic performance, yielding an open-circuit voltage (V_oc) of 0.72 V, short-circuit current density (J_sc) of 25.98 mA/cm², fill factor (FF) of 0.46, and power conversion efficiency (PCE) of 8.45%. And the incident photon-to-electron conversion efficiency (IPCE) is close to 85% between 400 and 600 nm visible region.     

Preparation of semiconducting copolymer and its application in nanocrystalline TiO2 solar cells

Abstract

The past decade has witnessed increasing attention in the nanocrystalline TiO₂ solar cells (TSSCs). In this work, we have studied a novel TiO₂/PCBM/PPy solar cell based on blends of the semiconducting copolymer polypyrrole (PPy) and [6,6]-phenyl C₆₁ butyric acid methyl (PCBM) coated titanium dioxide (TiO₂) nanocrystal film to substitute the I₃^?/I^? redox electrolyte and the dye using in DSSCs. The research by incident photon to current efficiency spectra shows that the TiO₂ films had a stronger absorption in 300–500 nm light range. The performance of the resulting photovoltaic devices was investigated, and the effects of the PCBM/PPy ratio by photocurrent–voltage characteristics were researched. By the optimised PCBM/PPy ratio was 3∶1, The TSSC exhibited a short circuit current of 1·28 mA cm^?2, an open circuit voltage of 0·788 V, a fill factor of 0·654 and a light to electric energy conversion efficiency of 0·622% under a simulated solar light irradiation of 100 mW cm^?2.     

GROWTH OF HETEROEPITAXIAL CdTe LAYERS ON REUSABLE Si SUBSTRATES AND A LIFT-OFF TECHNIQUE FOR THIN FILM SOLAR CELL FABRICATION

Heteroepitaxial (111) and (100) oriented CdTe layers have been grown on Si substrates by conventional molecular beam epitaxy (MBE) and photo-assisted MBE (PAMBE) using stacked BaF₂-CaF₂ as a buffer to overcome the 19% lattice mismatch between Si and CdTe. Heteroepitaxial As doped p-type CdTe(lOO) layers have been grown on BaF₂-CaF₂/Si(100). The dopant activation is accomplished using an extra Cd source and laser illumination of the substrate during growth. The growth kinetics and surface reconstructions have been studied using RHEED during CdTe growth under different conditions, and the induced effects on Te-desorption, Cd-migration, and As-substitution on Te-vacancy site have been correlated. The resistivity of As doped CdTe layers is down to 20 ohm cm. The 8 K photoluminescence spectra of such a layer shows a dominant (A^°, X) peak at 1.590 eV and the As acceptor level corresponds to a shallow level with = 60 me V activation energy. A lift-off technique has been used to separate the single crystal CdTe thin films from the Si wafer by dissolving the fluoride buffer. CdS/CdTe solar cells have been fabricated in these layers.     

Aqueous dye-sensitized solar cell based on new ruthenium diphenyl carbazide complexes

The major challenge of the operation of every solar cell based on dye including water splitting solar cell (WSSC) and dye sensitized solar cell (DSSC) is the using organic solvent medium which causes to decompose the solar cell structure, resulting environmental impact. Here, we synthesized and characterized two new ruthenium complexes with nitrogen and oxygen donor ligands for DSSC application which show good stability on TiO₂ surface in water solvent. Interestingly, the DSSC based on [Ru(dcbpy)₂(DPC)]Cl, where dcbpy = 4,4-dicarboxilic acid 2,2-bipyridin and DPC = diphenylcarbazide, was shown better efficiency in water than methanol dye loading as well as N3 as a benchmark sensitizer in the same condition. The DPC-based exhibited open circuit voltage (V_oc) of 0.63 V, short-circuit current density (J_sc) of 2.5 mA/cm² and fill factor (FF) of 70%, resulting an overall power efficiency of 1.12%. The incident-photon-to-current conversion efficiency (IPCE) value is also reached to 45% for [Ru(dcbpy)₂(DPC)]Cl in the same condition It is proposed that the ruthenium complex containing nitrogen and oxygen donor ligands is more stability on TiO₂ and prevent the decomposition of TiO₂ porous under water solvent condition.     

Wet-etch texturing of ZnO:Ga back layer on superstrate-type microcrystalline silicon solar cells

Surface wet etching is applied to the ZnO:Ga (GZO) back contact in μc-Si thin film solar cells. GZO transparency increases with increasing deposition substrate temperature. Texturing enhances reflective scattering, with etching around 5-6 s producing the best scattering, whereas etching around 5 s produces the best fabricated solar cells. Etching beyond these times produces suboptimal performance related to excessive erosion of the GZO. The best μc-Si solar cell achieves FF=68%, V_OC=471 mV and J_SC=21.48 mA/cm² (η=6.88%). Improvement is attributed to enhanced texture-induced scattering of light reflected back into the solar cell, increasing the efficiency of our lab-made single μc-Si solar cells from 6.54% to 6.88%. Improved external quantum efficiency is seen primarily in the longer wavelengths, i.e. 600-1100 nm. However, variation of the fabrication conditions offers opportunity for significant tuning of the optical absorption spectrum.     

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Texturing of silicon (Si) wafer surface is a key to enhance light absorption and improve the solar cell performance. While alkaline texturing of single-crystalline Si (sc-Si) wafers was well established, no chemical solution has been successfully developed for multi-crystalline Si (mc-Si) wafers. Reactive-ion-etch (RIE) is a promising technique for effective texturing of both sc-Si and mc-Si wafers, regardless of crystallographic characteristics, and more suitable for thin wafers. However, due to the use of plasma source generated by high power, the wafer surface gets a physical damage during the processing, which requires an additional subsequent damage-removal wet processing. In this work, we developed a damage-free RIE texturing for mc-Si solar cells. An improved self-masking RIE texturing process, developed in this study, produced ∼0.7% absolute efficiency gain on 156×156 mm² mc-Si cells, where the gas ratio and the plasma power density were keys to mitigate the plasma-induced-damage during the RIE processing while maintaining decent surface reflectance. In the self-masking RIE texturing, a mixture of SF₆/Cl₂/O₂ gases was found to significantly affect the surface morphology uniformity and reflectance, where an optimal etch depth was found to be 200-400 nm. We achieved J_sc gain of ∼1.3 mA/cm² while maintaining decent FFs of ∼0.78 without a V_oc loss after optimization of firing conditions.     

Near-infrared CdSexTe1-x@CdS “giant” quantum dots for efficient photoelectrochemical hydrogen generation

Colloidal quantum dots (QDs) have attracted a lot of attention due to their unique optoelectronic properties. They have been widely used as building block materials for solar technologies such as solar cell, and photoelectrochemical (PEC) water splitting. Hydrogen generation by using QDs as photocatalysts has emerged a promising application in PEC devices. However, it is still very challenging to obtain high-efficiency PEC devices due to the limited absorption wavelength of QDs and the existence of surface traps which prohibit the efficient charge transfer. In this work, we synthesized ternary CdSe_xTe_1-x/CdS (CdSeTe/CdS) “giant” QDs to extend the light absorption to near infrared, matched well with Sun's spectrum. The as-synthesized CdSeTe/CdS “giant” QDs exhibit quasi-type II band alignment as confirmed by its long lifetime and red-shifted emission peak compared with bare CdSeTe QDs. The wide absorption range of “giant” core/shell QDs and their long lifetime can improve the efficient absorption of Sun's spectrum and charge transfer. As a proof-of-concept, a PEC device using QDs sensitized TiO₂ mesoporous thin film as a photoanode was used for hydrogen production. The corresponding photocurrent density was increased to 3.0 mA/cm² with the introduction of CdS shell, which is 1.5 times higher than the PEC device using CdSeTe QDs. This study indicates that ternary or polynary alloyed core/shell QDs can be used as promising optoelectronic materials for applications of PEC devices.     

Computational modelling of the reflectivity of AlGaAs/GaAs and SiGe/Si quantum well solar cells

In this paper the modelling of the reflectivity of two quantum well solar cells (QWSC) are theoretically developed and computationally analysed. The new reflectivity model is based on the Modified Single Effective Oscillator model combined with Fresnel's equation. The model takes into consideration the effects of the design parameters including concentration levels, structural properties of the device (well length, etc.), operating temperature and electric field effects. The results generated are for a bare AlGaAs/GaAs cell and the same cell with a ZnS antireflection coating (ARC). Further investigations include a bare SiGe/Si cell and the same cell with a Ta₂O₅ ARC. The results generated are accurate and match with experimental data for similar cells. The analysis is performed for AM 1.5 spectrum. The model is intended to be an aid to QWSC designers.     

Solar assisted multi-generation system using nanofluids: A comparative analysis

In this comparative study, a parabolic trough solar collector and a parabolic dish solar collector integrated separately with a Rankine cycle and an electrolyzer are analyzed for power as well as hydrogen production. The absorption fluids used in the solar collectors are Al₂O₃ and Fe₂O₃ based nanofluids and molten salts of LiCl–RbCl and NaNO₃–KNO₃. The ambient temperature, inlet temperature, solar irradiance and percentage of nanoparticles are varied to investigate their effects on heat rate and net power produced, the outlet temperature of the solar receiver, overall energy and exergy efficiencies and the rate of hydrogen produced. The results obtained show that the net power produced by the parabolic dish assisted thermal power plant is higher (2.48 kW–8.17 kW) in comparison to parabolic trough (1 kW–6.23 kW). It is observed that both aluminum oxide (Al₂O₃) and ferric oxide (Fe₂O₃) based nanofluids have better overall performance and generate higher net power as compared to the molten salts. An increase in inlet temperature is observed to decrease the hydrogen production rate. The rate of hydrogen production is found to be higher using nanofluids as solar absorbers. The hydrogen production rate for parabolic dish thermal power plant and parabolic trough thermal power plant varies from 0.0098 g/s to 0.0322 g/s and from 0.00395 g/s to 0.02454 g/s, respectively.     

Dicyanovinyl substituted oligothiophenes: Thermal stability, mobility measurements, and performance in photovoltaic devices

A series of dicyanovinyl-oligothiophenes are investigated concerning their thermal stability, absorption in thin film, and hole mobility. Due to very high extinction coefficients, these materials are interesting for application as donor in solar cells. The quinquethiophene DCV₂-5T, which shows a hole mobility of 2.2×10^-5 cm²/Vs, is used as donor material in a flat heterojunction organic small molecule solar cells. Despite a very thin donor layer of only 6 nm, these devices exhibit in a planar heterojunction with 15 nm C₆₀ an efficiency of up to 2.8% with a fill factor of up to 58%, a short circuit current density of 5.2 mA/cm², an open circuit voltage of 1.03 V, and an external quantum efficiency of 30% in the green spectral range.     

Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination

Upconversion of sub-band-gap photons promises to increase solar cell efficiencies by making these photons useful. In this paper, we investigate the application of β-NaYF₄:20% Er³⁺ to silicon solar cells. We determine the external quantum efficiency of an upconverter silicon solar cell, both under monochromatic excitation and, for the first time in the context of silicon solar cells, under broad spectrum illumination as it is relevant for the application to harvest solar energy. The investigated upconverter silicon solar cell responds under broad spectrum illumination with an average upconversion efficiency of 1.07±0.13% in the spectral range from 1460 to 1600 nm. The resulting efficiency increase for the used solar cell with an overall efficiency of 16.7% is calculated to be 0.014% relative.     

A new light trapping TCO for nc-Si:H solar cells

A new transparent conducting light trapping structure with no free carrier absorption for solar cells is described. Indium oxide doped with molybdenum (IMO), prepared by the hollow cathode sputtering technique, exhibits high charge-carrier mobility up to 80 cm²/V s. No free-carrier absorption in the near infrared region has been found in the IMO. The superior long-wavelength transparency, however, is not sufficient for thin film Si solar cell applications. To obtain the highest possible short circuit current, the TCO needs to possess additional light trapping structure. Anisotropic etching of fiber texture oriented ZnO has been shown to result in an effective light trapping structure. Here we propose a bilayer structure that consists of light trapping-intrinsic ZnO and IMO (the ZnO/IMO bilayer). Both layers show low free-carrier absorption up to the wavelength of 1200 nm. We demonstrate the use of such a transparent conducting light trapping oxide (TCLO) in nanocrystalline (nc-Si:H) solar cells fabricated by a single chamber, batch-type PECVD process. Incorporation of such a transparent conducting light trapping bilayer can increase solar cell short-circuit current density (J_sc) by >30% compared to flat bilayers.     

Evaluating the Optical Properties of TiO2 Nanofluid for a Direct Absorption Solar Collector

Recent studies specify that designated nanofluids may increase the proficiency of direct absorption solar thermal collectors. To determine the efficiency of nanofluids in solar applications, their capability to change light energy to thermal energy must be identified (i.e., the absorption spectrum of the solar material). In view of that, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (200– 1100 nm). In the first decade of nanofluid research, most of the focus was on measuring and modeling the fundamental thermophysical properties of nanofluids (i.e., thermal conductivity, density, viscosity, and convection coefficients). Lately, considerable focus is given to the fundamental optical properties of nanofluids. However, the effect of particle size, shape, and volume fraction of nanoparticles as well as alternation of the base fluids, which can significantly affect scattering and absorption, have not been addressed to date in the literature. In this study, the effects of size and concentration of TiO₂ nanoparticles on the extinction coefficient were analyzed using the Rayleigh approach. The results show that smaller particle size (<20 nm) has a nominal effect on the optical properties of nanofluids. Volume fraction is linearly proportionate to the extinction coefficient. Considering a nanoparticle size of 20 nm, almost 0% transmissivity is obtained for wavelengths ranging from 200 to 300 nm. However, a sudden increase of 71% in transmissivity is noted from 400 nm, gradually increasing to 88% and becoming similar to that of water at 900 nm. Promising results are observed for volume fractions below 0.1%.     

Surface plasmon enhanced GaAs thin film solar cells

As a new method to improve the light trapping in solar cells, surface plasmon resonance (SPR) has attracted considerable attention because of its unique characteristics. Several studies have been reported on the photocurrent improvement of Si solar cells by surface plasmons, while little research has been done on III-V solar cells. In this work, we performed a systematic study of SPR on GaAs thin film solar cells with different sizes of Ag nanoparticles on the surface. The nanoparticles were fabricated by annealing E-beam evaporated Ag films in a N₂ atmosphere. It was found that the surface plasmon resonance wavelength does not undergo a simple red-shift with increasing metal thickness. It depends on the shape of the metal nanoparticles and the interparticle spacing. It is necessary to optimize the particle size to obtain an optimum enhancement throughout the visible spectrum for solar cells. We found that the optimum thickness of the Ag film was 6 nm under our experimental conditions. Furthermore, from the calculation based on the external quantum efficiency data, the short circuit current density of a GaAs solar cell with 6 nm Ag film after annealing was increased by 14.2% over that of the untreated solar cell.     

Kilogram-scale production of highly active chalcogenide photocatalyst for solar hydrogen generation

Solar photocatalytic hydrogen production from water has been regarded as an ideal way addressing world energy and environmental crises. The technology has long relied on the development of an efficient photocatalyst. In addition to its photocatalytic performance, the large-scale production of certain photocatalyst from the viewpoint of particle application remains a challenge yet has received insufficient focus. Herein, we report an efficient and practical batch preparation system based upon hydrothermal method to the scalable production of chalcogenide nanoparticle photocatalyst. Taking the synthesis of Cd_0.5Zn_0.5S (CZS) twinned photocatalyst as an example, the outcome of CZS photocatalyst could reach ～0.8 kg in this batched synthesis, which is about 390 times of the lab-scale production in mass amount. It was found that the twinned structure and visible-light absorption property were well maintained. Although further measurements toward the photocatalytic activity indicate slight decrement on solar H₂ generation compared to the lab-scale synthesized CZS photocatalyst, a high quantum efficiency of about 40.5% at 425 nm remained. The photocatalytic reaction could also stably proceed for 200 h without notable decay of H₂-evolution rate. This work thus provides a powerful means for facile scaling up the chalcogenide nanoparticle photocatalyst at the kilogram level with both high quality and good reproducibility.