Thermal damage in luminescent solar concentrators (LSC) for photovoltaic systems

The temperature dependence of the optical efficiency of luminescent solar concentrators (LSC) has been studied. It has been found that in the range room temperature (RT) ? 100 °C an important thermal degradation is produced.The damage depends not only on the temperature but also on the nature of the dye and can reach values as high as 37% in the case of RhB.The damage is partially recovered when the LSC is cooled to RT.     

The empirical relationship between global radiation and global ultraviolet (0.290–0.385) μm solar radiation components

Measurements of the monthly average daily global and global ultraviolet solar radiation over a period of three years (1985, 1986, 1987) in Kuwait are reported. Over the three years the computed yearly daily means for the above solar radiation components were 5.592 kW h/m² and 0.260 kW h/m². The effect of atmospheric dust on the measured solar radiation components is investigated. Employing the least-square linear regression analysis an estimated empirical function relating global radiation to ultraviolet radiation was proposed for the past three years' measurements.     

A novel procedure for the optical characterization of solar concentrators

A novel procedure for the optical characterization of solar concentrators is presented. The method is based on recording at night the light of a star reflected by the mirror. Images of the mirror taken from its focal region allow the reconstruction of the slope map. The application of this technique for the in situ characterization of heliostats is particularly simple and at very low cost. Results on first tests carried out with a heliostat of the CESA-I field at the Plataforma Solar de Almeria have shown the feasibility of this technique. Uncertainties in the reconstructed slopes of about 1.0 mrad have been estimated.     

Extension of the a-Si:H electronic transport model to μc-Si:H: use of the μτ product to correlate electronic transport properties and solar cell performances

The aim of this communication is to show that it is possible to extend the model of the electronic transport developed for amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). By describing the electronic transport with the μ⁰τ^R products (mobility×recombination time) as a function of the Fermi level, we observed the same behaviour for both materials, indicating a similar type of recombination. Moreover, applying the normalised μ⁰τ⁰ product (mobility×life-time) obtained by combining the photoconductivity (σ_photo) and the ambipolar diffusion length (L_amb) measured in individual layers, we are able, as in the case of a-Si:H, to predict the quality of the solar cells incorporating these layers as the active i layer.     

Control of μc-Si/c-Si interface layer structure for surface passivation of Si solar cells

Outstanding passivation properties for p-type crystalline silicon surfaces were obtained by using very thin n-type microcrystalline silicon (μc-Si) layers with a controlled interface structure. The n-type μc-Si layers were deposited by the RF PE-CVD method with an insertion of an ultra-thin oxide (UTO) layer or an n-type amorphous silicon (a-Si : H) interface layer. The effective surface recombination velocity (SRV) obtained was very small and comparable to that obtained using thermal oxides prepared at 1000°C. The structural studies by HRTEM and Raman measurements suggest that the presence of UTO produces a very thin a-Si : H layer under the μc-Si. A crystal lattice discontinuity caused by these interface layers is the key to a small SRV.     

Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

p-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm)⁻¹. The open circuit voltage V_oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J_sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J_sc=13.95 mA/cm², V_oc=834 mV, FF=65% and η=7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.     

Organic–inorganic hybrid composites for photovoltaics: Organic guest molecules embedded in μc-Si and ZnSe host matrices

The concept of organic–inorganic hybrid composites for bulk sensitization of inorganic semiconductors by organic dye molecules is introduced. The idea is either to increase the absorptivity of e.g. indirect semiconductors as μc-Si or to expand in a two-step process the absorption spectrum of wide gap semiconductors to photons of energy smaller than the band gap. The composites are prepared by vacuum-based codeposition. Raman and optical spectroscopy, and photoemission have been used to prove the stability of the organic molecules ZnPc and F₁₆ZnPc for the applied growth conditions. Enhancement of photoconductivity has been shown for ZnPc–Si bilayer. As a crucial parameter for the transfer of excited charges, the alignment of dye HOMO–LUMO states versus semiconductor band edges has been determined using photoelectron spectroscopy.     

Electronic transport properties of the μc-(Si,Ge) alloys prepared by ECR

Some μc-(Si,Ge):H alloys have been grown using low-pressure, reactive ECR plasma deposition with high H dilution and subtle (sub ppm) B-doping. Incorporating these high-quality materials into devices leads to low-gap μc-(Si,Ge) solar cells with acceptable performance. This justifies a detailed investigation of the electronic transport properties of μc-(Si,Ge):H alloys by employing the microwave photomixing technique.From the measurements of the electric field dependence of the drift mobility and lifetime, we have found strong evidence for the existence of long-range potential fluctuations in μc-(Si,Ge):H alloys. We determine the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of the deposition rate. It was found that the film transport properties do not degrade or enhance monotonically with increasing deposition rate; there exists a valley point where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again. The evidence indicates that it is the long-range potential fluctuations that result in the deterioration of the transport properties of μc-(Si,Ge):H alloys. Specifically, it is the increase in the depth, and a decrease in the length of the potential fluctuations, which lead to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that aside from the increase of charged scattering centers, compositional disorder in the alloys play an important role with the build-up of the potential fluctuations.     

Single-crystalline silicon solar cell with pp heterojunction of c-Si substrate and μc-Si : H film

Annealing effects of the single-crystalline silicon solar cells with hydrogenated microcrystaline silicon (μc-Si : H) film were studied to improve the conversion efficiency. Boron-doped (p⁺) μc-Si : H film was deposited in a RF plasma enhanced chemical vapor deposition system (RF plasma CVD) on the rear surface of the cell. With the optimized annealing conditions for the substrate, the conversion efficiency of 21.4% (AM1.5, 25°C, 100 mW/cm²) was obtained for 5 × 5 cm² area single crystalline-solar cell.     

A new strategy for the fabrication of cost-effective silicon solar cells

Fabrication of solar cells with very high efficiencies currently requires extremely complex processing. In order to make photovoltaics an economical large scale source of energy, very high efficiencies have to be achieved by low-cost processing. The innovative approach for the cost-effective production of highly efficient silicon solar cells presented in this paper is characterised by only four simple and environmentally safe large-area fabrication steps. The basic processing sequence consists of: (i) mechanical surface grooving, (ii) simple diffusion or inversion, (iii) shallow angle metal evaporation, and (iv) plasma silicon nitride deposition. Cell design, fabrication techniques and processing sequences for metal-insulator-semiconductor contacted diffused n⁺-p junction (MIS-n⁺p) and MIS-inversion-layer (MIS-IL) silicon solar cells are outlined. The new simple approach turned out to be most successful, as demonstrated by mechanically grooved MIS-n⁺p silicon solar cells with efficiencies above 21% using exclusively aluminium as metallisation.     

Absorption coefficient spectra of μc-Si in the low-energy region 0.4–1.2 eV

Optical absorption spectra in the low-energy region 0.4–1.2 eV is reported for μc-Si:H using a photothermal deflection spectroscopy technique. Absorption coefficient spectra in the low-energy region contain important information related to defects and hydrogen. It is demonstrated that there is a good correlation between electron spin densities and integrated absorption coefficient spectra from 0.7 to 1.2 eV. The amount of the hydrogen molecules in microvoids is much larger in μc-Si:H than that in a-Si:H. Light illumination effects in PDS spectra has also been studied from a view point of photo degradation of the μc-Si:H.     

Review of the phase change material (PCM) usage for solar domestic water heating systems (SDWHS)

The shortage in energy resources combined with the climb in greenhouse emissions is the main incentive beyond the deployment of solar energy resource in various applications. One of the most successful applications is the utilization of solar energy in the domestic water heating systems (DWHS) because 70% of the consumed energy in the residential segment is utilized for space heating and appliances in cold climates 1 . However, the full deployment of solar energy in domestic water heating is only possible when an energy storage system with acceptable price is available. Recently a new tendency for deploying phase change materials (PCMs) as an energy storage system is introduced in several solar DWHS. These systems are known as integrated PCM in solar DWHS and offer several advantages including high storage capacity, low storage volume, and isothermal operation during the charging and discharging phases. The present study reviews various techniques utilized for integrating the PCM in solar water heating systems and the utilized methods for enhancing the heat transfer characteristics of the PCM through the usage of extended surfaces and high conductive additives. Copyright © 2017 John Wiley & Sons, Ltd.     

An analytical approach for optimising a set of α and ε values for solar energy applications

An analytical method for optimising a set of α and ε values for specified set of operating conditions (plate temperature, ambient temperature, number of glass covers, wind velocity, solar flux etc.) has been proposed. The absorptivity (α)-emissivity (ε) curves obtained suggest the existence of an optimum (α, ε) set, which would maximise the useful energy from a collector.     

On the structural variations of Ru(II) complexes for dye-sensitized solar cells

Ruthenium(II) complexes with new phenanthrenyl ligand (TAPNB) have been synthesized and examined. The spectroscopic and electrochemical measurements showed that the excited states of those complexes matched the conduction band of titanium dioxide. The overall power conversion efficiencies of the solar cells utilized these new complexes as sensitizers for TiO₂ films were less than that of N3-sensitized cell. Although the open-circuit voltage was similar to that of N3-sensitized cell, the short-circuit current was one order lower. Such outcome may be attributed to the less amount of dyes adsorbed due to the steric congestion of the complex. When NCS ligand was replaced by pyridyl ligand, the energy of metal-to-ligand charge transfer (Ru(II)→TAPNB) increased and resulted in blue shift of the absorption band. Anchoring of carboxylic acid at the surface of TiO₂ slightly lowered the energy of Ru(II)→TAPNB charge transfer band. As carboxylic acid anchor was replaced by acetyl ester, the weaker interaction between the semiconductor and the ligand led to diminishing amount of the complex adsorbed and less photocurrent was detected.     

Numerical modelling of trap-assisted tunnelling mechanism in a-Si:H and μc-Si n/p structures and tandem solar cells

The trap-assisted tunnelling theory was developed to describe the tunnelling of charge carriers via bandgap energy levels in structures based on hydrogenated amorphous silicon and microcrystalline silicon. Its implementation into ASPIN numerical simulator is explained. Models that were verified on n/p single junctions were applied in the tunnel recombination junction area of a tandem solar cell. Thus, it is possible to study a multi-layer solar cell without separately simulating any of its components.     

Developing microencapsulated 12‐hydroxystearic acid (HSA) for phase change material use

Phase change materials (PCM) have an increasingly more important role as a thermal energy storage (TES) media. However, leakage problem of PCM causes limitation during their integration in TES systems. Therefore, the encapsulation of PCMs is attracting research interest to extend usage of PCMs in real TES applications in recent years. In this study, hydroxystearic acid (HSA) was encapsulated with polymethyl methacrylate (PMMA) and different PMMA comonomer shells via emulsion polymerization method for the first time in literature. HSA with high melting temperature range (74–78°C) can widen the scope of using PCMs, and the encapsulated form can make it more versatile. The chemical structures, morphologies, and thermophysical properties of capsules were determined by FT‐IR, SEM, DSC, TGA, and thermal infrared camera. Among the produced HSA capsule candidates, PMMA‐HEMA is the most promising with latent heat of 48.5 J/g with melting range of 47 to 85°C. SEM analysis indicated that the capsules have spherical shape with compact surface at nano‐micro (100–440 nm) size range; however, some capsules exhibited agglomeration.     

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B₂H₆ gas (diborane) phase ratio (Variation in B₂H₆ gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm)⁻¹) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).     

Hybrid organic–inorganic solar cells: Case of the all thin film PMeT(Y)/CdS(X) junctions

Hybrid organic–inorganic all thin film photovoltaic junctions PMeT(Y)/CdS(X) were investigated, where PMeT(Y) is the conducting polymer poly(3-methylthiophene) doped with various anions Y=CF₃SO₃⁻, ClO₄⁻, BF₄⁻, PF₆⁻, and CdS(X) cadmium sulfide doped with various elements X=Cu, Ni, Al, As and Sb. CdS(X) films were spray deposited on conducting and transparent indium-tin oxide (ITO) glass, and PMeT(Y) films were electrodeposited onto the CdS(X) film to form the junction. The electrochemical investigation of the mechanism of electrodeposition and growth of the PMeT(Y) films by means of chronoamperometry, and of the charge transfer behavior of the PMeT(Y)/CdS(X) junctions by means of cyclic voltammetry revealed a strong effect of the nature of Y and X. The same strong effect of Y was also found with PMeT(Y)/ITO junctions, and may have some generality. It was showed that the best quality of contact between the organic and inorganic phases, from an electrochemical viewpoint and in solution, was obtained with the junction, pointing towards a similarly better photovoltaic performance in solid state. This was indeed the case, and it was found that higher short-circuit current, open-circuit photovoltage and energy conversion efficiency, approaching 4%, could be obtained with this junction.