Sliver cells in thermophotovoltaic systems

Previous modelling has indicated that silicon solar cells should be thinner than 100 μm to be optimal for use in thermophotovoltaic (TPV) systems. Sliver cells are a novel type of thin photovoltaic cells fabricated from single crystal semiconductor wafers, with their contacts at the edges of the cell. A computational model was constructed to examine and compare the performance of silicon sliver cells with silicon conventional back-contact cells in TPV systems. Within the range of parameters investigated it was found that the lateral carrier transport resistance of sliver cell geometries limits their power output relative to conventional cells in TPV systems. In practical systems, the efficiencies are comparable.     

Charge transport study and spectral response of GaSb/GaAs heterojunctions prepared by MOVPE

The purpose of our work was the evaluation of GaSb/GaAs heterostructures grown on GaAs substrates for thermophotovoltaics (TPV). Heterojunctions p-GaSb/n-GaAs with p-layer prepared by metal organic vapour phase epitaxy (MOVPE) method at growth temperatures ranging from 500°C to 560°C were investigated. We have studied the charge transport in these structures and its influence on photovoltage spectral response of the cells. Measurement of I–V characteristics in the temperature range from 200 to 350 K show that the charge transport can be described by a combination of emission and diffusion processes. There is a spike and hence a discontinuity in the band diagram of the junction. The discontinuity increases with increasing GaSb growth temperature. Photovoltage spectral response shows higher signal from GaAs than that from GaSb. The experimental curves were compared with theoretically calculated ones accounting for the reduction of electron current crossing the barrier. The discontinuity is very probably connected with the lattice mismatch between both materials rather than with the affinity difference. Our results show that p-GaSb/n-GaAs heterojunctions prepared by this MOVPE method are not suitable enough for use in TPV.     

Characterisation of rare earth selective emitters for thermophotovoltaic applications

We investigate selective radiation emitters made from rare earth oxides suitable for thermophotovoltaic (TPV) systems. Yb₂O₃ and Er₂O₃ emitters were fabricated and their radiation power, temperature and emissivity were measured in the entire relevant spectral range. We found temperatures of 1735 K for the Yb₂O₃ emitter and of 1680 K for the Er₂O₃ emitter both heated with a 1.35 kW butane burner. The maximum emissivities of the selective peaks were 0.85 at 1.27 eV, and 0.82 at 0.80 eV for the Yb₂O₃ and the Er₂O₃ emitter, respectively. The emission spectra show gas emission lines originating from the combustion process in addition to the selective emission bands. An estimation based on a simplified combustion model show that a TPV system with a system efficiency of about 10% can be realised using an Yb₂O₃ emitter, silicon photocells and a perfect selective filter.     

Spectral emissivity of ytterbium oxide-based materials for application as selective emitters in thermophotovoltaic devices

The spectral emissivity of Yb³⁺ in a series of compounds has been investigated to study the effects of crystal structure type, chemical environment, ytterbium concentration, impurity concentration and temperature on the spectral radiant intensity of the selective emitter peak and emitted power from the material. A figure of merit has been defined which enables the compounds to be ranked for application in a practical thermophotovoltaic energy conversion device. It is shown that significant selective emission can be achieved from compounds in which the Yb³⁺ concentration is as low as 10 mol%. Apart from pure Yb₂O₃, the compounds Yb₃Al₅O₁₂, YbNbO₄ and Y_0.9Yb_0.1O_1.5 are found to have emission spectra suitable for efficient matching to silicon photovoltaic cells.     

Fabergé optics and edge filter for a wood powder fuelled thermophotovoltaic system

In order to achieve high efficiency in a TPV generator, it is important that a high fraction of emitted photons with energies below the TPV cell bandgap are reflected back to the emitter. This can be accomplished in several ways. We present the idea of an internally reflecting egg-shaped double cone with the emitter at one end, an edge filter at the wide center, and the TPV array at the other end. This geometry has so far been studied by means of both ray tracing analysis and by means of measurements with a simulated emitter. A sharp switchover from transmission to reflection in a multiple layer dielectric filter can be achieved only if the angles of incident rays are confined to a fairly narrow angular interval. The two methods both show that the studied optics can lower the angular spread of rays incident onto the filter and that some 96% of the emitted rays (in the ideal case) reach their goal without passing the filter or being reflected by the filter more than once. The concept of the whole of the wood powder fuelled TPV system is also given.     

Thermal design influence on the performance of solar thermophotovoltaic devices

The present theory integrates all components of a thermophotovoltaic (TPV) device (i.e. the primary lens, the absorber, the photovoltaic (PV) cell and a photon recuperator system). Energy balances are derived for the absorber and PV cell. The TPV efficiency is maximized by using three optimization parameters, namely, absorber temperature, PV cell temperature and cell voltage. An ad-hoc numerical constraint optimization procedure is developed which allows computation of an optimum absorber temperature within 5–10?K error. In case of an improved thermal design, the ‘ideal’ bandgap for TPV solar-energy conversion is in the range 0.5–1?eV, in reasonable good agreement with present knowledge. Further improvement in thermal-design quality moves the optimum bandgap toward higher values. Improving thermal-design quality decreases the influence of the concentration ratio, on both optimum absorber temperature and optimum PV cell voltage. An improved thermal design allows almost all narrow-bandgap materials to operate at positive voltage. The results prove existence of an optimum concentration ratio.     

The application of quantum well solar cells to thermophotovoltaics

We discuss the advantages of quantum well solar cells (QWSCs) for thermophotovoltaic (TPV) applications and illustrate them with InP/InGaAs and GaInAsP/InGaAs QWSCs which were designed for other applications and have not been optimised for TPV. It is shown that an InP p-i-n solar cell with 15 lattice matched InGaAs quantum wells (QWs) in the i region has an increase in open circuit voltage (V_oc) of (1.7 ± 0.1) times that of a control cell of InP with InGaAs in the i-region under an illuminating spectrum close to that expected from an ideal ytterbia emitter. Also, using an InGaAsP quaternary cell of band gap wavelength of 1.1 Am with 60 InGaAs QWs under the same illuminating spectrum the current density is increased by a factor of (2.4 ± 0.1) over that of the InP QWSC. The quaternary cell also absorbs longer wavelengths without any significant loss in V_OC. Better temperature coefficients for the former quantum well solar cell than the control cell are observed in a spectrum approximating a black body at 3000 K. Further advantages of QWs for narrow band and broad band illuminating spectra are discussed.     

A solar thermophotovoltaic converter using Pbs photovoltaic cells

A solar thermophotovoltaic converter using PbS photovoltaic cells is proposed. The converter is in the form of a flat plate consisting of a heat mirror, a black absorber, a cell filter and PbS photovoltaic cells. Theoretical analysis shows that, ideally, the efficiency of such a system is about 30%.     

Applications of carbon materials in photovoltaic solar cells

Carbon-based photovoltaic cells (PVCs) have attracted a great deal of interest for both scientific fundamentals and potential applications. In this paper, applications of various carbon materials in PVCs, especially in silicon-based solar cells, organic solar cells and dye-sensitized solar cells, are reviewed. The roles carbon materials played in the PVCs are discussed. Further research on solar cells comprised solely of carbon is prospected.     

Effect of copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

Comparison of charge accumulation and transport in nanostructured dye-sensitized solar cells with electrolyte or CuSCN as hole conductor

The charge transport properties of the dye-sensitized solar cells consisting of Ru(dcbpyH₂)₂(NCS)₂-sensitized nanostructured TiO₂ with either redox electrolyte or CuSCN as hole conductor have been compared. The electron transport time and the electron charge in the TiO₂ varies in a similar way with the incident light intensity for both hole conductors: electron transport becomes faster and electron accumulation increases with increasing light intensity. Electron transport in the CuSCN-based cells is significantly faster than in electrolyte cells under conditions where the accumulated charge is equal. An ultra-thin aluminum oxide layer on the nanocrystalline titanium oxide has a beneficial effect as it reduces the recombination and increases the open-circuit potential.     

Bias-dependent high saturation solar LBIC scanning of solar cells

A light beam-induced current measurement system that uses concentrated solar radiation as a beam probe to map spatially distributed defects on a solar cell has been developed and tested [F.J. Vorster, E.E. van Dyk, Rev. Sci. Instrum., submitted for review]. The induced current response from a flat plate EFG Si solar cell was mapped as a function of surface position and cell bias by using a solar light beam induced current (S-LBIC) mapping system while at the same time dynamically biasing the whole cell with an external voltage. This paper examines the issues relating to transient capacitive effects as well as the electrical behaviour of typical solar cell defect mechanisms under spot illumination. By examining the bias dependence of the S-LBIC maps, various defect mechanisms of photovoltaic (PV) cells under concentrated solar irradiance may be identified. The techniques employed to interpret the spatially distributed IV curves as well as initial results are discussed.     

Utilization of MEH-PPV as a sensitizer in titana-based photovoltaic cells

A conjugated polymer poly[2-methoxy-5(2-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) is utilized as the light-harvesting agent in a photovoltaic cell. Sensitized anodic and cathodic photocurrents are observed for CuCNS|MEH-PPV|TiO₂ cells and heterojunctions with MEH-PPV.     

ZnO-based nanocrystalline powders with applications in hybrid photovoltaic cells

In recent years there has been a growing interest in the development of hybrid photovoltaic cells consisting of new materials, such as devices based on the combination of a wide gap semiconductor and an organic dye (dye-sensitized solar cells, DSSC). In this paper we obtain nano-zinc oxide particles whose optical and electrical properties have been modified by the presence of small amounts of Al or In acting as dopants. The aim of this study is to improve the compatibility of each of the compounds present in the photovoltaic solar cell. The knowledge gained will provide input to guide the processes in the manufacture of hybrid solar cells.     

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.     

Synthesis and photovoltaic properties of soluble fulleropyrrolidine derivatives for organic solar cells

Organic solar cells made from bi-layer thin-film heterojunctions having poly((2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene)vinylene) (MEH-PPV) as an electron donor and fulleropyrrolidine derivatives as an electron acceptor were investigated. We synthesized soluble fulleropyrrolidine derivatives substituted different chain lengths for the organic solar cell. Due to the high solubility and sufficiently long chain length of fulleropyrrolidine derivatives, though those are monomers, a thin film (about 80–90 nm) could be fabricated individually by the spin-coating method. The fill factor of the bi-layer device was achieved to be 0.46, which is higher than that of the single-layer device by a polymer/fulleropyrrolidine derivative blend film of 0.37, due to the decrease of the recombination.     

Solar Cell Fabrication Techniques for Single and Semicrystalline Silicon Substrates

Several technologies for silicon solar cell fabrication are reviewed. More specifically, the characteristics of a process line, based on the use of thick film technology, are discussed. The technology, using screen printing and heat treatments in a conveyor furnace, is attractive for junction formation, metallization and deposition of the antireflective coating. The process can be implemented on single crystalline, semicrystalline and thin film crystalline silicon films, deposited on cheap substrates. The add-on cost of this solar cell fabrication process is 2 to 2.5 $/W for a 270 kW/year production line and can be as low as 0.5 to 0.6 $/W for a 10 MW/year capacity fully automated cell factory.     

Investigation of multi-donor bulk-heterojunction photovoltaic cells based on P3HT:PCBM system

(2,7-bis[5′-(9,9-dioctylfluorene-2-yl)-2,2′-dithienyl-5-yl]-9,9-dioctylfluorene) (F3Th4) was used as a secondary electron donor material in the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk-heterojunction photovoltaic cell. It is shown that the combination of F3Th4 with P3HT allows strong light absorption. The mechanism of charge transfer in the multi-donor PV cell was investigated; it shows that efficient energy transfer takes place from F3Th4 to P3HT. However, the short-circuit current (J_SC) of the multi-donor bulk-heterojunction photovoltaic (PV) cell still decreased. The possible reason for the smaller photocurrent is worsening of transport property after addition of F3Th4.     

微热光电系统带双环形翅片燃烧室的数值模拟

由于能量密度的极大优势,应用燃烧释放碳氢燃料的化学能并转化为电能或者机械能的微动力系统有巨大的发展潜力和广阔的应用前景。微热光电系统是其中的一种,而微燃烧室是该系统的核心部件。在实验验证的基础上,对带有双环形翅片燃烧室内的燃烧过程进行了数值模拟。结果表明,双环形翅片能增强微燃烧室内混合气体的湍流扰动,改善燃烧状况,有效地提高燃烧效率。     

Photovoltaic properties of octithiophene-based Schottky and p/n junction cells: Influence of molecular orientation

We report here on the photovoltaic properties of Schottky and p/n junction cells based on octithiophene (8T). In the solid state, 8T is an elongated planar molecule with efficient p-type transport properties. We investigate in particular the influence of molecular orientation of 8T films on light absorption, I/V characteristics and photocurrent spectra. We show that a substantial increase of the photocurrent density is obtained by aligning the 8T molecules horizontally and parallel to each other on the substrate by means of a simple rubbing technique. This improvement is attributed to a more efficient light absorption rather than to an improvement of the charge transport properties in the sandwich devices.