Volume 1, Issue 3-4 -

Achieving high-efficiency solar cells and at the same time driving down the cell cost has been among the key objectives for photovoltaic researchers to attain a lower levelized cost of energy (LCOE). While the performance of silicon (Si) based solar cells have almost saturated at an efficiency of ~25%, III–V compound semiconductor based solar cells have steadily shown performance improvement at ~1% (absolute) increase per year, with a recent record efficiency of 44.7%. Integration of such high-efficiency III–V multijunction solar cells on significantly cheaper and large area Si substrate has recently attracted immense interest to address the future LCOE roadmaps by unifying the high-efficiency merits of III–V materials with low-cost and abundance of Si. This review article will discuss the current progress in the development of III–V multijunction solar cell integration onto Si substrate. The current state-of-the-art for III–V-on-Si solar cells along with their theoretical performance projections is presented. Next, the key design criteria and the technical challenges associated with the integration of III–V multijunction solar cells on Si are reviewed. Different technological routes for integrating III–V solar cells on Si substrate through heteroepitaxial integration and via mechanical stacking approach are presented. The key merits and technical challenges for all of the till-date available technologies are summarized. Finally, the prospects, opportunities and future outlook toward further advancing the performance of III–V-on-Si multijunction solar cells are discussed. With the plummeting price of Si solar cells accompanied with the tremendous headroom available for improving the III–V solar cell efficiencies, the future prospects for successful integration of III–V solar cell technology onto Si substrate look very promising to unlock an era of next generation of high-efficiency and low-cost photovoltaics.

GaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I – V curve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.

Effects of growth conditions and buffer structures on crystal quality of 1.9-eV In 0.5 G 0.5 P, 1-eV In 0.3 Ga 0.7 As, and 0.75-eV In 0.52 Ga 0.48 As materials on misoriented GaAs substrate for inverted metamorphic solar cell grown by metalorganic chemical vapor deposition have been studied. Large lattice mismatch issue between the two lower bandgap 1.0-eV In 0.3 Ga 0.7 As and 0.75-eV In 0.52 Ga 0.48 As epilayers and the substrate has been resolved by using optimized step-graded buffer layers. Threading dislocation blocking mechanisms have been studied and discussed. It was indicated that threading dislocations have been significantly blocked in the designed InGaAs buffer layers through annihilation reactions between threading dislocations or through the formation of misfit dislocations. As a result, smooth surface In 0.3 Ga 0.7 As and In 0.52 Ga 0.48 As epifilms with threading dislocation density of about 1 × 10 6 cm −2 were obtained. For the growth of InGaP, the surface morphology, crystal quality, ordering parameter, and InGaP composition were significantly affected by growth temperature, V/III ratio, and III/III ratio. The almost lattice match to the substrate, high crystal quality, and smooth surface of In 0.495 Ga 0.505 P were obtained at growth temperature of 675°C and V/III ratio above 150. The ordering parameter was strongly dependent on growth temperature. Photoluminescence measurement indicated that the bandgap of the InGaP epilayer is 1.89 eV, indicating that the solid composition in the InGaP film is more disordered.

Industrial use of piezoelectric polymers is currently limited by low piezoelectric response and large performance losses at elevated operating temperatures. Leading polymers such as poly(vinylidene fluoride) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) possess piezoelectric d 33 constants around −31 pC/N, and drop rapidly above 100°C operating temperatures. In this work, we fabricate a composite of P(VDF-TrFE) and carbon nanoparticles that possess a d 33 piezoelectric constant of −40 pC/N at room temperature, 29% higher than pure P(VDF-TrFE). Additionally, the d 33 value of the composite is 80% higher than the pure polymer at 140°C, with a value of −34 pC/N as compared to −19 pC/N. These materials, which are lead free, nontoxic and solution processable, are composed of Buckminsterfullerene (C 60 ) nanoparticles functionalized with diamine (N–R–N) crosslinking chains composited into a P(VDF-TrFE) polymer matrix. The blended thermoplastic was spin-coated into films and thermally cured to covalently attach the amine-functionalized C 60 onto polymer backbones to form crosslinking “bridges.” X-ray diffraction spectra confirmed the presence of a crosslinked network, and differential scanning calorimetry thermographs show the disappearance of the ferroelectric–paraelectric (F-P) transition peak at 106°C, indicating retention of ferroelectricity until the material melts at 152°C. The loss of this transition was confirmed through P-E hysteresis testing at 140°C, where the pure polymer showed paraelectric behavior and the composite remained ferroelectric. In addition, the crosslinked composites showed significant increases in remnant and maximum polarization above the pure polymer. Piezoresponse force microscopy (PFM) was used to measure local piezoelectric and hysteretic effects of crosslinked P(VDF-TrFE)/C 60 composites and demonstrated improvement over neat P(VDF-TrFE) samples. These materials show promise toward the design of sensors for extreme environments such as structural sensors, monitoring sensors in automobiles, vibrational sensors for machinery supporting oil, gas, mining and manufacturing operations, and for harvesting human-based or environmental energy.

Piezoelectric microelectromechanical systems (MEMS) energy harvesting is an attractive technology for harvesting small energy from ambient vibrations. Increasing the operating frequency bandwidth of such devices is one of the major challenges to be solved for real-world applications. A MEMS-scale doubly clamped nonlinear beam resonator has demonstrated very wide bandwidth and high-power density among the energy harvesters reported. In this paper, a first complete theoretical discussion of nonlinear resonance-based piezoelectric energy harvesting is provided. The sectional behavior of the beam has been studied through the Classical Lamination Theory (CLT) specifically modified to introduce the piezoelectric coupling and nonlinear Green-Lagrange strain tensor. A lumped parameter model has been built through Rayleigh–Ritz method and the resulting nonlinear coupled equations have been solved in the frequency domain through the Harmonic Balance Method (HBM). Finally, the influence of external load resistance on the dynamic behavior has been studied. The theoretical model shows that nonlinear resonant harvesters have much wider power bandwidth than that of linear resonators but their maximum power is still bounded by the mechanical damping as is the case for linear resonating harvesters.

High-performance low-cost multilayer textured Pb(Mg 1/3 Nb 2/3 )O 3 –PbZrO 3 –PbTiO 3 (PMN–PZT) piezoelectric ceramic benders were fabricated by combining templated grain growth (TGG) and low-temperature co-firing ceramics (LTCC) process. The d × g values of textured samples were 700–800% higher than that of the random counterpart, which results in 500–600% increase in the output power from vibration energy harvesting. The output power and power density of tri-layer textured sample at the acceleration of 0.43 g were measured to be 903 µW and 15.5 mW/cm 3 , respectively. The results demonstrate that the multilayer structure results in an increase in output current and a decrease in the matching resistive load.

Small-scale energy harvesting has become a particularly hot topic for replacing batteries in autonomous or nomad systems. In particular, vibration energy harvesting using piezoelectric elements has experienced a significant amount of research over the last decade as vibrations are widely available in many environments and as piezoelectric materials can be easily embedded. However, the energy scavenging abilities of such systems are still limited and are very sensitive to the connected load. The purpose of this paper is to expose a new approach based on synchronous switching on resistive load, which allows both a significant enhancement of the energy harvesting capabilities as well as a high tolerance to a change of the impedance of the connected system, especially in the low value region. It is theoretically and experimentally shown that such an approach permits increasing the energy harvesting abilities by a factor 4 compared to classical DC energy harvesting approach. Furthermore, the self-powering possibility and automatic load adaptation of the proposed method is experimentally discussed, showing the realistic viability of the technique.

A long-standing encumbrance in the design of low-frequency energy harvesters has been the need of substantial beam length and/or large tip mass values to reach the low resonance frequencies where significant energy can be harvested from the ambient vibration sources. This need of large length and tip mass may result in a device that is too large to be practical. The zigzag (meandering) beam structure has emerged as a solution to this problem. In this letter, we provide comparative analysis between the classical one-dimensional cantilever bimorph and the two-dimensional zigzag unimorph piezoelectric energy harvesters. The results demonstrate that depending upon the excitation frequency, the zigzag harvester is significantly better in terms of magnitude of natural frequency, harvested power, and power density, compared to the cantilever configuration. The dimensions were chosen for each design such that the zigzag structure would have 25.4×25.4 mm 2 area, and the cantilever would have the same surface area. The zigzag prototype of 25.4×25.4 mm 2 area was capable of generating 65 μW/cm 3 at 32 Hz when subjected to 0.1 G base acceleration.

Vibrational energy harvesters are used for collecting electrical power from mechanical vibrations and supplying low power applications. Combining a piezoelectric generator with an electrodynamic generator creates a hybrid piezoelectrodynamic generator. In this paper, a model for this hybrid generator is proposed. The model allows the accurate simulation of the mechanical and electrical part of the generator. It also makes it possible to analyze the waveforms and phase relationships of the two electrical outputs of the hybrid generator and thus simulate power management circuits and their feedback on the generator.

We report correlation between the electro-mechanical, ferroelectric, optical and opto-electric behavior in Pb(Zn 1/3 Nb 2/3 )O 3 –Pb 0.97 La 0.03 (Zr,Ti)O 3 transparent ceramics. Optical interaction during current–voltage (I–V) measurements enhanced the current output and revealed its strong dependence on wavelength and power of the impinging radiation. Piezoresponse force microscopy revealed that the improved characteristics at low stoichiometric excess of various elements were related to nano-sized stripe domains. Raman analysis indicated the presence of short-range clusters with rhombohedral–tetragonal phase distributed at much above the Curie temperature. The results reveal the potential of these ceramics in wireless opto-electric devices operating over a wide-range of wavelengths and temperatures. These classes of ceramics have successfully demonstrated in literature for use in pyroelectric generators and UV light energy harvesting, piezoelectric transformers.

Two concepts of scaled micro-flywheel-energy-storage systems (FESSs): a flat disk-shaped and a thin ring-shaped (outer diameter equal to height) flywheel rotors were examined in this study, focusing on material selection, energy content, losses due to air friction and motor loss. For the disk-shape micro-FESS, isotropic materials like titanium, aluminum, steel and wolfram are shown to be suitable as a flywheel rotor. Wound fiber reinforced composite plastics (T1000-, T300-carbon fibers and carbon nanotubes “CNTs”) were investigated for the flywheel in a ring shape. It was shown that isotropic materials reach the highest energy densities in the shape of a Laval disk with a rim. A micro-FESS with wolfram flywheel would reach the highest half-time-periods due to its high density, and thus, it is the favored material to design a flat disk-shaped micro-FESS with low standby-losses. Fiber reinforced plastic flywheels in ring shape reach the highest energy densities, from 150 W h/kg (T300) to 2,600 W h/kg (CNT), but display higher standby-losses as well. A scaling of the rotors was done within this study and showed that air friction is influenced by the shape of the examined flywheel rotors and the material. A linear correlation of down scaling and air friction losses was shown. As a motor/generator type, an ironless air coil Halbach array motor was suggested. Motor losses due to eddy currents in the stator coil were estimated. Losses correlated in square with downscaling. FESSs with wolfram and CNT showed the lowest standby-losses due to eddy currents.

Ca 0.15 Zr 0.85 O 1.85 (CSZ) films were grown on Pt/Ti/SiO 2 /Si substrates at various temperatures using radio-frequency (RF) magnetron sputtering with a CaZrO 3 target. Crystalline CSZ phase was formed in the film grown even at room temperature (RT). For the film grown at 300°C, a dielectric constant ( k ) of value 27.6 and a low tan δ of 0.005 were obtained at 100 kHz, and a similar k value of 26.7 with a high Q -value of 646 was obtained at 2.0 GHz. This film also showed a large capacitance density (264 nF/cm 2 ), a small TCC (75 ppm/°C at 100 kHz), a low leakage current (1.44×10 −8 A/cm 2 at 1.5 MV/cm), and a large breakdown field strength (2.25 MV/cm). Moreover, a 600 nm-thick CSZ film grown on polyimide substrate at RT satisfied all the ITRS 2016 requirements for a capacitor on an organic substrate. Therefore, the CSZ film, grown at a temperature ≤300°C, is a good candidate for use in embedded capacitors in printed circuit boards.

If we accept temperature and entropy as primitive quantities, we can construct a direct approach to a dynamical thermal theory of spatially continuous and uniform processes. The theory of uniform models serves as a simple entry point for learners of modern thermodynamics. Such models can be applied fruitfully to an understanding of (the dynamics of) thermoelectric processes and devices. Entropy, temperature, charge, and voltage allow us to describe the role of energy concisely, and constitutive quantities can be given their natural entropic interpretation. In this paper, aggregate dynamical models of a Peltier device will be created and simulations will be compared to non-steady-state experimental data. Such overall models give us a simple image of the transport of charge and transport, production, and storage of entropy and can be easily extended to the spatially continuous case. Process diagrams for a uniform model can be used to visualize these processes and the role of energy. Device efficiency can be easily read from the model. Apart from external parameters such as load resistances or temperature differences, it depends upon three parameters of the device: internal electric resistance, entropy conductance, and Seebeck coefficient. The Second Law efficiency of a generator suggests how to define the figure of merit ( zT ) of the thermoelectric material. Distinction between ideal and dissipative processes and the rates at which energy is made available or used allows us to construct a simple argument for the equality of the Seebeck and Peltier coefficients.