Generation Photovoltaic Cells Research Articles

Tunneling in ZnO/ZnCdO quantum wells towards next generation photovoltaic cells

We report on the structural and photoluminescence (PL) properties of ZnO-based multiple quantum wells (MQWs) by metal organic vapor phase epitaxy. Two sets of ZnO/Zn0.75Cd0.25O MQW structures were grown on c- and r-plane oriented sapphire substrates to elucidate the carrier dynamics by varying the well (Zn0.75Cd0.25O) and barrier (ZnO) thicknesses. The high crystalline quality of the structures was confirmed by X-ray diffraction measurements. Importantly, no emission related to ZnO barriers could be resolved in PL spectra implying effective carrier confinement in the wells. This fact was further supported by a prominent blue-shift (∼0.5eV) of the dominant emission from the MQWs with respect to that in a single Zn0.75Cd0.25O layer. The structures with thicker wells are found to exhibit conventional quantum confinement, while samples with thinner wells demonstrate signs of excitonic wave functions penetrating into barrier layers according to results of carrier life-time measurements. It has been demonstrated that the well thickness also influences the tunneling in Zn1-xCdxO/ZnO MQWs.The estimated built-in electric field from optical transition is of the order of ∼1.75MV/cm. The observed spectral and carrier lifetime variations are discussed in terms of quantum confinement and internal electric field modulation.

Syntheses of near infrared absorbed phthalocyanines to utilize photosensitizers

Phthalocyanines have become of major interest as functional colorants for various applications. In order to use various applications especially photosensitizers, the absorption maxima called Q-band of phthalocyanines are required to be shifted to the near infrared region. Substituted phthalocyanine analog alkylbenzopiridoporphyrazins, especially zinc bis(1,4-didecylbenzo)-bis(3,4-pyrido)porphyrazine, and toroidal-shaped phthalocyanines having aminoamine dendric side chains such as toroidal zinc poly(aminoamine)phthalocyanine dendrons were synthesized. Phthalocyanines of two types reportedly use photosensitizers for photodynamic therapy of cancer. The respective efficacies of photodynamic therapy of cancer for zinc bis(1,4-didecylbenzo)-bis(3,4-pyrido)porphyrazine and its regioisomers were estimated using laser-flash photolysis. The capability of using photodynamic therapy for toroidal zinc poly(aminoamine)phthalocyanine dendrons was assessed using a cancer cell culture. Both phthalocyanines were suitable for the use as a photosensitizer as photodynamic therapy of cancer. Then, non-peripheral thioaryl substituted phthalocyanines, 1,4,8,11,15,18,22,25-octakis(thioaryl)phthalocyanines, such as 1,4,8,11,15,18,22,25-octakis(thiophenylmethyl)phthalocyanines, 1,4,8,11,15,18,22,25-octakis(thiophenylmethoxy)phthalocyanines, and 1,4,8,11,15,18,22,25-octakis(thiophenyl tert-butyl)phthalocyanines were also synthesized in order to develop next- generation photovoltaic cells and/or dye-sensitized solar cells. Non-peripheral substituted 1,4,8,11,15,18,22,25-octakis(thioaryl)phthalocyanines exhibited a Q-band in the near infrared region. Electrochemical measurements were performed on the above-mentioned 1,4,8,11,15,18,22,25-octakis(thioaryl)phthalocyanines described above to examine their electron transfer abilities and electrochemical mechanisms. The compounds 1,4,8,11,15,18,22,25-octakis(thioaryl)phthalocyanines are anticipated to be appropriate materials for use in the next generation of photovoltaic cells.

Modeling and Simulation of a Solar Power Source for a Clean Energy without Pollution

Photovoltaic cell generation is the technique which uses photovoltaic cell to convert solar energy into electrical energy. Now a days ,the photovoltaic generation is developing increasingly fast as a renewable energy source. The functioning of a photovoltaic cell as the power generator is equivalent to an electric circuit containing a current generator, diode, series resistance and shunt resistance. This paper presents a modeling and simulation of a photovoltaic system constitutes of a generator (PVG), DC-DC converter (boost chopper) to transfer the maximum power to a base transmitter station. The temperature and irradiance effects on the PVG will be studied, particularly on the variables such as the short circuit current Icc, the open circuit voltage Voc, the performance η and the fill factor FF. Depending on the load (BTS, I=60A, V=48V) profile and climatic factors influencing, we can find a highly gap between the maximum power supplied by the PVG and that actually transferred to the BTS. A maximum power point tracker (MPPT) based on a boost converter commanded by a Pulse Width Modulation (PWM) is used for extracting the maximum power from the PVG. Thus, a real time tracking of the optimal point of functioning (MPP: Maximum Power Point) is necessary to optimize the efficiency on the system. The modeling and simulation of the system (PVG, boost converter, PWM and MPPT algorithm Perturbation and Observation P&O) is then made with Matlab/Simulink software. DOI: http://dx.doi.org/10.11591/ijece.v3i5.3639

The importance of the TiO2/quantum dots interface in the recombination processes of quantum dot sensitized solar cells

Quantum dot sensitized solar cells (QDSSCs) present a promising technology for next generation photovoltaic cells, having exhibited a considerable leap in performance over the last few years. However, recombination processes occurring in parallel at the TiO(2)-QDs-electrolyte triple junction constitute one of the major limitations for further improvement of QDSSCs. Reaching higher conversion efficiencies necessitates gaining a better understanding of the mechanisms of charge recombination in these kinds of cells; this will essentially lead to the development of new solutions for inhibiting the described losses. In this study we have systematically examined the contribution of each interface formed at the triple junction to the recombination of the solar cell. We show that the recombination of electrons at the TiO(2)/QDs interface is as important as the recombination from TiO(2) and QDs to the electrolyte. By applying conformal MgO coating both above and below the QD surface, recombination rates were significantly reduced, and an improvement of more than 20% in cell efficiency was recorded.

Cyclic voltammetry of non-peripheral thioaryl substituted phthalocyanines

Abstract Phthalocyanines have become of major interest as functional colorants for dye-sensitized photovoltaic cells. Syntheses of non-peripheral (1,4,8,11,15,18,22,25) thioaryl substituted phthalocyanines have been reported. These phthalocyanines exhibited a Q band in the near-infrared region. In order to develop next generation photovoltaic cells, we have synthesized 1,4,8,11,15,18,22,25-octakis(thioaryl)phthalocyanines and estimated their electron transfer ability using cyclic voltammetry. In the cyclic voltammograms, these compounds showed reduction and oxidation potentials, which are attributed to the presence of the sulfur atoms in the thioaryl substituents at the peripheral positions of phthalocyanine. The electron transfer mechanisms of the phthalocyanines having cobalt and zinc as their central metal atoms were attributed to reduction and oxidation of their central metal, while electron transfer in the other metal-free compounds resulted from the highest occupied molecular orbital to the lowest unoccupied molecular orbital electron transitions.

Next Generation Photovoltaic Cells and System through MEMS Technology

We report on the application of MEMS and other microsystem technologies to photovoltaic (PV) cells, modules, and systems to take advantage of the numerous, significant beneficial effects that are realized as the size of solar cells decrease to sub-mm length scales. To demonstrate these effects, we have developed both crystalline silicon and III-V PV cells. These cells are from 2 to 20 microns thick and from 250 microns to one millimeter across. We have demonstrated conversion efficiencies of up to 14.9% for a 14 micron thick crystalline silicon PV cell. This work provides benefits for two broad PV applications: 1) highly flexible PV modules with conversion efficiencies greater than 20%, and 2) commercial/utility scale PV systems using moderate concentration flat plate modules with simple single-axis or coarse dual-axis tracking. Cost models indicate that systems based on these technologies can achieve unsubsidized energy costs of less than $0.10/kWh.

Development and Simulation of a Structure Based on CuIn1-xGaxSe2 Semiconductors Alloys for a New Generation of Photovoltaic Cells

The aim of our study is the development and simulation of thin films structure based on CuIn1-xGaxSe2 semiconductors alloys to improve the conversion efficiency of solar cells. We studied and simulated the variation effect of the Gallium concentration on some physical and electrical properties (lattice parameter, strain, energy gap, refractive index, absorption coefficient and J (V) characteristic), this in order to optimize the best possible performances for solar cells. Then, we studied the influence of surface state density and temperature on solar devices characteristics. For better performance of our structure based on CuIn1-xGaxSe2, the concentration of gallium must be less than 40% and this to avoid the lattice mismatch with CdS buffer layer. We have demonstrated by simulation models the possibility for efficiency to reach 22% in the case of CuIn1-xGaxSe2 thin films solar cells.

Fast Light-Induced Solid Phase Crystallization of Nanometer Thick Silicon Layers on Quartz

Multi-quantum wells (MQW) with nanometer thick crystalline Si layers are considered among the promising light absorbers for application in the next generation of photovoltaic cells. Proper crystallization of the initially amorphous Si (a-Si) layers in such MQW presents a challenge. Recently it was shown that light-induced solid-phase crystallization (LISPC) leads to almost complete crystallization of Si layers in the MQW. In this report we present and discuss recent results, problems and prospects related to the large-scale LISPC process of MQW structures on glass.

Method for fabricating third generation photovoltaic cells based on Si quantum dots using ion implantation into SiO2

In this paper, we report on the synthesis of silicon quantum dots for photovoltaic applications by means of ion implantation followed by annealing. Nucleation was achieved by implanting Si+ ions into SiO2 thin films, previously thermally grown on a Si(100) substrate, and annealing to 1100 °C. Passivation was used for photoluminescence (PL) measurements. The thickness of the oxide layer, the stoichiometry of the implanted layer, and the depth profiles of the implanted ions were determined for all samples by both Rutherford backscattering spectroscopy (RBS) and ellipsometry techniques. Characterization by transmission electron microscopy (TEM) indicates that the diameter of the silicon quantum dots (Si-QDs) varies from 2 to 4 nm, which is less than the Bohr radius of bulk crystalline Si(∼5 nm). Optical and electrical properties have been investigated by PL and I-V measurements. When passivated silicon nanocrystals (Si-nc) embedded into SiO2 are excited using a 450 nm diode laser, they exhibit a strong PL emission in the range of 650-1000 nm. Based on these investigations, p-type Si-QDs/n-type c-Si junctions were fabricated and electrically characterized in the dark as well as under an AM1.5G terrestrial solar spectrum for nonimplanted, as-implanted, and implanted-annealed samples for different implantation fluences. The electrical curves of the structures under illumination demonstrate the photovoltaic behavior of the Si-QDs. Despite the weak light conversion of these devices, these results remain very promising and offer potentially unprecedented, vast improvements to third generation solar cells.

Bacteriorhodopsin/TiO2 nanotube arrays hybrid system for enhanced photoelectrochemical water splitting

In recent years, considerable efforts have been made to improve the performance of photoactive nanostructured materials for water splitting applications. Herein, we report on the assembly and use of a bacteriorhodopsin (bR)/TiO2 nanotube array hybrid electrode system. Photoanode materials composed of ∼ 7μm long self-ordered and vertically oriented nanotube array of titanium dioxide films were fabricated via the anodization of Ti foil in formamide electrolytes containing NH4F at room temperature followed by sensitization of the electrodes with bR. The stability of bR on the TiO2 surface was found to depend on the pretreatment process of the TiO2 films. Our results demonstrate the opportunity to fabricate fairly stable bR/TiO2 hybrid electrodes that can be used as photoanodes for photoelectrochemical water splitting. Under AM 1.5 illumination (100 mW/cm2), the hybrid electrodes achieved a photocurrent density of 0.65 mA/cm2 which is a ∼ 50% increase over that measured for pure TiO2 nanotubes (0.43 mA/cm2) fabricated and tested under the same conditions. In the presence of a redox electrolyte, the photocurrent increased to 0.87 mA/cm2. To the best of our knowledge, this is the first report on the use of bR/TiO2 hybrid electrodes in photoelectrochemical water oxidation cells. We believe the proton pumping property of bR can be used in a variety of applications, especially those related to third generation photovoltaic cells.

Nanoscience and Nanostructures for Photovoltaics and Solar Fuels

Quantum confinement of electronic particles (negative electrons and positive holes) in nanocrystals produces unique optical and electronic properties that have the potential to enhance the power conversion efficiency of solar cells for photovoltaic and solar fuels production at lower cost. These approaches and applications are labeled third generation solar photon conversion. Prominent among these unique properties is the efficient formation of more than one electron-hole pair (called excitons in nanocrystals) from a single absorbed photon. In isolated nanocrystals that have three-dimensional confinement of charge carriers (quantum dots) or two-dimensional confinement (quantum wires and rods) this process is termed multiple exciton generation. This Perspective presents a summary of our present understanding of the science of optoelectronic properties of nanocrystals and a prognosis for and review of the technological status of nanocrystals and nanostructures for third generation photovoltaic cells and solar fuels production.

Influence of secondary phases during annealing on re-crystallization of CuInSe 2 electrodeposited films

Electrodeposited CuInSe 2 thin films are of potential importance, as light absorber material, in the next generation of photovoltaic cells as long as we can optimize their annealing process to obtain dense and highly crystalline films. The intent of this study was to gain a basic understanding of the key experimental parameters governing the structural–textural-composition evolution of thin films as function of the annealing temperature via X-ray diffraction, scanning/transmission electron microscopy and thermal analysis measurements. The crystallization of the electrodeposited CuInSe 2 films, with the presence of Se and orthorhombic Cu 2 − x Se (o-Cu 2 − x Se) phases, occurs over two distinct temperature ranges, between 220 °C and 250 °C and beyond 520 °C. Such domains of temperature are consistent with the melting of elemental Se and the binary CuSe phase, respectively. The CuSe phase forming during annealing results from the reaction between the two secondary species o-Cu 2 − x Se and Se (o-Cu 2 − x Se + Se → 2 CuSe) but can be decomposed into the cubic β-Cu 2 − x Se phase by slowing down the heating rate. Formation of liquid CuSe beyond 520°C seems to govern both the grain size of the films and the porosity of the substrate–CuInSe 2 film interface. A simple model explaining the competitive interplay between the film crystallinity and the interface porosity is proposed, aiming at an improved protocol based on temperature range, which will enable to enhance the film crystalline nature while limiting the interface porosity.

Photovoltaic and photoelectrochemical conversion of solar energy

The Sun provides approximately 100,000 terawatts to the Earth which is about 10000 times more than the present rate of the world's present energy consumption. Photovoltaic cells are being increasingly used to tap into this huge resource and will play a key role in future sustainable energy systems. So far, solid-state junction devices, usually made of silicon, crystalline or amorphous, and profiting from the experience and material availability resulting from the semiconductor industry, have dominated photovoltaic solar energy converters. These systems have by now attained a mature state serving a rapidly growing market, expected to rise to 300 GW by 2030. However, the cost of photovoltaic electricity production is still too high to be competitive with nuclear or fossil energy. Thin film photovoltaic cells made of CuInSe or CdTe are being increasingly employed along with amorphous silicon. The recently discovered cells based on mesoscopic inorganic or organic semiconductors commonly referred to as 'bulk' junctions due to their three-dimensional structure are very attractive alternatives which offer the prospect of very low cost fabrication. The prototype of this family of devices is the dye-sensitized solar cell (DSC), which accomplishes the optical absorption and the charge separation processes by the association of a sensitizer as light-absorbing material with a wide band gap semiconductor of mesoporous or nanocrystalline morphology. Research is booming also in the area of third generation photovoltaic cells where multi-junction devices and a recent breakthrough concerning multiple carrier generation in quantum dot absorbers offer promising perspectives.

Comparison of dilute nitride growth on a single- and 8×4-inch multiwafer MOVPE system for solar cell applications

The dilute nitrides like (GaIn)(NAs) or Ga(NAsSb) are attractive materials for the next generation of photovoltaic cells with four to six active junctions. Unfortunately, these compounds suffer from a low minority carrier diffusion length. This can be partially compensated by choosing suitable device structures with a reduced prerequisite on the necessary current density. (GaIn)(NAs) solar cells with a bandgap between 1.0 and 1.2 eV have been grown by metal organic vapor-phase epitaxy on single- and 8×4-in multiwafer reactors. Short-circuit current densities up to 10.9 mA/cm 2 (AM0) have been achieved for a (GaIn)(NAs) cell filtered with GaAs. This excellent value is sufficient for the application in five- or six-junction photovoltaic cells. Challenges are resulting from the transfer of growth conditions to a production size multi-wafer MOVPE reactor, which are discussed in this paper.

Photoelectrochemical cells

Until now, photovoltaics--the conversion of sunlight to electrical power--has been dominated by solid-state junction devices, often made of silicon. But this dominance is now being challenged by the emergence of a new generation of photovoltaic cells, based, for example, on nanocrystalline materials and conducting polymer films. These offer the prospect of cheap fabrication together with other attractive features, such as flexibility. The phenomenal recent progress in fabricating and characterizing nanocrystalline materials has opened up whole new vistas of opportunity. Contrary to expectation, some of the new devices have strikingly high conversion efficiencies, which compete with those of conventional devices. Here I look into the historical background, and present status and development prospects for this new generation of photoelectrochemical cells.