Efficiency Of Photovoltaic Solar Cells Research Articles

Tb3+/Yb3+ codoped silica–hafnia glass and glass–ceramic waveguides to improve the efficiency of photovoltaic solar cells

In this paper we present the investigation of the energy transfer efficiency between Tb3+ and Yb3+ ions in silica–hafnia waveguides. Cooperative energy transfer between these two ions allows to cut one 488nm photon in two 980nm photons and could have important applications in improving the performance of photovoltaic solar cells. Previous works revealed that for a given concentration of donors (Tb3+), increasing the number of acceptors (Yb3+) located near to the Tb3+ ion can increase the Tb–Yb transfer probability. However, when increasing the density of active ions, some detrimental effects due to cross-relaxation mechanisms become relevant. On the basis of this observation the sample doping was chosen keeping constant the molar ratio [Yb]/[Tb]=4 and the total rare earths contents were [Tb+Yb]/[Si+Hf]=5%, 7%, 9%. The choice of the matrix is another crucial point to obtain an efficient down conversion processes with rare earth ions. To this respect a 70SiO2–30HfO2 waveguide composition was chosen. The comparison between the glass and the glass–ceramic structures demonstrated that the latter is more efficient since it combines the good optical properties of glasses with the optimal spectroscopic properties of crystals activated by luminescent species. A maximum transfer efficiency of 55% was found for the highest rare earth doping concentration.

The effect of snow covering on photovoltaic solar cell efficiency

The effect of snow covering on photovoltaic solar cell efficiency

Energy conversion in Er/Eu:LiNbO3 for enhanced near-infrared and ultra-violet light harvesting

In order to enlarge the full solar spectrum and to increase the photovoltaic efficiency of solar cells, converting near-Infrared (NIR) and Ultra-violet (UV) light into visible emissions in Er/Eu:LiNbO3 was studied. Under 980nm/1550nm excitation, the visible green and red upconversion (UC) emissions were attributed to the 4S3/2/2H11/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ ion, respectively. The Eu3+ ions could convert the UV light (292nm/323nm/376nm/397nm) into the visible emissions. Both transitions of 7F0→5L5 of Eu3+ and 4I15/2→4G11/2 of Er3+ could absorb the 376nm UV light and convert into the visible fluorescence. The energy conversion mechanisms in Er/Eu:LiNbO3 were discussed under different wavelength laser excitation.

Photoluminescence of rare-earth ion (Eu3+, Tm3+, and Er3+)-doped and co-doped ZnNb2O6 for solar cells**Project supported by the National Natural Science Foundation of China (Grant Nos. 10572155 and 10732100) and the Research Fund for the Doctoral Program of Ministry of Education, China (Grant No. 20130171130003).

Visible converted emissions produced at an excitation of 286 nm in ZnNb2O6 ceramics doped with rare-earth ions (RE = Eu3+, Tm3+, Er3+ or a combination of these ions) were investigated with the aim of increasing the photovoltaic efficiency of solar cells. The structure of RE:ZnNb2O6 ceramics was confirmed by x-ray diffraction patterns. The undoped ZnNb2O6 could emit a blue emission under 286-nm excitation, which is attributed to the self-trapped excitons’ recombination of the efficient luminescence centers of edge-shared NbO6 groups. Upon 286-nm excitation, Eu:ZnNb2O6, Tm:ZnNb2O6, and Er:ZnNb2O6 ceramics showed blue, green, and red emissions, which correspond to the transitions of 5D0 → 7FJ (J = 1−4) (Eu3+), 1G4 → 3H6 (Tm3+), and 2H11/2/4S3/2 → 4I15/2 (Er3+), respectively. The calculated CIE chromaticity coordinates of Eu:ZnNb2O6, Tm:ZnNb2O6, and Er:ZnNb2O6 are (0.50, 0.31), (0.14, 0.19), and (0.29, 0.56), respectively. RE ion-co-doped ZnNb2O6 showed a combination of characteristic emissions. The chromaticity coordinates of Eu/Tm:ZnNb2O6, Eu/Er:ZnNb2O6, and Tm/Er:ZnNb2O6 were calculated to be (0.29, 0.24), (0.45, 0.37), and (0.17, 0.25).

Dual-band perfect metamaterial absorber for solar cell applications

The efficiency of solar photovoltaic (PV) cells has been one of the major problems impeding its global adoption as one of the sustainable substitutes to fossil fuel based technologies. Metamaterial (MTM) based solar cells offer an opportunity towards increasing the system efficiency by enhancing the total absorbed solar radiation incident on this device. In this study, a nanostructure-based MTM perfect absorber has been designed and simulated. By adjusting geometrical parameters and MTM structure properties, nearly perfect dual-band absorptions have been obtained with 99.99% and 99.90% absorption at 543.75 THz and 663.75 THz, respectively. The proposed structure is simple and more flexible for scaling, which helps achievement of multiple-band absorption. Implementation of the intended MTM structure can effectively lead to the realization of more efficient PV solar cells.

Radiation conversion in terbium acetylacetonate hydrate–doped poly methyl methacrylate

Terbium acetylacetonate hydrate (TAH)-doped poly methyl methacrylate (PMMA) with varying proportions was synthesized using the solvent evaporation method. Absolute spectral parameters of the samples under the excitation of a 308-nm light-emitting diode were derived adopting the integrating sphere method. The total radiant fluxes of typical visible emissions in 1, 2, and 3 wt% TAH doping PMMA samples are calculated to be 67.84, 80.16, and 93.90 μW, respectively, and the total luminous fluxes are determined to be 36.95, 43.16, and 50.18 millilumens. The results indicated that PMMA including TAH generates new applications as an efficient ultraviolet→visible conversion layer in enhancing the photovoltaic efficiency of solar cells.

Analysis of SnS2 hyperdoped with V proposed as efficient absorber material

Intermediate-band materials can improve the photovoltaic efficiency of solar cells through the absorption of two subband-gap photons that allow extra electron-hole pair formations. Previous theoretical and experimental findings support the proposal that the layered SnS2 compound, with a band-gap of around 2 eV, is a candidate for an intermediate-band material when it is doped with a specific transition-metal. In this work we characterize vanadium doped SnS2 using density functional theory at the dilution level experimentally found and including a dispersion correction combined with the site-occupancy-disorder method. In order to analyze the electronic characteristics that depend on geometry, two SnS2 polytypes partially substituted with vanadium in symmetry-adapted non-equivalent configurations were studied. In addition the magnetic configurations of vanadium in a SnS2 2H-polytype and its comparison with a 4H-polytype were also characterized. We demonstrate that a narrow intermediate-band is formed, when these dopant atoms are located in different layers. Our theoretical predictions confirm the recent experimental findings in which a paramagnetic intermediate-band material in a SnS2 2H-polytype with 10% vanadium concentration is obtained.

Correlation between ultrafast transient photomodulation spectroscopy and organic photovoltaic solar cell efficiency based on RR-P3HT/PCBM blends

Ultrafast transient spectroscopy was applied to various films of regio-regular polythiophene (RR-P3HT, donor-D) and C60 derivative (PCBM, acceptor-A) blends, in conjunction with organic photovoltaic (OPV) solar cell fabrication and evaluation based on the same blends, for investigating the existence of a correlation between the device efficiency and the transient photophysics characteristics. For our transient spectroscopy measurements we used the ps pump–probe transient photomodulation (PM) technique having a unique probe spectral range in the mid-IR (0.25–1.05eV). We found that the transient PM spectra contain photoinduced absorption bands of excitons in the donor polymer, charge transfer excitons (CTE) at the D–A interfaces, and free polarons. We compared the relative density of photogenerated CTE in D–A blends having various D–A weight ratio with the photocurrent density of fabricated solar cells based on the same blends. We found that the dissociation of CTE into free charges correlates well with the optoelectronic measurements of the corresponding solar cell. The more efficient CTE dissociation occurs in films having the optimum D–A weight ratio (which is 1.2:1 for the P3HT/PCBM system) that shows the highest OPV power conversion efficiency; this is due to the lowest CTE binding energy for this blend that results from the most suitable D- and A- grain sizes. We also show that the exciton lifetime is the shortest for the optimum blend, and this helps boosting the device efficiency by reducing energy loss.

Optimization the Efficiency Photovoltaic Solar Cells Using Synthesized TiO2 Semiconductor Nanomaterials and Functionalized Carbon Nanotubes.

In this project there were prepared solar cells using substrates of ITO on glass. The cell assembly was performed using carbon-ITO as cathode and, fourth types of anodes: (1) ITO-TiO2 commercial particles, (2) ITO-TiO2 thin film, (3) ITO-TiO2+SWCNT (functionalized single wall carbon nanotubes), (4) ITO- TiO2+ MWCNT (functionalized multi wall carbon nanotube), as anode respectively. For these types of solar cells was used solution organic dye Congo Red with ethanol, iodine electrolyte solution, between the two electrodes producing a photovoltaic cell Grätzel.Studies of X-Ray diffraction (XRD) in the synthesized nanoparticles of TiO2 showed peaks corresponding to anatase structure, with particle size of 10nm, and large surface area of 6 m2 / g, too these showed higher stability potential that TiO2 commercial particle.Preliminary studies showed that dye sensitized solar cell containing TiO2 nanoparticules and SWCNT showed the higher potential. Observed too that, the surface resistance is higher in the MWCNT then in the SWCNT.

Strong 1550 nm to visible luminescence in In/Er/Yb:LiNbO3 crystal considered as an upconverter for solar cells

Enhanced green and red upconversion emissions are observed in Er/Yb:LiNbO3 codoped with 3 mol% In3+ ions under both 1550 nm and 980 nm excitation, which provides more probabilities to increase efficiently the solar spectrum response and further increase the photovoltaic efficiency of solar cells. The energy transfer (ET: 2F7/2 Yb + 4I11/2 Er → 2F5/2 Yb + 4I15/2 Er) and the energy back transfer (EBT: 2F5/2 Yb + 4I13/2 Er → 2F7/2 Yb + 4F9/2 Er) processes are also responsible for the population of the red emitting 4F9/2 state (Er) under 1550 nm excitation. The increased red emission is attributed to the dissociation of Er3+ cluster sites in the In/Er/Yb:LiNbO3 crystal.

Upconversion emissions induced by 1550 nm in near-stoichiometric Er:LiNbO_3 crystal

To increase the photovoltaic efficiency of solar cells, green and red upconversion (UC) emissions produced at an excitation of 1550 nm were investigated. A drastically enhanced red UC emission was observed in near-stoichiometric LiNbO(3) crystal heavily doped with Er(3+) ions (Er:NSLN). Raman spectra showed that the maximum phonon energy shifted from 631 cm(-1) in Er(3+)-doped congruent LiNbO(3) (Er:CLN) to 871 cm(-1) in Er:NSLN crystal. The time decay of the (4)S(3/2)→(4)I(15/2) transition suggested that the Er(3+) cluster sites (Er(Li)(2+)-Er(Nb)(2-)) were dissociated in the Er:NSLN crystal. The visible UC luminescence converted from near infrared at the wavelength of 1550 nm was important to enlarge the solar spectrum response of solar cell.

The Dynamics of Singlet Fission in Crystalline Tetracene and Covalent Analogs

Singlet fission (SF) is a spin-allowed process in which an excited singlet state spontaneously splits into a pair of triplet excitons. This relaxation pathway is of interest as a mechanism for increasing the efficiency of photovoltaic solar cells, since ionization of the triplets could produce two charge carriers per absorbed photon. In this Account, we summarize our recent work on trying to understand how SF occurs using both covalent and noncovalent assemblies of tetracene. We first give a brief overview of the SF process and discuss why tetracene, where the singlet and triplet pair energies are nearly degenerate, is a particularly useful molecule for studying this process. Then we describe our experiments, beginning with the study of phenylene-linked tetracene dimers as covalent analogs for the crystal form, where SF is known to be very efficient. We found that only 2-3% of the initially excited singlets underwent SF in these dimers. These results motivated us to study crystalline tetracene in more detail. Transient absorption and photoluminescence experiments on polycrystalline thin films provided evidence for a delocalized singlet exciton that decays with a complicated temperature-dependence, but we were unable to unambiguously identify the signature of the triplet pair formed by SF. Then, using ultrathin single crystals, we observed quantum beats in the delayed fluorescence arising from recombination of spin-coherent triplet pairs. Analyzing these quantum beats revealed that SF proceeds through a direct one-step process occurring within 200 ps at room temperature. The product of this reaction is a pair of unperturbed triplets that have negligible interaction with each other. Looking at the overall SF process in tetracene, remaining issues that need to be clarified include the role of exciton diffusion, the temperature dependence of the SF rate, and how to use insights gained from the solid-state studies to generate design principles for high-efficiency covalent systems. Our experiments provide a good illustration of why the polyacenes, and tetracene in particular, play an important role as systems for the study of SF.

Computational Nano-Materials Design of Low Cost and High Efficiency Cu2ZnSn[Se1-xSx]4 Photovoltaic Solar Cells by Self-Organized Two-Dimensional Spinodal Nanodecomposition

We propose the possibility of spinodal nanodecomposition in Cu2ZnSn[Se1-xSx]4 (CZTSeS) for high efficiency photovoltaic solar cells, based on the first-principles calculations within the self-interaction-corrected local density approximation. By using the Korringa–Kohn–Rostoker coherent potential approximation method, electronic structures of CZTSeS are calculated. Due to the calculated positive mixing energy and type II band alignment between Cu2ZnSnSe4 and Cu2ZnSnS4, we can expect that the efficiency of CZTSeS becomes higher by spinodal nano-decomposition. Then we simulate the self-organized two-dimensional spinodal nanodecomposition by Monte Carlo method using the Ising model with chemical pair interactions calculated from the first principles.

Spin-enhanced organic bulk heterojunction photovoltaic solar cells

Recently, much effort has been devoted to improve the efficiency of organic photovoltaic solar cells based on blends of donors and acceptors molecules in bulk heterojunction architecture. One of the major losses in organic photovoltaic devices has been recombination of polaron pairs at the donor-acceptor domain interfaces. Here, we present a novel method to suppress polaron pair recombination at the donor-acceptor domain interfaces and thus improve the organic photovoltaic solar cell efficiency, by doping the device active layer with spin 1/2 radical galvinoxyl. At an optimal doping level of 3 wt%, the efficiency of a standard poly(3-hexylthiophene)/1-(3-(methoxycarbonyl)propyl)-1-1-phenyl)(6,6)C(61) solar cell improves by 18%. A spin-flip mechanism is proposed and supported by magneto-photocurrent measurements, as well as by density functional theory calculations in which polaron pair recombination rate is suppressed by resonant exchange interaction between the spin 1/2 radicals and charged acceptors, which convert the polaron pair spin state from singlet to triplet.

First-principles materials design of CuInSe2-based high-efficiency photovoltaic solar cells

Based on ab initio electronic structure calculations by self-interaction-corrected local-density-approximation (SIC-LDA) with the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), we propose a materials design for high efficiency photovoltaic solar cells (PVSCs). It is shown that (i) the concentration dependence of the mixing energy of CuIn1−xGaxSe2 shows upward convexity, thus this system favors phase separation. Due to the type II band alignment between CuInSe2 and CuGaSe2, efficient electron–hole separation is realized in decomposed phase of this system. (ii) CuIn1−xZn0.5xSn0.5xSe2 has a direct band gap and no impurity state appears in the gap. Therefore, cost reduction is possible by using Zn and Sn instead of In. (iii) n-type CuAl1−xSnxS2 and p-type Cu1−xVCuxAlS2 have negative activation energy for doped impurities and are expected to be low-resistive transparent conducting sulfides, which should be useful for CuInSe2-based PVSCs.

Dithiapyrannylidenes as Efficient Hole Collection Interfacial Layers in Organic Solar Cells

One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph(4)) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space-charge effects and favors recombination-limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph(4) compared to PEDOT:PSS. In ambient conditions, photovoltaic cells using DITPY-Ph(4) exhibit an 8% increase in the current density, although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we present a detailed analysis of the photocurrent generation, showing that DITPY-Ph(4) IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.

Multi-band silicon quantum dots embedded in an amorphous matrix of siliconcarbide

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized bya magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy,glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electronmicroscopy were used to characterize the chemical composition and the microstructuralproperties. The results show that the sizes and size distribution of silicon quantum dots canbe tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon inthe matrix. A physicochemical mechanism is proposed to demonstrate this formationprocess. Photoluminescence measurements indicate a multi-band configuration due to thequantum confinement effect of silicon quantum dots with different sizes. ThePL spectra are further widened as a result of the existence of amorphous siliconquantum dots. This multi-band configuration would be extremely advantageousin improving the photoelectric conversion efficiency of photovoltaic solar cells.

Self-Assembling Fullerenes for Improved Bulk-Heterojunction Photovoltaic Devices

The fullerene adducts 1a and 1b, whose molecular shapes either promote or hinder the formation of 1-D stacks, have been examined for their potential to form 1-D wire-like domains in bulk-heterojunction organic solar cells. The photovoltaic efficiency of solar cells based on blends of the stacking fullerene 1a with regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) is greatly enhanced compared to nonstacking model fullerene 1b.

Photovoltaic solar cells performance at elevated temperatures

It is well known that efficiency of photovoltaic solar cells decreases with an increase of temperature, and cooling is necessary at high illumination conditions such as concentrated sunlight, or cosmic or tropical conditions. The purpose of present study was to investigate the opposite option: to make a cell work at relatively high temperature (around 100–200 °C) and use the excessive heat in a hybrid system of some kind to increase the total efficiency of solar energy utilization. We studied the temperature dependence of the solar cell parameters both theoretically and experimentally, for the basic cells with p–n junction and the Schottky barrier, taking account of the different carrier transport mechanisms and recombination parameters of the cell material. The possibility of usage of the concentrated sunlight was also taken into account. The experiments conducted in the temperature interval of 25–170 °C and the calculated data show a real possibility of construction of a two-stage solar-to-electric energy converter with high-temperature second stage, having the overall conversion efficiency of 30–40%.

Theoretical Aspects of Doping of Photovoltaic Materials

AbstractThe low-resistive p- and n-type transparent conductors are necessary for the efficient photovoltaic (PV) solar cells. We review a new valence control method of co-doping with doping Ga- (or In-, or Alsubstitutional impurity, and Li-interstitial impurity) donor and N-acceptor at the same time for the fabrication of a low-resistive p-type ZnO based upon the ab initio calculation. We compare our materials design to fabricate a low resistive p -type ZnO with the recent successful co-doping experiments. The Delafossite structure of CuAlO2 has great potentiality far p-type transparent conducting oxide to apply the high efficient photovoltaic solar-cells combined with n-type transparent conducting oxides such as ITO (indium tin oxides), SnO2 or ZnO. We have calculated the electronic structure, impurity levels, and formation energy of Cu-vacancy, Al-vacancy, Be, Mg, Ca acceptors at the Al-site, and Be, Mg, Ca donors at the Cu-site. Calculated acceptor energy levels are as follows; Cu-vacancy (-300meV from the VBM [valence band maximum]), Al-vacancy (671 meV from the VBM), Be-acceptor (85 meV from the VBM), Mg-acceptor (200 meV from the VBM), and Ca-acceptor (960 meV from the VBM). We find that the Mg impurity at the Cu-site is more stable than the Al-site. Therefore, Mg impurity acts as donor at the Cu-site in the CuAlO2. This is the reason why Mg doping reduces the p-type conductivity in CuAlO2. We propose the following valence control method for the fabrication of p-type CuAlO2; we should dope the high concentration of Cu-vacancy in order to form the impurity band with reducing the Cu vapor pressure during the PLD or MBE crystal growth method, or we dope the Mg or Be acceptors at the Al-site with reducing the Al vapor pressure and increasing the Cu vapor pressure during the thermal non-equilibrium crystal growth method such as MBE or MOCVD. We compare our materials design with the available experimental data. Finally, we propose the co-doping by 2PS(Se)+InCu+VCu for the fabrication of p-type CuInS2 or CuInSe2 based upon ab initio electronic structure calculation.