Energy Conversion Efficiency Of Cells Research Articles

Design of conduction band structure of TiO2 electrode using Nb doping for highly efficient dye‐sensitized solar cells

ABSTRACTA barrier layer of undoped TiO2 was deposited on the Nb‐doped TiO2 electrode to suppress the recombination at the Nb‐doped TiO2/dye–electrolyte interface for highly efficient dye‐sensitized solar cells (DSCs). The Nb content in TiO2 was varied in a range of 0.7–3.5 mol% to modify the TiO2 energy‐band structure. Nb‐doped TiO2/dye interfaces were characterized by a combination of ultraviolet photoemission spectroscopy and optical absorption spectroscopy measurements, allowing the determination of the conduction band minimum (CBM) of the TiO2 electrode and the lowest unoccupied molecular orbital of the N719 dye. The lowering of TiO2 CBM by Nb doping induced the increase in short‐circuit current of DSCs. However, open‐circuit voltage and fill factor are decreased, and this result was ascribed to the enhanced recombination at the Nb‐doped TiO2/dye–electrolyte interface. The effect of doping on charge transport in DSCs was analyzed using electrochemical impedance spectroscopy. We have shown that by introducing of TiO2 barrier layer, the Nb doping content, which results in DSC highest efficiency, can be increased because of the suppression of the dopant‐induced recombination. The energy conversion efficiency of the solar cells increased from 7.8% to 9.0% when undoped TiO2 electrode is replaced with electrode doped with 2.7 mol% of Nb because of the improvement of the electron injection and collection efficiencies. The correlation between the electronic structure of the TiO2 electrode, charge transfer characteristics, and photovoltaic parameters of DSCs is discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Liquid carbon dioxide coating of CdS quantum-dots on mesoporous TiO2 film for sensitized solar cell applications

CdS quantum dots (QDs) were uniformly deposited on mesoporous TiO2 films by high-pressure coating with liquid carbon dioxide (l-CO2) as a coating solvent. The size and coverage of CdS QDs over the TiO2 surface were controlled by the number of l-CO2 coating cycles applied at a fixed precursor concentration. The formation of CdS QDs on the mesoporous TiO2 film was confirmed by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). In addition, depth profiling investigation revealed that CdS QDs were uniformly deposited throughout the ∼15μm-thick mesoporous TiO2 film. The energy conversion efficiency of a CdS QD-sensitized solar cell (QDSSC) was observed to increase up to ∼1.33% under standard conditions (1.5AM, 100mWcm−2) with an increase in the number of coating cycles to six. Meanwhile, a further increase in the number of coating cycles to ten caused a decrease in the cell performance, which may be attributed to the hindrance of electrolyte infiltration into the mesoporous TiO2 electrode caused by the blocking of the pores with the coated CdS. The cell performance of the l-CO2-coated CdS-QDSSC is slightly higher than that of the l-CO2-coated CdS-QDSSC prepared by a conventional successive ionic layer absorption and reaction (SILAR) method with ethanol as the solvent (1.16%).

Interface engineering: Boosting the energy conversion efficiencies for nanostructured solar cells

Nanostructured solar cells have attracted increasing attention in recent years because their low cost and ease of preparation offer unique advantages and opportunities unavailable with conventional single-crystalline solar cells. The efficiencies of this kind of solar cell largely depend on the interfacial structure owing to the large specific interface areas and the inherent high density of interface states. In this review article, strategies of interface engineering will be introduced in detail. The up-to-date progress and understanding of interface engineering and its role in influencing the efficiency of nanostructured solar cells will be discussed. Some of the representative examples of the interface engineering method will be presented wherever necessary. Continued boosting of the energy conversion efficiency for nanostructured solar cells is anticipated in the coming years and will bring this kind of solar cell to the status of commercialization.

Low temperature fabrication of high performance and transparent Pt counter electrodes for use in flexible dye-sensitized solar cells

High performance Pt counter electrode is prepared by using vacuum thermal decomposition at a relatively low (120°C) temperature on a flexible polyethylene naphthalate substrate coated with indium-doped tin oxide for use in flexible dye-sensitized solar cells. The obtained Pt counter electrode shows a good chemical stability, high light transmittance, and high electrocatalytic activity for the I −3 /I− redox reaction. The energy conversion efficiency of a flexible dye-sensitized solar cell based on the prepared Pt counter electrode and a TiO2/Ti photoanode reaches 5.14% under a simulated solar light irradiation with intensity of 100 mW cm−2.

Comparative experimental and simulative investigations of radial p–n junction Si microwire array solar cells

We carried out experimental and simulative investigations of the electrical and optical characteristics of radial p–n junction Si microwire (MW) array solar cells (diameter: 4μm, height: 12μm, and period: 12μm) prepared by electroless etching (EE) methods. Due to its unique geometry, the MW array had an enlarged junction area, enabling efficient carrier collection and optical reflectance in the wavelength range from 400 to 1000nm, which is lower than that of its planar counterpart. Despite these advantages, the energy conversion efficiency (CE) of the MW array solar cell (3.2%) was lower than that of a planar solar cell (4.2%). This comparative study clearly reveals the inherent potential of MW-based solar cells and the importance of extrinsic trap state control to realize low-cost, high-efficiency devices.

Impact ionization effects on the efficiency of the intermediate band solar cells

The thermodynamic limit of the maximum energy conversion efficiency of an intermediate band solar cell has been calculated taking into account the effect of impact ionization. We developed the calculations of impact ionization effect on the efficiency of the single-gap solar cells to intermediate band quantum dot solar cells by introducing the parameter probability (P) that a charge carrier will impact ionize. The parameter θ gives a criterion for the solar cell material selection to increase the probability and effectivity of the impact ionization by considering the effect of carriers effective masses on the energy conversion efficiency. A thermodynamic efficiency of about 81.2% is obtained, which is higher than the maximal efficiency of 63.2% for an intermediate band solar cell without Auger or impact ionization mechanism. The optimum bandgap and sub-bandgap is shifted towards the Silicon bandgap. Therefore, impact ionization effect is more applicable for sub-bandgap and bandgap energies below about 1eV and for photons with appropriate kinetic energies.

Dependence of energy conversion efficiency of dye-sensitized solar cells on the annealing temperature of TiO2 nanoparticles

Dye-sensitized solar cells (DSSCs) were fabricated from porous electrodes derived from sol–gel-synthesized (SGS) nanoparticles (NPs) of TiO2. Current–voltage measurements were performed to investigate performance characteristics of electrodes derived from SGS-NPs of TiO2 annealed at different temperatures. Experimental results indicate that the effects of bulk traps and surface states within TiO2 films on recombination of photo-injected electrons in DSSCs depend upon annealing temperature of SGS-TiO2 NPs. Moreover, electrodes fabricated from SGS-TiO2 showed higher photoelectric conversion efficiency than nonporous commercial (P25) TiO2 NPs. Porous structures within SGS-TiO2 NPs are of great benefit to sensitizer dye adsorption, and consequently to improvement of photo-electrochemical properties of DSSCs.

Efficiency enhancement of screen-printed multicrystalline silicon solar cells by integrating gold nanoparticles via a dip coating process

Multicrystalline silicon solar cells play an increasingly important role in the world photovoltaic market. Boosting the comparatively low energy conversion efficiency of multicrystalline silicon solar cells is of great academic and industrial significance. In this paper, Au nanoparticles of an optimized size, synthesized by the iterative seeding method, were integrated onto industrially available surface-textured multicrystalline silicon solar cells via a dip coating method. Enhanced performance of the light absorption, the external quantum efficiency and the energy conversion efficiency were consistently demonstrated, resulting from the light scattering by the sized-tailored Au nanoparticles placed on the front surface of the solar cells, particularly in the spectral range from 800 to 1200 nm, an enhancement of the external quantum efficiency by more than 11% near λ = 1150 nm and the short-circuit current by 0.93% were both observed. As a result, an increase in the energy conversion efficiency up to 1.97% under the standard testing conditions (25°C, global air mass 1.5 spectrum, 1000 Wm−2) was achieved. This study opens new perspectives for plasmonic nanoparticle applications for photon management in multicrystalline silicon solar cells.

Highly efficient nanoporous graphitic carbon with tunable textural properties for dye-sensitized solar cells

Nanoporous graphitic carbon (NGC) is synthesized by direct carbonization of iron phthalocyanine (Fe-PC) and 3D mesoporous silica (KIT-6) mixture for the first time, which exhibits tunable surface area and pore volume, as well as superior energy conversion efficiency for dye-sensitized solar cells compared to activated carbon (AC) and large porous carbon (LPC).

N-Ion-implanted TiO2 photoanodes in quantum dot-sensitized solar cells

Hierarchical nanostructured titanium dioxide (TiO(2)) clumps were fabricated using electrostatic spray with subsequent nitrogen-ion doping by an ion-implantation technique for improvement of energy conversion efficiency for quantum dot-sensitized solar cells (QDSCs). CdSe quantum dots were directly assembled on the produced N-ion-implanted TiO(2) photoanodes by chemical bath deposition, and their photovoltaic performance was evaluated in a polysulfide electrolyte with a Pt counter electrode. We found that the photovoltaic performance of TiO(2) electrodes was improved by nearly 145% upon N-ion implantation. The efficiency improvement seems to be due to (1) the enhancement of electron transport through the TiO(2) layer by inter-particle necking of primary TiO(2) particles and (2) an increase in the recombination resistance at TiO(2)/QD/electrolyte interfaces by healing the surface states or managing the oxygen vacancies upon N-ion doping. Therefore, N-ion-doped photoanodes offer a viable pathway to develop more efficient QD or dye-sensitized solar cells.

Metamaterial-Plasmonic Absorber Structure for High Efficiency Amorphous Silicon Solar Cells

We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.

Enhancement of the efficiency of dye-sensitized solar cell with multi-wall carbon nanotubes/polythiophene composite counter electrodes prepared by electrodeposition

For the purpose of increasing the energy conversion efficiency of dye-sensitized solar cells (DSSCs), multi-wall carbon nanotube (MWCNT)/polythiophene (PTh) composite film counter electrode has been fabricated by electrophoresis and cyclic voltammetry (CV) in sequence. The morphology and chemical structure have been characterized by transmission electron microscopy (TEM), scanning electron microscope (SEM), and Raman spectroscopy respectively. The overall energy conversion efficiency of the DSSC employing the MWCNT/PTh composite film has reached 4.72%, which is close to that of the DSSC with a platinum (Pt) counter electrode (5.68%). Compared with a standard DSSC with MWCNT counter electrode whose efficiency is 2.68%, the energy conversion efficiency has been increased by 76.12% for the DSSC with MWCNT/PTh counter electrode. These results indicate that the composite film with high conductivity, high active surface area, and good catalytic properties for I 3 − reduction can potentially be used as the counter electrode in a high-performance DSSC.

Dye-Sensitized Solar Cells Based on High Surface Area Nanocrystalline Zinc Oxide Spheres

High surface area nanocrystalline zinc oxide material is fabricated using mesoporous nanostructured carbon as a sacrificial template through combustion process. The resulting material is characterized by XRD, N2 adsorption, HR-SEM, and HR-TEM. The nitrogen adsorption measurement indicates that the materials possess BET specific surface area ca. 30 m2/g. Electron microscopy images prove that the zinc oxide spheres possess particle size in the range of 0.12 μm–0.17 μm. The nanocrystalline zinc oxide spheres show 1.0% of energy conversion efficiency for dye-sensitized solar cells.

Study on the Output Characteristics of Organic Photovoltaic Cells

Based on the analysis of the equivalent circuit model of organic photovoltaic (OPV) cells, the explicit expression of current, short-circuit current and open-circuit voltage was obtained by means of W-function, and the effects of internal resistances and diode quality factor on the output characteristics of OPV cells were studied. The results demonstrate that the series resistance can effect short-circuit current but not open-circuit voltage, and the shunt resistance and diode quality factor can influence open-circuit voltage but short-circuit current unchanged. The decrease in series resistance and the increase in shunt resistance and diode quality factor can enhance the fill factor and energy conversion efficiency of OPV cells. This conclusion is essential in preparing OPV cells and improving device performance.

Effects of passive cooling on performance of silicon photovoltaic cells

In this study, an experimental research concerning the effects of passive cooling on performance parameters of silicon solar cells was presented. An aluminum heat sink was used in order to dissipate waste heat from a photovoltaic (PV) cell. Dimensions of the heat sink were determined considering the results of a steady-state heat transfer analysis. The experiments were carried out for different ambient temperatures and various illumination intensities up to 1 sun under solar simulator. Experimental results indicate that energy, exergy and power conversion efficiency of the PV cell considerably increase with the proposed cooling technique. An increase of ∼20% in power output of the PV cell is achieved at 800 W/m-super-2 radiation condition. Maximum level of cooling is observed for the intensity level of 600 W/m-super-2. Performance of PV cells both with and without fins increases with decreasing ambient temperature. Copyright , Oxford University Press.

High Molar Extinction Coefficient Ru(II)-Mixed Ligand Polypyridyl Complexes for Dye Sensitized Solar Cell Application

Two new ruthenium(II) mixed ligand terpyridine complexes, “Ru(Htcterpy)(NCS)(L1) (N(C4H9)4), mLBD1” and Ru(Htcterpy)(NCS)(L2)(N(C4H9)4), mLBD2 were synthesized and fully characterized by UV-Vis, emission, cyclic voltammogram, and other spectroscopic means, and the structures of the compounds are confirmed by 1H-NMR, ESI-MASS, and FT-IR spectroscopes. The influence of the substitution of L1 and L2 on solar-to-electrical energy conversion efficiency (η) of dye-sensitized solar cells (DSSCs) was evaluated relative to reference black dye. The dyes showed molar extinction coefficients of 17600 M−1 cm−1 for mLBD1 and 21300 M−1 cm−1 for mLBD2 both at λ maximum of 512 nm, while black dye has shown 8660 M−1 cm−1 at λ maximum of 615 nm. The monochromatic incident photon-to-collected electron conversion efficiencies of 60.71% and 75.89% were obtained for mLBD1 and mLBD2 dyes, respectively. The energy conversion efficiencies of mLBD1 and mLBD2 dyes are 3.15% ( mA/cm2, mV, ) and 3.36% ( mA/cm2, mV, ), respectively, measured at the AM1.5G conditions, the reference black dye-sensitized solar cell, fabricated and evaluated under identical conditions exhibited η-value of 2.69% ( mA/cm2, mV, ).

Anatase TiO 2 sols derived from peroxotitanium acid and to form transparent TiO 2 compact film for dye-sensitized solar cells

Transparent and surfactant-free TiO 2 sols containing anatase nanocrystals were prepared by the hydrothermal treatment of water-soluble peroxotitanium acid (PTA) at a temperature of 120 °C. The TiO 2 nanocrystals were characterized by transmission electron microscopy (TEM). The TEM results indicated that the TiO 2 nanocrystals were nanorod-like with diameters of less than 7 nm after the subsequently hydrothermal treatment. A gradient layer between the transparent fluorine doped SnO 2 (FTO) layer and the porous titanium dioxide nanocrystalline film for dye-sensitized solar cells (DSSCs) photoelectrodes, was made with the as-prepared TiO 2 sols. The TiO 2 gradient layers were characterized by field-emission scanning electron microscopy and UV–vis absorption spectrometry. After the gradient layer deposition on the FTO coated glass, the composite multilayer film exhibited the visible light transmittance of 80% which approached to that of bare FTO glass. The photo-to-electric energy conversion efficiency of the N719 dye-sensitized solar cell had significantly improved from 4.2% to 5.6% in the presence of the compact layer between FTO and the porous TiO 2 nanocrystalline film under of AM1.5 illumination (100 mW/cm 2). The remarkable improvements in short-circuit current for the DSSCs was due to the effective gradient layer at the FTO–TiO 2 interface which prevented direct contact of electrolytes with FTO and consequently reduced charge recombination losses.

CdS quantum dots grown by in situ chemical bath deposition for quantum dot-sensitized solar cells

CdS quantum dots (QDs) are grown on mesoporous TiO2 films by 3 to 13 repeated cycles of in situ chemical bath deposition (CBD). The overall energy conversion efficiency of CdS quantum dot-sensitized solar cells (QD-SSCs) increases as the number of cycles increases to the eight — peaking at 2.13%. This is attributable to efficient light harvesting and charge-collection resulting from enhanced light absorption and faster charge transport. However a further increase of CBD cycles to thirteen reduces QD-SSCs performance, despite better light absorption in the long wavelengths. This is attributed to decreased charge-injection efficiency, which is due to reduced driving forces of carrier injection, increased trap-mediated recombination in the QDs, and hindered ion transport.

Realistic upconverter-enhanced solar cells with non-ideal absorption and recombination efficiencies

Upconverting materials can be used to increase the energy conversion efficiency of a solar cell. Such materials convert low-energy transmitted photons to higher-energy photons that can be absorbed by the cell, substantially reducing the spectral mismatch between the cell and the solar spectrum. Previously, the performance enhancements achievable with an ideal upconverter-solar cell system were theoretically investigated. Here, we perform a comprehensive analysis to determine the effects of non-ideal cell and upconverter systems, accounting for non-ideal absorption and radiative recombination. We also allow for realistic nonradiative relaxation within the upconverter. The system is modeled using a detailed balance approach, with the upconverter treated as a series connection of two small-bandgap solar cells and a large-bandgap light emitting diode. We demonstrate that significant improvements in efficiency are possible even for nonconcentrated light, as long as the upconverter includes a small nonradiative relaxation pathway. Furthermore, we show that the existence of a nonradiative relaxation event in the upconverter is necessary for improved power conversion when cell absorption efficiency is low. Our results indicate that the efficiencies of both conventional-Si and thin film photovoltaic cells can be substantially improved with upconverting materials, even including non-idealities.

Filling TiO 2 nanoparticles in the channels of TiO 2 nanotube membranes to enhance the efficiency of dye-sensitized solar cells

We have significantly enhanced the energy conversion efficiency of the dye-sensitized solar cells based on TiO 2 nanotube membranes by filling TiO 2 nanoparticles in the channels. By filling approximately 10 wt.% of TiO 2 nanoparticles to the membrane weight in the channels of the membrane, the energy conversion efficiency was improved from 5.2% to 6.3%, a 21% enhancement. By treating with TiCl 4 after filling TiO 2 nanoparticles in the channels, the efficiency was enhanced from 5.2% to 7.3% (a 40% enhancement). The efficiency enhancement is due to an increase in the total available surface area for dye adsorption.