Sufficient Light Absorption Research Articles

Recent Advances in Organic Photovoltaics: Device Structure and Optical Engineering Optimization on the Nanoscale.

Organic photovoltaic (OPV) devices, which can directly convert absorbed sunlight to electricity, are stacked thin films of tens to hundreds of nanometers. They have emerged as a promising candidate for affordable, clean, and renewable energy. In the past few years, a rapid increase has been seen in the power conversion efficiency of OPV devices toward 10% and above, through comprehensive optimizations via novel photoactive donor and acceptor materials, control of thin-film morphology on the nanoscale, device structure developments, and interfacial and optical engineering. The intrinsic problems of short exciton diffusion length and low carrier mobility in organic semiconductors creates a challenge for OPV designs for achieving optically thick and electrically thin device structures to achieve sufficient light absorption and efficient electron/hole extraction. Recent advances in the field of OPV devices are reviewed, with a focus on the progress in device architecture and optical engineering approaches that lead to improved electrical and optical characteristics in OPV devices. Successful strategies are highlighted for light wave distribution, modulation, and absorption promotion inside the active layer of OPV devices by incorporating periodic nanopatterns/nanostructures or incorporating metallic nanomaterials and nanostructures.

Enhancement of light absorption by using light scattering and emitting dyes in organic photovoltaics

To enhance the power conversion efficiency of organic photovoltaics (OPVs), sufficient light absorption in the active layer is necessary to generate a greater number of excitons. We propose incorporating luminescent dyes into hole transport layer of OPVs as a route of enhancing light absorption, minimizing changes in the morphology and structure of the active layer; dyes scatter incoming collimated light isotropically, thereby increasing the distance light travels in the active layer, which leads to enhanced light absorption. In addition, dyes can absorb light in the portion of the spectrum where the active layer is transparent and re‐emit light in the portion of the spectrum corresponding to the absorption peak of the active layer. Coumarin 102 (C102) is a dye that has an absorption wavelength of 389 nm, which corresponds to the weakest absorption region of P3HT:PCBM blends. C102 is then able to isotropically re‐emit visible light, in the region that corresponds to the absorption spectrum of P3HT, into the active layer. Based on this effect, we demonstrate that the short circuit current density can be increased by 22.8% compared to the reference sample without any dye.

Proposal of Integrated Sensitized Solar Cell Films Based on Sputtered ZnO Thin Films

The dye-sensitized solar cell (DSC) is expected to be the next-generation device for energy conversion.1 One of the issues in the DSC is the internal resistivity. In conventional DSCs, as shown in Fig. 1(a), porous semiconductors are used in order to increase the number of adsorbed dye molecules on the semiconductor surface. In this case, although sufficient light absorption is available, the porous structure degrades the crystalline quality, suppressing electron mobility, and makes the carrier path in the semiconductor narrow, increasing the internal resistivity of DSCs. To solve this problem, we proposed the waveguide-type sensitized solar cell2based on thin-film semiconductors shown in Fig. 1(b). In this cell, guided lights in the thin films are used to generate photocurrents instead of normally-incident lights. In such “guided light” configuration, the lights pass through a substantial number of dye molecules to enhance light absorption, consequently, enhance photocurrents. A schematic illustration of the proposed integrated sensitized solar cell film is shown in Fig. 2. The waveguide-type sensitized solar cells are embedded in a light beam collecting film consisting of two stacked films; a film with tapered vertical waveguides and a film with planar waveguides having vertical mirrors. Incident lights are collected by the tapered vertical waveguides into the planar waveguides through the vertical mirrors, and are guided to the waveguide-type sensitized solar cells. Following advantages are expected in the integrated sensitized solar cell film. -Reduction of semiconductor material consumption. -Availability of optical circuit functions. -Light-weight and flexible characteristics. In the present work, fabrication and evaluation of the tapered vertical waveguide films, and sensitization for ZnO thin films were investigated. A fabrication process of the tapered vertical waveguide films by the built-in mask method3is shown in Fig. 3. A photopolymer layer on an Al built-in mask is exposed to tilted ultra-violet (UV) beams from different directions to make cured regions having beveled walls. After development, by removing the removable layer, a tapered vertical waveguide film is obtained. In Fig. 4(a), a photograph for a top view of a tapered vertical waveguide film is presented. Light beam collecting efficiency of the tapered vertical waveguide is shown in Fig. 4(b). The efficiency was evaluated by introducing white light of a halogen lamp from the upper side and measuring the power of the light that transmits through the Al built-in mask window. The tapered vertical waveguide with a taper angle of 30othat is placed in front of the Al built-in mask increases the light beam collecting efficiency by 2~3 times, confirming the light beam collecting capability of the tapered vertical waveguides. The efficiency will be improved by optimizing the structure and the fabrication process. For sensitization of ZnO thin films, to date, we have investigated photocurrents generated in vacuum-evaporated ZnO thin films with two-dye stacked structures2 grown on the surface by the liquid-phase molecular layer deposition (LP-MLD)2,4,5, and demonstrated that guided lights propagated in the vacuum-evaporated ZnO thin films enhance the dye sensitization capability by 4~10 times compared to the normally-incident lights. One concern about the vacuum-evaporated ZnO thin films is that the film quality is not high, which limits the solar energy conversion efficiency. To improve the film quality of ZnO, we attempt to apply ZnO thin films deposited by the sputtering to the semiconductor instead of the vacuum-evaporated ZnO. Figs. 5(a) and (b) show two proposed sensitization mechanisms for the sputtered ZnO thin films. In Fig. 5(a), a two-dye stacked structure is grown using LP-MLD by sequentially providing p-type dye (rose bengal: RB) and n-type dye (crystal violet: CV) on sputtered ZnO thin film surfaces. This structure is expected to widen the photocurrent spectra as shown in Fig. 5(c) because electrons excided in the p-type dye are injected into ZnO directly, and electrons excided in the n-type dye are injected into ZnO through the p-type dye. In the model shown in Fig. 5(b), the two-dye stacked structure is replaced with a ZnO/Cr2O3 multi-layer, which is regarded as a multiple quantum well consisting of Cr2O3quantum wells with different widths. This structure is expected to widen the photocurrent spectra, too. Experiments on these attempts are under way. References 1 B. O’Regan and M. Gratzel, Nature 353, 737 (1991). 2 T. Yoshimura, H. Watanabe, and C. Yoshino, J. Electrochem. Soc., 58, 51 (2011). 3 T. Yoshimura, M. Ojima, Y. Arai, and K. Asama, IEEE J. Select. Topics in Quantum Electron., 9, 49 (2003). 4T. Yoshimura, Japanese Patent, Tokukai Hei3-60487 (1991) [in Japanese]. 5 T. Yoshimura, Japanese Patent, Tokukai 2009-60487 (2009) [in Japanese]. Figure 1

Photocatalytic generation of hydrogen by core-shell WO₃/BiVO₄ nanorods with ultimate water splitting efficiency.

Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it’s optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72 mA cm−2 under 1 sun illumination at 1.23 VRHE that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56 mA cm−2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.

A two-storey structured photoanode of a 3D Cu2ZnSnS4/CdS/ZnO@steel composite nanostructure for efficient photoelectrochemical hydrogen generation.

A two-storey structured photoanode of a 3D Cu2ZnSnS4(CZTS)/CdS/ZnO@steel composite nanostructure has been fabricated by using the solution method and demonstrated highly efficient photoelectrochemical hydrogen generation due to its contraption in the structure for sufficient light absorption as well as the three step-down band alignments for efficient charge separation and transport. This composite structure is composed of two storeys: the upper storey is the CZTS/CdS/ZnO hetero-nanorods (NRs) covered on the stainless steel mesh; the bottom storey is the CZTS/CdS/ZnO hetero-NRs grown on the FTO glass. The CZTS/CdS/ZnO hetero-NRs have cascade band gaps decreasing from 3.15 to 1.82 eV, which gives them efficient charge transfer and broad photoresponse in the UV to near-IR region, resulting in 47% IPCE in a wide light region from 400 to 500 nm; and the stainless steel mesh serves not only as a conductor for charge transport, but also as a skeleton of the grid structure for absorbing more light. The related mechanism has been investigated, which demonstrates that the two-storey CZTS/CdS/ZnO@steel composite nanostructure would have great potential as a promising photoelectrode with high efficiency and low cost for PEC hydrogen generation.

Nanostructuring methods for enhancing light absorption rate of Si-based photovoltaic devices: A review

In recent years, there has been growing consideration of renewable energy especially photovoltaic devices. A silicon (Si) based solar cell is the most popularly and frequently considered among the photovoltaic devices, but its bulk thickness issue lowers the performance and hinders widespread application due to the material cost. Also, this thick nature causes difference in length between minority carrier diffusion and sufficient light absorption. To mitigate the issues there have been many recent studies on Si photovoltaic devices adopting nanostructuring strategies to enhance the performance. Therefore, we report two different approaches on recent nanostructuring techniques for photovoltaic devices; bottom-up and top-down processes, which are composed of vapor-liquid-solid, solution-liquid-solid, reactive ion etching with Langmuir Blodgett and metal assisted chemical etching. Those fabrication processes enable the fabrication of nanostructures with a highly ordered and alignment structures leading to enhance the light absorption and have an appropriate thickness of Si substrate regressing Auger recombination. The fabricated nanowire and nanocone array structures outperform existing results with light absorption exceeding 90%.

Thick titania films with hierarchical porosity assembled from ultrasmall titania nanoparticles as photoanodes for dye-sensitized solar cells

Thin mesoporous titania films prepared by surfactant templating feature some of the highest light conversion efficiencies per thickness ratios as anodes in dye-sensitized solar cells (DSCs). However, the fabrication of thicker films required for sufficient light absorption is very challenging using this approach, often resulting in cracking and delamination of the films. Here we present a simple and scalable method to prepare thick mesoporous titania photoanodes via a surfactant-directed assembly of crystalline ultra-small TiO2 nanoparticles in combination with phase separation due to ethyl cellulose added to the coating solutions. Along with increasing film thickness, the ethyl cellulose introduces an interpenetrating macropore network into the films, leading to the formation of hierarchical porous films with bimodal porosity, with the smaller mesopores resulting from the structure-directing agent, Pluronic F127. In this way, films of up to 2 μm per layer without delamination can be produced, exhibiting a high surface area of 130 m2 g−1, about twice the value of films based on standard TiO2 nanoparticle paste. The preparation of multilayer films by a sequential spin-coating and calcination procedure enables the production of films with an overall thickness of up to 10 μm in only 5 steps, which showed high efficiencies of 7.7% in dye-sensitized solar cells.

Self-organized colloidal quantum dots and metal nanoparticles for plasmon-enhanced intermediate-band solar cells

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude. In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance. The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.

Optimisation of the physical properties of InAs/InGaAs/GaAs QDs heterostructures embedded p-i-n GaAs solar cell

In order to enhance absorption at infrared range for GaAs–based solar cell, we have grown multi–stacked InAs/InGaAs/GaAs quantum dots (QDs) heterostructure in the active region by Solid–Source Molecular Beam Epitaxy (SS–MBE). Two different families of dots were observed in the photoluminescence (PL) spectra. Temperature–dependent study was carried out at 10–240 K temperature range. Distinctive, asymmetric shape located in the high energy for the PL spectra of the QD solar cell sample can be deconvoluted in two sub–bands. From the temperature–dependent PL measurement, the two sub–bands are associated with the ground–state emission from the two families of InAs dots with different size. Besides, the spectral response of multi–stacked InAs/InGaAs QD solar cells extends the photo–absorption spectra towards a wavelength longer than the GaAs bandgap of 1280 nm. However, the QDs solar cell shows an enhanced short–circuits current density of 7.8 mA/cm² compared to the GaAs reference cell. The performance of the QD solar cells indicates that the InAs/InGaAs/GaAs QD heterostructures facilitate the fabrication of highly stacked QD layers that are suitable for solar cells devices requiring thick QD layers for sufficient light absorption.

A quantum-well superlattice solar cell for enhanced current output and minimized drop in open-circuit voltage under sunlight concentration

Insertion of quantum wells (QWs) extends the absorption edge to a longer wavelength than the value of a p-i-n cell without the QWs, which is preferable for the improved current matching of a InGaP/GaAs/Ge multijunction cell. The QWs, however, reduce the open-circuit voltage (Voc) and degrade the fill factor; the latter is significant for a large number of QWs that are mandatory for sufficient light absorption. As a structure to minimize these drawbacks, a QW superlattice, a strain-balanced In0.13Ga0.86As (4.7 nm)/GaAs0.57P0.43 (3.1 nm) stack, was implemented by metalorganic vapour-phase epitaxy. It brought about an enhancement in short-circuit current density (3.0 mA cm−2) with a minimal drop in Voc(0.03 V) compared with a p-i-n cell without the superlattice. The collection efficiency of photocarriers from the wells to an external circuit was evaluated: the efficiency was above 0.95 for the superlattice, while it was below 0.8 at a large forward bias for a conventional QW cell with thicker barriers. With the fast electron–hole separation in the superlattice owing to tunnelling transport, the superlattice cell exhibited a steeper increase in Voc as a function of the sunlight concentration ratio than the conventional QW cell: at the concentration ratio of 50, the value of Voc for the superlattice cell was almost equivalent to the value of the GaAs p-i-n cell without QWs. As a possible mechanism behind such an enhancement in Voc, photocurrent generation by two-step photon absorption was observed, using the electron ground state of the superlattice as an intermediate state.

Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells

We report ultra-high stacked InGaAs/GaAs quantum dot (QD) solar cells fabricated by the intermittent deposition of In0.4Ga0.6As under an As2 source using molecular beam epitaxy. We obtain a 400-stack In0.4Ga0.6As QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 1013 cm−2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In0.4Ga0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multi-stacked In0.4Ga0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.

Two-Dye-Molecule-Stacked Structures on ZnO Films Formed by Liquid-Phase Molecular Layer Deposition (MLD) for Waveguide-Type Photo-Voltaic Devices

We propose a waveguide-type two-dye-sensitized photo-voltaic device with flat thin film semiconductors. P-type dye molecule (rose bengal (RB)) and n-type dye molecule (crystal violet (CV)) were successively deposited on n-type ZnO thin films by the liquid-phase molecular layer deposition (MLD) to form a n/p/n structure of ZnO/p-type dye/n-type dye. The photocurrent spectra were widened in the n/p/n structure, where, electrons excited in the n-type dye molecules and the p-type dye molecules are injected into ZnO with large efficiency. The guided light configuration achieved sufficient light absorption in the dye molecules to enhance the photocurrents by about one-order of magnitude.

Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs QD structure of over 30 layers without using a strain compensation technique by the intermittent deposition of InGaAs layers. Moreover, there was no degradation in crystal quality. The external quantum efficiency of multi-stacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the quantum efficiency increases as the number of stacking layers increases. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers for sufficient light absorption.

Low temperature a-Si:H photodiodes and flexible image sensor arrays patterned by digital lithography

Hydrogenated amorphous silicon-based image sensor arrays were fabricated on polyethylene naphthalate substrates, with photodiodes optimized for process temperatures of 150°C. An optimal i-layer thickness was determined to minimize carrier recombination and to maintain sufficient light absorption and acceptable leakage current. Patterning of the thin-film transistor backplane was accomplished using ink-jet printed etch masks. A flexible image sensor is demonstrated with 75dots∕in. resolution over 180×180pixels and with sensitivity of 1.2pW∕cm2.

Electron diffusion in polymer:fullerene bulk heterojunctions

A method to study the bulk electron diffusion dynamics in poly2-methoxy-5-3 ,7 -dimethyloctyloxy1,4-phenylene-vinylene :1 -3-methoxycarbonyl-propyl-1-phenyl-6,6 C61 PCBM blend films with varying PCBM content is presented. The blends are spin coated on top of dense smooth TiO2 and excited with a visible laser pulse. The photogenerated electrons on PCBM diffuse through the blend film until they reach the interface with TiO2, which serves to collect the electrons. The resulting change in conductance of TiO2 is measured using the time-resolved microwave conductivity technique. Theoretical analysis of the timedependent photoconductance gives the electron diffusion constant De and the lower limit of the time for electrons to decay by trapping or recombination e. De increases from 7.010 13 m 2 /s at a weight fraction PCBM WPCBM=0.11 to 3.510 10 m 2 /s at WPCBM=0.50, while, interestingly, at WPCBM=0.75, a smaller value of 1.210 10 m 2 /s is found. The electron diffusion length increases with PCBM content. The presence of PCBM clusters in blends with a high PCBM content extends the time for electrons to decay, so that the lower limit of the electron diffusion length exceeds the film thickness of 100 nm, which is required for sufficient light absorption in a photovoltaic device.

Synthesis, characterization and fabrication of solar cells making use of [Ru(dcbpy)(tptz)X]X (where X = Cl −, SCN −, CN −) complexes

Dye-sensitized TiO 2 solar cells were fabricated using tridentate ligand ruthenium(II) complexes, [Ru(dcbpy)(tptz)X]X (where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, tptz = 2,4,6-Tris(2-pyridyl)- s-triazine and X = Cl −, SCN −, CN −) attached to sol–gel processed TiO 2 electrodes. The ligand tptz functions as spectator ligand and dcbpy functions as the anchoring ligand with sufficient visible light absorption. The synthesized complexes were characterized before using them in solar cells. The functioning of the solar cells fabricated using different conducting glasses was monitored and the current–voltage characteristics were measured. The efficiencies of different cells were calculated and compared.

On the mechanism of laser-induced desorption–ionization of organic compounds from etched silicon and carbon surfaces

The laser-induced desorption/ionization of organic compounds from etched carbon and silicon substrate surfaces was investigated. Two different etching procedures were used. Silicon surfaces were etched either by galvanostatic anodization to produce porous silicon or by a hyperthermal (∼5 eV) F-atom beam to produce nonporous silicon. Atomic force microscopy (AFM) images showed that both etching procedures yielded surfaces with sub-micrometer structures. Highly oriented pyrolytic graphite was etched with hyperthermal O atoms. A 337 nm ultraviolet (UV) laser and a 3.28 μm infrared (IR) laser were used for desorption. Analytes were deposited on the substrates either from the liquid or the gas phase. Mass spectra were obtained provided that three conditions were fulfilled. First, sufficient laser light had to be absorbed. When the IR laser was employed, a thin physisorbed solvent layer was required for sufficient laser light absorption to occur. Though the required fluence of IR and UV light differed by a factor of about 20, the calculated maximum surface temperatures were similar, about 1000 K. The second requirement was that the substrate had a “rough” surface. The third requirement, for the observation of protonated analytes, was that the aqueous pKa-value of the analyte be larger than about 4. These observations support the conclusion that the desorption–ionization mechanisms of analytes from porous and nonporous surfaces are very similar or essentially the same.

Quantitative cytochemistry using directly applied radioactive reagents.

WHILE radioactive isotopes have found valuable application in many cytochemical studies of metabolic incorporation processes, little use has so far been made of such labels in cytochemical reactions applied directly to the localization of substances in tissue specimens. Some qualitative applications to enzymic cytochemistry, for example, using metals or phosphorus-32 in deposited products, have been reported1. We have now examined the applicability of tritium-labelled reagents which form covalent bonds at specific sites in tissue components, as a method in quantitative cytochemistry. The advantages of such a method will include, inter alia (cf. ref. 2): (1) an escape from some of the problems of selectively introducing sufficient light-absorption at specific sites, since the label can be carried on many suitable simple reagent molecules; (2) autoradiographic measurement, which is free of certain of the problems of micro-spectrophotometry and is independent of the molecular environment of the target sites; (3) checks and calibrations of micro-spectrophotometric methods for the same components, and the possibility, at least in principle, of absolute determinations.