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THE INVESTIGATION OF ALTERNATIVE METALS OF Au, Ag, Al AND Cu FOR BACK CONTACT USED IN FABRICATED ZnS/SnS HETEROJUNCTION FOR SOLAR CELL APPLICATION

THE INVESTIGATION OF ALTERNATIVE METALS OF Au, Ag, Al AND Cu FOR BACK CONTACT USED IN FABRICATED ZnS/SnS HETEROJUNCTION FOR SOLAR CELL APPLICATION

Self-Assembled Monolayers for Improved Charge Injection of Silver Back Electrodes in Inverted Organic Electronic Devices.

Self-assembled monolayers (SAMs) formed from thiol compounds bound to Ag and Au electrodes have been used as an important strategy in improving the stability and efficiency of optoelectronic devices. Thiol compounds provide only one binding site with the metal electrode which limits their influence. Dithiolane/dithiol compounds can provide multiple binding sites and could be useful in enhancing the performance of the device. In this study, inverted organic semiconducting hole-only devices were fabricated by using Ag back electrodes in conjunction with SAMs formed from disulfide lipoic acid-based compounds and were compared to a long aliphatic chain thiol. The binding and the electronic properties as well as electrical characteristics of the SAMs on silver were studied to look at the influence of their structure on charge injection in the organic semiconductor devices. It was found that the SAMs formed with (±)-α-lipoic acid, isolipoic acid, and (±)-4-phenylbutyl 5-(1,2-dithiolan-3-yl) pentanoate significantly improved the charge injection by either changing the work function of the Ag or altering the physical interaction between the polymer and the metal surface. This study may lead to an understanding of how the nature of the functional groups of the SAM and the number of bonds formed between each SAM molecule and the metal electrode influence the contact resistance and the performance of organic semiconductor devices.

PCBM/Ag interface dipole management in inverted perovskite solar cells

In inverted perovskite solar cells, the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/Ag back interface is quite important because serious recombination occurs and H2O/O2 directly attacks this interface to induce device instability. Here, bi-functional dipole layers are applied at the interface aiming at improving the charge transport and device stability simultaneously. Silane with -CF3 end group anchored on the PCBM surface induces more effective dipole effect than that with (CH2)7-CH3 end functional silane, bending the interface energy level to promote electron transport and reduce recombination. The hydrophobic nature of -CF3 also enhances the durability of the device. This work highlights the back surface dipole management method to achieve efficient and stable perovskite solar cells.

Near-IR reconfigurable 1D Ag grating Fabry-Perot absorber hybridized with phase-change material GSST.

Chalcogenide phase-change materials (PCMs) offer a unique feature that can be used to dynamically control the response of the photonic devices and achieve fast, nonvolatile, reversible, multilevel, and specific optical modulation. The phase-change material Ge2Sb2Se4Te1 (GSST) has recently received a lot of attention due to the large index contrast between its amorphous and crystalline states with significantly low optical loss in the optical to near-IR spectrum. In this paper, we propose a tunable and reconfigurable hybrid PCM plasmonic nanostructure composed of a spacer layer of GSST sandwiched between a Ag back reflector and a 1D Ag Fabry-Perot grating structure. We use the finite element method (FEM) to numerically calculate the light absorption, absorption contrast, and figure of merit of the plasmonic nanostructure for both the amorphous and crystalline state of the GSST. Our calculations show that with constant structural variation the observed multimode absorption is drastically modified when the GSST undergoes a phase change from the amorphous to the crystalline state. The absorption contrast spectrum, which is defined as the absorption difference between the amorphous and crystalline state of GSST, shows four extrema modes between 70% and 89%. The figure of merit spectrum shows two large values of 44.39 and 37.78 at the 1502 nm and 2063 nm wavelengths, respectively. We also address the observed modes in the absorption contrast spectrum through spatial representation of the enhanced electric field distribution at their corresponding wavelengths. We show how the phase change in the GSST spacer can control the coupling between the optical cavity modes and the Ag surface plasmon resonance modes in the cavities and GSST spacer strip boundaries. The findings in this paper may open new avenues toward the design of next-generation photonic systems such as thermal emission controllers, sensors, ranging holograms, modulators and optical detection devices.

Solvothermal Crystallization of Ag/AgxO-AgCl Composites: Effect of Different Chloride Sources/Shape-Tailoring Agents

In the present work, AgCl microcrystals were obtained by solvothermal crystallization to investigate the effect of H+, Na+, K+, and different shape-tailoring agents (non-ionic: polyvinylpyrrolidone vs. anionic: sodium dodecyl sulfate) on the textural and photocatalytic properties of the samples. The crystallization process resulted in secondary products, such as AgxO or Ag, AgClO3, AgClO4, which were further transformed during the photocatalytic tests. The most efficient photocatalyst (assessed for methyl orange degradation) was synthesized using HCl, as a chloride source and polyvinylpyrrolidone, as a shape-tailoring agent. Therefore, the ability of polyvinylpyrrolidone to enhance the photocatalytic activity was also investigated, and it was found that the addition of 0.6 g polyvinylpyrrolidone resulted in the most efficient photocatalyst. Moreover, AgxO, being a charge separator, could play a critical role in the photocatalytic process, while reversibly transforming to Ag back and forth.

Design of efficient radiative emission and daytime cooling structures with Si3N4 and SiO2 nanoparticle laminate films.

Research on radiative cooling has attracted recent widespread interest owing to the potential for low-cost passive structures to enable large-scale thermal energy management. Using a generalized effective medium theory, we theoretically show that two-layer films comprised of SiO2 and Si3N4 nanoparticle layers on an Ag back reflector exhibit superior radiative cooling compared to single-layer or two-layer dense solid films, and can outperform other reported designs. The performance enhancement is a result of the ability to tune the nanoparticle fill fraction, which improves the spectral match between emissivity of this structure and the atmospheric transmission window. We also propose a standardized method for comparing the performance of radiative cooling structures reported by the research community.

Numerical optimization of 0.5‐μm‐thick Cu (In1-xGax) Se2 solar cell

This paper presents a numerical simulation of 2D ultra-thin Cu (In1-xGax) Se2 solar cell under drift-diffusion transport across heterojunction interfaces. The validation of the CIGS model is performed by matching the electrical characteristics of the experimental and simulation results. The surface recombination velocity of CdS/CIGS heterojunction interface and element composition Ga/(In + Ga) ratio effect on the optical properties and electrical performances of CIGS absorber are investigated. The thickness dependence and carrier concentration of the semiconductor layers on the cell performance are investigated. The nanostructured Ag, Au, or Cu back mirror is used to replace conventional Mo back contact to improve light absorption in the ultra-thin CIGS layer. Best power conversion efficiency of 21.74% has been obtained with a thinner absorber of about 0.5‐μm‐thick under AM1.5 illumination condition, 300 K.

Synthesis and characterization of AgPd alloy coatings as beneficial catalysts for low temperature fuel cells application

The Ag, Pd and AgPd alloys of different morphologies and compositions were electrodeposited onto Au and glassy carbon (GC) disc electrodes from the solution containing 0.001 M PdCl2 + 0.04 M AgCl + 0.1 M HCl + 12 M LiCl under the conditions of non-stationary (RPM = 0, samples AgPd1 and AgPd2) and convective diffusion (RPM = 1000, sample AgPd3). Electrodeposited alloy layers were characterized by the scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) was investigated on all coatings in 0.1 M NaOH solution saturated with oxygen. The j-E curves for the ORR were recorded by two procedures: (1) samples were cycled with 5 mV s−1 from open circuit potential (OCP) to −0.8 V for Ag and AgPd alloys (or −0.6 V for pure Pd) and back; (2) samples were cycled with 5 mV s−1 from open circuit potential to 0.45 V (formation of Ag2O, in the case of Pd formation of PdO and PdO2), from 0.45 V to −0.60 V and back to the OCP. Significant catalytic activity for the Ag and AgPd alloys was detected after cycling electrodes in the potential region of Ag2O formation and reduction. Increase of the catalytic activity for AgPd alloys was, for the first time in the literature, ascribed to the presence of a certain amount of Ag2O which could not be completely reduced during the reverse sweep from 0.45 V to −0.6 V. Catalytic activity of AgPd alloys was found to be closely related to the amount of non-reduced Ag2O (most probably in the form of Ag-hydroxide). In the absence of such treatment, the catalytic activity for the ORR on electrodeposited Ag and AgPd alloy coatings was not detected.

Ag back electrode bonding process for inverted organic solar cells

A new style of forming Ag back electrode for organic solar cells (OSCs) using a bonding process was compared with a conventional screen-printed Ag. All the fabrication process steps have been conducted under atmospheric conditions at room temperature without using a glove box. The OSCs with both screen-printed and bonded Ag back electrode exhibited comparable performances with the average power conversion efficiencies of 1.28% and 1.31%, respectively. The series and the shunt resistance values for the OSC with the bonded Ag were slightly better than those with the screen printed Ag. The binder resin at the interface between the PEDOT:PSS and the Ag may increase contact resistance for the screen-printed Ag process, while that between the Ag and the PET film for the bonded Ag process may not increase it. The bonding process for the Ag back electrode has the potential for future electronic device applications.

Silver versus white sheet as a back reflector for microcrystalline silicon solar cells deposited on LPCVD‐ZnO electrodes of various textures

AbstractWe compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5‐µm‐thick low‐pressure chemical vapor deposition (LPCVD)‐ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD‐ZnO layer (with thickness below 200 nm). For this latter design, the optical loss in the nano‐rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar‐plasma treatment. Because of its superior lateral conductivity, the thin‐ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD‐ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin‐ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active‐layer thickness of only 1.1 µm. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright © 2014 John Wiley & Sons, Ltd.

Optimization of a-Si1–xGex:H single-junction and a-Si:H/a-Si1–xGex:H tandem solar cells with enhanced optical management

This work aimed at improving the optical management of a-Si:H/a-Si1–xGex:H tandem cells and a-Si1–xGex:H single-junction cells. To improve the optical management, the effects of the a-Si1–xGex:H bandgap, the bandgap graded absorber and the n-type μc-SiOx:H back reflecting layer on the cell performance were investigated. Optical reflection spectra, internal quantum efficiency, external quantum efficiency (EQE), and cell performance were used to evaluate the improvement of the optical properties of solar cells. The tandem cells with a-Si1–xGex:H bandgap of 1.53 eV exhibited sufficient optical absorption from 630 to 900 nm and thus lead to higher JSC. Second, the EQE of a-Si1–xGex:H single-junction cell was significantly enhanced from 630 to 720 nm by employing bandgap graded absorber that relatively improved the JSC by 3.8% despite that the reduction in EQE from 720 to 900 nm compared to the cell without bandgap grading. Moreover, the μc-SiOx:H(n)/Ag back reflector showed higher optical reflection than a-Si:H(n)/Ag did, which relatively improved the JSC by 12.3%. The cell with μc-SiOx:H(n)/Ag back reflector exhibited a comparable JSC and efficiency to the cell with ITO–Ag. The previously mentioned approaches are relevant to enhance the optical management in cells and can be applied to silicon-based thin-film solar cells.

High roughness Ag back reflector on a metal underlayer for thin film solar cell applications

The roughness development of Ag film was investigated for potential as a back reflector material in thin film solar cells on flexible stainless steel (STS) substrates. The influence of metal underlayers was evaluated in order to obtain a rough Ag film at a low deposition temperature (≤400°C). By depositing Ag on a 100nm Al underlayer to induce Ag–Al alloying, the film roughness was increased three times more than that of Ag films on bare STS at 400°C. The Ag film deposited on an Al underlayer at 350°C exhibited 75nm roughness and uniformly distributed crystallites, which was effective for visible light scattering. The Ag–Al alloy phase was also controlled using the thickness ratio of Ag and Al. The present work clearly demonstrated that an Ag back reflector film with a higher roughness could be fabricated through inserting a metal underlayer at a deposition temperature much lower than the 500°C that has been reported in earlier works.

Angular reflection study to reduce plasmonic losses in the dielectrically displaced back reflectors of silicon solar cells

Metals are an integral part of solar cell structures—as a metal contact and/or a back reflector. However it is a less known fact that there are plasmonic losses associated with these structures and it is important to minimise such losses especially when placed in proximity to scattering media like textures on the front of the cell that can introduce an angular dependence to the light incident on the rear. This study investigates the losses in a metal reflector when placed adjacent to a dielectric layer in a typical solar cell rear geometry. The experimental measurement was realised using a novel custom built optical setup for characterising intensity of the reflected light at various internal incident angles at the Si–dielectric interface. Our results show that the thickness of the dielectric layer, the refractive index of the dielectric layer and the type of the rear metal can all affect the degree of such losses and the distribution of the angular reflection. Both measured and simulation results indicate that a 250–320nm SiO2 along with an Ag back reflector gives the best internal rear angular reflection in Si but is dependant on the wavelength of interest. It also shows that textured or scattering front interfaces with internal incident angles greater than 28° have the potential to provide light trapping through total internal reflection and at the same time minimise plasmonic losses at the metal reflector interface.

Properties of mixed phase n-doped silicon oxide layers and application in micromorph solar cells

We focus on the use of mixed-phase phosphorus-doped SiOx (n-SiOx) as advanced n-layer in both the junctions of micromorph tandem solar cells. The material, deposited by plasma enhanced chemical vapor deposition at two pressure values, has been fully characterized. Direct evidence of phase separation into nanocrystalline Si and amorphous oxygen-rich phases is reported, accompanied by information on the optical properties of the material as a whole and the two phases, as gathered by spectroscopic ellispometry. The multifunctional role of the n-SiOx layer, providing for beneficial optical effects at both the intermediate and back reflecting stage in micromorph solar cells, is established. In particular superior performance (improved fill factor and enhanced bottom solar cell spectral response) is demonstrated with use at the rear side of the device in combination with a simple Ag back reflector, with respect to the standard device employing the conventional ZnO/Ag reflector.

A low-cost high-efficiency crystalline silicon solar cell based on one-dimensional photonic crystal front surface textures

In this paper we analyze the influence of active layer thickness on the light absorption processes of c-Si solar cells by the finite-difference frequency-domain (FDFD) method and show that the ideal c-Si active layer thickness range is from 10 to 20 μm. By evaluating the absorption enhancement factor, we find that c-Si solar cells with thicknesses within this range can give full play to the synergistic effects of back reflectors and antireflection coatings. By using a simple light trapping scheme, which is composed of a Ag back reflector, a bilayer antireflection coating and a one-dimensional photonic crystal front surface texture structure with rectangular profile, a c-Si solar cell with an active layer thickness of 12 μm can obtain a higher photocurrent density of 32.6 mA cm−2 for normally incident sunlight and a broader acceptance angle range of −60°–60° for obliquely incident sunlight.

Design of a plasmonic back reflector for silicon nanowire decorated solar cells

This Letter presents a crystalline silicon thin film solar cell model with Si nanowire arrays surface decoration and metallic nanostructure patterns on the back reflector. The nanostructured Ag back reflector can significantly enhance the absorption in the near-infrared spectrum. Furthermore, by inserting a ZnO:Al layer between the silicon substrate and nanostructured Ag back reflector, the absorption loss in the Ag back reflector can be clearly depressed, contributing to a maximum J(sc) of 28.4 mA/cm(2). A photocurrent enhancement of 22% is achieved compared with a SiNW solar cell with a planar Ag back reflector.

Study of thin-film silicon solar cell back reflectors and potential of detached reflectors

Abstract The back reflector of thin-film silicon solar cells often consists of a textured metal surface separated from the silicon layers by a thin dielectric layer (e.g. ZnO). Despite the high reflectivity of the textured ZnO/Ag back reflector, parasitic absorption losses exist in both the ZnO and Ag layers. In order to better understand the origin of the optical losses at the rear side of thin-film silicon solar cells and study the potential of alternative back reflector designs, a new method is introduced. This method is based on the utilization of a highly conductive n-layer and a local electrical contacting scheme allowing the use of nonconductive rear dielectric layers or media. In this contribution, the cell absorption and the quantum efficiency of microcrystalline p-i-n solar cells with ZnO/Ag and SiO 2 /Ag back reflectors as well as with a textured Ag back reflector separated from the n-layer by an air gap of a few micrometers are compared.

Effect of Surface Morphology in ZnO:Al/Ag Back Reflectors for Flexible Silicon Thin Film Solar Cells on Light Scattering Properties

Changes in surface morphology and roughness of dc sputtered ZnO:Al/Ag back reflectors by varying the deposition temperature and their influence on the performance of flexible silicon thin film solar cells were systematically investigated. By increasing the deposition temperature from <TEX>$25^{\circ}C$</TEX> to <TEX>$500^{\circ}C$</TEX>, the grain size of Ag thin films increased from 100 nm to 1000 nm and the grain size distribution became irregular, which resulted in an increment of surface roughness from 6.6 nm to 46.6 nm. Even after the 100 nm thick ZnO:Al film deposition, the surface morphology and roughness of the ZnO:Al/Ag double structured back reflectors were the same as those of the Ag layers, meaning that the ZnO:Al films were deposited conformally on the Ag films without unnecessary changes in the surfacefeatures. The diffused reflectance of the back reflectors improved significantly with the increasing grain size and surface roughness of the Ag films, and in particular, an enhanced diffused reflectance in the long wavelength over 800 nm was observed in the Ag back reflectors deposited at <TEX>$500^{\circ}C$</TEX>, which had an irregular grain size distribution of 200-1000 nm and large surface roughness. The improved light scattering properties on the rough ZnO:Al/Ag back reflector surfaces led to an increase of light trapping in the solar cells, and this resulted in a noticeable improvement in the <TEX>$J_{sc}$</TEX> values from 9.94 mA/<TEX>$cm^2$</TEX> for the flat Ag back reflector at <TEX>$25^{\circ}C$</TEX> to 13.36 mA/<TEX>$cm^2$</TEX> for the rough one at <TEX>$500^{\circ}C$</TEX>. A conversion efficiency of 7.60% (<TEX>$V_{oc}$</TEX> = 0.93, <TEX>$J_{sc}$</TEX> = 13.36 mA/<TEX>$cm^2$</TEX>, FF = 61%) was achieved in the flexible silicon thin film solar cells at this moment.

Si microwire-array solar cells

Si microwire-array solar cells with Air Mass 1.5 Global conversion efficiencies of up to 7.9% have been fabricated using an active volume of Si equivalent to a 4 μm thick Si wafer. These solar cells exhibited open-circuit voltages of 500 mV, short-circuit current densities (Jsc) of up to 24 mA cm-2, and fill factors >65% and employed Al2O3 dielectric particles that scattered light incident in the space between the wires, a Ag back reflector that prevented the escape of incident illumination from the back surface of the solar cell, and an a-SiNx:H passivation/anti-reflection layer. Wire-array solar cells without some or all of these design features were also fabricated to demonstrate the importance of the light-trapping elements in achieving a high Jsc. Scanning photocurrent microscopy images of the microwire-array solar cells revealed that the higher Jsc of the most advanced cell design resulted from an increased absorption of light incident in the space between the wires. Spectral response measurements further revealed that solar cells with light-trapping elements exhibited improved red and infrared response, as compared to solar cells without light-trapping elements.

Ａｌ２Ｏ３微粒子を用いた光閉じ込め構造を有するａ‐Ｓｉ太陽電池の作製

The development of the high-efficiency a-Si:H solar cells has been achieved, fabricating the light trapping structures by the use of Al2O3 fine particles. As a result of the inserting the Al2O3 fine particles between the a-Si:H and the Ag back electrodes, the remarkable increase in the short circuit photocurrent density Jsc was achieved while no significant change was observed both in the open circuit voltage Voc and the fill factor. Observation of quantum efficiency spectra for the solar cells with Al2O3 particles indicates that this increase in the photocurrent density is due to the improvement of the sensitivity in the long wavelength regime (570-750 nm). It is concluded that Al2O3 particles cause random reflection of incident light at the interface, leading to an increase in the optical path.