Amorphous Silicon Thin-film Solar Cells Research Articles

Gaussian grating for enhancing light absorption by amorphous silicon thin-film solar cells

In this study, a grating with a Gaussian distribution was used to increase the absorption of light by amorphous silicon thin film solar cells. A grating is an effective structure for trapping light inside the active layer of a cell, so a two-dimensional Gaussian grating with a rectangular structure was placed on the front surface of the cell. The results obtained by using the finite element method showed that the Gaussian grating significantly enhanced the absorption of light in the visible and near-infrared ranges by a cell with a thickness of 0.5 μm compared with a cell without gratings and a cell with normal gratings. The maximum average light absorption by the cell with a Gaussian grating was 84.8%, which was 90% higher compared with the reference cell. In addition, the short-circuit current density and efficiency were determined as 34.2 and 17.6 mA/cm2, respectively, which were 72% and 72.5% higher, respectively, compared with the reference cell. The proposed structure could be used in a cell to convert more light into electricity.

Optical Sensing Using Hybrid Multilayer Grating Metasurfaces with Customized Spectral Response

Customized metasurfaces allow for controlling optical responses in photonic and optoelectronic devices over a broad band. For sensing applications, the spectral response of an optical device can be narrowed to a few nanometers, which enhances its capabilities to detect environmental changes that shift the spectral transmission or reflection. These nanophotonic elements are key for the new generation of plasmonic optical sensors with custom responses and custom modes of operation. In our design, the metallic top electrode of a hydrogenated amorphous silicon thin-film solar cell is combined with a metasurface fabricated as a hybrid dielectric multilayer grating. This arrangement generates a plasmonic resonance on top of the active layer of the cell, which enhances the optoelectronic response of the system over a very narrow spectral band. Then, the solar cell becomes a sensor with a response that is highly dependent on the optical properties of the medium on top of it. The maximum sensitivity and figure of merit (FOM) are SB = 36,707 (mA/W)/RIU and ≈167 RIU−1, respectively, for the 560 nm wavelength using TE polarization. The optical response and the high sensing performance of this device make it suitable for detecting very tiny changes in gas media. This is of great importance for monitoring air quality and thecomposition of gases in closed atmospheres.

Efficiency improvement in thin-film amorphous silicon solar cells based on PbS quantum dots

Efficiency improvement in thin-film amorphous silicon solar cells based on PbS quantum dots

Amorphous Silicon Thin-Film Solar Cells on Fabrics as Large-Scale Detectors for Textile Personal Protective Equipment in Active Laser Safety.

Laser safety is starting to play an increasingly important role, especially when the laser is used as a tool. Passive laser safety systems quickly reach their limits and, in some cases, provide inadequate protection. To counteract this, various active systems have been developed. Flexible and especially textile-protective materials pose a special challenge. The market still lacks personal protective equipment (PPE) for active laser safety. Covering these materials with solar cells as large-area optical detectors offers a promising possibility. In this work, an active laser protection fabric with amorphous silicon solar cells is presented as a large-scale sensor for continuous wave and pulsed lasers (down to ns). First, the fabric and the solar cells were examined separately for irradiation behavior and damage. Laser irradiation was performed at wavelengths of 245, 355, 532, and 808 nm. The solar cell sensors were then applied directly to the laser protection fabric. The damage and destruction behavior of the active laser protection system was investigated. The results show that the basic safety function of the solar cell is still preserved when the locally damaged or destroyed area is irradiated again. A simple automatic shutdown system was used to demonstrate active laser protection within 50 ms.

Dual metallic-based plasmonic back reflector to broaden the absorption spectra of a thin film amorphous silicon solar cell

Here, to obtain high efficiency in a thin film amorphous silicon solar cell, a dual metallic plasmonic back reflector was investigated. The designed dual metal plasmonic back reflector can improve the photocurrent and hence the efficiency. At first, a double layer SiO 2/Si3N4 as the antireflection (AR) was analyzed. The efficiencies of 8.74%, 11.38%, 12.53%, and 13.04% are obtained for a cell without AR, with SiO2, Si3N4, and SiO2/SiN 4 AR layers, respectively. A disk-ring and dual rings-based Ni and Ti-based plasmonic back reflector were designed. The study was carried out by a 3D finite difference time domain method and the optimized parameters were calculated to obtain higher efficiencies. Finally, a relatively higher photocurrent and conversion efficiency of 19.86mA/cm2 and 15.65% were achieved for the optimized structure with a dual Ni rings back reflector, respectively. Because of the importance of amorphous silicon solar cells due to the cost problem of crystalline silicon, here, the simulation steps present a valuable road map to design a novel plasmonic thin-film amorphous silicon solar cell.

Development of a dosimetry system for therapeutic X-rays using a flexible amorphous silicon thin-film solar cell with a scintillator screen.

To evaluate the dosimetric characteristics and applications of a dosimetry system composed of a flexible amorphous silicon thin-film solar cell and scintillator screen (STFSC-SS) for therapeutic X-rays. The real-time dosimetry system was composed of a flexible a-Si thin-film solar cell (0.2-mm thick), a scintillator screen to increase its efficiency, and an electrometer to measure the generated charge. The dosimetric characteristics of the developed system were evaluated including its energy dependence, dose linearity, and angular dependence. Calibration factors for the signal measured by the system and absorbed dose-to-water were obtained by setting reference conditions. The application and correction accuracy of the developed system were evaluated by comparing the absorbed dose-to-water measured using a patient treatment beam with that measured using the ion chamber. The responses of STFSC-SS to energy, field size, depth, and source-to-surface distance (SSD) were more dependent on measurement conditions than were the responses of the ion chamber, although the former dependence was due to the scintillator screen, not the solar cell. The signals of STFSC-SS were also dependent on dose rate, while the responses of solar cell alone and scintillator screen were not dependent on dose rate. The scintillator screen reduced the output of solar cell at 6 and 15 MV by 0.60 and 0.55%, respectively. The different absorbed dose-to-water measured using STFSC-SS for patient treatment beam differed by 0.4% compared to those measured using the ionization chamber. The uncertainties of the developed system for 6 and 15 MV photon beams were 1.8 and 1.7%, respectively, confirming the accuracy and applicability of this system. The thin-film solar cell-based detector developed in this study can accurately measure absorbed dose-to-water. The increased signal resulting from the use of the scintillator screen is advantageous for measuring low doses and stable signal output. In addition, this system is flexible, making it applicable to curved surfaces, such as a patient's body, and is cost-effective.

Performance improvement of an ultra-thin amorphous silicon solar cell using multilayers chirped diffraction back gratings

In this paper, a systematic design of a novel back reflector using chirped grating structures is investigated for thin-film amorphous silicon (a-Si) solar cells. At first, a cell with an optimum ITO antireflecting layer is simulated and a photocurrent of 17.67 mA cm−2 is obtained. Then a conventional grating from SiO2 is evaluated and the maximum photocurrent of 18.97 mA cm−2 is calculated. Then a thin-film solar cell based on the chirped back grating is designed and the photocurrent is increased to 19.30 mA cm−2. Also, their integrated optical absorption is compared for better understanding. To give a numerical comparison of the cells with a different number of chirped grating layers, a short circuit current for different numbers of grating layers is analyzed. It is shown that maximum current density is obtained for 5 layers grating with the chirped coefficient length of near 20 nm. It is believed that chirped back grating can be used to design higher performance thin film a-Si solar cells and the results are helpful for photovoltaic applications.

Evaluation method of the light-trapping structure for a transparent thin-film silicon solar cell with low-illuminance condition

Transparent hydrogenated amorphous silicon (a-Si:H) thin-film solar cells were fabricated for building-integrated photovoltaic (BIPV) windows, and light-trapping structures were formed on aluminum-doped zinc oxide (ZnO:Al) front electrodes to increase the output current and power under low-irradiance conditions in buildings. The degree of light-trapping was controlled by varying the etching time for texturing the ZnO:Al electrodes. As the etching time increased from 5 to 20 s, the surface area and average haze increased from 102.02 to 116.27 μm2, and 3.4 to 14.6% in the absorption wavelength range of 400–600 nm, respectively. The introduction of the light-trapping structure can increase the photogenerated current but causes defects in the cell. The current and defect density both depend on the light intensity; the highest power conversion efficiency of the cell was obtained in different light-trapping structures depended on the light intensity. To evaluate the light-trapping effect independent of the light intensity, we theoretically derived the effective absorbance and current density loss parameters using rate equations, which were in good agreement with the measurements. The presented methods can be applied to various thin-film solar cells requiring light-trapping structures to effectively utilize low-illuminance conditions.

Performance enhancement of amorphous silicon solar cell using 1D photonic crystal as back reflector

In the present work, design and optimization of a highly reflective light-trapping nanostructures were remarkably enhanced the efficiency of thin-film amorphous silicon (a-Si) solar cells by using rigorous coupled wave analysis (RCWA) method. One-dimensional photonic crystal (1DPC) is also known as distributed Bragg reflectors (DBRs) used as backside reflector. The property of photonic crystal has depends on the various parameters such as refractive index, thickness, center wavelength (λc) and the selection of the materials. In this work, the reflectivity of DBRs (Si/SiO2) was determined by various center (incidence) wavelength (λc) such as 400, 600, and 800 nm. The maximum reflectance of 100% achieved by 5DBR alternative layers (or stacks). The proposed solar cell consists of anti-reflection coating-ITO, top-grating (ITO), absorber layer (a-Si) and an optimized DBRs. Further, investigated the careful design optimization and presented the enhanced light trapping mechanism within 40 nm thick a-Si absorber region.

Light Propagation in Flexible Thin-Film Amorphous Silicon Solar Cells with Nanotextured Metal Back Reflectors.

Nanostructured metal back reflectors (BRs) are playing an important role in thin-film solar cells, which facilitates an increased optical path length within a relatively thin absorbing layer. In this study, three nanotextured plasmonic metal (copper, gold, and silver) BRs underneath flexible thin-film amorphous silicon solar cells are systematically investigated. The solar cells with BRs demonstrate an excellent light harvesting capability in the long-wavelength region. With the combination of hybrid cavity resonances, horizontal modes, and surface plasmonic resonances, more incident light is coupled into the photoactive layer. Compared to the reference cells, the three devices with plasmonic BRs show lower parasitic absorptions with different individual absorption distributions. Both experimental and simulated results indicate that the silver BR cells delivered the best performance with a promising power conversion efficiency of 7.26%. These rational designs of light harvesting nanostructures provide guidelines for high-performance thin-film solar cells and other optoelectronic devices.

High Haze Ratio-Based ITO Films Prepared on Periodic Textured Glass Surfaces for Thin Film Solar Cells

Light-trapping phenomenon is limited due to non-uniform surface structure of transparent conducting oxide (TCO) films. The proper control of surface structure with uniform cauliflower TCO films may be appropriate for efficient light trapping. We report a light-trapping scheme of SF6/Ar plasma-based textured glass surfaces for high root-mean-square (RMS) roughness and haze ratio of ITO films. It was observed that the variation in Ar flow ratio in SF6/Ar plasma during the inductive coupled plasma-reactive ion etching (ICP-RIE) process was an important factor to improve the haze ratio of textured glass. The SF6/Ar plasma textured glass showed low etching rates due to the presence of various metal elements, such as Al, B, F, and Na. The ITO films were deposited on SF6/Ar plasma-textured glass substrates showed the high RMS roughness (433 nm) and haze ratio (67.8%) in the visible wavelength region. The change in surface structure has a negligible influence on the electrical properties of ITO films. The TCO films deposited on periodic textured glass surfaces with high RMS roughness and haze ratio are proposed for high-efficiency amorphous silicon (a-Si) thin-film solar cells.

Toward self-powered and reliable visible light communication using amorphous silicon thin-film solar cells.

Enhancing robustness and energy efficiency is critical in visible light communication (VLC) to support large-scale data traffic and connectivity of smart devices in the era of fifth-generation networks. To this end, we demonstrate that amorphous silicon (a-Si) thin-film solar cells with a high light absorption coefficient are particularly useful for simultaneous robust signal detection and efficient energy harvesting under the condition of weak light in this study. Moreover, a first-generation prototype called AquaE-lite is developed that consists of an a-Si thin-film solar panel and receiver circuits, which can detect weak light as low as 1 µW/cm2. Using AquaE-lite and a white-light laser, orthogonal frequency-division multiplexing signals with data rates of 1 Mb/s and 908.2 kb/s are achieved over a 20-m long-distance air channel and 2.4-m turbid outdoor pool water, respectively, under the condition of strong background light. The reliable VLC system based on energy-efficient a-Si thin-film solar cells opens a new pathway for future satellite-air-ground-ocean optical wireless communication to realize connectivity among millions of Internet of Things devices.

Energy harvesting performance of bifacial and semitransparent amorphous silicon thin-film solar cells with front and rear transparent conducting oxide contacts

Bifacial and semitransparent hydrogenated amorphous silicon (a-Si:H) thin-film solar cells were prepared with front and rear transparent conducting oxide (TCO) contacts using a plasma-enhanced chemical vapor deposition process. Increased power conversion efficiencies (PCEs) and average visible transmissions (AVTs) in the visible wavelength range were realized in the cases of the bifacial and semitransparent solar cells based on the optimization of the device designs, such as front TCO contacts, intrinsic a-Si:H absorbers (i-a-Si:H), p-type hydrogenated nanocrystalline silicon carbide, and n-type hydrogenated nanocrystalline silicon layers. The dependencies of the performance parameters of the solar cells on the illumination direction and intensity were also investigated systematically. As the film thickness of the i-a-Si:H absorber layers increased from 150 nm to 450 nm, the PCEs of the solar cells respectively increased in the ranges of 6.01–7.50% and 5.25–6.25% when front and rear illuminations were used for a standard light intensity of 100 mW/cm2, while the AVTs decreased from 22.76% to 15.67% irrespective of illumination direction. The best PCEs of the bifacial and semitransparent solar cells that were fabricated with the use of optimal conditions were respectively equal to 7.94% and 6.20% when front and rear illumination conditions were used. The corresponding AVT was equal to 14.34%. The performance parameters of bifacial and semitransparent a-Si:H solar cells at varying illumination intensities in the range of 10–100 mW/cm2 were also investigated, and yielded consistent PCE values in the range of 6.39–6.62% with front illumination, and 4.95–5.15% with rear illumination.

Surface Modification of Al-Doped ZnO Transparent Conducive Thin Films with Polycrystalline Zinc Molybdenum Oxide.

High-work function (WF) transparent conductive thin films improve the performance of solar cells and organic light-emitting diodes by facilitating interfacial charge carrier transport. Al-doped ZnO (AZO) becomes a very promising transparent conductive material because of nontoxicity, abundant material resources, and low cost. To increase the WF of AZO without enhancing the series resistance of the device, a high-WF and low-resistance surface modifier of polycrystalline zinc molybdenum oxide (ZMO) was developed by utilizing thermal evaporation of MoO3 on the surface of AZO and a subsequent two-step annealing treatment. The first step of air annealing causes the formation of monoclinic ZnMoO4 nanocrystals in the ZMO modifier. This improves the WF of AZO from 3.83 to 4.86 eV by increasing the group electronegativity and cation oxidation state. Furthermore, the second step of N2 annealing decreases the resistivity of the polycrystalline ZMO by increasing the donor states of oxygen vacancies. The surface modification effect is verified by applying the ZMO-modified AZO to the front electrode of hydrogenated amorphous silicon thin-film solar cells. The low-resistance polycrystalline ZMO modifier not only increases light harvesting in the solar cells by improving interfacial refractive index matching but also improves the open-circuit voltage by modifying the interfacial band alignment. In particular, the modifier increases the fill factor by ca. 13% by reducing the series resistance of the device. These enable a gain of ca. 23% in photoelectric conversion efficiency compared to the unmodified AZO. The results suggest the feasibility to tune the WF and conductivity of a material independently.

Performance Improvement of Flexible Thin Film Si Solar Cells using Graphite Substrate

We investigated the characteristics of nano crystalline silicon(nc-Si) thin-film solar cells on graphite substrates. Amorphous silicon(a-Si) thin-film solar cells on graphite plates show low conversion efficiency due to high surface roughness, and many recombination by dangling bonds. In previous studies, we deposited barrier films by plasma enhanced chemical vapor deposition(PECVD) on graphite plate to reduce surface roughness and achieved ~7.8 % cell efficiency. In this study, we fabricated nc-Si thin film solar cell on graphite in order to increase the efficiency of solar cells. We achieved 8.45 % efficiency on graphite plate and applied this to nc-Si on graphite sheet for flexible solar cell applications. The characterization of the cell is performed with external quantum efficiency(EQE) and current density-voltage measurements(J-V). As a result, we obtain ~8.42 % cell efficiency in a flexible solar cell fabricated on a graphite sheet, which performance is similar to that of cells fabricated on graphite plates.

Electrical internal quantum efficiency improved by interval doping method.

Electrical internal quantum efficiency is an important parameter for evaluating the utilization degree of the photocurrent density. Amorphous silicon has a quite considerable absorption efficiency of the available incident light. However, the existence of substantial defects in heavily doping amorphous silicon limits the electrical internal quantum efficiency. In this paper, we propose the interval doping method for the amorphous silicon thin-film solar cells. Most of the hot carriers in the amorphous silicon layer will concentrate in the intrinsic region. Due to the lower recombination rate, more hot carriers could be collected by the electrodes. Through the coupled calculation of the optical field and the electric field, it is found that the proposed interval doping amorphous silicon thin-film solar cell's electrical internal quantum efficiency is significantly enhanced. If the interval doping method is applied to both the top and bottom heavily doping regions, the short-circuit current density will be improved from 9.77 to 12.30 mA/cm2, and the maximum output power will increase from 6.79 to 8.03 W/cm2. All these results indicate that the interval doping method is suitable for improving the performance of the amorphous silicon thin-film solar cells.

Unidirectional light scattering by up–down Janus dimers composed of gold nanospheres and silicon nanorods

Abstract In this paper, a time-domain finite-difference (FDTD) method is used to simulate the scattering properties of a Janus dimer that a gold nanosphere is putted on the top of a silicon nanorod. We have demonstrated that the Janus dimer exhibits unidirectional scattering in the whole wavelength region of the sunlight. The unidirectionality of the dimer will improve with the height increase of the silicon nanorod and the gap decrease between two adjacent dimers. In our simulation, the forward-to-backward ratio (F/B) of the Janus dimer calculated dividing forward scattering spectra by backward scattering spectra can achieve the maximum of 20 when the height of silicon nanorod is 300 nm. What is more, we have applied the Janus dimers to amorphous silicon thin-film solar cells as antireflection structures. The reflectivity of the solar cells reduces by 39.40% and the short circuit current density improves by 5.04% than those of the reference. Therefore, the Janus dimers has a great application prospect in photovoltaic devices.

Efficiency improvement of a-Si:H Thin-Film Solar Cells by phosphorus doping of absorption layer with a-Si:H buffer layer at p/i interface

Comparative properties of pin-type amorphous silicon (a-Si:H) thin-film solar cells with different PH3 flows for the i-layer and the presence of a buffer layer at the p–i interface were investigated. Doped a-Si:H films with different PH3 concentrations were fabricated by mixing PH3, SiH4, and H2 during i-layer deposition. The flows were changed within 0–4.2 sccm, the total thickness of the buffer and intrinsic layers was 200 nm, and the thickness of the buffer layer was varied from 0–30 nm. All amorphous materials on films were deposited by a plasma-enhanced chemical vapor deposition method. Output performances (open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, and conversion efficiency η) of the solar cells were characterized to optimize the structural features and manufactural processes. The maximum values of parameters were measured at a PH3 flow of 0.9 sccm. Excessive or insufficient flow rates of PH3 will deteriorate the solar cell characteristics because of the resulting large defect density and increased recombination rate, respectively. Performance of solar cells with the a-Si:H buffer layer (Voc = 0.682V, Jsc = 17.21 mA/cm2, FF = 0.51, and η = 6.03) improved compared with the results where phosphorus was lightly doped on the i-layer of pin-type a-Si:H solar cells without a buffer layer (Voc = 0.655 V, Jsc = 17.08 mA/cm2, FF = 0.46, and η = 5.11).

Absorption enhancement in a-Si thin-film solar cells based on silver nanopillar arrays

PurposeThe purpose of this paper is to investigate a light-trapping structure based on Ag nanograting for amorphous silicon (a-Si) thin-film solar cell. Silver nanopillar arrays on indium tin oxide layer of the a-Si thin-film solar cells were designed.Design/methodology/approachThe effects of the geometrical parameters such as nanopillar radius (R) and array period (P) were investigated by using the finite element simulation.FindingsThe optimization results show that the absorption of the solar cell with Ag nanopillar structure and anti-reflection film is enhanced up to 29.5 per cent under AM1.5 illumination in the 300- to 800-nm wavelength range compared with the reference cell. Furthermore, physical mechanisms of absorption enhancement at different wavelength range are discussed according to the electrical field amplitude distributions in the solar cells.Research limitations/implicationsThe research is still in progress. Further studies mainly focus on the performance of solar cells with different nanograting materials.Practical implicationsThis study provides a feasible method for light-trapping structure based on Ag nanograting for a-Si thin-film solar cell.Originality/valueThis study is promising for the design of a-Si thin-film solar cells with enhanced performance.

Transparent bifacial a-Si:H solar cells employing silver oxide embedded transparent rear electrodes for improved transparency

We developed a transparent oxide–metal–oxide (OMO) structure using aluminum-doped zinc oxide and oxidized silver (AgOx) as a transparent electrode of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell for use in building-integrated photovoltaic (BIPV) windows. The oxygen (O2) addition (O2 flow rate) was optimized for a metal-to-dielectric intermediate-phase AgOx OMO to have high transparency and high conductivity, which were confirmed by finite-difference time-domain simulation. Using the AgOx OMO as a rear electrode, transparent a-Si:H solar cells were fabricated for BIPV window application. The performance of the fabricated cells showed highest bifacial efficiency (b-η) of 7.87% at AgOx OMO of 1 sccm, and highest average transmittance (T500–800, i.e., wavelength range: 500–800 nm) of 21.9% at AgOx OMO of 3 sccm, i.e., improvements from b-η = 7.42% and T500–800 = 18.8% at Ag OMO of 0 sccm. The cell with the optimized AgOx OMO (3 sccm) achieved b-η = 7.69% and the best figure of merit (product of b-η and T500–800) of 169%, i.e., 30% higher than the Ag OMO cell (139%). The developed AgOx OMO electrodes could be used in BIPV windows or in other optical devices requiring both high transparency and high conductivity.