Intermediate Reflector Layer Research Articles

1-D Magnetic photonic crystal as intermediate reflective layer in tandem solar cells

The analysis of the optical properties of a 1-D binary magnetic photonic crystal (MPC) made up of Cu-doped YIG, a metal-doped magnetic composite material as one layer, and μc-SiOx:H as another layer is carried out. The refractive index of the Cu-YIG nanocomposite system is calculated using the Maxwell-Garnett equation. The transmission spectra of 1-D MPC is studied using the Transfer Matrix Method. The proposed 1-D MPC is designed in such a way that it is suitable to act as an intermediate reflective layer (IRL) in thin-film silicon tandem solar cells. The reference Bragg wavelength (λB) is set at 500 ​nm. The effect of copper doping in the magnetic photonic crystal on the photonic bandgap is studied. By varying the number of periods and the percentage of copper doping, the changes in the photonic bandgap are analysed. It is found that the photonic bandgap increases with the number of periods of the MPC and the percentage of copper doping, which tends to enhance the light trapping in tandem solar cells.

The Design and Optimization of an Anti-Reflection Coating and an Intermediate Reflective Layer to Enhance Tandem Solar Cell Photons Capture

We have theoretically demonstrated an efficient way to improve the optical properties of an anti-reflection coating (ARC) and an intermediate reflective layer (IRL) to enhance tandem solar cell efficiency by localizing the incident photons’ energy on a suitable sub-cell. The optimum designed ARC from a one-dimensional ternary photonic crystal, consisting of a layer of silicon oxynitride (SiON), was immersed between two layers of (SiO2); thicknesses were chosen to be 98 nm, 48 nm, and 8 nm, respectively. The numerical results show the interesting transmission properties of the anti-reflection coating on the viable and near IR spectrum. The IRL was designed from one-dimensional binary photonic crystals and the constituent materials are Bi4Ge3O12 and μc-SiOx: H with refractive indexes was 2.05, and 2.8, respectively. The numbers of periods were set to 10. Thicknesses: d1 = 62 nm and d2 = 40 nm created a photonic bandgap (PBG) in the range of [420 nm: 540 nm]. By increasing the second material thickness to 55 nm, and 73 nm, the PBG shifted to longer wavelengths: [520 nm: 630 nm], and [620 nm: 730 nm], respectively. Thus, by stacking the three remaining structures, the PBG widened and extended from 400 nm to 730 nm. The current theoretical and simulation methods are based on the fundamentals of the transfer matrix method and finite difference time domain method.

Computational study of film morphology impact on light absorption in particulate Ta3N5/Si photoanodes for water splitting

A large class of photoelectrodes for water splitting are processed by assembling nanoparticles onto a silicon solar cell substrate. A fundamental question is the optimal size of constituent nanoparticles that maximizes optical absorption in the photoanode. We use electromagnetic optical calculations to study the impact of particle size on optical absorption of Ta3N5/Si tandem photoanodes. We found that optical absorption efficiency dramatically increases when particle support Mie resonances, independent of particle shape. Subsequently, monolayers of resonant-size particulate and thin films of Ta3N5 on silicon substrate are studied. Ta3N5 limits overall performance of Ta3N5/Si tandem device in both devices with the particulate one showing better performance. We take into account material charge transport properties and conditions of imperfect surface coverage of Si solar cell with Ta3N5 nanoparticle. Furthermore, we found that an intermediate reflective layer placed between Ta3N5 and Si solar cell improves photocurrent density of Ta3N5 and the tandem device.

Chemical Stability and Performance of Doped Silicon Oxide Layers for Use in Thin-Film Silicon Solar Cells

Doped hydrogenated silicon oxide layers (SiO X :H) have recently been successfully integrated as front window layers, back reflector layers, intermediate reflector layers, passivation layers, and junction layers in thin-film silicon solar cells. Depending on the deposition conditions of the SiO X :H layers, some devices suffer from a degradation in performance in time. In this paper, we demonstrate the responsible mechanism involved. It is demonstrated that oxidation of the p-type doped (p-)SiO X :H with a high crystallinity and, therefore, poor passivation of crystalline grains is responsible for this degradation. The oxidation of p-SiO X :H is caused by the in-diffusion of water vapor from the ambient air. Stable p-SiO X :H can be obtained if the material is processed at higher pressure. In addition, the degradation can be prevented if the cell is well encapsulated, like using dense n-type (n-)SiO X :H in the back reflector of the cell.

Innovative Utilization of Improved n-doped μc-SiOx:H Films to Amplify the Performance of Micromorph Solar Cells

Due to in-situ deposition process doped SiOx material attracts the PV community as intermediate reflecting layer (IRL) for the less hazardous deposition process. Previously we have been shown that how to formulate a better quality SiOx layer using seeding technique. In this paper we have shown that how this superior quality material will be beneficial as IRL and improve the performance of micromorph solar cells. Enhanced performance is observed when SiOx material is deposited with the help of highly conducting seed layer. For improving the performance of the micromorph solar cell further we have eliminated the n-layer of the top constituent cell. In the context of substituting the top n-layer, SiOx layer grown on seed layer is more significant as of its suitable optoelectronics properties. Initial efficiency of ∼12% is perceived in micromorph solar cells with the improved SiOx based IRL used innovatively which is 7.9% higher than that of the cell with conventionally developed and typical use of IRL.

Versatility of doped nanocrystalline silicon oxide for applications in silicon thin-film and heterojunction solar cells

To optimize the optical response of a solar cell, specifically designed materials with appropriate optoelectronic properties are needed. Owing to the unique microstructure of doped nanocrystalline silicon oxide, nc-SiOx:H, this material is able to cover an extensive range of optical and electrical properties. However, applying nc-SiOx:H thin-films in photovoltaic devices necessitates an individual adaptation of the material properties according to the specific functions in the device. In this study, we investigated the detailed microstructure of doped nc-SiOx:H films via atom probe tomography at the sub-nm scale, thereby, for the first time, revealing the three-dimensional distribution of the nc-Si network. Furthermore, n- and p-type nc-SiOx:H layers with various optical and electrical properties were implemented as a window, back contact, and an intermediate reflector layer in silicon heterojunction and multi-junction thin-film solar cells with a focus on the key aspects for adapting the material properties to the specific functions. Here, nc-SiOx:H effectively reduced the parasitic absorption and opened new possibilities for the photon management in the solar cells, thereby, demonstrating the versatility of this material. Remarkably, using our adapted nc-SiOx:H layers in distinct functions enabled us to achieve a combined short circuit current density of 15.1mAcm−2 for the two a-Si:H sub-cells in an a-Si:H/a-Si:H/µc-Si:H triple-junction thin-film solar cell and an active area efficiency of 21.4% was realized for a silicon heterojunction solar cell.

Wafer Surface Tuning for a-Si:H/μc-Si:H/c-Si Triple Junction Solar Cells for Application in Water Splitting

Abstract Amorphous/microcrystalline/crystalline silicon (a-Si:H/μc-Si:H/c-Si) triple junction solar cells have been developed to directly split water, for which operating voltages (Voc) >1.5 V (depending on the catalysts) are needed. These cells are an inexpensive alternative to high efficiency, high voltage multi junction solar cells based on scarce elements like III-V semiconductors. To obtain a dense μc-Si:H middle cell with high Voc, we investigated a second acid-based polishing step to systematically smoothen the sharp V-shaped valleys present after standard KOH pyramid texturing. This way, we reduced defective regions in the 2 – 3 μm thick μc-Si:H middle and the 250 – 300 nm thick a-Si:H top cell. Quantitatively analyzing the surfaces by atomic force microscopy, we found that a reduction from 49° to 25° in the maximum of the angle distribution of the surface allowed an increase in Voc of >130 mV and an increase in fill factor (FF) from ∼50% to >70%. By further optimizing the nanocrystalline silicon oxide (nc-SiOx:H) intermediate reflector layer placed between bottom and middle cell, the nc-SiOx:H top cell p-layer as well as middle cell thickness, we could increase the solar to electricity conversion efficiency to 12.7%. Very promising first results are presented by utilizing the triple junction cell for stand-alone water splitting device in alkaline electrolysis as demonstrated by using a standard 3-electrode setup.

Optimal design of intermediate reflector layer in micromorph silicon thin-film solar cells

An intermediate reflector layer (IRL) serves as a spectrally selective layer between the top amorphous cell and bottom nanocrystalline cell in a micromorph silicon thin-film solar cell. In this paper, an IRL periodic design is proposed to achieve better conversion efficiency using thin active layers. The optically simulated short circuit current reaches 13.62 mA/cm 2 and three-dimensional electrical analysis shows a promising result. The design methodology used in this paper can be easily applied to different types of IRL materials and extended to triple thin-films solar cells. Finally, the results are compared with state-of-the-art design and further enhancement factors are discussed.

Development of wider bandgap n-type a-SiOx:H and μc-SiOx:H as both doped and intermediate reflecting layer for a-Si:H/a-Si1-xGex:H tandem solar cells

In this work, we developed a-SiOx:H(n) and μc-SiOx:H(n) films as n-type layer, intermediate reflecting layer (IRL), and back-reflecting layer (BRL) to improve the light management in silicon thin-film solar cells. In the development of SiOx:H films, by properly adjusting the oxygen content of the films, the optical bandgap of μc-SiOx:H(n) can be increased while maintaining sufficient conductivity. Similar effect was found for a-SiOx:H(n). In a-Si:H single-junction cells, employing a-SiOx:H(n) as the replacement for a-Si:H(n) resulted in a relative efficiency enhancement of 11.4% due to the reduced parasitic absorption loss. We have also found that μc-SiOx:H(n) can replace back ITO layer as BRL, leading to a relative efficiency gain of 7.6%. For a-Si:H/a-Si1-xGex:H tandem cell, employing μc-SiOx:H(n) as IRL increased the current density of top cell. In addition, employing a-SiOx:H(n) as a replacement of a-Si:H(n) in the top cell increased the current density of bottom cell due to the reduction of absorption loss. Combining all the improvements, the a-Si:H/a-Si1-xGex:H tandem cell with efficiency of 9.2%, VOC = 1.58 V, JSC = 8.43 mA/cm2, and FF = 68.4% was obtained.

Electrodeposition and Characterization of Selective Coatings Based on Black Cobalt for the Conversion of Solar-to-Thermal Energy

Solar-thermal energy conversion is achieved by selective coatings on a metallic substrate. The selective coating consist of a thin selectively absorbing film on a highly IR reflective underlying metal. The optical performance of the selective coating depends on the thickness of the respective films, which can be controlled through the electric charge applied during the electrodeposition process. Selective coatings have been deposited on copper substrates by the electrodeposition method. The coating consists of an intermediate reflective layer of electrodeposited bright nickel and an absorbing film of black cobalt. Different times were used for the galvanostatic deposition of black cobalt in order to optimize the optical properties. In order to evaluate the thermal stability, the coatings were exposed to temperature of 300oC. The coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The visible and IR film reflectance was evaluated before and after the thermal treatment through UV-Vis-NIR spectrophotometer and a Perkin Elmer FTIR spectrophotometer (Frontier MIR/NIR) with an integrating sphere from 2 µm to 15 µm. The spectrophotometry results showed little change in the film reflectance after of the thermal treatment, suggesting a good thermal stability.

Analysis of the light-induced degradation of differently matched tandem solar cells with and without an intermediate reflector using the Power Matching Method

We investigate the light-induced degradation of a-Si:H/µc-Si:H tandem solar cells by applying the Power-Matching-Method (PMM). The PMM allows for a characterization of a cell under various matching conditions by systematically varying the spectral distribution of the illumination. The top cell's thickness was varied from 100nm to 500nm for cells with and without SiOx intermediate reflector layer (IRL). The method provides a powerful means of comparing the degradation of differently matched tandem cells. We conclude that the IRL does not significantly affect the stability of our sub cells. The method reveals that only the top-cell degrades significantly under illumination. In addition, the study shows that the PMM is useful for outdoor performance estimations of tandem solar cells or modules.

Influence of Interface Textures on Light Management in Thin-Film Silicon Solar Cells With Intermediate Reflector

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

The Development and Application of High-Efficiency Low-Cost Silicon Thin Film Solar Cell

ABSTRACTFor the industrial application of silicon thin film solar cells, the current focus is on how to realize high-efficiency low-cost production process and minimize light-induced degradation effect, thus effectively reducing the balance-of-system (BOS) costs of system integration. In this paper, a brief introduction based on our development and application in this area is presented, highlighting in the achievement of some layers in a-Si:H/μc-Si:H tandem solar cell by optimizing the property of single layers, such as amorphous intrinsic layer, intermediate reflective layer and microcrystalline intrinsic layer. After transferring the process achievement to the industrial production line, we obtained the low-cost thin-film silicon solar cells with high photovoltaic conversion efficiency of 10.2%.

Enhanced Light Trapping in a-Si:H/μc-Si:H Tandem Solar Cells via Nanopatterning Top Absorber and Embedding Wavelength-Selective Intermediate Reflectors

Output photocurrent of a-Si:H/μc-Si:H tandem solar cells is usually determined by the unsatisfactory absorption of a-Si:H layer restricted by its thin thickness required by the very limited carrier diffusion length and low absorption coefficient at wavelengths over 550 nm. In this paper, we present a new configuration to enhance the light-trapping performance and the output photocurrent of a-Si:H/μc-Si:H tandem solar cells via the nanopatterning a-Si:H layer and embedding wavelength-selective intermediate reflector layer (WSIRL) between the top and bottom cells. Our simulations indicate that absorption and photocurrent of the tandem cells in the proposed scheme exhibit a significant and wide-angle enhancement compared with the planar cases with or without WSIRL and the nanopatterned case without WSIRL. An ultimate photocurrent density, which is obtained under normally incident AM1.5G solar irradiation and ideal internal quantum efficiency, is 37.56% higher than that of the planar counterpart with a normal ZnO intermediate reflector. Moreover, the attenuation ratio of the output photocurrent in the proposed configuration is only 2.95% when the incident angle alters from 0° to 60°.

Novel interconnection scheme for thin-film silicon solar modules with conductive intermediate reflector

Intermediate reflector layers are commonly used for light man-agement purposes in multi-junction silicon based devices containing a-Si:H top- and µc-Si:H bottom-sub-cells. A low resistance of such layers can have a severe impact on the solar module performance due to shunting of the bottom sub-cell by the P2 scribe. A common solution for this problem is the use of an additional scribe line. However, not only the additional processing step is disadvantageous but also the dead area losses are increased as well by the additional scribe. This work introduces a novel solar cell stripe interconnection scheme that requires only three scribing processes with similar dead area losses as they would be apparent in the standard interconnection scheme. An implementation to mini modules shows no negative impact on the electrical properties and simultaneously reducing the required number of scribing steps. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

High-Stable-Efficiency Tandem Thin-Film Silicon Solar Cell With Low-Refractive-Index Silicon-Oxide Interlayer

We report the recent advances and key requirements for high-efficiency tandem thin-film silicon solar cells composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell. The impact of inserting a low-refractive-index silicon-oxide (SiOx) film as intermediate reflecting layer (IRL) is highlighted. We show that refractive indexes as low as 1.75 can be obtained for layers still conducting enough to be implemented in solar cells, and without no additional degradation. This allows for high top-cell current densities with thin top cells, enabling low degradation rates. A micromorph cell with a certified efficiency of 12.63% (short-circuit current density of 12.8 mA/cm(2)) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm(2) are reported in the amorphous silicon top-cell of micromorph devices by combining a 150-nm- thick SiOx-based IRL and a textured antireflecting coating at the air-glass interface.

Optical and electrical properties of hydrogenated silicon oxide thin films deposited by PECVD

In this work, n-type amorphous silicon oxide thin films were deposited by RF-PECVD method using a gas mixture of SiH4, CO2, H2, and PH3. The deposition rate, refractive index, band gap, crystalline volume fraction, and conductivity of the silicon oxide thin films were determined and analyzed. The film with refractive index of 1.99, band gap of 2.6eV and conductivity of 10−7 S/cm was obtained, which was suitable for the intermediate reflector layer.

Micromorph silicon solar cell optical performance: Influence of intermediate reflector and front electrode surface texture

The optical performance of tandem a-Si:H/μc-Si:H (micromorph) thin film solar cell was investigated experimentally and by means of rigorous 3-D optical simulation. The interplay of intermediate reflectors, with different refractive indices and thicknesses, and front electrode surface texture was studied. Experiments and simulations show that LPCVD ZnO based front electrodes have the highest optical potential together with a low refractive index of the intermediate reflector. The intermediate reflector layer serves for redistribution of the mid-range solar spectrum between the top and bottom cell, while the sum of the top and bottom cell currents decreases with increasing IRL thickness. Additionally, promising concepts to increase the short-circuit current of the tandem solar cell are shown. The most important steps are related to lowering parasitic absorption in supportive layers by the introduction of silicon oxide layers and improving the light incoupling by introduction of anti-reflective layers.

Implications of TCO Topography on Intermediate Reflector Design for a-Si/μc-Si Tandem Solar Cells—Experiments and Rigorous Optical Simulations

The influence of the transparent conducting oxide (TCO) topography was studied on the performance of a silicon oxide intermediate reflector layer (IRL) in a-Si/μc-Si tandem cells, both experimentally and by 3-D optical simulations. Therefore, cells with varying IRL thickness were deposited on three different types of TCOs. Clear differences were observed regarding the performance of the IRL as well as its ideal thickness, both experimentally and in the simulations. Optical modeling suggests that a small autocorrelation length is essential for a good performance. Design rules for both the TCO topography and the IRL thickness can be derived from this interplay.

Low-refractive-index and high-transmittance silicon oxide with a mixed phase of n-type microcrystalline silicon as intermediate reflector layers for tandem solar cells

Low-refractive-index and high-transmittance silicon oxide (SiOx) with a mixed phase of n-type microcrystalline silicon was developed for intermediate reflector layers (IRLs) of high-efficiency amorphous Si and microcrystalline-Si tandem solar cells. The refractive index, crystalline fraction, and conductivity of the SiOx IRLs were characterized as functions of the deposition conditions. The SiOx films were prepared by plasma-enhanced chemical vapor deposition, at a high pressure of 9Torr and high hydrogen dilution ratio of 400, and using a narrow electrode gap of 12mm. The films showed a refractive index of 1.85, crystalline fraction of ~50%, and conductivity of ~10−7S/cm. Furthermore, the transmittance of the films was more than 90% at wavelengths between 600nm and 1100nm. We present here the procedure to optimize the SiOx IRLs, which is to compare the changes in the top and bottom cells' current densities of the tandem solar cells fabricated with 10–20% thinner absorption layers. A stabilized efficiency of 11.12% and light-induced degradation of −9.3% could be achieved in a tandem module containing an optimized IRL by enhancing the top cell current. The SiOx IRLs developed and the optimization procedure discussed in this paper can be very useful for the fabrication of high-efficiency thin-film Si tandem modules.