Microcrystalline Silicon Solar Cells Research Articles

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (µc-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated µc-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

Effect of Front TCO Layer on Properties of Substrate-Type Thin-Film Microcrystalline Silicon Solar Cells

Power conversion efficiency of thin-film microcrystalline silicon solar cells has been remarkably improved recently: from 10.1% to 11.8%. Front transparent conductive oxide (TCO) films have played an important role in this efficiency improvement. In this study, the impact of the front TCO films was investigated by comparing microcrystalline silicon solar cells with In2O3:Sn films grown by sputtering and ZnO:B films grown by metal-organic chemical vapor deposition (MOCVD). Improvement mechanisms in open-circuit voltages and fill factors in solar cells are investigated and discussed.

Improved efficiency of ultra-thin µc-Si solar cells with photonic-crystal structures.

We investigate the improvement of the conversion efficiency of ultra-thin (~500nm-thick) microcrystalline silicon (μc-Si) solar cells incorporating photonic-crystal structures, where light absorption is strongly enhanced by the multiple resonant modes in the photonic crystal. We focus on the quality of the intrinsic μc-Si layer deposited on the substrate, which is structured to form a photonic crystal at its upper surface with a period of several hundred nanometers. We first study the crystalline quality from the viewpoint of the crystalline fraction and show that the efficiency can be improved when the deposition conditions for the μc-Si layer are tuned to give an almost constant crystalline fraction of ~50% across the entire film. We then study the influence of the photonic-crystal structure on the crystalline quality. From transmission-electron microscope images, we show that the collision of μc-Si grains growing at different angles occurs when a photonic-crystal structure with an angular surface is used; this can be suppressed by introducing a rounded surface structure. As a result, we demonstrate an efficiency of 8.7% in a ~500-nm thick, homo-junction μc-Si solar cell, which has only ~1/4 the thickness of typical μc-Si solar cells. We also discuss the possibility of further improving the efficiency by performing calculations that focus on the absorption characteristics of the fabricated cell structure.

UV micro-imprint patterning for tunable light trapping in p-i-n thin-film silicon solar cells

In this paper, we used UV micro-imprint lithography periodic patterning combined with self-textured BZO films with a wide range of texture distributions for light trapping in thin-film silicon solar cells. It was found that the feature size of the periodic textures has a significant influence on the light trapping capacity of the glass substrate and the external quantum efficiency (EQE) of microcrystalline silicon (μc-Si:H) solar cells. Microcrystalline silicon solar cells, deposited on periodic textures of 5μm, showed an improved photocurrent density without any loss in the open-circuit voltage and fill factor; hence, resulting in an overall efficiency increase of 6.28%.

High-efficiency microcrystalline silicon solar cells on honeycomb textured substrates grown with high-rate VHF plasma-enhanced chemical vapor deposition

The potential of high-rate growth of high-quality microcrystalline silicon (µc-Si:H) films for solar cell applications is investigated by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) under a high-pressure SiH4 depletion condition. It is found that the morphology of textured substrates plays an important role in not only light trapping but also µc-Si:H film growth. A high conversion efficiency of 11.1% is attained in a substrate-type µc-Si:H cell on a substrate with honeycomb textures, which has rounded concaves in a honeycomb arrangement with an appropriate period. It is also clarified that ZnO:B films grown by metal organic chemical vapor deposition (MOCVD) are beneficial in terms of carrier collection compared with the standard In2O3:Sn (ITO) film grown by sputtering. On the basis of these findings, a new world-record µc-Si:H cell with a certified conversion efficiency of 11.8% is developed with a relatively high deposition rate of 1 nm/s.

Structural design of photonic crystal thin film silicon solar cells by sensitivity analysis: Inclusion of electrode absorption.

We carry out the structural design of photonic crystals (PCs) using sensitivity analysis for enhancing optical absorption of thin film microcrystalline silicon (μc-Si) solar cells. In this paper, we employ a model which includes absorption of not only the thin film μc-Si, but also the transparent conductive oxide and metal back reflector for design accuracy. We carry out structural design for this model using sensitivity analysis which maximizes optical absorption in μc-Si layer. As a result, we succeed in obtaining the maximum short circuit current density of 25.2 mA/cm2 for thin film (600-nm thick) μc-Si solar cells (1.4-fold increase compared to the case without a PC).

Highly transparent Zn1−xMgxO/ITO multilayer for window of thin film solar cells

Texture-etched Zn1−xMgxO films were fabricated and applied as front transparent electrodes for superstrate type thin film solar cells. The Zn0.65Mg0.35O film (x = 0.35) showed optical transparency better than commercially available Asahi VU and double-textured ZnO (WT-ZnO) substrates. To provide pertinent conductivity, ITO film was coated on the texture-etched Zn0.65Mg0.35O. By employing the Zn0.65Mg0.35O/ITO substrate instead of the SnO2, we demonstrated an enhancement of quantum efficiency for amorphous silicon thin film solar cell devices, resulted in efficiency improvement from 8.92 to 9.56%. We also examined effectiveness of the Zn0.65Mg0.35O/ITO substrate for the microcrystalline silicon solar cells which delivered an efficiency of 9.73% with proper anti-reflection coating. Our experimental results suggest that the Zn0.65Mg0.35O/ITO multilayer front contact can be beneficial for reinforcing performances of silicon-based thin film solar cell devices.

Improved performance of microcrystalline silicon solar cell with graded-band-gap silicon oxide buffer layer**Project supported by the Key Basic Research Project of Hebei Province, China (Grant Nos. 12963930D and 12963929D), the Natural Science Foundation of Hebei Province, China (Grant Nos. F2013201250 and E2012201059), and the Science and Technology Research Projects of the Education Department of Hebei Province, China (Grant No. ZH2012030).

Microcrystalline silicon (μc-Si:H) solar cell with graded band gap microcrystalline silicon oxide (μc-SiOx:H) buffer layer is prepared by plasma enhanced chemical vapor deposition and exhibits improved performance compared with the cell without it. The buffer layer moderates the band gap mismatch by reducing the barrier of the p/i interface, which promotes the nucleation of the i-layer and effectively eliminates the incubation layer, and then enhances the collection efficiency of the cell in the short wavelength region of the spectrum. The p/i interface defect density also decreases from 2.2 × 1012 cm−2 to 5.0 × 1011 cm−2. This graded buffer layer allows to simplify the deposition process for the μc-Si:H solar cell application.

Triple-junction thin-film silicon solar cell fabricated on periodically textured substrate with a stabilized efficiency of 13.6%

We report a high-efficiency triple-junction thin-film silicon solar cell fabricated with the so-called substrate configuration. It was verified whether the design criteria for developing single-junction microcrystalline silicon (μc-Si:H) solar cells are applicable to multijunction solar cells. Furthermore, a notably high short-circuit current density of 32.9 mA/cm2 was achieved in a single-junction μc-Si:H cell fabricated on a periodically textured substrate with a high-mobility front transparent contacting layer. These technologies were also combined into a-Si:H/μc-Si:H/μc-Si:H triple-junction cells, and a world record stabilized efficiency of 13.6% was achieved.

Mechanism insight into the effect of I/P buffer layer on the performance of NIP-type hydrogenated microcrystalline silicon solar cells

A simulation and experimental study on the effect of the buffer layer at the I/P interface on the performance of NIP-type hydrogenated microcrystalline silicon (μc-Si:H) single-junction solar cells is presented. Device-quality hydrogenated amorphous silicon (a-Si:H) material as a buffer layer at the I/P interface obviously improves the performance of NIP-type μc-Si:H single-junction solar cells. In addition to the well-known mechanism that an a-Si:H I/P buffer layer can reduce the recombination current density at I/P interfaces, the optically and electrically calibrated simulations and supporting experimental results in this study illustrate that the performance improvement also originates from the mitigation of the electric screening effect due to the reduced defect density at the I/P interfaces, which reinforces the bulk electric field. Integrating an optimized hydrogen profiling strategy and adding a-Si:H I/P buffer layer yielded an initial efficiency of 9.20% for μc-Si:H single-junction solar cells with an active area of 0.27 cm2. This study may provide new ideas of further improving the performance of NIP-type μc-Si:H single-junction solar cells by mitigating the electric screening effect.

Investigation of isochronal annealing on the optical properties of HWCVD amorphous silicon nitride deposited at low temperatures and low gas flow rates

Hydrogenated amorphous silicon nitride (a-SiNx:H) is used as anti-reflection coatings in commercial solar cells. A final firing step in the production of micro-crystalline silicon solar cells allows hydrogen effusion from the a-SiNx:H into the solar cell, and contributes to bulk passivation of the grain boundaries. In this study a-SiNx:H deposited in a hot-wire chemical vapour deposition (HWCVD) chamber with reduced gas flow rates and filament temperature compared to traditional deposition regimes, were annealed isochronally. The UV-visible reflection spectra of the annealed material were subjected to the Bruggeman Effective Medium Approximation (BEMA) treatment, in which a theoretical amorphous semiconductor was combined with particle inclusions due to the structural complexities of the material. The extraction of the optical functions and ensuing Wemple-DeDomenici analysis of the wavelength-dependent refractive index allowed for the correlation of the macroscopic optical properties with the changes in the local atomic bonding configuration, involving silicon, nitrogen and hydrogen.

Effect of I/N interface on the performance of superstrate hydrogenated microcrystalline silicon solar cells

A study on the effect of the n-type hydrogenated amorphous silicon (a-Si:H) buffer layers at the I/N interfaces on the overall performance of superstrate (pin-type) hydrogenated microcrystalline silicon (μc-Si:H) single-junction solar cells with wide band gap silicon oxide doped supporting layers is presented. By integrating the n-type a-Si:H materials with proper band gap and thickness values at the I/N interface, the band gap discontinuity between the μc-Si:H intrinsic and n-type nc-SiOx:H layers can be successfully compensated for, thereby improving the carrier collection, reducing the accumulation and recombination of photo-generated carriers near the I/N interface, and improving the overall current and power output of the μc-Si:H cells. Further adoption of an optimized hydrogen profiling technique based on the optimized n-type a-Si:H buffer layers yields an initial conversion efficiency of 10.05% for μc-Si:H single-junction solar cells with a thickness of 2μm.

A scalable and inexpensive surface-texturization method for advanced transparent front electrodes in microcrystalline and micromorph thin film silicon solar cells

As the thin film silicon solar cell technology reaches a pivotal point to keep competing in the photovoltaic industry where crystalline silicon and other technologies currently dominate, it has become an urgent task to revolutionize some of its state-of-the-art key processes which have reached their cost barriers for decades. We have devised a more cost-effective method for mass production of transparent front electrodes for thin film silicon solar modules. It involves sputtering deposition and a novel surface-texturization process. The new method produces microscopic U-shaped surface textures of aluminum-doped zinc oxide (AZO) and other transparent conductive oxides (TCOs) which lead to higher open circuit voltages, higher fill factors, and comparable short circuit current densities for microcrystalline and micromorph silicon solar cells. We experimentally demonstrate that solar cells using these TCO front electrodes reach comparable efficiency levels to those using any other commercialized TCO electrodes such as fluorine-doped tin oxide (FTO) and boron-doped zinc oxide (BZO). An analysis shows that the manufacturing costs of the new method can be significantly lower than those of the commercial counterparts. The possibilities brought about by this method may pave a new path for future developments of thin film silicon solar cells.

Microcrystalline silicon solar cells with passivated interfaces for high open‐circuit voltage

We introduce passivating hetero‐interfaces in single‐junction microcrystalline silicon (μc‐Si:H) solar cells. We investigate the effect of different i–n layer stacks in thin μc‐Si:H devices, in which recombination is significant at the interfaces as well as in the bulk material. By applying amorphous silicon passivating layers at the μc‐Si:H i–n interface, we show a device with a high open‐circuit voltage (Voc) of 608 mV, for a standard Raman crystalline fraction of the i‐layer (>50%). This Voc is the highest reported value for a state‐of‐the‐art μc‐Si:H device made by plasma‐enhanced chemical vapor deposition. We also report an efficiency of 9.45% for a solar cell with an absorber layer as thin as 650 nm on an area greater than 1 cm2, and show with a simple crystalline silicon model that for such thin μc‐Si:H devices or μc‐Si:H devices with a very high bulk‐material quality, well‐mastered interfaces and doped layers are of paramount importance for high efficiency.

Photonic crystal microcrystalline silicon solar cells

AbstractEnhancing the absorption of thin‐film microcrystalline silicon solar cells over a broadband range in order to improve the energy conversion efficiency is a very important challenge in the development of low cost and stable solar energy harvesting. Here, we demonstrate that a broadband enhancement of the absorption can be achieved by creating a large number of resonant modes associated with two‐dimensional photonic crystal band edges. We utilize higher‐order optical modes perpendicular to the silicon layer, as well as the band‐folding effect by employing photonic crystal superlattice structures. We establish a method to incorporate photonic crystal structures into thin‐film (~500 nm) microcrystalline silicon photovoltaic layers while suppressing undesired defects formed in the microcrystalline silicon. The fabricated solar cells exhibit 1.3 times increase of a short circuit current density (from 15.0 mA/cm2 to 19.6 mA/cm2) by introducing the photonic crystal structure, and consequently the conversion efficiency increases from 5.6% to 6.8%. Moreover, we theoretically analyze the absorption characteristics in the fabricated cell structure, and reveal that the energy conversion efficiency can be increased beyond 9.5% in a structure less than 1/400 as thick as conventional crystalline silicon solar cells with an efficiency of 24%. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

Numerical simulation of light absorption enhancement in microcrystalline silicon solar cells with Al nanoparticle arrays

Metal nanoparticles with low cost and high performance have good potential applications in light-trapping of solar cells. In this paper, a three-dimensional model is proposed to simulate the light absorption of microcrystalline silicon (μc-Si:H) thin film solar cells. The effects of spherical and hemispherical Al nanoparticle arrays located on the front surfaces of solar cells are investigated, and the particle radius and array period are optimized by the finite element method. The results show that the optimal Al nanoparticle arrays can enhance broadband absorption in thin film solar cells. For spherical particle arrays, the key parameter that influences light absorption in solar cells is period/radius ratio (P/R) or particle surface coverage. When P/R=4-5, the optimum integrated absorption enhancement (Eabs) is over 20% under AM1.5 illumination compared with the solar cell without nanoparticles. The value of Eabs is small and decreases with the increase of P/R when P/R>5, and Eabs is less than zero when P/RP=500 nm and R=120 nm, the spectral absorption rate as a function of wavelength shows broadband absorption including four distinct peaks, which are attributed to quadrupole plasmon resonance mode, dipole resonance mode and waveguide mode respectively according to the electric field distribution in the solar cell. For hemispherical particle arrays, the maximum value of Eabs is 24.5%, which is higher than that of the solar cell with optimized spherical particle arrays. This is due to the high coupling efficiencies of the particles, so that most of the scattered light is directly coupled into the substrate. However, the value of Eabs is very sensitive to the hemispherical particle radius. As the radius decreases, the scattering cross-section and scattering efficiency of the particle decrease dramatically. As the radius increases, the dipole plasmon resonance wavelength rapidly shifts towards longer wavelength (red shift). Both of these are detrimental to absorption enhancement of solar cells. Thus we conclude that spherical Al particle arrays are more preferable in actually fabricating the light-trapping of solar cells.

Highly transparent front electrodes with metal fingers for p-i-n thin-film silicon solar cells

The optical and electrical properties of transparent conductive oxides (TCOs), traditionally used in thin-film silicon (TF-Si) solar cells as front-electrode materials, are interlinked, such that an increase in TCO transparency is generally achieved at the cost of reduced lateral conductance. Combining a highly transparent TCO front electrode of moderate conductance with metal fingers to support charge collection is a well-established technique in wafer-based technologies or for TF-Si solar cells in the substrate (n-i-p) configuration. Here, we extend this concept to TF-Si solar cells in the superstrate (p-i-n) configuration. The metal fingers are used in conjunction with a millimeter-scale textured foil, attached to the glass superstrate, which provides an antireflective and retroreflective effect; the latter effect mitigates the shadowing losses induced by the metal fingers. As a result, a substantial increase in power conversion efficiency, from 8.7% to 9.1%, is achieved for 1-μm-thick microcrystalline silicon solar cells deposited on a highly transparent thermally treated aluminum-doped zinc oxide layer combined with silver fingers, compared to cells deposited on a state-of-the-art zinc oxide layer.

Post-deposition treatment of microcrystalline silicon solar cells for improved performance on rough superstrates

In this contribution, we investigate the effect of post-deposition treatments on finished non-encapsulated thin-film microcrystalline silicon solar cells and show that annealing in vacuum leads to improved electrical properties of the solar cells, particularly for cells deposited on rough superstrates. Our results suggest that both curing of intrinsic defects in the silicon, which can appear during the deposition of the ZnO back electrode, as well as an improvement of the ZnO back-electrode conductivity itself, occur during an annealing in vacuum, leading to large improvements of the open-circuit voltage and fill factor values. An improvement of the porous zones in the absorber layer, as induced by rough superstrates, is also observed by Fourier-transform photocurrent spectroscopy, implying that these porous zones cannot be considered as being purely bi-dimensional, but have a spatial extension within the absorber layer.

Modulated surface textured glass as substrate for high efficiency microcrystalline silicon solar cells

In this paper, we investigate three different procedures for making modulated surface textures (MST) on glass substrates and show the application of these substrates to microcrystalline silicon solar cells. MST substrates have broader surface angle distribution (AD) than a single textured glass substrate and this distribution peaks at higher angles. Therefore, (i) the haze in reflection of MST substrates covered with an Ag layer is over 99% in the wavelength range above 350nm, which is higher than in the case of a single textured glass substrate; (ii) more reflected light is scattered into higher angles, resulting in a broader reflective angular intensity distribution (AID) in air. These properties lead to a higher Jsc for n-i-p microcrystalline silicon solar cells deposited on the MST substrates compared to cells deposited on single textured glass substrate. A solar cell deposited on a MST glass substrate with smoother peaks shows the highest electrical performance (Voc×FF value), because of a lower density of defective regions. The best cell achieved in this work has an active-area efficiency of 10.49%, with a Voc of 0.532V, a Jsc of 26.36mA/cm2, and a FF of 74.8% for a 3-μm thick absorber layer.

Development of Double-Textured ZnO:B Substrates for Improving Microcrystalline Silicon Solar Cell Performance

Zinc oxide (ZnO) films are widely used as front transparent conductive oxide films in thin-film silicon solar cells. Double-textured B-doped zinc oxide (W-textured ZnO:B) substrates, which are formed by reactive ion etching (RIE) and metal-organic chemical vapor deposition, have a high haze ratio over a wide wavelength range. In this study, we have investigated the RIE conditions necessary to modify the surface morphology and the optical properties of a glass substrate. By increasing the pressure during the RIE process, the surface morphology of etched glass was changed from circular to ellipsoidal shapes, and its texture size increased. A newly developed W-textured ZnO:B substrate fabricated using the modified glass exhibited a higher haze ratio than a conventional W-textured ZnO:B substrate and achieved a higher short-circuit current in microcrystalline silicon solar cells without significant deterioration of open-circuit voltage and fill factor.