Design of a Highly Efficient Subwavelength Antireflective Structure for Solar Cells

We present a low-cost method to implement a single-layer antireflective coating (ARC) including metallic nanoparticles embedded into a polymer matrix. This ARC shows an almost flat behaviour in the whole visible spectral range (400–750 nm). The metamaterial is formed by a 1% in weight of Aluminium nanoparticles (NPs) randomly dispersed into a transparent UV curable photopolymer matrix that can be deposited on different substrates, which extends its application not only to solar cells but also to the protective glass covers. The NPs dispersion method strongly affects the optical properties, being the reflectance reduction more efficient when the cluster density in the metamaterial is low enough. Our results show an average reflectance reduction up to 50% in the whole VIS spectral range when the substrate is a soda-lime glass slide. The deposition method, based on the doctor-blade technique, is simple and inexpensive, since does not require clean-room nor vacuum equipment, being able to roll out the ARC layer even on already existing devices. The NPs were also deposited on top of a fully operative InN on Si thin-film solar cell. Optical measurements showed an average surface reflectance reduction of a 24% when incorporating the NPs. Furthermore, the photovoltaic performance increased under 1 sun of standard AM1.5G solar illumination when adding the ARC layer, pointing to a relative 9.0% increment in the conversion efficiency.

Light harvesting is essential for high performance thin-film solar cells made of amorphous (a-Si:H) or microcrystalline silicon (μc-Si:H). In present solar cells randomly textured TCO substrates (e.g. SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or ZnO) serving as front contacts and reflective metal back contacts (e.g. Ag) are used. Recently, the use of plasmonic effects by implementation of metallic nanoparticles has become a focus to enhance the photocurrent. Depending on the size of the Ag nanostructures, the physical effects leading to an enhancement differ. For large Ag nanostructures (>; 50 nm) the light scattering provided by localised plasmonic resonances is shown to improve the light-trapping of a solar cell for wavelengths above the band gap of the absorber material. For small Ag nanoparticles (diameter <; 40 nm) a photocurrent enhancement in the sub band gap wavelength region of a-Si:H is reported. In this contribution, the progress in the research of both effects is presented. While for large Ag nanostructures improvements of light trapping in thin-film silicon solar cells was presented, the inclusion of small Ag nanoparticles in a-Si:H is promising for extending the spectral range of thin-film silicon based detectors.

Thin film solar cells, due to the low cost, high efficiency, long-term stability, and consumer applications, have been widely applied for harvesting green energy. All of these thin film solar cells generally adopt various metal thin films as the back electrode, like Mo, Au, Ni, Ag, Al, graphite, and so forth. When they contact with p-type layer, it always produces a Schottky contact with a high contact potential barrier, which greatly affects the cell performance. In this work, we report for the first time to find an appropriate p-type conductive semiconductor film, digenite Cu9S5 nanocrystalline film, as the back electrode for CdTe solar cells as the model device. Its low sheet resistance (16.6 Ω/sq) could compare to that of the commercial TCO films (6-30 Ω/sq), like FTO, ITO, and AZO. Different from the traditonal metal back electrode, it produces a successive gradient-doping region by the controllable Cu diffusion, which greatly reduces the contact potential barrier. Remarkably, it achieved a comparable power conversion efficiency (PCE, 11.3%) with the traditional metal back electrode (Cu/Au thin films, 11.4%) in CdTe cells and a higher PCE (13.8%) with the help of the Au assistant film. We believe it could also act as the back electrode for other thin film solar cells (α-Si, CuInS2, CIGSe, CZTS, etc.), for their performance improvement.

AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract We have developed a new light-trapping scheme for a thin-film Si stacked module (Si HYBRID PULS module), where a (a-Si:H/transparent interlayer/microcrystalline Si) thin-film was integrated into a large-area solar cell module. An initial aperture efficiency of 13·1% has been achieved for a 910 × 455 mm Si HYBRID PLUS module, which was independently confirmed by AIST. This is the first report of the independently confirmed efficiency of a large-area thin-film Si module with an interlayer. The 19% increase of short-circuit current of this module was obtained by the introduction of a transparent interlayer that caused internal light-trapping. A mini-module was shown to exhibit a stabilized efficiency of 12%. Outdoor performance of a Si HYBRID (a-Si:H / micro-crystalline Si stacked) solar cell module has been investigated for over 4 years with two different kinds of module (top and bottom cell limited, respectively). The HYBRID modules limited by the top cell have exhibited a more efficient performance than the modules limited by the bottom cell, in natural sunlight at noon. Copyright © 2005 John Wiley & Sons, Ltd. REFERENCES 1 Yamamoto K, Suzuki T, Yoshimi M, Nakajima A. Low temperature fabrication of thin film polycrystalline Si solar cell on the glass substrate and its application to the a-Si:H/Polycrystalline Si tandem solar cell. Proceedings of the 25th IEEE Photovoltaic Energy Conversion, Conference, 1996; 661– 664. 2 Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, Okamoto Y, Nakajima A. Very thin film crystalline silicon solar cells on glass substrate fabricated at low temperature. IEEE Transactions Electron Devices 1999; 46: 2041– 2047. 3 Yamamoto K, Yoshimi M, Tawada Y, Okamoto Y, Nakajima A. Cost effective and high-performance thin film Si solar cell towards the 21st century. Solar Energy Materials and Solar Cells 2001; 66: 117– 125. 4 Meier J, et al. On the way towards high-efficiency thin film silicon solar cells by the ‘micromorph’ concept. Proceedings of the Material, Research Society Symposium, Spring Meeting, San Francisco, 1996; 420: 3– 14. 5 Yamamoto K, et al. Novel HYBRID thin film silicon solar cell and module. Proceedings of the 3rd World Conference on Photovoltaic Solar Energy Conversion, 2003; Paper S20B903. 6 Nakajima A, et al. Improvement on actual output power of thin film silicon HYBRID module. Proceedings of the 3rd World Conference on Photovoltaic Solar Energy Conversion, 2003; Paper 60B1104. Citing Literature Volume13, Issue6September 2005Pages 489-494 ReferencesRelatedInformation

Trapping light inside the solar cells can significantly boost their efficiency by minimizing the reflection losses at the front interface as well as the absorption losses in the solar cell by increasing the path length of incoming light. This study focuses on light trapping using antireflection coating (ARC). One of the simplest and most efficient types of ARC is called the quarter-wave transformer. Its great results make it a good and cheaper alternative for using metallic nanoparticles (MNPs). However, using MNPs with appropriate parameters might have a positive effect on the overall enhancement. In this work it was found that placing the NPs on the ARC surface give the best results compared to the other positions, whereas decreasing the overall refractive index of the device improves its behavior.

This paper describes a freeware program that computes the optical losses associated with the front surface of a silicon solar cell. The optical losses are not trivial to assess because (i) the refractive index and extinction coefficient of silicon, antireflection coatings (ARCs), and encapsulants varies with wavelength; (ii) cells are usually textured such that the light reflects multiple times from the front surface; (iii) light can be polarised, particularly after the first ‘bounce’ from a textured surface; and (iv) the incident spectrum and the cell's quantum efficiency varies with wavelength. The freeware program takes all of these aspects into account to calculate reflection from the solar cell, absorption in the ARCs, transmission into the silicon, and the equivalent current that is generated for any given spectrum. When modelling textured silicon, the program is restricted to normally incident light and pyramidal morphologies. The program computes solutions within one second for regular upright pyramids, regular inverted pyramids, and random upright pyramids—making it much faster than ray tracing. We provide an example of how the freeware can be employed to determine the optimal thickness of an ARC with and without encapsulation. The example demonstrates that the optimal thickness cannot be determined from reflection measurements when absorption in the ARCs is significant. The program is readily adaptable to assess ARCs on glass and thin-film solar cells.

Reduced adverse effects on Si thin film solar cells caused by growth chamber air exposure

I Introduction 1. Introduction to thin-film photovoltaics II Device characterization 2. Fundamental electrical characterization of thin-film solar cells 3. Electroluminescence analysis of thin-film solar modules 4. Capacitance spectroscopy of thin-film solar cells III Materials characterization 5. Characterizing the light trapping properties of textured surfaces with scanning near-field optical microscopy 6. Ellipsometry 7. Photoluminescence analysis of Si and chalcopyrite-type thin films for solar cells 8. Steady state photocarrier grating method 9. Time-of-flight analysis 10. Electron Spin Resonance on Si thin films for solar cells 11. Scanning probe microscopy on thin films for solar cells 12. Electron microscopy on thin films for solar cells 13. X-ray and neutron diffraction of materials for thin film solar cells 14. Raman Spectroscopy on thin films for solar cells 15. Soft x-ray and electron spectroscopy: a unique tool chest to characterize the chemical and electronic properties of surfaces and interfaces 16. Elemental distribution profiling of thin films for solar cells 17. Hydrogen effusion experiments IV Materials and device modelling 18. Ab-initio modelling of semiconductors 19. One-dimensional electro-optical simulations of thin film solar cells 20. Two-dimensional electrical simulations of thin film solar cells

An overview of the applications of porous silicon (PS) thin films, as antireflection coatings (ARC) in silicon solar cells and transducers in biosensors, is presented. The reflectance spectra of PS films have been compared with other conventional ARCs (such as SiNx TiO2/MgF2 and ZnS), and optimal PS ARC with minimum reflectance has been obtained. The implementation of PS into an industrially compatible screen-printed (SP) solar cell by both the electrochemical etching (ECE) and chemical etching (CE) methods are reviewed. Porous silicon films, formed via ECE for short anodization times, on textured n+ emitter ofc-Si solar cell having SP front and back contacts, lead to improvements in the performance of solar cells and demonstrate their viability in industrial applications.

Investigation on antireflection coatings for Al:ZnO in silicon thin-film solar cells

Cost-effective and lightweight solar cells are currently demanded in strategic fields such as space applications or integrated-wearable devices. A reduction of the active layer thickness, producing thin-film devices, has been a traditional solution to accomplish both requirements. However, this solution also reduces the efficiency of the device. For this reason, alternative strategies are being proposed. In this work, light trapping effects of an array of semiconductor nanoparticles located on the top surface of a thin-film GaAs solar cell are investigated to improve the optical absorption and current density in active layer, under the standard AM-1.5 solar spectrum. The numerical results are compared with other previous proposals such as an aluminum nanoparticle array, as well as conventional solar cells with and without a standard anti-reflective coating (ARC). The inclusion of semiconductor nanoparticles (NPs) shows an improved response of the solar cells at different angles of incidence in comparison to solar cell with an ARC. Furthermore, the efficiency increases a 10% respect to the aluminum nanoparticles (NPs) architecture, and a 21% and a 30% respect to solar cells with and without ARC, respectively.

Present status of thin film solar cells in Japan

In this work, a numerical assessment of the optoelectrical properties of the ZnO–ZnSe–CdSe heterojunction for a thin and cost-effective solar cell was made by using the PC1D simulation software. The photovoltaic (PV) properties have been optimized by varying thicknesses of the absorber layer of the p-CdSe layer, the window layer of n-ZnSe, and the antireflection coating (ARC) layer of ZnO, a transparent conductive oxide with enhanced light trapping, and wide bandgap engineering. There is a positive conduction band offset (CBO) of ΔEc = 0.25 eV and a negative valence band offset (VBO) of ΔEv = 1.2 − 2.16 = − 0.96 eV. The positive CBO prevents the flow of electrons from the CdSe to the ZnSe layer. Further, the impact of doping concentration on the performance of solar cells has been analyzed. The simulation results reveal the increase in the efficiency of solar cells by adding an ARC. The rapid and sharp increase in the efficiency with the thickness of the window layer beyond 80 nm is interesting, unusual, and unconventional due to the combined effect of morphology and electronics on a macro-to-micro scale. The thin-film solar cell with the structure of ZnO/ZnSe/CdSe exhibited a high efficiency of 11.98% with short-circuit current (Isc) = 1.72 A, open-circuit voltage (Voc) = 0.81 V and fill factor (FF) = 90.8% at an optimized thickness of 2 μm absorber layer, 50 nm window layer, and 78 nm ARC layer. The EQE of solar cells has been observed at about 90% at a particular wavelength at 470 nm (visible light range). Around 12% of efficiency from such a thin-layered solar cell is highly applicable.

Transparent ZnO thin-films are prepared using the RF magnetron sputtering and spray pyrolysis techniques on the glass substrates. Reflectance spectra and thin films heights are measured using spectrophotometer and stylus surface profiler, respectively. Measured optical data is used for investigating the effect of the ZnO prepared by above two processes on the performance of Cu2ZnSnS4 (CZTS) thin films solar cell (TFSC). One dimensional simulation approach is considered using the simulation program, SCAPS. External quantum efficiency and J-V characteristics of CZTS TFSC is simulated on the basis of optical reflectance data of ZnO films with and without ZnO thin-films as antireflection coating (ARC). Study shows that ARC coated CZTS TFSC provides a better fill factor (FF) as compared to other ARC material such as MgF2. Sprayed ZnO thin-films as ARC show comparable performance with the sputtered samples.

Thin-film solar cell technology based on hydrogenated amorphous silicon has matured over the last two decades and is capable to deliver commercial modules with almost 10 % stabilized efficiency. Moreover attention on hydrogen fuel cells is increasing as the range of potential commercial applications expands, with more than 3800 fuel cell systems being installed and operated worldwide. These systems attain high theoretical efficiencies in the range of 43.7-70.4 %, with an additional 20% as heat recovery, and operating at carbon-free conditions between 50 °C and 900 °C. In this paper the status of thin-film silicon solar cell technology is reviewed by comparing it to other major solar cell technologies. A discussion is also made upon the benefits obtained from the few potential applications, as long-term potential actions for sustainable development, and the future of Solid Oxide Fuel Cells (SOFCs). Applications of thin-film silicon modules and SOFCs are presented. EPIA (European Photovoltaic Industry Association) estimates that half of the growth in the next two years will come from thin-film technology.