High-efficiency Thin-film Silicon Solar Cells Research Articles

Combining light-harvesting with detachability in high-efficiency thin-film silicon solar cells.

Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in achieving good energy-conversion efficiency and integrating light-management into the solar cell design. In this report a technique to circumvent these obstacles is presented: transferability and an efficient light-harvesting scheme are combined for thin-film silicon solar cells by the incorporation of a NaCl layer. Amorphous silicon solar cells in p-i-n configuration are fabricated on reusable glass substrates coated with an interlayer of NaCl. Subsequently, the solar cells are detached from the substrate by dissolution of the sacrificial NaCl layer in water and then transferred onto a plastic sheet, with a resultant post-transfer efficiency of 9%. The light-trapping effect of the surface nanotextures originating from the NaCl layer on the overlying solar cell is studied theoretically and experimentally. The enhanced light absorption in the solar cells on NaCl-coated substrates leads to significant improvement in the photocurrent and energy-conversion efficiency in solar cells with both 350 and 100 nm thick absorber layers, compared to flat-substrate solar cells. Efficient transferable thin-film solar cells hold a vast potential for widespread deployment of off-grid photovoltaics and cost reduction.

Combining randomly textured surfaces and one-dimensional photonic crystals as efficient light-trapping structures in hydrogenated amorphous silicon solar cells

One of the foremost challenges in achieving high-efficiency thin-film silicon solar cells is in devising an efficient light trapping system because of the short optical path length imposed by the inherent thin absorption layers. In this paper, an efficient light trapping system is proposed using a combination of randomly textured surfaces and a one-dimensional photonic crystal (randomly textured photonic crystal; RTPC). The influence of the texture on the optical performance of RTPCs is discussed using the results of an experiment and a finite-difference time-domain simulation. This RTPC back reflector (BR) can provide high reflectivity and strong light scattering, resulting in an increased photocurrent density of the hydrogenated amorphous silicon (a-Si:H) solar cell. As a result, the highly textured RTPC BR yielded an efficiency of 9.6% for a-Si:H solar cell, which is much higher than the efficiency of 7.6% on flat AZO/Ag BR and 9.0% on textured AZO/Ag BR. This RTPC BR provides a new approach for creating high-efficiency, low-cost thin-film silicon solar cells.

High-efficiency thin-film silicon solar cells realized by integrating stable a-Si:H absorbers into improved device design

We report that thin-film silicon solar cells exhibiting high stabilized efficiencies can be obtained by depositing hydrogenated amorphous silicon (a-Si:H) absorbers using triode-type plasma-enhanced chemical vapor deposition. The improved light-soaking stability and performance of solar cells are also realized by optimizing the device design, such as p and p–i buffer layers. As a result, we attain independently confirmed stabilized efficiencies of 10.1–10.2% for a-Si:H single-junction solar cells (absorber thickness: ti = 220–310 nm) and 12.69% for an a-Si:H (ti = 350 nm)/hydrogenated microcrystalline silicon (µc-Si:H) tandem solar cell fabricated using textured SnO2 and ZnO substrates, respectively. The relative efficiency degradations of these solar cells are ∼10 and 3%, respectively, under 1 sun illumination at 50 °C for 1000 h.

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.

11.0%-Efficient Thin-Film Microcrystalline Silicon Solar Cells With Honeycomb Textured Substrates

In this paper, we present our latest results toward high-efficiency thin-film silicon solar cells. Owing to the superior light trapping capability of periodic textures combined with other technologies, a new world record was achieved for the efficiency of single-junction microcrystalline silicon solar cells, with a conversion efficiency of 11.0%, independently confirmed by the Advanced Industrial Science and Technology (AIST) Characterization, Standards, and Measurement (CSM) team.

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.

Tailoring the surface morphology of zinc oxide films for high-performance micromorph solar cells

Self-textured zinc oxide polycrystalline films prepared by metalorganic low-pressure chemical vapor deposition combine excellent transparency, conductivity and light-scattering ability when used as electrodes for high-efficiency thin-film silicon solar cells. However, the growth of silicon layers with low defect density, which are necessary for high-performance solar cells, often requires the rough surface morphology of as-deposited zinc oxide films to be smoothened. This is usually achieved by a post-deposition argon plasma-etching treatment. We investigate here an alternative method to modify the surface morphology by changing the zinc oxide growth conditions over only the last hundreds of nanometers of the total film thickness. We discuss two types of zinc oxide cap layers, one grown with the addition of ethanol and the other with an enhanced diethylzinc (DEZ) precursor flow. We show that the presence of either type of cap layer does not alter the layer׳s electrical properties, but does slightly diminish the layer׳s light-scattering ability. The cap layer grown with ethanol leads to a more pronounced leveling of the film texture, while the high-DEZ-grown layer better preserves the sharp features of the underlying ZnO. Finally, zinc oxide films with cap layer are used as front electrodes for tandem amorphous/microcrystalline silicon solar cells, and are compared to rough films treated with an argon plasma. With rising thickness of the capping layers, the efficiency of tandem cells increases reaching values over 12%, and approaches that of cells on Ar plasma treated ZnO films.

Quantitative measurement and design of texture morphology for high-efficiency thin-film silicon solar cells

To decouple the trade-off relationship between short-circuit current ISC and open-circuit voltage VOC of thin-film silicon solar cells, which vary with texture morphology, it is necessary to first clarify the relationship between a solar cell’s properties and its texture morphology. We have developed a method for quantitatively measuring the texture morphology, which has enabled us to identify novel indices on the basis of texture width and angle individually correlated to ISC and VOC. A texturing process based on these indices should allow ISC and VOC to be improved independently.

Colloidal Self-assembled Nanosphere Arrays for Plasmon-enhanced Light Trapping in Thin Film Silicon Solar Cells

To realize high-efficiency thin-film silicon solar cells it is crucial to develop light-trapping methods that can increase absorption of the near- bandgap light in the silicon material. That can be achieved using the far-field scattering properties of metal nanoparticles (MNP) sustaining surface plasmons. The MNPs should be inserted in the back of the cell, embedded in the transparent conductive oxide (TCO) layer which separates the rear mirror from the silicon layers. In this way, a plasmonic back reflector (PBR) is constructed that can redirect light at angles away from the incidence direction and thereby increase its path length in the cell material.In this work, a novel technique is presented to fabricate PBRs (composed of Ag mirror/TCO/MNPs/TCO) containing colloidal gold MNPs patterned with a self-assembly wet-coating method. The method allows the construction of long-range ordered arrays of MNPs with monodisperse size and shape using fast, scalable, low-cost and low-temperature (<120°C) procedures.Colloidal MNPs are synthesized with spherical shapes, so their scattering properties are analytically modeled with Mie theory. Such formalism allowed the computation of the preferential MNP sizes that provide the best scattering performance for light-trapping in amorphous and microcrystalline thin-film silicon solar cells.

Management of light-trapping effect for a-Si:H/µc-Si:H tandem solar cells using novel substrates, based on MOCVD ZnO and etched white glass

Abstract The authors have successfully developed “W-textured” ZnO:B-coated soda-lime glass substrates with higher haze value and lower reflectance and thus, improved light-trapping effect, especially, in the long wavelength region. In order to obtain higher efficiency the authors have applied here this technique to a white glass substrate which has higher transmittance. The authors have investigated the relationship between its refractive index on each side of the glass before RIE etching and the RMS roughness of ZnO:B layer coated by the MOCVD method (on either side of the glass) after etching. For the surface morphology of the ZnO:B layer deposited on the side of the RIE-etched white glass with lower refractive index, it was found, in this study, that it shows mountain-chain-like double texture with higher haze value. Whereas ZnO:B layers deposited on soda-lime glass (SLG) investigated in a previous study, have cauliflower-like morphology and lower haze values. By using the newly developed white glass substrates, a higher short-circuit current density Jsc could be obtained corresponding to an improved spectral response, especially in the long wavelength regions (>500 nm). As a result, the a-Si:H/µc-Si:H solar cell fabricated on this newly developed substrate has an initial efficiency of 13.2% which is higher than the one for substrates based on soda-lime glass. This shows that this newly developed W-textured substrate is a promising solution for the deposition of high-efficiency thin-film silicon solar cells.

Advanced materials processing for high-efficiency thin-film silicon solar cells

We report on recent developments of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) for high-efficiency thin-film silicon solar cells. For a-Si:H, the light absorber layers were grown by a remote plasma technique using a triode electrode configuration in plasma-enhanced chemical vapor deposition (PECVD). Despite the relatively low deposition rate (0.01–0.03nm/s) compared to the conventional diode-type PECVD process (~0.2nm/s), the light-induced degradation in conversion efficiency (Δη/ηini) of single-junction solar cell is substantially reduced (e.g., Δη/ηini~10% at an absorber thickness of 250nm). As a result, we have obtained confirmed stabilized efficiencies of 9.6% and 11.9% for a-Si:H single-junction and a-Si:H/μc-Si:H tandem solar cells, respectively. Meanwhile, for μc-Si:H solar cells, we have investigated the structural properties of the μc-Si:H absorber layers grown at high deposition rates (>2nm/s). Several design criteria for the device grade μc-Si:H are proposed in terms of crystallographic orientation, grain size and grain boundary passivation.

New progress in the fabrication of n–i–p micromorph solar cells for opaque substrates

In this paper, we investigate tandem amorphous/microcrystalline silicon solar cells with asymmetric intermediate reflectors grown in the n–i–p substrate configuration. We compare different types of substrates with respect to their light-trapping properties as well as their influence on the growth of single-junction microcrystalline cells. Our most promising back reflector combines a textured zinc oxide film grown by low-pressure chemical vapor deposition, a silver film for reflection, and a zinc oxide buffer layer. Grown on this substrate, microcrystalline cells exhibit excellent response in the infrared while keeping high open-circuit voltage and fill factor, leading to efficiencies of up to 10.0%. After optimizing the morphology of the asymmetric intermediate reflector, we achieve an n–i–p micromorph solar cell stabilized efficiency of 11.6%, using 270nm and 1.7μm of silicon for the absorber layer of the amorphous top cell and the microcrystalline bottom cell, respectively. Using this original device architecture, we reach efficiencies close to those of state-of-the-art n–i–p and p–i–n micromorph devices, demonstrating a promising route to deposit high-efficiency thin-film silicon solar cells on opaque substrates.

Combined Optical and Electrical Design of Plasmonic Back Reflector for High-Efficiency Thin-Film Silicon Solar Cells

A back reflector (BR) that can efficiently scatter weakly absorbed light is essential to obtain high-efficiency thin-film silicon solar cells. We present the design routes of plasmonic BR based on self-assembled silver nanoparticles (Ag NPs) for high-efficiency thin-film silicon solar cells. Both optical and electrical effects on solar cells are considered. The shape of Ag NPs, the thickness of ZnO:Al spacer layers, materials on top of Ag NPs, and nanoparticle size are crucial for the performance of plasmonic BR. Increased annealing temperature lead to the formation of more appropriate shapes (more spherical and regular shapes) for a good light scattering and, thus, increase the photocurrent. The ZnO:Al layer between the Ag NPs and the Ag planar film has an optical effect on solar cells, while the ZnO:Al layer between the Ag NPs and the doped a-Si:H has both optical and electrical influence on the device. Larger NPs have less parasitic absorption and can preferentially scatter light into larger angles, thus increasing the spectral response in the solar cell. However, for larger Ag NPs, the fill factor deteriorates due to the rougher surface in the plasmonic BR, indicating a compromise between light trapping and electrical performance. Following the design routes, we obtained 8.4% high-efficiency plasmonic a-Si:H solar cell.

Nanoimprint Lithography for High-Efficiency Thin-Film Silicon Solar Cells

We demonstrate high-efficiency thin-film silicon solar cells with transparent nanotextured front electrodes fabricated via ultraviolet nanoimprint lithography on glass substrates. By replicating the morphology of state-of-the-art nanotextured zinc oxide front electrodes known for their exceptional light trapping properties, conversion efficiencies of up to 12.0% are achieved for micromorph tandem junction cells. Excellent light incoupling results in a remarkable summed short-circuit current density of 25.9 mA/cm(2) for amorphous top cell and microcrystalline bottom cell thicknesses of only 250 and 1100 nm, respectively. As efforts to maximize light harvesting continue, our study validates nanoimprinting as a versatile tool to investigate nanophotonic effects of a large variety of nanostructures directly on device performance.

Large-scale, high-efficiency thin-film silicon solar cells fabricated by short-pulsed plasma CVD method

Abstract A novel growth technology of microcrystalline silicon ( μ c-Si:H) thin films has been developed using short-pulsed VHF plasma CVD method [K. Nomoto, et al., Short-pulse VHF plasma-enhanced CVD of high-deposition-rate a-Si:H films, in: Proceedings of the 14th European Photovoltaic Solar Energy Conference and Exhibition, 1997, pp. 1226–1230]. The homogeneity of crystallinity in a film over square meter was improved by using this technology. We applied this technology to intrinsic layers of μ c-Si:H single solar cell, and confirmed that the homogeneity of cell's characteristics on a large-scale substrate was improved. And using this novel fabrication technology to intrinsic layers of a-Si:H/ μ c-Si:H tandem thin-film Si solar cell, initial conversion efficiency of 12.1%, corresponding to about 11% stabilized conversion efficiency was obtained with a large-scale substrate size of 925 mm × 560 mm .

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p ++-type CZ silicon substrates. Growth was performed from In Ga solutions, and in a purified Ar 4% H 2 forming gas ambient, rather than pure H 2. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm 2. Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10 16/cm 3 could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.