Thin Si solar cell with N type absorber based on epitaxial lateral overgrowth

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

Nanopyramid-based absorber to boost the efficiency of InGaN solar cells

Direct energy conversion from sun light to electricity using crystalline silicon based solar cell modules has been commercially available for several years. Above 20% conversion efficiencies have been realised in laboratory scale since 1990 [1]. The more important issue in the present solar cell fabrication development is to adopt such procedures and equipment that allow practical module efficiencies with less material and processing costs. One way to cut the material costs is to use thinner silicon wafers, which gives more wafers per length of a silicon log.In a high conversion efficiency cell the thickness has to be well below electron diffusion length for collection of the energy from long wavelength part of the solar spectrum. The diffusion length is an indicator of the material quality. Thin crystalline silicon solar cell technology gives the opportunity to choose cheaper, lower quality silicon as a starting material for the cell process without severe reduction in the conversion efficiency.Optimisation of a thin crystalline silicon solar cell design includes efficient light trapping and passivation of the cell surfaces. The proportion of a cell surface area to the cell volume increases as thickness is reduced. This magnifies the relative importance of surface recombination of the charge carriers as compared to the volume recombination in a solar cell. Although the quality demand on the cell material is relaxed, high conversion efficiency thin solar cell technology puts more weight on the quality of the surface passivation.In this paper, the fabrication procedure of thin crystalline silicon solar cell samples used in H.U.T. Electron Physics Laboratory is presented. The sample material is thinned using a 20% hot KOH bath to a nominal 1007 μm thickness. The standard cell sample is a 24 mm × 24 mm square with a p-type substrate, aluminium doped back surface field and phosphorous front diffusion. Aluminium evaporation through a stainless steel mask is adopted as a quick and simple metallisation scheme for both front and back contacts [2].The current-voltage-characteristics together with spectral response data of a typical thin crystalline silicon solar cell is also illustrated for the evaluation of the process. The measurement data is evaluated using the procedures and software developed by the author [2, 3]. The software includes diode modelling of the solar cell current-voltage data and breaking the spectral response to contributions from the emitter, depletion region and the base with numerical estimates on diffusion length, surface recombination velocities and charge carrier lifetimes.

Chapter IB-4 - Thin Silicon Solar Cells

<p>The fabrication of low cost, high efficiency solar cells is imperative in competing with existing energy technologies. Many research groups have explored using III-V materials and thin-film technologies to create high efficiency cells; however, the materials and manufacturing processes are very costly as compared to monocrystalline silicon (Si) solar cells. Since commercial Si solar cells typically have efficiencies in the range of 17-19%, techniques such as surface texturing, depositing a surface-passivating film, and creating multi-junction Si cells are used to improve the efficiency without significantly increasing the manufacturing costs. This research focused on two of these techniques: (1) a tandem junction solar cell comprised of a thin-film perovskite top cell and a wafer-based Si bottom cell, and (2) Si solar cells with single- and double-layer silicon nitride (SiNx) anti-reflection coatings (ARC).</p><p>The perovskite/Si tandem junction cell was modeled using a Matlab analytical program. The model took in material properties such as doping concentrations, diffusion coefficients, and band gap energy and calculated the photocurrents, voltages, and efficiencies of the cells individually and in the tandem configuration. A planar Si bottom cell, a cell with a SiNx coating, or a nanostructured black silicon (bSi) cell can be modeled in either an n-terminal or series-connected configuration with the perovskite top cell. By optimizing the bottom and top cell parameters, a tandem cell with an efficiency of 31.78% was reached.</p><p>Next, planar Si solar cells were fabricated, and the effects of single- and double-layer SiNx films deposited on the cells were explored. Silicon nitride was sputtered onto planar Si samples, and the refractive index and thicknesses of the films were measured using ellipsometry. A range of refractive indices can be reached by adjusting the gas flow rate ratios of nitrogen (N2) and argon (Ar) in the system. The refractive index and thickness of the film affect where the minimum of the reflection curve is located. For Si, the optimum refractive index of a single-layer passivation film is 1.85 with a thickness of 80nm so that the minimum reflection is at 600nm, which is where the photon flux is maximized. However, using a double-layer film of SiNx, the Si solar cell performance is further improved due to surface passivation and lowered surface reflectivity. A bottom layer film with a higher refractive index passivates the Si cell and reduces surface reflectivity, while the top layer film with a smaller refractive index further reduces the surface reflectivity. The refractive indices and thicknesses of the double-layer films were varied, and current-voltage (IV) and external quantum efficiency (EQE) measurements were taken. The double-layer films resulted in an absolute value increase in efficiency of up to 1.8%.</p>

The value of a high performance thin silicon solar cell is based on high open circuit voltage (Voc) which is highly dependent upon surface and interface recombination. A microelectronic approach with the series and parallel fabrication of different device structures is presented. This approach includes the fabrication of planar solar cells based on different solar cell designs that maximize Voc. Subsequently, using the same solar cell design, more advanced epitaxial growth test structures were fabricated on the same substrate to understand recombination losses and Voc. In high performance thin silicon solar cells, Voc is highly sensitive to surface recombination. Achieving a good surface and interface passivation in epitaxial silicon solar cells is a challenge, especially for the absorber back surface. In this work, we used test structures embedded in the solar cells to independently optimize Voc and the back surface of a thin epitaxial absorber as well as other silicon interfaces. Most epitaxial thin silicon solar cells are fabricated with in-situ processes that lead to difficulties in the independent optimization of such surfaces. To contribute to the design space in thin silicon solar cells, we tested basic device designs and test structures that lead to high Voc. The design of basic structures is important to measure parameters that limit voltage. This is a diagnostic tool for the fabrication of high performance thin silicon solar cells. This work reports device results that exhibit a path to high open circuit voltage, determined by device design, interface recombination, and bulk lifetime. With independent optimization of parameters that limit Voc and therefore performance, this work contributes to the understanding of thin silicon solar cells.

In this work, we describe the fabrication and operating characteristics of GaAs/AlGaAs thin film solar cells processed by the epitaxial liftoff (ELO) technique. This technique allows the transfer of these cells onto glass substrates. The performance of the lifted-off solar cell is demonstrated by means of electrical measurements under both dark and illuminated conditions. We have also optimized the light trapping conditions in this direct-gap material. The results show that good solar absorption is possible in active layers as thin as 0.32 μm. In such a thin solar cell, the open circuit voltage would be enhanced. We believe that the combination of an epitaxial liftoff thin GaAs film, and nano-texturing can lead to record breaking performance.

We demonstrated the fabrication of modulated surface nanostructures as a new surface texture design for thin wafer solar cells. Using a combination of conventional alkali etching and colloidal lithography, we fabricated surface textures with micrometer and nanometre scales on a Si substrate. These modulated surface nanostructures exhibit reduced surface reflection in a broad spectral range, compared with conventional micrometer textures. We investigated optical absorption using a rigorous coupled wave analysis simulation, which revealed a significant reduction in surface reflection over a broad spectral range and efficient light trapping (comparable to that of conventional micrometer-scale textures) for the modulated nanostructures. We found that the modulated surface nanostructures have a high potential of improving the performance of thin wafer crystalline Si solar cells.

Novel separation process for free-standing silicon thin-films

In this work, we investigated a passivation of both surface and bulk of polycrystalline silicon films by water vapour. To assess the passivation effect we used Suns‐VOCmethod to measure the open‐circuit voltageVOCof polycrystalline silicon thin film solar cells. The treatment conditions, i.e. temperature, steam pressure, duration, were systematically varied to optimise the passivation process. According to our results, there is a parameter interaction because the same passivation effect can be achieved at different treatment conditions. A sufficiently high temperature (350–450 °C) is necessary for a successful silicon passivation. The impact of this parameter cannot be replaced either by an elevated steam pressure or a prolonged exposure time. Nevertheless, the passivation effect of steam can be strengthened by an elevated steam pressure. Different gases were tested beside water vapour, e.g. H2, H2 + H2O, O2 + H2O, air, but none of them resulted in higherVOCthan pure steam (360 mV from starting 220 mV). Results from Fourier transform infrared spectroscopy indicate that water vapour passivation is rather oxidation while hydrogen plays a significant supporting role in the process. We conclude that the water vapour passivation is able to passivate defects in the whole silicon volume. However, its passivation effect is not strong enough to become an adequate alternative to the plasma hydrogenation with the best result ofVOC∼497 mV. On the other hand, it provides advantage of simplicity (no vacuum system and deionised water steam as the only input).Recombination activity of defects in polycrystalline Si can be suppressed by their saturation for instance in hydrogen plasma (Si–H) or in water vapour (Si–O–Si).

Roll-over behavior in current-voltage curve introduced by an energy barrier at the front contact in thin film CdTe solar cell

An 18 μm thin crystalline silicon solar cell was demonstrated, and its best open circuit voltage is 642.3 mV. However, this value is far from the cell’s theoretical upper limit in an ideal case. This paper explores the open circuit voltage losses of the thin silicon solar cell, starting from the ideal case, through first principle calculation and experiments. The open circuit voltage losses come from the introduced recombination due to the non-ideal surface passivation and contacts integration on front and rear surfaces, and edge isolation. This paper presents a roadmap of the open circuit voltage reduction from an ideal case of 767.0 mV to the best measured value of 642.3 mV.

We theoretically explain and experimentally demonstrate light trapping in thin-film solar cells through guided-mode resonance (GMR) effects. Resonant field enhancement and propagation path elongation lead to enhanced solar absorption. We fabricate nanopatterned solar cells containing embedded 300-nm period, one-dimensional gratings. The grating pattern is fabricated on a glass substrate using laser interference lithography followed by a transparent conducting oxide coating as a top contact. A ∼320-nm thick p-i-n hydrogenated amorphous silicon solar cell is deposited over the patterned substrate followed by bottom contact deposition. We measure optical and electrical properties of the resonant solar cells. Compared to a planar reference solar cell, around 35% integrated absorption enhancement is observed over the 450 to 750-nm wavelength range. This light-management method results in enhanced short-circuit current density of 14.8 mA/cm 2 , which is a ∼40% improvement over planar solar cells. Our experimental demonstration proves the potential of simple and well-designed GMR features in thin-film solar cells.

In this paper, we present a novel silicon (Si) subwavelength-scale surface structure (SWSS) fabrication process using the silica sphere (SS) lithography technique, which allows controllable geometries. The process involves a new cost-effective solvent-controlled spin-coating method that deposits SS on large Si surface areas with enhanced SS monolayer coverage and uniformity compared to conventional methods. The larger areas and rapid, low cost processing allow colloidal sphere lithography to be realistically used for solar cells. We successfully demonstrate 1.57 μm diameter SS on a 2-inch round Si wafer with more than 95% SS monolayer coverage and great uniformity. Using these deposited SS, a SWSS fabrication process was designed and successfully demonstrated Si inverted pyramid structures with dimension on the order of 1.1 μm, thus potentially providing a new technique for effective light-management of thin crystalline Si solar cells.

Mechanical fragility and insufficient light absorption are two major challenges for thin flexible crystalline Si-based solar cells. Flexible hybrid single-walled carbon nanotube (SWNT)/Si solar cells are demonstrated by applying scalable room-temperature processes for the fabrication of solar-cell components (e.g., preparation of SWNT thin films and SWNT/Si p-n junctions). The flexible SWNT/Si solar cells present an intrinsic efficiency ≈7.5% without any additional light-trapping structures. By using these solar cells as model systems, the charge transport mechanisms at the SWNT/Si interface are investigated using femtosecond transient absorption. Although primary photon absorption occurs in Si, transient absorption measurements show that SWNTs also generate and inject excited charge carriers to Si. Such effects can be tuned by controlling the thickness of the SWNTs. Findings from this study could open a new pathway for designing and improving the efficiency of photocarrier generation and absorption for high-performance ultrathin hybrid SWNT/Si solar cells.