Thin Crystalline Silicon Solar Cells Research Articles

박형 결정질 실리콘 태양전지에서의 휨현상 감소를 위한 알루미늄층 두께 조절

Crystalline silicon solar cell remains the major player in the photovoltaic marketplace with 80% of the market, despite the development of various thin film technologies. Silicon`s excellent efficiency, stability, material abundance and low toxicity have helped to maintain its position of dominance. However, the cost of silicon materials remains a major barrier to reducing the cost of silicon photovoltaics. Using the crystalline silicon wafer with thinner thickness is the promising way for cost and material reduction in the solar cell production. However, the thinner the silicon wafer is, the worse bow phenomenon is induced. The bow phenomenon is observed when two or more layers of materials with different temperature expansion coefficiencies are in contact, in this case silicon and aluminum. In this paper, the solar cells were fabricated with different thicknesses of Al layer in order to reduce the bow phenomenon. With less amount of paste applications, we observed that the bow could be reduced by up to 40% of the largest value with 120 micron thickness of the wafer even though the conversion efficiency decrease by 0.5% occurred. Since the bowed wafers lead to unacceptable yield losses during the module construction, the reduction of bow is indispensable on thin crystalline silicon solar cell. In this work, we have studied on the counterbalance between the bow and conversion efficiency and also suggest the formation of enough back surface field (BSF) with thinner Al layer application.

RIE surface texturing for optimum light trapping in multicrystalline silicon solar cells

Optical losses by reflection and transmission of the incident light should be reduced to improve the efficiency of solar cells. Compared with antireflection coatings, surface texturing is a more persistent and effective solution aiming at reducing light reflection losses. Alkali (NaOH, KOH) or acidic (HF, HNO3, CH3COOH) chemicals are used in conventional solar cell production lines for wet chemical texturing. However, Surface texturing is too difficult to apply to solar cell fabrication with thinner wafers due to the large amount of silicon loss caused by saw damage removal (SDR) and the texturing process for multicrystalline silicon (mc-Si). In order to solve the problems, reactive ion etching (RIE) has been applied for surface texturing of solar cell wafers. The RIE method can be effective in the reducing surface reflection with low silicon loss. In this study, we, therefore, fabricated a large-area (243.3 cm2) mc-Si solar cell by maskless surface texturing using a SF6/O2 RIE process. Also, we achieved a conversion efficiency (Eff), open circuit voltage (Voc), short circuit current density (Jsc) and fill factor (FF) as high as 17.2%, 616 mV, 35.1 mA/cm2, and 79.6%, respectively, which are suitable for fabricating thin crystalline silicon solar cells at low cost and with high efficiency.

Front side plasmonic effect on thin silicon epitaxial solar cells

We study the effect of metal nanoparticles, showing localised plasmonic resonances, on the spectrally resolved efficiency of thin film crystalline silicon solar cells. We investigate model structures: silver (Ag) nanodiscs on the surface of epitaxial cells grown on highly doped silicon substrates, with a controlled micron-scale thickness. The cells have no back reflector in order to exclusively study the effect of the front surface on their optical properties. The nanodiscs were deposited through hole-mask colloidal lithography, which is a low-cost, bottom-up and extremely versatile technique. As opposed to many other works, we use as benchmarks both bare silicon cells and cells with a dielectric antireflection coating. We optically observe a resonance showing an absorption increase, found to be controllable by the discs parameters. We also see an increase in short-circuit current with respect to bare cells, but we see a decrease in efficiency with respect to cells with a dielectric antireflection coating, due to losses at short wavelengths. As the material properties are not notably affected by the particles deposition, we show that the main loss mechanisms are an important parasitic absorption in the nanoparticles and destructive interferences.

Optimal wavelength scale diffraction gratings for light trapping in solar cells

Dielectric gratings are a promising method of achieving light trapping for thin crystalline silicon solar cells. In this paper, we systematically examine the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO2) gratings using numerical simulations. The square pyramid structure with silicon nitride coating provides the best light trapping among all the symmetric structures investigated, with 89% of the expected short circuit current density of the Lambertian case. For structures where the grating is at the rear of the cell, we show that the light trapping provided by the square pyramid and the checkerboard structure is almost identical. Introducing asymmetry into the grating structures can further improve their light trapping properties. An optimized Si skewed pyramid grating on the front surface of the solar cell results in a maximum short circuit current density, Jsc, of 33.4 mA cm−2, which is 91% of the Jsc expected from an ideal Lambertian scatterer. An optimized Si skewed pyramid grating on the rear performs as well as a rear Lambertian scatterer and an optimized TiO2 grating on the rear results in 84% of the Jsc expected from an optimized Si grating. The results show that submicron symmetric and skewed pyramids of Si or TiO2 are a highly effective way of achieving light trapping in thin film solar cells. TiO2 structures would have the additional advantage of not increasing recombination within the cell.

Photon frequency management for trapping & concentration of sunlight

This paper considers a range of techniques which – within the realm of classical optics – can be used to enhance light capture as a first step in photovoltaic energy conversion. Examples include a simple case of downshifting, fluorescent collectors which reduce the size of a light beam, and a novel form of light trapping to increase the path length of light within the solar cell. The results are discussed using a thermodynamic framework where the energy exchange with an absorbing/fluorescent medium allows the entropy of the captured photon gas to be lowered, reducing the étendue of the emitted beam. We show that frequency management represents a powerful tool, allowing enhancement in light trapping above the Yablononovitch limit, leading to potentially highly efficient but very thin crystalline silicon solar cells.

Effect of an in-Situ H2 Plasma Pretreatment on the Minority Carrier lifetime of a-Si:H(i) Passivated Crystalline Silicon

In the future, thin (<100um) crystalline silicon (c-Si) solar cells based on amorphous heterostructures (a-Si:H/c-Si) are likely to become an essential branch of wafer-based photovoltaic industry. Within this thin technology, the surface passivation will have a larger influence on effective minority charge carrier lifetime, therefore an accurate analysis of the a-Si:H/c-Si interface is paramount to achieve high efficiencies. In this paper we studied an in-situ hydrogen (H2) plasma pretreatment before a-Si:H deposition, obtaining a surface recombination velocity (Srec) of 3.8±1.0cm/s at 1x1015 minority charge carriers/cm3 on moderately-doped n-type <100> silicon wafers and, therefore, we show a significant drop compared to the reference value (6.9±1.4cm/s). The trend is related to a change in surface hydrogen configuration toward lower hydrides (from SiH3 to SiH2 and SiH) and, thus, it indicates an atomic reconstruction more suited for the growth of a-Si:H(i) passivating layer. However, if the treatment is extended above an optimum time, the Srec increases abruptly above 10cm/s which suggests the introduction of defects. These effects are explained by hydrogen-induced reactions on the c-Si surface during the H2 pretreatment. The exposure to H2 plasma improves surface passivation without relevant modifications in the a-Si:H deposition process itself: hence, it represents an interesting option to be implemented in future heterojunction solar cells.

Surface Texturing for Crystalline Silicon Solar Cell Using RIE Equipped with Metal-Mesh

Surface texturing is an important process to enhance light absorption and to improve efficiency of a solar cell. Reactive ion etching (RIE) process is a very effective process and low-cost process, which is applicable during the dry etching processes for thin crystalline silicon solar cells with large areas. In this study, we studied a dry and free mask texturing process on crystalline silicon wafer using SF6/O2plasmas and metal mesh in a RIE system, with special attention to the effect of the metal mesh and RIE conditions on the texture of the silicon surface. In particular, we have found an optimized RIE conditions by increasing the distance between the metal mesh and silicon wafer. We have also found that by increasing the RIE process time, with an optimized SF6/O2ratio, pressure and RF power, it is possible to switch from a random texture, to a nm-size pyramid texture and finally to an um-size pyramid texture. This RIE system textured a crystalline wafer surface that formed about 1~2 μm pyramidal black silicon with 7~10% of reflectivity.

Thin crystalline silicon solar cells based on epitaxial films grown at 165 °C by RF-PECVD

We report on heterojunction solar cells whose thin intrinsic crystalline absorber layer has been obtained by plasma enhanced chemical vapor deposition at 165 °C on highly doped p-type (1 0 0) crystalline silicon substrates. We have studied the effect of the epitaxial intrinsic layer thickness in the range from 1 to 2.5 μm. This absorber is responsible for photo-generated current whereas highly doped wafer behave like electric contact, as confirmed by external quantum efficiency measurements and simulations. A best conversion efficiency of 7% is obtained for a 2.4 μm thick cell with an area of 4 cm 2, without any light trapping features. Moreover, the achievement of a fill factor as high as 78.6% is a proof that excellent quality of the epitaxial layers can be produced at such low temperatures.

The effect of rear surface polishing to the performance of thin crystalline silicon solar cells

As the thickness of crystalline silicon solar cells decreases, light loss cannot be avoided due to the absorption limit in long wavelength light. Internal rear side reflection can be enhanced by polishing the rear surface. The rear polishing processes are performed before the texturing and before and after doping the emitter layer to optimize the solar cell fabrication process sequences. All cells made by rear surface polishing showed improved light trapping in long wavelength region (900–1100 nm) compared to that in the conventional cells. However, silicon solar cells fabricated by rear polishing before and after doping have similar (35.5 mA/cm 2) or lower (35.26 mA/cm 2) short circuit current density compared to the cells produced by the conventional process (35.59 mA/cm 2) due to pore damage to the anti-reflection layer and the surface of the emitter layer during rear polishing. This surface damage was effectively prevented adapting the rear surface polishing before the front surface texturing, which led to increasing the current density from 35.59 to 36.29 mA/cm 2.

Thin film silicon solar cells by AIC on foreign substrates

This paper presents the fabrication of thin film crystalline silicon solar cells on foreign substrates like alumina, glass–ceramic (GC) and metallic foils (ferritic steel—FS) using seed layer approach, which employs aluminium induced crystallisation (AIC) of amorphous silicon. Effect of hydrogen content in a-Si:H precursor films on the AIC process has been studied and the results showed that defects in the AIC grown films increased with increase of hydrogen content. At the optimal thermal annealing conditions, the AIC grown poly-Si films showed an average grain size of 7.6, 26, and 8.1 μm for the films synthesised on alumina, GC, and FS, respectively. The grains were (1 0 0) oriented with a sharp Raman peak around 520 cm −1. Similarly, n-type seed layers were also fabricated by over-doping of as-grown AIC layers using a highly phosphorus doped glass solution. The resistivity of as-grown films reduced from 8.4×10 −2 Ω cm (p-type) to 4.1×10 −4 Ω cm (n-type) after phosphorus diffusion. These seed layers of n-type/p-type were thickened to form an absorber layer by vapour phase epitaxy or solid phase epitaxy. The passivation step was applied before the heterojunction formation, while it was after in the case of homojunction. Open circuit voltage of the junctions showed a strong dependence on the hydrogenation temperature and microwave (μW) power of electron cyclotron resonance (ECR) plasma of hydrogen. Effective passivation was achieved at a μW power of 650 W and hydrogenation temperature of 400 °C. Higher values of solar conversion efficiencies of 5% and 2.9% were achieved for the n-type and p-type heterojunction cells, respectively fabricated on alumina substrates. The analysis of the results and limiting factors are discussed in detail.

Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells

We examine light trapping in thin silicon nanostructures for solar cell applications. Using group theory, we design surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays closely follows the Lambertian limit for isotropic incident radiation. These effects correspond to a reduction in silicon mass by 2 orders of magnitude, pointing to the promising future of thin crystalline silicon solar cells.

Thin silicon films synthesis by AIC on ONO coated metal foils

Abstract Thin film crystalline silicon solar cells on flexible metallic substrates are being considered to be potential for low cost production. In the present investigation, metallic substrates (MS) coated with dielectric diffusion barrier layer (BL) of ONO (SiO 2 /SiN/SiO 2 ) have been used for the fabrication of polycrystalline silicon ( pc -Si) thin films using aluminium induced crystallisation (AIC) for the first time. The crystallographic quality of pc -Si layers synthesised by AIC has been studied using Raman and UV reflection spectroscopy. Similarly, grain size and defect distribution were characterised using electron backscattered diffraction (EBSD) analysis. The AIC grown films showed a symmetric peak around 519–520 cm −1 with an average grain size of 8.1 μm. The thermal stability of the diffusion barrier has been evaluated by annealing the structure, MS/BL/ pc -Si, at higher temperatures with subsequent chemical analysis of AIC grown pc -Si template. The potential impurities are identified to be Fe and Ni on a limited area. These results open the potential of fabrication of large grain silicon seed layer on flexible substrates for thin film solar cells technology.

A new back surface passivation stack for thin crystalline silicon solar cells with screen-printed back contacts

In order to manufacture high-efficiency Si solar cells with a passivated rear surface and local contacts, it is necessary to develop both an excellent rear-passivation scheme compatible with screen-printing technology and a robust patterning technique for local contact formation. In this work, we have fabricated Si solar cells on ∼130 μm thick substrates using manufacturable processing, where rear side was passivated with a plasma-enhanced chemical vapor deposited SiO x /SiN x /SiO x N y stack and local back contacts using laser. As a result of both the rear surface passivation stack and the laser-fired local contacts, cell efficiencies of up to 17.6% on a 148.6 cm 2 Float-zone Si wafer and 17.2% for a 156.8 cm 2 multicrystalline Si wafer were achieved. PC-1D calculations revealed that the cells had a back surface recombination velocity (BSRV) of ∼400 cm/s and a back surface reflectance (BSR) of over 90%, as opposed to standard full Al-BSF cells having a BSRV of ∼800 cm/s and a 70% BSR. This result clearly indicates that the new technique of the passivation scheme and the patterning using laser developed in this study are promising for manufacturing high-efficiency PERC-type thin Si solar cells.

Analysis of thin-film silicon solar cells with plasma textured front surface and multi-layer porous silicon back reflector

The optical performance of thin crystalline silicon solar cells with plasma textured front surface and with porous silicon stack back reflector is analysed. A rigorous analytical ray tracing model, that uses an accurately estimated porous silicon refractive index, is elaborated to predict the optical absorption, carrier generation, external quantum efficiency, and total photogenerated current in the cell. Calculated results best fitted to those measured for an experimental demonstration cell show that surface recombination is a major loss factor in the demonstration cell. The study also concludes that the plasma textured front surface has poor light diffusion properties, and predicts that the maximum gain in the photocurrent from back reflection at the porous silicon stack equals 10% compared to 20% if the back reflector is ideal. However, the proposed cell is realistic and promises excellent performance when state of the art front surface texturing, passivation and front metallization techniques are implemented.

High-efficiency photonic crystal solar cell architecture

Thin silicon solar cells suffer from low light absorption compared to their thick counterparts, especially in the near infra-red regime. In order to obtain high energy conversion efficiency in thin solar cells, an efficient light trapping scheme is required. In this paper, we theoretically demonstrate significant enhancement in efficiency of thin crystalline silicon solar cells by using photonic crystals as the light absorbing layer. In particular, a relative increase of 11.15% and 3.87% in the energy conversion efficiency compared to the optimized conventional design is achieved for 2 microm and 10 microm thicknesses, respectively.

Flexible silicon solar cells

In order to be useful for certain niche applications, crystalline silicon solar cells must be able to sustain either one-time flexure or multiple non-critical flexures without significant loss of strength or efficiency. This paper describes experimental characterisation of the behaviour of thin crystalline silicon solar cells, under either static or repeated flexure, by flexing samples and recording any resulting changes in performance. Thin SLIVER cells were used for the experiment. Mechanical strength was found to be unaffected after 100,000 flexures. Solar conversion efficiency remained at greater than 95% of the initial value after 100,000 flexures. Prolonged one-time flexure close to, but not below, the fracture radius resulted in no significant change of properties. For every sample, fracture occurred either on the first flexure to a given radius of curvature, or not at all when using that radius. In summary, for a given radius of curvature, either the flexed solar cells broke immediately, or they were essentially unaffected by prolonged or multiple flexing.

Epitaxially grown emitters for thin film crystalline silicon solar cells

In recent years, research on epitaxial growth for thin film crystalline silicon solar cells has gained in importance due to the alarming feedstock shortage of electronic grade polysilicon. This work concentrates on the epitaxial growth of the emitter, as a replacement for emitter diffusion, on top of an epitaxially grown base. The main advantage of emitter formation by CVD is more controllable doping profile, which gives the opportunity to form an excellent front surface field, leading to high open-circuit voltages. In this paper, the doping profile and thickness of the epitaxial layers are optimized. A solar cell process including a light trapping mechanism resulted in solar cell efficiencies close to 15%.

A simple analytical model of thin films crystalline silicon solar cell with quasi-monocrystalline porous silicon at the backside

An analytical model that simulates the performance of an elementary thin silicon solar cell with a thin film quasi-monocrystalline porous silicon (QMPS) at the backside reflector is developed. A complete set of equations for the photocurrent generated under the effect of the reflected light is solved analytically in each region. The collection of the light absorbed by the QMPS layer has been discussed and the analytical solution of the light-generated current in this layer is derived. The maximum of the photocurrent density calculated in the present study is in accordance with the numerical values established by Bergmann et al. Furthermore, the influence that the layer's number of double porosities and high porosity have on the photovoltaic parameters is studied. It is demonstrated that the photovoltaic parameters increase with the number of double porosities that the layer might have in a given structure. When the QMPS layer is formed by three double-porosity layers 20%/80% and for a 5-μm-thick film c-Si, the backside reflector gives a total improvement of about 6mA/cm2 for the photocurrent density and 3.2% for the cell efficiency.

RIE texturing optimization for thin c-Si solar cells in SF6/O2 plasma

Dry etching plasma parameters were optimized for texturing single crystalline thin silicon solar cells and hence for high efficiency. In reactive ion etching (RIE) texturing, a low etch depth (∼2 µm) was obtained compared with the etch depth that occurred in the wet chemical texturing process. For the flow ratios (SF6/O2) of 2 and 3, needle-like and cylindrical type structured surfaces were obtained. In the RIE process, the effects of working pressure, flow ratio, and etching time on reflectance and electrical properties of single crystalline silicon solar cells were investigated. The textured c-Si wafer with needle-like structure has good antireflectance behaviour. The interface defect density (Dit) in these textured silicon wafers increased with etching time. But an improvement in the reduction of interface trap density was observed through the annealing effect. Single crystalline solar cells with cylindrical type texture have higher values for open circuit voltage, short circuit current and efficiency in spite of a higher reflectance as compared with those needle-like structures. It is observed that the optimized SF6/O2 based chemistry in the RIE method is more suitable for thin crystalline silicon solar cells instead of the conventional wet texturization processes.

Simulation and implementation of a porous silicon reflector for epitaxial silicon solar cells

AbstractOne of the main challenges in the ongoing development of thin film crystalline silicon solar cells on a supporting silicon substrate is the implementation of a long‐wavelength reflector at the interface between the epitaxial layer and the substrate. IMEC has developed such a reflector based on electrochemical anodization of silicon to create a multi‐layer porous silicon stack with alternating high and low porosity layers. This innovation results in a 1–2% absolute increase in efficiency for screenprinted epitaxial cells with a record of 13·8%. To reach a better understanding of the reflector and to aid in its continued optimization, several extensive optical simulations have been performed using an in‐house‐developed optical software programme. This software is written as a Microsoft Excel workbook to make use of its user‐friendliness and modular structure. It can handle up to 15 individual dielectric layers and is used to determine the influence of the number and the sequence of the layers on the internal reflection. A sensitivity analysis is also presented. A study of the angle at which the light strikes the reflector shows separate regions in the physical working of the reflector which include a region where the Bragg effect is dominant as well as a region where total internal reflection plays the largest role. The existence of these regions is proved using reflection measurements. Based on these findings, an estimate is made for the achievable current gain with an ideal reflector and the potential of epitaxial silicon solar cells is determined. Copyright © 2008 John Wiley &amp; Sons, Ltd.