Si Thin Film Solar Cells Research Articles

Influence of Substrate on Crystal Orientation of Large-Grained Si Thin Films Formed by Metal-Induced Crystallization

Producing large-grained polycrystalline Si (poly-Si) film on glass substrates coated with conducting layers is essential for fabricating Si thin-film solar cells with high efficiency and low cost. We investigated how the choice of conducting underlayer affected the poly-Si layer formed on it by low-temperature (500°C) Al-induced crystallization (AIC). The crystal orientation of the resulting poly-Si layer strongly depended on the underlayer material: (100) was preferred for Al-doped-ZnO (AZO) and indium-tin-oxide (ITO); (111) was preferred for TiN. This result suggests Si heterogeneously nucleated on the underlayer. The average grain size of the poly-Si layer reached nearly 20 µm for the AZO and ITO samples and no less than 60 µm for the TiN sample. Thus, properly electing the underlayer material is essential in AIC and allows large-grained Si films to be formed at low temperatures with a set crystal orientation. These highly oriented Si layers with large grains appear promising for use as seed layers for Si light-absorption layers as well as for advanced functional materials.

Stacked nanoparticle-transparent conductive oxide substrate combining high haze with low surface roughness for improvement of thin film Si solar-cell performance

Surface-textured morphology of transparent conductive oxide (TCO) substrates enhances light absorption in Si thin films by scattering and trapping light, which results in an increase in the current density of Si thin film solar cells. However, the highly textured morphology often induces the formation of structural defects within the Si layer, which decreases the photovoltaic performance parameters of solar cells such as the open-circuit voltage (VOC) and fill factor (FF). To overcome this trade-off, we propose a flattened light-scattering TCO substrate based on a strongly light-scattering metal-oxide nanoparticle (NP) layer in conjunction with a surface TCO layer of low surface roughness. Individual layers of ZnO, TiO2, and stacked TiO2/ZnO metal-oxide nanoparticles (NP-TiO2/NP-ZnO), which functioned as a light-scattering layer, were formed between a glass substrate and a low-resistivity Al-doped zinc oxide (AZO) layer to serve as substrates for Si thin film solar cells. After deposition of the AZO film, the nanoparticle AZO substrate demonstrated good flatness and high haze values, which were nearly equivalent to the haze values of the nanoparticle layers before coating by AZO. For thin film hydrogenated amorphous Si (a-Si:H) solar cells fabricated on the stacked AZO/NP-TiO2/NP-ZnO substrate, the short-circuit current increased with no corresponding decrease in the VOC or FF. We attribute this result to an improved spectral response in a wavelength range λ>500nm and a decrease in the structural defects within the Si layers.

Light trapping in hybrid nanopyramid and nanohole structure silicon solar cell beyond the Lambertian limit

We propose a hybrid nanostructure that comprises nanopyramids and nanoholes for thin film silicon (Si) solar cells. The hybrid structure demonstrates a stronger light trapping ability that is beyond the Lambertian limit. This is achieved with the smaller dimension nanohole structure which effectively reduces shorter wavelength light reflection, and the larger dimension nanopyramid structure which significantly enhances longer wavelength light trapping. An ultimate efficiency of 38.3% is yielded for a 2 μm thick Si cell incorporated with the hybrid structure, which is higher than that achievable corresponding to the Lambertian limit. Moreover, the high ultimate efficiency is retained as the incident angle increases from normal incidence to 50° for TM polarized sunlight. Therefore, the proposed hybrid structure is very promising to enhance the performance of thin film Si solar cells.

Light-trapping performance of silicon thin-film plasmonics solar cells based on indium nanoparticles and various TiO2 space layer thicknesses

Thin-film solar cells have the potential to substantially reduce the material cost of photovoltaic devices. However, to increase the amount of light absorbed in the thin active layer, light trapping is a critical concern in developing thin-film solar cells. In this study, we investigated the suitability of using localized surface plasmons of indium nanoparticles (In NPs) on TiO2 space layers of various thicknesses to enhance the absorption of silicon (Si) thin-film solar cells. The experimental results demonstrated how the combined effects of the incident light plasmonics scattering, surface passivation, and antireflection of In NPs affect the photovoltaic performance of the TiO2 space layer. The optical reflectance, dark current, photocurrent, and external quantum efficiency were measured and compared. Compared with bare-type Si thin-film solar cells, the proposed cells with In NPs on a 59.5-nm-thick TiO2 space layer demonstrated a short-circuit current enhancement of 45.7% (from 2.56 to 3.73 mA) and a conversion efficiency enhancement of 36.2% (from 7.56 to 10.3%).

Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells

We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se2 as well as from μc-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se2, from tail-to-tail transitions in μc-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band–tail density of states in Cu(In,Ga)Se2 is compatible with the reciprocity relation at room temperature while the shallower band–tail density of states in μc-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity.

Investigation on growth behavior of multiphase silicon carbon film for front contact layer in a Si thin film solar cell

Boron doped hydrogenated multiphase silicon–carbon (multiphase silicon–carbon) film has been grown by a plasma enhanced chemical vapour deposition (PECVD) method to obtain the properties of high conductivity and a low absorption coefficient. It consists of amorphous carbon, amorphous silicon and a crystalline silicon-like clustering phase. It has the advantage of reducing optical loss due to the wider band gap of amorphous carbon compared to amorphous silicon carbide. The film was fabricated in conditions of low power density with a high hydrogen flow rate to increase the ratio of the amorphous carbon. This result is able to be achieved because the reaction of Si-based and C-based radicals is suppressed by the deposition condition of low electron temperature (Te) of the plasma and the short residence time of the gases. The multiphase silicon–carbon showed high electrical conductivity and a low optical absorption coefficient in the short wavelength region. Applying it for use as a front contact layer in a Si thin film solar cell, it showed an improvement in the conversion efficiency due to the increase in the quantum efficiency in the short wavelength region.

Bilayer contacts composed of amorphous and solid-phase crystallized transparent conducting oxides for solar cells

We fabricated three types of transparent conductive In–Zn–O:H/In–O:H multilayer film with amorphous/amorphous (a-/a-), amorphous/polycrystalline (a-/poly-), and polycrystalline/polycrystalline (poly-/poly-) interfaces, and investigated the effects of the interface structures on their transport properties. The amorphous multilayer films were fabricated by a sputtering method, and the other films were fabricated by post-annealing of the amorphous multilayer films, utilizing the difference in crystallization temperature for the constituent layers. The characterization of the transport properties of these films revealed that the a-/poly- interface as well as the a-/a- interface does not scatter free electrons significantly, and bilayer transparent conductive oxide films consisting of a-/poly- and a-/a- structures show superior transport properties compared with those consisting of the poly-/poly- structure. We applied the poly-In–O:H/a-In–Zn–O:H bilayer film as a front electrode of a hydrogenated microcrystalline Si thin-film solar cell and confirmed its usefulness in the solar cell.

High conductive and transparent Al doped ZnO films for a-SiGe:H thin film solar cells

Al doped zinc oxide (AZO) films were prepared by mid-frequency magnetron sputtering for silicon (Si) thin film solar cells. Then, the influence of deposition parameters on the electrical and optical properties of the films was studied. Results showed that high conductive and high transparent AZO thin films were achieved with a minimum resistivity of 2.45 × 10–4 Ω·cm and optical transmission greater than 85% in visible spectrum region as the films were deposited at a substrate temperature of 225°C and a low sputtering power of 160 W. The optimized films were applied as back reflectors in a-SiGe:H solar cells. A relative increase of 19% in the solar cell efficiency was achieved in comparison to the cell without the ZnO films doped with Al (ZnO:Al).

Nanoimprinted backside reflectors for a-Si:H thin-film solar cells: critical role of absorber front textures.

The development of optimal backside reflectors (BSRs) is crucial for future low cost and high efficiency silicon (Si) thin-film solar cells. In this work, nanostructured polymer substrates with aluminum coatings intended as BSRs were produced by positive and negative nanoimprint lithography (NIL) techniques, and hydrogenated amorphous silicon (a-Si:H) was deposited hereon as absorbing layers. The relationship between optical properties and geometry of front textures was studied by combining experimental reflectance spectra and theoretical simulations. It was found that a significant height variation on front textures plays a critical role for light-trapping enhancement in solar cell applications. As a part of sample preparation, a transfer NIL process was developed to overcome the problem of low heat deflection temperature of polymer substrates during solar cell fabrication.

Enhanced absorption in elliptical silicon nanowire arrays for solar energy harvesting

A crystalline Si thin film solar cell with elliptical silicon nanowire (SiNW)-decorated surface is presented. The elliptical SiNW solar cell performs broadband light absorption in the low energy region while maintaining strong absorption in the high energy region. For the elliptical SiNW solar cells with 1-μm underlying Si substrate, J sc can be improved to 24.2 mA/cm 2 , which is 16% higher than that of the circular SiNW-decorated solar cells. After integration with an Ag reflector, the elliptical SiNW solar cells can achieve higher J sc of 26.3 mA/cm 2 , demonstrating a photocurrent enhancement of 12.6% as compared with circular SiNW control device.

Investigation of Laser-fired Rear-side Point Contacts of Laser-crystallized Silicon Thin-film Solar Cells by Conductive Probe Atomic Force Microscopy

The influence of a subsequent ns laser-firing process on aluminium rear-side point contacts of laser-crystallized silicon thin-film solar cells was investigated by means of conductive probe atomic force microscopy. A significant increase in conductivity was observed in the laser-fired contact area. The spatial uniformity of this enhanced conductivity as well as changes in the aspect ratio of the induced pit allowed us to derive a suitable parameter window for firing a 100nm thin aluminium layer typically used in such thin film Si solar cells devices.

Structural and Optical Properties of Smooth Surface TCO Thin Films Deposited on Different-Sized Staked Nanoparticle Layers for Window Electrode of Thin Film Si Solar Cells

To improve the light-scattering capacity of transparent conductive oxide (TCO) films without increasing in surface roughness, we formed a stacked nanoparticle layers of ZnO (NP-ZnO) and TiO2 (NP-TiO2), which serve as light-scattering and surface-modifying layers, respectively, between the glass substrate and low-resistive TCO layers. The stacked TCO/NP/glass (NP-TCO) substrate indicated strong light-scattering capacity, with a relatively low surface root mean roughness of approximately 10nm. In addition, the haze value of NP-TCO substrates increased with an increase in the component particle diameter of the underlying NP-ZnO layers from about 20% (33nm) to 40% (96nm) at a wavelength of 550nm. [doi:10.2320/matertrans.M2014208]

Amorphous and micromorph Si solar cells: current status and outlook

An overview of the current status and prospects of thin-film Si photovoltaics, including both hydrogenated amorphous and microcrystalline Si as well their combination known as micromorph solar cells, with a major focus on the technological development is given. Although thin-film Si solar cells have been one of the first commercially successful photovoltaic devices, today they face a tremendous challenge from variety of bulk Si technologies (monoand multicrystalline Si) and compound-semiconductor thin-film solar cells, both of which have managed to substantially reduce the production cost and improve power conversion efficiency. Thin-film Si photovoltaics benefiting from the mighty mainstream Si industry have demonstrated the ability to reduce the production cost and cost per peak power; however, the performance improvement of amorphous-Si panels has been only marginal and further progress is hard to expect. The power conversion efficiencies of micromorph solar cells have already exceeded those of amorphous-Si devices. However, to improve their market penetration and even to hold their current share, the micromorph Si solar modules need to improve their performance, which today is approaching 11%, along the path to potential 15%, while keeping the manufacturing cost low. Further progress will require the improvement of light-trapping technologies and contact performance.

Broadband and omnidirectional antireflection of Si nanocone structures cladded by SiN film for Si thin film solar cells

Si nanocone structures cladded by SiN material are fabricated by nanosphere lithography, inductively coupled plasma (ICP) etching and plasma enhanced chemical vapor deposition (PECVD). Their broadband omnidiectional antireflection performance is investigated through measurement and simulation by rigorous coupled-wave analysis (RCWA). Both the measurement and simulation results show that the reflectivity of SiN passivated Si nanocone is lower than that of original Si nanocone structure over broadband and wide view. The average reflectivity data further indicate that such reflectivity seems to be linearly reduced with SiN thickness when it is less than 90nm. After that, the reflectivity of such composite nanocone structure is less dependent on SiN thickness. The minimum average reflectivity of 1.84% is achieved in Si nanocone structure cladded by 90nm thick SiN film, which is only 10.3% of the average reflectivity of the same Si nanocone structure. In addition, the comparison further shows that such SiN cladding layer can attenuate the bad influence of random nanosphere mask induced by self-assembly processing on their reflection. Therefore, great antireflection and good passivation performance in such SiN/Si composite nanocone structures are expected, which benefit them to be actually employed in Si thin film solar cells and improve their conversion efficiency finally.

Thin film solar cells on honeycomb-structured substrates for photovoltaic building blocks

Honeycomb-structured solar cell is proposed for photovoltaic building block applications. Honeycomb-like substrates were prepared either by a conventional semiconductor processing or by a low cost wet-chemical method, and amorphous Si thin film solar cells were fabricated on these substrates. We have demonstrated one of the essential requirements for building block application, which is the low sensitivity of the light incidence angles on the power conversion efficiency; and we have identified the critical processing issues through the experimental study using various thin film deposition methods. This honeycomb-structured solar cell is a promising candidate for the future photovoltaic building block applications enabling the inherent high strength-to-weight ratio and higher efficiency at an oblique light incidence.

Efficient light incoupling into silicon thin-film solar cells by anti-reflecting MgO/high-compact-AZO with air-side textured glass

Light incoupling effects have been enhanced at front interfaces of silicon (Si) thin-film solar cells. Firstly, a MgO thin film was introduced at glass substrate/Al-doped ZnO (AZO) interface for anti-reflection effect. We additionally found that the surface morphology of AZO films grown on MgO film after texture-etching is dependent on the compactness of AZO. For high-compact AZO films, the texture-etched MgO/AZO double layer exhibited significantly enhanced light-scattering capability. Secondly, we made textured surfaces at air/glass interface through simple plasma-etching without sacrificial layers or masks by optimizing the etching pressure. The additional air-side texture contributed to further improvement of total light scattering from the MgO/AZO-coated glass substrate. Fabricated microcrystalline Si thin-film solar cells employing the MgO coated glass with air-side surface texture showed decreased cell reflectance and increased quantum efficiency. The Jsc increased from 21.7 to 26.5 mA cm−2 and final efficiency of 9.49% was achieved. Based on our experimental results, the suggested structure, the MgO coating on glass substrate of which air-side surface is texture-etched, can offer a promising approach to improve the light incoupling and efficiency of Si thin-film solar cells.

Broadband Light Harvesting Enhancement in Thin Film Solar Cells with Surface and Back Reflector Grating Nanostructures

Nanostructures with top subwavelength hybrid dielectric gratings and metallic triangle back reflector in Si thin film solar cells (TFSCs) are introduced in this paper. Compared with the studied light-trapping structures with normal surface and bottom gratings, our optimized structure has a better performance. We can achieve above 26 percent average cell efficiency enhancement in contrast to the normal flat cells and about 40 percent increase with the transverse magnetic (TM) polarized irradiation only.

Demonstration of enhanced absorption in thin film Si solar cells with periodic microhemisphere hole arrays

This paper reports a study about how to enhance the performance of thin film Si solar cells. Periodic microhemisphere hole (PMHH) arrays with a period of 10μm and a depth of ~4μm were made on the glass wafers. We found that the PMHH arrays can trap light via testing the transmittance of glass with PMHH arrays. Comparing with the original cell, the thin film Si solar cell covered by this kind of PMHH arrays glass is different from the following two perspectives: the short-circuit current (Jsc) increased by 7.5% and the conversion efficiency (Eff) enhanced by 5.2%. The design based on light trapping scheme on the external of cell is feasible and worth promoting.

Characterization of μc-Si:H/a-Si:H tandem solar cell structures by spectroscopic ellipsometry

In order to perform the structural characterization of Si thin-film solar cells having submicron-size rough textured surfaces, we have developed an optical model that can be utilized for the spectroscopic ellipsometry (SE) analysis of a multilayer solar cell structure consisting of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) layers fabricated on textured SnO2:F substrates. To represent the structural non-uniformity in the textured structure, the optical response has been calculated from two regions with different thicknesses of the Si layers. Moreover, in the optical model, the interface layers are modeled by multilayer structures assuming two-phase composites and the volume fractions of the phases in the layers are controlled by the structural curvature factor. The polarized reflection from the μc-Si:H layer that shows extensive surface roughening during the growth has also been modeled. In this study, a state-of-the-art solar cell structure with the textured μc-Si:H (2000nm)/ZnO (100nm)/a-Si:H (200nm)/SnO2:F/glass substrate structure has been characterized. The μc-Si:H/a-Si:H textured structure deduced from our SE analysis shows remarkable agreement with that observed by transmission electron microscopy. From the above results, we have demonstrated the high-precision characterization of highly-textured μc-Si:H/a-Si:H solar cell structures.

Plasmonic fractals: ultrabroadband light trapping in thin film solar cells by a Sierpinski nanocarpet

Plasmonic Sierpinski nanocarpet as back structure for a thin film Si solar cell is investigated. We demonstrate that ultra-broadband light trapping can be obtained by placing square metallic nanoridges with Sierpinski pattern on the back contact of the thin film solar cell. The multiple-scale plasmonic fractal structure allows excitation of localized surface plasmons and surface plasmon polaritons in multiple wavelengths leading to obvious absorption enhancements in a wide frequency range. Full wave simulations show that 109 % increase of the short-circuit current density for a 200 nm thick solar cell, is achievable by the proposed fractal back structure. The amount of light absorbed in the active region of this cell is more than that of a flat cell with semiconductor thickness of 1,000 nm.