Silicon thin‐film solar cells with rectangular‐shaped grating couplers

As an alternative to randomly textured transparent conductive oxides as front contact for thin‐film silicon solar cells the application of transparent grating couplers was studied. The grating couplers were prepared by sputtering of aluminium‐doped zinc oxide (ZnO) on glass substrate, a photolithography and a lift‐off process and were used as periodically textured substrates. The period size and groove depth of these transparent gratings were tuned independently from each other and varied between 1 and 4 μm and 100–600 nm. The optical properties of rectangular‐shaped gratings and the opto‐electronic behaviour of amorphous and microcrystalline silicon solar cells with integrated grating couplers as a function of the grating parameters (period size P and groove depth h_g) are presented. The optical properties of the gratings are discussed with respect to randomly textured substrates and the achieved solar cell results are compared with the opto‐electronic properties of solar cells deposited on untextured (flat) and randomly textured substrates. Copyright © 2005 John Wiley & Sons, Ltd.     

Broadband light trapping with disordered photonic structures in thin‐film silicon solar cells

We theoretically investigate light trapping with disordered 1D photonic structures in thin‐film crystalline silicon solar cells. The disorder is modelled in a finite‐size supercell, which allows the use of rigorous coupled‐wave analysis to calculate the optical properties of the devices and the short‐circuit current density J_sc. The role of the Fourier transform of the photonic pattern in the light trapping is investigated, and the optimal correlation between size and position disorder is found. This result is used to optimize the disorder in a more effective way, using a single parameter. We find that a Gaussian disorder always enhances the device performance with respect to the best ordered configuration. To properly quantify this improvement, we calculate the Lambertian limit to the absorption enhancement for 1D photonic structures in crystalline silicon, following the previous work for the 2D case [M.A. Green, Progr. Photovolt: Res. Appl. 2002; 10 (4), pp. 235–241]. We find that disorder optimization can give a relevant contribution to approach this limit. Finally, we propose an optimal disordered 2D configuration and estimate the maximum short‐circuit current that can be achieved, potentially leading to efficiencies that are comparable with the values of other thin‐film solar cell technologies. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterisation of rough reflecting substrates incorporated into thin‐film silicon solar cells

Four different categories of rough reflecting substrates as well as a single periodic grating are incorporated and tested within n‐i‐p type amorphous silicon (a‐Si:H) solar cells. Each category is characterised by its own texture shape; dimensions were varied within the categories. Compared to flat reflecting substrates, gains in short‐circuit current density (J_sc) up to 20% have been obtained on rough reflecting plastic substrates. As long as (1) the characteristic dimensions of the textures are lower than the involved light wavelengths, (2) the textures do not present any defects i.e. as long as they do not have large craters or bumps spread over the surface, the root mean square roughness (δ_RMS) as well as the ratio of average feature height to average period can be used to evaluate the gain in J_sc; if each category of randomly textured substrates is considered separately, the haze factor can be used to estimate δ_RMS and thereby the gains in J_sc. Copyright © 2006 John Wiley & Sons, Ltd.     

Highly transparent modulated surface textured front electrodes for high‐efficiency multijunction thin‐film silicon solar cells

To further increase the efficiency of multijunction thin‐film silicon (TF‐Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high‐quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light‐trapping structures in multijunction TF‐Si solar cells. The MST substrates comprise a micro‐textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub‐micron nano‐textured ZnO layer grown by low‐pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in‐coupling and light trapping for the top amorphous silicon (a‐Si:H) solar cell while minimizing the parasitic absorption losses. The crater‐shaped micro‐textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high‐quality nanocrystalline silicon (nc‐Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high‐quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single‐textured state‐of‐the‐art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a‐Si:H/nc‐Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state‐of‐the‐art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high‐efficiency multijunction solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Nanoparticle‐enhanced light trapping in thin‐film silicon solar cells

A systematic investigation of the nanoparticle‐enhanced light trapping in thin‐film silicon solar cells is reported. The nanoparticles are fabricated by annealing a thin Ag film on the cell surface. An optimisation roadmap for the plasmon‐enhanced light‐trapping scheme for self‐assembled Ag metal nanoparticles is presented, including a comparison of rear‐located and front‐located nanoparticles, an optimisation of the precursor Ag film thickness, an investigation on different conditions of the nanoparticle dielectric environment and a combination of nanoparticles with other supplementary back‐surface reflectors. Significant photocurrent enhancements have been achieved because of high scattering and coupling efficiency of the Ag nanoparticles into the silicon device. For the optimum light‐trapping scheme, a short‐circuit current enhancement of 27% due to Ag nanoparticles is achieved, increasing to 44% for a “nanoparticle/magnesium fluoride/diffuse paint” back‐surface reflector structure. This is 6% higher compared with our previously reported plasmonic short‐circuit current enhancement of 38%. Copyright © 2011 John Wiley & Sons, Ltd.     

TCOs for nip thin film silicon solar cells

Substrate configuration allows for the deposition of thin film silicon (Si) solar cells on non‐transparent substrates such as plastic sheets or metallic foils. In this work, we develop processes compatible with low T_g plastics. The amorphous Si (a‐Si:H) and microcrystalline Si (µc‐Si:H) films are deposited by plasma enhanced chemical vapour deposition, at very high excitation frequencies (VHF‐PECVD). We investigate the optical behaviour of single and triple junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of low pressure CVD (LP‐CVD) ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 µm thick LP‐CVD ZnO. We observe that, for a‐Si:H, the cell performance depends critically on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a‐Si:H, a thick LP‐CVD ZnO front contact provides more light trapping on the 2D periodic substrate. Then, we investigate the influence of the thick and thin TCOs in conjunction with thick absorbers (µc‐Si:H). Because of the different nature of the optical systems (thick against thin absorber layer), the antireflection effect of ITO becomes more effective and the structure with the flat TCO provides as much light trapping as the rough LP‐CVD ZnO. Finally, the conformality of the layers is investigated and guidelines are given to understand the effectiveness of the light trapping in devices deposited on periodic gratings. Copyright © 2008 John Wiley & Sons, Ltd.     

A thin‐film silicon solar cell and module

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.     

On the interplay of cell thickness and optimum period of silicon thin‐film solar cells: light trapping and plasmonic losses

Light trapping and photon management in honeycomb‐textured microcrystalline silicon solar cells are investigated experimentally and by modeling of the manufacturing process and the optical wave propagation. The solar cells on honeycomb‐textured substrates exhibit short circuit current densities exceeding 30 mA/cm² and energy conversion efficiencies of up to 11.0%. By controlling the fabrication process, the period and height of the honeycomb‐textured substrates are varied. The influence of the honeycomb substrate morphology on the interfaces of the individual solar cell layers and the quantum efficiency is determined. The optical wave propagation is calculated using 3D finite difference time domain simulations. A very good agreement between the optical simulation and experimental results is obtained. Strategies are discussed on how to increase the short circuit current density beyond 30 mA/cm². In particular, the influence of plasmonic losses of the textured silver (Ag) reflector on the short circuit current and quantum efficiency of the solar cell is discussed. Finally, solar cell structures with reduced plasmonic losses are proposed. Copyright © 2015 John Wiley & Sons, Ltd.     

Photocurrent enhancement in thin‐film silicon solar cells by combination of anti‐reflective sub‐wavelength structures and light‐trapping textures

Dielectric films with anti‐reflective sub‐wavelength structures are applied to thin‐film silicon solar cells to improve the light incoupling at the front surface. It is verified that modification of the refractive index of the incident medium using dielectric films with sub‐wavelength structures is beneficial to reduce the average reflectivity of Si solar cells with an anti‐reflective coating based on optical interference. It is also shown that the sub‐wavelength structure must be combined with a proper light‐trapping texture to enhance the absorption within thin‐film silicon solar cells. The effectiveness of dielectric films with sub‐wavelength structures is demonstrated by an increase of the short‐circuit current density of a microcrystalline silicon cell from 29.1 to 30.4 mA/cm² in a designated area of 1 cm². The optical interplay between the dielectric films and the light‐trapping textures is also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

UV nanoimprint for the replication of etched ZnO:Al textures applied in thin‐film silicon solar cells

In this work, we present a technology for a high precision nanostructure replication process based on ultraviolet nanoimprint lithography for the application in the field of thin‐film photovoltaics. The potential of the technology is demonstrated by the fabrication of microcrystalline silicon thin‐film prototype solar cells. The high accuracy replication of random microstructures made from sputtered and etched ZnO:Al, used to scatter the incident light in thin solar cells, is shown by local topography investigations of the same 7.5 × 7.5 µm² area on the master and the replica. Different types of imprint resists and imprint moulds were investigated to find the optimal, high precision replication technology. Two types of thin‐film silicon solar cells, in p‐i‐n and n‐i‐p configuration, were fabricated to study the potential of the imprint technology for different applications. It is shown that solar cells deposited on an imprinted glass hold similar performances compared with reference solar cells fabricated with a standard process on textured ZnO:Al. Thus, it is demonstrated that the replication of light scattering structures by using an imprint process is an attractive method to decouple the scattering properties from the layer forming the electrical front contact. Because a simple and cheap high throughput process is used, this study additionally proves the relevance for the industrial mass production in the field of photovoltaics. Copyright © 2013 John Wiley & Sons, Ltd.     

Potential of light trapping in microcrystalline silicon solar cells with textured substrates

The real and potential benefits of light trapping in microcrystalline silicon single‐junction solar cells are studied theoretically. Effects of a hypothetical high haze parameter of textured transparent conductive oxide on quantum efficiency and short‐circuit current of the solar cell are analysed by numerical simulation. The role of the angular distribution function of scattered light as a second important scattering parameter is shown. The potential thickness reductions of the intrinsic layer due to enhanced light trapping are demonstrated. Copyright © 2003 John Wiley & Sons, Ltd.     

3‐D optical modeling of thin‐film silicon solar cells on diffraction gratings

An effective rigorous 3‐D optical modeling of thin‐film silicon solar cells based on finite element method (FEM) is presented. The simulation of a flat single junction thin‐film silicon solar cell on thick glass (i.e., superstrate configuration) is used to validate a commercial FEM‐based package, the High Frequency Structure Simulator (HFSS). The results are compared with those of the reference software, Advanced Semiconductor Analysis (ASA) program, proving that the HFSS is capable of correctly handling glass as an incident material within very timely, short, and numerically stable calculations. By using the HFSS, we simulated single junction thin‐film silicon solar cells on glass substrates textured with one‐dimensional (1‐D) and two‐dimensional (2‐D) trapezoid‐shaped diffraction gratings. The correctness of the computed results, with respect to an actual device, is discussed, and the impact of different polarizations on spectral response and optical losses is examined. From the simulations carried out, optimal combinations for period and height in both 1‐D and 2‐D grating configurations can be indicated, leading to short‐circuit current percentage increase with respect to a flat cell of, respectively, 25.46% and 32.53%. With very limited computer memory usage and computational time in the order of tens of minutes for a single simulation, we promote the usage of 3‐D FEM as a rigorous and efficient way to simulate thin‐film silicon solar cells. Copyright © 2012 John Wiley & Sons, Ltd.     

5% Efficient evaporated solid‐phase crystallised polycrystalline silicon thin‐film solar cells

The first energy conversion efficiencies of over 5% are reported for evaporated solid‐phase crystallised (SPC) polycrystalline silicon thin‐film solar cells. All cells have a size of 2 cm² and are formed on planar glass superstrates. Back surface reflectance is provided by a simple coating with commercial white paint. The best cells have short‐circuit current densities of about 19 mA/cm² and external quantum efficiencies peaking at above 80%. The diffusion length in the base of the solar cells is larger than the base thickness, providing significant room for further efficiency improvements via an increased thickness of the base layer. Additional improvements are expected via the use of textured glass sheets, boosting the light trapping capabilities of the cells. Copyright © 2009 John Wiley & Sons, Ltd.     

Photocurrent enhancement in thin film amorphous silicon solar cells with silver nanoparticles

Silver nanoparticles embedded in a dielectric material have strong scattering properties under light illumination, due to localized surface plasmons. This effect is a potential way to achieve light trapping in thin‐film solar cells. In this paper we study light scattering properties of nanoparticles on glass and ZnO, and on silver coated with ZnO, which represent the back reflector of a solar cell. We find that large nanoparticles embedded in the dielectric at the back contact of amorphous silicon solar cells lead to a remarkable increase in short circuit current of 20% compared to co‐deposited cells without nanoparticles. This increase is strongly correlated with the enhanced cell absorption in the long wavelengths and is attributed to localized surface plasmons. We also discuss the electrical properties of the cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites for highly efficient nanocrystalline silicon thin‐film solar cells

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites (p‐nc‐SiC:H) were prepared using a plasma‐enhanced chemical vapor deposition system with a mixture of CH₄, SiH₄, B₂H₆ and H₂ gases. The influence of hydrogen dilution on the material properties of the p‐nc‐SiC:H films was investigated, and their roles as window layers in hydrogenated nanocrystalline silicon (nc‐Si:H) solar cells were examined. By increasing the R_H (H₂/SiH₄) ratio from 90 to 220, the Si―C bond density in the p‐nc‐SiC:H films increased from 5.20 × 10¹⁹ to 7.07 × 10¹⁹/cm³, resulting in a significant increase of the bandgap from 2.09 to 2.23 eV in comparison with the bandgap of 1.95 eV for p‐nc‐Si:H films. For the films deposited at a high R_H ratio, the Si nanocrystallites with a size of 3–15 nm were formed in the amorphous SiC:H matrix. The Si nanocrystallites played an important role in the enhancement of vertical charge transport in the p‐nc‐SiC:H films, which was verified by conductive atomic force microscopy measurements. When the p‐nc‐SiC:H films deposited at R_H = 220 were applied in the nc‐Si:H solar cells, a high conversion efficiency of 8.26% (V_oc = 0.53 V, J_sc = 23.98 mA/cm² and FF = 0.65) was obtained compared to 6.36% (V_oc = 0.44 V, J_sc = 21.90 mA/cm² and FF = 0.66) of the solar cells with reference p‐nc‐Si:H films. Further enhancement in the cell performance was achieved using p‐nc‐SiC:H bilayers consisting of highly doped upper layers and low‐level doped bottom layers, which led to the increased conversion efficiency of 9.03%. Copyright © 2015 John Wiley & Sons, Ltd.     

High‐efficiency microcrystalline silicon single‐junction solar cells

This short communication highlights our latest results towards high‐efficiency microcrystalline silicon single‐junction solar cells. By combining adequate cell design with high‐quality material, a new world record efficiency was achieved for single‐junction microcrystalline silicon solar cell, with a conversion efficiency of 10.69%, independently confirmed at ISE CalLab PV Cells. Such significant conversion efficiency could be achieved with only 1.8 µm of Si. Copyright © 2013 John Wiley & Sons, Ltd.     

Silica nanospheres as back surface reflectors for crystalline silicon thin‐film solar cells

In this paper, fabrication of a non‐continuous silicon dioxide layer from a silica nanosphere solution followed by the deposition of an aluminium film is shown to be a low‐cost, low‐thermal‐budget method of forming a high‐quality back surface reflector (BSR) on crystalline silicon (c‐Si) thin‐film solar cells. The silica nanosphere layer has randomly spaced openings which can be used for metal‐silicon contact areas. Using glass/SiN/p⁺nn⁺ c‐Si thin‐film solar cells on glass as test vehicle, the internal quantum efficiency (IQE) at long wavelengths (>900 nm) is experimentally demonstrated to more than double by the implementation of this BSR, compared to the baseline case of a full‐area Al film as BSR. The improved optical performance of the silica nanosphere/aluminium BSR is due to reduced parasitic absorption in the Al film. Copyright © 2007 John Wiley & Sons, Ltd.     

Recent progress of high efficiency Si thin‐film solar cells in large area

Si thin‐film solar cells are suitable to the sunbelt region due to a low temperature coefficient and to building integrated photovoltaics owing to flexible size, easily controllable transmittance, and an aesthetic design. Nevertheless, the application is limited until now due to their low conversion efficiency. We have developed a triple junction cell (a‐Si:H/a‐SiGe:H/µc‐Si:H) providing efficient light utilization. For the high efficiency, we have focused on the smoothing of high haze TCO, a low absorption window layer, a low refractive index interlayer, uniformity control of the thickness and crystalline volume fraction in the microcrystalline silicon layer, and a low absorption back reflector. Through these activities, we have achieved a world record of 13.4% stabilized efficiency in the small size cell (1 cm²) and 10.5% stabilized efficiency in the large area module (1.1 × 1.3 m²), certificated by the National Renewable Energy Laboratory and Advanced Industrial Science and Technology, respectively. This result was presented in solar cell efficiency tables (Version 41). At this moment, we have increased a stabilized efficiency of 11.2% (Output power 160 W) in the large area module. We will report on the advanced materials in detail for high efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

Polycrystalline silicon thin‐film solar cells on glass by aluminium‐induced crystallisation and subsequent ion‐assisted deposition (ALICIA)

A research project is under way at The University of New South Wales aiming at the realisation of a novel type of polycrystalline silicon thin‐film solar cell on glass. The idea is to first create a thin large‐grained polycrystalline seed layer on glass by aluminium‐induced crystallisation of amorphous silicon and then to epitaxially thicken the seed layer with ion‐assisted deposition. By mid‐2003 this ALICIA project had achieved laboratory cells with voltages of up to 163 mV, as reported elsewhere. In the present paper we give an overview of recent progress (improved Si epitaxy process, improved control of base doping profile due to the use of phosphorus dopants instead of gallium, hydrogen passivation) that has improved the voltages of ALICIA solar cells to 270 mV. Furthermore, the strategy for further voltage improvements is presented. At the present point in time only the voltages of ALICIA cells are known, but obviously solar cells also require current for good efficiency. Hence much improvement in both voltage and current is still needed. Copyright © 2004 John Wiley & Sons, Ltd.