Design principles for light trapping in thin silicon films with embedded dielectric nanoparticles

We investigate the light‐trapping effects of dielectric nanoparticles embedded within the active semiconductor layer of a thin‐film solar cell. The baseline model consists of a 1·0 µm slab of crystalline silicon on an aluminum back contact topped with a 75 nm Si₃N₄ anti‐reflective coating. Using finite‐difference time‐domain simulations, we calculate the absorption gain due to a periodic array of SiO₂ nanospheres with characteristic depth, diameter, and pitch. Under optimal conditions, absorption gain due to embedded spheres can reach as high as 23·4% relative to the baseline geometry. Using Au‐core/SiO₂‐shell nanoparticles, it is even possible to reach 30%. We then infer a series of design principles from our data that include trade‐offs between broadband scattering efficiency, poor absorption at long wavelengths, and semiconductor displacement. We also find that the optimal spacing between particles is approximately 400 nm. Above this distance, each scatterer acts in near isolation from any neighboring particles, and absorption gain is approximately linear with area coverage. Such gains are also expected for disordered as well as ordered arrays. These results demonstrate the potential of embedded dielectric nanoparticles as a tool for enhancing carrier generation in thin silicon solar cells. 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.     

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.     

Utilization of geometric light trapping in thin film silicon solar cells: simulations and experiments

In this study, we present a new light absorption enhancement method for p‐i‐n thin film silicon solar cells using pyramidal surface structures, larger than the wavelength of visible light. Calculations show a maximum possible current enhancement of 45% compared with cells on a flat substrate. We deposited amorphous silicon (a‐Si) thin film solar cells directly onto periodically pyramidal‐structured polycarbonate (PC) substrates, which show a significant increase (30%) in short‐circuit current over reference cells deposited on flat glass substrates. The current of the cells on our pyramidal structures on PC is only slightly lower than that of cells on Asahi U‐type TCO glass (Asahi Glass Co., Tokyo, Japan), but suffer from a somewhat lower open circuit voltage and fill factor. Because the used substrates have a locally flat surface area due to the fabrication process, we believe that the current enhancement in the cells on structured PC can be increased using larger or more closely spaced pyramids, which can have a smaller flat surface area. Copyright © 2012 John Wiley & Sons, Ltd.     

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.     

Angular behavior of the absorption limit in thin film silicon solar cells

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n² limit can be potentially exceeded in a wide angular and wavelength range using two‐dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure, but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure that is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances that can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one‐dimensional gratings, and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to obtain more practical values for the absorption enhancement, we consider the effect of parasitic loss that can significantly reduce the enhancement factor. Copyright © 2013 John Wiley & Sons, Ltd.     

Optical properties of thin‐film silicon solar cells with grating couplers

The effect of grating couplers on the optical properties of silicon thin‐film solar cells was studied by a comparison of experimental results with numerical simulations. The thin‐film solar cells studied are based on microcrystalline silicon (μc‐Si:H) absorber layers of thickness in the micrometer range. To investigate the light propagation in these cells, especially in the red wavelength region, three‐dimensional power loss profiles are simulated. The influence of different grating parametres—such as period size, groove height, and shape of the grating—was studied to gain more insight into the light propagation within thin‐film silicon solar cells and to determine an optimized light trapping scheme. The effect of the TCO front and TCO back side layer thickness was investigated. The calculated quantum efficiencies and short‐circuit current densities are in good agreement with the experimental data. The simulations predict further optimization criteria. Copyright © 2006 John Wiley & Sons, Ltd.     

柔性衬底硅薄膜太阳电池中ZGO薄膜的应用

在室温下,采用孪生对靶直流磁控溅射工艺,在玻璃衬底上制备出高质量的Ga掺杂ZnO(ZnO:Ga)透明导电膜。研究了薄膜厚度对薄膜的结构、光学及电学特性的影响。制备的ZnO:Ga是具有六角纤锌矿结构的多晶薄膜,最佳择优取向为(002)方向。随着薄膜厚度的增加,衍射峰明显增强,晶粒增大。优化反应条件,薄膜的电阻率达到4.69×10-4Ω.cm,在可见光范围内平均透过率达到了85%以上。将不同厚度的ZnO:Ga薄膜(350～820 nm)在柔性聚酰亚胺衬底nip非晶硅(a-Si)薄膜太阳电池中,随厚度的增加,电池的填充因子和效率都得到了提高,得到聚酰亚胺衬底效率7.09%的a-Si薄膜太阳电池。     

Experimental and simulation study of thin film silicon solar cells with intermediate reflector

Optical and electrical simulations were carried out for thin film silicon solar tandem cells with intermediate reflector layer (IRL) between top and bottom cell and compared with experimental external quantum efficiency and current voltage characteristics results. Reference data were collected from a series of tandem cells with different thicknesses of the top cell absorber layer (160–240 nm), the bottom cell absorber layer (1750–2100 nm), and the transparent conductive oxides based IRL (10–80 nm). It turned out that for capturing correctly the influence of the IRL on the light management as a function of the IRL thickness, the conventional semicoherent approach is not sufficient. Whereas the optical properties of a very thin IRL are governed by interference effects that are best calculated using a fully coherent model, increasingly thicker IRL show a more and more incoherent behavior. By taking into account, the interface morphology and angular light distribution within the cell stack an algorithm for the effective IRL reflectivity was proposed that explains the experimental findings very well. The consecutive electrical simulations were carried out with the device simulator ASA. The dependence of short circuit current density j_sc and fill factor FF on the thickness d_IRL of the IRL is in qualitative agreement between simulation and experiment showing coincident extrema in j_sc(d_IRL) and FF(d_IRL) at the current matching point. Copyright © 2013 John Wiley & Sons, Ltd.     

Improvement of hydrogenated amorphous silicon germanium thin film solar cells by different p-type contact layer

In this study, we report an appreciably increased efficiency from 6% up to 9.1% of hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells by using a combination of different p-doped window layers, such as boron doped hydrogenated amorphous silicon (p-a-Si:H), amorphous silicon oxide (p-a-SiO_x:H), microcrystalline silicon (p-µc-Si:H), and microcrystalline silicon oxide (p-µc-SiO_x:H). Optoelectronic properties and the role of these p-layers in the enhancement of a-SiGe:H cell efficiency were also examined and discussed. An improvement of 1.62 mA/cm² in the short-circuit current density (J_sc) is attributed to the higher band gap of p-type silicon oxide layers. In addition, an increase in open-circuit voltage (V_oc) by 150 mV and fill factor (FF) by 6.93% is ascribed to significantly improved front TCO/p-layer interface contact.     

Analysis of short circuit current gains by an anti‐reflective textured cover on silicon thin film solar cells

The influence of a retro‐reflective texture cover on light in‐coupling and light‐trapping in thin film silicon solar cells is investigated. The texture cover is applied to the front glass of the cell and leads to a reflectance as low as r ≈ 3% by reducing the reflection at the air/glass interface and indirectly also reducing the reflections from the internal interfaces. For weakly absorbed light in the long wavelength range, the texture also enhances the light‐trapping in the solar cell. We demonstrate an increase of the short circuit current density of exemplary investigated thin film silicon tandem solar cells by up to 0.95 mA cm⁻² and of the conversion efficiency by up to 0.74% (absolute). For a planar microcrystalline solar cell, the enhancement of light‐trapping was determined from the reduced reflection in the long wavelength range to be up to 17%, leading to an increase of the external quantum efficiency of up to 12%. Copyright © 2012 John Wiley & Sons, Ltd.     

a-Si/μc-Si叠层电池中间层材料SiO_x:H制备研究

为提高a-Si/μc-Si叠层太阳电池的效率,采用射频等离子体增强化学气相沉积(RF-PECVD)技术,制备了系列n型掺磷硅氧(SiOx:H)薄膜作为中间层,研究了CO2/Si H4气体流量比、沉积功率和PH3掺杂浓度等工艺参数对材料光电特性的影响,获得了折射率、电导率和禁带宽度能够在较大范围内调控的SiOx:H薄膜。     

Modeling of current-voltage characteristics of thin film solar cells

An analytical model is developed to study the current-voltage characteristics of thin film solar cells by incorporating exponential photon absorption, carrier trapping and carrier drift in the absorber layer. An analytical expression for the external voltage dependent photocurrent is derived by solving the continuity equation for both electrons and holes assuming the electric field remains uniform in the absorber layer. The analytical results are verified with the numerical self-consistent solution of the steady-state continuity equations and the Poisson’s equation. The overall load current is calculated considering the actual solar spectrum. It is found that the solar cell efficiency critically depends on the transport properties of the carriers that drift towards the bottom contact. The recombination current dominates over the ideal diode current in CdTe based solar cells. The theoretical model is fitted with the published experimental data on various thin film solar cells and shows a very good agreement.     

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.     

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.     

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.     

Theory and experiments on the back side reflectance of silicon wafer solar cells

New passivation layers for the back side of silicon solar cells have to show high performance in terms of electrical passivation as well as high internal reflectivity. This optical performance is often shown as values for the back side reflectance R_b which describes the rear internal reflection. In this paper, we investigate in detail the meaning of this single‐value parameter, its correct determination and the use in one‐dimensional simulations with PC1D. The free‐carrier‐absorption (FCA) as non‐carrier‐generating absorption channel is analyzed for solar cells with varying thickness. We apply the optical analysis to samples with different thickness, silicon oxide layer thickness, rear side topography as well as passivation layers (SiO₂, SiN_x, SiC and stack systems). Additionally, the optical influence of the laser‐fired contacts (LFC) process is experimentally investigated. Finally, we show that with correct parameters, the one‐dimensional simulation of very thin silicon solar cells can successfully be performed. Copyright © 2007 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.     

n-Doped-layer free amorphous silicon thin film solar cells fabricated with the CuMg alloy as back contact metal

The feasibility of using CuMg alloy as back contact metal for n⁺-doped-layer free a-Si:H thin film solar cell (TFSC) has been investigated in this work. The ohmic-contact characteristic has been achieved by using the CuMg alloy as back contact metal. The proposed structure showed the typical solar cell current-voltage (I-V) characteristic. An initial efficiency of 4.3% has been obtained with a open-circuit voltage V_oc = 0.79 V, short-circuit current J_sc = 13.4 mA/cm² and fill factor F.F. = 0.40. Furthermore, the experimental results also showed the CuMg alloy was suitable for the replacement of n⁺-doped-layer with the production cost reduction of a-Si:H TSFC.     

非晶硅薄膜太阳能电池应用开发新进展

非晶硅（ａ－Ｓｉ）薄膜太阳能电池是取之不尽的洁净能源－太阳能的光电元（组）件。文章详述了ａ－Ｓｉ薄膜太阳能电池的工艺优势，市场开发状况，可能应用领域，存在问题和展望。