Large area ZnO:Al films with tailored light scattering properties for photovoltaic applications

Aluminum-doped zinc oxide films on glass are promising substrates for use in thin film solar cells based on amorphous and amorphous/microcrystalline silicon absorber material. The films can be produced by magnetron sputtering on large scale at relative low cost. Especially reactive sputtering of metallic Zn/Al compound targets is a cheap way to produce films at high deposition rate. One drawback of amorphous silicon is the low absorption in the near infrared spectral range. Wet chemical etching has been used to produce a rough TCO surface that enables light trapping in the absorber. The etching behaviour of ZnO:Al films can be tuned by changing oxygen partial pressure during deposition. The etching behaviour is compared to ZnO structure and discussed regarding the performance of solar cells deposited onto the etched films.     

Texturization of ZnO:Al surface by reactive ion etching in SF<Subscript>6</Subscript>/Ar,CHF<Subscript>3</Subscript>/Ar plasma for application in thin film silicon solar cells

As-deposited sputtered ZnO:Al (AZO) thin films having high transparency (T?≥?85% at 550 nm of wavelength) and good electrical properties (ρ?=?2.59?×?10^?04 Ω cm) are etched to get suitable light trapping in thin film solar cells, using reactive ion etching method in sulfur hexafluoride–argon (SF₆/Ar) plasma and trifluoromethane–argon (CHF₃/Ar) plasma to texture their surface. Though the electrical properties of the films are not affected much by the etching process but significant increment in the average haze values in the wave length range of 350–1100 nm in the etched AZO films (19.21% for SF₆/Ar and 22.07% for CHF₃/Ar plasma etched) are found compared to as-deposited AZO films (5.61%). Increment in haze value is due to more scattering of light from the textured surface. These textured substrates are used as front transparent conducting oxide electrode for the fabrication of amorphous silicon solar cells. Solar cells fabricated on etched AZO substrates show 7.76% increase in conversion efficiency compared to as-deposited AZO substrates.     

陷光结构在GaAs薄膜太阳电池中的应用

陷光结构由于其独特的光学特性,在光伏器件中发挥的作用越来越重要。目前硅基太阳电池中陷光结构的应用很常见,然而在GaAs薄膜太阳电池中陷光结构的报道并不多。详细介绍了陷光结构的原理及其在GaAs薄膜电池中的研究现状和应用情况。综述了GaAs薄膜太阳能电池中常用的三类陷光结构:正面陷光结构(包括纳米颗粒、纳米线、纳米锥等)、背面陷光结构(如镜面背反射层)以及混合陷光结构。大量研究表明,陷光结构的使用可以进一步提高GaAs薄膜电池的光电转换效率,一定程度上达到降低电池生产成本的目的。     

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.     

High-efficiency ordered silicon nano-conical-frustum array solar cells by self-powered parallel electron lithography

Nanostructured silicon thin film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top-down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of 99% (upper bound) over wavelengths 400-1100 nm by a thickness of only 5 μm, is realized by our recently reported technique self-powered parallel electron lithography that has high-throughput and sub-35-nm high resolution. Moreover, high-efficiency (up to 10.8%) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. These related fabrication techniques can also be transferred to low-cost substrate solar energy harvesting device applications.     

Roll to roll fabrication of thin film silicon solar cells on nano-textured substrates

ECN is developing a novel fabrication process for thin film silicon solar cells on steel foil. Key features in this process are: (1) application of an insulating barrier layer which enables monolithic interconnection and texturization of the rear contact with submicron structures for light trapping; (2) Si deposition with remote, linear PECVD; (3) series interconnection by laser scribing and printing after deposition of all layers, which reduces the total number of process steps. The barrier layer is essential for the monolithic series interconnection of cells, but we show that it also enables optimum light trapping in the solar cells. We can fabricate any arbitrary sub-micron surface profile by hot embossing the barrier layer. For deposition of doped and intrinsic silicon layers we use novel remote, linear plasma sources, which are excellently suited for continuous roll-to-roll processing. We have been able to fabricate device-quality amorphous and microcrystalline silicon layers with these sources. The first nip a-Si cells were made on steel substrates with flat barrier layer and had initial efficiencies of 6.3%, showing the potential of the concept.     

陷光结构应用于太阳能电池的研究进展

在太阳能电池中引入陷光结构是提高光电转换效率的一种重要方法。本文主要从晶体硅太阳能电池、薄膜太阳能电池和其他新型太阳能电池三方面,综述了近年来国内外陷光结构用于太阳能电池的最新研究进展,分析了陷光结构对各类太阳能电池性能的影响、陷光作用的原理及工艺手段,最后指出其发展潜力及未来的研究方向。     

TCAD simulations for thin film solar cells with nanoplate structures

The novel thin film solar cell with a nanoplate structure that can solve the conflict between the light absorption and the carrier transport in amorphous silicon thin film solar cell was investigated by TCAD simulations. This new structure has n-type amorphous silicon nanoplate array on the substrate, and p-type amorphous silicon-carbon as window layer and intrinsic amorphous silicon as absorption layer are sequentially grown along the surface of each n-type amorphous silicon nanoplate. Under AM 1.5 G sunlight illumination, the light is absorbed along the vertical direction of nanoplate while the carrier transport is along the horizontal direction. Therefore, nanoplate with the larger height can absorb most of the sunlight. The advantage of this novel structure is that the thickness of the solar cell can be used as thin as possible for effective transport of photo-generated carriers in comparison with the planer one.     

Influence of working pressure on ZnO:Al films from tube targets for silicon thin film solar cells

Mid-frequency magnetron sputtering of aluminum doped zinc oxide films (ZnO:Al) from tube ceramic targets has been investigated for silicon based thin film solar cell applications. The influence of working pressure on structural, electrical, and optical properties of sputtered ZnO:Al films was studied. ZnO:Al thin films with a minimum resistivity of 3.4 × 10⁻⁴ Ω cm, high mobility of 50 cm²/Vs, and high optical transmission close to 90% in visible spectrum region were achieved. The surface texture of ZnO:Al films after a chemical etching step was investigated. A gradual increase in feature sizes (diameter and depth) was observed with increasing sputter pressure. Silicon based thin film solar cells were prepared using the etched ZnO:Al films as front contacts. Energy conversion efficiencies of up to 10.2% were obtained for amorphous/microcrystalline silicon tandem solar cells.     

Radial junction amorphous silicon solar cells on PECVD-grown silicon nanowires

Constructing radial junction hydrogenated amorphous silicon (a-Si:H) solar cells on top of silicon nanowires (SiNWs) represents a promising approach towards high performance and cost-effective thin film photovoltaics. We here develop an all-in?situ strategy to grow SiNWs, via a vapour-liquid-solid (VLS) mechanism on top of ZnO-coated glass substrate, in a plasma-enhanced chemical vapour deposition (PECVD) reactor. Controlling the distribution of indium catalyst drops allows us to tailor the as-grown SiNW arrays into suitable size and density, which in turn results in both a sufficient light trapping effect and a suitable arrangement allowing for conformal coverage of SiNWs by subsequent a-Si:H layers. We then demonstrate the fabrication of radial junction solar cells and carry on a parametric study designed to shed light on the absorption and quantum efficiency response, as functions of the intrinsic a-Si:H layer thickness and the density of SiNWs. These results lay a solid foundation for future structural optimization and performance ramp-up of the radial junction thin film a-Si:H photovoltaics.     

N-type crystalline silicon films free of amorphous silicon deposited on glass by HCl addition using hot wire chemical vapour deposition

Since n-type crystalline silicon films have the electric property much better than those of hydrogenated amorphous and microcrystalline silicon films, they can enhance the performance of advanced electronic devices such as solar cells and thin film transistors (TFTs). Since the formation of amorphous silicon is unavoidable in the low temperature deposition of microcrystalline silicon on a glass substrate at temperatures less than 550 degrees C in the plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition (HWCVD), crystalline silicon films have not been deposited directly on a glass substrate but fabricated by the post treatment of amorphous silicon films. In this work, by adding the HCl gas, amorphous silicon-free n-type crystalline silicon films could be deposited directly on a glass substrate by HWCVD. The resistivity of the n-type crystalline silicon film for the flow rate ratio of [HCl]/[SiH4] = 7.5 and [PH3]/[SiH4] = 0.042 was 5.31 x 10(-4) ohms cm, which is comparable to the resistivity 1.23 x 10(-3) ohms cm of films prepared by thermal annealing of amorphous silicon films. The absence of amorphous silicon in the film could be confirmed by high resolution transmission electron microscopy.     

Film adhesion in amorphous silicon solar cells

A major issue encountered during fabrication of triple junction a-Si solar cells on polyimide substrates is the adhesion of the solar cell thin films to the substrates. Here, we present our study of film adhesion in amorphous silicon solar cells made on different polyimide substrates (Kapton VN, Upilex-S and Gouldflex), and the effect of tie coats on film adhesion.     

Preparation and properties of surface textured ZnO: Al films by direct current pulse magnetron sputtering

Transparent conductive surface textured Al-doped zinc oxide (ZnO:Al, AZO) thin films were prepared on glass substrates by direct current pulse magnetron sputtering at substrate temperature of 270 °C and post-etching in NaOH solution at room temperature. The effects of Ar flow rate on the structural, optical, electrical properties and light trapping ability were investigated systematically. With the increasing of Ar flow rate from 10 to 50 sccm, different surface features ranging from honeycomb-like to crater-like structures were observed. The relationship between surface textured structures and Ar flow rate was discussed. The AZO film deposited with Ar flow rate in 50 sccm displayed fine optoelectronic properties, improved figure of merit and effective surface textured structures for light trapping, which could be applied as a transparent conducting electrode in silicon-based thin film solar cells.     

Graphene/silicon nanowire Schottky junction for enhanced light harvesting

Schottky junction solar cells are assembled by directly coating graphene films on n-type silicon nanowire (SiNW) arrays. The graphene/SiNW junction shows enhanced light trapping and faster carrier transport compared to the graphene/planar Si structure. With chemical doping, the SiNW-based solar cells showed energy conversion efficiencies of up to 2.86% at AM1.5 condition, opening a possibility of using graphene/semiconductor nanostructures in photovoltaic application.     

Enhanced efficiency of graphene-silicon Schottky junction solar cell through inverted pyramid arrays texturation

Nanostructures of silicon are gradually becoming hot candidate due to outstanding capability for trapping light and improving conversion efficiency of solar cell. In this paper, silicon nanowires (SiNWs) and silicon inverted pyramid arrays (SiIPs) were introduced on surface of Gr-Si solar cell through silver and copper-catalyzed chemical etching, respectively. The effects of SiNWs and SiIPs on carrier lifetime, optical properties and efficiency of Gr-SiNWs and Gr-SiIPs solar cells were systematically analyzed. The results show that the inverted pyramid arrays have more excellent ability for balancing antireflectance loss and surface area enlargement. The power conversion efficiency (PCE) and carrier lifetime of Gr-SiIPs devices respectively increase by 62% and 34% by comparing with that of Gr-SiNWs solar cells. Finally, the Gr-SiIPs cell with PCE of 5.63% was successfully achieved through nitric acid doping. This work proposes a new strategy to introduce the inverted pyramid arrays for improving the performance of Gr-Si solar cells.     

Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications

This paper presents analysis of the optical absorption in silicon nanowire arrays that have potential applications in solar cells. The effects of wire diameter, length, and filling ratio on the absorptance of nanowire arrays are simulated. The study reveals that nanowire arrays with moderate filling ratio have much lower reflectance compared to thin films. In a high-frequency regime, nanowire arrays have higher absorptance than their thin film counterparts. In low-frequency regime, nanowire arrays absorb less but can be designed to approach that of the film by changing the filling ratio.     

Surface textured molybdenum zinc oxide for light diffusion enhancement

Surface textures have been fabricated on a molybdenum doped zinc oxide (MZO) film using a shadow mask in a co-sputter process. The surface textures yielded 5.3% and 10.1% of light diffusion in the visible light region for MZO films with a thickness of 100 nm and 200 nm, respectively. Light diffusion in the near infra-red region was slightly less with 4.5% for the 100 nm MZO film and 8.9% for the 200 nm MZO film. The enhanced light diffusion will be beneficial to the light trapping efficiency of a-Si/µ-Si based thin film solar cells.     

GZO厚度对非晶硅电池中复合背电极的影响

在非晶硅太阳能电池中加入复合背电极是提高非晶硅太阳能电池光电转换效率和稳定性的有效手段.本文利用磁控溅射技术在非晶硅薄膜太阳能电池上制备了ZnO :Ga(GZO)/Al复合背电极,研究了GZO厚度对GZO薄膜光电性质及非晶硅电池中GZO/Al复合背电极性能的影响.研究表明：随着GZO层厚度的增加,GZO薄膜的光电性质均表现出较高水平,适合制备GZO/Al复合背电极;相较于单层Al背电极的非晶硅太阳能电池,具有GZO/Al复合背电极的太阳能电池性能大幅提高.当GZO层厚度为100 nm时,太阳能电池的短路电流(I_SC)、开路电压(V_OC)和填充因子(FF)分别达到8.66 mA,1.62 V和54.7%.     

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10^- 6, correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.     

Dielectric core-shell optical antennas for strong solar absorption enhancement

We demonstrate a new light trapping technique that exploits dielectric core-shell optical antennas to strongly enhance solar absorption. This approach can allow the thickness of active materials in solar cells lowered by almost 1 order of magnitude without scarifying solar absorption capability. For example, it can enable a 70 nm thick hydrogenated amorphous silicon (a-Si:H) thin film to absorb 90% of incident solar radiation above the bandgap, which would otherwise require a thickness of 400 nm in typical antireflective coated thin films. This strong enhancement arises from a controlled optical antenna effect in patterned core-shell nanostructures that consist of absorbing semiconductors and nonabsorbing dielectric materials. This core-shell optical antenna benefits from a multiplication of enhancements contributed by leaky mode resonances (LMRs) in the semiconductor part and antireflection effects in the dielectric part. We investigate the fundamental mechanism for this enhancement multiplication and demonstrate that the size ratio of the semiconductor and the dielectric parts in the core-shell structure is key for optimizing the enhancement. By enabling strong solar absorption enhancement, this approach holds promise for cost reduction and efficiency improvement of solar conversion devices, including solar cells and solar-to-fuel systems. It can generally apply to a wide range of inorganic and organic active materials. This dielectric core-shell antenna can also find applications in other photonic devices such as photodetectors, sensors, and solid-state lighting diodes.