18.45%‐Efficient Multi‐Crystalline Silicon Solar Cells with Novel Nanoscale Pseudo‐Pyramid Texture

Silicon‐based cells could convert more solar energy to electrical energy if the cells could absorb more light. However, the nanostructured cells have demonstrated relatively low power conversion efficiency even when its reflection is very low; thus, they are still far from becoming real products of the photovoltaic industry. Here, nanoscale pseudo‐pyramid textured multi‐crystalline silicon (Pmc‐Si) solar cells, with the best efficiency of ≈18.45%, are fabricated by using a metal‐catalyzed chemical etching plus a post alkaline etching on an industrial production line. Such Pmc‐Si solar cells have showed similar light trapping ability as single crystalline silicon solar cells of micrometer pyramid texture, and the improved efficiency is mainly ascribed to its enhanced light absorption while the nanostructured surface still keeps acceptable passivation quality, that is, the short‐circuit current density has an increase of ≈300 mA cell^–1, while the open‐circuit voltage has only a slight decrease of ≈1 mV. Further elevations of the efficiency are expected by optimizing both micrometer‐ and nanotextures, and exploring more effective passivation technique. More excitingly, the technique presented here has been verified in the production line for several batches as a real technique of low cost and high efficiency.     

用于太阳能电池的多晶硅激光表面织构化研究

介绍了利用激光制备多晶硅表面织构的研究结果。采用激光在硅片表面刻蚀,然后利用化学方法去除残渣和损伤,制得均匀的表面陷光结构。通过扫描电子显微镜,HitachiU-4100分光光度计和Semilab WT2000少子寿命仪分析了表面织构化后硅片的表面形貌、反射率和少子寿命。通过调节激光和化学腐蚀参数得到很好的陷光效果,表面反射率最低可以降到约10%。但是激光刻蚀对硅片性能仍有一定损伤,有待改进。激光表面织构为多晶硅的减反射处理提供有效的途径。     

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 novel method for etching trenches in silicon carbide

In this paper, a novel trench etching technique for silicon carbide is described. In this technique, ion implantation is used to first create an amorphous silicon carbide region. The amorphous layer is then etched away by wet chemical etching. Trenches of 0.3 to 0.8 μ have been obtained using a single implantaion/etching step. It has been demonstrated that deeper trenches can be obtained by repeating the implantation/etching step with platinum as a masking material. The etched surface was found to be smooth when compared with reactive ion etched surfaces reported for silicon carbide.     

Low-Roughness Plasma Etching of HgCdTe Masked with Patterned Silicon Dioxide

A novel mask technique utilizing patterned silicon dioxide films has been exploited to perform mesa etching for device delineation and electrical isolation of HgCdTe third-generation infrared focal-plane arrays (IRFPAs). High-density silicon dioxide films were deposited at temperature of 80°C, and a procedure for patterning and etching of HgCdTe was developed by standard photolithography and wet chemical etching. Scanning electron microscopy (SEM) showed that the surfaces of inductively coupled plasma (ICP) etched samples were quite clean and smooth. Root-mean-square (RMS) roughness characterized by atomic force microscopy (AFM) was less than 1.5 nm. The etching selectivity between a silicon dioxide film and HgCdTe in the samples masked with patterned silicon dioxide films was greater than 30:1. These results show that the new masking technique is readily available and promising for HgCdTe mesa etching.     

A model of front-illuminated n+-p-p+high-efficiency silicon solar cell

A realistic model of a front-illuminated n⁺-p-p⁺ silicon solar cell is developed by solving the current continuity equations for minority carriers in the quasi-neutral regions in steady state, assuming the light in the cell is trapped as a result of multiple reflections at the front and the back of the cell. This model is used to study the effects of the front emitter thickness and doping level and the light trapping on the J-V characteristic and thereby on the open-circuit voltage, short-circuit current density, curve factor, and the efficiency of the cell. A textured cell with an emitter thickness in the range of 0.3-1.0 μm with its doping ≈5×10¹⁸ cm^-3 and the recombination velocities of minority carriers as large as 200 cm/s at the n⁺ front surface and 10 cm/s at the back of the p base can exhibit an efficiency in excess of 26% (under AM 1.5 sunlight of 100 mW/cm² intensity) at 25°C if the light reflection losses at the front surface can be made small     

A novel way of texturing glass for microcrystalline silicon thin film solar cells application

In this paper, we present a novel way of texturing glass facilitated by ZnO:Al thin film as sacrificial layer for thin film silicon solar cell application. We name this technique zinc oxide‐induced texturing (ZIT). The texturing of glass was achieved by wet etching of ZnO:Al covered glass with HF and HNO₃ as etchants. We investigated the influence of the ZnO:Al layer sputtering condition, the layer thickness, and the etchant composition on the surface morphology of the textured glass. We demonstrate that we are able to control the roughness of the ZIT glass over a wide roughness range, ranging from 20 to 400 nm. Highly efficient microcrystalline silicon n‐i‐p solar cells were deposited on ZIT glass. The influence of the substrate morphology on the solar cell performance is also discussed. The highest efficiency for a single junction n‐i‐p microcrystalline silicon solar cell obtained in this work is 10.64% (Active area). Copyright © 2014 John Wiley & Sons, Ltd.     

表面粗化提高红光LED的光提取效率

介绍了通过出光表面粗糙化来减少全反射的方法,实验中使用化学湿法腐蚀的技术获得预计的粗糙形貌,结果给出不同参数下的光强和光辐射功率比较,器件的外量子效率得到了约29%的提高。从理论和测试结果两方面阐述了表面粗糙化对提高红光LED外量子效率的机理。     

A novel silicon texturization method based on etching through a silicon nitride mask

We present a new texturing technique applicable to silicon solar cells. The technique is based on the isotropic etching of silicon through a very thin layer of silicon nitride, deposited by low‐pressure chemical vapor deposition. Spectrophotometry measurements show that the resulting surface texture displays low reflectivity after encapsulation behind glass, and nearly ideal light‐trapping behaviour. The surfaces can also be well passivated using standard passivation techniques. Emitter dark saturation currents in the range 4–5 × 10⁻¹⁴ A/cm² have been measured by quasi‐steady‐state photoconductance following the growth of a thermal oxide. Copyright © 2005 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.     

Characteristics of large‐scale nanohole arrays for thin‐silicon photovoltaics

Nanostructured crystalline silicon is promising for thin‐silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive‐ion etching (RIE) techniques for large‐area processes. A post‐RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti‐reflective properties, where an AM1.5 G spectrum‐weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three‐dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin‐silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.     

Silver versus white sheet as a back reflector for microcrystalline silicon solar cells deposited on LPCVD‐ZnO electrodes of various textures

We compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5‐µm‐thick low‐pressure chemical vapor deposition (LPCVD)‐ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD‐ZnO layer (with thickness below 200 nm). For this latter design, the optical loss in the nano‐rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar‐plasma treatment. Because of its superior lateral conductivity, the thin‐ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD‐ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin‐ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active‐layer thickness of only 1.1 µm. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright © 2014 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Multicrystalline silicon solar cells exceeding 20% efficiency

This paper presents the first conversion efficiency above 20% for a multicrystalline silicon solar cell. The application of wet oxidation for rear surface passivation significantly reduces the process temperature and therefore prevents the degradation of minority‐carrier lifetime. The excellent optical properties of the dielectrically passivated rear surface in combination with a plasma textured front surface result in a superior light trapping and allow the use of substrates below 100 μm thickness. A simplified process scheme with laser‐fired rear contacts leads to conversion efficiencies of 20·3% for multicrystalline and 21·2% for monocrystalline silicon solar cells on small device areas (1 cm²). Copyright © 2004 John Wiley & Sons, Ltd.     

Increasing the extraction efficiency of AlGaInP LEDs via n-side surface roughening

An n-side-up AlGaInP-based light-emitting diode (LED) with a triangle-like surface morphology was fabricated using the adhesive layer bonding technique, followed by wet etching to roughen the surface. The light output power of the roughened-surface LED was 1.6 times higher than that of a flat-surface LED at an injection current of 20 mA, i.e., a significant improvement attributed to the ability of the roughened surface to not only reduce the internal reflection between the rear mirror system and the semiconductor-air interface, but also to effectively scatter the light outside the LED device.     

Efficient light trapping in silicon solar cells by ultrafast‐laser‐induced self‐assembled micro/nano structures

A novel ultrafast laser processing technique is used to create self‐assembled micro/nano structures on a silicon surface for efficient light trapping. Under appropriate experimental conditions, light reflection (including scattering) of the Si surface has been reduced to less than 3% for the entire solar spectrum and the material appears completely black to the naked eye. A post‐chemical cleaning is applied to remove laser‐redeposited material and induced defects. Optical, morphological, and structural characterizations have been carried out on as‐laser‐treated and post‐chemically cleaned surfaces. Finally, we report for the first time the total efficiency of over 14% and high external quantum efficiency (EQE) results on photovoltaic devices fabricated on the ultrafast‐laser‐induced micro/nano structured silicon wafer, which can be further improved upon process optimization. Copyright © 2011 John Wiley & Sons, Ltd.     

亚硝酸钠刻蚀液对多晶硅表面陷阱坑形貌的影响

酸刻蚀多晶硅表面技术是当前太阳能研究的热点之一。利用亚硝酸钠比硝酸钠氧化能力弱的特点,在普通酸刻蚀液中用亚硝酸钠取代硝酸配制多晶硅表面刻蚀液,然后在相同的工艺条件下刻蚀多晶硅表面。实验样品的SEM显示:含有NaNO2酸刻蚀液使多晶硅表面能布满蚯蚓状的腐蚀坑,腐蚀坑的深度比传统的酸刻蚀的陷阱坑深,而且密度分布比较均匀,样品平均反射率下降到23.5%,与传统配方酸刻蚀液刻蚀的多晶硅表面相比,平均反射率下降了8%左右。     

Wet Etching of Ion-Implanted Silicon Dioxide Monitored by Atomic-Force Microscopy

An experiment using an atomic-force microscope to monitor the wet etching of silicon dioxide subjected to local ion implantation is presented. The adequate agreement between the measured and the computed values of the etching rate as a function of depth strongly indicates that the technique proposed allows one to determine the thickness of the radiation-damaged layer and to accurately evaluate the degree of sputtering or swelling for the implantation layer.     

应用于中红外波段的阵列化多孔硅一维光子晶体的制备

首先通过光刻工艺制作了阵列化岛状硅衬底,然后利用交替变换阳极腐蚀电流,通过合理地控制制备参数,适当的热氧化条件,成功地制备了禁带中心位于5μm、6μm、7μm、10μm的阵列化多孔氧化硅一维光子晶体.随后在其表面淀积一层低应力的Si3N4,通过原子力显微镜(AFM)和傅里叶红外反射谱(FTIR)测试证明,沉积Si3N4后该结构仍然具有良好的平整度和较高的反射特性.该阵列结构不但具有较好的隔热和高反射特性,而且岛状的阵列结构可使其与其他器件互联变得简单易行,必将为制备多功能、一体化器件提供有利条件.     

干法腐蚀在硅微波功率晶体管研制中的应用

干法腐蚀与传统的湿法腐蚀相比,具有明显的各向异性,并且具有较好的可重复性、可控性及在硅片加工中易实现连续生产等优点,已成为目前硅微波功率晶体管研制和生产的关键技术.本文对目前干法腐蚀在硅微波大功率晶体管中的应用进行了分析,指出随着硅微波功率晶体管工作频率的不断提高,对高性能各向异性的干法腐蚀技术要求也更加迫切.     

Nano‐cones on micro‐pyramids: modulated surface textures for maximal spectral response and high‐efficiency solar cells

The front‐side reflection represents a significant optical loss in solar cells. One way to minimize this optical loss is to nano‐texture the front surface. Although nano‐textured surfaces have shown a broad‐band anti‐reflective effect, their light scattering and surface passivation properties are found to be generally worse than those of standard micro‐textured surfaces. To overcome these setbacks in crystalline silicon solar cells, advanced texturing and passivation approaches are here presented. In the first approach, we propose a modulated surface texture by superimposing nano‐cones on micro‐pyramidal surface texture. This advanced texture applied at the front side of crystalline silicon wafers completely suppresses the reflection in a broad wavelength range from 300 nm up to 1000 nm and efficiently scatters light up to 1200 nm. In the second approach, we show a method to minimize recombination at nano‐textured surfaces by using defect‐removal etching followed by dry thermal oxidation. These two approaches are applied here in an interdigitated back‐contacted crystalline silicon solar cell and result in decoupling of the interplay between the mechanisms behind short‐circuit current density and open‐circuit voltage. The device exhibits a conversion efficiency equal to 19.8%, record external quantum efficiency (78%) at short wavelengths (300 nm), and electrical performance equal to the performance of the reference interdigitated back‐contacted device based on front‐side micro‐pyramids. Copyright © 2015 John Wiley & Sons, Ltd.