Nickel silicide contacts formed by excimer laser annealing for high efficiency solar cells

Driven by the relatively high cost of silver (Ag), interest has grown in the photovoltaic (PV) industry to substitute conventional screen printed (SP) Ag front contacts with copper (Cu) plated contacts. The approach chosen here applies selective laser ablation of the front anti‐reflection coating (ARC), then forming self‐aligned nickel silicides (NiSi_x) contacts, and thickening the lines by Cu plating to achieve the desired line conductivity. A successful implementation of this scheme requires annealing to form NiSi_x with low contact resistance. However, it has been shown that industrial shallow emitters can be damaged severely upon conventional annealing of nickel. In this paper, we show that by using large area excimer laser annealing (ELA), NiSi_x contacts can be formed on industrial shallow emitters without the associated junction degradation. On the basis of sheet resistance, transmission electron microscopy, and lifetime measurements, we demonstrate that NiSi_x formation by ELA can be achieved in narrow contact openings without damaging the passivation and reflectance properties of the neighboring ARC. In addition, the thresholds for NiSi_x formation for different Ni thicknesses are quantified by rigorous finite element simulations and compared with experimental data. Finally, high efficiency passivated emitter and rear cell type solar cells featuring a shallow 85 Ω/sq emitter have been processed on large area CZ–Si using laser ablation of the ARC and subsequent NiSi_x formation by ELA. These cells show an average efficiency gain of 0.4%_abs compared with cells processed with reference SP contacts. In this work, the best performing cell with the ELA process reached 20.0% energy conversion efficiency. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of precursor layer ablation on laser doping of silicon

This paper presents a numerical model, which quantitatively demonstrates that ablation and partial recondensation of the dopant precursor layer are some of the dominating physical processes in laser doping (LD) of crystalline silicon. Our pulsed LD process uses a line focused laser beam, enabling the creation of solar cell emitters without the generation of dislocations, if the width w of the short axis of the line focus is w < 10 μm. The concentration profiles of the dopant atoms strongly depend on the pulse energy density E_p, the pulse to pulse separation Δx and the number of laser scans N_s. By comparing measured with modeled concentration profiles, we are able to evaluate the ablation width as well as the amount of the ablated precursor layer. In case of a sputtered phosphorus precursor layer, the ablation width w_a is w_a = 6 μm, whereas the width of the molten silicon layer w_m is w_m = 5 μm. The model also explains the dependence of experimental dopant concentration profiles on the number of subsequent laser scans N_s and pulse to pulse separation Δ_x. Copyright © 2010 John Wiley & Sons, Ltd.     

高能纳秒激光烧蚀单晶硅的微观结构

高能纳秒激光烧蚀硅时,往往会伴随着相爆炸过程,它决定了烧蚀的形貌特征。基于相爆炸的物理过程,分析了相应的烧蚀形貌：相爆炸的发生会使得硅材料发生大范围的去除,高温熔化材料的混合物向外喷溅,冷却形成放射状的冷却条,期间分布着冷却的球状微粒;超热液体的去除部位形成一个类似花瓣状坑,在坑中由于入射光与散射光的干涉形成条纹,其周期与激光光波相似。     

飞秒激光烧蚀加工恒弹性合金的规律研究

恒弹性合金的加工对表面质量和加工精度的要求越来越高,为了实现对恒弹合金的精密定量去除,本文探索了采用飞秒激光烧蚀的加工新方法。首先,分析计算了在高强度飞秒激光辐照加工下,恒弹性合金材料的烧蚀阈值;其次,实验研究了飞秒激光脉冲能量和脉冲个数对该材料上烧蚀加工微坑的直径和深度的影响,结果表明：恒弹性合金的飞秒激光烧蚀阈值为0.167 J/cm2;可以通过增大脉冲能量来增大烧蚀坑直径,通过增大脉冲数来增大烧蚀坑深度。脉冲烧蚀坑直径上限为150.64 μm,运用飞秒激光旋切加工方法,可获得直径为500 μm的微孔,提高了飞秒激光烧蚀加工的能力。     

Influence of silver paste rheology and screen parameters on the front side metallization of silicon solar cell

Four commercially available silver pastes for silicon solar cells front side metallization were examined and compared. Both rheological and printing experiments were used to correlate pastes properties and screens characteristics with printing results. Theoretical printability and spreading behavior were first characterized by means of rotational and oscillatory rheological experiments. Further on, the pastes were printed, dried, and fired in a standard solar cell process on an industrial line using a full factorial design of experiments. This methodology allows to study the effect of the paste, the screen, the aperture, and their interactions on the printing results. Especially, the finger aspect ratio, the finger cross section and its relative standard deviation, which are respectively linked to solar cell efficiency, silver consumption, and process stability, were systematically measured. This work allows finding general guidelines for process optimization. Finally, a confirmation test was carried out, leading to 19% efficiency on monocrystalline silicon solar cells in an industrial environment.     

20·5% efficient silicon solar cell with a low temperature rear side process using laser‐fired contacts

The paper presents a rear side structure for crystalline silicon solar cells, which is processed at a maximum temperature of 220°C. Using two different material compositions for electrical and optical needs, the layer system has excellent passivation properties, enhances light trapping and allows for a good ohmic contact. With this structure we achieve an independently confirmed conversion efficiency η=20·5% on a 250 μm thick silicon solar cell. Due to the fact that the maximum process temperature is 220°C, this layer system enables new solar cell concepts. Copyright © 2006 John Wiley & Sons, Ltd.     

超快激光制备PMMA微流道机理及工艺研究

由于传统加工方法不能解决PMMA微流道的加工质量不好和效率低的问题,本文对超快激光直写PMMA制备微流道的烧蚀机理和工艺参数进行了研究。根据实验分析不同的激光功率、加工速度和加工次数对微流道的宽度和横截面的影响规律,利用超快激光加工系统制备微流道,并采用超景深三维显微镜观测微流道的表面形貌。实验结果表明,当超快激光的加工速度为20 mm/s时,激光功率为1.5 W时,制备的微流道的宽度较小、宽度趋势比较平稳;当超快激光作用PMMA的次数一样,由于加工速度逐渐增加,制备的微流道其宽度和激光的加工速度保持线性增加。当加工速度越大时,微流道的宽度较小、且壁面趋势相对平缓,而当加工速度一定,超快激光的输出功率在1.5 W时,微流道内壁区域不易出现残渣堆积和气泡隆起现象。本文通过优化超快激光加工系统的工艺参数,从而加工出尺寸精度高、表面光滑、宽度为20~90 μm的微流道芯片。     

激光烧蚀法制备纳米粉的现状及前景

本文综述了激光烧蚀法制备纳米颗粒的发展现状，展望了该技术的发展前景。     

Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

Silicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post‐metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum‐coated oxide‐passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide‐passivated rear surface. Measuring dark and illuminated IV‐curves of standard industrial aluminum back surface field (Al‐BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV. One‐dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide‐passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide‐passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm² Czochralski (Cz) silicon wafers. Copyright © 2009 John Wiley & Sons, Ltd.     

飞秒激光加工镍钛合金的烧蚀阈值实验研究

利用飞秒激光微加工系统对镍钛合金的烧蚀阈值进行理论和试验研究.设计了不同功率和扫描速度下的镍钛合金多脉冲累积烧蚀实验,应用扫描电子显微镜观测并分析试件烧蚀区域的形貌特征,计算得到了不同扫描速度下的烧蚀阈值分别为2.02、2.40、3.04、5.46、15.44 J/cm2.然后分析了多脉冲的累计效应对镍钛合金烧蚀阈值的...     

红外激光蚀除生物组织过程的时间演化规律

在描述激光蚀除生物组织过程的多层结构模型的基础上,研究了激光蚀除生物组织过程中组织的温度场、热损伤程度等表征蚀除过程和效果的主要物理量的时间演化规律.以10 W/mm2 的CO2激光蚀除人体皮肤组织为例进行了数值计算,得到了组织发生汽化脱水、干化和碳化等重要变化的时刻、蚀除界面移动和蚀除进入准稳定状态的时刻以及蚀除过程中组织的温度场、蚀除界面和各层组织间相界面等物理量的时间演化规律.     

激光对不同物质消融的研究

本文报道了用308nm XeCl激光消融聚合物及生物组织和角膜刻划的研究结果,并与1.06μm和0.53μm的纳秒、皮秒YAG激光消融生物组织的结果作了对比。     

Effect of carbon containing SiNx antireflection coating on the screen‐printed contact and low illumination performance of silicon solar cell

Screen‐printed metal contact formation through a carbon containing antireflection coating was investigated for silicon solar cells by fabricating conventional carbon‐free SiN_x and carbon‐rich SiC_xN_y film. An appreciable difference was found in the average shunt resistance (R_sh), which was about an order of magnitude higher for SiC_xN_y‐coated solar cells relative to the counterpart SiN_x‐coated solar cells. Series resistance (R_s) and fill factor (FF) were comparable for both antireflection coatings but the starting efficiency of SiC_xN_y‐coated cell was ~0·2% lower because of slightly inferior surface passivation. However, SiC_xN_y‐coated solar cells showed less degradation under lower illumination (<1000 W/m²) compared with the SiN_x‐coated cells due to reduced FF degradation under low illumination. Theoretical calculations in this paper support that this is a direct result of high R_sh. Detailed photovoltaic system and cost modeling is performed to quantify the enhanced energy production and the reduced levelized cost of electricity due to higher shunt resistance of the SiC_xN_y‐coated cells. It is shown that R_sh value below 30 Ω (7000 Ω cm² for 239 cm² cell) can lead to appreciable loss in energy production in regions of low solar insolation. Copyright © 2011 John Wiley & Sons, Ltd.     

激光等离子体烧蚀推力的机理研究

随着航天空航空天技术的发展,激光等离子体推进受到了人们的普遍关注.基于激光与物质相互作用产生等离子体的空间分布,本文从微观、宏观和简化模型推导了碳靶的烧蚀压与激光强度的幂函数关系,幂指数分别是0.60493、2/3和2/3.实验测得碳靶的最大偏移量与激光能量的幂函数关系,幂指数为0.61936.实验与微观模型的幂指数相...     

球端面光纤耦合界面烧蚀现象的实验研究

本文提出大功率半导体激光器(LD)对光纤耦合界面的烧蚀的理论假设,得出系统耦合效率随时间的变化关系为ηt=η0t-αβmsk=η0t-αβρ。制作出三种不同直径的光纤端面小球,并对其防烧蚀性能进行了实验研究。实验证明激光器对光纤的烧蚀主要集中在粘接剂区,且大直径的球端面光纤能有效的减小烧蚀对系统耦合效率的影响。从而证明了公式的正确性。     

脉冲激光烧蚀金属氧化物的物理化学过程及反应机理

本文回顾了前人有关脉冲激光烧蚀过程的各种理论模型，重点综述了脉冲激光烧蚀金属氧化物的物理化学过程及反应机理，并对激光能量不很高时金属氧化物的烧蚀过程提出了热控制机制的新观点。     

KrF准分子激光烧蚀氧化镁单晶的实验研究

本文采用光谱学和离子探针等检测手段,对KrF准分子激光烧蚀氧化镁单晶的物理过程进行了实验研究。结果表明,激光烧蚀等离子体内的主要粒子形态是镁原子和一价的镁离子,其粒子的动能在30～45eV范围。从烧蚀部位的扫描电镜形貌分析表明,在激光能量密度近于烧蚀阈值时,激光辐照的氧化镁单晶表面呈现出一定程度的层离特征。     

激光烧蚀法制备准一维纳米材料

介绍了当前激光烧蚀法制备纳米丝、纳米管和纳米电缆的研究现状,讨论了准一维纳米材料激光烧蚀法制备的系统设备和制备条件,分析了激光烧蚀法制备准一维纳米材料的特点和发展趋势.认为准一维纳米材料是气相粒子在高温催化剂作用下生成的.激光烧蚀法具有洁净、可控性强及适应面广的特点.     

纳米硅镶嵌薄膜的光学性质及对1342nm激光的被动调Q

采用射频磁控溅射技术和热退火处理制备了石英衬底的纳米Si镶嵌SiN<,x>(ne-SiN<,x>)薄膜.通过对薄膜的小角度X射线衍射和吸收光谱测试,确定了硅晶粒尺寸和光学带隙.采用Z-扫描技术研究了薄膜的三阶非线性光学特性,结果表明薄膜的非线性吸收属于双光子吸收.把ne-Si/SiN<,x>薄膜作为可饱和吸收体插入LD...     

Theoretical approach to estimate laser process parameters for drilling in crystalline silicon

This paper presents a simple method for estimating the ablation rate during drilling processes in crystalline silicon wafers. Using data from literature, the physical process of material heating, melting, and ejection can be estimated on a time scale and leads to a temporal separation of the pulse duration in two parts. The latter one offers a theoretical determination of the expected average ablation rate. The results of this approximation coincide with experimental values and offer solutions to increase drilling efficiency. The experiments were done with a q‐switched solid‐state laser emitting at 1064 nm wavelength at pulse durations between 810 ns and 1440 ns. Copyright © 2010 John Wiley & Sons, Ltd.