硅表面钝化及对异质结太阳电池特性的影响

研究了不同的晶体硅表面钝化方法,测试分析了硅片的少数载流子寿命以及对晶体硅/非晶硅异质结(HIT)太阳电池性能的影响。发现适当时间的HF溶液处理、氢等离子体处理和表面覆盖约3nm的本征非晶硅层能有效提高硅片的少子寿命,从而提高HIT太阳电池的开路电压。对电池制备工艺综合优化后,得到了基于n型晶体硅的光电转换效率为16.75%(Voc=0.596V,Jsc=41.605mA/cm2,FF=0.676,AM1.5,25℃)的HIT太阳电池。     

HIT太阳能电池的发展概况

阐述了异质结(HIT)太阳能电池的结构与特征,并从异质结能带结构的优化、非晶硅层的制备方法、背面场(BSF)的研究、衬底材料的选取以及发射极材料的革新等方面综述了HIT太阳能电池的技术发展状况。概述了HIT电池的产业化应用研究现状,并展望了HIT太阳能电池的未来发展趋势。     

对非晶硅薄膜进行快速磷扩散以获得本征薄层异质结

本征薄层异质结(HIT)太阳能电池具有优异的性能,包括效率高、成本低、稳定性好、制备温度低等。本研究利用磁控溅射技术在p型单晶硅(pc-Si)衬底上制备一定厚度的本征非晶硅薄膜(i-a-Si),以磷纸为扩散源,通过快速热扩散(RTD)方法进行扩散得到具有p-n结的掺杂非晶硅层(n+-a-Si),最终得到n+-a-Si/i-a-Si/c-Si的异质结结构。系统地研究了扩散过程对a-Si膜(包括i-a-Si和n+-a-Si)晶化程度以及p-n结深度的影响,利用拉曼光谱(Raman)、X射线衍射(XRD)仪、台阶仪、扫描电镜(SEM)等对a-Si膜进行表征,并利用金相显微镜测量p-n结(采用磨角染色法染色)深度,从而获得制备p-n结的最佳扩散温度和时间。     

用于HIT太阳能电池的本征非晶硅薄膜

带本征薄层的异质结（HIT）太阳能电池要求本征非晶硅薄膜具有生长速率低，暗电导大，光学带隙宽的特点。采用等离子增强化学气相沉积（PECVD）制备符合HIT太阳能电池要求的本征非晶硅薄膜，并通过分析薄膜的透射光谱，采用Tauc法计算了薄膜的光学带隙，为约1．87eV，衬底温度为180℃，放电功率为80W时获得的薄膜性能最佳。     

纳米硅薄膜制备及HIT太阳能电池

在较高工作气压(332.5～399Pa)下,采用等离子增强化学气相沉积(PECVD)工艺制备了优质的本征纳米硅薄膜及掺磷的纳米硅薄膜,并采用X射线衍射(XRD)、拉曼散射(Raman) 测试技术对其进行了测试和分析.结果表明纳米硅薄膜的XRD谱中存在(111)、(220)和(331)峰位;Raman谱中显示出其薄膜中的晶粒的大小(2～5nm)符合纳米晶的要求.将制备的纳米硅薄膜初步用于栅极/ITO/n-nc-Si∶H/i-nc-Si∶H/p-c-Si/Al/Ag结构的异质结(HIT)太阳能电池,开路电压(Voc)达404mV,短路电流密度(Jsc)可达到34.2mA/cm2(AM1.5,100mW/cm2,25℃).     

异质结太阳能电池的仿真模拟研究和界面态密度对效率的影响

大量文献已经报道非晶硅/晶体硅异质结电池每层材料的参数对电池的影响,如厚度、掺杂浓度等,但是并没有进一步给出透明导电氧化物薄膜TCO的功函数对电池的影响以及如何选择合适的TCO,也很少有报道界面态密度对异质结太阳能电池的影响机理。本研究表明,对于n型单晶硅片为衬底的异质结电池,发射场的TCO功函数越大越好,最佳范围是5.4～6.3 eV。对于背场的TCO功函数越小越好,最佳范围是3.6～4.0 eV。另外研究表明,对于n型衬底的非晶硅/晶体硅异质结电池(HIT电池),与衬底背面与非晶硅的界面态(Dit2)相比,衬底前表面与非晶硅的界面态(Dit1)是影响电池性能的主要因素,并且Dit1和Dit2态中,与类施主态相比,对电池效率起到主要影响作用的都是类受主态。     

硅异质结太阳能电池用透明导电氧化物薄膜的研究现状及发展趋势

硅异质结(SHJ)太阳能电池是目前光伏产业中的重要组成部分,其由于具有高开路电压(V_oc)等优点而引起了广泛的关注。在硅异质结太阳能电池中,透明导电氧化物(TCO)薄膜层的光学性能和电学性能分别影响着电池的短路电流(J_sc)、填充因子(FF),进而影响电池的转换效率。近年来,SHJ电池中TCO层的研究主要集中于掺杂的In₂O₃和ZnO体系。本文从硅异质结太阳能电池的不同结构出发,概述了TCO薄膜的光电性能(透过率、禁带宽度、方块电阻、载流子浓度、迁移率和功函数)以及与相邻层的接触对电池性能的影响,介绍了不同体系的透明导电氧化物薄膜在硅异质结太阳能电池中的应用及研究现状,并展望其未来的发展趋势。     

高效单晶硅太阳电池的最新进展及发展趋势

晶体硅太阳电池是目前光伏市场的主流产品,其又可分为多晶硅电池和单晶硅(c-Si)电池。目前,多晶硅电池成本较低,市场份额较大,但其效率较低;单晶硅电池成本相对偏高,但其效率更高,市场份额小于多晶硅电池。随着硅材料和硅片切割技术的进步,单晶硅片的成本持续下降,且未来市场对高效率的高端光伏产品需求日益增长。因此,高效率的单晶硅电池将受到更多的关注。为进一步提高单晶硅太阳电池的效率,近几年的研究工作主要集中于提高硅片质量来降低体缺陷,寻找新型钝化材料来降低表面和界面缺陷,开发先进的减反技术(新型的绒面陷光结构和材料)以提高光的利用率,引入低电阻金属化技术降低串联电阻,优化PN结制备技术以及器件结构等。2014年至今,单晶硅太阳电池的转换效率得到连续突破。目前,最高效率是日本Kaneka公司创造的26.6%,其他效率达到或者超过25%的晶硅电池包括钝化发射极背面局部场接触(PERL)电池、交叉指式背接触(IBC)电池、硅异质结(SHJ)电池、交叉指式背接触异质结(HBC)电池、隧穿氧化层钝化接触(TOPCon)电池、多晶硅氧化物选择钝化接触(POLO)电池等。分析这些典型电池的关键技术可以发现,栅线电极与c-Si的金属-半导体接触复合成为影响电池效率的关键因素。为减小这些复合,一方面通过电池背面局部开孔来减小金属与c-Si直接接触的面积,包括钝化发射极背场点接触(PERC)、PERL、钝化发射极和背面全扩散(PERT)等电池。另一方面则是开发既能够实现优异的表面钝化,同时又无需开孔便可分离与输运载流子的新型载流子选择性钝化接触技术,如SHJ电池、TOPCon电池等。此外,采用交叉指式背接触技术与其他电池结构结合则是最大限度提高光利用率的必然选择,包括IBC电池和HBC电池。本文介绍了当前国际上转化效率达到或超过25%的典型高效单晶硅太阳电池,分别对其器件结构、核心工艺、关键材料等进行了分析,在总结这些高效单晶硅太阳电池各自特点的基础上对该领域的发展前景进行了展望。     

氧化锌/氧化亚铜异质结太阳能电池的研究进展

因氧化亚铜(Cu2O)、氧化锌(ZnO)能级和晶格匹配较好,近年来较多的研究者将两者复合制备异质结太阳能电池。异质结的形成可提高光生电子-空穴对的分离效率,同时拓展复合结构的光响应范围,从而有效提高太阳能电池性能。介绍了3类主流的ZnO/Cu2O异质结结构,分别阐述主要的进展,综述了异质结结构中Cu2O、ZnO的制备方法以及制备条件对电池效率的影响,讨论了电池性能的改进措施,并对ZnO/Cu2O异质结太阳能电池未来的发展前景进行展望。     

微晶硅同质结薄膜太阳电池的数值模拟分析

采用太阳电池电容模拟软件(简称SCAPS)对p-i-n结构的微晶硅同质结薄膜太阳电池进行了数值模拟。研究了本征层的厚度和缺陷态浓度及窗口层的厚度等参数对电池性能的影响。得到的主要结论如下:(1)随着本征层缺陷态浓度Nt的增加,电池的各性能参数均单调下降。(2)随着本征层厚度的增加,长波段的光谱响应逐渐改善,但该层过厚则导致中波段的光谱响应急剧下降,在Nt=1.0×1016/cm3的条件下,本征层厚度在1.5～2.0μm范围内电池效率均可达到7.0%以上。(3)p型窗口层的厚度对短波段的光谱响应及短路电流密度JSC有较大影响。     

Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells

High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highly-efficient p-i-n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10-40 W) and pressure (20-50 Pa) used during the deposition of absorber layers in p-i-n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD.     

Analysis of single junction a-Si:H solar cells grown on different TCO’s

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/p-type μc-Si:H (∼5 nm)/p-type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H₂]/[SiH₄] = 20 (300 nm)/n-type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO₂:F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing.It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.     

Effect on thickness of Al layer in poly-crystalline Si thin films using aluminum(Al) induced crystallization method

The polycrystalline silicon (poly-Si) thin films were prepared by aluminum induced crystallization. Aluminum (Al) and amorphous silicon (a-Si) layers were deposited using DC sputtering and plasma enhanced chemical vapor deposition method, respectively. For the whole process Al properties of bi-layers can be one of the important factors. In this paper we investigated the structural and electrical properties of poly-crystalline Si thin films with a variation of Al thickness through simple annealing process. All samples showed the polycrystalline phase corresponding to (111), (311) and (400) orientation. Process time, defined as the time required to reach 95% of crystalline fraction, was within 60 min and Al(200 nm)/a-Si(400 nm) structure of bi-layer showed the fast response for the poly-Si films. The conditions with a variation of Al thickness were executed in preparing the continuous poly-Si films for solar cell application.     

The temperature dependence of the characteristics of crystalline-silicon-based heterojunction solar cells

Technical Physics Letters - Temperature dependences of the photovoltaic characteristics of (p)a-Si/(i)a-Si:H/(n)c-Si singlecrystalline- silicon based heterojunction-with-intrinsic-thin-layer (HIT)...     

Defects in amorphous and solid phase epitaxial silicon

The microstructural morphology of amorphous Si (a-Si) layers deposited in ultrahigh vacuum, as well as crystalline Si grown by solid phase epitaxy (SPE), was studied as a function of Al doping and vapour beam incidence angle. The microstructure of the films was investigated using cross-section transmission electron microscopy. All a-Si layers have a columnar structure, with an average column width of 5 nm. The direction of the columns abruptly changes with the change of deposition direction and shows local column tilts and void formation at substrate surface irregularities. These built-in defects in the a-Si films also influence the defect structure in epitaxial Si films grown by SPE. Voids are initially aligned along the column directions and extra voids form owing to irregularities of the columnar structure. Doping of amorphous Si with Al to 10¹⁸−10²⁰ cm⁻³ does not leave detectable effects in the amorphous structure itself, but will increase the void density of the re-grown SPE Si layers. Furthermore, segregation of Al resulting in metallic inclusions in the amorphous crystalline interface causes metal induced crystallization of Si at temperatures far below the normal SPE regrowth temperature, thus preventing the formation of single crystalline silicon in a single-step process.     

Aluminum-induced crystallization of amorphous silicon films deposited by hot wire chemical vapor deposition on glass substrates

Aluminum-induced crystallization of amorphous silicon films is discussed. Amorphous Si films were deposited by hot wire chemical vapor deposition onto Al coated glass substrates at 430 °C. Complete crystallization of a-Si films was achieved during a-Si deposition by controlling Al and Si layer thicknesses. The grain structure of the poly-Si films formed on glass substrate was evaluated by optical and electron microscopy. Continuous poly-Si films were obtained using Al layers with a thickness of 500 nm or less. The average grain size was found to be 10-15 μm, corresponding to a grain size/thickness ratio greater than 20.     

Formation and microstructure of various thin film chromium silicide phases

The formation of the various chromium silicide phases, as predicted by the Cr-Si phase diagram for bulk materials, was studied when thin Cr/a-Si (where a-Si is amorphous silicon) bilayers were annealed in situ in a transmission electron microscope using a limited supply of a-Si. These results are compared with the silicide formed when unlimited a-Si or single-crystal silicon was used. The specimens were analysed using electron micrographs and diffraction patterns.The detailed studies of the bilayers were preceded by studies of single layers of chromium and a-Si. The chromium single layers proved to be continuous for films as thin as approximately 2 nm. The as-deposited films with thickness in excess of about 5 nm were crystalline with b.c.c. structure and comprised very small grains. A phase change occured at about 450°C from the b.c.c to a simple cubic lattice structure. The silicon single layers were completely amorphous in their as-deposited state and crystallized around 600°C. Very large grains formed.The self-supporting Cr/a-Si bilayers with a typical total thickness of about 50 nm, where the relative film thickness were adjusted to yield Cr:a-Si atomic ratios of 1:2, 1:1, 5:3 and 3:1, were prepared by sequential electron gun vacuum deposition of chromium and silicon onto photoresist-covered glass slides. In the early stages of phase formation, when both unreacted chromium and silicon were present, the CrSi₂ phase was formed at about 450°C for all of these specimens. This was the end phase for the 1:2 ratio specimen. For all the other specimens the metal- rich Cr₅Si₃ phase was next to grow at about 550°C. For the 5:3 ratio specimen this was the end phase.Upon further heating of specimens with a Cr:a-Si atomic ration of 3:1, a more chromium-rich phase of Cr₃Si was formed at about 650°C. In the 1:1 ratio specimen, however, the next and end phase observed was CrSi, also growing at about 650°C. The end phase was thus determined by the availability of chromium and silicon during the reactions and could be predicted from the phase diagram.When using an unlimited supply of a-Si (or of single-crystal silicon), instead of limiting it to the thickness necessary for the predetermined ratios mentioned above, the only phase that ever formed was CrSi₂.The grain sizes observed in the various final phase specimens were as follows: CrSi₂, 25 nm; Cr₃Si, 40 nm; Cr₅Si₃, 50 nm; CrSi, 100 nm.     

Hydrogen-related mechanical stress in amorphous silicon and plasma-deposited silicon nitride

Hydrogen concentrations in amorphous silicon (a-Si) prepared by electron beam evaporation or plasma deposition as well as in plasma-deposited silicon nitride were measured as a function of annealing conditions using the photon-proton scattering method. Comparison with IR evaluations of SiH bonds shows the existence of “quasi-free” (IR inactive) hydrogen in a-Si. The amount of this component can be altered by annealing. The dependence of the mechanical stress on the annealing conditions was determined interferometrically, and the intrinsic contribution was separated in the case of plasma-deposited silicon nitride. As the hydrogen content decreases monotonically with increasing annealing temperature, the initial compressive stress of plasma-deposited a-Si or silicon nitride films changes to a tensile stress. For these films a linear correlation between the amount of mechanical stress and the number of SiH or NH bonds is shown. Evaporated a-Si films show a totally different stress behaviour from that of plasma-deposited a-Si. No correlation of stress with the hydrogen concentration was found for the evaporated a-Si films.     

Nanocrystalline silicon thin films on PEN substrates

We study the structural and electrical properties of intrinsic layer growth close to the transition between amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H), deposited on glass and PEN without intentional heating. These samples showed different behaviour in Raman shift and XRD spectra when compared with that of samples deposited at 200 °C. Electrical properties of these films also reflect the transition between a-Si:H and nc-Si:H, and put in evidence some differences between the microstructure of the films grown on PEN and on glass.P- and n-doped layers were deposited onto glass substrate without intentional heating and at 100 °C with thicknesses ranging from 1000 nm to 35 nm. Conductivity measurements indicate the capability of doping this material, but, for very thin layers, substrate heating was found to be essential.     

非晶硅薄膜表面与非晶须特性

利用透射电子显微镜(TEM),观测到未见报道的须状微结构,称为非晶须。提出非晶须可能的生长条件是淀积系统中出现大的过饱和气压。X 射线光电子能谱分析表明,非晶硅与其表面SiO_2之间存在化学成分为 SiO_x(0相似文献