摘要
提高光电转换效率、降低成本是太阳能电池技术发展中不变的主题。太阳能电池陷光薄膜及陷光结构可以有效降低电池表面太阳光的反射率,增加进入到电池内部的光子数量,进而提高太阳能电池的光电转换效率。现有的陷光结构不能在广角谱、宽光谱范围内降低电池光学损失。该问题在薄膜电池、以及第三代太阳能电池中更加突出,为太阳能电池技术发展的瓶颈之一。
本文以TiO2单层、双层及梯度折射率减反膜为研究对象。研究了薄膜制备工艺、沉积模式、显微结构及光学特性之间的相互关系；提出了TiO2陷光薄膜的相变模型；探索了TiO2薄膜折射率调控及纳米结构化的新方法,并研究了杂质光伏效应与陷光结构之间的关系；为高效太阳能电池技术发展提供了理论及实验支撑。
采用直流反应磁控溅射技术制备了TiO2薄膜,研究了工艺条件对薄膜沉积模式、晶体结构、表面形貌、光学特性的影响,研究了晶体结构、表面形貌对光学特性的影响。结果表明：薄膜中悬空键的光吸收作用、晶界散射和薄膜表面的光散射作用为影响薄膜光透过率的主要因素。薄膜结构特性和沉积速率由沉积过程中产生的Ti+、TiO+、TiO2+数量决定,当氧流量小于3mL/min,且溅射功率大于400W时,Ti+主导沉积过程,沉积模式为金属模式,获得的薄膜为Ti金属色泽；当氧流量在3mL/min与5mL/min之间时,Ti+、TiO+主导沉积过程,薄膜表现为金黄色；当氧流量大于7mL/min时,TiO2+主导沉积过程,薄膜为透明状态。
首次制备了锐钛矿相与金红石相混合结构的沉积态TiO2薄膜,并对TiO2薄膜中存在的相变过程进行了研究。结果发现：纳米柱状结构对相变过程有抑制作用,相变后薄膜晶粒尺寸为相变前的二倍。薄膜相变过程分为三个阶段：第一阶段,在退火过程中薄膜中的锐钛矿相晶粒首先生长,直至彼此相接；第二阶段,金红石相在两个锐钛矿相晶粒的相接处成核；第三阶段,金红石相晶粒向外扩张,最后将两个锐钛矿相晶粒转化为一个金红石相晶粒。
首次在直流反应磁控溅射中采用改进的倾斜沉积方法对TiO2薄膜折射率进行调控研究。结果表明：当沉积角度增加到30°时,薄膜中的柱状颗粒开始分离；当沉积角度增加到60°时,薄膜中出现分离的柱状颗粒,特别是增加到80°时,薄膜中出现了贯穿薄膜的孔洞。薄膜中的柱状结构的倾斜角度β与沉积角度α之间有β=tan-1(0.347tan α)关系,薄膜的折射率n与沉积角度α有n=2.50619-1.1×10-42关系。改进的倾斜沉积方法获得了折射率在1.81-2.53之间可调的TiO2薄膜。该方法可降低薄膜晶粒尺寸,并减小薄膜柱体结构半径,两者分别提高了可见光中长波段与短波段的透过率。
对TiO2薄膜的单层、双层及梯度折射率减反膜的电池输出特性进行了仿真研究。发现减反膜可有效提高太阳能电池的短路电流密度和转换效率,双层减反膜的减反射效果优于单层减反膜。TiO2单层减反膜的光伏电池短路电流密度与转换效率分别提高了66%与65%,加权平均反射率最低达7.8%。SiO2/TiO2、TiO2/TiO2双层减反膜具有更低的反射率,加权平均反射率可降低至1.5%与4.6%。当玻璃封装的太阳能电池梯度折射率减反膜全部由TiO2组成,其折射率梯度满足三次方关系,加权平均反射率为1.5%,太阳能电池短路电流密度和开路电压分别提高了76%和75%；当太阳光入射角小于70°时,全TiO2结构的梯度折射率减反膜在全波段范围内都能起到很好的减反射效果。未封装的太阳能电池中,全Ti02结构的梯度折射率由于缺少低折射率材料,减反射效果仅与双层减反膜效果相当,采用了SiO2薄膜后,梯度折射率减反膜效果更佳。研究发现深能级杂质能有效提高电池对红外光波段光子的吸收。随深能级杂质铊掺入量的增加,短路电流密度与转换效率先增加后下降,最优掺杂浓度为Nt=ND=1017cm-3。深能级杂质在晶硅太阳能电池内引入两个主要的光激发过程,一是光子将电子从硅的价带激发到铊能带,另一个是光子将电子从铊能带激发到硅的导带,这两个光激发过程相互竞争共同完成以能量小于硅带隙的光子将电子从硅价带激发到导带的过程；它提高了晶硅电池的红外光波段特别是1000～1400nm波段响应性能。良好的陷光结构是发挥杂质光伏效应的重要条件。在良好的陷光结构以及最优化铊掺杂浓度条件下,杂质光伏效应将晶硅太阳能电池的短路电流密度提高了9mA/cm2,转换效率增加值△η达到2%。
Improving conversion efficiency and reducing cost are the focus of the research on the solar cell. The light trapping layers and structure can minimize the light reflection and enhance the transmittance, therefore, improve the solar collection efficiency and the overall solar-to-electricity efficiency. However, the light trapping structure, which has been developed in the solar cell, works only at a certain spectral range and at the normal incidence. The omni-directional broadband antireflection characteristic is important, especially in thin film solar cell and third generation solar cell.
In this present study, the TiO2single layer, double layers and gradient-index antireflection coatings were investigated. The preparation parameters, deposition model, structure, optical properties of the TiO2thin films and their correlationships have been investigated; The mechanism for the transition of anatase to rutile has been found; The novel methods, which are developed to adjust the refractive index of the TiO2films, have been used to prepare the nanostructured antireflection coatings; The effect of light trapping structure on the impurity photovoltaics effect has been studied. These researches have provided theoretical and experimental support for the development of the high efficient solar cells.
TiO2thin films were deposited by direct current reactive magnetron sputtering. The relationship between the deposition model, structure, optical properties and the preparation parameters were investigated. It is demonstrated that the light absorption of the dangling bond, and the scattering of the grain boundaries and surface reduce the transmission of the films. The deposition model of the TiO2thin films is determined by the intensity of the Ti+, TiO+and TiO2+present in the plasma. When the oxygen flow rate is less than3mL/min and the sputtering power is greater than400W, the deposition is dominated by Ti+, this deposition is in the metal model. When the oxygen flow rate increases from3to5mL/min, the deposition is dominated by both Ti+and TiO+, the films are golden yellow. When the oxygen flow rate is bigger than7mL/min, the deposition is dominated by TiO2+and the films are transparent.
The films with mixtures of anatase and rutile phases were deposited for the first time. The research on the transition of anatase to rutile demonstrates that the columnar structure restricts the transition, and the crystallite size is doubled by the transition. It is because that the transition of anatase to rutile was carried out in three stages:Firstly, the anatase particles grow large enough to meat each other during the annealing. Secondly, the rutile starts to locally nucleate at the boundary of two anatase particles. Thirdly, the rutile further migrates towards the exterior of the new larger particle, and two anatase particles combine together to form one rutile particle.
The oblique angle deposition method was developed to adjust the nano structure and the refractive index of the TiO2thin films. Results confirm that the column particles start to separate from each other at the deposition angle of30°, some separated column particles appeared in the films at deposited angle60°, many holes penetrated through the films at deposition angel of80°. The column structure angle (3has a relationship of β=tan-1(0.347tanα) with the deposition angle a, while the relationship between the films refractive index n and deposition angle is n=2.50619-1.1×10-4α2. The oblique angle deposition methods can prepare the film with a refractive index of1.81～2.53, this controllable refractive index allows the realization of grade-index profile. With this method, the decrease of the grain size increases the optical transmittance in the visible-light region, while the decrease of the nanorod diameter increases the optical transmittance in the ultraviolet region.
Properties of the solar cell with the single layer, double layers and gradient-index TiO2based antireflection coating were simulated. The results show that, the solar cell with antireflection coating has a main improvement in the short-circuit current density and the efficiency, and this improvement is more remarkable in double layers antireflection coating. The least weighted average reflection can be acquired is7.8%in single layer TiO2antireflection coating, and an increase of66and65%can be gotten for short-circuit current density and the efficiency respectively. A weighted average reflection of1.5%and4.6%can be got in the SiO2/TiO2and TiO2/TiO2double layers antireflection coating, respectively. When used in the encapsulated solar cell, the gradient-index antireflection coating constructed all by TiO2has a very small weighted average reflection of1.5%. The short-circuit current density and the efficiency have an increase of76%and75%, respectively; This gradient-index antireflection coating has a cubic-index profile, and can work well if the light incidence angle is lower than70°. Used in the non-encapsulated solar cell, its reflection increases due to lake of low index material. After using of SiO2thin films, the antireflection property was improved.
The solar cell with dopping of the deep level impurity can absorb more near-infrared photons. The short-circuit current density and the efficiency increase first then decrease as the increase of T1impurity concentration. The highest efficiency was acquired at Nt=ND=1017cm-1. In the impurity photo voltage effect, there are two competing absorption mechanisms for the photons with energy between mid gap and the full band gap. One photon is needed to excite an electron from the valence band to the defect level, and another photon is needed to excite the electron from the defect level to the conduction band. The impurity photo voltage effect of thallium extends the spectral sensitivity in the sub-band gap range from1000nm to about1400nm. If higher values of the light trapping were possible, the IPV effect becomes appreciable (ΔJsc≈9mA/cm2and Δη≈2%).
引文
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