【作者】 岳之浩；

【导师】 沈鸿烈；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2013， 博士

【摘要】 众所周知，在当前光伏市场中，晶体硅太阳电池仍然占据着绝大部分份额。随着人类需求的增加，人们对晶体硅太阳电池光电转换效率的要求也越来越高。在众多提高太阳电池转换效率的方法中，降低硅片表面反射率是一种非常有效的方式，于是出现了黑硅这个概念。黑硅是一种在紫外至近红外波长范围内都具有极低反射率的材料，正是黑硅的这个特性使得其吸引了业界广泛的关注，同时黑硅的制备方法和性能也成为领域内的研究热点。本论文首先采用SF6和O2作为反应气体，通过反应离子刻蚀（Reactive ion etching, RIE）法在已制备金字塔的单晶硅表面上制备了黑硅结构。相对于现有的RIE工艺，本实验是在室温、不加脉冲偏压且没有掩膜的情况下进行的，研究了不同刻蚀参数对黑硅性能的影响规律，并结合RIE法制备黑硅结构的机理对所得实验结果进行了分析。研究结果表明，刻蚀功率、SF6和O2流量比（F SF6/FO2）以及刻蚀时间对所制备的黑硅的结构形貌、表面反射率以及少子寿命有着非常重要的影响。最终在刻蚀功率为150W、F SF6/FO2为18sccm/6sccm和刻蚀时间为20min的条件下制备出了在400-900nm范围内表面反射率为4.94%的单晶黑硅样品，同时该样品的少子寿命为8μs。但是，RIE法制备黑硅时需要在真空环境中进行，且RIE设备昂贵，制备周期长，所以该方法不是很适合低成本产业化生产。本文选用了工艺简单、制备周期短和低成本的金属辅助化学腐蚀（Metal-assisted chemical etching, MACE）法。在现有的MACE法中所采用的辅助金属都属于贵金属，如铂（Pt）和金（Au）等。因此，本文采用更廉价的金属Ni来作为辅助金属，研究了不同工艺参数对所制备的黑硅性能的影响，同时结合实验结果分析了Ni辅助化学腐蚀法制备黑硅结构的原理。研究结果表明，腐蚀温度、腐蚀液中H2O2浓度和腐蚀时间对黑硅的结构形貌、表面反射率和光致发光性能有着非常重要的影响，经过优化工艺参数，我们制备出了表面反射率为1.59%（400-900nm）的单晶黑硅。同时，由于Ni的电负性只有1.91，只比Si（1.90）大0.01，因此，在Ni辅助化学腐蚀硅的过程中，Ni纳米颗粒从Si中得电子的速度小于腐蚀液从Ni中得电子的速度，致使Ni本身的电子被夺去，最终导致Ni纳米颗粒在辅助腐蚀过程中消失。而Pt（2.28）和Au（2.54）的电负性比Si（1.90）大更多，因此Pt和Au从Si中得电子的速度要比Ni从Si中得电子的速度快，从而可以保证自身的电子不会失去。另外，结合实验结果可以分析出，Ni辅助化学腐蚀法制备黑硅结构的过程中，其腐蚀方向倾向于<111>方向。由于上述Ni纳米颗粒是采用磁控溅射法制备的，为了降低制备成本，本文尝试了采用化学法在硅片表面制备Ni纳米颗粒。但发现化学法制备的Ni极易成膜，很难形成纳米颗粒分布在硅片表面，且其与硅片的接触不是很好，导致后期腐蚀过程中Ni膜直接脱落，无法进行辅助腐蚀。因此，本着低成本产业化的目的，本文采用全化学法的Ag辅助化学腐蚀法制备了单晶黑硅结构，研究了不同工艺参数对黑硅性能的影响规律。实验结果表明，Ag颗粒沉积时间、腐蚀液中H2O2浓度、腐蚀温度和腐蚀时间对样品的结构形貌、表面反射率、光致发光性能有着极其重要的影响，并结合实验结果分析了不同腐蚀深度和不同腐蚀孔径的硅纳米结构的减反射原理。最终经过工艺参数的优化，制备出了在400nm至900nm波长范围内平均反射率为0.98%的单晶黑硅，且发现Ag纳米颗粒优先在金字塔的顶端和四条棱边处形核和长大。为了进一步降低成本和简化工艺，本文采用一步腐蚀法并将AgNO3浓度降低为原来1/200，最终制备了400nm至900nm范围内表面反射率为1.11%的黑硅结构。目前光伏市场上主要以多晶硅太阳电池为主，主要原因是其价格相对单晶硅较便宜。同时，在表面反射率方面，制绒后的单晶硅为13%，沉积完SiNx减反射膜后仅为4%，而制绒后的多晶硅的表面反射率则大于20%，沉积完SiNx减反射膜后仍有10%。因此，多晶硅的表面反射率的可降低空间更大，又由于其价格相对较便宜，所以本文采用全化学法的Ag辅助化学腐蚀法来制备多晶黑硅结构，同时，在Ag辅助化学腐蚀法制备黑硅电池的工艺中，由于该工艺制备的腐蚀孔洞的孔径偏小，以至于后期PECVD法沉积的SiNx薄膜不能完全沉积到腐蚀孔洞底部，致使腐蚀孔洞底部出现大量的裸硅表面，最终导致光生载流子在此处被复合。因此，本文采用稀释的NaOH溶液对多晶黑硅表面进行扩孔处理，以便于腐蚀孔洞底部能被彻底钝化。研究了NaOH扩孔时间对所制备的黑硅结构形貌以及光学性能的影响规律，最终采用Ag辅助化学腐蚀法制备的多晶黑硅作为衬底硅片制备了最高光电转换效率为18.03%的多晶黑硅太阳电池，其Voc、Jsc和FF分别为632mV、36.08mA/cm2和79.07%，该效率比传统的酸制绒多晶硅太阳电池要高0.64%。更多还原

【Abstract】 As well known, the crystalline silicon solar cells take up most of photovoltaic market. Along withthe increasing requirement from human in the whole world, higher photoelectricity conversionefficiency of crystalline silicon solar cells is requested all the time. And, among all the ways forincreasing the efficiency, to reduce the reflectance of silicon surface is a much efficient one. Therefore,the concept of black silicon comes out. Black silicon is one kind of materials which can nearly absorball the light from ultraviolet to near-infrared, due to which the black silicon attracts extensive attentionfrom related fields. Also, the preparation methods and properties of black silicon become the hotresearching topics.In this thesis, we first used reactive ion etching (RIE) method with sulfur hexafluoride (SF6) andoxygen (O2) as reactive gases to prepare the black silicon on pyramid-structured single-crystallinesilicon (sc-Si) surface. Comparing with the current RIE processes, the experiments in this study werecarried out at room temperature, without pulse bias voltage and without any mask before the etching.The effects of different etching parameters on the properties of black silicon were studied. Also, weanalyzed the obtained results according to the mechanism of RIE process. The research resultsshowed that the etching power, flow ratio of SF6and O2(F SF6/FO2) as well as the etching timepresented very important effects on the morphology, surface reflectance and minority carrier lifetimeof black silicon. Finally, under the etching power of150W,F SF6/FO2of18sccm/6sccm andetching time of20min, we obtained a black silicon sample with a surface reflectance of4.94%in thewavelength range of400-900nm and this sample showed a minority carrier lifetime of8μs.However, the RIE process needs to be carried out in vacuum environment. Besides, the price of aRIE facility is too expensive and it needs a long preparation period. In consideration of thesedisadvantages of RIE process for fabricating black silicon, it is not suitable for low-costindustrialization. Therefore, we selected the metal-assisted chemical etching (MACE) method toprepare the black silicon due to its simple process, short preparation period and low cost. However,the current used metals are all noble ones, such as platinum (Pt) and gold (Au). Therefore, in order tolower the cost of MACE process for fabricating black silicon, we used nickel (Ni) instead of Pt andAu as assisted-metal. The effects of different process parameters on the properties of black siliconwere studied and the formation mechanism of black silicon by Ni-assisted chemical etching was alsoinvestigated according to the experimental results. The results showed that the etching temperature,H2O2concentration and etching time presented important effects on the morphologies, surface reflectance and PL property of black silicon. After the optimization of process parameters, a blacksc-Si with a reflectance of1.59%from400nm to900nm was obtained. Besides, according to theexperimental results, we found that Ni could be etched off during the assisted etching process, whichindicated that the transfer rate of electrons from Si to Ni is slower than that from Ni to O-. The reasonfor this phenomenon is that the electronegativity of Ni is1.91, which is just a little higher than that ofsilicon (1.90) in contrast to the phenomena happened when using higher electronegativity metal likePt (2.28) or Au (2.54) as assist metals. Additionally, it was found that the etching direction tended to<111>.On the other side, the forementioned Ni nanoparticles (Ni NPs) were fabricated by magnetronsputtering method, which is not a low-cost one. Consequently, we used chemical method to fabricatethe Ni NPs. But only Ni thin film was formed by chemical method instead of nanoparticle shape andthe Ni film fell off easily from the silicon wafer. Therefore, in order to lower the preparation cost,silver (Ag) was chosen as the assisted metal for the fabrication of black silicon on pyramid-structuredsc-Si surface and the effects of different parameters on the properties of black silicon was investigated.The results showed that the morphologies, surface reflectance and photoluminescence (PL) propertyof black silicon were greatly affected by the deposition time of Ag nanoparticles, H2O2concentration,etching temperature and etching time. In addition, the anti-reflective mechanisms of siliconnanostructures with different etching depth and different etching diameter were investigated accordingto the experimental results. Finally, after the optimization of etching parameters, black sc-Si withreflectance of0.98%from400nm to900nm was obtained. In order to lower cost further and simplifythe preparation process, we used one-step etching process and reduced the AgNO3concentration to1/200of the original one. Finally, black sc-Si with reflectance of1.11%from400nm to900nm wasobtained.In the current photovoltaic market, multi-crystalline silicon (mc-Si) solar cell takes up more thansc-Si solar cell, which is due to the lower cost of mc-Si. Besides, in the aspect of surface reflectance,pyramid-structured sc-Si is13%and4%before and after the deposition of silicon nitride (SiNx) from400nm to900nm. However, the reflectance of acid-textured mc-Si wafer is higher than20%and itstill shows10%after the deposition of SiNx. Consequently, the surface reflectance of mc-Si can bereduced much more than that of sc-Si. Therefore, in consideration of the price and reflectance, weused Ag-assisted chemical etching method to fabricate the black mc-Si solar cell. However, there wasmuch non-well covered silicon surface which was not covered with SiNx layer due to the pores withsmall diameter by Ag-assisted etching. In other words, the non-well covered silicon surface was therecombination centers of carriers. As a result, dilute sodium hydroxide solution (NaOH) was used to widen the etched pores for a better passivation of the bottom of pores. The effects of NaOH treatmenttime on the morphologies and optical property were investigated. Finally, black mc-Si solar cells withbest conversion efficiency of18.03%were fabricated by Ag-assisted chemical etching method, ofwhich the Voc，Jsc and FF were632mV,36.08mA/cm2and79.07%, respectively.And, the efficiencyof18.03%is0.64%higher than that of the traditional mc-Si solar cell.更多还原