【作者】 张伟英；

【导师】 傅竹西；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2008， 博士

【摘要】 ZnO是一种直接带隙的宽禁带半导体,在常温下的禁带宽度为3.37eV,由于其在可见光区域有很高的透过率,因此可以成为替代ITO和SnO₂等传统透明导电氧化物薄膜（TCO）的新一代材料;因此ZnO薄膜在太阳能电池中的广泛应用是作为透明窗口。此外,ZnO在非故意掺杂下能易形成电子浓度较高的n型材料,并且很容易通过掺杂使电子浓度达到10²⁰/cm³以上。结合以上两个特点,作者主要抓住ZnO薄膜在异质结光电转换过程中所起的作用进行一些研究探索。本论文的主要工作是研究ZnO/Si异质结的光电转换特性,其主要内容如下:第一章综述了ZnO薄膜材料的基本性质,并着重介绍了太阳能电池的发展概况及ZnO薄膜在这个领域的应用研究状况,指出了本论文的中心思想。第二章详细讲述了用直流反应溅射方法制各ZnO薄膜的实验过程并对薄膜的基本性质进行了相关的表征。在n型Si（100）衬底上制备出了高度沿c轴取向表面粗糙度较小的强紫外发射ZnO薄膜;并通过薄膜的拉曼散射谱分析得出薄膜中沿c轴方向存在张应力。第三章研究了各种生长条件对ZnO/Si异质结光电转换特性的影响,包括生长温度、生长气氛和生长时间。结果表明随着生长温度的提高,光生电流和光生电压都是迅速增大,这主要是和ZnO薄膜的结晶质量密切相关。随着生长气氛中氧含量的变化,ZnO/Si异质结的光生电压基本维持不变;随氧含量的增加,光生电流先是增加,当氧含量达到一定值（22.2%）时,而后又迅速的下降。这个过程主要因为生长过程中的氧含量影响了ZnO薄膜的本征缺陷浓度。此外对于生长时间的影响,光生电压也是基本维持不变,随着生长时间的增加,光生电流同样是先增加而后减小。当薄膜的厚度较小时,影响异质结电输运性质的主要因素是薄膜的结晶质量;当薄膜的厚度大到一定程度后,少子的扩散长度成为影响光电流的主要因素。第四章通过测量ZnO薄膜的光电流响应谱和异质结的光电压的响应谱,分析了ZnO薄膜的禁带宽度和ZnO薄膜光电流光谱响应的关系;ZnO:Al/Si异质结的光电压响应谱的测量发现在可见光照射条件下异质结两侧所产生的光电压方向相反,这可能是这种简单异质结电池光电转换效率比较低的原因。提出了解决问题的方法,可以通过后期退火以及加过渡层的方法来改善。第五章研究了各种退火条件对ZnO/Si异质结光电转换特性的影响,包括退火温度、退火气氛和退火时间。通过改变退火温度,我们发现400℃退火能够很好的改善异质结的光电转换特性,光生电流比没有退火前的增加了2倍多;但同时也发现当退火温度大于500℃时,异质结的光生电压和光生电流都大幅度的下降,直至减小至0。对于退火气氛对异质结光生伏特效应的影响,实验结果表明N₂气退火后异质结的光电转换得到大幅度的提高,air气退火次之,O₂气退火提高的最小。退火时间实验表明其改变对异质结的光电转换特性影响不大。更多还原

【Abstract】 ZnO is a wide band semiconductor material with a band gap of 3.37eV at room temperature, it is considered to be a new transparent conductive oxide instead of ITO and SnO₂ because of its high transmittance in visible light region. Otherwise, native ZnO films demonstrate n type conduction resulting from its intrinsic defects （such as V_o and O_i）, the electron concentration of n type ZnO can easily arrive over 10²⁰/cm³ by doping. Considering these research background, the main work of this thesis is to study photovoltaic effect of ZnO/Si heterojunction. In this case, the contents of the dissertation are listed as follows:In chapter one, a comprehensive review was given on ZnO in the crystal structure, basic physical and chemical properties, optical and electrical properties; secondly introduce the method of preparation ZnO films briefly, such as sputtering, pulse laser deposition, chemical vapor deposition et al. Finally, introduction of the application of ZnO films in solar cells was given, indicating the core of the thesis.In chapter two, ZnO film has been prepared by DC reactive sputtering on n type Si（100） substrate. X-ray diffraction and morphology studies revealed that the films demonstrated a strain stress in c axis, which is consistent with the result obtained from Raman scattering. A strong ultraviolet emission with no distinct visible emissions was observed in room temperature photo luminescence （PL） spectrum.In chapter three, influence of growth conditions on the photovoltaic property of ZnO/Si heteroj unction had been investigated, including substrate temperature, content of oxygen, growth time. The results demonstrated that the photo current and photo voltage are increase rapidly with the growth temperature, which was connected closely with the crystal quality of ZnO films; the effect of Ar/O₂ ratio on the photovoltaic property was also discussed related to the concentration of intrinsic defects in ZnO films. The photo voltage maintained 350mv with the change of oxygen partial pressure, and photo current initially increased with it, and then decreased sharply. When the Ar/O₂ ratio was 7/2, the short circuit current reached its maximum as 700μA. Open circuit voltage and short circuit current with the ZnO films thickness were investigated. Open circuit voltage remained almost constant at～320 mv with the thickness of ZnO films. Short circuit current reached its maximum as 626μA when the film thickness is 150nm. The mechanism of photovoltaic effect was also discussed.In chapter four, the photo current response spectrum of ZnO films and photo voltage response spectrum of ZnO: Al/Si heterojunction were investigated to analyzed the mechanism of photovoltaic effect. The result demonstrated that the direction of photovoltage generated in ZnO and Si was converse, this was the possible reason for the low photovoltaic efficiency of this simple structure solar cell. We also put forward the method to solve the problem,In chapter five, influence of annealing condition on the photovoltaic effect of ZnO/Si heterojunction was investigated, including annealing temperature, annealing atmosphere, annealing time. Photovoltaic effect was improved when the heterojunction was annealed in N₂ 400℃, the photocurrent increased by two magnitudes than that of unannealed. Conversly, when it was annealed over 500℃, the photovoltage and photocurrent decreased sharply, to Zero at last. For annealing atmosphere, the photovoltaic effect was improved greatly in N₂, subsequently air, finally O₂. Influence of photovoltaic effect by annealing time was not obvious.更多还原