AlkaCIGS - Effects of Alkali elements in Cu(In,Ga)Se2 thin films and solar cells

碱金属元素在多晶Cu（In，Ga）Se2（CIGS）薄膜太阳电池中起着重要作用。钠（Na）对CIGS太阳能电池性能的积极影响是通过使用钠钙玻璃代替无钠基板偶然发现的。后来钾（K）似乎是细胞效率的进一步助推器。目前，较重的碱金属元素铷（Rb）和铯（Cs）已被确定为提高性能的进一步候选物。已经做了许多实验来找出Na主要作用于晶界还是在晶粒的本体中或两者。原子探针断层扫描（APT）的测量结果表明，钠是内部的晶粒，以及在晶界，在那里它强烈偏析。但到目前为止，还不清楚是否钝化的晶界是钠的主要效果，或如果增加的p掺杂的晶粒体中的钠是主要的效果。Na和其他碱元素对CIGS缺陷物理学的影响也是未知的，因为在大多数以前的实验中，在Na存在下生长的CIGS层与没有Na生长的层进行了比较。因此，Na对CIGS层生长过程中缺陷形成的影响不能与其对光电性能的影响分开。通过在CIGS生长完成后用碱元素对CIGS层进行后沉积处理，如本项目所实现的，碱对CIGS层和太阳能电池的缺陷物理和光电特性的影响可以清楚地识别。该项目的目的是更好地了解钠和其他碱金属对CIGS层和太阳能电池性能的积极影响，并研究不同碱金属元素之间的相互作用。这包括区分晶界、晶粒体积和界面效应的任务。更好地理解CIGS中碱金属元素的作用有助于进一步提高电池效率。

Alkali elements play an important role in polycrystalline Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The positive effect of sodium (Na) on CIGS solar cell performance was found by accident by using soda lime glass instead of sodium free substrates. Later potassium (K) appeared as a further booster for cell efficiency. Currentliy, the heavier alkali elements rubidium (Rb) and cesium (Cs) have been identified as further candidates to increase the performance. Many experiments have been done to find out if Na mainly acts at the grain boundaries or in the bulk of the grains or both. Atomic Probe Tomography (APT) measurements revealed that Na is inside the grains as well as in the grain boundaries, where it strongly segregates. But so far it is unclear whether passivation of grain boundaries is the dominating effect of Na or if the increase of p doping in the grain bulk by Na is the main effect. The effect of Na and other alkali elements on the defect physics of CIGS is also unknown, since in most former experiments CIGS layers grown in the presence of Na were compared to layers grown without Na. Therefore, the effect of Na on the formation of defects during the CIGS layer growth could not be separated from its effect on opto-electronic properties. By post deposition treatment of CIGS layers with alkali elements after CIGS growth is finished, as to be realized in this project, the effect of alkalis on defect physics and opto-electronic properties of CIGS layers and solar cells can be clearly identified. The aim of the project is to better understand the positive effect of sodium and other alkali metals on the CIGS layer and solar cell properties and to investigate the interaction between the different alkali elements. This includes a task to distinguish between grain boundary, grain bulk and interface effects. The better fundamental understanding of the effects of alkali elements in CIGS could help to further improve cell efficiency.