Barium disilicide as a promising thin-film photovoltaic absorber: structural, electronic, and defect properties
Barium disilicide as a promising thin-film photovoltaic absorber: structural, electronic, and defect properties
复制标题
DOI：
10.1039/c7ta08312b
复制
发表时间：
2017-12-28
影响因子：
11.9
通讯作者：
Imai, Motoharu
中科院分区：
文献类型：
--
作者：
Kumar, Mukesh;Umezawa, Naoto;Imai, Motoharu
二硅化钡（BaSi2）是一种极具潜力的薄膜太阳能电池吸收材料，其元素丰富且价格低廉。在本研究中，密度泛函理论计算表明，BaSi2属于具有四面体Si-4共价单元混合特征的Zintl相，具有（2Ba(2+)）（Si-4）(4)（-）描述的离子性质。基于电子结构对分子轨道图进行了解析，表明从Si p到Ba d的电荷转移跃迁极大地增强了光吸收。利用先进的激发态计算，包括激子效应，证实了一个大的光吸收系数。BaSi2的电离电位小于硅和锗，表明BaSi2/Si或BaSi2/ Ge的能带边缘位置适合p型电导率和II型异质结。化学计量BaSi2薄膜稳定生长的化学势窗口非常窄，在实际生长条件下研究了天然缺陷的稳定性。Si空位、Ba取代Si对位和Si间隙缺陷占主导地位，但不会导致大量载流子的产生。计算得到的费米能级被固定在整个硅化学势范围和较宽生长温度范围的带隙中间，表明双极掺杂的可行性，这有利于p-n结的制造。
Barium disilicide (BaSi2), composed of abundant and inexpensive elements, is a potential absorber material for thin-film solar cells. In this study, density-functional theory calculations show that BaSi2 belongs to a Zintl phase with a mixed character of covalent units of tetrahedral Si-4 with an ionic nature described by (2Ba(2+))(Si-4)(4)(-). The molecular orbital diagram is elucidated based on the electronic structures, suggesting that the charge transfer transition from Si p to Ba d greatly enhances optical absorption. A large photoabsorption coefficient is confirmed using advanced excited state calculations that include excitonic effects. The ionization potential of BaSi2 is smaller than that of silicon or germanium, suggesting that the band edge positions are suitable for p-type conductivity and type II heterojunctions for BaSi2/Si or BaSi2/ Ge. The chemical potential window for the stable growth of a stoichiometric BaSi2 film is very narrow, and the stability of native defects is investigated under realistic growth conditions. Si vacancy, Ba substituted for Si antisite, and Si interstitial defects are predominant but do not cause the generation of a significant number of carriers. The calculated Fermi level is pinned in the middle of the band gap for the entire silicon chemical potential range and a wide growth temperature range, indicating the feasibility of bipolar doping, which is advantageous for fabricating p-n junctions.