Effective and Noneffective Recombination Center Defects in Cu2ZnSnS4: Significant Difference in Carrier Capture Cross Sections
Effective and Noneffective Recombination Center Defects in Cu2ZnSnS4: Significant Difference in Carrier Capture Cross Sections
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Cu2ZnSnS4 中有效和无效复合中心缺陷：载流子捕获截面的显着差异
DOI：
10.1021/acs.chemmater.8b03933
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发表时间：
2019-02-12
影响因子：
8.6
通讯作者：
Wei, Su-Huai
中科院分区：
文献类型：
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作者：
Li, Jiqiang;Yuan, Zhen-Kun;Wei, Su-Huai
结合电子-声子耦合效应和静态耦合形式，通过第一性原理方法计算了Cu 2 ZnSnS 4中三种可能的非辐射复合中心（NRRC）缺陷的载流子俘获截面。这些值目前是不可用的，但关键的了解少数载流子寿命的限制因素和模拟的光伏器件。我们发现Sn-Zn（2+）俘获一个电子（a（+2/+1）跃迁）和[Cu-Zn-Sn-Zn]（+）俘获一个电子（a（+1/0）跃迁）的截面都很大，而Sn-Zn（+）俘获一个电子（a（+1/0）跃迁）的截面要小几个数量级.当Sn-Zn（2+）和[Cu-Zn-Sn-Zn]（+）的浓度大于10（15）cm（-3）时，少数载流子寿命小于1 ns，是有效的非负阻效应，而当Sn-Zn（+）浓度相同时，少数载流子寿命可达10 μ s，是无效的非负阻效应。声子模分析表明，该截面与缺陷周围Sn-S键的振动模式及其与缺陷态局域波函数的耦合密切相关。Sn-Zn（2+）和[Cu-Zn-Sn-Zn]（+）具有短而强的Sn-S键，具有高频振动模式，并且这两种缺陷在捕获电子后经历大的结构畸变，这降低了载流子捕获的势垒，从而产生大的截面。相比之下，Sn-Zn（+）具有较软的Sn-S振动模式，因此具有高得多的电子捕获势垒。我们的计算不仅确定了两个有效的NRRCs，这提供了为什么贫铜，富锌，贫锡的生长条件，被广泛采用，用于制造高效率的Cu 2 ZnSnS 4太阳能电池背后的机制，但也表明，一个非常大的差异可以存在于载流子捕获截面相同的缺陷在不同的电荷状态（Sn-Zn（2+）与Sn-Zn（+））。我们提出，深能级缺陷可能有大的载流子俘获截面，如果它们被强键包围，并在俘获载流子后经历相当大的结构弛豫，这可以作为一个经验标准，用于快速识别有效的NRRC。
By combining the electron-phonon coupling effect and the static coupling formalism, we calculate, through the first-principles methods, the carrier capture cross sections of the three possible nonradiative recombination center (NRRC) defects in Cu2ZnSnS4. These values are currently unavailable but critical for understanding the limiting factors of the minority carrier lifetime and simulating the photovoltaic devices. We show that the cross sections for Sn-Zn(2+) capturing one electron (a (+2/+1) transition) and for [Cu-Zn-Sn-Zn](+) capturing one electron (a (+1/0) transition) are both very large, whereas for Sn-Zn(+) capturing one electron (a (+1/0) transition) is much smaller by several orders of magnitude. The minority carrier lifetime will be limited to below 1 ns if the concentrations of Sn-Zn(2+) and [Cu-Zn-Sn-Zn](+) are higher than 10(15) cm(-3), so they are effective NRRCs, whereas the lifetime can be as long as 10 mu s with the same concentration of Sn-Zn(+), so Sn-Zn(+) is a noneffective NRRC. The phonon mode analysis shows that the cross section is strongly correlated with the vibration mode of Sn-S bonds around the defects and its coupling with the localized wavefunction on the defect state. Sn-Zn(2+) and [Cu-Zn-Sn-Zn](+) have a short and strong Sn-S bond with a high-frequency vibration mode, and these two defects undergo a large structural distortion after capturing an electron, which decreases the barrier for carrier capture and thus produces a large cross section. In contrast, Sn-Zn(+) has a softer Sn-S vibration mode and thus much higher barrier for electron capture. Our calculations not only identify two effective NRRCs, which provide the mechanism behind why the Cu-poor, Zn-rich, Sn-poor growth condition, were widely adopted for fabricating high-efficiency Cu2ZnSnS4 solar cells but also show that a very large difference can exist in the carrier capture cross sections for the same defect in different charge states (Sn-Zn(2+) vs Sn-Zn(+)). We propose that the deep-level defects may have large carrier capture cross sections if they are surrounded by strong bonds and undergo considerable structural relaxations after capturing a carrier, which can be used as an empirical criterion for the quick identification of effective NRRCs.