Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea.
Phys Chem Chem Phys. 2012 Apr 14;14(14):4789-95. doi: 10.1039/c2cp40355b. Epub 2012 Mar 1.
The electronic band structure at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface was investigated for its potential application in Cd-free Cu(In,Ga)Se(2) thin film solar cells. Zn(1-x)Mg(x)O thin films with various Mg contents were grown by atomic layer deposition on Cu(In(0.7)Ga(0.3))Se(2) absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn(0.9)Mg(0.1)O and Zn(0.8)Mg(0.2)O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn(1-x)Mg(x)O (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn(1-x)Mg(x)O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn(1-x)Mg(x)O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn(1-x)Mg(x)O films results in the flat (0 eV) and cliff (-0.23 eV) conduction band alignments at the Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) and Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) interfaces, respectively. The experimental results suggest that the bandgap energy of Zn(1-x)Mg(x)O films is the main factor that determines the conduction band offset at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface. Based on these results, we conclude that a Zn(1-x)Mg(x)O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn(1-x)Mg(x)O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn(1-x)Mg(x)O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.
我们研究了 Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) 界面的电子能带结构，以期将其应用于无镉 Cu(In,Ga)Se(2) 薄膜太阳能电池。通过原子层沉积技术，在通过共蒸发 Cu、In、Ga 和 Se 元素源沉积的 Cu(In(0.7)Ga(0.3))Se(2) 吸收体上生长了不同 Mg 含量的 Zn(1-x)Mg(x)O 薄膜。使用 X 射线和紫外光电子能谱技术的深度剖析技术测量了价带和芯能级的电子发射。对于 Zn(0.9)Mg(0.1)O 和 Zn(0.8)Mg(0.2)O 薄膜，价带最大值约为 3.17 eV，而 CIGS 的价带最大值为 0.48 eV。因此，Zn(1-x)Mg(x)O（x = 0.1 和 x = 0.2）体区和 CIGS 体区之间的价带偏移值为 2.69 eV。考虑到界面区的小能带弯曲，在 Zn(1-x)Mg(x)O/CIGS 界面处，价带偏移值为 2.55 eV。随着 Mg 含量从 0%增加到 25%，Zn(1-x)Mg(x)O 薄膜的能带隙能量从 3.25 增加到 3.76 eV。Zn(1-x)Mg(x)O 薄膜的价带偏移值和能带隙能量的组合导致 Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) 和 Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) 界面处的平带（0 eV）和悬崖（-0.23 eV）导带对齐。实验结果表明，Zn(1-x)Mg(x)O 薄膜的能带隙能量是决定 Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) 界面处导带偏移的主要因素。基于这些结果，我们得出结论，为了在 Zn(1-x)Mg(x)O/CIGS 界面获得合适的导带偏移，需要具有较高能带隙能量的 Zn(1-x)Mg(x)O 薄膜来创建合适的导带偏移，以获得稳健的异质结。此外，ALD Zn(1-x)Mg(x)O 薄膜可以被认为是一种有前途的替代有毒 CdS 的缓冲材料，以确保环境安全。
