Our group studied the effects of conduction band offset of window/Cu(In,Ga)Se2 (CIGS) layers on CIGS-based solar cell performance. To control the conduction band offset, we considered the use of a window layer of Zn1−xMgxO thin film with a controllable band gap as an alternative to the conventional window layer using CdS film. From the measurement of valence band offset between Zn1−xMgxO/CIGS layers and the band gap of each layer, we confirmed that the conduction band offset of Zn1−xMgxO/CIGS layers could be controlled by changing the Mg content of the Zn1−xMgxO film. The CIGS-based solar cells prepared for this study consisted of an ITO/Zn1−xMgxO/CIGS/Mo/soda-lime glass structure. When the conduction band minimum of Zn1−xMgxO was higher than that of CIGS, the performance of CIGS-based solar cells with a Zn1−xMgxO window layer was equivalent to that of CIGS-based solar cells with CdS window layers. We confirmed that the control of the conduction band offset of the window/CIGS layers decreases the majority carrier recombination via the Zn1−xMgxO/CIGS interface defects.
2,4,6-trimethylphenyl)amine] (PTAA) [7] as the hole transport material (HTM). However, there is still a gap of more than 5% between the record efficiency and the maximum efficiency of thermally stable PSCs. So far, the thermal stability of perovskite absorber materials has been improved by engineering perovskite composition; both cation [8][9][10][11] and halide [10][11][12][13] composition engineering have been conducted. Also, 2D-3D incorporated perovskite materials for high thermal stability have been recently suggested. [14][15][16] On the other hand, multication approach is one of the promising ways to achieve highly efficient PSCs. [7,17] However, at the same time, it is more and more difficult to understand and control such a large number of components for highly efficient and highly stable PSCs. In case of our previous study for quadruple cation PSCs, [7] seven components of formamidinium (FA), methylammonium (MA), Cs, Rb, Pb, I, and Br should be controlled. Furthermore, not only effect of single element but also combination effect of these elements should be important. Thus, careful studies are required for further development. Indeed, even though more and more studies have been reported on multication approach, [18][19][20][21] there are still no reports of PSCs that meets international standard (IEC 61215); 85 °C/85% relative humidity (RH) stress test with high efficiency. Here we focus on the dependence of device thermal stability on perovskite composition by using state-of-the-art highly efficient PSCs (≈20%) to achieve compatibility of high efficiency and high thermal stability.We tested the thermal stability of devices with a structure of indium tin oxide (ITO)/compact titanium dioxide (TiO 2 )/ mesoporous TiO 2 /perovskite/PTAA/gold (Au). The starting perovskite composition used here was Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb (I 0.83 Br 0.17 ) 3 because this composition has been reported to display high efficiency and high operational stability. [17] We chose PTAA as the HTM because its high efficiency [22][23][24] and high thermal stability [7] have been reported respectively. In addition, we used a low doping concentration of 3 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in PTAA in this experiment to minimize any adverse effects of the additives on the device thermal stability reported so far. [25][26][27] First, we examined the relationship between efficiency and thermal stability for devices with different PbI 2 ratios in the Perovskite solar cells have received great attention because of their rapid progress in efficiency, with a present certified highest efficiency of 23.3%. Achieving both high efficiency and high thermal stability is one of the biggest challenges currently limiting perovskite solar cells because devices displaying stability at high temperature frequently suffer from a marked decrease of efficiency. In this report, the relationship between perovskite composition and device thermal stability is examined. It is revealed that Rb can suppress the growth of PbI 2 even un...
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