- 건국대학교

	Prof. Man-Jong Lee Reported Unprecedented and Novel Analysis Method for the Investigation of Quantum Dots Solar Cells
	관리자		528		2019.05.27

<img style="margin-left: auto; display: block; margin-right: auto;" alt="" src="ArticleFile.do?id=11763883" /> Professor Man-Jong Lee (Department of Chemistry, Konkuk University) is studying quantum dot solar cells (QDSCs) that are theoretically estimated to have the highest power conversion efficiency (PCE). The co-first authors, Kyungeun Jung (Ph.D. candidate) and Jeongwon Lee (MS graduate) analyzed the PCE enhancing mechanism of Mn-doped CdS/CdSe based QDSCs by a new method employing the internal nano-strain relaxation of QD nano-composites, which is completely different with the conventional viewpoint of electro-optical analysis. This interpretation is approved and published in the online edition of ‘Composites Science and Technology (May 2019)’, the 1st ranked international journal in the field of ‘Materials science, composites’. This paper is published in July (vol. 179 (2019) 79-87). (Title: Effect of manganese dopants on defects, nano-strain, and photovoltaic performance of Mn–CdS/CdSe nanocomposite-sensitized ZnO nanowire solar cells) DOI) doi.org/10.1016/j.compscitech.2019.05.002 When doping Mn into the CdS quantum dots, the conventional analytical method for the improvement of PCE is interpreted as an effect of suppressing recombination by forming a mid-band (4T1−6A1) due to the d orbital of Mn, but no evidence thereof is presented. In other words, for the first time, through the quantitative calculation of the nanoscale stress field in Mn-doped quantum dots and Mn-free quantum dots, we suggested that relaxation of the nano-stress field is the main cause of efficiency improvement. In addition, the main cause of the relaxation of the nano-stress field is the ion exchange triggered by the difference of the solubility product (ksp) when Mn is added, and it alleviates the stress by eliminating the defects inside the QD complex, which improved the PCE. This calculation is very complicated. Briefly, phase (P, phase) and displacement (u, displacement) are extracted using Pg(r) = -2pgᆞu(r) from a high-resolution electron microscope (HRTEM) image (quantum dot atomic image), and strain can be calculated by the partial differentiation of the displacement (Fig. 1).
<img style="margin-left: auto; display: block; margin-right: auto;" alt="" src="ArticleFile.do?id=11763880" width="626" height="401" /> 
Fig. 1. Calculation results using geometric phase analysis (GPA)   Fig. 2a and b are ZnO nanowires acting as solar cell electrodes. In Fig. 2c, the addition of Mn significantly increases the quantum efficiency. In order to understand these two-dimensional results (Fig. 1) in a simple one-dimensional manner, we indicated the spatial distribution of the internal nano-strain of the QD composites, and the results are shown in Fig. 2c. In this study, it is demonstrated that the reduction of the nanoscale stress is one of the important causes as shown in Figs. 2c and d. This work was support by KU research fund. <img style="margin-left: auto; display: block; margin-right: auto;" alt="" src="ArticleFile.do?id=11763881" width="653" height="453" /> 
Fig. 2. (a, b) ZnO NWs acting as photoanode, (c) Quantum efficiencies, (d, e) 1D strain distribution


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