Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Publication date: 23rd October 2018
Colloidal quantum dots (CQDs) solar cells have attracted much attention as a candidate of next generation solar cells [1]. The CQDs are covered with oleic acid (OA) ligands after preparation, which are so long that they hinder the carrier transfer in the CQDs film. Therefore, we need to exchange the OA ligands with short ligands. For the ligand exchange, salts which provide ligands are used. Recent studies reported that these salts affected alrgely the photovoltaic properties of CQDs solar cells[2]. However, it is unclear why and how the salts affect the photovoltaic properties. Here we investigate the mechanism of this phenomenon. Previous studies have reported that iodide salts with high acidity increased removal rate of the OA in the PbS CQDs and increased the carrier mobility in PbS CQDs film[3]. So, it is expected that the acidity of the iodide salts can also affect the photovoltaic properties. In this study, PbS QDs films were prepared by using different ligands of methylammonium iodide (MAI), ammonium iodide (NH4I), tetrabutylammonium iodide (TBAI) and 1-Ethyl-3-methylimidazolium iodide (EMII). TBAI and EMII have lower acidity than NH4I and MAI. The PCE of the PbS CQDs solar cells treated with MAI (1.5%) and NH4I (1.3%) were lower than those treated with TBAI (5.4%) and EMII (7.6%). This result suggests that the acidity of the iodide salts affects the photovoltaic properties. The carrier lifetimes in the CQD films treated with MAI and NH4I were about one third of those in the CQD film treated with EMII and TBAI. The decrease in carrier lifetimes indicates an increase of charge recombination probability. Therefore, it was suggested that iodide salts with high acidity induce defects in the PbS CQDs films. The Pb amount in the PbS CQDs film treated with MAI and NH4I became smaller than the amount of those treated with TBAI and EMII. This result shows that the iodide salts with high acidity remove more Pb atoms from the surface of the CQDs, which result in more surface defects on the CQDs, the reduction of the carrier lifetime in the CQDs film and decrease in PCE of the PbS CQDs solar cells. Based on the above results, it is suggested that the acidity of the iodide salts is one key property for the effects on the physical properties of the CQDs films and the PCEs of the CQDs solar cells.
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