The lead chalcogenide quantum dots with a size smaller than 3 nm and good chemical stability were difficult to synthesize due to the poor chemical reactivity of commonly used phosphine-free chalcogen and lead sources. Here we report the synthesis of small and surface passivated PbSe quantum dots (QDs) by addition of SnCl2 as the nucleation promoting agent and PbCl2 as the in situ passivation agent. Common PbCl2 (dissolved in oleylamine, oleic acid, and 1-octadecene) and elemental Se (dissolved in oleylamine) were used to synthesize small PbSe QDs by addition of SnCl2 to promote the fast nucleation of PbSe nanocrystals. The sizes of the PbSe QDs are between 3 and 8 nm and are tunable by controlling the ratio of PbCl2/SnCl2 and the reaction time. High photostability of PbSe QDs approved by the XPS analysis and photoluminescence spectra of PbSe QDs was also achieved by the in situ formation of thin halide adlayers on the surface of QDs. This new method is green with accessible raw materials and simple in situ passivation scheme and will enable the broader utilization of PbSe QDs in ambient conditions in many optoelectronic applications.
Qiulian Li,Shizhong Sun,Xinyu Li,Xiang Li,Xiu Liu,Daoyong Zhang,Na Yu,Shurong Wang
Kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) is emerging as one of the most promising materials owing to its low-cost, non-toxicity, similar electronic properties with Cu 2 (In,Ga)Se 2 (CIGS). However, the current highest power conversion efficiency (PCE) for CZTSSe device is far below the CIGS because of the high open-circuit voltage (V oc ) deficit and low fill factor (FF). Here, we fabricated CZTSSe thin film from 2-methoxyethanol solutions and investigated the effects of different Sn (Sn 2+ vs Sn 4+ ) oxidation states of precursor solution and selenization temperature on the crystal quality of the CZTSSe thin film and device performances. By characterizations and analyses, we found that incomplete redox reaction caused by the off-stoichiometric component in Sn 2+ precursor solution led to the formation of detrimental secondary phases such as SnSe, Cu 2 Se, and SnSe 2 in the CZTSSe thin film. The existence of volatile SnSe could cause the formation of voids and thus affected the FF and V oc . The reaction from Sn 4+ precursor solution to absorber material avoided the formation of SnSe. Ultimately, the Sn 4+ thin film showed better features in terms of improving crystal quality and reducing V oc deficit that limited the efficiency of kesterite CZTSSe thin film solar cells. Besides, we demonstrated that an appropriate selenization temperature could further reduce the number of secondary phases and improve the surface morphology and crystallinity of the CZTSSe thin film. As a result, we obtained CZTSSe thin film solar cells with 8.27% PCE from the Sn 4+ precursor solution under the selenization temperature of 580 °C.
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