The surface-dominant nature endows colloidal quantum dots (CQDs) extreme sensitivity towards ambient chemical environments. It has been widely observed that the CQD surface hydroxylation is strongly linked to the trapping behavior in CQD films, which, to date, is a widely concerned issue in building high-efficient CQD photovoltaics. However, the trapping mechanism is still unclear. The strong bonding between surface hydroxyls and atomic Pb on the polar facet fails to explain the hydroxyl-related trapping mechanism by the traditional non-stoichiometry issue in CQDs, namely, the formation of surface vacancies. Thus, it remains an urgent challenge to understand the hydroxyl-related trap mechanism and its effect on the performance of CQD electronics.
Recently, the research group of Prof. Wanli Ma, collaborated with Prof. Zeke Liu found that H-bonded water is strongly adsorbed on the partially hydroxylated surface of PbS CQDs, as shown in Figure a. The adsorbed water could govern the temperature-dependent change of CQD surface chemistry through water desorption, hydroxylation, and dehydration process. More importantly, the transmission electron microscopy (TEM) in Figure b-f and the molecular dynamic (MD) simulations in Figure g-h suggest that the water molecules could potentially work as H-bonded bridges between CQD, promoting the subsequent CQD close attachment and fusion. The CQD fusion triggered by ambient water leads to inter-band traps in atomic passivated CQD, significantly enhances the Stokes shift and reduces the carrier lifetime in CQD solids. We found that the deposition technique of CQD solids has a critical impact on the film stabilities under humid ambient. The CQD film achieved by the meniscus-guided coating technique (MGC) shows more compact film morphology, which improves the device performance and, at the same time, suppress the negative effect of ambient water on solar cell performance. We believe our works not only elucidate hydroxyl involved CQD surface chemistry, but also achieve a comprehensive understanding of the impact of ambient water on CQD solar cells, which may provide general insight into the practical manufacturing of stable CQD based electronics under ambient conditions.
The first author, Dr. Guozheng Shi, is from FUNSOM, Soochow University.
We cooperated with two scientists in FUNSOM, Prof. Steffen Duhm, and Prof. Liang Zhang. The XPS and XAS results provide experimental evidences for the water adsorption on CQD surfaces. We surprisingly found that under humid ambient with gentle heating, the CQD significantly fused with each other. This randomly fused CQD with large sizes shows less CQD confinement and thus small bandgaps, which lead to optically sensitive traps. Eventually, Prof. Lu Wang and Prof. Shiyun Xiong in FUNSOM performed critical DFT calculations and molecular dynamics to depict the fusion mechanism.
Link to Paper: https://www.nature.com/articles/s41467-021-24614-7.pdf
Link to Prof. Ma’s Group: http://funsom.suda.edu.cn/7f/97/c2735a32663/page.htm
Acknowledgements: This work was supported by the National Key Research Projects (Grant No. 2016YFA0202402), the National Natural Science Foundation of China (Grant Nos. 22161142003, 61911530158, 52002260 and 51803144), the Natural Science Foundation of Jiangsu Province of China (BK20200872). This work is supported by Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project. The authors would like to thank SSRF (beamline 02B02) and TLS (beamline 20A1) for the allocation of synchrotron beamtime. This research also made use of resources at the Advanced Photon Source and the Center for Nanoscale Materials at Argonne National Laboratory, which was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. T. L. thanks the NIU start-up fund for support. H.W and T.K thank the Japan Society for the Promotion of Science (JSPS) for a Grant-in-Aid for Scientific Research (No. 16H03824 and 19H02534). Q. S. would like to thank the support of the MEXT KAKENHI (Grants 17H02736 and 20H02565). T. S. would like to thank the support of the MEXT KAKENHI (Grants 17H03536).
Editor: Danting Xiang