Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP20)
Publication date: 14th October 2019
Quantum dot solar cells (QDSCs) are considered to be one of the most promising candidates for the new-generation photovoltaics due to multiple exciton generation (MEG) in quantum dots (QDs).[1] In the MEG process, a single high energy photon can generate more than two excitons (electron-hole pairs) across the band gap, which is potentially vital for improving the photocurrent and thus the energy conversion efficiency of the solar devices. Designed surface treatment of QDs can improve the mobility of carriers and reduce the non-radiative carrier recombination in QDs solid film. In particular, in the QD solid films, the interparticle distance between the QDs can be controlled by the length of surface ligand of the QDs. It was reported that QD solid film treated with short ligands has strong electronic interaction between QDs which can improve the carrier mobility. However, to date, the deeper reason behind this result has not been revealed and the charge transfer mechanism of multiple exciton between adjacent QDs is not fully understood. Thus, the study of dynamics of multiple exciton in QD solid films is essential to realize the extraction of excitons and high performance of QDSCs based on MEG.
Here, we systematically investigate the multiple exciton charge transfer behaviour in PbS QD solid films by using ultrafast transient absorption spectroscopy.[2] We observe that multiple exciton charge transfer rate within QD ensembles is exponentially enhanced as the interparticle distance between QDs decreases. The biexciton and triexciton dissociation between adjacent QDs occur via charge transfer tunneling effect just like single exciton, and charge tunneling constants of single exciton (b1: 0.67 ± 0.02 nm-1), biexciton (b2: 0.68 ± 0.05 nm-1) and triexciton (b3: 0.71 ± 0.01 nm-1) are obtained. More importantly, for the first time, interparticle distance limit (≤ 4.3 nm) of multiple exciton charge transfer between adjacent QDs is found for extracting the multiple exciton rapidly before the occurrence of Auger recombination. This result points out a vital and necessary condition for using the multiple exciton produced in PbS QD films especially for their applications in QDSCs.
This research was supported by the Japan Science and Technology Agency (JST) PRESTO and CREST programs, Beijing Advanced Innovation Center for Future Urban Design, Beijing University of Civil Engineering and Architecture (Grant UDC2018031121), and the MEXT KAKENHI Grant (Grants 26286013, 17H02736)
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