Abstract-Time-resolved terahertz spectroscopy reveals the influence of charged sensitizing quantum dots on the electron dynamics in ZnO

Sesha Bamini N.,^ab   Hynek Němec,^c   Karel Žídek,^a  Mohamed Abdellah,^ad   Mohammed J. Al-Marri,^e  Pavel Chábera,^a   Carlito Ponseca,^a   Kaibo Zheng*^ae and  Tönu Pullerits*^a  

http://pubs.rsc.org/en/content/articlelanding/2017/cp/c6cp07509f#!divAbstract

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Corresponding authors

^a

Department of Chemical Physics and NanoLund, Lund University, Box 124, 22100 Lund, Sweden
 E-mail: kaibo.zheng@chemphys.lu.se,tonu.pullerits@chemphys.lu.se

^b

National Center for Ultrafast Processes, University of Madras, Chennai 600113, India

^c

Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 18221 Prague, Czech Republic

^d

Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt

^e

Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

Photoinitiated charge carrier dynamics in ZnO nanoparticles sensitized by CdSe quantum dots is studied using transient absorption spectroscopy and time-resolved terahertz spectroscopy. The evolution of the transient spectra shows that electron injection occurs in a two-step process, where the formation of a charge transfer state (occurring in several picoseconds) is followed by its dissociation within tens of picoseconds. The photoconductivity of electrons injected into the ZnO nanoparticles is lower than that of charges photogenerated directly in ZnO. We conclude that the motion of injected electrons in ZnO nanoparticles is strongly influenced by their interaction with positive charges left in the sensitizing quantum dots.

Abstract-Ultrafast Dynamics of Hole Injection and Recombination in Organo Metal Halide Perovskite Using Nickel Oxide as p-Type Contact Elec-Trode

Alice Corani, Ming-Hsien Li, Po-Shen Shen, Peter Chen, Tzung-Fang Guo, Amal El Nahhas, Kaibo Zheng, Arkady Yartsev, Villy Sundstrom, and Carlito S. Ponseca

J. Phys. Chem. Lett., Just Accepted Manuscript

DOI: 10.1021/acs.jpclett.6b00238

Publication Date (Web): March 4, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.6b00238?journalCode=jpclcd

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organo-metal halide perovskite based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the sub-picosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of ps to few ns. Due to the low conduc-tivity of NiO(np), holes are pinned at the interface and it is electrons that determines the recombination rate. This recom-bination competes with charge collection and therefore must be minimized. Doping NiO that promotes higher mobility of holes is desirable in order to prevent back recombination.