Charge Carrier Diffusion Dynamics in Multisized Quaternary Alkylammonium-Capped CsPbBr3 Perovskite Nanocrystal Solids.

Sol Gutiérrez Álvarez,Weihua Lin,Jie Meng,Tõnu Pullerits,Karel Žídek,Mohamed Abdellah,Kaibo Zheng

doi:10.1021/acsami.1c11676

Abstract

CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid (OA) and oleylamine (OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be ∼239 ± 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm2/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.

Highlights

CsPbBr3 colloidal perovskite quantum dots (QDs) have recently attracted great interest to the research community due to their outstanding optoelectronic properties.[1,2] The facile solution-processed synthesis, size-tunable optical bandgap, and near-unity photoluminescence quantum yields (PLQYs), together with narrow emission bandwidth, render these materials great potential in different applications such as photodetectors, solar cells, and especially light-emitting diodes (LEDs).[1,3−6]Despite substantial materials and device engineering, the transport of charge carriers in QD solids remains a challenge for LED applications.[6]
We studied the electron and hole transport dynamics in densely packed QD films using two different sizes of quaternary alkylammonium-capped CsPbBr3 QD mixtures via the transient absorption (TA) spectroscopy analysis
Using the Singular value decomposition (SVD) analysis, we reveal that the photoinduced electrons and holes are less mobile in small-sized QD-4, whereas they are more freely diffused within QD-10 clusters and recombine with charges in QD-4 at the interface

Summary

Introduction

CsPbBr3 colloidal perovskite quantum dots (QDs) have recently attracted great interest to the research community due to their outstanding optoelectronic properties.[1,2] The facile solution-processed synthesis, size-tunable optical bandgap, and near-unity photoluminescence quantum yields (PLQYs), together with narrow emission bandwidth, render these materials great potential in different applications such as photodetectors, solar cells, and especially light-emitting diodes (LEDs).[1,3−6]Despite substantial materials and device engineering, the transport of charge carriers in QD solids remains a challenge for LED applications.[6]. One promising solution is to replace them with smaller capping ligands, which has been done with molecules like octylamine[7] or di-dodecyl dimethyl ammonium bromide (DDAB).[8,9] DDAB is a quaternary alkylammonium with a halide ion pair that provides strong anchoring to the QD surface This leads to superior surface passivation with an enhanced PLQY.[10,11] In addition, shorter alkyl chains of DDAB compared with OA and OLA are believed to facilitate the charge carrier transport.[12,13] Significant improvement in LED external quantum efficiency (EQE) using DDAB-capped QDs has been reported recently, with the recording value reaching 13.4%.14,15. A systematic characterization of the charge carrier transport dynamics in a DDAB-capping QD solid film is of great importance to rationalize the underlying mechanism of the optoelectronic devices

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Conclusion