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

Abstract

CsPbBr 3 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 CsPbBr 3 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 CsPbBr 3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm 2 /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.

Stabilization of CsPbBr3quantum dots for photocatalysis, imaging and optical sensing in water and biological medium: a review

“…CPQDs exhibit several interesting electrical and optical properties that are best suited for the utilization of solar energy like long charge carrier diffusion lengths, 140 a high absorption coefficient 141 and shallow energy states from intrinsic defects. The relative positions of the CPQD conduction band (CB) and valence band (VB) are depicted with the redox potentials in Fig.…”

Mixed Ligand Passivation as the Origin of Near-Unity Emission Quantum Yields in CsPbBr3 Nanocrystals

“…NCs are then resurfaced by exposing them to a solution of quaternary ammonium cations such as didodecyl dimethyl ammonium (DDDMA + ) whose intrinsic resistance to deprotonation results in improved ligand stability constants. , This leads to reproducible syntheses of near-unity QY NCs with excellent colloidal stability. Syntheses involving quaternary ammonium salts have therefore become the prevailing approach for producing high-QY CsPbBr 3 NCs. − …”

Fluctuations at Metal Halide Perovskite Grain Boundaries Create Transient Trap States: Machine Learning Assisted Ab Initio Analysis

“…Lead halide perovskites are extensively investigated for optoelectronic and photovoltaic applications − due to facile and cheap solution-based fabrication process, long electron–hole diffusion distances, , high charge carrier mobility, high optical absorption, and straightforward bandgap tuning, , which have led to the outstanding 25.5% power conversion efficiency of hybrid organic–inorganic perovskites (HOIPs) . Though the photovoltaic performance of the optimized HOIPs is already high, it is still below the Shockley–Queisser limit (∼33%), and manipulation and control of charge traps can help approach the efficiency limit .…”