Light-Emitting Electrochemical Cells of Single Crystal Hybrid Halide Perovskite with Vertically Aligned Carbon Nanotubes Contacts

Abstract

Based on the reported ion migration under electric field in hybrid lead halide perovskites we have developed a bright, light emitting electrochemical cell with CH3NH3PbBr3 single crystals directly grown on vertically aligned carbon nanotube (VACNT) forests as contact electrodes. Under the applied electric field, charged ions in the crystal drift and accumulate in the vicinity of the electrodes, resulting in an in operando formed p-i-n heterojunction. The decreased interface energy 2 barrier and the strong charge injection due to the CNT tip enhanced electric field, result in a bright green light emission up to 1800 cd/m 2 at room temperature (average ≈ 60 cd/m 2 ). Beyond the light emission, this original device architecture points to the possibility of implementing vertically aligned CNTs as electrodes in operationally-stable perovskite-based optoelectronic devices.

Operational stability of perovskite light emitting diodes

“…The degradation mechanisms of perovskite LEDs are divided into three categories: (i) Ion migration, (ii) electrochemical reactions and (iii) interfacial reactions, as summarized in figure 1. As the first order root cause of device degradation, ion migration inside perovskites can lead to defect migration [41][42][43][44], annihilation and creation of halide Frenkel defects [45][46][47][48][49], modification on charge injection [50][51][52][53][54] and distortion of crystal lattice [35,43,49,55] (figure 1(a)), while ion migration across the interface can lead to corrosion of electrodes [36,[56][57][58][59][60][61] (figure 1(b)). Electrochemical reactions driven by charge injection lead to a decomposition of perovskites into PbI 2 at the anode interface [35,49,62] and a formation of deep traps (Pb 0 interstitials) at the cathode interface [63] (figure 1(c)).…”

“…On the other hand, the activation energy of Pb 2+ migration has been calculated to be larger than 1.0 eV [64], which is too large to be mobile. The low activation energy of halide ion migration makes it the most likely mobile ionic species in perovskites, and the main reason for various phenomena reported, including hysteresis in PVs [72], giant switchable photovoltaic effect [73], fluorescence intermittency [74], photo-or-electric-induced enhancement in photoluminescence (PL) [45,47], polarityswitchable charge injection [50,52], and phase segregation [75,76].…”

“…Ion migration inside perovskites can lead to: (i) defect migration [41][42][43][44], (ii) annihilation and creation of Frenkel defects [45][46][47][48][49], (iii) modification on charge injection [50][51][52][53][54] and (iv) distortion of crystal lattice [35,43,49] ( figure 1(a)). In the following discussion, we will show that the first three consequences can give rise to partial reversible changes in device efficiency, while the last one can occur under high driving voltage or prolonged operation, resulting in a completely irreversible degradation.…”

“…Lastly, ion migration can modify the charge injection in perovskite LEDs, leading to a reversible change in device efficiency [50][51][52][53][54]. Studies on how ion migration affects charge injection and electroluminescence (EL) of perovskites have been reported by several groups.…”

Light-induced charge transfer at the CH3NH3PbI3/TiO2 interface—a low-temperature photo-electron paramagnetic resonance assay

“…Therefore, for serving specific purposes, their architectures required a stack of various kinds of functional thin layers. In consequence, besides MHP-based light-harvesting layers, the other most investigated elements of the stacked planar architectures were the ETLs and HTLs, as well as the device terminal electrodes [11,[17][18][19][20][21].…”