All‐Inorganic Perovskite Nanocrystals with Remarkably Enhanced Optoelectronic Properties Realized by an Alkene‐Free Solvent Strategy and Their Electroluminescence

Abstract

highly luminescent CsPbX 3 PeNCs are widely prepared through hot-injection methods using long-chain hydrocarbons oleic acid (OA) and oleylamine (OAm) as the surface capped ligands and octadecene (ODE) as the high-temperature reaction medium. [4][5][6] These long-chain hydrocarbons will significantly impact the optical and electrical properties of PeNCs, thereby determining the performance of perovskite LEDs (PeLEDs).The abundant organic ligands can effectively passivate the surface defects of PeNCs and provide colloidal stability in the solutions; however, they also form an insulating layer when transforming into solid films, which is deleterious to the injection of carriers into PeNCs emissive layers in PeLEDs. [7,8] Many efforts have been devoted to enhancing the carrier transport capabilities of PeNCs via resurfacing, ligand exchange, and/or surface ligand density control for improving the electroluminescence (EL) performance of the perovskite NCs based devices. [9][10][11] Zeng et al. regulated the antisolvent and optimized the purification cycles to control the ligand density, which realized 50-fold efficiency improvement. [10] Li and coworkers developed the combined method of metal salt treatment and short-chain ligand exchange to improve the carrier transport of PeNCs. [11] Sargent et al. recently achieved a bipolar shell to replace the OAm ligand for increasing the carrier mobility and reducing the trap density, which remarkably enhanced the external quantum efficiency (EQE) of PeLEDs to 22%. [9] Nonetheless, the effects of the dielectric alkene-based solvents on the optoelectric properties of PeNCs and thus, the performance of the PeLEDs is still unrevealed.Given the ionic nature of perovskites, ethyl acetate (EtOAc) with moderate polarity is usually adopted to purify PeNCs for maintaining their structural integrity and excellent optical properties. [12] However, the optoelectric properties of PeNCs will suffer from the residual ODE molecules, which lead to the formation of insulating barrier for injection and transport of carriers. Following the similarity solution principle, [13] it is difficult for the ODE molecules to be removed completely due to their incompatible with EtOAc, and they exist on the surface Hot-injection methods are widely used for the synthesis of colloidal perovskite nanocrystals (PeNCs). However, the long-chain hydrocarbon solvents with alkene groups, such as octadecene (ODE), cannot be removed completely due to the strong van der Waals force between ODE and surface ligands as well as the large difference in the polarity with ethyl acetate, which hinders the charge transport characteristic of PeNCs and thus reduces the performance of PeNC-based light-emitting diodes (PeLEDs). Herein, an alkene-free solvent-assisted strategy is reported to synthesize high-quality CsPbBr 3 PeNCs in toluene and it is found that the alkene-free solvent can significantly influence the morphology, composition, and optoelectronic properties of PeNCs. Compared to the traditional ODE solvent with alkene gro...

Molecular design for perovskite solar cells

“…Thirdly, solvent engineering is considered as another consideration to molecular engineer the halide perovskite materials. The solvent‐based molecular engineering should consider how to add, remove, exchange, coordinate, and react solvent during the perovskite formation stage process 108‐110 . The following types of solvent molecules have been investigated to tailor for specific tasks and their molecular origin that gives rise to the final performance has been studied.…”