Elucidating the role of two-dimensional cations in green perovskite light emitting diodes

Perovskite light emitting diodes (PeLEDs) have emerged as promising candidates for applications requiring visible and near-infrared emission. Lead-based quasi-2D perovskite materials are commonly employed as an emissive layer for PeLEDs as they promote radiative emission via spatially confining excitons and funnelling of charge carriers. Designing efficient emissive layers based on these quasi-2D perovskites requires that the ratio between inorganic [PbX6]4-component and the large organic cations (spacers) is carefully optimised. In this work, we demonstrate the importance of such compositional tuning using three different 2D cations namely phenylethylammonium (PEA), its monofluorinated analogue FPEA and a custom-made bulkier cation BPEA containing an extra phenyl ring. Our results show that the tuning of the ratio between 2D cation and the [PbX6]4-provides a trade-off between electrical transport in the device and the emission properties of the emissive layer. Generally, a large excess of cations is required to enhance the external quantum efficiency (EQE) of PeLEDs. Among the various cations, FPEA leads to PeLEDs with the highest EQE up to 7.7%, while BPEA resulted in the smallest EQE. Atomic force microscopy, space-charge-limited current, steady-state and time-resolved photo-luminescence results revealed that FPEA-based perovskite have lower number of pinholes and traps compared to PEA, while it is also likely to have better charge transport properties due to its smaller size in contrast to BPEA. Our results provide guidelines regarding the multiple functions an organic spacer needs to fulfil for fabricating efficient PeLEDs.