Understanding the Passivation Mechanisms and Opto-Electronic Spectral Response in Methylammonium Lead Halide Perovskite Single Crystals

Attaining control over the surface traps in halide perovskites is critical for the tunability of ultimate device characteristics. Here, we present a study on the modulation of photophysical properties, surface traps, and recombination in MAPbI ₃ single crystals (MSCs) with methylamine (MA) vapor surface treatment. Transient photoluminescence spectroscopy in conjunction with density functional theory calculations reveals that nonradiative recombination related to Pb ²⁺ becomes mitigated after MA vaporing while radiative recombination via bimolecular path tends to increase, which originates from the passivation of Pb ions with the Lewis base nitrogen in MA. In contrast to the broad photoresponse in the pristine MSC photodiodes, application of MA surface treatments leads to a spectral narrowing effect (SNE) in MSCs with the response peak width <40 nm. On the basis of the examined photon-cycling effect with MA treatment that indicates a reduction of exciton diffusion into the interior region of MSCs, we attempt to propose an operation mechanism for the SNE which can be related to the overall stronger surface recombination and resulting severe photocarrier losses, such that the charge collection and quantum efficiency from the above-band gap absorption decrease. This work provides a facile approach with chemical means to tune the surface properties and eventual spectral selectivity in MSCs that are promising for photon-detection device applications.