Edge-Up Oriented Colloidal CdSe Nanoplatelets Facilitate Faster Response in Vertical Photodetectors

Integrating self-assembled colloidal nanocrystals with uniform orientation into optoelectronic devices may allow for the significant improvement of charge transport processes. In this work, for this purpose, using the liquid-air interface self-assembly technique and selectively controlling the orientation of CdSe nanoplatelets (NPLs) with short 2-ethylhexane-1-thiol (EHT) ligands in a vertical-configuration photodetector device, we show that the photoconductivity response is substantially improved by the selective arrangement of CdSe NPLs into two distinct assemblies of edge-up (EO) and face-down (FO) orientations compared to the randomly oriented (RO) NPL film deposited by spin-coating. Our devices reveal that EO nanoplatelets significantly enhance response speed, while RO yields higher photocurrent, responsivity and detectivity. The assembled devices, consisting of one-monolayer EO and three-monolayer FO NPL films of comparable vertical film thickness, demonstrate superior photocurrent responses of 8 ms/11.3, 13 ms/4.5, and 15.2 ms for the rise/decay time constants, respectively, compared to 17 ms/18, and 80 ms for the RO for the rise/decay time constants. Despite using only a monolayer of EO NPLs, we achieved a responsivity of 21.04 mAW^-1 and a detectivity of 5.77 × 10¹⁰ Jones, compared with the best results from CdSe-based photodetectors reported in the literature. This work provides critical insight for charge transportation management in solution-processed photodetection devices by adjusting the orientation of the two-dimensional quantum structures, paving the way toward fast and atomically thin functional optoelectronic devices. This also demonstrates that facet-specific metal-semiconductor interfaces are another critical factor, in addition to the charge transportation pathway, which can modulate the interfacial electronic structure and recombination dynamics in vertical configuration photodetectors.