Manipulating Electronic Energy Disorder in Colloidal Quantum Dot Solids for Enhanced Charge Carrier Transport

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the individual CQDs. This inhomogeneity in electronic energy levels can disrupt the charge hopping transport in CQD fi lms and degrade the transport rate signifi cantly. In addition, a suffi ciently narrow CQD size dispersion is essential to achieve the periodic supercrystals that have shown dramatic enhancement in carrier mobility in CQD fi eld effect transistors due to the reduced interparticle spacing and increased exchange coupling from the ligand treatments. [ 32 ] The impact of CQD size dispersion on the charge carrier mobility in CQD fi lms is little understood, and apparently contradictory results such as a lack of correlation between polydispersity and carrier mobility [ 21,33 ] versus the high carrier mobility observed using monodisperse CQDs, [ 32 ] calls for further investigation into the impact of radius distribution of CQDs on the carrier mobility in CQD fi lms. Furthermore, while recent work showed that CQD solar cell performance was not infl uenced by polydispersity since a high density of deep trap states degraded the charge transport in CQD active layers regardless of their size dispersion, [ 34 ] new concepts involving small ionic ligands and hybrid passivation strategies could successfully inactivate those trap states, [ 23 ] motivating the need for detailed studies on the impact of the CQD polydispersity, and for the establishment of design rules for polydisperse CQD layers. In this paper, we analyze the impact of the electronic energy disorder originating from the size dispersion of CQDs on the charge carrier mobility in CQD fi lms under a hopping transport regime using computational approaches, in which we determine the equilibrium in the charge continuity equations for each CQD by taking into account all the possible hopping transport pairs within a given cut-off distance. We show how the manipulation of CQD size dispersion can effectively control the carrier mobility in CQD fi lms. In particular, our results confi rm that both the energy barriers and the diffusion barriers work simultaneously, blocking the charge carrier transfer in the case that a gradient exists in the radius profi le along the electric fi eld direction. We suggest that well-controlled doping helps circumvent the mobility drops from these coexisting barriers, and then show that forming networks of large CQDs through radius rearrangement in the direction perpendicular to the electric fi eld promotes fast charge hopping transport effi ciently. Manipulating Electronic Energy Disorder in Colloidal Quantum Dot Solids for Enhanced Charge Carrier Transport