Unidirectional ordering of colloidal semiconductor nanorods (NRs) is key to taking advantage of anisotropic optical properties of NRs in solid-state devices. In this study, we report long-range orientation of colloidal CdSe/ CdS heterostructure NRs by spin coating a mixture of NRs and poly(9-vinylcarbazole) (PVK) molecules on an offcenter position of a substrate. Addition of 1,2-dichlorobenzene slows the evaporation of solvent resulting in better orientation with contrast ratio of 2.5 in linear photoluminescence polarization and 2.0 in electroluminescence. Our calculation reveals that the stabilization of the NR assembly is likely a result of depletion energy during spin coating. The retarded evaporation enables the depletion interaction to come into play in the process of NR stacking, hence highly oriented and closely packed NR films. Our findings on unidirectionally assembled NRs in uniform film offer an insight for ordering of anisotropic colloidal nanoparticles.