Tuning carrier mobilities and polarity of charge transport in films of CulnSe_xS_2-x quantum dots

Functional CISeS QD FETs were demonstrated with fairly high charge carrier mobilities. CISeS QDs were synthesized with 60% Se cations. The absorption onset is characterized by a similar wavelength. To fabricate FETs a heavily doped silicon substrate was used as a bottom gate electrode and a 500 nm layer of SiO₂ (dry thermal oxide) provides a low-leakage gate dielectric. Films of CISeS QDs were deposited onto the substrate by spin coating at 2500 rpm. In order to make a consistent QD layer thickness of ?50 nm, 4-5 cycles of spin coating separated by 5 min of annealing of the film at 100°C were repeated. For ligand exchange, a 0.5 mmol solution of the new ligand in methanol was spin coated during each cycle after spin coating QDs. Without any treatments, the as-synthesized QD films are conductive despite the presence of bulky ligands and exhibit p-type transport. The mobility increases after treatment with short EDT molecules that replace the original passivation. Interestingly, the QD treatment with Cd²⁺ ions changes the nature of charge transport and results in ambipolar behavior for both electrons and holes.