Bandlike Transport in Strongly Coupled and Doped Quantum Dot Solids: A Route to High-Performance Thin-Film Electronics

Abstract

We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm(2)/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperature-dependent measurements show bandlike transport to 220 K on a SiO(2) gate insulator that is extended to 140 K by reducing the interface trap density using an Al(2)O(3)/SiO(2) gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics.

Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors

“…To prepare the NC dispersion for spincoating, monodisperse CdSe NCs with as-synthesized long insulating ligands were treated in solution with ammonium thiocyanate in a nitrogen glovebox 16,20 , replacing the long ligands with the compact thiocyanate ligands, while maintaining solution dispersibility. The thiocyanate-exchanged NCs were redispersed in dimethylformamide and spincast atop the flexible substrates to form uniform, crack-free, randomly close-packed NC thin-film semiconducting channels.…”

“…1a,b). We have also fabricated back-gate/bottom-contact devices, but these devices typically suffer from larger contact resistance and therefore display poorer device performance (Supplementary We translated our previous work on high-performance CdSe NCFETs on rigid wafers operating at high voltages 20 to plastic substrates operating at low voltages. These FETs show high I ON /I OFF over 10 6 , low subthreshold swing (S ¼ 0.28±0.09 V dec À 1 ), low threshold voltage (V T ¼ 0.38 ± 0.15 V) and low hysteresis (DV T ¼ 0.25±0.07 V) at V DS ¼ 2 V. We attribute the low hysteresis to passivation of the NC surface by indium and the selection of Al 2 O 3 as the gate dielectric material, which we have shown to reduce the density of trap states at the NC surface and at the semiconductor-gate dielectric interface that give rise to hysteresis in NC-FETs.…”

“…These FETs show high I ON /I OFF over 10 6 , low subthreshold swing (S ¼ 0.28±0.09 V dec À 1 ), low threshold voltage (V T ¼ 0.38 ± 0.15 V) and low hysteresis (DV T ¼ 0.25±0.07 V) at V DS ¼ 2 V. We attribute the low hysteresis to passivation of the NC surface by indium and the selection of Al 2 O 3 as the gate dielectric material, which we have shown to reduce the density of trap states at the NC surface and at the semiconductor-gate dielectric interface that give rise to hysteresis in NC-FETs. 20 As such, the small variation in device parameters and large-area uniformity of these NC-FETs enables their integration in flexible NCICs.…”

“…Although there has been significant progress in developing single high-mobility NC-FETs that further operate with low hysteresis 18,20 at low voltage 19 , these high-performance NC-FETs have not been integrated into NC circuits in the literature. All circuit demonstrations have been limited to two single separate FETs connected externally to form an inverter 19,21 .…”

“…We previously introduced high-mobility CdSe NC-FETs that operated at high voltages on rigid substrates, achieved through both strong coupling, by introducing the compact ligand ammonium thiocyanate, and through doping, by thermal diffusion of indium at mild temperatures 20 . Thiocyanate is an environmentally benign and non-corrosive ligand, allowing solution deposition of NC devices on a variety of substrates, including flexible plastic substrates, which we have previously demonstrated 21 .…”