A tunable colloidal quantum dot photo field-effect transistor

We fabricate and investigate field-effect transistors in which a light-absorbing photogate modulates the flow of current along the channel. The photogate consists of colloidal quantum dots that efficiently transfer photoelectrons to the channel across a charge-separating (type-II) heterointerface, producing a primary and sustained secondary flow that is terminated via electron back-recombination across the interface. We explore colloidal quantum dot sizes corresponding to bandgaps ranging from 730 to 1475 nm and also investigate various stoichiometries of aluminum-doped ZnO (AZO) channel materials. We investigate the role of trap state energies in both the colloidal quantum dot energy film and the AZO channel. Most photodetectors function either as photodiodes or as photoconductors: photodiodes offer fast response and low dark current, while photoconductors provide built-in gain associated with the use of long-lived sensitizing centers. The concept of a phototransistor, such as a photo-field-effecttransistor (photoFET), 1 offers an attractive possibility: gain united with a lowered dark current compared to the photoconductor, achieved if the thickness of the current-carrying channel can be chosen independently from the thickness of the light-absorbing layer. Here, we report a versatile photoFET that benefits from a broad spectrum of detection, spanning the visible and portions of the short-wavelength infrared spectral region. We present two of the four ingredients necessary to the realization of a highly sensitive photodetector based on the photo-FET concept: we successfully transfer photoelectrons from the sensitizing material to the electron-accepting channel (EAC), and we obtain photocurrents through the recirculation of secondary injected photocarriers flowing along the channel. Further work will be required to realize a sensitizing layer having sufficiently high electron mobility that a fully light-absorbing thickness of this medium can efficiently inject all carriers into the channel; and to deposit an ultrathin channel compatible with low dark current. Our device uses colloidal quantum dots (CQDs) as the sensitizing material to form the photo gate. CQDs offer tunability of the bandgap, and thus the spectral range of light sensing in a photodetector, through the size of the nanoparticles. Since CQDs are solution processable, they are compatible with low-cost, large-area processes. We utilize a sub-monolayer film of PbS CQD of a variety of sizes as the sensitizing material. Aluminum-doped zinc oxide (AZO) serves as the electron-accepting channel. We employ no conductivity-enhancing treatments to the CQD films-instead of employing crosslinking treatments using short dithiols, 2 we leave the quantum dots capped with oleic acid. This, combined with our use of a submonolayer of CQDs, ensures that our study focuses on electron injection into the channel, followed by conduction within the channel, since the photogate material has negligible conductance compared to the channel. As schematically drawn i