circuits. [4][5][6][7] PbS CQDs are well-suited to SWIR photodetection in light of their sizetuned bandgap and strong absorption in the IR region. The latest CQD photodetectors have adopted the photodiode architecture consisting of a CQD-based active layer, a CQD-based hole-transport layer (HTL), and a metal oxide-based electrontransport layer (ETL). [7][8][9][10] High-efficiency photodetectors require accurate control of the optoelectronic properties of each layer. To reach a peak sensitivity in the wavelength range of 1400-1500 nm, small bandgap PbS CQDs are used as active layers, and their relatively deep conduction band position limits the selection of ETs. Known metal oxides such as ZnO or TiO 2 do not offer the combination of properties needed for SWIR CQD detectors: the requisite union of band alignment at the interface, and stable performance at the operating wavelengths. [8,10] CQD-based transport layers benefit from tunable optoelectronic properties through surface ligand engineering and quantum-size effect tuning. While CQD HTLs have been extensively studied and improved, CQD ETLs are rarely explored. [11][12][13] Here we develop n-type CQDs as ETLs and tailor their band-edge positions for efficient charge extraction at the active layer/ETL interface. We fabricate all-CQD photo detectors in which CQDs of the same composition but with different sizes and ligand passivation were used to produce functionalities optimized separately for the active layer and charge-transport layers. The CQD photodetectors operating at 1450 nm exhibit a high external quantum efficiency (EQE) of 66% and a low dark current of ≈1 × 10 −3 mA cm −2 at 1 V, which has not been realized simultaneously in the prior works that used metal oxides as ETLs.We find that the surface passivation of CQDs is crucial to suppressing ion migration and preventing performance degradation in CQD photodetectors. Therefore, we develop a strategy to improve the passivation of ETL CQDs using strongly bound organic ligands. Specifically, we demonstrate that ETLs employing a strong organic ligand trans-4-(trifluoromethyl) cinnamic acid (TFCA) improve the dark current stability of CQD photodetectors by 50× compared to ETLs employing a weakly bound inorganic ligand tetrabutylammonium iodide (TBAI). Solution-processed photodetectors based on colloidal quantum dots (CQDs)are promising candidates for short-wavelength infrared light sensing applications. Present-day CQD photodetectors employ a CQD active layer sandwiched between carrier-transport layers in which the electron-transport layer (ETL) is composed of metal oxides. Herein, a new class of ETLs is developed using n-type CQDs, finding that these benefit from quantum-size effect tuning of the band energies, as well as from surface ligand engineering. Photodetectors operating at 1450 nm are demonstrated using CQDs with tailored functionalities for each of the transport layers and the active layer. By optimizing the band alignment between the ETL and the active layer, CQD photodetectors that combine a low d...

Colloidal quantum dots (CQD) have emerged as attractive materials for infrared (IR) photodetector (PD) applications because of their tunable bandgaps and facile processing. Presently, zinc oxide is the electron‐transport layer (ETL) of choice in CQD PDs; however, ZnO relies on continuous ultraviolet (UV) illumination to remove adsorbed oxygen and maintain high external quantum efficiency (EQE), speed, and photocurrent. Here, it is shown that ZnO is dominated by electropositive crystal planes which favor excessive oxygen adsorption, and that this leads to a high density of trap states, an undesired shift in band alignment, and consequent poor performance. Over prolonged operation without UV exposure, oxygen accumulates at the electropositive planes, trapping holes and degrading performance. This problem is addressed by developing an electroneutral plane composition at the ZnO surface, aided by atomic layer deposition (ALD) as the means of materials processing. It is found that ALD ZnO has 10× lower binding energy for oxygen than does conventionally deposited ZnO. IR CQD PDs made with this ETL do not require UV activation to maintain low dark current and high EQE.

Infrared photodetection enables depth imaging techniques such as structured light and time-of-flight. Traditional photodetectors rely on silicon (Si); however, the bandgap of Si limits photodetection to wavelengths shorter than 1100 nm. Photodetector operation centered at 1370 nm benefits from lower sunlight interference due to atmospheric absorption. Here, we report 1370 nm-operating colloidal quantum dot (CQD) photodetectors and evaluate their outdoor performance. We develop a surface-ligand engineering strategy to tune the electronic properties of each CQD layer and fabricate photodetectors in an inverted (PIN) architecture. The strategy enables photodetectors with an external quantum efficiency of 75% and a low dark current (1 μA/cm 2 ). Outdoor testing demonstrates that CQD-based photodetectors combined with a 10 nm-line width bandpass filter centered at 1370 nm achieve over 2 orders of magnitude (140× at incident intensity 1 μW/cm 2 ) higher signal-to-background ratio than do Si-based photodetectors that use an analogous bandpass filter centered at 905 nm.

III‐V colloidal quantum dots (CQDs) are promising materials for optoelectronic applications, for they avoid heavy metals while achieving absorption spanning the visible to the infrared (IR). However, the covalent nature of III‐V CQDs requires the development of new passivation strategies to fabricate conductive CQD solids for optoelectronics: this work shows herein that ligand exchanges, previously developed in II‐VI and IV‐VI quantum dots and employing a single ligand, do not fully passivate CQDs, and that this curtails device efficiency. Guided by density functional theory (DFT) simulations, this work develops a co‐passivation strategy to fabricate indium arsenide CQD photodetectors, an approach that employs the combination of X‐type methyl ammonium acetate (MaAc) and Z‐type ligands InBr3. This approach maintains charge carrier mobility and improves passivation, seen in a 25% decrease in Stokes shift, a fourfold reduction in the rate of first‐exciton absorption linewidth broadening over time‐under‐stress, and leads to a doubling in photoluminescence (PL) lifetime. The resulting devices show 37% external quantum efficiency (EQE) at 950 nm, the highest value reported for InAs CQD photodetectors.

Solution-processed colloidal quantum dots (CQDs) are promising materials for photodetectors operating in the short-wavelength infrared region (SWIR). Devices typically rely on CQD-based hole transport layers (HTL), such as CQDs treated using 1,2-ethanedithiol. Herein, we find that these HTL materials exhibit low carrier mobility, limiting the photodiode response speed. We develop instead inverted (p-i-n) SWIR photodetectors operating at 1370 nm, employing NiOx as the HTL, ultimately enabling 4× shorter fall times in photodiodes (∼800 ns for EDT and ∼200 ns for NiOx). Optoelectronic simulations reveal that the high carrier mobility of NiOx enhances the electric field in the active layer, decreasing the overall transport time and increasing photodetector response time.