Optimization of charge carrier extraction in colloidal quantum dots short-wave infrared photodiodes through optical engineering

Colloidal quantum dots (QDs) have attracted scientific interest for infrared (IR) optoelectronic devices due to their bandgap tunability and the ease of fabrication on arbitrary substrates. In this work, short-wave IR photodetectors based on lead sulfide (PbS) QDs with high detectivity and low dark current is demonstrated. Using a combination of time-resolved photoluminescence, carrier transport, and capacitance-voltage measurements, it is proved that the charge carrier diffusion length in the QD layer is negligible such that only photogenerated charges in the space charge region can be collected. To maximize the carrier extraction, an optical model for PbS QD-based photodiodes is developed, and through optical engineering, the cavity at the wavelength of choice is optimized. This universal optimization recipe is applied to detectors sensitive to wavelengths above 1.4 mu m, leading to external quantum efficiency of 30% and specific detectivity (D*) in the range of 10(12) Jones

Thin-film quantum dot photodiode for monolithic infrared image sensors

Imaging in the infrared wavelength range has been fundamental in scientific, military and surveillance applications. Currently, it is a crucial enabler of new industries such as autonomous mobility (for obstacle detection), augmented reality (for eye tracking) and biometrics. Ubiquitous deployment of infrared cameras (on a scale similar to visible cameras) is however prevented by high manufacturing cost and low resolution related to the need of using image sensors based on flip-chip hybridization. One way to enable monolithic integration is by replacing expensive, small-scale III-V-based detector chips with narrow bandgap thin-films compatible with 8- and 12-inch full-wafer processing. This work describes a CMOS-compatible pixel stack based on lead sulfide quantum dots (PbS QD) with tunable absorption peak. Photodiode with a 150-nm thick absorber in an inverted architecture shows dark current of 10(-6) A/cm(2) at 2 V reverse bias and EQE above 20% at 1440 nm wavelength. Optical modeling for top illumination architecture can improve the contact transparency to 70%. Additional cooling (193 K) can improve the sensitivity to 60 dB. This stack can be integrated on a CMOS ROIC, enabling order-of-magnitude cost reduction for infrared sensors

Phonon-mediated and weakly size-dependent electron and hole cooling in CsPbBr3 nanocrystals revealed by atomistic simulations and ultrafast spectroscopy

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over similar to 10 ps) of "warm" electrons in the vicinity (within similar to 0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps(-1) carrier(-1), increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers

Light absorption coefficient of CsPbBr3 perovskite nanocrystals

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV−vis absorption spectroscopy and transmission electron microscopy to determine the size, composition, and intrinsic absorption coefficient μi of 4 to 11 nm sized colloidal CsPbBr3 nanocrystals (NCs). The ICP-MS measurements demonstrate the nonstoichiometric nature of the NCs, with a systematic excess of lead for all samples studied. Rutherford backscattering measurements indicate that this enrichment in lead concurs with a relative increase in the bromide content. At high photon energies, μi is independent of the nanocrystal size. This allows the nanocrystal concentration in CsPbBr3 nanocolloids to be readily obtained by a combination of absorption spectroscopy and the CsPbBr3 sizing curve

Monolithic near infrared image sensors enabled by quantum dot photodetector

status: publishe

Probing solvent-ligand interactions in colloidal nanocrystals by the NMR line broadening

Although solvent-ligand interactions play a major role in nanocrystal synthesis, dispersion formulation, and assembly, there is currently no direct method to study this. Here we examine the broadening of H-1 NMR resonances associated with bound ligands and turn this poorly understood descriptor into a tool to assess solvent-ligand interactions. We show that the line broadening has both a homogeneous and a heterogeneous component. The former is nanocrystal-size dependent, and the latter results from solvent-ligand interactions. Our model is supported by experimental and theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of oxide, sulfide, and selenide nanocrystals. Our findings thus put forward NMR line-shape analysis as an indispensable tool to form, investigate, and manipulate nanocolloids

Solar cells using N,N’-substituted thiourea-based lead sulfide quantum dots

Typically lead sulfide quantum dots (PbS QDs) for solar cell incorporation are synthesized with the aid of the highly toxic and reactive sulfur precursor bis(trimethylsilyl) sulfide (TMS) (Hines et al. Adv. Mater. 2003). Recently, Hendricks et al. (Science 2015) proposed a new synthesis scheme that makes use of a wide variety of N,N'-substituted thioureas. By varying the thiourea substituents, the reaction rate can be tuned and this allows control over the QD size while maintaining extremely narrow size dispersions and full conversion of reactants. This, together with a high reproducibility makes it more appealing for larger volume synthesis than the TMS-based synthesis. However, various synthesis methods for the same material can give raise to different QD surface terminations which, in turn, will influence the final device performance. Perhaps, because of that, there have been no reports on PbS solar cells, which make use of this thiourea-based synthesis. In this contribution, we synthetized PbS QDs with a ~1.35 eV band gap by both TMS and thioureas and incorporated them into solar cells. Using nuclear magnetic resonance and Rutherford backscattering spectroscopy, we show that in both cases, synthesis and workup methods can be tuned to yield PbS with identical surface chemistry. Under such conditions, the solar cells show similar performance. That finding tells us that the resulting QD surface chemistry is key for high performance and not the synthesis route it-self