Lead sulfide (PbS) colloidal quantum dots (QDs) have received attentions as materials for near-infrared (NIR) photodetection in view of their strong and tunable absorption in the NIR region and room-temperature solution processability. However, the realization of high-speed PbS QD photodetection has been severely hindered by the extremely low carrier mobility (∼10 −5 to 10 −2 cm 2 V −1 s −1 ). Here, an ultrafast PbS QD photodiode fabricated with low mobility QDs (∼10 −3 cm 2 V −1 s −1 ) is demonstrated, which has rise/fall times as short as 0.33 μs at zero voltage bias. The fast response is achieved by engineering resistor−capacitor (RC) time delay and charge transport in the device. The photodiode also has an external quantum efficiency (EQE) exceeding 100% under voltage bias, which is possibly due to the photoconductive gain induced by hole transport layer (HTL). The photoconductive gain combined with low noise current enables high sensitivity with a specific detectivity value up to 3.2 × 10 11 Jones at 1125 nm.

exploited for the purpose of uniform backlighting, brightness enhancement, efficiency improvement, and increased sensitivity in liquid crystal displays (LCDs), [1][2][3] light-emitting diodes (LEDs), [4][5][6] solar cells, [7][8][9] and photodetectors, [10][11][12] all via excellent light scattering properties of optical diffusers.As a nontoxic and biodegradable material, cellulose nanocrystals (CNCs) are environment friendly. They are directly extracted from natural resources such as wood and other fiber supplies available in plants. As a high-molecular-weight linear polymer formed of monomers linked together by glycosidic oxygen bridges, [13] they offer desirable bulk and nanoscale properties (e.g., high tensile strength, high surface area for interaction with surrounding species) [14] that make them suited for use as substrates in devices such as sensors, [15] solar cells, [16] LEDs, [17] and transistors. [18] In addition, recently, cellulose nanofiber (CNF)-based films and papers have attracted attention for their light scattering capabilities. [9,19,20] Both CNCs and CNFs are very similar in terms of chemical composition and can be derived from the same source but are morphologically different in that CNFs are long (in order of micrometers) and flexible composed of both crystalline and amorphous parts while CNCs are short rod-like crystals with lengths in the range of a few hundreds of nanometers. [21] Here, we propose, fabricate, and characterize a new class of optical diffusers based on CNP hybrid films. While some of the recently proposed volumetric diffusers are made of rare earth materials such as cerium [22] which can drive up the cost of fabrication, the proposed diffuser relies on a ubiquitous and environment-friendly material, cellulose, which is cost-effective to produce. Our hybrid diffuser achieves high haze values (up to 85%), which is significantly higher than cellulose nanofiber/ wood pulp diffusers, [9,20] while maintaining a high degree of transparency (≈85%). In addition, unlike previously reported cellulose nanofiber/wood pulp optical diffusers which require an additional index-matching layer between the diffuser and the device for efficient light coupling, [9,20] no index-matching is required for the diffuser here due to the hydrophobic surfaces A novel optical diffuser based on cellulose nanocrystals (CNCs) embedded in polydimethylsiloxane (PDMS) matrix, or cellulose nanocrystal:polymer (CNP) is reported. By offering simple low-cost fabrication process as well as compatibility with large-scale production, the proposed optical diffuser is a better choice for integration into optoelectronic devices for light management compared to other cellulose-based diffusers due to its physical durability and the lack of requirement for index-matching between the diffuser and the optoelectronic device thanks to the unique surface properties, mechanical flexibility, and optical transparency offered by PDMS as the matrix material. It is demonstrated that CNCs are an excellent filler material to pair up w...

Sensitizing crystalline silicon (c-Si) with an infrared-sensitive material, such as lead sulfide (PbS) colloidal quantum dots (CQDs), provides a straightforward strategy for enhancing the infrared-light sensitivity of a Si-based photodetector. However, it remains challenging to construct a high-efficiency photodetector based upon a Si:CQD heterojunction. Herein, we demonstrate that Si surface passivation is crucial for building a high-performance Si:CQD heterojunction photodetector. We have studied one-step methyl iodine (CH 3 I) and two-step chlorination/methylation processes for Si surface passivation. Transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements reveal that the two-step passivated Si:CQD interface exhibits fewer trap states and decreased recombination rates. These passivated substrates were incorporated into prototype Si:CQD infrared photodiodes, and the best performance photodiode based upon the two-step passivation shows an external quantum efficiency (EQE) of 31% at 1280 nm, which represents a near 2-fold increase over the standard device based upon the one-step CH 3 I passivated Si.

Microscopic lenses are paramount in solar energy harvesting, optical devices, and imaging technologies. This work reports an extraordinary focusing effect exhibited by a surface nanolens (i.e., with at least one dimension of subwavelength) that is situated in an evanescent field from the total internal reflection (TIR) of light illuminated to the supporting substrate above the critical angle. Our measurements show that the position, shape, and size of the surface area with enhanced light intensity are determined by the geometry of the nanolens and the incident angle, in good agreement with simulation results. This strong focusing effect of the surface nanolens is shown to significantly promote the plasmonic effect of deposited gold nanoparticles on the lens surface inlight conversion and to vaporize surrounding water to microbubbles by using low laser power. This work further demonstrates that the light redistribution by the surface nanolens in TIR enables a range of novel applications in selectively local visualization of specimens in fluorescence imaging, optical trapping of colloids from an external flow, and selective materials deposition from photoreactions.

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