Hybrid graphene–quantum dot phototransistors with ultrahigh gain

Graphene, which can detect a broad spectrum from ultraviolet to terahertz, is a promising photodetector material because it offers a broad spectral bandwidth and fast response times. However, the nature of weak light absorption has limited the responsivity of graphene-based photodetectors. Here, we demonstrate a responsivity of up to ∼6.7×103 A/W in a hybrid photodetector that consists of monolayer or bilayer graphene covered with a thin film of colloidal quantum dots. At the same time, benefits from gate-tunability, the device can response from the short-wavelength infrared to the visible, and compatibility with current circuit technologies.

Weak light absorption of graphene has limited the responsivity of graphene-based photodetectors. On the other hand, the slow response of PbSe as a mid-infrared range (MIR) detector makes this type of detector unsuitable as a commercial detector. Here, we report a fast MIR detector based on hybrid graphene-PbSe nanorods. For this purpose, a few-layer graphene piece was synthesized using a simple, scalable, and economical method on a cobalt layer, the synthesized graphene was transferred onto interdigitated copper electrodes, and then synthesized nanorods were spin coated on the transferred graphene. Strong and tunable light absorption in the quantum dot layer creates electric charges, which are transferred to the graphene, and due to the high charge mobility of graphene and long trapped-charge lifetimes in the quantum dot layer, they recirculate many times. The fabricated device has high speed and responsivity. The gain of fabricated detectors based on hybrid graphene quantum dots is 10.3 times more, their response time is 14.3 times faster, and their responsivity is 10 times more than conventional nanorod-based detectors. From the point of view of spectral selectivity, tuning the size of the nanorods helps optical detection from the IR to mid-IR.

Colloidal quantum dots (QDs) are of great interest in optoelectronic and photovoltaic devices with low-cost processing, strong light absorption, and size tunable direct band gap. However, their limited carrier mobility and short diffusion length limit efficient charge collection and transport. The short diffusion length in QD solid films, 100–200 nm, limits their thickness to t≈200–300 nm, resulting in poor absorption in the near-infrared, λ>800 nm, wasting part of sunlight and reducing power conversion efficiency. Recently, a novel architecture based on multiple graphene monolayers (Gr) intercalated inside QD films was reported to improve charge extraction beyond QDs diffusion length. The intercalated graphene layers ensure efficient charge collected in QD films thicker than the diffusion length. However, this architecture still fails to collect carriers from the QDs when the thickness is >~200 nm due to the poor vertical conductivity of the devices. Herein, we present the fabrication, optimization, and implementation of intercalated devices with vertical interconnecting contacts, increasing carrier collection, and photocurrent, aiming to develop a novel architecture for improved photodetection and photovoltaics with QDs. First, we analyze the individual roles of Gr and QDs, studying the evolution of light absorption, photocurrent (Iph), and conductivity as successive QD and Gr layers are added. We find the optimal interspacing between graphene layers in the intercalated system, aiming for the best compromise between light absorption and efficient charge collection. Our main contribution is the implementation of vertical interconnect access (VIAs) to each graphene layer to ensuring efficient charge transfer from Gr to the gold electrical contacts for efficient current collection. We show that for 850 nm wavelength illumination, photocurrent of intercalated devices show a ~10 fold improvement over devices without VIAs. We also use a back-gate voltage to monitor Fermi level shift in Gr and charge transfer from QDs to Gr. The intercalated configuration with VIAs contacts herein presented is a significant improvement in charge collection for QD optoelectronic applications as well as a promising architecture to enhance the power conversion efficiency for QD solar cells.

Quantum dots (QDs) offer several advantages in optoelectronics such as easy solution processing, strong light absorption and size tunable direct bandgap. However, their major limitation is their poor film mobility and short diffusion length (<250 nm). This has restricted the thickness of QD film to ∼200-300 nm due to the restriction that the diffusion length imposes on film thickness in order to keep efficient charge collection. Such thin films result in a significant decrease in quantum efficiency for λ > 700 nm in QDs photodetector and photovoltaic devices, causing a reduced photoresponsivity and a poor absorption towards the near-infrared part of the sunlight spectrum. Herein, we demonstrate 1 μm thick QDs photodetectors with intercalated graphene charge collectors that avoid the significant drop of quantum efficiency towards λ > 700 nm observed in most QD optoelectronic devices. The 1 μm thick intercalated QD films ensure strong light absorption while keeping efficient charge extraction with a quantum efficiency of 90%-70% from λ = 600 nm to 950 nm using intercalated graphene layers as charge collectors with interspacing distance of 100 nm. We demonstrate that the effect of graphene on light absorption is minimal. We achieve a time-modulation response of <1 s. We demonstrate that this technology can be implemented on flexible PET substrates, showing 70% of the original performance after 1000 times bending test. This system provides a novel approach towards high-performance photodetection and high conversion photovoltaic efficiency with quantum dots and on flexible substrates.

For the past few decades, silicon solar-cells have been researched to improve PCE through surface texturing, anti-reflection coating, plasma doping, selective emitter, back contact cell, local contact cell, and metallization. Eventually, silicon solar-cells has been saturated with the maximum PCE value of ~25 %.[1] Recently, the research on the energy-down-shift via implementing quantum-dots (QDs) in silicon solar-cells has been proposed to overcome the saturation of the PCE of silicon solar−cells since QDs are able to absorb UV light and emit visible light. However, they have not reported an evident PCE improvement and clear mechanism. Thus, we implemented the energy-down-shift via Cd0.5Zn0.5S/ZnS core/shell ODs on silicon solar-cells.The core/shell QDs were spin-coated on SiNX film deposited textured p-type silicon solar-cells, as shown in Fig. 1(a). They were well coated along the SiNX film textured surface where Al and Pt were deposited on the QDs layer to avoid the focus-ion-beam damage during the TEM sample preparation, as shown in Fig. 1(b). The core/shell QDs showed a spherical shape, were well crystallized, and well dispersed with each other, as shown in Fig. 1(c). The average size of the Cd0.5Zn0.5S/ZnS core/shell QDs was 6.6 nm, as shown in Fig. 1(d). The chemical composition of the core QD and shell layer coated on the core QD were Cd0.5Zn0.5S and ZnS, and the diameter of the core QD and thickness of the shell coated on the core QD were 4.2 and 1.2 nm, respectively, as shown in Fig. 1(e), analyzed by EDX line-scan profile. The Cd0.5Zn0.5S/ZnS core/shell QDs in the quartz-cuvette with 0.05 wt% absorbed 100% UV light in wavelength from 450 to 250 nm and emitted the 442 nm PL peak−signal with the quantum yield of 80%. These results evidently indicate that the Cd0.5Zn0.5S/ZnS core/shell QDs emitted the blue visual light when the UV light was absorbed, proving the energy-down-shift of the Cd0.5Zn0.5S/ZnS core/shell QDs. The dependency of the light absorption amount of the Cd0.5Zn0.5S/ZnS core/shell QDs on the concentration (wt %) of the QD solution was estimated as a function of the wavelength when the QD solutions were spin-coated on glass, as shown in Fig. 2(b), The absorption amount of the UV light in wavelength between 250 and 450 nm increased with the concentration of the QD solution. For p-type silicon solar-cells coated with the QDs layer, the dependency of PV performance on the average QDs layer thickness was shown in Fig. (3). The value of JSC increased abruptly from 34.70 to 36.94 mA/cm2 as the QDs layer thickness increased up to 8.8 nm, which was a 6.45 % increase compared to the reference without the QDs layer. Finally, PCE increased from 16.92 to 18.00 % as the QDs layer thickness increased up to 8.8 nm corresponds to relative 6.4 % PCE enhancement compared with that in the reference. These results indicate that the coating of Cd0.5Zn0.5S/ZnS core/shell QDs on the SiNX film textured surface for p-type silicon solar-cells affect JSC due to the energy-down-shift effect of the QDs but it does not affect VOCand FF.*This work was financially supported by the Brain Korea 21 plus Project in 2014, Korea.Fig. 1. Design of energy-down-shift via Cd0.5Zn0.5S/ZnS QDs coated on SiNx film textured p-type silicon solar cell.Fig. 2. Optical characteristics for Cd0.5Zn0.5S/ZnS QDs.Fig. 3. Photo-voltaic performance for p-type silicon solar-cells coated with Cd0.5Zn0.5S/ZnS core/shell QDs Reference [1] M. Tuan Trinh. et al., Nature Photonics 6, 316–321 (2012)[2] Bae. W K. et al., Chem Master 205307-5313 (2008)

Quantum dot (QD)-based white light-emitting diodes (LEDs) or white QD-LEDs were successfully fabricated by stacking QD films of primary colors (i.e., red, green, and blue). An ultrathin ZnO film was deposited between each QD layer to facilitate the formation of a tricolor-stacked QD structure. Such stacked QD films effectively suppress the Förster resonance energy transfer between QDs and enable efficient blue light emission, unlike randomly mixed QD films of different colors. The photoluminescence (PL) spectra of various QD films were obtained to study the change in the corresponding color gamut with applied voltage. In addition, QD-LEDs were fabricated based on the PL behavior and electronic band diagrams of QDs of different colors. Maximum luminance and peak EQE of white QD-LED shows 5700 cd/m2 and 1.1% compared to 2200 cd/m2 and 0.3% of QD-LED with mixed QD layer. Finally, a smooth color transition and white light emission were achieved using a blue/green/red stacking sequence for the QDs, which successfully suppressed the energy transfer effect. The overall performance and brightness of the white QD-LEDs synthesized in this study can be further enhanced by developing high-performance blue QDs.

Colloidal quantum dot‐based light‐emitting diodes (QD‐LEDs) are one of the potential future self‐emissive displays owing to their large‐scale solution‐processibility and high color purity. For the industrial application of QD‐LEDs, high‐performance QD‐LED and high‐resolution patterning of quantum dot (QD) films are required. Photolithography is an ideal tool for patterning QD films. Previously, the high‐resolution patterning of QD films using direct photolithography by ultra‐thin atomic layer deposition of ZnO on the QD surface is reported. The patterning process is acceptable for Cd‐based QD films, but the photoresist severely deteriorates the photoluminescence (PL) intensity of InP‐based QD films owing to the presence of sulfonic groups in the photoactive compound. Herein, a non‐destructive direct photolithography process for QD film patterning using a negative photoresist that does not affect the PL intensities of Cd‐ and InP‐based QD films is reported. The effect of the photoresist is also verified by a PL lifetime study. Extremely bright Cd‐ and InP‐based QD films are successfully patterned using a softer photoresist, and micropatterning of InP‐based QD films is reported for the first time in this work using photolithography. A QD electroluminescence device is also successfully fabricated using the patterning method.

The micropatterning of layers of colloidal quantum dots (QDs) stabilized by inorganic ligands is demonstrated using PbS core and CdSe/CdS core/shell QDs. A layer-by-layer approach is used to assemble the QD films, where each cycle involves the deposition of a QD layer by dip-coating, and the replacement of the native organic ligands by inorganic moieties, such as OH− and S2−, followed by a thorough cleaning of the resulting film. This results in a smooth and crack-free QD film on which a photoresist can be spun. The micropatterns are defined by a positive photoresist, followed by the removal of uncovered QDs by selective wet etching with an HCl/H3PO4 mixture. The resulting patterns can have submicron feature dimensions, limited by the resolution of the lithographic process, and can be formed on planar and 3D substrates. It is shown that the photolithography and wet etching steps have little effect on the photoluminescence quantum yield of CdSe/CdS QDs. Compared with the unpatterned CdSe/CdS QD film, only a 10% degradation in the quantum yield is observed. These results demonstrate the feasibility of the proposed micropatterning method to implement the large-scale device integration of colloidal quantum dots.

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage U r. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-Å-thick or 40-Å-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 Å, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage U r applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.

Quantum dots (QDs) have extraordinary strong light absorption and size tunable bandgap. However, QD films are typically limited to ~200-300 nm due to their poor charge mobility. This severely limits the quantum efficiency of QD devices for λ <750 nm (infrared). Herein, we report a record 1 μm thick QD film using intercalated graphene layers as transparent current extractors. This overcomes QD poor mobility, ensuring both effective light absorption and charge extraction towards the near-infrared reaching quantum efficiency (EQE) of 90%. The short diffusion length (LD<200 nm) of QDs limits their useful thickness to ~200-300 nm1–4 , resulting in poor infrared light absorption. To overcome this limitation, we have built a 1 µm thick QD film with intercalated transparent graphene electrodes that keep high charge collection efficiency. As a result, the 1 µm intercalated devices show a superior EQE reaching 90% at λ ~800 nm without the drop of quantum efficiency at λ ~700 nm observed in most QD devices. The EQE of intercalated devices improves over the entire λ~ 600-1100 nm spectrum as the thickness increases from 100 nm to 1 μm, clearly breaking the restriction that the diffusion length of QDs imposes on the film thickness. This improves absorption and charge collection in the infrared.

The luminance and color properties of backlight units with quantum-dot (QD) films were investigated through experimentation and optical simulation. Red and green QD films were arranged in four different configurations above and below the light guide plate (LGP). The on-axis luminance of the backlight units with QD films was much larger when diffuse reflectors were used instead of specular reflectors, irrespective of the QD configuration. This result is attributed to the higher color conversion efficiency and the outcoupling efficiency caused by the spreading of the light reflected from the diffuse reflectors. The ‘red QD film-LGP-green QD film’ combination exhibited the highest luminance performance among the four configurations, which can be explained in terms of the reduced radiation load on the two QD films, besides the nearly zero absorption of the red light emitted from the red QD film by the green QD film. The simulation results reproduced all these main results, which indicates that optical simulation can be a useful tool for the optimization of the optical structure of backlight units with QD films in advance of experiment and fabrication.

Enhancement of near ultraviolet spectral range responsibility of silicon photodetectors via additional fluorescent InP/ZnS quantum dots layer

The diffusion length of quantum dot (QD) films is a critical parameter to improve the performance of QD-based optoelectronic devices. The dot-to-dot hopping transport mechanism results in shorter diffusion lengths compared to bulk solids. Herein, we present an experimental method to measure the diffusion length in PbS QD films using single layer graphene as a charge collector to monitor the density of photogenerated carriers. By producing devices with different thicknesses, we can construct light absorption and photocarrier density profiles, allowing extracting light penetration depths and carrier diffusion lengths for electrons and holes. We realized devices with small (size: ∼2.5nm) and large (size: ∼4.8nm) QDs, and use λ = 532nm and λ = 635nm wavelength illumination. For small QDs, we obtain diffusion lengths of 180nm for holes and 500nm for electrons. For large QDs, we obtain diffusion lengths of 120nm for holes and 150nm for electrons. Our results show that films made of small QD films have longer diffusion lengths for holes and electrons. We also observe that wavelength illumination may have a small effect, with electrons showing a diffusion length of 500 and 420nm under λ = 532nm and λ = 635nm illumination, respectively, which may be due to increased interactions between photocarriers for longer wavelengths with deeper penetration depths. Our results demonstrate an effective technique to calculate diffusion lengths of photogenerated electrons and holes and indicate that not only QD size but also wavelength illumination can play important roles in the diffusion and electrical transport of photocarriers in QD films.

High-resolution patterning of quantum dot (QD) films is one of the preconditions for the practical use of QD-based emissive display platforms. Recently, inkjet printing and transfer printing have been actively developed; however, high-resolution patterning is still limited owing to nozzle-clogging issues and coffee ring effects during the inkjet printing and kinetic parameters such as pickup and peeling speed during the transfer process. Consequently, employing direct optical lithography would be highly beneficial owing to its well-established process in the semiconductor industry; however, exposing the photoresist (PR) on top of the QD film deteriorates the QD film underneath. This is because a majority of the solvents for PR easily dissolve the pre-existing QD films. In this study, we present a conventional optical lithography process to obtain solvent resistance by reacting the QD film surface with diethylzinc (DEZ) precursors using atomic layer deposition. It was confirmed that, by reacting the QD surface with DEZ and coating PR directly on top of the QD film, a typical photolithography process can be performed to generate a red/green/blue pixel of 3000 ppi or more. QD electroluminescence devices were fabricated with all primary colors of QDs; moreover, compared to reference QD-LED devices, the patterned QD-LED devices exhibited enhanced brightness and efficiency.

Lead sulfide quantum dots (QDs) have been widely used for various optoelectronic devices due to their high absorption coefficient and tunable bandgap. However, the low mobility of QD films results in poor charge collection and device performance. By combining QDs with graphene into hybrid graphene/QD photodetectors, photocarriers from QDs are transferred to graphene, improving charge collection and transport, drastically increasing the photoresponsivity. Herein, we carry a systematic analysis on how critical tuning parameters such as QD size and QD film thickness affect responsivity, spectral response, and time response. We report the absorption coefficient, refractive index (n, k), penetration depth, and energy bandgap of PbS QDs of different sizes. We study systematically how the photocurrent, photoresponsivity, time response, and power density dependence vary with QD size in hybrid Gr/QD. The bandgap of lead sulfide quantum dots was size-tuned between 0.86 and 1.39 eV. The time response shows that subsecond modulation can be achieved for different QD sizes with a responsivity up to 107 A/W at power densities of 10-5 mW/cm2. We also studied how the performance of the photodetectors is affected by the thickness, discussing the limitations on the thickness by the compromise between light absorption and charge collection. We describe how the optical response shifts toward the infrared as QD films get thicker. Time responses below 1 s are obtained for graphene/QD devices with thickness from 150 nm to 1 μm. This systematic study provides important guidelines to design hybrid graphene/QD photodetectors and tune their spectral response and performance.