Colloidal Quantum Dot Layer Research Articles

Light-controllable fiber interferometer utilizing photoexcitation dynamics in colloidal quantum dot.

The development of highly efficient light-controlled functional fiber elements has become indispensable to optical fiber communication systems. Traditional nonlinearity-based optical fiber devices suffer from the demerits of complex/expensive components, high peak power requirements, and poor efficiency. In this study, we utilize colloidal quantum dots (CQDs) to develop a light-controlled optical fiber interferometer (FI) for the all-optical control of the transmission spectrum. A specially designed exposed-core microstructure fiber (ECMF) is utilized to form the functional structure. Two types of PbS CQDs with absorption wavelengths around 1180 nm and 1580 nm, respectively, are deposited on the ECMF to enable the functional FI. The wavelength and power of control light are key factors for tailoring the FI transmission spectrum. A satisfactory recovery property and linear relationship between the spectrum shift and the power of control light at certain wavelength are achieved. The highest wavelength shift sensitivity of our light-controlled FI is 4.6 pm/mW, corresponding to an effective refractive index (RI) change of 5 × 10-6 /mW. We established a theoretical model to reveal that the RI of the CQD layer is governed by photoexcitation dynamics in CQD with the light absorption at certain wavelength. The concentration of charge carriers in the CQD layer can be relatively high under light illumination owing to their small size-related quantum confinement, which implies that low light power (mW-level in this work) can change the refractive index of the CQDs. Meanwhile, the absorption wavelength of quantum dots can be easily tuned via CQD size control to match specific operating wavelength windows. We further apply the CQD-based FI as a light-controllable fiber filter (LCFF) in a 50-km standard single-mode fiber-based communication system with 12.5-Gbps on-off keying direct modulation. Chirp management and dispersion compensation are successfully achieved by using the developed LCFF to obtain error-free transmission. CQDs possess excellent solution processability, and they can be deposited uniformly and conformally on various substrates such as fibers, silicon chips, and other complex structure surfaces, offering a powerful new degree of freedom to develop light control devices for optical communication.

Wet‐Transfer of Freestanding Dense Colloidal Quantum Dot Films and Their Photonic Device Application

AbstractA method of wet‐transferring a densely packed colloidal quantum dot (CQD) film is reported herein. Layers of poly(vinyl alcohol) (sacrificial layer) and poly(methyl methacrylate) (temporary mechanical support) are spin‐coated below and above the CQD film, respectively; the two layers are eventually removed, resulting in a single CQD film on an alien substrate. CQD films as large as 1 cm × 1 cm are transferred to any substrate, including flexible and prepatterned ones, with very little mechanical or optical damage. The wet‐transfer method can be repeated to form a CQD heterostructure. A CQD distributed feedback (DFB) laser is also demonstrated by transferring a thick CdSe‐based CQD layer (for both optical gain and waveguiding) on top of a surface grating structure. The resultant CQD structure exhibits a single‐mode DFB lasing action, with a high spontaneous emission factor (β ≈ 10−3), owing to a strong vertical mode confinement and smooth surface morphologies on both sides of the CQD waveguide layer.

Chloride Passivation of ZnO Electrodes Improves Charge Extraction in Colloidal Quantum Dot Photovoltaics.

The tunable bandgap of colloidal quantum dots (CQDs) makes them an attractive material for photovoltaics (PV). The best present-day CQD PV devices employ zinc oxide (ZnO) as an electron transport layer; however, it is found herein that ZnO's surface defect sites and unfavorable electrical band alignment prevent devices from realizing their full potential. Here, chloride (Cl)-passivated ZnO generated from a solution of presynthesized ZnO nanoparticles treated using an organic-solvent-soluble Cl salt is reported. These new ZnO electrodes exhibit decreased surface trap densities and a favorable electronic band alignment, improving charge extraction from the CQD layer and achieving the best-cell power conversion efficiency (PCE) of 11.6% and an average PCE of 11.4 ± 0.2%.

Enhancing optoelectronic properties of SiC-grown graphene by a surface layer of colloidal quantum dots

We report a simultaneous increase of carrier concentration, mobility and photoresponsivity when SiC-grown graphene is decorated with a surface layer of colloidal PbS quantum dots, which act as electron donors. The charge on the ionised dots is spatially correlated with defect charges on the SiC-graphene interface, thus enhancing both electron carrier density and mobility. This charge-correlation model is supported by Monte Carlo simulations of electron transport and used to explain the unexpected 3-fold increase of mobility with increasing electron density. The enhanced carrier concentration and mobility give rise to Shubnikov-de Haas oscillations in the magnetoresistance, which provide an estimate of the electron cyclotron mass in graphene at high densities and Fermi energies up to 1.2 × 1013 cm−2 and 400 meV, respectively.

Mobility enhancement of CVD graphene by spatially correlated charges

We present a strategy for enhancing the carrier mobility of single layer CVD graphene (CVD SLG) based on spatially correlated charges. Our Monte Carlo simulations, numerical modeling and the experimental results confirm that spatial correlation between defects with opposite charges can provide a means to control independently the carrier concentration and mobility of planar field effect transistors in which graphene is decorated with a layer of colloidal quantum dots (QDs). We show that the spatial correlation between electrically charged scattering centres close to the graphene/SiO2 interface and the localised charges in a QD layer can smooth out the electrostatic potential landscape, thus reducing scattering and enhancing the carrier mobility. The QD capping molecules influence the distribution and correlation of electrical charges in the vicinity of SLG and provide a means of tuning the carrier concentration and increasing the carrier mobility in graphene. These results represent a significant conceptual advance and provide a novel strategy for control of the electronic properties of 2D materials that could accelerate their utilization in optoelectronic devices.

Color-tuned and transparent colloidal quantum dot solar cells via optimized multilayer interference.

Colloidal quantum dots (CQDs), are a promising candidate material for realizing colored and semitransparent solar cells, due to their band gap tunability, near infrared responsivity and solution-based processing flexibility. CQD solar cells are typically comprised of several optically thin active and electrode layers that are optimized for their electrical properties; however, their spectral tunability beyond the absorption onset of the CQD layer itself has been relatively unexplored. In this study, we design, optimize and fabricate multicolored and transparent CQD devices by means of thin film interference engineering. We develop an optimization algorithm to produce devices with controlled color characteristics. We quantify the tradeoffs between attainable color or transparency and available photocurrent, calculate the effects of non-ideal interference patterns on apparent device color, and apply our optimization method to tandem solar cell design. Experimentally, we fabricate blue, green, yellow, red and semitransparent devices and achieve photocurrents ranging from 10 to 15.2 mA/cm2 for the colored devices. We demonstrate semitransparent devices with average visible transparencies ranging from 27% to 32%, which match our design simulation results. We discuss how our optimization method provides a general platform for custom-design of optoelectronic devices with arbitrary spectral profiles.

Role of Carrier Mobility and Band Alignment Engineering on the Efficiency of Colloidal Quantum Dot Solar Cells

We investigate the physics-based design of colloidal quantum dot (CQD) solar cells using self-consistent computational modeling. The significance of band alignment engineering and optimized carrier mobility is quantitatively explored as a function of subbandgap defect density ( Nt ) in the bulk CQD. For Nt $ \leq $ 10 15 cm-3, band alignment engineering near the interface of CQD and the metal contact could significantly improve open-circuit voltage by suppressing the forward-bias dark current. This effect could enhance cell efficiency up to ∼37% for thinner ( $ 1 μm) CQD layers. For the thicker ( $ > $ 1 μm) CQD layer, the effect of band engineering is diminished, as the forward-bias dark current becomes diffusion limited and less dependent on the interfacial band offsets. An optimal carrier mobility in CQD lies in the range ∼10-2–100 cm2/V⋅s and shows variation as a function of CQD layer thickness and the interfacial band offset. For Nt $ \approx {10^{14}}$ cm-3, an optimally designed cell could provide $\sim$ 20% efficiency under AM1.5G solar spectrum without employing advanced structural optimizations such as the nanostructured electrodes. These physical insights contribute to a better understanding of the quantum dot solar cell design that is needed for improving the efficiency for this emerging low-cost solar cell technology.

Utilizing light trapping interference effects in microcavity structured colloidal quantum dot solar cells: A combined theoretical and experimental approach

Colloidal quantum dot (CQD) solar cells are promising devices for solar energy conversion. However, usually the CQD layer in the devices needs to be thin to avoid recombination of the photogenerated charges before reaching the contacts. A microcavity structure with metal layers on both sides on the CQD film gives light interference effects in the CQD layer, which may increase the light absorption in the solar cell. However, such interference effects are not well known for CQD films, and differences compared to solid materials may appear. In this work the interference effects in CQD solar cells with different thicknesses of the CQD layer is investigated and the results are compared to simulations of the electro-optical field in the devices. The results show that the interference effects can be used to enhance the efficiency of the devices and the interference in the microcavity structure is favorable for efficient photocurrent generation also in thin CQD layers. Using the results, solar cells based on thin CQD layers on flexible or glass substrates are prepared and the results are very promising.

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Numerical Study of Complementary Nanostructures for Light Trapping in Colloidal Quantum Dot Solar Cells.

We have investigated two complementary nanostructures, nanocavity and nanopillar arrays, for light absorption enhancement in depleted heterojunction colloidal quantum dot (CQD) solar cells. A facile complementary fabrication process is demonstrated for patterning these nanostructures over the large area required for light trapping in photovoltaic devices. The simulation results show that both proposed periodic nanostructures can effectively increase the light absorption in CQD layer of the solar cell throughout the near-infrared region where CQD solar cells typically exhibit weak light absorption. The complementary fabrication process for implementation of these nanostructures can pave the way for large-area, inexpensive light trapping implementation in nanostructured solar cells.

Simultaneous Improvement of Charge Generation and Extraction in Colloidal Quantum Dot Photovoltaics Through Optical Management

Inverted structure heterojunction colloidal quantum dot (CQD) photovoltaic devices with an improved performance are developed using single‐step coated CQD active layers with a thickness of ≈60 nm. This improved performance is achieved by managing the device architecture to simultaneously enhance charge generation and extraction by raising optical absorption within the depletion region. The devices are composed of an ITO/PEDOT:PSS/PbS‐CQD/ZnO/Al structure, in which the p–n heterojunction is placed at the rear (i.e., opposite to the side of illumination) of the devices (denoted as R‐Cell). Sufficient optical generation is achieved at very low CQD layer thicknesses of 45–60 nm because of the constructive interference caused by the insertion of ZnO between the CQD and the Al electrode. The power conversion efficiency (PCE) of R‐Cells containing a thin CQD layers (≈60 nm) is much higher (≈6%) than that of conventional devices containing CQD layers with a thickness of ≈300 nm (PCE ≈4.5%). This optical management strategy provides a general guide to obtain the optimal trade‐off between generation and extraction in planar p–n junction solar cells. In terms of device engineering, all the layers in our R‐Cells are fabricated using single coating, which can lead to compatibility with high‐throughput processes.

Enhancing light absorption within the carrier transport length in quantum junction solar cells.

Colloidal quantum dot (CQD) solar cells have attracted tremendous attention because of their tunable absorption spectrum window and potentially low processing cost. Recently reported quantum junction solar cells represent a promising approach to building a rectifying photovoltaic device that employs CQD layers on each side of the p-n junction. However, the ultimate efficiency of CQD solar cells is still highly limited by their high trap state density in both p- and n-type CQDs. By modeling photonic structures to enhance the light absorption within the carrier transport length and by ensuring that the carrier generation and collection efficiencies were both augmented, our work shows that overall device current density could be improved. We utilized a two-dimensional numerical model to calculate the characteristics of patterned CQD solar cells based on a simple grating structure. Our calculation predicts a short circuit current density as high as 31 mA/cm2, a value nearly 1.5 times larger than that of the conventional flat design, showing the great potential value of patterned quantum junction solar cells.

Suppression of dark current through barrier engineer for solution-processed colloidal quantum-dots infrared photodetectors

In an attempt to suppress the dark current, the barrier layer engineer for solution-processed PbSe colloidal quantum-dot (CQD) photodetectors has been investigated in the present study. It was found that the dark current can be significantly suppressed by implementing two types of carrier blocking layers, namely, hole blocking layer and electron blocking layer, sandwiched in between two active PbSe CQD layers. Meanwhile no adverse impact has been observed for the photo current. Our study suggests that this improvement resides on the transport pathway created via carrier recombination at intermediate layer, which provides wide implications for the suppression of dark current for infrared photodetectors.

PbS/Cd3P2 quantum heterojunction colloidal quantum dot solar cells

Here, we demonstrated the quantum heterojunction colloidal quantum dot (CQD) solar cells employing the PbS CQDs/Cd3P2 CQDs architecture in which both the p-type PbS and n-type Cd3P2 CQD layers are quantum-tunable and solution-processed light absorbers. We synthesized well-crystallized and nearly monodispersed tetragonal Cd3P2 CQDs and then engineered their energy band alignment with the p-type PbS by tuning the dot size and hence the bandgap to achieve efficient light absorbing and charge separation. We further optimized the device through the Ag-doping strategy of PbS CQDs that may leverage an expanded depletion region in the n-layer, which greatly enhances the photocurrent. The resulting devices showed an efficiency of 1.5%.

The micropatterning of layers of colloidal quantum dots with inorganic ligands using selective wet etching

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.

Producing white light with multiple layers of colloidal quantum dots

Producing white light with multiple layers of colloidal quantum dots

Planar PbS quantum dot/C60 heterojunction photovoltaic devices with 5.2% power conversion efficiency

Of interest for both photovoltaic and photodetector applications is the ability of colloidal quantum dot (CQD) devices to provide response further into the infrared than is typical for other solution-processable materials. Here, we present a simple heterojunction diode structure that utilizes the extended infrared absorption of PbS CQDs. We show that device performance benefits from a discontinuous exciton blocking layer which improves charge separation without limiting charge extraction. By enhancing charge carrier mobility in the CQD layer, we demonstrate a planar heterostructure device with a power conversion efficiency of 5.2% under 1 sun illumination.

All solid state multiply layered PbS colloidal quantum-dot-sensitized photovoltaic cells

The combination of solution processable colloidal quantum dots (CQDs) and organic conjugated polymers has attracted growing interest in the past few years, because devices fabricated using this combination can have potentially low cost and high efficiency. In this paper, we propose a new device architecture of multiply layered PbS CQD-sensitized photovoltaic cells. The multiple layers of PbS CQDs were formed by the repeated spin-coating of oleic-acid-capped PbS CQD solution and 1 wt% 1,2-ethanedithiol (EDT) solution onto mesoporous (mp)-TiO2 as photoanode. Through the structural advantage of a sensitized type device and post-treatment in 10 wt% EDT solution for 16 h, we could fabricate efficient photovoltaic cells having an overall energy conversion efficiency of 2.9% under 1 sun illumination, and an external quantum efficiency of over 15% at zero bias with a signal attenuation to frequency of 10 kHz levels in the near-infrared (NIR) range.

Thin film photodiodes fabricated by electrostatic self-assembly of aqueous colloidal quantum dots

We demonstrate a thin film photodiode structure consisting of multi layers of colloidal quantum dots (QDs) which has application in photovoltaics and photodetection. The CdTe QDs with either positively or negatively charged capping ligands are self-assembled layer-by-layer on an indium tin oxide (ITO) substrate by electrostatic attraction in aqueous solution. A photolithographically patterned photoresist window defines the device active area and an evaporated aluminum (Al) thin film serves as the top electrode. The built-in electric field due to the work function difference between Al and ITO separates photo-excited electron-hole pairs and generates photocurrent. Since the ligands used for QD synthesis are short (less than 0.5 nm), no additional steps of ligand exchange or annealing is needed for enhancing the thin film photoconductivity. Thiol passivation and self-assembly in an inert environment help reduce surface traps, leading to less fermi-level pinning which also improves the device performance.

Depleted-Heterojunction Colloidal Quantum Dot Solar Cells

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced. The first-the Schottky device-was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type CQD film. The second-the excitonic device-employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate CQD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety.