Quantum Dot Heterojunction Research Articles

Manipulating Charge Transfer from Core to Shell in CdSe/CdS/Au Heterojunction Quantum Dots.

The photophysics of charge-transfer and recombination mechanisms in a heterojunction structure of CdSe/CdS/Au quantum dots (QDs) are studied by temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). We manipulate the charge transfer from core to shell surface by varying the tunneling barrier height resulting from temperature variation and the barrier width resulting from shell thickness variation. The charge-transfer process, which can be described by a tunneling transmission model, is manifested by two competitive recombination processes, an intrinsic exciton emission and a trap emission in the near-infrared (NIR) range. Our study establishes the photophysics foundation for the core/shell/metal application in photocatalyst and optoelectronics.

Engineering the Band Alignment in QD Heterojunction Films via Ligand Exchange.

Colloidal quantum dots (QDs) allow great flexibility in the design of optoelectronic devices, thanks to their size-dependent optical and electronic properties and the possibility to fabricate thin films with solution-based processing. In particular, in QD-based heterojunctions, the band gap of both components can be controlled by varying the size of the QDs. However, control over the band alignment between the two materials is required to tune the dynamics of carrier transfer across a heterostructure. We demonstrate that ligand exchange strategies can be used to control the band alignment of PbSe and CdSe QDs in a mixed QD solid, shifting it from a type-I to a type-II alignment. The change in alignment is observed in both spectroelectrochemical and transient absorption measurements, leading to a change in the energy of the conduction band edges in the two materials and in the direction of electron transfer upon photoexcitation. Our work demonstrates the possibility to tune the band offset of QD heterostructures via control of the chemical species passivating the QD surface, allowing full control over the energetics of the heterostructure without requiring changes in the QD composition.

Temperature dependent photovoltaic performance of TiO2/PbS heterojunction quantum dot solar cells

A planner heterojunction quantum dot solar cells (QDSCs) structure of FTO/TiO2/PbS-EMII/PbS-EDT/Au is fabricated via layer-by-layer spin coating method, and then the temperature dependent photovoltaic performance of QDSCs is studied. The results indicate that the environment temperature has great influence on the current density-voltage (J-V) characteristics of quantum dot solar cell. The short-circuit photocurrent density (JSC), open-circuit voltage (VOC) and fill factor (FF) are all increased when the temperature decreases from 353 K to 253 K. A top value of power conversation efficiency (PCE, 9.78%) is obtained for the QDSCs when the environment temperature is lowered to 253 K, with a VOC of 0.63 V, JSC of 33.1 mA/cm2 and FF of 0.47, which is 33% above the PCE at room temperature (7.34%). In conclusion, it is necessary to cool the device for keeping the high efficiency operation of solar cell.

P-GaN/n-ZnO Nanoplate/CsPbBr3 Quantum Dots Heterojunction Light-Emitting Diode for Dual-Wavelength Emission

In this paper, we report a p-GaN/n-ZnO nanoplate/CsPbBr3 quantum dots (QDs) heterojunction light-emitting diode (LED) for dual-wavelength emission. ZnO nanoplates were prepared by vapor phase transport on the GaN thin film to form the p-GaN/n-ZnO nanoplate heterojunction. Afterwards, green CsPbBr3 QDs (band gap of 2.25 eV) were deposited on ZnO nanoplates to realize green light emission. The structure and photoluminescence of the ZnO nanoplates and CsPbBr3 QDs were characterized. The as-prepared LED device with turn-on voltage of ∼ 2.7 V displays a typical rectification behavior. The electroluminescence spectra with narrow emission peaks reveal the device presents commendable dual-wavelength electroluminescence performance at ∼ 385 nm and ∼ 512 nm and the electroluminescence intensity saturates at about 65 mA/cm2. Moreover, the detailed electroluminescence mechanism including emission originate of n-ZnO/p-GaN heterojunction was discussed based on the band diagram. Thus, our work indicates compelling potential for the practical application of perovskite LEDs.

Self-driven visible-near infrared photodetector with vertical CsPbBr3/PbS quantum dots heterojunction structure

Self-driven photodetectors are widely used in communication and imaging. As a newly developed semiconductor material, perovskite quantum dots (QDs) are not only bandgap tunable, but also easily combined with other materials. In this paper, a vertical structure self-driven photodetector based on heterojunction of CsPbBr3 QDs and PbS QDs is proposed, and the device is prepared by solution spin coating method. The device can work in visible and near infrared (400–1130 nm) regions, and has excellent performance, such as ultrafast response speed (rise and decay time are 0.4 μs/0.73 μs under 532 nm laser irradiation in self-driven mode, the estimated response time under 1064 nm laser irradiation is about 11.5 μs), more than 100 dB linear dynamic range for both visible and infrared regions, and good stability. Similarly, the responsivity of the photodetector can also reach an average of 10 mA W−1, and the detectivity is 1.13 × 1010 Jones at 0 V bias for 1064 nm laser irradiation. The device combines two kinds of QDs revealing its good prospects and great advantages in self-driven photodetectors and high-speed optical communication devices.

Controlling fluorescence of a nano-Al2O3 film enabled by CdSe quantum dots on CdSe/Al2O3 heterojunctions

The wide-spectrum enhanced fluorescence of a nano-Al2O3 film was studied by using CdSe quantum dots (QDs) on CdSe/Al2O3 heterojunctions. The CdSe QDs were deposited on the surface of a nano-Al2O3 film to prepare the CdSe/Al2O3 heterojunctions via a microfluidics-electrochemical deposition (M-ECD) method. The wide-spectrum enhanced fluorescence effect of nano-Al2O3 film was observed due to the transfer of photo-generated carriers from the CdSe QDs to the surface of nano-Al2O3 film. Moreover, owing to the addition of depositing time, the binding sites on the surface of nano-Al2O3 film were continually consumed; more CdSe QDs were deposited on the surface of nano-Al2O3 film to result in the increase of photo-generated carriers for boosting fluorescence and facilitating the blue shift of fluorescence peak. These results could be considered a new method to enhance and control the fluorescence of QDs heterojunction in photodiode, LED and photovoltaic cell.

High efficiency Si quantum dot heterojunction solar cells using a single SiOX:B layer

Si quantum dots (QDs) have been fabricated from SiO2/SiOx multilayer structures to create a homogeneous size. However, this structure achieved much lower efficiencies than would be expected in the Si QD photovoltaic field. This is because Si QD generation and photoexcited carrier transport is restricted by the adjacent SiO2 layer. In this study, we applied a single SiOx:B layer fabrication method to the Si QD heterojunction solar cells. The number of generated Si QDs and the photo-excited carrier lifetime was maximized when the oxygen partial pressure and boron doping concentration parameters were 2.7 × 10−5 Torr and 2.27 × 1021 atoms cm−3, respectively. As a result, in excess of 17% power conversion efficiency for the Si QD heterojunction solar cell was achieved using the single layer method.

PbS QDs as Electron Blocking Layer Toward Efficient and Stable Perovskite Solar Cells

Colloidal PbS quantum dots (QD), which possess size-tunable bandgap, multiple exciton generation effect, and low-cost synthesis process, are regarded as potential candidates in thin film photovoltaic field. Meanwhile, perovskite solar cells (PSCs) are prospecting photovoltaic devices with high power conversion efficiency (PCE). However, the high-performance PSCs largely rely on expensive and unstable organic hole transporting material (HTM), such as spiro-OMeTAD. Here, we design a new PSC architecture with a perovskite/PbS quantum dots (QDs) heterojunction film, which promotes hole extraction and decreases carrier recombination. By optimizing the concentration of colloidal PbS QDs, a PCE of 11.32% can be achieved, which is far higher than the control HTM-free PSCs (6.96% in PCE). In addition, the device structure exhibits higher stability than the conventional PSCs based on the control one. These findings along with low-temperature fabrication ( <100 °C) offer an alternative route in HTM-free heterojunction structure for low-cost and high-performance flexible PSCs.

G-C3N4/B doped g-C3N4 quantum dots heterojunction photocatalysts for hydrogen evolution under visible light

Developing high activity and eco-friendly photocatalysts for water splitting is still a challenge in solar energy conversion. In this paper, B doped g-C3N4 quantum dots (BCNQDs) were prepared via a facile molten salt method using melamine and boron oxide as precursors. By introducing BCNQDs onto the surface of g-C3N4, g-C3N4/BCNQDs heterojunction was constructed via hydrothermal treatment. The resulting g-C3N4/BCNQDs heterojunction exhibits enhanced hydrogen evolution performance for water splitting under visible light irradiation. The mechanism underlying the improved photocatalytic activity was explored and discussed based on the formation of heterojunction between g-C3N4 and BCNQDs with well-matched band structure.

Solution phase surface functionalization of PbS nanoparticles with organic ligands for single-step deposition of p-type layer of quantum dot solar cells

Semiconducting p-type layers of heterojunction quantum dot solar cells (QDSCs) are often prepared by the layer-by-layer (LBL) method. Due to the intermittent nature of quantum dot (QD) deposition, solid-state ligand exchange and washing steps, the LBL process typically involves a high waste of materials and a relatively low surface quality of the applied layers. To introduce a simplified method for deposition of QD layer, this paper investigates the preparation of pre-exchanged QD inks for the single-step deposition of QD layers of QDSCs. Various pre-exchanged QD inks were prepared through the solution phase exchange of oleate ligands with organic ligands of different functional groups, chain lengths and chain structures. Oxidation and colloidal stability of the prepared inks during long-time air exposure storage were investigated by the means of UV–Vis-NIR spectroscopy, photoluminescence (PL) and zeta potential measurements. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR) and combustion CHNS analyses were used to study the surface chemistry of the exchanged QDs and the mechanism of ligand exchange. Various inks of pre-exchanged QDs were deposited on different substrates in a single-step method. The best surface topography and the lowest oxidation rate were achieved for layers prepared by the single-step deposition of 3-mercaptopropionic acid (MPA)-capped QD ink. The QDSCs fabricated by such layers had higher efficiency than those fabricated by the common LBL method. Furthermore, annealing the MPA-capped QD layer increases the power conversion efficiency of the ink based solar cells up to 3.22% under simulated AM1.5 solar illuminations.

Hot-electron transfer in quantum-dot heterojunction films

Thermalization losses limit the photon-to-power conversion of solar cells at the high-energy side of the solar spectrum, as electrons quickly lose their energy relaxing to the band edge. Hot-electron transfer could reduce these losses. Here, we demonstrate fast and efficient hot-electron transfer between lead selenide and cadmium selenide quantum dots assembled in a quantum-dot heterojunction solid. In this system, the energy structure of the absorber material and of the electron extracting material can be easily tuned via a variation of quantum-dot size, allowing us to tailor the energetics of the transfer process for device applications. The efficiency of the transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale.

Quantum Dot Sensitized Solar Cells Based on TiO2/AgInS2

Quantum dot heterojunctions with type-II band alignment can efficiently separate photogenerated electron-hole pairs and, hence, are useful for solar cell studies. In this study, a quantum dot sensitized solar cell (QDSSC) made of TiO2/AgInS2 is achieved to boost the photoconversion efficiency for the TiO2-based system by varying the AgInS2 layer’s thickness. The TiO2 nanorods array film is prepared by using a simple hydrothermal technique. The formation of a AgInS2 QD-sensitized TiO2-nanorod photoelectrode is carried out by successive ionic layer adsorption and reaction (SILAR) technique. The effect of the QD layer on the performance of the solar cell is studied by varying the SILAR cycles of the QD coating. The synthesized electrode materials are characterized by using X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, high resolution transmission electron microscopy and solar cell performances. The results indicate that the nanocrystals have effectively covered the outer surfaces of the TiO2 nanorods. The interfacial structure of quantum dots (QDs)/TiO2 is also investigated, and the growth interface is verified. A careful comparison between TiO2/AgInS2 sensitized cells reveals that the trasfer of electrons and hole proceeds efficiently, the recombination is suppressed for the optimum thickness of the QD layer and light from the entire visible spectrum is utilised. Under AM 1.5G illumination, a high photocurrent of 1.36 mAcm−2 with an improved power conversion efficiency of 0.48% is obtained. The solar cell properties of our photoanodes suggest that the TiO2 nanorod array films co-sensitized by AgInS2 nanoclusters have potential applications in solar cells.

Efficiency enhancement for solution-processed PbS quantum dots solar cells by inserting graphene oxide as hole-transporting and interface modifying layer

Abstract Interface modifying between the active layer and anode solar cells is a key technique to improve the device performance. In this paper, the enhancement of power conversion efficiency (PCE) of ZnO/PbS heterojunction quantum dot (QD) solar cells Au/PbS-TBAI/ZnO/ITO was achieved by incorporating a graphene oxide (GO) layer between the PbS-TBAI active film and the Au anode. Our experimental data showed the GO interlayer was partially reduced to graphene after its post-annealing at 140 °C and it played the role of hole-transporting layer and the interface-modifying layer. In this way, the defects existed at the contact interface of PbS-TBAI/Au were reduced significantly after inserting and post-annealing GO interlayer, resulting to the enhancement of holes transport and collection efficiency, showing a PCE enhancement of 12.87% as compared to that of the control device Au/PbS-TBAI/ZnO/ITO. The influence of post-annealing treatment on the whole device, as the reduction method for GO, on the performance of the QD solar cells was also explored and discussed.

Efficiency improvement of Si quantum dot solar cells by activation with boron implantation

The activation of Si quantum dots (QDs) in general Si QD heterojunction solar cells fabricated by the co-deposition of Si and B is typically not effective, as a high concentration of B is required for effective activation. In order to address this issue, B atoms were injected by ion implantation after the formation of Si QDs in a SiO2 matrix. A Si QD solar cell was successfully realized when a Si QD layer was activated by the implantation of B ions followed by an annealing step. As a result, the power conversion efficiency of a Si QD solar cell realized by the ion implantation of B increased from 13.17% to 13.92%. The open-circuit voltage also increased from 523.03 mV to 529.64 mV in the Si QD solar cell created with the ion implantation of B.

Nonadiabatic Molecular Dynamics Simulation of Charge Separation and Recombination at a WS2/QD Heterojunction

Two-dimensional transition metal dichalcogenides (TMDs), such as WS2, are appealing candidates for optoelectronics and photovoltaics. The strong Coulomb interaction in TMDs is however known to prevent electron–hole pairs from dissociating into free electron and hole. The experiment demonstrates that combination of WS2 and quantum dots (QDs) can achieve efficient charge separation and enhance photon-to-electron conversion efficiency. Using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics, we model electron and hole transfer dynamics at a WS2/QD heterojunction. We demonstrate that both electron and hole transfer are ultrafast due to strong donor–acceptor coupling. The photoexcitation of the WS2 leads to a 75 fs electron transfer, followed by a 0.45 eV loss within 90 fs. The photoexcitation of QD results in 240 fs hole transfer, but loses only 0.15 eV of energy within 1 ps. The strong charge–phonon coupling and a broad range of phonon modes involved in electron...

Porous Co3O4/SnO2 quantum dot (QD) heterostructures with abundant oxygen vacancies and Co2+ ions for highly efficient gas sensing and oxygen evolution reaction.

Porous Co3O4/SnO2 quantum dot (QD) heterojunctions with a strong synergistic effect are successfully synthesized in this paper. Owing to the strong synergistic effect between Co3O4 and SnO2QDs, Co3O4/SnO2QD heterostructures possess more Co2+ ions for a faster Co2+/Co0 redox reaction in the process of sensing of reducing gases and electrochemical reactions, and more oxygen vacancies for more active sites and reduced charge transfer resistance on the surface. These advantages are demonstrated to significantly enhance the gas sensitivity to xylene and greatly improve the catalysis for the oxygen evolution reaction (OER). As a catalyst for the OER, Co3O4/SnO2QD (1 : 1) heterostructures exhibit the highest current density, lowest onset potential, largest active surface area and remarkable durability in alkaline electrolytes. The sensitivity of Co3O4/SnO2QD (1 : 1) heterostructures to 100 ppm xylene is almost 10 times higher than that of pure Co3O4 nanosheets and 3 times higher than that of SnO2QDs. In addition, Co3O4/SnO2QD (1 : 1) heterostructure sensors exhibit excellent gas selectivity, long-term stability and markedly high response to low concentrations of xylene at low operating temperatures.

Mixed-quantum-dot solar cells

Colloidal quantum dots are emerging solution-processed materials for large-scale and low-cost photovoltaics. The recent advent of quantum dot inks has overcome the prior need for solid-state exchanges that previously added cost, complexity, and morphological disruption to the quantum dot solid. Unfortunately, these inks remain limited by the photocarrier diffusion length. Here we devise a strategy based on n- and p-type ligands that judiciously shifts the quantum dot band alignment. It leads to ink-based materials that retain the independent surface functionalization of quantum dots, and it creates distinguishable donor and acceptor domains for bulk heterojunctions. Interdot carrier transfer and exciton dissociation studies confirm efficient charge separation at the nanoscale interfaces between the two classes of quantum dots. We fabricate the first mixed-quantum-dot solar cells and achieve a power conversion of 10.4%, which surpasses the performance of previously reported bulk heterojunction quantum dot devices fully two-fold, indicating the potential of the mixed-quantum-dot approach.

Determination of Band Alignment in the Synergistic Catalyst of Electronic Structure-Modified Graphitic Carbon Nitride-Integrated Ceria Quantum-Dot Heterojunctions for Rapid Degradation of Organic Pollutants

We engineered novel heterojunction ceria (CeO2) QDs decorated on the surfaces of graphitic carbon nitride (g-C3N4) nanosheets by a facile in situ hydrothermal synthetic route. Using core-level/valence-band X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy, and work function measurements of the materials, we constructed the energy band alignment at the heterojunction. The band alignment has a Type-II alignment between organic (g-C3N4) and inorganic (CeO2 QDs) semiconductors junction with valence/conduction band offsets (VBO/CBO) of −0.07/–0.31 eV. The calculated band alignment parameters of the heterojunction were compared with the experimental values of g-C3N4/CeO2 QD composite and a new energy band diagram was proposed for the electronic structure-modified g-C3N4/CeO2 QDs heterojunction. The newly constructed heterojunction is formed by carbon-vacancy-promoted g-C3N4 coupled to lower defect-mediated (oxygen vacancies) CeO2, as determined by high-resolution XPS analysis. Moreover, t...

Recombination Suppression in PbS Quantum Dot Heterojunction Solar Cells by Energy-Level Alignment in the Quantum Dot Active Layers.

Using spatial energy-level gradient engineering with quantum dots (QDs) of different sizes to increase the generated carrier collection at the junction of a QD heterojunction solar cell (QDHSC) is a hopeful route for improving the energy-conversion efficiency. However, the results of current related research have shown that a variable band-gap structure in a QDHSC will create an appreciable increase, not in the illumination current density, but rather in the fill factor. In addition, there are a lack of studies on the mechanism of the effect of these graded structures on the photovoltaic performance of QDHSCs. This study presents the development of air atmosphere solution-processed TiO2/PbS QDs/Au QDHSCs by engineering the energy-level alignment (ELA) of the active layer via the use of a sorted order of differently sized QD layers (four QD sizes). In comparison to the ungraded device (without the ELA), the optimized graded architecture (containing the ELA) solar cells exhibited a great increase (21.4%) in short-circuit current density ( Jsc). As a result, a Jsc value greater than 30 mA/cm2 has been realized in planar, thinner absorption layer (∼300 nm) PbS QDHSCs, and the open-circuit voltage ( Voc) and power-conversion efficiency (PCE) were also improved. Through characterization by the light intensity dependences of the Jsc and Voc and transient photovoltage decay, we find that (i) the ELA structure, serving as an electron-blocking layer, reduces the interfacial recombination at the PbS/anode interface, and (ii) the ELA structure can drive more carriers toward the desirable collection electrode, and the additional carriers can fill the trap states, reducing the trap-assisted recombination in the PbS QDHSCs. This work has clearly elucidated the mechanism of the recombination suppression in the graded QDHSCs and demonstrated the effects of ELA structure on the improvement of Jsc. The charge recombination mechanisms characterized in this work would be able to shed light on further improvements of QDHSCs, which could even benefit other types of solar cells.

Electrical and optical characteristics of solution-processed MoOx and ZnO QDs heterojunction

Abstract