QD Films Research Articles

Open-circuit voltage deficit, radiative sub-bandgap states, and prospects in quantum dot solar cells.

Quantum dot photovoltaics (QDPV) offer the potential for low-cost solar cells. To develop strategies for continued improvement in QDPVs, a better understanding of the factors that limit their performance is essential. Here, we study carrier recombination processes that limit the power conversion efficiency of PbS QDPVs. We demonstrate the presence of radiative sub-bandgap states and sub-bandgap state filling in operating devices by using photoluminescence (PL) and electroluminescence (EL) spectroscopy. These sub-bandgap states are most likely the origin of the high open-circuit-voltage (VOC) deficit and relatively limited carrier collection that have thus far been observed in QDPVs. Combining these results with our perspectives on recent progress in QDPV, we conclude that eliminating sub-bandgap states in PbS QD films has the potential to show a greater gain than may be attainable by optimization of interfaces between QDs and other materials. We suggest possible future directions that could guide the design of high-performance QDPVs.

Atomic layer deposition effect on the electrical properties of Al2O3-passivated PbS quantum dot field-effect transistors

We have investigated the effect of atomic layer deposition (ALD) on the electrical properties of colloidal PbS quantum dot field-effect transistors (PbS QD-FETs). Low-temperature Al2O3 ALD process was used to fill up the pore spaces of PbS QD films containing 1, 2-ethanedithiol ligands. Upon deposition of Al2O3 on PbS film, the PbS QD-FETs showed ambipolar behavior. The treated film retained this property for over 2 months, despite of exposure to air. This change in the electrical properties of the PbS QD-FETs is attributed to the formation of electron channels in the Al2O3-passivated PbS film. We conclude that these electron transport channels in the AlxOy-PbS film are formed due to substitution of the Pb sites by Al metal and chemical reduction of Pb2+ ions, as determined by an analysis of the depth profile of the film using secondary ion mass spectrometry and X-ray photoelectron spectroscopy.

High-efficiency, low turn-on voltage blue-violet quantum-dot-based light-emitting diodes.

We report high-efficiency blue-violet quantum-dot-based light-emitting diodes (QD-LEDs) by using high quantum yield ZnCdS/ZnS graded core-shell QDs with proper surface ligands. Replacing the oleic acid ligands on the as-synthesized QDs with shorter 1-octanethiol ligands is found to cause a 2-fold increase in the electron mobility within the QD film. Such a ligand exchange also results in an even greater increase in hole injection into the QD layer, thus improving the overall charge balance in the LEDs and yielding a 70% increase in quantum efficiency. Using 1-octanethiol capped QDs, we have obtained a maximum luminance (L) of 7600 cd/m(2) and a maximum external quantum efficiency (ηEQE) of (10.3 ± 0.9)% (with the highest at 12.2%) for QD-LEDs devices with an electroluminescence peak at 443 nm. Similar quantum efficiencies are also obtained for other blue/violet QD-LEDs with peak emission at 455 and 433 nm. To the best of our knowledge, this is the first report of blue QD-LEDs with ηEQE > 10%. Combined with the low turn-on voltage of ∼2.6 V, these blue-violet ZnCdS/ZnS QD-LEDs show great promise for use in next-generation full-color displays.

Improving Polymer/Nanocrystal Hybrid Solar Cell Performance via Tuning Ligand Orientation at CdSe Quantum Dot Surface

Achieving superior solar cell performance based on the colloidal nanocrystals remains challenging due to their complex surface composition. Much attention has been devoted to the development of effective surface modification strategies to enhance electronic coupling between the nanocrystals to promote charge carrier transport. Herein, we aim to attach benzenedithiol ligands onto the surface of CdSe nanocrystals in the "face-on" geometry to minimize the nanocrystal-nanocrystal or polymer-nanocrystal distance. Furthermore, the "electroactive" π-orbitals of the benzenedithiol are expected to further enhance the electronic coupling, which facilitates charge carrier dissociation and transport. The electron mobility of CdSe QD films was improved 20 times by tuning the ligand orientation, and high performance poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):CdSe nanocrystal hybrid solar cells were also achieved, showing a highest power conversion efficiency of 4.18%. This research could open up a new pathway to improve further the performance of colloidal nanocrystal based solar cells.

Depleted Bulk Heterojunctions in Thermally Annealed PbS Quantum Dot Solar Cells

We have studied the detailed interface structure and energy conversion behavior of TiO2/PbS heterojunction solar cells. Nanoscale structure and composition analysis have revealed that thermal annealing causes intermixing of the TiO2 and PbS phases and influences the morphologies and optical properties of the heterojunction film. This intermixing increased the junction area within the depleted bulk heterojunction (DBH) layer and promoted the carrier extraction from PbS QDs to TiO2. In addition, the thermal annealing caused interparticle necking between PbS QDs and increased the crystallinity of the PbS QD film. Compared with unannealed PbS/TiO2 heterojunction solar cells, the formation of the DBH layer and the partial sintering of PbS QDs led to a doubling of the short-circuit current (Jsc) and an improved energy conversion efficiency, by 39%. Electric force microscopy analysis confirmed the presence of a DBH layer. The electron lifetime and fill factor (FF) of the solar cells decreased when the TiO2/PbS mixed film was thermally annealed, and this was assigned to a lower recombination resistance in the DBH layer. Post-treatment of PbS/TiO2 DBH films with ethanedithiol was found to increase the recombination resistance at PbS/TiO2 interface and to enhance the energy conversion efficiency to ∼4%.

Activation of CdSe Quantum Dots after Exposure to Polysulfide

Polysulfide electrolyte is the most commonly utilized electrolyte in quantum dot-sensitized solar cells (QDSSCs). Recently, however, a redshift in the relative positions of the absorbance and incident photon conversion efficiencies of CdSe QDs in polysulfide solutions has been reported. We conduct extensive analyses of CdSe QD films comparing those exposed to polysulfide solution with unexposed films; indeed, a redshift in the absorbance is observed after exposure. Most strikingly, we observe that a photocurrent response from films immersed in sodium sulfite is observed only after exposure to polysulfide solution. We demonstrate this is due to a combination of both QD ripening and a change in the QD capping layer. The results demonstrate that a polysulfide processing step significantly alters the photoelectrochemical performance of QDs.

Fabrication of p–i–n devices consisting of ZnO quantum dots embedded in Ga2ZnO4 film

ZnO films have been prepared at substrate temperatures of 125 to 200 °C by chemical vapor deposition with a precursor of Zn4O(CH3COO)6. As the substrate temperature decreases, the grain size of ZnO crystals decreases. From transmission electron microscopic observations, the film grown at 125 °C is found to be constructed from ZnO quantum dots with a diameter of ca. 5 nm. Ultraviolet emissions are seen at 367, 375, 380, and 384 nm in photoluminescence spectra of the ZnO films grown at 125, 150, 175, and 200 °C, respectively. The blue shift of the ultraviolet emissions is ascribed to the quantum confinement effect. After the deposition of a Ga2ZnO4 film with a thickness of 20 nm on the ZnO QD film grown at 125 °C, a visible emission disappears and the ultraviolet emission intensity significantly increases. Such an increase is due to the partial embedding of ZnO QDs in Ga2ZnO4 films. The insertion of the ZnO QD films in the p–i–n devices constructed from In1.8Zn0.2O3/InGaZnO4/Ga2ZnO4/Ni0.7Zn0.3O/Ni does not improve their rectification properties, and electroluminescence is not observed in these devices at forward voltages of 10 V or more.

Efficient inverted quantum-dot light-emitting devices with TiO_2/ZnO bilayer as the electron contact layer

We have demonstrated an efficient inverted CdSe/CdS/ZnS core/shell quantum-dot light-emitting device (QD-LED) using a solution-processed sol-gel TiO2 and ZnO nanoparticle composite layer as an electron-injection layer with controllable morphology and investigated the electroluminescence mechanism. The introduction of the ZnO layer can lead to the formation of spin-coated uniform QD films and fabrication of high-luminance QD-LEDs. The TiO2 layer improves the balance of charge injection due to its lower electron mobility relative to the ZnO layer. These results offer a practicable platform for the realization of a trade-off between the luminance and efficiency in the inverted QD-LEDs with TiO2/ZnO composites as the electron contact layer.

Condensable InP quantum dots solid

Colloidal thioacetic acid-capped InP quantum dots (InP-TAA QDs) and their spin coated-films have been examined in comparison with those of myristic acid-capped InP (InP-MA) QDs. While the QDs are far away from each other in the InP-MA QD films, even in a InP-MA QD film cured at 250 °C, upon thermal annealing a film of InP-TAA QDs at 250 °C, the indium thioacetate groups on the surfaces QDs likely condensed, thus resulting in the QD film that consists of individual and proximally packed InP QDs. The structures of the films of InP-MA QDs or InP-TAA QDs were characterized by means of TEM, XRD, and XPS. The current through the film of InP-TAA QDs cured at 250 °C was about 2–5 orders of magnitude higher than that of the film of InP-MA film annealed by the sample conditions. We, newly in this letter, define this kind of materials architecture as the condensable QDs solid concept.

QD-LED devices using ZnSnO as an electron-transporting layer

We have investigated the light-emitting diodes based on colloidal CdSe quantum dots (QD-LEDs), in which inorganic ZnSnO thin films and organic TPD thin films were used as the electron-transporting layer (ETL) and hole-transporting layer (HTL), respectively. The quantum dots were embedded between the inorganic ETL and organic HTL to form a sandwich structure. ZnSnO ETL was made by magnetron sputtering, while the TPD and QD films were made by spin-coating method. The QD-LEDs display sharp interface and smooth morphology. Optical and electrical characterizations show that QD-LEDs have low turn-on voltage, good monochromaticity, bright electroluminescence and good stability in atmosphere ambient. These characteristics are attributed to the utility of high electron mobility and low carrier concentration of the ZnSnO films used as the ETL. To investigate the devices operation mechanism, the conductivity of ZnSnO was varied during deposition to realize equal injection rate for both electrons and holes, which allows the device to operate optimally.

SiOx nanowires with intrinsic nC-Si quantum dots: the enhancement of the optical absorption and photoluminescence

We report a very simple method for the fabrication of amorphous silicon rich oxide nanowires (a-SiOx NWs) with intrinsic nanocrystalline silicon quantum dots (nC-Si QDs) with a high density and controlled size, by thermally annealing thin films of nC-Si quantum dots embedded in the a-SiOx matrix, overcoated by an ultra-thin Au catalyst layer. The size and density of the Si-QDs in the basic matrix are controllable by changing the plasma parameters during the growth of the nC-Si/a-SiOx thin films. The diameter of the grown SiOx nanowires is controlled by merely varying the thickness of the Au-coating. The formation of the 1D NWs with rigid boundaries influences a shrinkage in the size of the intrinsic 0D QDs; the size-reduction of the QDs is more prominent in narrower NWs obtained from thinner catalyst layers. This unique quantum-dot/nanowire composite system demonstrates a significantly improved optical absorption and reduced reflection in the entire UV-visible range of the solar spectrum compared to its thin film structure, and those are again pronounced by reducing the dimension of the nanowires. The photoluminescence properties of this composite system demonstrate a strong room temperature emission band in the range of 400–600 nm with a peak at ∼519 nm. The PL peak of the NW film undergoes a large blue shift by 150 nm from that of the annealed QD films which has been attributed to the reduction in the average size of the intrinsic nC-Si QDs during the growth of the a-SiOx NWs. These novel a-SiOx NWs with intrinsic nC-Si QDs, produced by a simple synthesis technique involving a solid–liquid–solid (SLS) growth process, deserve enough promise in the field of photovoltaics and light emitting devices.

Unconventional gap state of trapped exciton in lead sulfide quantum dots

Exciton states in lead selenide (PbSe) and lead sulfide (PbS) quantum dots have beenstudied extensively. However, relatively less attention has been paid to the states within thequantum dot bandgap. Our experimental results have revealed a single in-gap state whichbears confinement dependence yet cannot be explained by dark exciton theory, nor is it atrap state related to quantum dot surface defects as previously observed. A detailedanalysis of the temperature dependence of photoluminescence, Stokes shift, absorptionand photoinduced absorption indicates the unconventional GS is a new stateof a trapped exciton in a QD film. With appropriate design engineering, thesetrapped excitons might be harvested in solar cells and other optoelectronic devices.

Supercrystals of CdSe Quantum Dots with High Charge Mobility and Efficient Electron Transfer to TiO2

Thermal annealing of thin films of CdSe/CdS core/shell quantum dots induces superordering of the nanocrystals and a significant reduction of the interparticle spacing. This results in a drastic enhancement of the quantum yield for charge carrier photogeneration and the charge carrier mobility. The mobile electrons have a mobility as high as 0.1 cm(2)/(V x s), which represents an increase of 4 orders of magnitude over non-annealed QD films and exceeds existing literature data on the electron mobility in CdSe quantum dot films. The lifetime of mobile electrons is longer than that of the exciton. A fraction of the mobile electrons gets trapped at levels below the conduction band of the CdSe nanocrystals. These electrons slowly diffuse over 50-300 nm on longer times up to 20 micros and undergo transfer to a TiO2 substrate. The yield for electron injection in TiO2 from both mobile and trapped electrons is found to be >16%.

ENHANCED LIGHT EXTRACTION FROM COLLOIDAL ZnO QD FILM BY ADDING ON PHOTONIC CRYSTALS USING UV-NANOIMPRINT LITHOGRAPHY

A significant enhancement in light extraction from colloidal ZnO QD films was observed on an addition of a layer of two-dimensional photonic crystal (2D-PC) structure. The PCs with a rectangular-lattice air-hole array pattern with the period of 1.4 μm and the hole-size of 400 nm were fabricated by UV-nanoimprint lithography (UV-NIL). Unlike the commonly utilized electron-beam lithographic technique, the UV-NIL process can make patterns over a large area with high throughput. The resultant colloidal zinc oxide ( ZnO ) QD films with a PC pattern add-on had nearly double the output power, as measured from the top of the film.

Multi-color colloidal quantum dot based light emitting diodes micropatterned on siliconhole transporting layers

We present a colloidal quantum dot based light emitting diode (QD-LED) which utilizesthe p-type silicon substrate as the hole transporting layer. A microcontact printingtechnique was introduced to pattern self-assembled CdSe/ZnS QD films, whichallowed creation of an LED with well-defined geometry suitable for monolithicintegration on silicon substrates. Our QD-LED consists of multi-layers of inorganicmaterials: a combination of Au (thickness: 5 nm) and Ag (12 nm) as the cathode, aZnO:SnO2 mixture (ratio 3:1, 40 nm) as the electron transporting layer, CdSe/ZnS QDs as the light emission layer,1 nm SiO2 as an energy barrier layer, and p-type silicon as the hole transporting layer. These printedQD-LEDs are capable of multi-color emission peaked at wavelengths of 576 nm, 598 nm, and622 nm, corresponding to sizes of the embedded QDs with the diameters of 8.4 nm, 9.0 nm,and 9.8 nm respectively. The optimal thickness of the quantum dot layers needed for lightemission is characterized using atomic force microscopy: for 8.4 nm QDs, the value is 33 nm (± 5 nm)or ∼4 ML (monolayers). Larger turn on voltages were measured (2, 4 and 5 V) for thesmaller average particle diameters (9.8 nm, 9.0 nm and 8.4 nm, respectively). Themixture ratio of Zn and Sn was optimized (40% Zn and 25% Sn) to maintain properhole–electron recombination at the QD layer and avoid the yellowish-white emission fromZnO/SnO2.

In Situ Assembly of CuxS Quantum-Dots into Thin Film: A Highly Conductive P-Type Transparent Film

Copper sulfide (CuxS) nanocrystals ranging from ∼2 to 4 nm in diameter were in situ assembled on surface-functionalized substrates at room-temperature to realize novel transparent p-type conducting film that combines good optical transparency and high electrical conductivity. The CuxS film exhibited superior homogeneity, highly compact microstructure and good adhesion to quartz-glass and polyethylene substrates. Results of X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and electron diffraction revealed that the film consisted of uniform fine Cu7S4 nanocrystals with copper vacancies. Good optical transparency and large bandgap blue-shift were observed in the CuxS film attributed to the dominant quantum effect. The CuxS QD film turned out to be a p-type semiconducting film with a large conductivity up to 105 S·m−1 at room temperature, which originated from the numerous defects stabilized by grain boundaries. A mechanism of the CuxS QD film formation has been proposed base...