Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

Construction of Robust Iridium(III) Complex-Based Photosensitizer for Boosting Hydrogen Evolution

Developing a photosensitizer with high efficiency and long-term stability for photocatalytic hydrogen evolution is highly desirable yet remains a challenge. Herein, a novel Ir(III) complex-based photosensitizer (Ir3) bearing coumarin and triphenylamine groups is designed. Ir3 exhibits record activity and durability among reported transition metal complexes for photocatalytic hydrogen evolution, with a TON of 198,363 and a duration of 214 h. The excellent photocatalytic performance of Ir3 can be attributed to the synergistic effect of coumarin and triphenylamine, which improves the visible light absorption, charge separation, and electron transfer capacity of photosensitizers. This is an efficient and long-lived Ir(III) photosensitizer constructed on the basis of a synergistic approach, which could provide a new insight for the development of high-performance Ir(III) photosensitizers at the molecular level.

Heterogeneous stimuli induced nonassociative learning behavior in ZnO nanowire memristor

Nonassociative learning is a biologically essential and evolutionarily adaptive behavior in organisms. The bionic simulation of nonassociative learning based on electronic devices is essential to the neuromorphic computing. In this work, nonassociative learning is mimicked by a ZnO nanowire memristor without any other peripheral control circuit. The memristor demonstrates habituation and sensitization behaviors at the electrical and optical stimuli. Typical network-level parametric characteristics of habituation in neuroscience are realized in the memristor. When the heterogeneous stimuli are applied coincidentally, sensitization pulse could be identified by the exceptional response current. The results show that the natural selection rules could be simulated by the current single memristor. A possible mechanism based on the trapping states and adsorption of oxygen at the interface of Au/ZnO is proposed. The implementation of nonassociative learning in a single memristor device paves the way for building neuromorphic systems by simple electronic devices.

Exciton-Delocalizing Ligands Can Speed Up Energy Migration in Nanocrystal Solids

Researchers have long sought to use surface ligands to enhance energy migration in nanocrystal solids by decreasing the physical separation between nanocrystals and strengthening their electronic coupling. Exciton-delocalizing ligands, which possess frontier molecular orbitals that strongly mix with nanocrystal band-edge states, are well-suited for this role because they can facilitate carrier-wave function extension beyond the nanocrystal core, reducing barriers for energy transfer. This report details the use of the exciton-delocalizing ligand phenyldithiocarbamate (PDTC) to tune the transport rate and diffusion length of excitons in CdSe nanocrystal solids. A film composed of oleate-terminated CdSe nanocrystals is subjected to a solid-state ligand exchange to replace oleate with PDTC. Exciton migration in the films is subsequently investigated by femtosecond transient absorption. Our experiments indicate that the treatment of nanocrystal films with PDTC leads to rapid (∼400 fs) downhill energy migration (∼80 meV), while no such migration occurs in oleate-capped films. Kinetic Monte Carlo simulations allow us to extract both rates and length scales for exciton diffusion in PDTC-treated films. These simulations reproduce dynamics observed in transient absorption measurements over a range of temperatures and confirm excitons hop via a Miller-Abrahams mechanism. Importantly, our experiments and simulations show PDTC treatment increases the exciton hopping rate to 200 fs, an improvement of 5 orders of magnitude relative to oleate-capped films. This exciton hopping rate stands as one of the fastest determined for CdSe solids. The facile, room-temperature processing and improved transport properties offered by the solid-state exchange of exciton-delocalizing ligands show they offer promise for the construction of strongly coupled nanocrystal arrays.

Balancing the Electron and Hole Transfer for Efficient Quantum Dot Light-Emitting Diodes by Employing a Versatile Organic Electron-Blocking Layer

The electron-blocking layer (EBL) is important to balance the charge carrier transfer and achieve highly efficient quantum dot light-emitting diodes (QLEDs). Here, we report the utilization of a soluble tert-butyldimethylsilyl chloride-modified poly( p-phenylene benzobisoxazole) (TBS-PBO) as an EBL for simultaneous good charge carrier transfer balance while maintaining a high current density. We show that the versatile TBS-PBO blocks excess electron injection into the quantum dots (QDs), thus leading to better charge carrier transfer balance. It also restricts the undesired QD-to-EBL electron-transfer process, which preserves the superior emission capabilities of the emitter. As a consequence, the TBS-PBO device delivers an external quantum efficiency (EQE) maximum of 16.7% along with a remarkable current density as high as 139 mA/cm² with a brightness of 5484 cd/m². The current density of our device is higher than those of insulator EBL-based devices because of the higher conductivity of the TBS-PBO versus insulator EBL, thus helping achieve high luminance values ranging from 1414 to 20 000 cd/cm² with current densities ranging from 44 to 648 mA/cm² and EQE > 14%. We believe that these unconventional features of the present TBS-PBO-based QLEDs will expand the wide use of TBS-PBO as buffer layers in other advanced QLED applications.

Understanding chemically processed solar cells based on quantum dots

Photovoltaic energy conversion is one of the best alternatives to fossil fuel combustion. Petroleum resources are now close to depletion and their combustion is known to be responsible for the release of a considerable amount of greenhouse gases and carcinogenic airborne particles. Novel third-generation solar cells include a vast range of device designs and materials aiming to overcome the factors limiting the current technologies. Among them, quantum dot-based devices showed promising potential both as sensitizers and as colloidal nanoparticle films. A good example is the p-type PbS colloidal quantum dots (CQDs) forming a heterojunction with a n-type wide-band-gap semiconductor such as TiO2 or ZnO. The confinement in these nanostructures is also expected to result in marginal mechanisms, such as the collection of hot carriers and generation of multiple excitons, which would increase the theoretical conversion efficiency limit. Ultimately, this technology could also lead to the assembly of a tandem-type cell with CQD films absorbing in different regions of the solar spectrum.

Reliable and Low-Power Multilevel Resistive Switching in TiO2 Nanorod Arrays Structured with a TiOx Seed Layer

The electrical performance of TiO₂ nanorod array (NRA)-based resistive switching memory devices is examined in this paper. The formation of a seed layer on the fluorine-doped tin oxide (FTO) glass substrate after treatment in TiCl₄ solution, before the growth of TiO₂ NRAs on the FTO substrate via a hydrothermal process, is shown to significantly improve the resistive switching performance of the resulting TiO₂ NRA-based device. As fabricated, the Al/TiO₂ NRA/TiO_x layer/FTO device displayed electroforming-free bipolar resistive switching behavior while maintaining a stable ON/OFF ratio for more than 500 direct sweeping cycles over a retention period of 3 × 10⁴ s. Meanwhile, the programming current as low as ∼10^-8 A and 10^-10 A for low resistance state and high resistance state respectively makes the fabricated devices suitable for low-power memristor applications. The TiO_x precursor seed layer not only promotes the uniform and preferred growth of TiO₂ nanorods on the FTO substrate but also functions as an additional source layer of trap centers due to its oxygen-deficient composition. Our data suggest that the primary conduction mechanism in these devices arises from trap-mediated space-charge-limited current (SCLC). Multilevel memory performance in this new device is achieved by varying the SET voltage. The origin of this effect is also discussed.

A microwave synthesized CuxS and graphene oxide nanoribbon composite as a highly efficient counter electrode for quantum dot sensitized solar cells

To boost the photoconversion efficiency (PCE) of ever promising quantum dot sensitized solar cells (QDSSCs), and to improve the design of photoanodes, the ability of the counter electrode (CE) to effectively reduce the oxidized electrolyte needs special attention. A composite of a 15 wt% graphene oxide nanoribbon (GOR), obtained by unzipping multi-walled carbon nanotubes (MWCNTs), and CuxS intersecting hexagonal nanoplates, synthesized by a low cost, facile and scalable microwave synthesis route, is reported as a fascinating CE for QDSSCs. The best performing Cu1.18S-GOR CE could notably achieve a record PCE of ∼3.55% for CdS sensitized QDSSCs, ∼5.42% for in situ deposited CdS/CdSe co-sensitized QDSSCs and ∼6.81% for CdTe/CdS/CdS dual sensitized QDSSCs, apart from increasing the PCE of previously reported QDSSCs. A systematic investigation of the CE design revealed the high electrocatalytic activity of GOR due to the presence of organic functional groups, graphitic edge sites and a quasi-one-dimensional (quasi-1D) structure, which increases the interfacial charge transfer kinetics from the CE to the polysulfide electrolyte. The highly stable Cu1.18S-GOR CE has the added advantage of a favourable energy band alignment with the redox potential of the polysulfide electrolyte, which reduces the loss of charge carriers and thus can increase the PCE of QDSSCs.

Poly(3-hexylthiophene-2,5-diyl) as a Hole Transport Layer for Colloidal Quantum Dot Solar Cells

Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (Jsc) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO2 or ZnO). Here, we show the use of semiconducting polymer poly(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better Jsc and FF.

Anisotropic growth and structure-dependent photoresponse activity of multi-level one-dimensional PbS nano-architectures

PbS nanostructures with three-fold hierarchy in 〈111〉 directions for designing optoelectronic devices.

Trap-Assisted Transport and Non-Uniform Charge Distribution in Sulfur-Rich PbS Colloidal Quantum Dot-based Solar Cells with Selective Contacts

This study reports evidence of dispersive transport in planar PbS colloidal quantum dot heterojunction-based devices as well as the effect of incorporating a MoO3 hole selective layer on the charge extraction behavior. Steady state and transient characterization techniques are employed to determine the complex recombination processes involved in such devices. The addition of a selective contact drastically improves the device efficiency up to 3.15% (especially due to increased photocurrent and decreased series resistance) and extends the overall charge lifetime by suppressing the main first-order recombination pathway observed in device without MoO3. The lifetime and mobility calculated for our sulfur-rich PbS-based devices are similar to previously reported values in lead-rich quantum dots-based solar cells. Nevertheless, strong Shockley-Read-Hall mechanisms appear to keep restricting charge transport, as the equilibrium voltage takes more than 1 ms to be established.

Boosting Photon Harvesting in Organic Solar Cells with Highly Oriented Molecular Crystals via Graphene-Organic Heterointerface

Photon harvesting in organic solar cells is highly dependent on the anisotropic nature of the optoelectronic properties of photoactive materials. Here, we demonstrate an efficient approach to dramatically enhance photon harvesting in planar heterojunction solar cells by using a graphene-organic heterointerface. A large area, residue-free monolayer graphene is inserted at anode interface to serve as an atomically thin epitaxial template for growing highly orientated pentacene crystals with lying-down orientation. This anisotropic orientation enhances the overall optoelectronic properties, including light absorption, charge carrier lifetime, interfacial energetics, and especially the exciton diffusion length. Spectroscopic and crystallographic analysis reveal that the lying-down orientation persists until a thickness of 110 nm, which, along with increased exciton diffusion length up to nearly 100 nm, allows the device optimum thickness to be doubled to yield significantly enhanced light absorption within the photoactive layers. The resultant photovoltaic performance shows simultaneous increment in Voc, Jsc, and FF, and consequently a 5 times increment in the maximum power conversion efficiency than the equivalent devices without a graphene layer. The present findings indicate that controlling organic-graphene heterointerface could provide a design strategy of organic solar cell architecture for boosting photon harvesting.

Solution processed flexible and bending durable heterojunction colloidal quantum dot solar cell

A flexible hybrid heterojunction PbS colloidal quantum dot solar cell, where the photoactive layers are deposited using a solution processed fabrication approach under ambient condition and at room temperature is presented. The bending stability of the obtained solar cell is evaluated. The results show that the solar cell exhibits high bending stability and even under the bent state the cell also maintains a high performance, which shows the potential of the quantum dot solar cell toward a lightweight, bendable power source with many possible applications.

Reduced energy offset via substitutional doping for efficient organic/inorganic hybrid solar cells

Charge carrier transport in bulk heterojunction that is central to the device performance of solar cells is sensitively dependent on the energy level alignment of acceptor and donor. However, the effect of energy level regulation induced by nickel ions on the primary photoexcited electron transfer and the performance of P3HT/TiO₂ hybrid solar cells remains being poorly understood and rarely studied. Here we demonstrate that the introduction of the versatile nickel ions into TiO₂ nanocrystals can significantly elevate the conduction and valence band energy levels of the acceptor, thus resulting in a remarkable reduction of energy level offset between the conduction band of acceptor and lowest unoccupied molecular orbital of donor. By applying transient photoluminescence and femtosecond transient absorption spectroscopies, we demonstrate that the electron transfer becomes more competitive after incorporating nickel ions. In particular, the electron transfer life time is shortened from 30.2 to 16.7 ps, i.e., more than 44% faster than pure TiO₂ acceptor, thus leading to a notable increase of power conversion efficiency in organic/inorganic hybrid solar cells. This work underscores the promising virtue of engineering the reduction of 'excess' energy offset to accelerate electron transport and demonstrates the potential of nickel ions in applications of solar energy conversion and photon detectors.

Direct electron injection into an oxide insulator using a cathode buffer layer

Injecting charge carriers into the mobile bands of an inorganic oxide insulator (for example, SiO₂, HfO₂) is a highly complicated task, or even impossible without external energy sources such as photons. This is because oxide insulators exhibit very low electron affinity and high ionization energy levels. Here we show that a ZnO layer acting as a cathode buffer layer permits direct electron injection into the conduction bands of various oxide insulators (for example, SiO₂, Ta₂O₅, HfO₂, Al₂O₃) from a metal cathode. Studies of current–voltage characteristics reveal that the current ohmically passes through the ZnO/oxide-insulator interface. Our findings suggests that the oxide insulators could be used for simply fabricated, transparent and highly stable electronic valves. With this strategy, we demonstrate an electrostatic discharging diode that uses 100-nm SiO₂ as an active layer exhibiting an on/off ratio of ∼10⁷, and protects the ZnO thin-film transistors from high electrical stresses.

Carrier injection from a metallic electrode into an oxide insulator component is made difficult by the large energy level offset between the two. Here, the authors show that an intermediary zinc-oxide layer enables an Ohmic electrical contact between a metal and an oxide insulator layer.

Inverted quantum dot light emitting diodes using polyethylenimine ethoxylated modified ZnO

Colloidal quantum dots (QDs) are an emerging class of new materials due to their unique physical properties. In particular, colloidal QD based light emitting diodes (QDLEDs) have been extensively studied and developed for the next generation displays and solid-state lighting. Among a number of approaches to improve performance of the QDLEDs, the most practical one is optimization of charge transport and charge balance in the recombination region. Here, we suggest a polyethylenimine ethoxylated (PEIE) modified ZnO nanoparticles (NPs) as electron injection and transport layer for inverted structure red CdSe-ZnS based QDLED. The PEIE surface modifier, incorporated on the top of the ZnO NPs film, facilitates the enhancement of both electron injection into the CdSe-ZnS QD emissive layer by lowering the workfunction of ZnO from 3.58 eV to 2.87 eV and charge balance on the QD emitter. As a result, this device exhibits a low turn-on voltage of 2.0–2.5 V and has maximum luminance and current efficiency values of 8600 cd/m² and current efficiency of 1.53 cd/A, respectively. The same scheme with ZnO NPs/PEIE layer has also been used to successfully fabricate green, blue, and white QDLEDs.

Classification of solar cells according to mechanisms of charge separation and charge collection

This paper elaborates a general description of solar cells based on a single absorber material, according to the mechanisms of charge separation and charge collection.

Engineering charge injection and charge transport for high performance PbSe nanocrystal thin film devices and circuits

We study charge injection and transport in PbSe nanocrystal thin films. By engineering the contact metallurgy and nanocrystal ligand exchange chemistry and surface passivation, we demonstrate partial Fermi-level pinning at the metal-nanocrystal interface and an insulator-to-metal transition with increased coupling and doping, allowing us to design high conductivity and mobility PbSe nanocrystal films. We construct complementary nanocrystal circuits from n-type and p-type transistors realized from a single nanocrystal material by selecting the contact metallurgy.

p-i-n Heterojunction solar cells with a colloidal quantum-dot absorber layer

A quantum-dot (QD) p-i-n heterojunction solar cell with an increased depletion region is demonstrated by depleting the QD layer from both the front and back junctions. Due to a combination of improved charged extraction and increased light absorption, a 120% increase in the short-circuit current is achieved compared with that of conventional ZnO/QD devices.

Heterojunction PbS nanocrystal solar cells with oxide charge-transport layers

Oxides are commonly employed as electron-transport layers in optoelectronic devices based on semiconductor nanocrystals, but are relatively rare as hole-transport layers. We report studies of NiO hole-transport layers in PbS nanocrystal photovoltaic structures. Transient fluorescence experiments are used to verify the relevant energy levels for hole transfer. On the basis of these results, planar heterojunction devices with ZnO as the photoanode and NiO as the photocathode were fabricated and characterized. Solution-processed devices were used to systematically study the dependence on nanocrystal size and achieve conversion efficiency as high as 2.5%. Optical modeling indicates that optimum performance should be obtained with thinner oxide layers than can be produced reliably by solution casting. Room-temperature sputtering allows deposition of oxide layers as thin as 10 nm, which enables optimization of device performance with respect to the thickness of the charge-transport layers. The best devices achieve an open-circuit voltage of 0.72 V and efficiency of 5.3% while eliminating most organic material from the structure and being compatible with tandem structures.