S-Rich PbS Quantum Dots:A Promising p-Type Material for Optoelectronic Devices

PbS colloidal quantum dots (CQDs) are versatile building blocks for bottom-up fabrication of various optoelectronic devices. The transport properties of thin films of this class of materials depend on the size of the CQDs, their surface ligands, and stoichiometry. The most common synthetic methods yield PbS CQDs with an excess of Pb atoms, which induces n-type transport properties in CQD films. In this work, we developed a new synthesis, which offers S-rich PbS CQDs. Thanks to their sufficient colloidal stability in nonpolar solvents, we established a protocol for the integration of these CQDs into thin field-effect transistors and found strong hole-dominated transport with a hole mobility of about 1 × 10–2 cm2/Vs. Moreover, we were able to enhance the electron mobility for almost two orders of magnitude while keeping the hole mobility nearly the same. This approach allows us to obtain reliably p-doped PbS CQDs, which can be used for the fabrication of various electronic and optoelectronic devices.ISSN:0897-475

Double Gate PbS Quantum Dot Field-Effect Transistors for Tuneable Electrical Characteristics

In this work colloidal quantum dots double gate transistors are introduced. A high-k (k = 43) relaxor ferroelectric polymer is used as a dielectric material for the top gate in a device where the other gate is fabricated from SiO2. The device in double gate configuration is characterized by reduced hysteresis in the transfer curves measured by separately sweeping the voltage of the SiO2 and of the polymer gate. Gating with the relaxor polymer leads to mobility values of μe = 1.1 cm2 V−1s−1 and μh = 6 × 10−3 cm2 V−1s−1 that exceed those extracted from the SiO2 gating: μe = 0.5 cm2 V−1s−1 and μh = 2 × 10−3 cm2 V−1s−1. Measurements under double gating conditions prove that the device works in a single channel mode that is delocalized over the whole film thickness. Double gating allows for shifting the threshold voltage into a desired position and also allows increasing the on-current of the devices

Opto-electronics of PbS quantum dot and narrow bandgap polymer blends

Here we report on the interaction between the narrow bandgap polymer [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and lead sulphide (PbS) colloidal quantum dots (CQDs) upon photoexcitation. We show that the presence of both materials in a blend leads to a significant reduction of photoluminescence (PL) lifetime of the polymer. This observation is attributed, supported by transient absorption (TA) data, to an efficient electron transfer towards the QDs for excitons generated on the polymer. Furthermore, the ligand capping the QD surface exhibits a great impact on the dynamics of the PL, with the long-chain oleic acid (OA) largely suppressing any kind of interaction. By means of external quantum efficiency (EQE) measurements we find evidence that both components give rise to a contribution to the photocurrent, making this an interesting blend for future applications in hybrid organic-inorganic solar cells.</p

Scalable fabrication of efficient p-n junction lead sulfide quantum dot solar cells

Nowadays, the best lead sulfide (PbS) colloidal quantum dot (CQD) solar cells are primarily demonstrated in the n-p structure, while the p-n structure is significantly less developed. This technological gap between the n-p and p-n structures is much more distinct than in cases of other solution-processable photovoltaic technologies like perovskites and polymers. Here, we propose a scalable fabrication strategy for efficient PbS QD solar cells with p-n structure. An industrially suited blade-coating technique has been used to deposit both n-type and p-type QD layers. The obtained solar cells demonstrated power conversion efficiency of 9%, thus, commensurate to the record device efficiency with this architecture fabricated with a non-scalable technique. The availability of both p-n and n-p structures fabricated from scalable methods may promote the future integration of the PbS QDs into tandem devices together with other solution-processable materials to exploit the most prominent benefits of the PbS QDs, such as infrared absorption.</p

Electroluminescence Generation in PbS Quantum Dot Light-Emitting Field-Effect Transistors with Solid-State Gating

The application of light-emitting field-effect transistors (LEFET) is an elegant way of combining electrical switching and light emission in a single device architecture instead of two. This allows for a higher degree of miniaturization and integration in future optoelectronic applications. Here, we report on a LEFET based on lead sulfide quantum dots processed from solution. Our device shows state-of-the-art electronic behavior and emits near infrared photons with a quantum yield exceeding 1% when cooled. We furthermore show how LEFETs can be used to simultaneously characterize the optical and electrical material properties on the same device and use this benefit dot film. to investigate the charge transport through the quantum dot film

Stoichiometric control of the density of states in PbS colloidal quantum dot solids

Colloidal quantum dots, and nanostructured semiconductors in general, carry the promise of overcoming the limitations of classical materials in chemical and physical properties and in processability. However, sufficient control of electronic properties, such as carrier concentration and carrier mobility, has not been achieved until now, limiting their application. In bulk semiconductors, modifications of electronic properties are obtained by alloying or doping, an approach that is not viable for structures in which the surface is dominant. The electronic properties of PbS colloidal quantum dot films are fine-tuned by adjusting their stoichiometry, using the large surface area of the nanoscale building blocks. We achieve an improvement ofmore than two orders ofmagnitude in the holemobility, from below 10(-3) to above 0.1 cm(2)/N.s, by substituting the iodide ligands with sulfide while keeping the electron mobility stable (similar to 1 cm(2)/V.s). This approach is not possible in bulk semiconductors, and the developed method will likely contribute to the improvement of solar cell efficiencies through better carrier extraction and to the realization of complex (opto) electronic devices

Scalable PbS Quantum Dot Solar Cell Production by Blade Coating from Stable Inks

The recent development of phase transfer ligand exchange methods for PbS quantum dots (QD) has enhanced the performance of quantum dots solar cells and greatly simplified the complexity of film deposition. However, the dispersions of PbS QDs (inks) used for film fabrication often suffer from colloidal instability, which hinders large-scale solar cell production. In addition, the wasteful spin-coating method is still the main technique for the deposition of QD layer in solar cells. Here, we report a strategy for scalable solar cell fabrication from highly stable PbS QD inks. By dispersing PbS QDs capped with CH3NH3PbI3 in 2,6-difluoropyridine (DFP), we obtained inks that are colloidally stable for more than 3 months. Furthermore, we demonstrated that DFP yields stable dispersions even of large diameter PbS QDs, which are of great practical relevance owing to the extended coverage of the near-infrared region. The optimization of blade-coating deposition of DFP-based inks enabled the fabrication of PbS QD solar cells with power conversion efficiencies of up to 8.7%. It is important to underline that this performance is commensurate with the devices made by spin coating of inks with the same ligands. A good shelf life-time of these inks manifests itself in the comparatively high photovoltaic efficiency of 5.8% obtained with inks stored for more than 120 days

Free carrier generation and recombination in PbS quantum dot solar cells

Time Delayed Collection Field and Bias Assisted Charge Extraction (BACE) experiments are used to investigate the charge carrier dynamics in PbS colloidal quantum dot solar cells. We find that the free charge carrier creation is slightly field dependent, thus providing an upper limit to the fill factor. The BACE measurements reveal a rather high effective mobility of 2 x 10(-3) cm(2)/Vs, meaning that charge extraction is efficient. On the other hand, a rather high steady state non-geminate recombination coefficient of 3 x 10(-10) cm(3)/s is measured. We, therefore, propose a rapid free charge recombination to constitute the main origin for the limited efficiency of the PbS colloidal quantum dots cells. (C) 2016 AIP Publishing LLC

High performance photoelectrochemical hydrogen generation and solar cells with a double type II heterojunction

We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO2/CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under -0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster

Exploiting the lability of metal halide perovskites for doping semiconductor nanocomposites

Cesium lead halides have intrinsically unstable crystal lattices and easily transform within perovskite and nonperovskite structures. In this work, we explore the conversion of the perovskite CsPbBr into CsPbBr in the presence of PbS at 450 °C to produce doped nanocrystal-based composites with embedded CsPbBr nanoprecipitates. We show that PbBr is extracted from CsPbBr and diffuses into the PbS lattice with a consequent increase in the concentration of free charge carriers. This new doping strategy enables the adjustment of the density of charge carriers between 10 19 and 10 20 cm -3, and it may serve as a general strategy for doping other nanocrystal-based semiconductors