Enabling Ambipolar to Heavy n‑Type Transport in PbS Quantum Dot Solids through Doping with Organic Molecules

PbS quantum dots (QDs) are remarkable semiconducting materials, which are compatible with low-cost solution-processed electronic device fabrication. Understanding the doping of these materials is one of the great research interests, as it is a necessary step to improve the device performance as well as to enhance the applicability of this system for diverse optoelectronic applications. Here, we report the efficient doping of the PbS QD films with the use of solution-processable organic molecules. By engineering the energy levels of the donor molecules and the PbS QDs through the use of different cross-linking ligands, we are able to control the characteristics of PbS field-effect transistors (FETs) from ambipolar to strongly n-type. Because the doping promotes trap filling, the charge carrier mobility is improved up to 0.64 cm² V^–1 s^–1, which is the highest mobility reported for low-temperature processed PbS FETs employing SiO₂ as the gate dielectric. The doping also reduces the contact resistance of the devices, which can also explain the origin of the increased mobility

Quasi-epitaxial Metal-Halide Perovskite Ligand Shells on PbS Nanocrystals

Epitaxial growth techniques enable nearly defect free heterostructures with coherent interfaces, which are of utmost importance for high performance electronic devices. While high-vacuum technology-based growth techniques are state-of-the art, here we pursue a purely solution processed approach to obtain nanocrystals with eptaxially coherent and quasi-lattice matched inorganic ligand shells. Octahedral metal-halide clusters, respectively 0-dimensional perovskites, were employed as ligands to match the coordination geometry of the PbS cubic rock-salt lattice. Different clusters (CH₃NH₃⁺)_{(6–<i>x</i>)}[M^(<i>x</i>+)Hal₆]^{(6–<i>x</i>)–} (M^<i>x</i>+ = Pb­(II), Bi­(III), Mn­(II), In­(III), Hal = Cl, I) were attached to the nanocrystal surfaces <i>via</i> a scalable phase transfer procedure. The ligand attachment and coherence of the formed PbS/ligand core/shell interface was confirmed by combining the results from transmission electron microscopy, small-angle X-ray scattering, nuclear magnetic resonance spectroscopy and powder X-ray diffraction. The lattice mismatch between ligand shell and nanocrystal core plays a key role in performance. In photoconducting devices the best performance (detectivity of 2 × 10¹¹ cm Hz ^1/2/W with > 110 kHz bandwidth) was obtained with (CH₃NH₃)₃BiI₆ ligands, providing the smallest relative lattice mismatch of <i>ca</i>. −1%. PbS nanocrystals with such ligands exhibited in millimeter sized bulk samples in the form of pressed pellets a relatively high carrier mobility for nanocrystal solids of ∼1.3 cm²/(V s), a carrier lifetime of ∼70 μs, and a low residual carrier concentration of 2.6 × 10¹³ cm^–3. Thus, by selection of ligands with appropriate geometry and bond lengths optimized quasi-epitaxial ligand shells were formed on nanocrystals, which are beneficial for applications in optoelectronics

Crystal Phase Transitions in the Shell of PbS/CdS Core/Shell Nanocrystals Influences Photoluminescence Intensity

We reveal the existence of two different crystalline phases, i.e., the metastable <i>rock salt</i> and the equilibrium <i>zinc blende</i> phase within the CdS-shell of PbS/CdS core/shell nanocrystals formed by cationic exchange. The chemical composition profile of the core/shell nanocrystals with different dimensions is determined by means of anomalous small-angle X-ray scattering with subnanometer resolution and is compared to X-ray diffraction analysis. We demonstrate that the photoluminescence emission of PbS nanocrystals can be drastically enhanced by the formation of a CdS shell. Especially, the ratio of the two crystalline phases in the shell significantly influences the photoluminescence enhancement. The highest emission was achieved for chemically pure CdS shells below 1 nm thickness with a dominant metastable <i>rock salt</i> phase fraction matching the crystal structure of the PbS core. The metastable phase fraction decreases with increasing shell thickness and increasing exchange times. The photoluminescence intensity depicts a constant decrease with decreasing metastable <i>rock salt</i> phase fraction but shows an abrupt drop for shells above 1.3 nm thickness. We relate this effect to two different transition mechanisms for changing from the metastable <i>rock salt</i> phase to the equilibrium <i>zinc blende</i> phase depending on the shell thickness

From Highly Monodisperse Indium and Indium Tin Colloidal Nanocrystals to Self-Assembled Indium Tin Oxide Nanoelectrodes

Indium tin oxide (ITO) nanopatterned electrodes are prepared from colloidal solutions as a material saving alternative to the industrial vapor phase deposition and top down processing. For that purpose highly monodisperse In_1–<i>x</i>Sn_<i>x</i> (<i>x</i> < 0.1) colloidal nanocrystals (NCs) are synthesized with accurate size and composition control. The outstanding monodispersity of the NCs is evidenced by their self-assembly properties into highly ordered superlattices. Deposition on structured substrates and subsequent treatment in oxygen plasma converts the NC assemblies into transparent electrode patterns with feature sizes down to the diameter of single NCs. The conductivity in these ITO electrodes competes with the best values reported for electrodes from ITO nanoparticle inks

High Infrared Photoconductivity in Films of Arsenic-Sulfide-Encapsulated Lead-Sulfide Nanocrystals

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH₄)₃AsS₃-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH₄)₃AsS₃ capping ligands were converted into a thin layer of As₂S₃, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2 × 10¹³ Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids

Hydrogen-Bonded Organic Semiconductor Micro- And Nanocrystals: From Colloidal Syntheses to (Opto-)Electronic Devices

Organic pigments such as indigos, quinacridones, and phthalocyanines are widely produced industrially as colorants for everyday products as various as cosmetics and printing inks. Herein we introduce a general procedure to transform commercially available insoluble microcrystalline pigment powders into colloidal solutions of variously sized and shaped semiconductor micro- and nanocrystals. The synthesis is based on the transformation of the pigments into soluble dyes by introducing transient protecting groups on the secondary amine moieties, followed by controlled deprotection in solution. Three deprotection methods are demonstrated: thermal cleavage, acid-catalyzed deprotection, and amine-induced deprotection. During these processes, ligands are introduced to afford colloidal stability and to provide dedicated surface functionality and for size and shape control. The resulting micro- and nanocrystals exhibit a wide range of optical absorption and photoluminescence over spectral regions from the visible to the near-infrared. Due to excellent colloidal solubility offered by the ligands, the achieved organic nanocrystals are suitable for solution processing of (opto)­electronic devices. As examples, phthalocyanine nanowire transistors as well as quinacridone nanocrystal photodetectors, with photoresponsivity values by far outperforming those of vacuum deposited reference samples, are demonstrated. The high responsivity is enabled by photoinduced charge transfer between the nanocrystals and the directly attached electron-accepting vitamin B2 ligands. The semiconducting nanocrystals described here offer a cheap, nontoxic, and environmentally friendly alternative to inorganic nanocrystals as well as a new paradigm for obtaining organic semiconductor materials from commercial colorants

Size-Dependent Electron Transfer from Colloidal PbS Nanocrystals to Fullerene

We investigate a promising organic/inorganic hybrid composite for solution-processable optoelectronics made by lead sulphide nanoparticles and fullerene derivatives, which combine the sensitivity of PbS to the infrared spectrum with the good electron transport properties of fullerenes. Charge separation is the crucial process that determines whether the heterojunction can be the building block for devices converting photogenerated excitons into free charges flowing in a circuit. Subpicosecond spectroscopy techniques on bulk heterojunctions between PbS nanocrystals of various sizes and [6,6]-phenyl-61-butyric acid methyl ester (PCBM) were employed to reveal the ultrafast dynamics of photoexcited carriers, particularly transfer of photoexcited electrons from nanocrystals to PCBM. Electron transfer is found to critically depend on nanoparticle size, occurring for nanocrystals with diameter 4.4 nm and smaller, not for larger ones. Our findings are relevant to the engineering of hybrid solar cells and light detectors based on PbS nanocrystal/fullerene bulk heterojunctions

Random Lasing with Systematic Threshold Behavior in Films of CdSe/CdS Core/Thick-Shell Colloidal Quantum Dots

While over the past years the syntheses of colloidal quantum dots (CQDs) with core/shell structures were continuously improved to obtain highly efficient emission, it has remained a challenge to use them as active materials in laser devices. Here, we report random lasing at room temperature in films of CdSe/CdS CQDs with different core/shell band alignments and extra thick shells. Even though the lasing process is based on random scattering, we find systematic dependencies of the laser thresholds on morphology and laser spot size. To minimize laser thresholds, optimizing the film-forming properties of the CQDs, proven by small-angle X-ray scattering, was found to be more important than the optical parameters of the CQDs, such as biexciton lifetime and binding energy or fluorescence decay time. Furthermore, the observed systematic behavior turned out to be highly reproducible after storing the samples in air for more than 1 year. These highly reproducible systematic dependencies suggest that random lasing experiments are a valuable tool for testing nanocrystal materials, providing a direct and simple feedback for further development of colloidal gain materials toward lasing in continuous wave operation