High mobility and low density of trap states in dual-solid-gated PbS nanocrystal field-effect transistors

Advancing Silver Bismuth Sulfide Quantum Dots for Practical Solar Cell Applications

Colloidal quantum dots (CQDs) show unique properties that distinguish them from their bulk form, the so-called quantum confinement effects. This feature manifests in tunable size-dependent band gaps and discrete energy levels, resulting in distinct optical and electronic properties. The investigation direction of colloidal quantum dots (CQDs) materials has started switching from high-performing materials based on Pb and Cd, which raise concerns regarding their toxicity, to more environmentally friendly compounds, such as AgBiS2. After the first breakthrough in solar cell application in 2016, the development of AgBiS2 QDs has been relatively slow, and many of the fundamental physical and chemical properties of this material are still unknown. Investigating the growth of AgBiS2 QDs is essential to understanding the fundamental properties that can improve this material's performance. This review comprehensively summarizes the synthesis strategies, ligand choice, and solar cell fabrication of AgBiS2 QDs. The development of PbS QDs is also highlighted as the foundation for improving the quality and performance of AgBiS2 QD. Furthermore, we prospectively discuss the future direction of AgBiS2 QD and its use for solar cell applications.

PbI2 Passivation of Three Dimensional PbS Quantum Dot Superlattices Toward Optoelectronic Metamaterials

Lead chalcogenide colloidal quantum dots are one of the most promising materials to revolutionize the field of short-wavelength infrared optoelectronics due to their bandgap tunability and strong absorption. By self-assembling these quantum dots into ordered superlattices, mobilities approaching those of the bulk counterparts can be achieved while still retaining their original optical properties. The recent literature focused mostly on PbSe-based superlattices, but PbS quantum dots have several advantages, including higher stability. In this work, we demonstrate highly ordered 3D superlattices of PbS quantum dots with tunable thickness up to 200 nm and high coherent ordering, both in-plane and along the thickness. We show that we can successfully exchange the ligands throughout the film without compromising the ordering. The superlattices as the active material of an ion gel-gated field-effect transistor achieve electron mobilities up to 220 cm2 V-1 s-1. To further improve the device performance, we performed a postdeposition passivation with PbI2, which noticeably reduced the subthreshold swing making it reach the Boltzmann limit. We believe this is an important proof of concept showing that it is possible to overcome the problem of high trap densities in quantum dot superlattices enabling their application in optoelectronic devices.

PbS Quantum Dots Ink with Months-Long Shelf-Lifetime Enabling Scalable and Efficient Short-Wavelength Infrared Photodetectors

The phase-transfer ligand exchange of PbS quantum dots (QDs) has substantially simplified device fabrication giving hope for future industrial exploitation. However, this technique when applied to QDs of large size (>4 nm) gives rise to inks with poor colloidal stability, thus hindering the development of QDs photodetectors in short-wavelength infrared range. Here, it is demonstrated that methylammonium lead iodide ligands can provide sufficient passivation of PbS QDs of size up to 6.7 nm, enabling inks with a minimum of ten-week shelf-life time, as proven by optical absorption and solution-small angle X-ray scattering. Furthermore, the maximum linear electron mobility of 4.7 × 10-2 cm2 V-1 s-1 is measured in field-effect transistors fabricated with fresh inks, while transistors fabricated with the same solution after ten-week storage retain 74% of the average starting electron mobility, demonstrating the outstanding quality both of the fresh and aged inks. Finally, photodetectors fabricated via blade-coating exhibit 76% external quantum efficiency at 1300 nm and 1.8 × 1012 Jones specific detectivity, values comparable with devices fabricated using ink with lower stability and wasteful methods such as spin-coating.

Approaching Bulk Mobility in PbSe Colloidal Quantum Dots 3D Superlattices

3D superlattices made of colloidal quantum dots are a promising candidate for the next generation of optoelectronic devices as they are expected to exhibit a unique combination of tunable optical properties and coherent electrical transport through minibands. While most of the previous work was performed on 2D arrays, the control over the formation of these systems is lacking, where limited long-range order and energetical disorder have so far hindered the potential of these metamaterials, giving rise to disappointing transport properties. Here, it is reported that nanoscale-level controlled ordering of colloidal quantum dots in 3D and over large areas allows the achievement of outstanding transport properties. The measured electron mobilities are the highest ever reported for a self-assembled solid of fully quantum-confined objects. This ultimately demonstrates that optoelectronic metamaterials with highly tunable optical properties (in this case in the short-wavelength infrared spectral range) and charge mobilities approaching that of bulk semiconductor can be obtained. This finding paves the way toward a new generation of optoelectronic devices.

Highly Sensitive Broadband Phototransistors Based on Gradient Tin/Lead Mixed Perovskites

Highly sensitive broadband photodetectors are critical to numerous cutting-edge technologies such as biomedical imaging, environment monitoring, and night vision. Here, phototransistors based on mixed Sn/Pb perovskites are reported, which demonstrate ultrahigh responsivity, gain and specific detectivity in a broadband from ultraviolet to near-infrared region. The interface properties of the perovskite phototransistors are optimized by a special three-step cleaning-healing-cleaning treatment, leading to a high hole mobility in the channel. The highly sensitive performance of the mixed Sn/Pb perovskite phototransistors can be attributed to the vertical compositional heterojunction automatically formed during the film deposition, which is helpful for the separation of photocarriers thereby enhancing a photogating effect in the perovskite channel. This work demonstrates a convenient approach to achieving high-performance phototransistors through tuning compositional gradient in mixed-metal perovskite channels.

Evidence of band filling in PbS colloidal quantum dot square superstructures

PbS square superstructures are formed by the oriented assembly of PbS quantum dots (QDs), reflecting the facet structures of each QD. In the square assembly, the quantum dots are highly oriented, in sharp contrast to the conventional hexagonal QD assemblies, in which the orientation of QDs is highly disordered, and each QD is connected through ligand molecules. Here, we measured the transport properties of the oriented assembly of PbS square superstructures. The combined electrochemical doping studies by electric double layer transistor (EDLT) and spectroelectrochemistry showed that more than fourteen electrons per quantum dot are introduced. Furthermore, we proved that the lowest conduction band is formed by the quasi-fourth degenerate quantized (1S_e) level in the PbS QD square superstructures.

All-Solution-Processed Quantum Dot Electrical Double-Layer Transistors Enhanced by Surface Charges of Ti3C2Tx MXene Contacts

Fully solution-processed, large-area, electrical double-layer transistors (EDLTs) are presented by employing lead sulfide (PbS) colloidal quantum dots (CQDs) as active channels and Ti₃C₂T_x MXene as electrical contacts (including gate, source, and drain). The MXene contacts are successfully patterned by standard photolithography and plasma-etch techniques and integrated with CQD films. The large surface area of CQD film channels is effectively gated by ionic gel, resulting in high performance EDLT devices. A large electron saturation mobility of 3.32 cm² V^-1 s^-1 and current modulation of 1.87 × 10⁴ operating at low driving gate voltage range of 1.25 V with negligible hysteresis are achieved. The relatively low work function of Ti₃C₂T_x MXene (4.42 eV) compared to vacuum-evaporated noble metals such as Au and Pt makes them a suitable contact material for n-type transport in iodide-capped PbS CQD films with a LUMO level of ∼4.14 eV. Moreover, we demonstrate that the negative surface charges of MXene enhance the accumulation of cations at lower gate bias, achieving a threshold voltage as low as 0.36 V. The current results suggest a promising potential of MXene electrical contacts by exploiting their negative surface charges.

On the Colloidal Stability of PbS Quantum Dots Capped with Methylammonium Lead Iodide Ligands

Phase-transfer exchange of pristine organic ligands for inorganic ones is essential for the integration of colloidal quantum dots (CQDs) in optoelectronic devices. This method results in a colloidal dispersion (ink) which can be directly deposited by various solution-processable techniques to fabricate conductive films. For PbS CQDs capped with methylammonium lead iodide ligands (MAPbI₃), the most commonly employed solvent is butylamine, which enables only a short-term (hours) colloidal stability and thus brings concerns on the possibility of manufacturing CQD devices on a large scale in a reproducible manner. In this work, we studied the stability of alternative inks in two highly polar solvents which impart long-term colloidal stability of CQDs: propylene carbonate (PC) and 2,6-difluoropyridine (DFP). The aging and the loss of the ink's stability were monitored with optical, structural, and transport measurements. With these solvents, PbS CQDs capped with MAPbI₃ ligands retain colloidal stability for more than 20 months, both in dilute and concentrated dispersions. After 17 months of ink storage, transistors with a maximum linear mobility for electrons of 8.5 × 10^-3 cm²/V s are fabricated; this value is 17% of the one obtained with fresh solutions. Our results show that both PC- and DFP-based PbS CQD inks offer the needed shelf life to allow for the development of a CQD device technology.

Rapid Photonic Processing of High-Electron-Mobility PbS Colloidal Quantum Dot Transistors

Recent advances in solution-processable semiconducting colloidal quantum dots (CQDs) have enabled their use in a range of (opto)electronic devices. In most of these studies, device fabrication relied almost exclusively on thermal annealing to remove organic residues and enhance inter-CQD electronic coupling. Despite its widespread use, however, thermal annealing is a lengthy process, while its effectiveness to eliminate organic residues remains limited. Here, we exploit the use of xenon flash lamp sintering to post-treat solution-deposited layers of lead sulfide (PbS) CQDs and their application in n-channel thin-film transistors (TFTs). The process is simple, fast, and highly scalable and allows for efficient removal of organic residues while preserving both quantum confinement and high channel current modulation. Bottom-gate, top-contact PbS CQD TFTs incorporating SiO₂ as the gate dielectric exhibit a maximum electron mobility of 0.2 cm² V^-1 s^-1, a value higher than that of control transistors (≈10^-2 cm² V^-1 s^-1) processed via thermal annealing for 30 min at 120 °C. Replacing SiO₂ with a polymeric dielectric improves the transistor's channel interface, leading to a significant increase in electron mobility to 3.7 cm² V^-1 s^-1. The present work highlights the potential of flash lamp annealing as a promising method for the rapid manufacture of PbS CQD-based (opto)electronic devices and circuits.

Exclusive Electron Transport in Core@Shell PbTe@PbS Colloidal Semiconductor Nanocrystal Assemblies

Assemblies of colloidal semiconductor nanocrystals (NCs) in the form of thin solid films leverage the size-dependent quantum confinement properties and the wet chemical methods vital for the development of the emerging solution-processable electronics, photonics, and optoelectronics technologies. The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. Here we demonstrate a strategy to render the solids of narrow-bandgap NC assemblies exclusively electron-transporting by creating a type-II heterojunction via shelling. Electronic transport of molecularly cross-linked PbTe@PbS core@shell NC assemblies is measured using both a conventional solid gate transistor and an electric-double-layer transistor, as well as compared with those of core-only PbTe NCs. In contrast to the ambipolar characteristics demonstrated by many narrow-bandgap NCs, the core@shell NCs exhibit exclusive n-type transport, i.e., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms a type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level and simultaneously strongly localizes the holes within the NC core valence level. This strongly enhanced n-type transport makes these core@shell NCs suitable for applications where ambipolar characteristics should be actively suppressed, in particular, for thermoelectric and electron-transporting layers in photovoltaic devices.

Surface-Modified Substrates for Quantum Dot Inks in Printed Electronics

Printed electronics fill the niches for low-cost, flexible devices in electronics. Developing substrates suitable for various printable electronic inks becomes an important topic in both academia and industry. Because of their extraordinary properties like solution processability, colloidal quantum dots (QDs) are gradually emerging in this field as promising candidates for electronic inks. In recent years, researchers have successfully produced high quality PbS QD inks in polar solvents. However, the incorporation of electronic inks onto a well-passivated substrate remains challenging due to the processing incompatibility between polar solvents and hydrophobic substrates. Here, we propose a surface modification strategy by using chlorine to achieve both trap-site suppression and a hydrophilic surface. The chlorine can effectively passivate the surface dangling bonds and charged hydroxyls while creating a hydrophilic surface. On this modified substrate, the contact angle between the water droplet and the SiO₂ substrate can be as small as 20° and this strategy is also feasible for other polymer and inorganic substrates. For a proof-of-concept demonstration, we fabricated a PbS QD ink-based field-effect transistor on a Cl-passivated substrate, and the device showed a mobility as high as 4.36 × 10^-3 cm²/V s, which indicates effective trap-site suppression. This device also enables the potential of the Cl-passivated substrates for QD inks with water or other polar solvents.

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 to investigate the charge transport through the quantum dot film.

FeIn2S4 Nanocrystals: A Ternary Metal Chalcogenide Material for Ambipolar Field-Effect Transistors

An ambipolar channel layer material is required to realize the potential benefits of ambipolar complementary metal-oxide-semiconductor field-effect transistors, namely their compact and efficient nature, reduced reverse power dissipation, and possible applicability to highly integrated circuits. Here, a ternary metal chalcogenide nanocrystal material, FeIn2S4, is introduced as a solution-processable ambipolar channel material for field-effect transistors (FETs). The highest occupied molecular orbital and the lowest unoccupied molecular orbital of the FeIn2S4 nanocrystals are determined to be -5.2 and -3.75 eV, respectively, based upon cyclic voltammetry, X-ray photoelectron spectroscopy, and diffraction reflectance spectroscopy analyses. An ambipolar FeIn2S4 FET is successfully fabricated with Au electrodes (EF = -5.1 eV), showing both electron mobility (14.96 cm2 V-1 s-1) and hole mobility (9.15 cm2 V-1 s-1) in a single channel layer, with an on/off current ratio of 105. This suggests that FeIn2S4 nanocrystals may be a promising alternative semiconducting material for next-generation integrated circuit development.

Printed Electronics Based on Inorganic Semiconductors: From Processes and Materials to Devices

Following the ever-expanding technological demands, printed electronics has shown palpable potential to create new and commercially viable technologies that will benefit from its unique characteristics, such as, large-area and wide range of substrate compatibility, conformability and low-cost. Through the last few decades, printed/solution-processed field-effect transistors (FETs) and circuits have witnessed immense research efforts, technological growth and increased commercial interests. Although printing of functional inks comprising organic semiconductors has already been initiated in early 1990s, gradually the attention, at least partially, has been shifted to various forms of inorganic semiconductors, starting from metal chalcogenides, oxides, carbon nanotubes and very recently to graphene and other 2D semiconductors. In this review, the entire domain of printable inorganic semiconductors is considered. In fact, thanks to the continuous development of materials/functional inks and novel design/printing strategies, the inorganic printed semiconductor-based circuits today have reached an operation frequency up to several hundreds of kilohertz with only a few nanosecond time delays at the individual FET/inverter levels; in this regard, often circuits based on hybrid material systems have been found to be advantageous. At the end, a comparison of relative successes of various printable inorganic semiconductor materials, the remaining challenges and the available future opportunities are summarized.

Solution Processed Metal Oxide High-κ Dielectrics for Emerging Transistors and Circuits

The electronic functionalities of metal oxides comprise conductors, semiconductors, and insulators. Metal oxides have attracted great interest for construction of large-area electronics, particularly thin-film transistors (TFTs), for their high optical transparency, excellent chemical and thermal stability, and mechanical tolerance. High-permittivity (κ) oxide dielectrics are a key component for achieving low-voltage and high-performance TFTs. With the expanding integration of complementary metal oxide semiconductor transistors, the replacement of SiO2 with high-κ oxide dielectrics has become urgently required, because their provided thicker layers suppress quantum mechanical tunneling. Toward low-cost devices, tremendous efforts have been devoted to vacuum-free, solution processable fabrication, such as spin coating, spray pyrolysis, and printing techniques. This review focuses on recent progress in solution processed high-κ oxide dielectrics and their applications to emerging TFTs. First, the history, basics, theories, and leakage current mechanisms of high-κ oxide dielectrics are presented, and the underlying mechanism for mobility enhancement over conventional SiO2 is outlined. Recent achievements of solution-processed high-κ oxide materials and their applications in TFTs are summarized and traditional coating methods and emerging printing techniques are introduced. Finally, low temperature approaches, e.g., ecofriendly water-induced, self-combustion reaction, and energy-assisted post treatments, for the realization of flexible electronics and circuits are discussed.

Intra- and inter-nanocrystal charge transport in nanocrystal films

The exploitation of semiconductor nanocrystal (NC) films in novel electronic and optoelectronic applications requires a better understanding of charge transport in these systems. Here, we develop a model of charge transport in NC films, based on a generalization of the concept of transport energy level ET to nanocrystal assemblies, which considers both intra- and inter-NC charge transfer processes. We conclude that the role played by each of these processes can be probed from temperature-dependent measurements of charge carrier density n and mobility μ in the same films. The model also enables the determination of the position of the Fermi energy level EF with respect to ET, an important parameter of charge transport in semiconductor materials, from the temperature dependence of n. Moreover, we provide support to an essentially temperature-independent intra-NC charge carrier mobility, considered in the transport level concept, and consequently the frequently observed temperature dependence of the overall mobility μ in NC films results from a temperature variation of the inter-NC charge transport processes. Importantly, we also conclude that the temperature dependence of conductivity in NC films should result in general from a combination of temperature variations of both n and μ. By applying the model to solution-processed Si NC films, we conclude that transport within each NC is similar to that in amorphous Si (a-Si), with charges hopping along band tail states located below the conduction band edge. For Si NCs, we obtain values of ET - EF of ∼0.25 eV. The overall mobility μ in Si NC films is significantly further reduced with respect to that typically found in a-Si due to the additional transport constraints imposed by inter-NC transfer processes inherent to a nanoparticulate film. Our model accounting for inter- and intra-NC charge transport processes provides a simple and more general description of charge transport that can be broadly applied to films of semiconductor NCs.

In Situ Passivation for Efficient PbS Quantum Dot Solar Cells by Precursor Engineering

Current efforts on lead sulfide quantum dot (PbS QD) solar cells are mostly paid to the device architecture engineering and postsynthetic surface modification, while very rare work regarding the optimization of PbS synthesis is reported. Here, PbS QDs are successfully synthesized using PbO and PbAc₂  · 3H₂ O as the lead sources. QD solar cells based on PbAc-PbS have demonstrated a high power conversion efficiency (PCE) of 10.82% (and independently certificated values of 10.62%), which is significantly higher than the PCE of 9.39% for PbO-PbS QD based ones. For the first time, systematic investigations are carried out on the effect of lead precursor engineering on the device performance. It is revealed that acetate can act as an efficient capping ligands together with oleic acid, providing better surface coverage and replace some of the harmful hydroxyl (OH) ligands during the synthesis. Then the acetate on the surface can be exchanged by iodide and lead to desired passivation. This work demonstrates that the precursor engineering has great potential in performance improvement. It is also pointed out that the initial synthesis is an often neglected but critical stage and has abundant room for optimization to further improve the quality of the resultant QDs, leading to breakthrough efficiency.

High Speed Ultraviolet Phototransistors Based on an Ambipolar Fullerene Derivative

Combining high charge carrier mobility with ambipolar transport in light-absorbing organic semiconductors is highly desirable as it leads to enhanced charge photogeneration, and hence improved performance, in various optoelectronic devices including solar cells and photodetectors. Here we report the development of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM)-based ultraviolet (UV) phototransistors with balanced electron and hole transport characteristics. The latter is achieved by fine-tuning the source-drain electrode work function using a self-assembled monolayer. Opto/electrical characterization of as-prepared ambipolar PC₆₁BM phototransistors reveals promising photoresponse, particularly in the UV-A region (315-400 nm), with a maximum photosensitivity and responsivity of 9 × 10³ and 3 × 10³ A/W, respectively. Finally, the temporal response of the PC₆₁BM phototransistors is found to be high despite the long channel length (10 s of μm) with typical switching times of <2 ms.

Colloidal Quantum Dot Inks for Single-Step-Fabricated Field-Effect Transistors: The Importance of Postdeposition Ligand Removal

Colloidal quantum dots are a class of solution-processed semiconductors with good prospects for photovoltaic and optoelectronic applications. Removal of the surfactant, so-called ligand exchange, is a crucial step in making the solid films conductive, but performing it in solid state introduces surface defects and cracks in the films. Hence, the formation of thick, device-grade films have only been possible through layer-by-layer processing, limiting the technological interest for quantum dot solids. Solution-phase ligand exchange before the deposition allows for the direct deposition of thick, homogeneous films suitable for device applications. In this work, fabrication of field-effect transistors in a single step is reported using blade-coating, an upscalable, industrially relevant technique. Most importantly, a postdeposition washing step results in device properties comparable to the best layer-by-layer processed devices, opening the way for large-scale fabrication and further interest from the research community.

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.

Broadening of Distribution of Trap States in PbS Quantum Dot Field-Effect Transistors with High-k Dielectrics

We perform a quantitative analysis of the trap density of states (trap DOS) in PbS quantum dot field-effect transistors (QD-FETs), which utilize several polymer gate insulators with a wide range of dielectric constants. With increasing gate dielectric constant, we observe increasing trap DOS close to the lowest unoccupied molecular orbital (LUMO) of the QDs. In addition, this increase is also consistently followed by broadening of the trap DOS. We rationalize that the increase and broadening of the spectral trap distribution originate from dipolar disorder as well as polaronic interactions, which are appearing at strong dielectric polarization. Interestingly, the increased polaron-induced traps do not show any negative effect on the charge carrier mobility in our QD devices at the highest applied gate voltage, giving the possibility to fabricate efficient low-voltage QD devices without suppressing carrier transport.

Trap-mediated electronic transport properties of gate-tunable pentacene/MoS2 p-n heterojunction diodes

We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS₂) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS₂ and pentacene. The pentacene/MoS₂ p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.

Counterion-Mediated Ligand Exchange for PbS Colloidal Quantum Dot Superlattices

In the past years, halide capping became one of the most promising strategies to passivate the surface of colloidal quantum dots (CQDs) in thin films to be used for electronic and optoelectronic device fabrication. This is due to the convenient processing, strong n-type characteristics, and ambient stability of the devices. Here, we investigate the effect of three counterions (ammonium, methylammonium, and tetrabutylammonium) in iodide salts used for treating CQD thin films and shed light on the mechanism of the ligand exchange. We obtain two- and three-dimensional square-packed PbS CQD superlattices with epitaxial merging of nearest neighbor CQDs as a direct outcome of the ligand-exchange reaction and show that the order in the layer can be controlled by the nature of the counterion. Furthermore, we demonstrate that the acidity of the environment plays an important role in the substitution of the carboxylates by iodide ions at the surface of lead chalcogenide quantum dots. Tetrabutylammonium iodide shows lower reactivity compared to methylammonium and ammonium iodide due to the nonacidity of the cation, which eventually leads to higher order but also poorer carrier transport due to incomplete removal of the pristine ligands in the QD thin film. Finally, we show that single-step blade-coating and immersion in a ligand exchange solution such as the one containing methylammonium iodide can be used to fabricate well performing bottom-gate/bottom-contact PbS CQD field effect transistors with record subthreshold swing.