Cleavable Ligands Enable Uniform Close Packing in Colloidal Quantum Dot Solids.

Uniform close packing in colloidal quantum dot solids is critical for high-optical density, high-mobility optoelectronic devices. A hybrid-ligand strategy is developed, combining the advantages of solid state and solution-phase ligand exchanges. This strategy uses a medium length thioamide ligand that is readily cleaved in a single chemical treatment, leading to quantum dot solids with uniformly packed domains 3 times larger than those observed in ligand-exchanged films.

Record Charge Carrier Diffusion Length in Colloidal Quantum Dot Solids via Mutual Dot-To-Dot Surface Passivation.

Through a combination of chemical and mutual dot-to-dot surface passivation, high-quality colloidal quantum dot solids are fabricated. The joint passivation techniques lead to a record diffusion length for colloidal quantum dots of 230 ± 20 nm. The technique is applied to create thick photovoltaic devices that exhibit high current density without losing fill factor.

Efficient spray-coated colloidal quantum dot solar cells.

A colloidal quantum dot solar cell is fabricated by spray-coating under ambient conditions. By developing a room-temperature spray-coating technique and implementing a fully automated process with near monolayer control-an approach termed as sprayLD-an electronic defect is eliminated resulting in solar cell performance and statistical distribution superior to prior batch-processed methods along with a hero performance of 8.1%.

Electronically active impurities in colloidal quantum dot solids.

Colloidal quantum dot films have seen rapid progress as active materials in photodetection, light emission, and photovoltaics. Their processing from the solution phase makes them an attractive option for these applications due to the expected cost reductions associated with liquid-phase material deposition. Colloidally stable nanoparticles capped using long, insulating aliphatic ligands are used to form semiconducting, insoluble films via a solid-state ligand exchange in which the original ligands are replaced with short bifunctional ligands. Here we show that this ligand exchange can have unintended and undesired side effects: a high molecular weight complex can form, containing both lead oleate and the shorter conductive ligand, and this poorly soluble complex can end up embedded within the colloidal quantum dot (CQD) active layer. We further show that, by adding an acidic treatment during film processing, we can break up and wash away these complexes, producing a higher quality CQD solid. The improved material leads to photovoltaic devices with reduced series resistance and enhanced fill factor relative to controls employing previously reported CQD solids.

Effect of solvent environment on colloidal-quantum-dot solar-cell manufacturability and performance.

The absorbing layer in state-of-the-art colloidal quantum-dot solar cells is fabricated using a tedious layer-by-layer process repeated ten times. It is now shown that methanol, a common exchange solvent, is the main culprit, as extended exposure leaches off the surface halide passivant, creating carrier trap states. Use of a high-dipole-moment aprotic solvent eliminates this problem and is shown to produce state-of-the-art devices in far fewer steps.

The complete in-gap electronic structure of colloidal quantum dot solids and its correlation with electronic transport and photovoltaic performance.

The direct observation of the complete electronic band structure of a family of PbS CQD solids via photoelectron spectroscopy is reported. We investigate how materials processing strategies, such as the latest passivation methods that produce record-performance photovoltaics, achieve their performance advances. Halide passivated films show a drastic reduction in states in the midgap, contributing to a marked improvement in the device performance.

Role of bond adaptability in the passivation of colloidal quantum dot solids.

Colloidal quantum dot (CQD) solids are attractive materials for photovoltaic devices due to their low-cost solution-phase processing, high absorption cross sections, and their band gap tunability via the quantum size effect. Recent advances in CQD solar cell performance have relied on new surface passivation strategies. Specifically, cadmium cation passivation of surface chalcogen sites in PbS CQDs has been shown to contribute to lowered trap state densities and improved photovoltaic performance. Here we deploy a generalized solution-phase passivation strategy as a means to improving CQD surface management. We connect the effects of the choice of metal cation on solution-phase surface passivation, film-phase trap density of states, minority carrier mobility, and photovoltaic power conversion efficiency. We show that trap passivation and midgap density of states determine photovoltaic device performance and are strongly influenced by the choice of metal cation. Supported by density functional theory simulations, we propose a model for the role of cations, a picture wherein metals offering the shallowest electron affinities and the greatest adaptability in surface bonding configurations eliminate both deep and shallow traps effectively even in submonolayer amounts. This work illustrates the importance of materials choice in designing a flexible passivation strategy for optimum CQD device performance.

Directly deposited quantum dot solids using a colloidally stable nanoparticle ink.

We develop a photovoltaic colloidal quantum dot ink that allows for lossless, single-step coating of large areas in a manufacturing-compatible process. Our materials strategy involves a solution-phase ligand exchange to transport compatible linkers that yield 1-thioglycerol-capped PbS quantum dots in dimethyl sulfoxide with a photoluminescence quantum yield of 24%. A proof-of-principle solar cell made from the ink exhibits 2.1% power conversion efficiency.

Hybrid passivated colloidal quantum dot solids.

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

A donor-supply electrode (DSE) for colloidal quantum dot photovoltaics.

The highest-performing colloidal quantum dot (CQD) photovoltaics (PV) reported to date have relied on high-temperature (>500°C) annealing of electron-accepting TiO2. Room-temperature processing reduces energy payback time and manufacturing cost, enables flexible substrates, and permits tandem solar cells that integrate a small-bandgap back cell atop a low-thermal-budget larger-bandgap front cell. Here we report an electrode strategy that enables a depleted-heterojunction CQD PV device to be fabricated entirely at room temperature. We find that simply replacing the high-temperature-processed TiO2 with a sputtered version of the same material leads to poor performance due to the low mobility of the sputtered oxide. We develop instead a two-layer donor-supply electrode (DSE) in which a highly doped, shallow work function layer supplies a high density of free electrons to an ultrathin TiO2 layer via charge-transfer doping. Using the DSE we build all-room-temperature-processed small-bandgap (1 eV) colloidal quantum dot solar cells having 4% solar power conversion efficiency and high fill factor. These 1 eV bandgap cells are suitable for use as the back junction in tandem solar cells. The DSE concept, combined with control over TiO2 stoichiometry in sputtering, provides a much-needed tunable electrode to pair with quantum-size-effect CQD films.

Implications of partial tryptic digestion in organic-aqueous solvent systems for bottom-up proteome analysis.

For bottom-up MS, the digestion step is critical and is typically performed with trypsin. Solvent-assisted digestion in 80% acetonitrile has previously been shown to improve protein sequence coverage at shorter digestion times. This has been attributed to enhanced enzyme digestion efficiency in this solvent. However, our results demonstrate that tryptic digestion in 80% acetonitrile is less efficient than that of conventional (aqueous) digestion. This is a consequence of decreased enzyme activity beyond ~40% acetonitrile, increased enzyme autolysis and lower protein solubility in 80% acetonitrile. We observe multiple missed cleavages and reduced concentration of fully cleaved digestion products. Nonetheless we confirm, through room temperature solvent-assisted digestion, a consistent improvement in protein sequence coverage when analyzed by mass spectrometry. These results are explained through the increased number of unique digestion products available for detection. Thus, while solvent-assisted digestion has clear merits for proteome analysis, one should be aware of the inefficiency of protein digestion though this protocol, particularly with absolute protein quantitation experiments.

Combinatorial synthesis of mixed transition metal oxides for lithium-ion batteries.

Many methods exist for the synthesis of lithium metal oxides for use as positive electrode materials in lithium-ion batteries; however, no such method to date is both combinatorial (able to process broad ranges of compositional space quickly and efficiently) and well-studied to ensure confidence in the procedure. This study develops a procedure for microliter-scale solution-processing synthesis using a combinatorial solutions-processing robot. Two compositional systems (LiNi(x)Mn(2-x)O(4) and Li-Al-Mn oxide) were synthesized to compare the method to existing syntheses to ensure confidence in the procedure. Samples produced by this new synthetic procedure have crystal structures and lattice constants that closely match those of bulk-prepared samples found in the literature.