High-Throughput Screening of Lead-Free Perovskite-like Materials for Optoelectronic Applications

We use high-throughput density functional theory calculations to screen lead-free perovskite-like materials with compositions A₂BB′X₆, ABX₄, and A₃B₂X₉ for optoelectronic performance. We screen monovalent A and B′ cations from Na, K, Rb, Cs Cu, and Ag, trivalent B cations from Ga, In, and Sb, and monovalent anions from Cl, Br, and I. Our screening procedure is based on formation energy and hybrid HSE06 functional predicted bandgaps. We screened more than 480 compounds and found 10 compounds that have bandgaps in the 1.5–2.5 eV range. Of these 10 compounds, seven are new, not having been reported before. We further characterize effective masses, density of states, and absorption coefficients of these selected compounds for their suitability in optoelectronic applications. All 10 of these selected compounds are lead-free and are solution processable. These compounds pave a path forward for lead-free photovoltaics and light emission devices

Measuring Charge Carrier Diffusion in Coupled Colloidal Quantum Dot Solids

Colloidal quantum dots (CQDs) are attractive materials for inexpensive, room-temperature-, and solution-processed optoelectronic devices. A high carrier diffusion length is desirable for many CQD device applications. In this work we develop two new experimental methods to investigate charge carrier diffusion in coupled CQD solids under charge-neutral, <i>i.e.</i>, undepleted, conditions. The methods take advantage of the quantum-size-effect tunability of our materials, utilizing a smaller-bandgap population of quantum dots as a reporter system. We develop analytical models of diffusion in 1D and 3D structures that allow direct extraction of diffusion length from convenient parametric plots and purely optical measurements. We measure several CQD solids fabricated using a number of distinct methods and having significantly different doping and surface ligand treatments. We find that CQD materials recently reported to achieve a certified power conversion efficiency of 7% with hybrid organic–inorganic passivation have a diffusion length of 80 ± 10 nm. The model further allows us to extract the lifetime, trap density, mobility, and diffusion coefficient independently in each material system. This work will facilitate further progress in extending the diffusion length, ultimately leading to high-quality CQD solid semiconducting materials and improved CQD optoelectronic devices, including CQD solar cells

Hybridization of Phenylthiolate- and Methylthiolate-Adatom Species at Low Coverage on the Au(111) Surface

Using scanning tunneling microscopy we observed reaction products of two chemisorbed thiolate species, methylthiolate and phenylthiolate, on the Au(111) surface. Despite the apparent stability, organometallic complexes of methyl- and phenylthiolate with the gold-adatom (RS‑Au‑SR, with R as the hydrocarbon group) undergo a stoichiometric exchange reaction, forming hybridized CH₃S‑Au‑SPh complexes. Complementary density functional theory calculations suggest that the reaction is most likely mediated by a monothiolate RS‑Au complex bonded to the gold surface, which forms a trithiolate RS‑Au‑(SR)‑Au‑SR complex as a key intermediate. This work therefore reveals the novel chemical reactivity of the low-coverage “striped” phase of alkanethiols on gold and strongly points to the involvement of monoadatom thiolate intermediates in this reaction. By extension, such intermediates may be involved in the self-assembly process itself, shedding new light on this long-standing problem

Ultrafast Carrier Trapping in Thick-Shell Colloidal Quantum Dots

It has previously been found that Auger processes can lead to femtosecond carrier trapping in quantum dots, limiting their performance in optoelectronic applications that rely on radiative recombination. Using atomistic simulations, we investigate whether a shell can protect carriers from Auger-assisted trapping. For these studies we investigate CdSe/CdS core–shell quantum dots having total diameters reaching up to 10 nm. We find trapping lifetimes as fast as 1 ps for 2 nm shells, and we report that shells as thick as 6 nm are required to suppress trapping fully. The most efficient recombination mechanism is found to proceed through shallow empty traps, suggesting it can be suppressed by filling the traps through doping or external electrochemical potential. Our findings suggest that to achieve efficient light emission, surface traps have to be completely eliminated, even in thick-shell quantum dots

Solar Cells Based on Inks of n‑Type Colloidal Quantum Dots

New inorganic ligands including halide anions have significantly accelerated progress in colloidal quantum dot (CQD) photovoltaics in recent years. All such device reports to date have relied on halide treatment during solid-state ligand exchanges or on co-treatment of long-aliphatic-ligand-capped nanoparticles in the solution phase. Here we report solar cells based on a colloidal quantum dot ink that is capped using halide-based ligands alone. By judicious choice of solvents and ligands, we developed a CQD ink from which a homogeneous and thick colloidal quantum dot solid is applied in a single step. The resultant films display an n-type character, making it suitable as a key component in a solar-converting device. We demonstrate two types of quantum junction devices that exploit these iodide-ligand-based inks. We achieve solar power conversion efficiencies of 6% using this class of colloids

Computational Study of Magic-Size CdSe Clusters with Complementary Passivation by Carboxylic and Amine Ligands

The electronic and optical properties of tetrahedral CdSe magic clusters (average diameter ∼1.5 nm) protected by carboxyl and amine ligands, which correspond to previously reported experimental structures, are studied using density functional theory. We find extreme ligand packing densities, capping every single dangling bond of the inorganic core, strong dependence of the Z-type metal carboxylate binding on the amount of excess amine, and potential for improved photoluminescence upon replacing phenyl ligands with alkanes. The computed absorption spectra of the Cd₃₅Se₂₀ cluster agree well with experiments, resolving the 0.2 eV splitting of the first exciton peak due to spin–orbit coupling. We discuss the origin of the significant broadening of the optical spectra as due to phonons and structural variations in the ligand configurations and inorganic core apexes

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

Small-Band-Offset Perovskite Shells Increase Auger Lifetime in Quantum Dot Solids

Colloidal quantum dots (CQDs) enable low-cost, high-performance optoelectronic devices including photovoltaics, photodetectors, LEDs, and lasers. Continuous-wave lasing in the near-infrared remains to be realized based on such materials, yet a solution-processed NIR laser would be of use in communications and interconnects. In infrared quantum dots, long-lived gain is hampered by a high rate of Auger recombination. Here, we report the use of perovskite shells, grown on cores of IR-emitting PbS CQDs, and we thus reduce the rate of Auger recombination by up to 1 order of magnitude. We employ ultrafast transient absorption spectroscopy to isolate distinct Auger recombination phenomena and study the effect of bandstructure and passivation on Auger recombination. We corroborate the experimental findings with model-based investigations of Auger recombination in various CQD core–shell structures. We explain how the band alignment provided by perovskite shells comes close to the optimal required to suppress the Auger rate. These results provide a step along the path toward solution-processed near-infrared lasers

Controlling C₆₀ Organization through Dipole-Induced Band Alignment at Self-Assembled Monolayer Interfaces

Understanding the structural organization and growth of organic molecules on self-assembled monolayers (SAMs) is crucial for creating high-performance SAM-based electronic devices. We report herein C₆₀ adsorption onto benzenethiol (BT), pentafluorobenzenethiol (PFBT), and octanethiol (C8SH) SAM-modified Au(111) studied using scanning tunneling microscopy at the liquid–solid interface. A continuous film of C₆₀ molecules forms at a much faster rate (<i>k</i> = 3.3 × 10^–7 s^–1) on PFBT compared to that on BT (<i>k</i> = 7.2 × 10^–9 s^–1) and C8SH SAMs (<i>k</i> = 9.5 × 10^–9 s^–1). On the basis of density functional theory calculations, we propose that the difference in C₆₀ growth behavior originates from the dipole-induced dipole interactions between the SAM and C₆₀. This may be further augmented by an inverse charge transfer from C₆₀ to SAM. This work provides new insights into the self-assembly behavior of next-generation electronic materials

Atomistic Design of CdSe/CdS Core–Shell Quantum Dots with Suppressed Auger Recombination

We design quasi-type-II CdSe/CdS core–shell colloidal quantum dots (CQDs) exhibiting a suppressed Auger recombination rate. We do so using fully atomistic tight-binding wave functions and microscopic Coulomb interactions. The recombination rate as a function of the core and shell size and shape is tested against experiments. Because of a higher density of deep hole states and stronger hole confinement, Auger recombination is found to be up to six times faster for positive trions compared to negative ones in 4 nm core/10 nm shell CQDs. Soft-confinement at the interface results in weak suppression of Auger recombination compared to same-bandgap sharp-interface CQDs. We find that the suppression is due to increased volume of the core resulting in delocalization of the wave functions, rather than due to soft-confinement itself. We show that our results are consistent with previous effective mass models with the same system parameters. Increasing the dot volume remains the most efficient way to suppress Auger recombination. We predict that a 4-fold suppression of Auger recombination can be achieved in 10 nm CQDs by increasing the core volume by using rodlike cores embedded in thick shells