The efficiency of nanoparticle (NP) solar cells has grown impressively in recent years, exceeding 16%. However, the carrier mobility in NP solar cells, and in other optoelectronic applications remains low, thus critically limiting their performance. Therefore, carrier transport in NP solids needs to be better understood to further improve the overall efficiency of NP solar cell technology. However, it is technically challenging to simulate experimental scale samples, as physical processes from atomic to mesoscopic scales all crucially impact transport. To rise to this challenge, here we report the development of TRIDENS: the Transport in Defected Nanoparticle Solids Simulator, that adds three more hierarchical layers to our previously developed HINTS code for nanoparticle solar cells. In TRIDENS, we first introduced planar defects, such as twin planes and grain boundaries into individual NP SLs superlattices (SLs) that comprised the order of 10³ NPs. Then we used HINTS to simulate the transport across tens of thousands of defected NP SLs, and constructed the distribution of the NP SL mobilities with planar defects. Second, the defected NP SLs were assembled into a resistor network with more than 10⁴ NP SLs, thus representing about 10⁷ individual NPs. Finally, the TRIDENS results were analyzed by finite size scaling to explore whether the percolation transition, separating the phase where the low mobility defected NP SLs percolate, from the phase where the high mobility undefected NP SLs percolate drives a low-mobility-to-highmobility transport crossover that can be extrapolated to genuinely macroscopic length scales. For the theoretical description, we adapted the Efros-Shklovskii bimodal mobility distribution percolation model. We demonstrated that the ES bimodal theory's two-variable scaling function is an effective tool to quantitatively characterize this low-mobility-to-high-mobility transport crossover.

Epitaxially-fused superlattices of colloidal quantum dots (QD epi-SLs) may exhibit electronic minibands and high-mobility charge transport, but electrical measurements of epi-SLs have been limited to large-area, polycrystalline samples in which superlattice grain boundaries and intragrain defects suppress/obscure miniband effects. Systematic measurements of charge transport in individual, highly-ordered epi-SL grains would facilitate the study of minibands in QD films. Here, we demonstrate the air-free fabrication of microscale field-effect transistors (μ-FETs) with channels consisting of single PbSe QD epi-SL grains (2-7 μm channel dimensions) and analyze charge transport in these single-grain devices. The eight devices studied show p-channel or ambipolar transport with a hole mobility as high as 3.5 cm² V^-1 s^-1 at 290 K and 6.5 cm² V^-1 s^-1 at 170-220 K, one order of magnitude larger than that of previous QD solids. The mobility peaks at 150-220 K, but device hysteresis at higher temperatures makes the true mobility-temperature curve uncertain and evidence for miniband transport inconclusive.

The authors regret the following minor errors in the published article: In Section 2.1 (Materials), two chemicals, lead iodide and ethanol, were accidentally omitted from the list. The revised text should read:

Oriented attachment of nanocrystals is an important route to constructing epitaxially-connected nanocrystal superlattices for various applications. During oriented attachment of semiconductor nanocrystals, neck can be formed between nanocrystals and it strongly influences the properties of the resulting superlattice. However, the neck formation mechanism is poorly understood. Here, we use in situ liquid cell transmission electron microscopy to directly observe the initiation and growth of homoepitaxial necks between PbSe nanocrystals with atomic details. We find that neck initiation occurs slowly (~ 10 s) when two nanocrystals approach to each other within an edge-to-edge distance of 0.6 nm. During neck initiation, Pb and Se atoms defuse from other facets into the gap, forming “dynamic reversible” filaments. Once the filament (neck) width is larger than a critical size of 0.9 nm, it gradually (15 s) widens into a 3-nm-wide neck. The atomic structure of the neck is further obtained using ex situ aberration-corrected scanning TEM imaging. Neck initiation and growth mechanisms are elucidated with density functional theory calculations. Our direct unveiling of the atomic pathways of neck formation during oriented attachment shed light into the fabrication of nanocrystal superlattices with improved structural order and electronic properties. [Figure not available: see fulltext.].

Three dimensional epitaxially-fused colloidal quantum dot (QD) superlattices (epi-SLs) feature exceptional electronic coupling and spatial order and are promising systems for studying the emergence of delocalized states and mini-band charge transport in self-assembled solids. However, energy disorder arising from structural defects and aperiodicity has so far resulted in charge carrier localization and slow hopping transport. Detailed 3D structural characterization is critical for rationally improving epi-SL structural perfection to trigger the formation of mini-bands. Here, we analyze the 3D structure of a 120 × 38 nm disc-shaped region of a PbSe QD epi-SL using full-tilt high-angle annular dark-field electron tomography. The high spatial resolution of the tomographic reconstruction (0.65 nm) enables determination of the center-of-mass coordinates of all 1846 QDs in the sample as well as the size and shape of the thousands of epitaxial connections (necks) between the QDs. The tomogram reveals the detailed crystallography and internal positional disorder of the three SL grains that constitute this sample. A map of the neck network is used to quantify relationships between neck number (the number of necks each QD possesses), average neck diameter, QD location in the film, and the nearest neighbor inter-QD distance and distance distribution. We find a strong positive correlation between neck number and local spatial order, suggesting that future improvements in neck connectivity are likely to simultaneously enhance the overall structural perfection of the epi-SLs. A kinetic Monte Carlo model is employed to estimate the electron mobility of the tomography sample and assess the impact of grain boundaries on charge transport. Our electron tomography study establishes a baseline for the quantitative statistical analysis of structural defects in 3D QD epi-SLs.

Highly ordered epitaxially fused colloidal quantum dot (QD) superlattices (epi-SLs) promise to combine the size-tunable photophysics of QDs with the efficient charge transport of bulk semiconductors. However, current epi-SL fabrication methods are crude and result in structurally and chemically inhomogeneous samples with high concentrations of extended defects that localize carriers and prevent the emergence of electronic mini-bands. Needed fabrication improvements are hampered by inadequate understanding of the ligand chemistry that causes epi-SL conversion from the unfused parent SL. Here we show that epi-SL formation by the conventional method of amine injection into an ethylene glycol subphase under a floating QD film occurs by deprotonation of glycol by the amine and subsequent exchange of oleate by glycoxide on the QD surface. By replacing the amine with hydroxide ion, we demonstrate that any Brønsted-Lowry base that creates a sufficient dose of glycoxide can produce the epi-SL. We then introduce an epi-SL fabrication method that replaces point injection of a base with contactless and uniform illumination of a dissolved photobase. Quantitative mapping of multilayer (3D) films shows that our photobase-made epi-SLs are chemically and structurally uniform and have much lower concentrations of bulk defects compared to the highly inhomogeneous and defect-rich epi-SLs produced by amine point injection. The structural-chemical uniformity and structural perfection of photobase-made epi-SLs make them leading candidates for achieving emergent mini-band charge transport in a self-assembled mesoscale solid.

The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole-dipole interactions.