Charge Overlap Interaction in Quantum Dot Films: Time Dependence and Suppression by Cyanide Adsorption

Chemical bath deposited films of CdSe nanocrystals (<4 nm) are shown to exhibit time-dependent spectral red shifts, caused by increasing overlap of the electron wave functions in adjacent nanocrystals. Treatment of these “aggregated” films with aqueous KCN solution results in repulsion of the wave functions due to the strongly adsorbed negatively charged cyanide and thus electronic decoupling of the physically connected nanocrystals. The previously reported band gap increase due to cyanide adsorption on nominally uncoupled nanocrystals is also described here in more detail

Photoluminescence Flickering of Micron-Sized Crystals of Methylammonium Lead Bromide: Effect of Ambience and Light Exposure

Recent reports on temporal photoluminescence (PL) intensity fluctuations (<i>blinking</i>) within localized domains of organo-metal lead halide (hybrid) perovskite microcrystals have invoked considerable interest to understand their origins. Using PL microscopy, we have investigated the effect of atmospheric constituents and photoillumination on spatially extended intensity fluctuations in methylammonium lead bromide (MAPbBr₃) perovskite materials, explicitly for micrometer (ca. 1–2 μm)-sized crystals. Increase in the relative humidity of the ambience results in progressive reduction in the PL intensity, and beyond a threshold value, individual microcrystalline grains exhibit multistate PL intermittency (<i>flickering</i>), which is characteristically different from quasi two-state blinking observed in nanocrystals. Such flickering disappears upon removal of moisture, accompanied by considerable enhancement of the overall PL efficiency. We hypothesize that initiation of moisture-induced degradation marked by the lowering of PL intensity correlates with the appearance of PL flickering, and such processes further accelerate in the presence of oxygen as opposed to an inert (nitrogen) environment. We find that the intrinsic defects not only increase the threshold level of ambient moisture needed to initiate flickering but also modulate the nature of PL intermittency. Our results therefore establish a strong correlation between initiation of material degradation and PL flickering of hybrid perovskite microcrystals, induced by transient defects formed via interaction with the ambience

Novel Plasma-Assisted Low-Temperature-Processed SnO₂ Thin Films for Efficient Flexible Perovskite Photovoltaics

The recent evolution of solution-processed hybrid organic–inorganic perovskite-based photovoltaic devices opens up the commercial avenue for high-throughput roll-to-roll manufacturing technology. To circumvent the thermal limitations that hinder the use of metal oxide charge transport layers on plastic flexible substrates in such technologies, we employed a relatively low-power nitrogen plasma treatment to achieve compact SnO₂ thin-film electrodes at near room temperature. The perovskite photovoltaic devices thus fabricated using N₂ plasma-treated SnO₂ performed on par with thermally annealed SnO₂ electrodes and resulted in a power conversion efficiency (PCE) of ca. 20.3% with stabilized power output (SPO) of ca. 19.1% on rigid substrates. Furthermore, the process is extended to realize flexible perovskite solar cells on indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates with champion PCE of 18.1% (SPO ca. 17.1%), which retained ca. 90% of its initial performance after 1000 bending cycles. Our investigations reveal that deep ultraviolet irradiation associated with N₂ and N₂O plasma emission plays a major role in obtaining good quality metal oxide thin films at lower temperatures and offers promise toward facile integration of a wide variety of metal oxides on flexible substrates

Atomic Layer Deposited Molybdenum Nitride Thin Film: A Promising Anode Material for Li Ion Batteries

Molybdenum nitride (MoN_<i>x</i>) thin films are deposited by atomic layer deposition (ALD) using molybdenum hexacarbonyl [Mo­(CO)₆] and ammonia [NH₃] at varied temperatures. A relatively narrow ALD temperature window is observed. <i>In situ</i> quartz crystal microbalance (QCM) measurements reveal the self-limiting growth nature of the deposition that is further verified with <i>ex situ</i> spectroscopic ellipsometry and X-ray reflectivity (XRR) measurements. A saturated growth rate of 2 Å/cycle at 170 °C is obtained. The deposition chemistry is studied by the <i>in situ</i> Fourier transform infrared spectroscopy (FTIR) that investigates the surface bound reactions during each half cycle. As deposited films are amorphous as observed from X-ray diffraction (XRD) and transmission electron microscopy electron diffraction (TEM ED) studies, which get converted to hexagonal-MoN upon annealing at 400 °C under NH₃ atmosphere. As grown thin films are found to have notable potential as a carbon and binder free anode material in a Li ion battery. Under half-cell configuration, a stable discharge capacity of 700 mAh g⁻¹ was achieved after 100 charge–discharge cycles, at a current density of 100 μA cm^–2

Inorganic Hole Conducting Layers for Perovskite-Based Solar Cells

Hybrid organic–inorganic semiconducting perovskite photovoltaic cells are usually coupled with organic hole conductors. Here, we report planar, inverse CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>-based cells with inorganic hole conductors. Using electrodeposited NiO as hole conductor, we have achieved a power conversion efficiency of 7.3%. The maximum <i>V</i>_OC obtained was 935 mV with an average <i>V</i>_OC value being 785 mV. Preliminary results for similar cells using electrodeposited CuSCN as hole conductor resulted in devices up to 3.8% in efficiency. The ability to obtain promising cells using NiO and CuSCN expands the presently rather limited range of available hole conductors for perovskite cells

Effects of Solution pH and Surface Chemistry on the Postdeposition Growth of Chemical Bath Deposited PbSe Nanocrystalline Films

Chemical bath deposited PbSe films were subjected to postdeposition treatment with aqueous (typically 0.25−0.5 M) KOH. For films deposited using a citrate complex, this treatment resulted in dissolution of surface lead oxides (seen from XPS and EXAFS measurements) and growth of the nanocrystals (from ca. 5 to as much as 20 nm, measured by XRD and TEM) by an Ostwald ripening mechanism and formation of a porous network. For films deposited using KOH-complexed Pb, this growth did not occur. The latter films are made up of PbSe crystals (ca. 4 nm) embedded in an amorphous matrix of lead oxide. Successful etching of the crystallite surface passivation is found to be critical for the growth progress. While the KOH treatment removed most of this matrix, the individual crystals of PbSe still remained passivated with a surface where Pb was apparently bonded to both O and Se. With use of a concentrated KOH solution (3 M) for long periods of time (>1 h), this surface could be removed and crystal growth occurred to give a network of PbSe crystals several tens of nanometers in size. This study, besides explaining the very different chemical behaviors of the two types of PbSe films, demonstrates the important role of what appear to be small differences in surface chemistries in determining the chemical properties of nanocrystals

Atomic Layer Deposition of Transparent and Conducting p‑Type Cu(I) Incorporated ZnS Thin Films: Unravelling the Role of Compositional Heterogeneity on Optical and Carrier Transport Properties

Optically transparent and highly conducting p-type Cu­(I) incorporated ZnS (Cu:ZnS) films are deposited by stacking individual layers of CuS and ZnS using atomic layer deposition. The deposition chemistry and growth mechanism are studied by in situ quartz crystal microbalance. Compositional disorder in atomic scale is observed with increasing Cu incorporation in the films that results in systematic decrease in the optical transmittance in the visible spectrum. Again the conductivity also emphatically depends on the volume fraction of phase-segregated conducting covellite phase. An illustrious correlation prevailing the interplay between the optical transparency and the charge transport mechanism is established. The hole transport mechanism that indicates insulator-to-metal transition with increasing Cu incorporation in the composite is explained in terms of an inhomogeneously disordered system. Under optimized conditions, the material having moderately high optical transmission with degenerate carrier concentration lies exactly at the confluence between the metallic and insulating regime. The lowest resistivity that is obtained here (1.3 × 10^–3 Ω cm) with >90% (after reflection correction) transmission is highly comparable to the best ones that are reported in the field and probably analogous to the commercially available n-type transparent conductors

Photonically Cured Solution-Processed SnO₂ Thin Films for High-Efficiency and Stable Perovskite Solar Cells and Minimodules

High-throughput fabrication of metal oxide thin films is always a bottleneck for solution-processed perovskite solar cells. Here, we report a rapid photonic curing process, with a well-controlled train of short light pulses, to develop bilayer (colloidal and blocking layer) SnO2 thin films used as electron transport layers in perovskite ((FA0.83MA0.17)0.95Cs0.05PbI2.5Br0.5, 1.62 eV band gap) photovoltaic devices (n–i–p architecture) with an optimized efficiency of 21.1% alongside good ambient and operational (MPPT) stability. The strong dependency of the photonic curing pulse parameters on device properties is investigated, and we established a corroboration between the chemical properties of the as-cured SnO2 and the optoelectronic performance of the devices and the interface quality. Furthermore, we show that the futile removal of the chloride species in photonically cured SnO2 is an added advantage against the thermally annealed ones regarding charge transport and lower interfacial recombination. Furthermore, the process is impeccably scaled up to demonstrate a series-connected minimodule (16 cm2) with 18.2% efficiency

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared