Colloidal quantum dot photovoltaics: the effect of polydispersity

The size-effect tunability of colloidal quantum dots enables facile engineering of the bandgap at the time of nanoparticle synthesis. The dependence of effective bandgap on nanoparticle size also presents a challenge if the size dispersion, hence bandgap variability, is not well-controlled within a given quantum dot solid. The impact of this polydispersity is well-studied in luminescent devices as well as in unipolar electronic transport; however, the requirements on monodispersity have yet to be quantified in photovoltaics. Here we carry out a series of combined experimental and model-based studies aimed at clarifying, and quantifying, the importance of quantum dot monodispersity in photovoltaics. We successfully predict, using a simple model, the dependence of both open-circuit voltage and photoluminescence behavior on the density of small-bandgap (large-diameter) quantum dot inclusions. The model requires inclusion of trap states to explain the experimental data quantitatively. We then explore using this same experimentally tested model the implications of a broadened quantum dot population on device performance. We report that present-day colloidal quantum dot photovoltaic devices with typical inhomogeneous linewidths of 100-150 meV are dominated by surface traps, and it is for this reason that they see marginal benefit from reduction in polydispersity. Upon eliminating surface traps, achieving inhomogeneous broadening of 50 meV or less will lead to device performance that sees very little deleterious impact from polydispersity.     

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.     

Bridge Pier Scour in Clay-Sand Mixed Sediments at Near-Threshold Velocity for Sand

Local scour at circular bridge piers embedded in a clay-sand-mixed bed was investigated in laboratory flume experiments. The effects of clay content, water content, and sand size on maximum equilibrium scour depth, equilibrium scour hole geometry, scouring process, and time variation of scour were studied at velocities close to the threshold velocities for the sand in the clay-sand mixture. It was observed that clay content and water content were the key parameters that effect the scouring process, scour hole geometry, and maximum equilibrium scour depth. The bridge pier scouring process in clay-sand mixtures involved different dominating modes for removal of sediment from scour hole: chunks-of-aggregates, aggregate-by-aggregate, and particle-by-particle. Regression-based equations for estimation of nondimensional maximum scour depth and scour hole diameter for piers embedded in clay-sand mixtures having clay content of <40% and water content of <40% were proposed as functions of pier Froude number, clay content, water content, and bed shear strength.     

Compression behaviour of jute–polypropylene blended needle-punched nonwoven under wet conditions

The subject of this paper is the compression behaviour of jute–polypropylene blended needle-punched nonwoven under wet conditions. It is essential to know the compression behaviour of the needle-punched fabrics under wet conditions because of its various applications like geotextiles and carpets. Needle-punched fabric samples were prepared based on the three-factor three-level Box–Behnken design of experiment to study the individual as well as interaction effects of process parameters of fabric weight, needling density and blend proportion of jute and polypropylene fibre blend on compression properties under wet conditions. Initial thickness, percentage compression, percentage thickness loss and percentage compression resilience are the compression properties considered. The compression properties under wet conditions were compared with their respective dry conditions. This study reveals that most of the wet samples show a slightly higher value of initial thickness, percentage compression and thickness loss than dry samples, irrespective of processing variables, though for compression resilience value it is vice versa. The effect of blend does not have much influence on percentage compression in wet conditions. There is optimum level of needling density (around 250 punches/cm²) and blend proportions (around 75% polypropylene) for minimum percentage thickness loss of wet fabric under higher fabric weight. The compression resilience increases with increase in the content of polypropylene in the blend of polypropylene and jute. Optimum compression properties (highest percentage compression and compression resilience with minimum percentage thickness loss) can be achieved at fabric weight of 450 g/m² with the combination of around 250 punches/cm² needling density and around 75% polypropylene content.     

How similar are two-unit bicycle and motorcycle crashes?

This paper explores the similarities and differences between bicycle and motorcycle crashes with other motor vehicles. If similar treatments can be effective for both bicycle and motorcycle crashes, then greater benefits in terms of crash costs saved may be possible for the same investment in treatments. To reduce the biases associated with under-reporting of these crashes to police, property damage and minor injury crashes were excluded. The most common crash type for both bicycles (31.1%) and motorcycles (24.5%) was intersection from adjacent approaches. Drivers of other vehicles were coded most at fault in the majority of two-unit bicycle (57.0%) and motorcycle crashes (62.7%). The crash types, patterns of fault and factors affecting fault were generally similar for bicycle and motorcycle crashes. This confirms the need to combat the factors contributing to failure of other drivers to yield right of way to two-wheelers, and suggest that some of these actions should prove beneficial to the safety of both motorized and non-motorized two-wheelers. In contrast, child bicyclists were more often at fault, particularly in crashes involving a vehicle leaving the driveway or footpath. The greater reporting of violations by riders and drivers in motorcycle crashes also deserves further investigation.     

Implication of molecular orientation on charge transport and gas sensing characteristics of cobalt–phthalocyanine thin films

Charge transport and gas sensing characteristics of cobalt phthalocyanine films deposited along (ATB) and perpendicular (PTB) to the natural twin boundaries of (0 0 1) LaAlO₃ substrate have been investigated. The charge carrier mobility of ATB films (∼5 cm² V⁻¹ s⁻¹) is five orders of magnitude higher compared to that of PTB films (∼7 × 10⁻⁵ cm² V⁻¹ s⁻¹), suggesting that twin boundaries acts like a template for ordering of molecules. The ATB films on exposure to ammonia showed a reversible increase of resistance, with fast response and recovery. In contrast PTB films showed same sensitivity, but exhibits base resistance drift along with sluggish response.     

Existence of positive solutions of singular elliptic boundary value problems in a ball

This paper deals with existence theorems of positive solutions for singular elliptic boundary value problems in a ball. The results of this paper are generalizations of those proved by authors cited in the references.     

Evaluation of Multipoint Dosing Strategy in a Miniaturized Tubular Reactor: Nitration of Salicylic Acid

A 2D non‐isothermal reaction engineering model for a tubular reactor was developed for a complex reaction network of salicylic acid nitration. The influence of different operating and design parameters was studied to minimize the amount of secondary nitration products and limit the maximum temperature inside the reactor. Critical temperature effects were observed for larger reaction tubes, whereas close to isothermal conditions were perceived in smaller tubes. With single‐point dosing of both reactants, complete conversion can be achieved but formation of secondary nitration products cannot be avoided. For a given number of dosing points, a suitable combination of the operating parameters allowed to achieve complete conversion and better yield of the desired product.     

Investigation of thermal performance of SAC variables using fuzzy logic based expert system

The thermal performance of a solar air collector (SAC) is investigated experimentally under the different climatic conditions of north eastern India using fuzzy logic based expert system (FLES). The FLES based on subtractive clustering (SC) with the fuzzy logic method where here, SC is used for extraction of optimal fuzzy IF-THEN rules while a fuzzy logic is used for modeling of SAC variables. This work considered four input variables [like mass flow rate (m), collector tilt angles (θ), solar radiation (Q), temperature (T)] and the four output variables [i.e. efficiency (η), exergetic efficiency (η_II), temperature rise (∆T), and pressure drop (∆P)]. First, 272 trials of experimentation on SAC are performed by varying m from 0.0078 to 0.0118 kg/s and θ from 30 to 60°, whereas the variation of metrological data is obtained in different working days. Then modeling and parametric analysis is carried out for SAC. Experimental results reveal that the value of η increases with the increase in m, Q, T and θ up to 45°. The higher value of m results in a higher value of ∆P and that reduces the value of η_II. Also, FLES model provides comparable and acceptable values for SAC. At last, validation of the FLES model is done via published data to confirm the results.