Large Diffusion Length Research Articles

Low Density of Conduction and Valence Band States Contribute to the High Open-Circuit Voltage in Perovskite Solar Cells

Hybrid perovskites are widely used for high-performance solar cells. Large diffusion lengths and long charge carrier lifetimes are considered two main factors for their high performance. Here, we argue that not only large diffusion lengths and long carrier lifetimes but also the low densities of the conduction and valence band states (Nc, Nv) contribute to high-performance perovskite solar cells. We estimated Nc and Nv of silicon, CdTe, and typical perovskites with two different methods. It was found that Nc and Nv of perovskites and CdTe are much lower than that of silicon. Using numerical models, we found that the solar cell of a material with same characteristics but lower Nc and Nv can realize a higher open-circuit voltage (Voc) and higher power conversion efficiency (PCE). We put forward and proved that the low Nc and Nv in hybrid perovskite is one of the factors for its high performance. This provides a new guideline for finding and developing new photovoltaic candidate materials.

Engineering band gap and electronic transport in organic-inorganic halide perovskites by superlattices.

Organic/inorganic lead and tin halide perovskites (CH3NH3PbI3 and CH3NH3SnI3) have been promising for photovoltaics because of their high charge carrier mobility, and large absorption coefficient and diffusion length. Both these perovskites also have a notable Seebeck coefficient, depending on the doping level, indicating their potential as thermoelectrics. We create superlattices of these hybrid organic-inorganic halide perovskites and investigate electronic transport through them using first principles computations and experiments. While the transverse components of electrical and electronic thermal conductivities for the superlattices are higher than those for simple perovskite lattices, their longitudinal counterparts are 103 times smaller resulting in overall lower transport coefficients. The superlattice structures have more carriers, but with less average energy compared to pure perovskites causing a lower Seebeck coefficient. However, with the impedance to thermal conduction being relatively stronger than that to charge transfer, the electronic thermoelectric figure of merit of superlattices is higher. Our results lead towards a unique opportunity to engineer the band gap of perovskites by nanostructuring for thermoelectric and optoelectronic applications.

Communication—Large Electron-Hole Diffusion Lengths in Methylammonium Lead Triiodide Perovskite Films Prepared by an Electrochemical-Chemical Approach

Methylammonium lead triiodide (CH3NH3PbI3) perovskite films were generated by chemical conversion of PbO2 layers electro-deposited over NiO hole selective contact, by simple immersion in CH3NH3I isopropanol solution under ambient conditions. 2.4 μm charge carrier diffusion lengths were measured by surface photovoltage, being these very similar to the size of the as formed perovskite crystals, showing that there are not deleterious encounters with defects or impurities. The electrochemical procedure used here demonstrates the capacity of this technique for the fabrication of perovskite films with large electron-hole diffusion lengths by solution based processes using only water and isopropanol.

Ionic liquid induced surface trap-state passivation for efficient perovskite hybrid solar cells

The highly crystalline methylammonium lead halides (MALHs) perovskite has large carrier diffusion length and thus minimal charge loss within the MALHs layers. But the charge extraction from the perovskite to the charge extraction layers can be significantly influenced by the interfacial contact. Since the fast-crystalized MALHs usually have a rough surface with numerous trap-states and crystal grain boundaries near the top surface, which will deteriorate the electrical coherence and physical contact with the top electron extraction layer (EEL) in the conventional device structure. In this study, we introduced an ionic liquid methyltrioctylammonium trifluoromethanesulfonate (MATS) to passivate the traps and boundaries at the interface of the i-n junction. The photoluminance results demonstrated an improved electron extraction upon the MATS surface treatment. And the impedance measurements also showed a reduced charge transfer resistance within the MATS treated device. Consequently, the conventional planar heterojunction solar cells based on the MALHs perovskite treated by MATS, showed an enhanced device efficiency of 16.10%. Moreover, after heating at elevated temperatures under 2 V forward bias and cooling down to room temperature, we found the MATS modified solar cell devices exhibit a further efficiency improvement to 17.51%.

Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis.

The large diffusion lengths recurrently measured in perovskite single crystals and films signal small bulk nonradiative recombination flux and locate the dominant carrier recombination processes at the outer interfaces. Surface recombination largely determines the photovoltaic performance, governing reductions under short-circuit current and open-circuit voltage. Quantification of recombination losses is necessary to reach full understanding of the solar cell operating principles. Complete impedance model is given, which connects capacitive and resistive processes to the electronic kinetics at the interfaces. Carrier collection losses affecting the photocurrent have been determined to equal 1%. Photovoltage loss is linked to the decrease in surface hole density, producing 0.3 V reduction with respect to the ideal radiative limit. Our approach enables a comparison among different structures, morphologies, and processing strategies of passivation and buffer layers.

Carbonized Electrospun Nanofiber Sheets for Thermophones.

Thermoacoustic performance of thin freestanding sheets of carbonized poly(acrylonitrile) and polybenzimidazole nanofibers are studied as promising candidates for thermophones. We analyze thermodynamic properties of sheets using transport parameters of single nanofibers and their aligned and randomly electrospun thin film assemblies. The electrical and thermal conductivities, thermal diffusivity, heat capacity, and infrared blackbody radiation are investigated to extract the heat exchange coefficient and enhance the energy conversion efficiency. Spectral and power dependencies of sound pressure in air are compared with carbon nanotube sheets and theoretical prediction. Despite lower thermoacoustic performance compared to that of CNT sheets, the mechanical strength and cost-effective production technology of thermophones make them very attractive for large-size sound projectors. The advantages of carbonized electrospun polymer nanofiber sheets are in the low frequency domain (<1000 Hz), where the large thermal diffusion length diminishes the thermal inertia of thick (∼200 nm) nonbundled fibers and the high intrinsic thermal conductivity of fibers enhances the heat exchange coefficient. Applications of thermoacoustic projectors for loudspeakers, high power SONAR arrays, and sound cancellation are discussed.

Overcoming Charge Collection Limitation at Solid/Liquid Interface by a Controllable Crystal Deficient Overlayer

Bulk and surface charge recombination of photoelectrode are two key processes that significantly hinder solar‐to‐fuel conversion of photoelectrochemical cell (PEC). In this study, the function of a “crystal‐deficient” overlayer is unveiled, which outperforms a traditionally used amorphous or crystalline overlayer in PEC water splitting by exhibiting a high conductivity and large electron diffusion length to enable unlimited electron collection. The optimized ≈2.5 nm thickness of the “crystal‐deficient” shell results in a depletion layer with a width of 3 nm, which overcomes the flat band limitation of the photovoltage and increases the light absorptivity in the wavelength range from 300 to 420 nm. In addition, a 50‐fold increase in the conductivity yields a one‐order‐of‐magnitude increase in the diffusion length of an electron (Ln)(≈20 μm), allowing for unlimited electron collection in the 1.9 μm TiO2 nanowire array with the “crystal‐deficient” shell. The controllable “crystal‐deficient” overlayer in rutile TiO2 nanowires photoanode achieves a photocurrent density greater than 2.0 mA cm−2 at 1.23 V versus reversible hydrogen electrode (RHE), a 1.18% applied bias photon‐to‐current efficiency at 0.49 V versus RHE, a faradaic efficiency greater than 93.5% at 0.6 V versus Pt under air mass 1.5G simulated solar light illumination (100 mW cm−2).

Room-Temperature Micron-Scale Exciton Migration in a Stabilized Emissive Molecular Aggregate.

We report 1.6 ± 1 μm exciton transport in self-assembled supramolecular light-harvesting nanotubes (LHNs) assembled from amphiphillic cyanine dyes. We stabilize LHNs in a sucrose glass matrix, greatly reducing light and oxidative damage and allowing the observation of exciton-exciton annihilation signatures under weak excitation flux. Fitting to a one-dimensional diffusion model, we find an average exciton diffusion constant of 55 ± 20 cm2/s, among the highest measured for an organic system. We develop a simple model that uses cryogenic measurements of static and dynamic energetic disorder to estimate a diffusion constant of 32 cm2/s, in agreement with experiment. We ascribe large exciton diffusion lengths to low static and dynamic energetic disorder in LHNs. We argue that matrix-stabilized LHNS represent an excellent model system to study coherent excitonic transport.

Static and dynamic properties of a GaAs p-i-n position-sensitive detector (PSD)

A GaAs-based p-i-n structure which is layer-compatible to a commercial heterojunction bipolar transistor (HBT) was employed to fabricate a visible-light position-sensitive detector (PSD) with a long distance of 14mm between two coplanar contacts. Sensing properties of the GaAs p-i-n PSD operated in the lateral photovoltaic mode were obtained by using visible-light sources with a power of 1–3mW. The measured sensitivity was 14.3±0.05mV/mm, resulting in a sensitivity-distance product of 200±0.5mV, for the fabricated PSD responding to a 3mW 638nm red light. The estimated nonlinearities were as small as 0.9±0.05%–1.9±0.05%, 1.8±0.05%–2.3±0.05%, and 0.3±0.05%–1.3±0.05% for the PSD tested under the 405nm, 532nm, and 638nm lights, respectively. Such a long linear distance is possibly attributed to carrier’s large diffusion lengths associated with the GaAs p-i-n layers. Another PSD was fabricated with three-terminal configuration using the same GaAs-based p-i-n structure. Thus, the three-terminal PSD can be operated in both of the lateral and transverse photovoltaic modes. A PSD’s differential output from two transverse photovoltaic voltages showed a sensitivity of 11.1±0.05mV/mm with a 0.3±0.05% nonlinearity. Furthermore, transient responses of the PSD’s LPV to the illumination and interdiction of the visible lights were included. Response time of 60±4μs for the 532nm green light compared to those of 134±2 and 99±2μs for the 405nm blue and 638nm red lights, respectively, will be investigated to realize the proposed visible-light PSD.

Bulk- and Powder Surface Exchange Coefficients of (La,Sr)CoO3-d

1. Introduction La0.6Sr0.4CoO3-δ (LSC64) has been recognized as a promising cathode for SOFC, which has high mixed conductivity of electron hole and oxide ion. Its high mixed conductivity makes oxygen reduction reaction (ORR) possible to take place not only at triple-phase boundary but also on two-phase boundary of electrode surface and gas phase. This two-phase boundary reaction proceeds: oxygen gas diffusion to the electrode surface, adsorption, dissociation of the oxygen atom, incorporation into the electrode, then bulk diffusion. In the case of LSC64 thin film, the rate limiting step is the surface reaction [1]. It is necessary to understand the surface reaction kinetics to improve ORR on LSC64 cathode. Surface exchange coefficients of LSC64 bulk are reported in several papers including ref. 1. However ORR kinetics of LSC64 porous cathode can’t be explained by the reported values. In this study, therefore, surface exchange coefficients of LSC64 powder are determined, which is better to simulate surface reaction kinetics of porous electrode. 2. Experimental Surface exchange coefficients, k, of LSC64 powder were determined by a pulse isotope exchange (PIE) method [2]. The measurements were performed in oxygen partial pressure range from 10-3to 10-2 bar at 573-773 K. Sample powder was loaded at the center of a quartz tubular micro reactor (ϕ2 mm). Typical values of the powder mass and length of the powder bed were 0.03-0.05 g and 13-15 mm, respectively. A 16O2-N2 gas mixture was used as the carrier gas. A six-port valve was used for injection of the 18O2/He pulse gas (500 μL) into the 16O2/N2 carrier gas. The response to an 18O2-enriched pulse gas was analyzed by an on-line mass spectrometry (OmniStar TM GSD320, Pfeiffer-Vacuum). The pulse experiments are carried out under following conditions. (i) The number of 18O atoms is negligibly small compared to that of the exchangeable oxygens in the oxide, hence f(O18) ≈ 0. (ii)There is a large diffusion length of 18O in the non-porous oxide particle. Under these conditions, mass balance analysis yields a simple equation for surface exchange coefficients [2]: k = nV m/t r*ln(f(18g.i)/f(18g.e)) where f(g.i18) and f(g.e18) are the 18O isotope molar fractions in the pulse at the inlet and exit of the reactor, respectively; n is the total number of oxygen atoms in the free volume of the packed bed; V m is molecular volume; t r is the average residence time of the labeled gas in the reactor. 3. Results and discussion Temperature dependence of k for LSC64 powder are shown in Fig. 1. Surface exchange coeffiicients k of LSC64 powder showed temperature dependence at 300-400℃. In comparison to LSC64 bulk, surface exchange coefficients, k, has different tendency. This suggests that surface reaction kinetics of LSC powder (or porous LSC64) might be different.

Terahertz Conductivity within Colloidal CsPbBr3 Perovskite Nanocrystals: Remarkably High Carrier Mobilities and Large Diffusion Lengths

Colloidal CsPbBr3 perovskite nanocrystals (NCs) have emerged as an excellent light emitting material in last one year. Using time domain and time-resolved THz spectroscopy and density functional theory based calculations, we establish 3-fold free carrier recombination mechanism, namely, nonradiative Auger, bimolecular electron-hole recombination, and inefficient trap-assisted recombination in 11 nm sized colloidal CsPbBr3 NCs. Our results confirm a negligible influence of surface defects in trapping charge carriers, which in turn results into desirable intrinsic transport properties, from the perspective of device applications, such as remarkably high carrier mobility (∼4500 cm(2) V(-1) s(-1)), large diffusion length (>9.2 μm), and high luminescence quantum yield (80%). Despite being solution processed and possessing a large surface to volume ratio, this combination of high carrier mobility and diffusion length, along with nearly ideal photoluminescence quantum yield, is unique compared to any other colloidal quantum dot system.

Spin Funneling for Enhanced Spin Injection into Ferromagnets.

It is well-established that high spin-orbit coupling (SOC) materials convert a charge current density into a spin current density which can be used to switch a magnet efficiently and there is increasing interest in identifying materials with large spin Hall angle for lower switching current. Using experimentally benchmarked models, we show that composite structures can be designed using existing spin Hall materials such that the effective spin Hall angle is larger by an order of magnitude. The basic idea is to funnel spins from a large area of spin Hall material into a small area of ferromagnet using a normal metal with large spin diffusion length and low resistivity like Cu or Al. We show that this approach is increasingly effective as magnets get smaller. We avoid unwanted charge current shunting by the low resistive NM layer utilizing the newly discovered phenomenon of pure spin conduction in ferromagnetic insulators via magnon diffusion. We provide a spin circuit model for magnon diffusion in FMI that is benchmarked against recent experiments and theory.

Organolead trihalide perovskite materials for efficient light emitting diodes

Organolead trihalide perovskite materials have been attracting increasing attention due to their promising role in solid solar cells. Several advantages make them potential candidates for optoelectronics: (1) solution- or/and vapor-processed preparation at low temperature; (2) tunable optical bandgap, wide absorption spectrum but narrow photoluminescence peaks; (3) long carrier life time, large diffusion length and high charge mobility; (4) various nanostructures via tuning capping agents and solvents. In this review, we summarize recent attempts toward efficient LEDs based on organolead trihalide perovskite materials. The strategies of materials science, device design and interface engineering are highlighted. Recent development and future perspectives are summarized for practical perovskite light technologies.

Extended defect study in Si: EBIC versus LBIC

A comparative study of multicrystalline Si and plastically deformed single crystalline Si by the electron (EBIC) and laser beam induced current (LBIC) methods has been carried out. It is shown that the LBIC contrast of two-dimensional defects in crystals with large diffusion length can be a few times higher than that in the EBIC method. Thus, in recent Si structures the LBIC method allows to reveal grain boundaries with the lower recombination velocity than the EBIC on. The advantages of LBIC for the dislocation trail study in Si and for backside metallization effects in solar cells are demonstrated.

Large diffusion lengths of excitons in perovskite and TiO2 heterojunction

Solar cells based on organometal halide perovskites have recently become very promising among other materials because of their cost-effective character and improvements in efficiency. Such performance is primarily associated with effective light absorption and large diffusion length of charge carriers. Our paper is devoted to the explanation of large diffusion lengths in these systems. The transport mean free path of charged carriers in a perovskite/TiO2 heterojunction that is an important constituent of the solar cells have been analyzed. Large transport length is explained by the planar diffusion of indirect excitons. Diffusion length of the coupled system increases by several orders compared to single carrier length due to the correlated character of the effective field acting on the exciton.

The nature of free-carrier transport in organometal halide perovskites.

Organometal halide perovskites are attracting great attention as promising material for solar cells because of their high power conversion efficiency. The high performance has been attributed to the existence of free charge carriers and their large diffusion lengths, but the nature of carrier transport at the atomistic level remains elusive. Here, nonadiabatic quantum molecular dynamics simulations elucidate the mechanisms underlying the excellent free-carrier transport in CH3NH3PbI3. Pb and I sublattices act as disjunct pathways for rapid and balanced transport of photoexcited electrons and holes, respectively, while minimizing efficiency-degrading charge recombination. On the other hand, CH3NH3 sublattice quickly screens out electrostatic electron-hole attraction to generate free carriers within 1 ps. Together this nano-architecture lets photoexcited electrons and holes dissociate instantaneously and travel far away to be harvested before dissipated as heat. This work provides much needed structure-property relationships and time-resolved information that potentially lead to rational design of efficient solar cells.

An STM and Monte Carlo study of the AlF3 thin film growth on Cu(1 1 1)

We report measurements of AlF3 thin film growth on Cu(1 1 1) at room temperature by means of scanning tunneling microscopy. The growth proceeds by the formation of fractal islands characterized by a very corrugated surface. Through uncovered zones and island density we determined a diffusion length of ~25 nm for the adsorbed molecules. Even with this large diffusion length the step-edges do not appear fully decorated. These experimental results are contrasted with simulations based on a limited diffusion aggregation model and Metropolis Monte Carlo.Additionally, the results of the AlF3 sub-monolayer growth on Cu(1 1 1) are compared with our previous results on Cu(1 0 0), finding that both systems show more differences than similarities. Thus, while the growth on Cu(1 0 0) shows fully decorated step-edges, on Cu(1 1 1), they present non-covered zones even at coverages as high as 0.7 monolayers. Supported on MC simulations we suggest that the qualitative difference between both faces is due to different step-edge behaviour.

Excellent spin transport in spin valves based on the conjugated polymer with high carrier mobility.

Organic semiconductors (OSCs) are characteristic of long spin-relaxation lifetime due to weak spin-orbit interaction and hyperfine interaction. However, short spin diffusion length and weak magnetoresistance (MR) effect at room temperature (RT) was commonly found on spin valves (SVs) using an organic spacer, which should be correlated with low carrier mobility of the OSCs. Here, N-type semiconducting polymer P(NDI2OD-T2) with high carrier mobility is employed as the spacer in the SV devices. Exceedingly high MR ratio of 90.0% at 4.2 K and of 6.8% at RT are achieved, respectively, via improving the interface structure between the polymer interlayer and top cobalt electrode as well as optimal annealing of manganite bottom electrode. Furthermore, we observe spin dependent transport through the polymeric interlayer and a large spin diffusion length with a weak temperature dependence. The results indicate that this polymer material can be used as a good medium for spintronic devices.

Advancements in all-solid-state hybrid solar cells based on organometal halide perovskites

Over the past several years, organic–inorganic hybrid perovskites have gained considerable research attention due to their direct band gap, large absorption coefficient, ambipolar diffusion and long carrier diffusion length, and have revolutionized the prospects of emerging photovoltaic technologies, with the highest power conversion efficiency of over 19% achieved under laboratory conditions.

Kerfless layer-transfer of thin epitaxial silicon foils using novel multiple layer porous silicon stacks with near 100% detachment yield and large minority carrier diffusion lengths

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin (<50µm) silicon foils for future silicon solar modules are addressed in this work: achieving high detachment yield and high minority carrier diffusion lengths. The detachment characteristics of the porous silicon-based lift-off process is studied using finite element modeling as well as experiments. It is shown that for easy detachment and high detachment yield, a low density of thin silicon pillars must be attained in the high porosity detachment layer (HP-DL) after high temperature sintering. This is elegantly achieved by increasing the thickness of the low porosity template layer (LP-TL) which acts as the vacancy supply to increase the post-anneal porosity of the HP-DL. In this way, near 100% detachment yield has been achieved. However, a thicker LP-TL results in a poorer quality epitaxial growth surface. To circumvent this trade-off, novel triple and quadruple layer porous silicon stacks are introduced which decouple the function of the LP-TL that acts as both the template for epitaxy and as the vacancy supply for the HP-DL. In these new stacks, a surface zone of very low void size and density (nearly void-free) is created which allows high quality epitaxy on easily-detachable porous silicon stacks. Minority carrier lifetime measurements on epitaxial foils grown on such a triple layer stack has resulted in an effective lifetime of ~350µs at the injection level of 1015cm−3 which corresponds to a minimum minority carrier diffusion length of ~670µm (> 16 times the silicon thickness). With such high quality epitaxial foils combined with high detachment yield, very high efficiency solar devices on thin silicon substrates would be a reality in the near future.