Long-range charge carrier mobility in metal halide perovskite thin-films and single crystals via transient photo-conductivity.

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA0.83Cs0.17Pb(I0.9Br0.1)3, (FA0.83MA0.17)0.95Cs0.05Pb(I0.9Br0.1)3 and CH3NH3PbI3-xClx polycrystalline films in the range of 0.3 to 6.7 cm2 V-1 s-1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.

Utilizing Nonpolar Organic Solvents for the Deposition of Metal-Halide Perovskite Films and the Realization of Organic Semiconductor/Perovskite Composite Photovoltaics.

Having captivated the research community with simple fabrication processes and staggering device efficiencies, perovskite-based optoelectronics are already on the way to commercialization. However, one potential obstacle to this commercialization is the almost exclusive use of toxic, highly coordinating, high boiling point solvents to make perovskite precursor inks. Herein, we demonstrate that nonpolar organic solvents, such as toluene, can be combined with butylamine to form an effective solvent for alkylammonium-based perovskites. Beyond providing broader solvent choice, our finding opens the possibility of blending perovskite inks with a wide range of previously incompatible materials, such as organic molecules, polymers, nanocrystals, and structure-directing agents. As a demonstration, using this solvent, we blend the perovskite ink with 6,6-phenyl-C-61-butyric acid methyl ester and show improved perovskite crystallization and device efficiencies. This processing route may enable a myriad of new possibilities for tuning the active layers in efficient photovoltaics, light-emitting diodes, and other semiconductor devices.

Highly Absorbing Lead-Free Semiconductor Cu2AgBiI6 for Photovoltaic Applications from the Quaternary CuI-AgI-BiI3 Phase Space.

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu2AgBiI6: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 105 cm-1 near the absorption onset, several times that of CH3NH3PbI3. Solution-processed Cu2AgBiI6 thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm2 V-1 s-1), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI3. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs2AgBiI6 thin films have not been synthesized to date, Cu2AgBiI6 is a valuable example of a stable Ag+/Bi3+ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

A piperidinium salt stabilizes efficient metal-halide perovskite solar cells.

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

Revealing Factors Influencing the Operational Stability of Perovskite Light-Emitting Diodes.

Light-emitting diodes (LEDs) made from metal halide perovskites have demonstrated external electroluminescent quantum efficiencies (EQEEL) in excess of 20%. However, their poor operational stability, resulting in lifetimes of only tens to hundreds of hours, needs to be dramatically improved prior to commercial use. There is little consensus in the community upon which factors limit the stability of these devices. Here, we investigate the role played by ammonium cations on the operational stability. We vary the amount of phenylethylammonium bromide, a widely used alkylammonium salt, that we add to a precursor solution of CsPbBr3 and track changes in stability and EQEEL. We find that while phenylethylammonium bromide is beneficial in achieving high efficiency, it is highly detrimental to operational stability. We investigate material properties and electronic characteristics before and after degradation and find that both a reduction in the radiative efficiency of the emitter and significant changes in current-voltage characteristics explain the orders of magnitude drop in the EQEEL, which we attribute to increased ionic mobility. Our results suggest that engineering new contacts and further investigation into materials with lower ionic mobility should yield much improved stability of perovskite LEDs.

Charge-Carrier Trapping Dynamics in Bismuth-Doped Thin Films of MAPbBr3 Perovskite.

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr3 host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials.

Microsecond Carrier Lifetimes, Controlled p-Doping, and Enhanced Air Stability in Low-Bandgap Metal Halide Perovskites.

Mixed lead-tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite-perovskite tandem solar cells. Previous reports on lead-tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 µs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.

Facile Synthesis of Stable and Highly Luminescent Methylammonium Lead Halide Nanocrystals for Efficient Light Emitting Devices.

Metal halide perovskites are promising candidates for use in light emitting diodes (LEDs), due to their potential for color tunable and high luminescence efficiency. While recent advances in perovskite-based light emitting diodes have resulted in external quantum efficiencies exceeding 12.4% for the green emitters, and infrared emitters based on 3 D/2D mixed dimensional perovskites have exceeded 20%, the external quantum efficiencies of the red and blue emitters still lag behind. A critical issue to date is creating highly emissive and stable perovskite emitters with the desirable emission band gap to achieve full-color displays and white LEDs. Herein, we report the preparation and characterization of a highly luminescent and stable suspension of cubic-shaped methylammonium lead triiodide (CH3NH3PbI3) perovskite nanocrystals, where we synthesize the nanocrystals via a ligand-assisted reprecipitation technique, using an acetonitrile/methylamine compound solvent system to solvate the ions and toluene as the antisolvent to induce crystallization. Through tuning the ratio of the ligands, the ligand to toluene ratio, and the temperature of the toluene, we obtain a solution of CH3NH3PbI3 nanocrystals with a photoluminescence quantum yield exceeding 93% and tunable emission between 660 and 705 nm. We also achieved red emission at 635 nm by blending the nanocrystals with bromide salt and obtained perovskite-based light emitting diodes with maximum electroluminescent external quantum efficiency of 2.75%.

Impact of Bi3+ Heterovalent Doping in Organic-Inorganic Metal Halide Perovskite Crystals.

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

Consolidation of the optoelectronic properties of CH3NH3PbBr3 perovskite single crystals.

Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH3NH3PbBr3), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH3NH3PbBr3 perovskite single crystals is 1015 cm-3, only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.Metal halide perovskites for optoelectronic devices have been extensively studied in two forms: single-crystals or polycrystalline thin films. Using spectroscopic approaches, Wenger et al. show that polycrystalline thin films possess similar optoelectronic properties to single crystals.

Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap.

A2BB'X6 halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs2BiAgX6 (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B3+ and B+ cations are In3+ and Ag+, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs2InAgX6 (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs2InAgCl6 and Cs2InAgBr6, and we succeed to form the hitherto unknown double perovskite Cs2InAgCl6. X-ray diffraction yields a double perovskite structure with space group Fm3̅m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs2InAgX6 and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs2InAg(Cl1-xBrx)6 for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

Mechanism for rapid growth of organic-inorganic halide perovskite crystals.

Optoelectronic devices based on hybrid halide perovskites have shown remarkable progress to high performance. However, despite their apparent success, there remain many open questions about their intrinsic properties. Single crystals are often seen as the ideal platform for understanding the limits of crystalline materials, and recent reports of rapid, high-temperature crystallization of single crystals should enable a variety of studies. Here we explore the mechanism of this crystallization and find that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization. We use this knowledge to demonstrate a broader range of processing parameters and show that these can lead to improved crystal quality. Our findings are therefore of central importance to enable the continued advancement of perovskite optoelectronics and to the improved reproducibility through a better understanding of factors influencing and controlling crystallization.

Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals.

Lead-based halide perovskites are emerging as the most promising class of materials for next-generation optoelectronics; however, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs2BiAgCl6, perform structural refinement using single-crystal X-ray diffraction, and characterize its optical properties via optical absorption and photoluminescence measurements. This new perovskite belongs to the Fm3̅m space group and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-vis and photoluminescence measurements we obtain an indirect gap of 2.2 eV.

Structured Organic-Inorganic Perovskite toward a Distributed Feedback Laser.

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.

Controlling mesopore size and processability of transparent enzyme-loaded silica films for biosensing applications.

Silica-based nanoporous thin films including large mesopores are relevant as enzyme supports for applications in biosensing. The diffusion and immobilization of large biomolecules such as enzymes in such porous films require the presence of large mesopores. Creating such morphologies based on a bottom-up synthesis using colloidal templates is a challenge in view of the combination of desired material properties and the robustness of the casting process for the fabrication of thin films. Here a strategy to reproducibly synthesize transparent porous silica thin films with submicrometer thickness and homogeneously distributed porosity is presented. For this purpose, polystyrene-poly-2-vinylpyridine (PS-P2VP) amphiphilic block copolymers are used as porogenic templates. Low-chain alcohols are employed as both selective solvents for the P2VP blocks and reaction media for silica synthesis. Rheology measurements reveal a strong influence of the block copolymer length on the behavior of PS-P2VP micelles in suspension. The pore distribution and accessibility into the film are controlled by adjusting the silica to block copolymer weight ratio. The solvent choice is shown to control not only the micelle size and the generated pore morphology but also the structural homogeneity of the films. Finally, the suitability of the synthesized films as supports for enzymes is tested using a model enzyme, horseradish peroxidase EC 1.11.1.7. Our approach is innovative, robust, and reproducible and provides a convenient alternative to synthesize large mesopores up to small macropores (20-100 nm) in nanostructured thin films with applications in biosensing and functional coatings.

Au-labeled antibodies to enhance the sensitivity of a refractometric immunoassay: detection of cocaine.

An integrated platform for a very sensitive detection of cocaine based on a refractometric biosensor is demonstrated. The system uses a waveguide grating biosensor functionalized with a cocaine multivalent antigen-carrier protein conjugate. The immunoassay scheme consists of the competitive binding of cocaine-specific antibodies to the immobilized conjugates. A 1000-fold enhancement of the sensor's sensitivity is achieved when using gold conjugated monoclonal antibodies instead of free antibodies. Together with the optimization of the assay conditions, the setup is designed for a quick identification of narcotics using automated sampling. The results show that the presence of cocaine in a liquid sample could be identified down to a concentration of 0.7 nM within one minute. This value can be reduced even further when longer binding time is allowed (0.2 nM after 15 min). Application of the system to detection of narcotics at airport security control points is discussed.

Integrated optical biosensor for in-line monitoring of cell cultures.

An analytical detection platform was developed to evaluate the induced toxicity in cell cultures exposed to foreign agents like growth factors or nanoparticles. Connecting a biosensing detection device to the cell culture flasks allows analyzing the composition of cell medium in real-time. The analysis relies on the quantification of inflammatory cytokines released by cells into the cell culture medium, by means of solid-phase immunoassays analyzed with the wavelength interrogated optical sensing (WIOS) instrument. A fluidic system for in situ measurements allows detecting cytokines in real-time, with a sensitivity of 1-100 ng/mL depending on the cytokine. In addition, integration of an in-line optical absorbance measurement unit, in combination with the standard AB cell proliferation assay, provides information on the cell viability in the culture. Fluidic connections between the cell culture flasks, the optical biosensor and the absorbance measurement unit simultaneously allow quantifying up to three cytokines (interleukin 8, interleukin 6 and the monocyte chemotactic protein), assessing cellular proliferation, and thus discriminating between naïve cells and cells exposed to foreign agents such as growth factors (tumor necrosis factor alpha) or nanoparticles. This analytical tool presents a high potential for assessing the cytotoxicity of nanoparticles and other chemicals in vitro.

Inexpensive and fast wafer-scale fabrication of nanohole arrays in thin gold films for plasmonics.

In this paper, a fast and inexpensive wafer-scale process for the fabrication of arrays of nanoscale holes in thin gold films for plasmonics is shown. The process combines nanosphere lithography using spin-coated polystyrene beads with a sputter-etching process. This allows the batch fabrication of several 1000 microm(2) large hole arrays in 200 nm thick gold films without the use of an adhesion layer for the gold film. The hole size and lattice period can be tuned independently with this method. This allows tuning of the optical properties of the hole arrays for the desired application. An example application, refractive index sensing, is demonstrated.

High molar extinction coefficient heteroleptic ruthenium complexes for thin film dye-sensitized solar cells.

Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-hexyloxystyryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-methoxystyryl)-2,2'-bipyridine) (NCS)2, coded as K-19 and K-73, respectively, have been synthesized and characterized by 1H NMR, FTIR, UV-vis absorption, and emission spectroscopy and excited-state lifetime and spectroelectrochemical measurements. The introduction of the alkoxystyryl group extends the conjugation of the bipyridine donor ligand increasing markedly their molar extinction coefficient and solar light harvesting capacity. The dynamics of photoinduced charge separation following electronic excitation of the K-19 dye was scrutinized by time-resolved laser spectroscopy. The electron transfer from K-19 to the conduction band of TiO2 is completed within 20 fs while charge recombination has a half-life time of 800 s. The high extinction coefficients of these sensitizers enable realization of a new generation of a thin film dye sensitized solar cell (DSC) yielding high conversion efficiency at full sunlight even with viscous electrolytes based on ionic liquids or nonvolatile solvents. An unprecedented yield of over 9% was obtained under standard reporting conditions (simulated global air mass 1.5 sunlight at 1000 W/m2 intensity) when the K-73 sensitizer was combined with a nonvolatile "robust" electrolyte. The K-19 dye gave a conversion yield of 7.1% when used in conjunction with the binary ionic liquid electrolyte. These devices exhibit excellent stability under light soaking at 60 degrees C. The effect of the mesoscopic TiO2 film thickness on photovoltaic performance has been analyzed by electrochemical impedance spectroscopy (EIS).

Rationale for kinetic heterogeneity of ultrafast light-induced electron transfer from Ru(II) complex sensitizers to nanocrystalline TiO2.

Because of their successful use in dye-sensitized solar cells, Ru(II) polypyridyl complex dyes adsorbed on nanocrystalline TiO2 films have been regarded as model systems for the experimental study of the ultrafast dynamics of interfacial light-induced electron transfer. Most studies have reported charge injection kinetics from Ru(dcbpyH2)2(NCS)2 (N3) to take place with a fast (sub-100 fs) phase, followed by a slower (0.7-100 ps) multiexponential component. This complex, multiphasic behavior observed for the electron injection process has prevented the development of a satisfying kinetic model and has led to often contradicting conclusions. Here, we show that the observed kinetic heterogeneity can result from the aggregation of sensitizer molecules on the surface. Carefully controlled deposition of Ru(II) complex dye molecules onto nanocrystalline titania consistently yields a monophasic injection dynamics with a time constant shorter than 20 fs. The latter figure suggests the process is beyond the scope of vibration-mediated electron transfer kinetic models and might be controlled by the electron dephasing in the solid.