Tin(IV) Oxide Electron Transport Layer via Industrial-Scale Pulsed Laser Deposition for Planar Perovskite Solar Cells

Electron transport layers (ETL) based on tin(IV) oxide (SnO₂) are recurrently employed in perovskite solar cells (PSCs) by many deposition techniques. Pulsed laser deposition (PLD) offers a few advantages for the fabrication of such layers, such as being compatible with large scale, patternable, and allowing deposition at fast rates. However, a precise understanding of how the deposition parameters can affect the SnO₂ film, and as a consequence the solar cell performance, is needed. Herein, we use a PLD tool equipped with a droplet trap to minimize the number of excess particles (originated from debris) reaching the substrate, and we show how to control the PLD chamber pressure to obtain surfaces with very low roughness and how the concentration of oxygen in the background gas can affect the number of oxygen vacancies in the film. Using optimized deposition conditions, we obtained solar cells in the n-i-p configuration employing methylammonium lead iodide perovskite as the absorber layer with power conversion efficiencies exceeding 18% and identical performance to devices having the more typical atomic layer deposited SnO₂ ETL.

Highly Luminescent Transparent Cs2Ag x Na1-x Bi y In1-y Cl6 Perovskite Films Produced by Single-Source Vacuum Deposition

Thermal deposition of halide perovskites as a universal and scalable route to transparent thin films becomes highly challenging in the case of lead-free double perovskites, requiring the evaporation dynamics of multiple metal halide sources to be balanced or a single-phase precursor preliminary synthesized to achieve a reliable control over the composition and the phase of the final films. In the present Letter, the feasibility of the single-source vacuum deposition of microcrystalline Cs₂Ag _x Na_1-x Bi _y In_1-y Cl₆ double perovskites into corresponding transparent nanocrystalline films while preserving the bulk spectral and structural properties is shown. The perovskite films produced from the most emissive powders with x = 0.40 and y = 0.01 revealed a photoluminescence quantum yield of 85%, highlighting thermal evaporation as a promising approach to functional perovskite-based optical materials.

Perovskite/Perovskite Tandem Solar Cells in the Substrate Configuration with Potential for Bifacial Operation

Perovskite/perovskite tandem solar cells have recently exceeded the record power conversion efficiency (PCE) of single-junction perovskite solar cells. They are typically built in the superstrate configuration, in which the device is illuminated from the substrate side. This limits the fabrication of the solar cell to transparent substrates, typically glass coated with a transparent conductive oxide (TCO), and adds constraints because the first subcell that is deposited on the substrate must contain the wide-bandgap perovskite. However, devices in the substrate configuration could potentially be fabricated on a large variety of opaque and inexpensive substrates, such as plastic and metal foils. Importantly, in the substrate configuration the narrow-bandgap subcell is deposited first, which allows for more freedom in the device design. In this work, we report perovskite/perovskite tandem solar cells fabricated in the substrate configuration. As the substrate we use TCO-coated glass on which a solution-processed narrow-bandgap perovskite solar cell is deposited. All of the other layers are then processed using vacuum sublimation, starting with the charge recombination layers, then the wide-bandgap perovskite subcell, and finishing with the transparent top TCO electrode. Proof-of-concept tandem solar cells show a maximum PCE of 20%, which is still moderate compared to those of best-in-class devices realized in the superstrate configuration yet higher than those of the corresponding single-junction devices in the substrate configuration. As both the top and bottom electrodes are semitransparent, these devices also have the potential to be used as bifacial tandem solar cells.

Ionic multiresonant thermally activated delayed fluorescence emitters for light emitting electrochemical cells

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent compound. For instance, DiKTa-OBuIm emits in the green (λ_PL = 499 nm, 1 wt % in mCP) while DiKTa-DPA-OBuIm emits in the red (λ_PL = 577 nm, 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed lifetimes of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 10³ s⁻¹ and 3.04 × 10³ s⁻¹. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as active emitters showing green (λ_max = 534 nm) and red (λ_max = 656 nm) emission, respectively.

Vacuum-Deposited Cesium Tin Iodide Thin Films with Tunable Thermoelectric Properties

Most current thermoelectric materials have important drawbacks, such as toxicity, scarceness, and peak operating temperatures above 300 °C. Herein, we report the thermoelectric properties of different crystalline phases of Sn-based perovskite thin films. The 2D phase, Cs₂SnI₄, is obtained through vacuum thermal deposition and easily converted into the black β phase of CsSnI₃ (B-β CsSnI₃) by annealing at 150 °C. B-β CsSnI₃ is a p-type semiconductor with a figure of merit (ZT) ranging from 0.021 to 0.033 for temperatures below 100 °C, which makes it a promising candidate to power small electronic devices such as wearable sensors which may be interconnected in the so-called Internet of Things. The B-β phase is stable in nitrogen, whereas it spontaneously oxidizes to Cs₂SnI₆ upon exposure to air. Cs₂SnI₆ shows a negative Seebeck coefficient and an ultralow thermal conductivity. However, the ZT values are 1 order of magnitude lower than for B-β CsSnI₃ due to a considerably lower electrical conductivity.

Dimensionality Controls Anion Intermixing in Electroluminescent Perovskite Heterojunctions

Metal halide perovskites have emerged as a promising group of materials for optoelectronic applications such as photovoltaics, light emission, and photodetectors. So-far, in particular, the stability of light-emitting devices is limited, which is in part attributed to the intrinsic ionic conductivity of these materials. High-performance devices inevitably contain heterojunctions similar to other optoelectronic devices based on oxide perovskites, II-VI, or III-V group semiconductors. To enable efficient heterojunctions, ion exchange at the interface between different layers should be controlled. Herein, we report a method that enables to control and monitor the extent of anion intermixing between solution-processed lead bromide and vacuum-deposited lead chloride perovskite films. Taking advantage of the ability to fine tune the layer thicknesses of the vacuum-deposited films, we systematically study the effect of film thickness on anionic intermixing. Using these multiple layers, we prepare proof of principle light-emitting devices exhibiting green and blue electroluminescence.

Quadruple-Cation Wide-Bandgap Perovskite Solar Cells with Enhanced Thermal Stability Enabled by Vacuum Deposition

Vacuum processing of multicomponent perovskites is not straightforward, because the number of precursors is in principle limited by the number of available thermal sources. Herein, we present a process which allows increasing the complexity of the formulation of vacuum-deposited lead halide perovskite films by multisource deposition and premixing both inorganic and organic components. We apply it to the preparation of wide-bandgap CsMAFA triple-cation perovskite solar cells, which are found to be efficient but not thermally stable. With the aim of stabilizing the perovskite phase, we add guanidinium (GA⁺) to the material formulation and obtained CsMAFAGA quadruple-cation perovskite films with enhanced thermal stability, as observed by X-ray diffraction and rationalized by microstructural analysis. The corresponding solar cells showed similar performance with improved thermal stability. This work paves the way toward the vacuum processing of complex perovskite formulations, with important implications not only for photovoltaics but also for other fields of application.

Simple approach for an electron extraction layer in an all-vacuum processed n-i-p perovskite solar cell

Vacuum processing is considered to be a promising method allowing the scalable fabrication of perovskite solar cells (PSCs). In vacuum processed PSCs, the n-i-p structure employing organic charge transport layers is less common than the p-i-n structure due to limited options to achieve an efficient electron extraction layer (EEL) on indium tin oxide (ITO) with vacuum thermal evaporation. There are a number of specific applications where an n-i-p structure is required and therefore, it is of interest to have alternative solutions for the n-type contact in vacuum processed PSCs. In this work, we report an efficient vacuum deposited EEL using a mixture of conventional organic small molecules, C₆₀ and bathocuproine (BCP). Incorporation of BCP into C₆₀ does not result in conventional n-doping; however, we observed enhanced charge extraction, which significantly increased the power conversion efficiency (PCE) from 13.1% to 18.1% in all-vacuum processed PSCs. The C₆₀:BCP mixed (co-sublimated) film most likely results in shifted energy levels leading to better alignment with the electrodes.

C₆₀:BCP (bathocuproine) mixture, significantly improved electron extraction in an all-vacuum processed n-i-p perovskite solar cell.

A counterion study of a series of [Cu(P^P)(N^N)][A] compounds with bis(phosphane) and 6-methyl and 6,6'-dimethyl-substituted 2,2'-bipyridine ligands for light-emitting electrochemical cells

The syntheses and characterisations of a series of heteroleptic copper(I) compounds [Cu(POP)(Mebpy)][A], [Cu(POP)(Me2bpy)][A], [Cu(xantphos)(Mebpy)][A] and [Cu(xantphos)(Me2bpy)][A] in which [A]- is [BF4]-, [PF6]-, [BPh4]- and [BArF4]- (Mebpy = 6-methyl-2,2'-bipyridine, Me2bpy = 6,6'-dimethyl-2,2'-bipyridine, POP = oxydi(2,1-phenylene)bis(diphenylphosphane), xantphos = (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane), [BArF4]- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) are reported. Nine of the compounds have been characterised by single crystal X-ray crystallography, and the consequences of the different anions on the packing interactions in the solid state are discussed. The effects of the counterion on the photophysical properties of [Cu(POP)(N^N)][A] and [Cu(xantphos)(N^N)][A] (N^N = Mebpy and Me2bpy) have been investigated. In the solid-state emission spectra, the highest energy emission maxima are for [Cu(xantphos)(Mebpy)][BPh4] and [Cu(xantphos)(Me2bpy)][BPh4] (λemmax = 520 nm) whereas the lowest energy λemmax values occur for [Cu(POP)(Mebpy)][PF6] and [Cu(POP)(Mebpy)][BPh4] (565 nm and 563 nm, respectively). Photoluminescence quantum yields (PLQYs) are noticeably affected by the counterion; in the [Cu(xantphos)(Me2bpy)][A] series, solid-state PLQY values decrease from 62% for [PF6]-, to 44%, 35% and 27% for [BF4]-, [BPh4]- and [BArF4]-, respectively. This latter series of compounds was used as active electroluminescent materials on light-emitting electrochemical cells (LECs). The luminophores were mixed with ionic liquids (ILs) [EMIM][A] ([EMIM]+ = [1-ethyl-3-methylimidazolium]+) containing the same or different counterions than the copper(I) complex. LECs containing [Cu(xantphos)(Me2bpy)][BPh4] and [Cu(xantphos)(Me2bpy)][BArF4] failed to turn on under the LEC operating conditions, whereas those with the smaller [PF6]- or [BF4]- counterions had rapid turn-on times and exhibited maximum luminances of 173 and 137 cd m-2 and current efficiencies of 3.5 and 2.6 cd A-1, respectively, when the IL contained the same counterion as the luminophore. Mixing the counterions ([PF6]- and [BF4]-) of the active complex and the IL led to a reduction in all the figures of merit of the LECs.

Pulsed Laser Deposition of Cs2AgBiBr6: from Mechanochemically Synthesized Powders to Dry, Single-Step Deposition

Cs₂AgBiBr₆ has been proposed as a promising lead-free and stable double perovskite alternative to hybrid and lead-based perovskites. However, the low solubility of precursors during wet synthesis, or the distinct volatility of components during evaporation, results in complex multistep synthesis approaches, hampering the widespread employment of Cs₂AgBiBr₆ films. Here, we present pulsed laser deposition of Cs₂AgBiBr₆ films as a dry, single-step and single-source deposition approach for high-quality film formation. Cs₂AgBiBr₆ powders were prepared by mechanochemical synthesis and pressed into a solid target maintaining phase purity. Controlled laser ablation of the double perovskite target in vacuum and a substrate temperature of 200 °C results in the formation of highly crystalline Cs₂AgBiBr₆ films. We discuss the importance of deposition pressure to achieve stoichiometric transfer and of substrate temperature during PLD growth to obtain high-quality Cs₂AgBiBr₆ films with grain sizes > 200 nm. This work demonstrates the potential of PLD, an established technique in the semiconductor industry, to deposit complex halide perovskite materials while being compatible with optoelectronic device fabrication, such as UV and X-ray detectors.

Wide-Bite-Angle Diphosphine Ligands in Thermally Activated Delayed Fluorescent Copper(I) Complexes: Impact on the Performance of Electroluminescence Applications

We report a series of seven cationic heteroleptic copper(I) complexes of the form [Cu(P^P)(dmphen)]BF₄, where dmphen is 2,9-dimethyl-1,10-phenanthroline and P^P is a diphosphine chelate, in which the effect of the bite angle of the diphosphine ligand on the photophysical properties of the complexes was studied. Several of the complexes exhibit moderately high photoluminescence quantum yields in the solid state, with Φ_PL of up to 35%, and in solution, with Φ_PL of up to 98%. We were able to correlate the powder photoluminescence quantum yields with the % V_bur of the P^P ligand. The most emissive complexes were used to fabricate both organic light-emitting diodes and light-emitting electrochemical cells (LECs), both of which showed moderate performance. Compared to the benchmark copper(I)-based LECs, [Cu(dnbp)(DPEPhos)]⁺ (maximum external quantum efficiency, EQE_max = 16%), complex 3 (EQE_max = 1.85%) showed a much longer device lifetime (t_1/2 = 1.25 h and >16.5 h for [Cu(dnbp)(DPEPhos)]⁺ and complex 3, respectively). The electrochemiluminescence (ECL) properties of several complexes were also studied, which, to the best of our knowledge, constitutes the first ECL study for heteroleptic copper(I) complexes. Notably, complexes exhibiting more reversible electrochemistry were associated with higher annihilation ECL as well as better performance in a LEC.

Amplified spontaneous emission in thin films of quasi-2D BA3MA3Pb5Br16 lead halide perovskites

Quasi-2D (two-dimensional) hybrid perovskites are emerging as a new class of materials with high photoluminescence yield and improved stability compared to their three-dimensional (3D) counterparts. Nevertheless, despite their outstanding emission properties, few studies have been reported on amplified spontaneous emission (ASE) and a thorough understanding of the photophysics of these layered materials is still lacking. In this work, we investigate the ASE properties of multilayered quasi-2D BA₃MA₃Pb₅Br₁₆ films through the dependence of the photoluminescence on temperature and provide a novel insight into the emission processes of quasi-2D lead bromide perovskites. We demonstrate that the PL and ASE properties are strongly affected by the presence, above 190 K, of a minor fraction of the high temperature (HT) phase. This phase dominates the PL spectra at low excitation density and strongly affects the ASE properties. In particular, ASE is only present between 13 K and 230 K, and, at higher temperatures, it is suppressed by absorption of charge transfer states of the HT phase. Our results improve the understanding of the difficulties to obtain ASE at room temperature from these quasi-2D materials and are expected to guide possible materials improvement in order to exploit their excellent emission properties also for the realization of low threshold optically pumped lasers.

Zero-Dimensional Hybrid Organic-Inorganic Lead Halides and Their Post-Synthesis Reversible Transformation into Three-Dimensional Perovskites

Zero-dimensional (0D) mixed-halide hybrid organic-inorganic MA₄PbX₆·2H₂O (MA = CH₃NH₃⁺; X = Br_1 - xI_x with 0 < x < 1) has been synthesized by a solvent-free mechanochemical approach. It has been shown that this 0D phase with sharp absorption features in the near-UV is a hydrated structure, which can be reversibly transformed into the three-dimensional perovskite phase MAPbX₃ by simple thermal annealing (dehydration) in air. This work reveals a new approach to hybrid organic-inorganic perovskites and related 0D structures, which have so far only been thoroughly studied for the inorganic Cs₄PbX₆ compounds.

Efficient Wide-Bandgap Mixed-Cation and Mixed-Halide Perovskite Solar Cells by Vacuum Deposition

Vacuum deposition methods are increasingly applied to the preparation of perovskite films and devices, in view of the possibility to prepare multilayer structures at low temperature. Vacuum-deposited, wide-bandgap solar cells based on mixed-cation and mixed-anion perovskites have been scarcely reported, due to the challenges associated with the multiple-source processing of perovskite thin films. In this work, we describe a four-source vacuum deposition process to prepare wide-bandgap perovskites of the type FA_1-n Cs _n Pb(I_1-x Br _x )₃ with a tunable bandgap and controlled morphology, using FAI, CsI, PbI₂, and PbBr₂ as the precursors. The simultaneous sublimation of PbI₂ and PbBr₂ allows the relative Br/Cs content to be decoupled and controlled, resulting in homogeneous perovskite films with a bandgap in the 1.7-1.8 eV range and no detectable halide segregation. Solar cells based on 1.75 eV bandgap perovskites show efficiency up to 16.8% and promising stability, maintaining 90% of the initial efficiency after 2 weeks of operation.

Deposition Kinetics and Compositional Control of Vacuum-Processed CH3NH3PbI3 Perovskite

Halide perovskites have generated considerable research interest due to their excellent optoelectronic properties in the past decade. To ensure the formation of high-quality semiconductors, the deposition process for the perovskite film is a critical issue. Vacuum-based processing is considered to be a promising method, allowing, in principle, for uniform deposition on a large area. One of the benefits of vacuum processing is the control over the film composition through the use of quartz crystal microbalances (QCMs) that monitor the rates of the components in situ. In metal halide perovskites, however, one frequently employed component or precursor, CH₃NH₃I, exhibits nonstandard sublimation properties. Here, we study in detail the sublimation properties of CH₃NH₃I and demonstrate that by correcting for its complex adsorption properties and by modeling the film growth, accurate predictions of the stoichiometry of the final perovskite film can be obtained.

Efficient Vacuum Deposited P-I-N Perovskite Solar Cells by Front Contact Optimization

Hole transport layers (HTLs) are of fundamental importance in perovskite solar cells (PSCs), as they must ensure an efficient and selective hole extraction, and ohmic charge transfer to the corresponding electrodes. In p-i-n solar cells, the ITO/HTL is usually not ohmic, and an additional interlayer such as MoO₃ is usually placed in between the two materials by vacuum sublimation. In this work, we evaluated the properties of the MoO₃/TaTm (TaTm is the HTL N4,N4,N4″,N4″-tetra([1,1′-biphenyl]-4-yl)-[1,1′:4′,1″-terphenyl]-4,4″-diamine) hole extraction interface by selectively annealing either MoO₃ (prior to the deposition of TaTm) or the bilayer MoO₃/TaTm (without pre-treatment on the MoO₃), at temperature ranging from 60 to 200°C. We then used these p-contacts for the fabrication of a large batch of fully vacuum deposited PSCs, using methylammonium lead iodide as the active layer. We show that annealing the MoO₃/TaTm bilayers at high temperature is crucial to obtain high rectification with low non-radiative recombination, due to an increase of the electrode work function and the formation of an ohmic interface with TaTm.

Highly Photoluminescent Blue Ionic Platinum-Based Emitters

New cycloplatinated N-heterocyclic carbene (NHC) compounds with chelate diphosphines (P^P) as ancillary ligands: [Pt(R-C^C*)(P^P)]PF₆ (R = H, P^P = dppm (1A), dppe (2A), dppbz (3A); R = CN, P^P = dppm (1B), dppe (2B), dppbz (3B)) have been prepared from the corresponding starting material [{Pt(R-C^C*)(μ-Cl)}₂] (R = H, A, R = CN, B) and fully characterized. The new compound A has been prepared by a stepwise protocol. The photophysical properties of 1A-3A and 1B-3B have been widely studied and supported by the time-dependent-density functional theory. These compounds show an efficient blue (dppe, dppbz) or cyan (dppm) emission in PMMA films (5 wt %), with photoluminescence quantum yield (PLQY) ranging from 30% to 87% under an argon atmosphere. This emission has been assigned mainly to transitions from ³ILCT [π(NHC) → π*(NHC)] excited states with some ³LL'CT [π(NHC) → π*(P^P)] character. The electroluminescence of these materials in proof-of-concept solution-processed organic light-emitting diodes containing 3A and 3B as dopants was investigated. The CIE coordinates for devices based on 3A (0.22, 0.41) and 3B (0.24, 0.44) fit within the sky blue region.

The shiny side of copper: bringing copper(i) light-emitting electrochemical cells closer to application

Heteroleptic [Cu(P^P)(N^N)][PF₆] complexes, where N^N is 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me₂bpy), 4,5,6-trimethyl-2,2′-bipyridine (4,5,6-Me₃bpy), 6-(tert-butyl)-2,2′-bipyridine (6-tBubpy) and 2-ethyl-1,10-phenanthroline (2-Etphen) and P^P is either bis(2-(diphenylphosphino)phenyl)ether (POP, PIN [oxydi(2,1-phenylene)]bis(diphenylphosphane)) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos, PIN (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)) have been synthesized and their NMR spectroscopic, mass spectrometric, structural, electrochemical and photophysical properties were investigated. The single-crystal structures of [Cu(POP)(5,5′-Me₂bpy)][PF₆], [Cu(xantphos)(5,5′-Me₂bpy)][PF₆], [Cu(POP)(6-tBubpy)][PF₆], [Cu(POP)(4,5,6-Me₃bpy)][PF₆]·1.5Et₂O, [Cu(xantphos)(4,5,6-Me₃bpy)][PF₆]·2.33CH₂Cl₂, [Cu(POP)(2-Etphen)][PF₆] and [Cu(xantphos)(2-Etphen)][PF₆] are described. While alkyl substituents in general exhibit electron-donating properties, variation in the nature and substitution-position of the alkyl group in the N^N chelate leads to different effects in the photophysical properties of the [Cu(P^P)(N^N)][PF₆] complexes. In the solid state, the complexes are yellow to green emitters with emission maxima between 518 and 602 nm, and photoluminescence quantum yields (PLQYs) ranging from 1.1 to 58.8%. All complexes show thermally activated delayed fluorescence (TADF). The complexes were employed in the active layer of light-emitting electrochemical cells (LECs). The device performance properties are among the best reported for copper-based LECs, with maximum luminance values of up to 462 cd m⁻² and device half-lifetimes of up to 98 hours.

Heteroleptic copper(i) complexes with bisphosphanes and astutely tuned N^N chelating ligands as emitters give bright LECs with record-breaking stability.

High voltage vacuum-processed perovskite solar cells with organic semiconducting interlayers

In perovskite solar cells, the choice of appropriate transport layers and electrodes is of great importance to guarantee efficient charge transport and collection, minimizing recombination losses. The possibility to sequentially process multiple layers by vacuum methods offers a tool to explore the effects of different materials and their combinations on the performance of optoelectronic devices. In this work, the effect of introducing interlayers and altering the electrode work function has been evaluated in fully vacuum-deposited perovskite solar cells. We compared the performance of solar cells employing common electron buffer layers such as bathocuproine (BCP), with other injection materials used in organic light-emitting diodes, such as lithium quinolate (Liq), as well as their combination. Additionally, high voltage solar cells were obtained using low work function metal electrodes, although with compromised stability. Solar cells with enhanced photovoltage and stability under continuous operation were obtained using BCP and BCP/Liq interlayers, resulting in an efficiency of approximately 19%, which is remarkable for simple methylammonium lead iodide absorbers.

The effect of n-type interlayers and electrodes on the voltage and stability of fully vacuum-deposited perovskite solar cells is evaluated.

Dipole reorientation and local density of optical states influence the emission of light-emitting electrochemical cells

Herein, we analyze the temporal evolution of the electroluminescence of light-emitting electrochemical cells (LECs), a thin-film light-emitting device, in order to maximize the luminous power radiated by these devices. A careful analysis of the spectral and angular distribution of the emission of LECs fabricated under the same experimental conditions allows describing the dynamics of the spatial region from which LECs emit, i.e. the generation zone, as bias is applied. This effect is mediated by dipole reorientation within such an emissive region and its optical environment, since its spatial drift yields a different interplay between the intrinsic emission of the emitters and the local density of optical states of the system. Our results demonstrate that engineering the optical environment in thin-film light-emitting devices is key to maximize their brightness.

Correction: A comparative study of Ir(iii) complexes with pyrazino[2,3-f][1,10]phenanthroline and pyrazino[2,3-f][4,7]phenanthroline ligands in light-emitting electrochemical cells (LECs)

Correction for 'A comparative study of Ir(iii) complexes with pyrazino[2,3-f][1,10]phenanthroline and pyrazino[2,3-f][4,7]phenanthroline ligands in light-emitting electrochemical cells (LECs)' by Iván González et al., Dalton Trans., 2015, 44, 14771-14781.

Short Photoluminescence Lifetimes in Vacuum-Deposited CH3NH3PbI3 Perovskite Thin Films as a Result of Fast Diffusion of Photogenerated Charge Carriers

It is widely accepted that a long photoluminescence (PL) lifetime in metal halide perovskite films is a crucial and favorable factor, as it ensures a large charge diffusion length leading to a high power conversion efficiency (PCE) in solar cells. It has been recently found that vacuum-evaporated CH₃NH₃PbI₃ (eMAPI) films show very short PL lifetimes of several nanoseconds. The corresponding solar cells, however, have high photovoltage (>1.1 V) and PCEs (up to 20%). We rationalize this apparent contradiction and show that eMAPI films are characterized by a very high diffusion coefficient D, estimated from modeling the PL kinetics to exceed 1 cm²/s. Such high D values are favorable for long diffusion length as well as fast transport of carriers to film surfaces, where they recombine nonradiatively with surface recombination velocity S ∼ 10⁴ cm/s. Possible physical origins leading to the high D values are also discussed.

Room-Temperature Cubic Phase Crystallization and High Stability of Vacuum-Deposited Methylammonium Lead Triiodide Thin Films for High-Efficiency Solar Cells

Methylammonium lead triiodide (MAPI) has emerged as a high-performance photovoltaic material. Common understanding is that at room temperature, it adopts a tetragonal phase and it only converts to the perfect cubic phase around 50-60 °C. Most MAPI films are prepared using a solution-based coating process, yet they can also be obtained by vapor-phase deposition methods. Vapor-phase-processed MAPI films have significantly different characteristics than their solvent-processed analogous, such as relatively small crystal-grain sizes and short excited-state lifetimes. However, solar cells based on vapor-phase-processed MAPI films exhibit high power-conversion efficiencies. Surprisingly, after detailed characterization it is found that the vapor-phase-processed MAPI films adopt a cubic crystal structure at room temperature that is stable for weeks, even in ambient atmosphere. Furthermore, it is demonstrated that by tuning the deposition rates of both precursors during codeposition it is possible to vary the perovskite phase from cubic to tetragonal at room temperature. These findings challenge the common belief that MAPI is only stable in the tetragonal phase at room temperature.

Low-dimensional iodide perovskite nanocrystals enable efficient red emission

We report herein a simple ligand-assisted reprecipitation method at room temperature to synthesize mixed-cation hybrid organic-inorganic perovskite nanocrystals with low structural dimensionality. The emission wavelength of iodide-based perovskites is thus tuned from the near-infrared to the red part of the visible spectrum. While this is mostly achieved in the literature by addition of bromide, we demonstrate here a controllable blueshift of the band gap by varying the chain length of the alkylammonium ligands. Furthermore, an antisolvent washing step was found to be crucial to purify the samples and obtain single-peak photoluminescence with a narrow linewidth. The so-formed nanocrystals exhibit high and stable photoluminescence quantum yields exceeding 90% over 500 hours, making these materials ideal for light-emitting applications.

Consistent Device Simulation Model Describing Perovskite Solar Cells in Steady-State, Transient, and Frequency Domain

A variety of experiments on vacuum-deposited methylammonium lead iodide perovskite solar cells are presented, including JV curves with different scan rates, light intensity-dependent open-circuit voltage, impedance spectra, intensity-modulated photocurrent spectra, transient photocurrents, and transient voltage step responses. All these experimental data sets are successfully reproduced by a charge drift-diffusion simulation model incorporating mobile ions and charge traps using a single set of parameters. While previous modeling studies focused on a single experimental technique, we combine steady-state, transient, and frequency-domain simulations and measurements. Our study is an important step toward quantitative simulation of perovskite solar cells, leading to a deeper understanding of the physical effects in these materials. The analysis of the transient current upon voltage turn-on in the dark reveals that the charge injection properties of the interfaces are triggered by the accumulation of mobile ionic defects. We show that the current rise of voltage step experiments allow for conclusions about the recombination at the interface. Whether one or two mobile ionic species are used in the model has only a minor influence on the observed effects. A delayed current rise observed upon reversing the bias from +3 to -3 V in the dark cannot be reproduced yet by our drift-diffusion model. We speculate that a reversible chemical reaction of mobile ions with the contact material may be the cause of this effect, thus requiring a future model extension. A parameter variation is performed in order to understand the performance-limiting factors of the device under investigation.

Phosphomolybdic acid as an efficient hole injection material in perovskite optoelectronic devices

Phosphomolybdic acid (PMA) is a solution-processable high work function material which can be used as hole-injection interface in perovskite solar cells and LEDs.

Phosphane tuning in heteroleptic [Cu(N^N)(P^P)]+ complexes for light-emitting electrochemical cells

The effects on photo-and electroluminescent properties of structurally modifying the bisphosphane in [Cu(N^N)(P^P)]⁺ complexes (N^N = bpy, 6-Mebpy, 6,6′-Me₂bpy) are described.

Efficient Perovskite Light-Emitting Diodes: Effect of Composition, Morphology, and Transport Layers

Organic-inorganic metal halide perovskites are emerging as novel materials for light-emitting applications due to their high color purity, band gap tunability, straightforward synthesis, and inexpensive precursors. In this work, we improve the performance of three-dimensional perovskite light-emitting diodes (PeLEDs) by tuning the emissive layer composition and thickness and by using small-molecule transport layers. Additionally, we correlate PeLED efficiencies to the perovskite structure and morphology. The results show that the PeLEDs containing perovskites with an excess of methylammonium bromide (MABr) to lead bromide (PbBr₂) in a 2:1 ratio and a layer thickness of 80 nm have the highest performance. The optimized device exhibits a peak luminance of 17 600 cd/m² and an external quantum efficiency of 3.9%. Structural and morphological studies reveal a reduction in crystallite size and surface roughness with decreasing perovskite layer thickness and increasing ratio of MABr to PbBr₂. Balanced charge injection, spatial charge confinement, and reduction in nonradiative sites can explain the enhanced performance by virtue of favorable morphology and transport layer choice.

Efficient Photo- and Electroluminescence by Trap States Passivation in Vacuum-Deposited Hybrid Perovskite Thin Films

Methylammonium lead iodide (MAPI) has excellent properties for photovoltaic applications, although it typically shows low photoluminescence quantum yield. Here, we report on vacuum-deposited MAPI perovskites obtained by modifying the methylammonium iodide (MAI) to PbI₂ ratio during vacuum deposition. By studying the excitation density dependence of the photoluminescence lifetime, a large concentration of trap states was deduced for the stoichiometric MAPI films. The use of excess MAI during vacuum processing is capable of passivating these traps, resulting in luminescent films which can be used to fabricate planar light-emitting diodes with quantum efficiency approaching 2%.

Perovskite-Perovskite Homojunctions via Compositional Doping

One of the most important properties of semiconductors is the possibility of controlling their electronic behavior via intentional doping. Despite the unprecedented progress in the understanding of hybrid metal halide perovskites, extrinsic doping of perovskite remains nearly unexplored and perovskite-perovskite homojunctions have not been reported. Here we present a perovskite-perovskite homojunction obtained by vacuum deposition of stoichiometrically tuned methylammonium lead iodide (MAPI) films. Doping is realized by adjusting the relative deposition rates of MAI and PbI₂, obtaining p-type (MAI excess) and n-type (MAI defect) MAPI. The successful stoichiometry change in the thin films is confirmed by infrared spectroscopy, which allows us to determine the MA content in the films. We analyzed the resulting thin-film junction by cross-sectional scanning Kelvin probe microscopy (SKPM) and found a contact potential difference (CPD) of 250 mV between the two differently doped perovskite layers. Planar diodes built with the perovskite-perovskite homojunction show the feasibility of our approach for implementation in devices.

Bis(arylimidazole) Iridium Picolinate Emitters and Preferential Dipole Orientation in Films

The straightforward synthesis and photophysical properties of a new series of heteroleptic iridium(III) bis(2-arylimidazole) picolinate complexes are reported. Each complex has been characterized by nuclear magnetic resonance, UV-vis, cyclic voltammetry, and photoluminescent angle dependency, and the emissive properties of each are described. The preferred orientation of transition dipoles in emitter/host thin films indicated more preferred orientation than homoleptic complex Ir(ppy)₃.

Interfacial Modification for High-Efficiency Vapor-Phase-Deposited Perovskite Solar Cells Based on a Metal Oxide Buffer Layer

Vacuum deposition is one of the most technologically relevant techniques for the fabrication of perovskite solar cells. The most efficient vacuum-based devices rely on doped organic contacts, compromising the long-term stability of the system. Here, we introduce an inorganic electron-transporting material to obtain power conversion efficiencies matching the best performing vacuum-deposited devices, with open-circuit potential close to the thermodynamic limit. We analyze the leakage current reduction and the interfacial recombination improvement upon use of a thin (<10 nm) interlayer of C₆₀, as well as a more favorable band alignment after a bias/ultraviolet light activation process. This work presents an alternative for organic contacts in highly efficient vacuum-deposited perovskite solar cells.

CF3 Substitution of [Cu(P^P)(bpy)][PF6 ] Complexes: Effects on Photophysical Properties and Light-Emitting Electrochemical Cell Performance

Herein, [Cu(P^P)(N^N)][PF₆ ] complexes (P^P=bis[2-(diphenylphosphino)phenyl]ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos); N^N=CF₃ -substituted 2,2'-bipyridines (6,6'-(CF₃ )₂ bpy, 6-CF₃ bpy, 5,5'-(CF₃ )₂ bpy, 4,4'-(CF₃ )₂ bpy, 6,6'-Me₂ -4,4'-(CF₃ )₂ bpy)) are reported. The effects of CF₃ substitution on their structure as well as their electrochemical and photophysical properties are also presented. The HOMO-LUMO gap was tuned by the N^N ligand; the largest redshift in the metal-to-ligand charge transfer (MLCT) band was for [Cu(P^P){5,5'-(CF₃ )₂ bpy}][PF₆ ]. In solution, the compounds are weak yellow to red emitters. The emission properties depend on the substitution pattern, but this cannot be explained by simple electronic arguments. Among powders, [Cu(xantphos){4,4'-(CF₃ )₂ bpy}][PF₆ ] has the highest photoluminescence quantum yield (PLQY; 50.3 %) with an emission lifetime of 12 μs. Compared to 298 K solution behavior, excited-state lifetimes became longer in frozen Me-THF (77 K; THF=tetrahydrofuran), thus indicating thermally activated delayed fluorescence (TADF). Time-dependent (TD)-DFT calculations show that the energy gap between the lowest-energy singlet and triplet excited states (0.12-0.20 eV) permits TADF. Light-emitting electrochemical cells (LECs) with [Cu(POP)+(6-CF₃ bpy)][PF₆ ], [Cu(xantphos)(6-CF₃ bpy)][PF₆ ], or [Cu(xantphos){6,6'-Me₂ -4,4'-(CF₃ )₂ bpy}][PF₆ ] emit yellow electroluminescence. The LEC with [Cu(xantphos){6,6'-Me₂ -4,4'-(CF₃ )₂ bpy}][PF₆ ] had the fastest turn-on time (8 min), and the LEC with the longest lifetime (t_1/2 =31 h) contained [Cu(xantphos)(6-CF₃ bpy)][PF₆ ]; these LECs reached maximum luminances of 131 and 109 cd m^-2 , respectively.

Solution processed organic light-emitting diodes using a triazatruxene crosslinkable hole transporting material

A cross-linkable triazatruxene that leads to insoluble films upon thermal annealing at temperatures compatible with flexible substrates is presented. The films were used as the hole transporting and electron blocking layer in partially solution processed phosphorescent organic light-emitting diodes, reaching power conversion efficiencies of 24 lm W⁻¹, an almost 50% improvement compared to the same OLEDs without the cross-linkable hole transporting layer.

A cross-linkable triazatruxene that leads to insoluble films upon thermal annealing at temperatures compatible with flexible substrates is presented.

Incorporation of potassium halides in the mechanosynthesis of inorganic perovskites: feasibility and limitations of ion-replacement and trap passivation

Potassium halides (KX; X = I, Br, or Cl) were incorporated as partial replacements of CsBr in the mechanosynthesis of CsPbBr₃. This led to partial substitution of both monovalent ions forming mixed Cs_1−xK_xPbBr_3−yX_y perovskites. Longer photoluminescence lifetimes were also observed, possibly linked to the formation of a non-perovskite KPb₂X₅ passivating layer.

Potassium halides are used for cation-exchange, anion-exchange and trap passivation of mechanosynthesized perovskites.

High Photoluminescence Quantum Yields in Organic Semiconductor-Perovskite Composite Thin Films

One of the obstacles towards efficient radiative recombination in hybrid perovskites is a low exciton binding energy, typically in the orders of tens of meV. It has been shown that the use of electron-donor additives can lead to a substantial reduction of the non-radiative recombination in perovskite films. Herein, the approach using small molecules with semiconducting properties, which are candidates to be implemented in future optoelectronic devices, is presented. In particular, highly luminescent perovskite-organic semiconductor composite thin films have been developed, which can be processed from solution in a simple coating step. By tuning the relative concentration of methylammonium lead bromide (MAPbBr₃ ) and 9,9spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1), it is possible to achieve photoluminescent quantum yields (PLQYs) as high as 85 %. This is attributed to the dual functions of SPPO1 that limit the grain growth while passivating the perovskite surface. The electroluminescence of these materials was investigated by fabricating multilayer LEDs, where charge injection and transport was found to be severely hindered for the perovskite/SPPO1 material. This was alleviated by partially substituting SPPO1 with a hole-transporting material, 1,3-bis(N-carbazolyl)benzene (mCP), leading to bright electroluminescence. The potential of combining perovskite and organic semiconductors to prepare materials with improved properties opens new avenues for the preparation of simple lightemitting devices using perovskites as the emitter.

Preface to Special Issue of ChemSusChem on Perovskite Optoelectronics

This Editorial introduces one of two companion Special Issues on "Halide Perovskites for Optoelectronics Applications" in ChemSusChem and Energy Technology following the ICMAT 2017 Conference in Singapore. More information on the other Special Issue can be found in the Editorial published in Energy Technology.

Luminescent osmium(ii) bi-1,2,3-triazol-4-yl complexes: photophysical characterisation and application in light-emitting electrochemical cells

The series of osmium(ii) complexes [Os(bpy)3-n(btz)n][PF6]2 (bpy = 2,2'-bipyridyl, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 0, n = 1, n = 2, n = 3), have been prepared and characterised. The progressive replacement of bpy by btz leads to blue-shifted UV-visible electronic absorption spectra, indicative of btz perturbation of the successively destabilised bpy-centred LUMO. For , a dramatic blue-shift relative to the absorption profile for is observed, indicative of the much higher energy LUMO of the btz ligand over that of bpy, mirroring previously reported data on analogous ruthenium(ii) complexes. Unlike the previously reported ruthenium systems, heteroleptic complexes and display intense emission in the far-red/near-infrared (λmax = 724 and 713 nm respectively in aerated acetonitrile at RT) as a consequence of higher lying, and hence less thermally accessible, (3)MC states. This assertion is supported by ground state DFT calculations which show that the dσ* orbitals of to are destabilised by between 0.60 and 0.79 eV relative to their Ru(ii) analogues. The homoleptic complex appears to display extremely weak room temperature emission, but on cooling to 77 K the complex exhibits highly intense blue emission with λmax 444 nm. As complexes to display room temperature luminescent emission and readily reversible Os(ii)/(iii) redox couples, light-emitting electrochemical cell (LEC) devices were fabricated. All LECs display electroluminescent emission in the deep-red/near-IR (λmax = 695 to 730 nm). Whilst devices based on and show inferior current density and luminance than LECs based on , the device utilising shows the highest external quantum efficiency at 0.3%.

Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions

Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p-i-n and n-i-p CH₃NH₃PbI₃ solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current-voltage (J-V) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the J-V hysteresis.

Pyrene-fused bisphenazinothiadiazoles with red to NIR electroluminescence

Deep red and NIR electroluminescence from pyrene-fused bisphenazinothiadiazoles.

Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells

Two cationic Ir(iii) complexes bearing bithiazole-type ancillary ligands have been synthesised and tested as deep red-to-near-infrared emitters in solution-processed electroluminescent devices.

Photoluminescence quantum yield exceeding 80% in low dimensional perovskite thin-films via passivation control

The effects of the processing conditions and of the relative content of methylammonium (MA) and butylammonium (BA) cations on the properties of lead bromide quasi-2D perovskite thin-films were studied.

Chiral Iridium(III) Complexes in Light-Emitting Electrochemical Cells: Exploring the Impact of Stereochemistry on the Photophysical Properties and Device Performances

Despite hundreds of cationic bis-cyclometalated iridium(III) complexes having been explored as emitters for light-emitting electrochemical cells (LEECs), uniformly their composition has been in the form of a racemic mixture of Λ and Δ enantiomers. The investigation of LEECs using enantiopure iridium(III) emitters, however, remains unprecedented. Herein, we report the preparation, the crystal structures, and the optoelectronic properties of two families of cyclometalated iridium(III) complexes of the form of [(C^N)₂Ir(dtBubpy)]PF₆ (where dtBubpy is 4,4'-di-tert-butyl-2,2'-bipyridine) in both their racemic and enantiopure configurations. LEEC devices using Λ and Δ enantiomers as well as the racemic mixture of both families have been prepared, and the device performances were tested. Importantly, different solid-state photophysical properties exist between enantiopure and racemic emitters, which are also reflected in the device performances.

Strontium Insertion in Methylammonium Lead Iodide: Long Charge Carrier Lifetime and High Fill-Factor Solar Cells

The addition of Sr²⁺ in CH₃ NH₃ PbI₃ perovskite films enhances the charge carrier collection efficiency of solar cells leading to very high fill factors, up to 85%. The charge carrier lifetime of Sr²⁺ -containing perovskites is in excess of 40 μs, longer than those reported for perovskite single crystals.