Stable Green Electroluminescence from an Iridium Tris-Heteroleptic Ionic Complex

An ionic tris-heteroleptic iridium complex gives green light-emitting electrochemical cells (LECs) with unprecedented performances for this part of the visible spectrum. The devices are very bright (>1000 cd m^–2), efficient (∼3%), and stable (>55 h). The novel complex is prepared using a new and efficient synthetic procedure. We show that there is a mixed orbital formation originating from the two different orthometalating ligands resulting in photophysical properties that lie between those of its two bis-heteroleptic analogs. Therefore, tris-heteroleptic complexes provide new avenues for fine-tunning the emission properties and to bridge gaps between a series of bis-heteroleptic complexes

Tris(2-(1<i>H</i>-pyrazol-1-yl)pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors and Its Application in Highly Efficient Solid-State Dye-Sensitized Solar Cells

Chemical doping is an important strategy to alter the charge-transport properties of both molecular and polymeric organic semiconductors that find widespread application in organic electronic devices. We report on the use of a new class of Co(III) complexes as p-type dopants for triarylamine-based hole conductors such as spiro-MeOTAD and their application in solid-state dye-sensitized solar cells (ssDSCs). We show that the proposed compounds fulfill the requirements for this application and that the discussed strategy is promising for tuning the conductivity of spiro-MeOTAD in ssDSCs, without having to rely on the commonly employed photo-doping. By using a recently developed high molar extinction coefficient organic D-π-A sensitizer and p-doped spiro-MeOTAD as hole conductor, we achieved a record power conversion efficiency of 7.2%, measured under standard solar conditions (AM1.5G, 100 mW cm^–2). We expect these promising new dopants to find widespread applications in organic electronics in general and photovoltaics in particular

Subnanometer Ga₂O₃ Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells

Herein, we present the first use of a gallium oxide tunnelling layer to significantly reduce electron recombination in dye-sensitized solar cells (DSC). The subnanometer coating is achieved using atomic layer deposition (ALD) and leading to a new DSC record open-circuit potential of 1.1 V with state-of-the-art organic D-π-A sensitizer and cobalt redox mediator. After ALD of only a few angstroms of Ga₂O₃, the electron back reaction is reduced by more than an order of magnitude, while charge collection efficiency and fill factor are increased by 30% and 15%, respectively. The photogenerated exciton separation processes of electron injection into the TiO₂ conduction band and the hole injection into the electrolyte are characterized in detail

Influence of Donor Groups of Organic D−π–A Dyes on Open-Circuit Voltage in Solid-State Dye-Sensitized Solar Cells

In solid-state dye-sensitized solar cells (ssDSCs), the poor pore filling of the mesoporous semiconductor and the short diffusion length of charge carriers in the hole-transport material (HTM) have limited the mesoscopic titania layer to a thickness of 2–3 μm. To increase the amount of light harvested by ssDSCs, organic dyes with high molar extinction coefficients are of great importance and have been the focus of intensive research. Here we investigate ssDSCs using an organic D−π–A dye, coded Y123, and 2,2′,7,7′-tetrakis(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene as a hole-transport material, exhibiting 934 mV open-circuit potential and 6.9% efficiency at standard solar conditions (AM1.5G, 100 mW cm^–2), which is a significant improvement compared to the analogue dyes C218, C220, and JK2 (<i>V</i>_oc values of 795, 781, and 914 mV, respectively). An upward shift in the conduction band edge was observed from photovoltage transient decay and impedance spectroscopy measurements for devices sensitized with Y123 and JK2 dyes compared to the device using C220 as sensitizer, in agreement with the high photovoltage response of the corresponding ssDSCs. This work highlights the importance of the interaction between the HTM and the dye-sensitized TiO₂ surface for the design of ssDSCs

Correlating the Lifetime and Fluorine Content of Iridium(III) Emitters in Green Light-Emitting Electrochemical Cells

In light-emitting electrochemical cells, the lifetime of the device is intrinsically linked to the stability of the phosphorescent emitter. In this study, we present a series of ionic iridium­(III) emitters based on cyclometalating phenylpyridine ligands whose fluorine substituents are varied in terms of position and number. Importantly, despite these structural modifications, the emitters exhibit virtually identical electrochemical and spectroscopic properties, which allows for proper comparison in functional devices. Quantum-chemical calculations support the properties measured in solution and suggest great similarities regarding the electronic structures of the emitters. In electroluminescent devices, the initial luminance, efficiency, and efficacy are also relatively unaffected throughout the series. However, a shorter device lifetime is obtained upon increasing the fluorine content of the emitter, which suggests drawbacks of such electron-withdrawing substituents for the design of ionic iridium­(III) emitters

Acid-Induced Degradation of Phosphorescent Dopants for OLEDs and Its Application to the Synthesis of Tris-heteroleptic Iridium(III) Bis-cyclometalated Complexes

Investigations of blue phosphorescent organic light emitting diodes (OLEDs) based on [Ir­(2-(2,4-difluorophenyl)­pyridine)₂(picolinate)] (FIrPic) have pointed to the cleavage of the picolinate as a possible reason for device instability. We reproduced the loss of picolinate and acetylacetonate ancillary ligands in solution by the addition of Brønsted or Lewis acids. When hydrochloric acid is added to a solution of a [Ir­(C^∧N)₂(X^∧O)] complex (C^∧N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)­pyridine (diFppy) and X^∧O = picolinate (pic) or acetylacetonate (acac)), the cleavage of the ancillary ligand results in the direct formation of the chloro-bridged iridium­(III) dimer [{Ir­(C^∧N)₂(μ-Cl)}₂]. When triflic acid or boron trifluoride are used, a source of chloride (here tetrabutylammonium chloride) is added to obtain the same chloro-bridged iridium­(III) dimer. Then, we advantageously used this degradation reaction for the efficient synthesis of tris-heteroleptic cyclometalated iridium­(III) complexes [Ir­(C^∧N¹)­(C^∧N²)­(L)], a family of cyclometalated complexes otherwise challenging to prepare. We used an iridium­(I) complex, [{Ir­(COD)­(μ-Cl)}₂], and a stoichiometric amount of two different C^∧N ligands (C^∧N¹ = ppy; C^∧N² = diFppy) as starting materials for the swift preparation of the chloro-bridged iridium­(III) dimers. After reacting the mixture with acetylacetonate and subsequent purification, the tris-heteroleptic complex [Ir­(ppy)­(diFppy)­(acac)] could be isolated with good yield from the crude containing as well the bis-heteroleptic complexes [Ir­(ppy)₂(acac)] and [Ir­(diFppy)₂(acac)]. Reaction of the tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic chloro-bridged iridium dimer [{Ir­(ppy)­(diFppy)­(μ-Cl)}₂], which can be used as starting material for the preparation of a new tris-heteroleptic iridium­(III) complex based on these two C^∧N ligands. Finally, we use DFT/LR-TDDFT to rationalize the impact of the two different C^∧N ligands on the observed photophysical and electrochemical properties

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect