Karttikay Moudgil

Heteroannulated acceptors based on benzothiadiazole

Increasing the acceptor strength of the widely used acceptor benzothiadiazole (BT) by extending the heterocyclic core is a promising strategy for developing new and stronger acceptors for materials in organic electronics and photonics. In recent years, such heteroannulated BT acceptors have been incorporated into a wide variety of materials that have been used in organic electronic and photonic devices. This review critically assesses the properties of these materials. Although heteroannulation to form acceptors, such as benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazole (BBT), does result in materials with significantly higher electron affinity (EA) relative to BT, in many cases the extended BT systems also exhibit lower ionization energy (IE) than BT. Both the significantly higher EA and lower IE limit the efficacy of these materials in applications such as bulk heterojunction organic photovoltaics (BHJ-OPV) based on C60. Although the relatively high EA may enable some applications such as air stable organic field effect transistors (OFET), more widespread use of heteroannulated BT acceptors will likely require the ability to moderate or retain the high EA while increasing IE.

Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structures energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structures radiative emission yield sevenfold, validating the relevance of our approach.

Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors

Chemical doping of organic semiconductors using molecular dopants plays a key role in the fabrication of efficient organic electronic devices. Although a variety of stable molecular p-dopants have been developed and successfully deployed in devices in the past decade, air-stable molecular n-dopants suitable for materials with low electron affinity are still elusive. Here we demonstrate that photo-activation of a cleavable air-stable dimeric dopant can result in kinetically stable and efficient n-doping of host semiconductors, whose reduction potentials are beyond the thermodynamic reach of the dimers effective reducing strength. Electron-transport layers doped in this manner are used to fabricate high-efficiency organic light-emitting diodes. Our strategy thus enables a new paradigm for using air-stable molecular dopants to improve conductivity in, and provide ohmic contacts to, organic semiconductors with very low electron affinity.

Dimers of Nineteen‐Electron Sandwich Compounds: Crystal and Electronic Structures, and Comparison of Reducing Strengths

The dimers of some Group 8 metal cyclopentadienyl/arene complexes and Group 9 metallocenes can be handled in air, yet are strongly reducing, making them useful n-dopants in organic electronics. In this work, the X-ray molecular structures are shown to resemble those of Group 8 metal cyclopentadienyl/pentadienyl or Group 9 metal cyclopentadienyl/diene model compounds. Compared to those of the model compounds, the DFT HOMOs of the dimers are significantly destabilized by interactions between the metal and the central CC σ-bonding orbital, accounting for the facile oxidation of the dimers. The lengths of these CC bonds (X-ray or DFT) do not correlate with DFT dissociation energies, the latter depending strongly on the monomer stabilities. Ru and Ir monomers are more reducing than their Fe and Rh analogues, but the corresponding dimers also exhibit much higher dissociation energies, so the estimated monomer cation/neutral dimer potentials are, with the exception of that of [RhCp2 ]2 , rather similar (-1.97 to -2.15 V vs. FeCp2 (+/0) in THF). The consequences of the variations in bond strength and redox potentials for the reactivity of the dimers are discussed.

Controlled n-Type Doping of Carbon Nanotube Transistors by an Organorhodium Dimer

Single-walled carbon nanotube (SWCNT) transistors are among the most developed nanoelectronic devices for high-performance computing applications. While p-type SWCNT transistors are easily achieved through adventitious adsorption of atmospheric oxygen, n-type SWCNT transistors require extrinsic doping schemes. Existing n-type doping strategies for SWCNT transistors suffer from one or more issues including environmental instability, limited carrier concentration modulation, undesirable threshold voltage control, and/or poor morphology. In particular, commonly employed benzyl viologen n-type doping layers possess large thicknesses, which preclude top-gate transistor designs that underlie high-density integrated circuit layouts. To overcome these limitations, we report here the controlled n-type doping of SWCNT thin-film transistors with a solution-processed pentamethylrhodocene dimer. The charge transport properties of organorhodium-treated SWCNT thin films show consistent n-type behavior when characterized in both Hall effect and thin-film transistor geometries. Due to the molecular-scale thickness of the organorhodium adlayer, large-area arrays of top-gated, n-type SWCNT transistors are fabricated with high yield. This work will thus facilitate ongoing efforts to realize high-density SWCNT integrated circuits.

Organometallic Dimers: Application to Work-Function Reduction of Conducting Oxides

The dimers of pentamethyliridocene and ruthenium pentamethylcyclopentadienyl mesitylene, (IrCp*Cp)2 and (RuCp*mes)2, respectively, are shown here to be effective solution-processable reagents for lowering the work functions of electrode materials; this approach is compared to the use of solution-deposited films of ethoxylated poly(ethylenimine) (PEIE). The work functions of indium tin oxide (ITO), zinc oxide, and gold electrodes can be reduced to 3.3-3.4 eV by immersion in a toluene solution of (IrCp*Cp)2; these values are similar to those that can be obtained by spin-coating a thin layer of PEIE onto the electrodes. The work-function reductions achieved using (IrCp*Cp)2 are primarily attributable to the interface dipoles associated with the formation of submonolayers of IrCp*Cp(+) cations on negatively charged substrates, which in turn result from redox reactions between the dimer and the electrode. The electrical properties of C60 diodes with dimer-modified ITO cathodes are similar to those of analogous devices with PEIE-modified ITO cathodes.

Thermo-cross-linkable fullerene for long-term stability of photovoltaic devices

In this study, a highly soluble PCBM-based thermo-cross-linkable fullerene precursor has been synthesized for use in bulk heterojunction based organic solar cells. The cross-linking was achieved using a thermally activated benzocyclobutene (BCB) molecule. The thermo-crosslinking reaction is initiated at temperatures as low as 150 °C. Compared to PCBM, the cross-linked fullerene is highly insoluble and has a diffusional mobility in poly(3-hexylthiophene) (P3HT) that is an order of magnitude slower than PCBM. Its electron mobility is comparable to that of PCBM and organic photovoltaic (OPV) devices consisting of bulk heterojunction active layers with P3HT or PTB7 and this fullerene show very similar efficiencies. Devices prepared either with pure cross-linked fullerene or its mixture with PCBM as acceptors in OPVs have been shown to be highly stable to accelerated aging with little loss in device efficiency up to 48 hours of aging at 150 °C. This compares to a loss of 60% of initial efficiency in identically prepared devices when using PCBM as the acceptor. Optical microscopy and grazing incidence wide angle X-ray scattering (GIWAXS) shows that a probable cause for this excellent stability in the cross-linked fullerene containing BHJs is associated with a significant inhibition of formation of crystals of fullerene.

Redox-active molecules as electrical dopants for OLED transport materials (Conference Presentation)

Electrical doping of organic semiconductors increases conductivity and reduces injection barriers from electrode materials, both of which effects can improve the performance of organic light-emitting diodes (OLEDs). However, the low electron affinities of typical OLED electron-transport materials make the identification of suitable n-dopants particularly challenging; electropositive metals such as the alkali metals are not easily handled and form monoatomic ions that are rather mobile in host materials, whereas molecular dopants that operate as simple one-electron reductants must have low ionization energies, which leads to severe air sensitivity. This presentation will discuss approaches to circumventing this issue by coupling electron transfer to other chemical reactivity. In particular, dimers formed by certain highly reducing organometallic sandwich compounds and organic radicals can be handled in air, yet have effective reducing potentials, corresponding to formation of the corresponding monomeric cations and contribution of two electrons to the semiconductor, of ca. –2.0 V vs. ferrocene. These values fall a little short of what is required for typical OLED materials; approaches to further extending the doping reach of these dimers will be described. One such approach involving photoirradiation of a dimer:semiconductor blend leads to metastable doping of a material with a redox potential of –2.24 V, which allows the fabrication of efficient OLEDs in which even high-workfunction electrodes, such as indium tin oxide, can be used as electron-injection contacts.

Corrigendum: Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors

This corrects the article DOI: 10.1038/nmat5027.

Efficient light emission from hybrid inorganic/organic semiconductor structures by energy level optimization

Innovative hybrid inorganic/organic structures (HIOS) should implement exciton creation by electrical injection in inorganic semiconductors followed by resonant energy transfer and light emission from the organic semiconductor. An inherent obstacle of such designs is the typically unfavorable energy level alignment at HIOS interfaces, which assists in exciton separation thus quenching light emission. Here, we introduce a technologically relevant method to optimize the hybrid structures energy levels: ZnO and a tailored ladder-type oligophenylene. Using an organometallic donor interlayer the ZnO work function is substantially lowered eliminating the ZnO - L4P-sp3 interfacial energy level offsets enhancing the hybrid structures radiative emission yield sevenfold.