Showing papers by "Stephen Barlow published in 2021"

Persistent Conjugated Backbone and Disordered Lamellar Packing Impart Polymers with Efficient n‐Doping and High Conductivities

Abstract: Solution-processable highly conductive polymers are of great interest in emerging electronic applications. For p-doped polymers, conductivities as high a nearly 105 S cm-1 have been reported. In the case of n-doped polymers, they often fall well short of the high values noted above, which might be achievable, if much higher charge-carrier mobilities determined could be realized in combination with high charge-carrier densities. This is in part due to inefficient doping and dopant ions disturbing the ordering of polymers, limiting efficient charge transport and ultimately the achievable conductivities. Here, n-doped polymers that achieve a high conductivity of more than 90 S cm-1 by a simple solution-based co-deposition method are reported. Two conjugated polymers with rigid planar backbones, but with disordered crystalline structures, exhibit surprising structural tolerance to, and excellent miscibility with, commonly used n-dopants. These properties allow both high concentrations and high mobility of the charge carriers to be realized simultaneously in n-doped polymers, resulting in excellent electrical conductivity and thermoelectric performance.

High-Efficiency Ion-Exchange Doping of Conducting Polymers

Abstract: Molecular doping-the use of redox-active small molecules as dopants for organic semiconductors-has seen a surge in research interest driven by emerging applications in sensing, bioelectronics and thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox-active character of these materials. A recent breakthrough was a doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. Here, we study the equilibrium and kinetics of ion exchange doping in a model system, PBTTT doped with FeCl 3 and BMP TFSI, which reaches conductivities in excess of 1000 S/cm and ion exchange efficiencies above 99%. We demonstrate several factors which enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl 3. In this high ion exchange efficiency regime, we illustrate a simple connection between electrochemical doping and ion exchange, and show that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential.

Understanding how Lewis acids dope organic semiconductors: a “complex” story

Abstract: We report on computational studies of the potential of three borane Lewis acids (LAs) (B(C6F5)3 (BCF), BF3, and BBr3) to form stable adducts and/or to generate positive polarons with three different semiconducting π-conjugated polymers (PFPT, PCPDTPT and PCPDTBT). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on range-separated hybrid (RSH) functionals provide insight into changes in the electronic structure and optical properties upon adduct formation between LAs and the two polymers containing pyridine moieties, PFPT and PCPDTPT, unravelling the complex interplay between partial hybridization, charge transfer and changes in the polymer backbone conformation. We then assess the potential of BCF to induce p-doping in PCPDTBT, which does not contain pyridine groups, by computing the energetics of various reaction mechanisms proposed in the literature. We find that reaction of BCF(OH2) to form protonated PCPDTBT and [BCF(OH)]−, followed by electron transfer from a pristine to a protonated PCPDTBT chain is highly endergonic, and thus unlikely at low doping concentration. The theoretical and experimental data can, however, be reconciled if one considers the formation of [BCF(OH)BCF]− or [BCF(OH)(OH2)BCF]− counterions rather than [BCF(OH)]− and invokes subsequent reactions resulting in the elimination of H2.

Nanosecond-Pulsed Perovskite Light-Emitting Diodes at High Current Density.

Abstract: While metal-halide perovskite light-emitting diodes (PeLEDs) hold the potential for a new generation of display and lighting technology, their slow operation speed and response time limit their application scope. Here, high-speed PeLEDs driven by nanosecond electrical pulses with a rise time of 1.2 ns are reported with a maximum radiance of approximately 480 kW sr-1 m-2 at 8.3 kA cm-2 , and an external quantum efficiency (EQE) of 1% at approximately 10 kA cm-2 , through improved device configuration designs and material considerations. Enabled by the fast operation of PeLEDs, the temporal response provides access to transient charge carrier dynamics under electrical excitation, revealing several new electroluminescence quenching pathways. Finally, integrated distributed feedback (DFB) gratings are explored, which facilitate more directional light emission with a maximum radiance of approximately 1200 kW sr-1 m-2 at 8.5 kA cm-2 , a more than two-fold enhancement to forward radiation output.

A Naphthalene Diimide Covalent Organic Framework: Comparison of Cathode Performance in Lithium-Ion Batteries with Amorphous Cross-linked and Linear Analogues, and Its Use in Aqueous Lithium-Ion Batteries

Abstract: We report a two-dimensional (2D) imine-linked covalent organic framework (COF) containing naphthalene diimide (NDI) redox groups, TAPB-NDI COF. Lithium-ion batteries (LIBs) with TAPB-NDI COF-based ...

Highly air-stable, n-doped conjugated polymers achieved by dimeric organometallic dopants

Abstract: Chemical doping is a key process for controlling the electronic properties of molecular semiconductors, including their conductivity and work function. A common limitation of n-doped polymers is their instability under ambient conditions, which has imposed restrictions on the characterisation and device application of n-doped polymers. In this study, sequential n-doping with organometallic dopants was performed on thin films of polymeric semiconductors with naphthalene diimide and perylene diimide-based backbones. Moderate ambient stability was achieved with (RuCp*Mes)2, {Cp* = pentamethylcyclopentadienyl; Mes = 1,3,5-trimethylbenzene}, which is in striking contrast to the unstable, n-doped state obtained with cobaltocene, a simple one-electron reductant. The highly cathodic, effective redox potential of (RuCp*Mes)2, ca. −2.0 V vs. ferrocene, suppresses the back electron transfer reaction and the subsequent dopant loss in air, which gives rise to the observed air stability. It also allows a perylene diimide-based polymer to be reduced to a state in which the repeat units are largely dianionic. Photoelectron measurements show that the ionization potential of the heavily doped polymer is ca. 3.9 eV. Our findings show that chemical doping with (RuCp*Mes)2 is an effective method to produce highly stable, n-doped conjugated polymers.

A naphthalene diimide side-chain polymer as an electron-extraction layer for stable perovskite solar cells

Abstract: Poly(N-(5-(5-norbornene-2-carbonyl)oxy)pentyl)-N′-n-hexyl-naphthalene-1,8:4,5-bis(dicarboximide) has been synthesized by esterification of (N-(5-hydroxypentyl)-N′-n-hexyl-naphthalene-1,8:4,5-bis(dicarboximide)) with exo-5-norbornene-2-carboxylic acid, and has been polymerized using the first-generation Grubbs initiator. This side-chain polymer exhibits good transparency throughout the visible (absorption onset at ca. 400 nm), good solubility in common low- and medium-polarity organic solvents, good resistance to dimethylformamide, and appropriate electron affinity for use as an electron-extraction layer in lead-halide perovskite solar cells. The performance of this polymer in n-i-p perovskite solar cells was compared to that of several small-molecule naphthalene-1,8:4,5-bis(dicarboximide) derivatives and of SnO2. Solar cells using the polymer exhibited open-circuit voltages of up to 1.02 V, short-circuit currents of over 21 mA cm−2, and power conversion efficiencies (PCE) reaching 14% which stabilize at 13.8% upon 90 s of illumination. Meanwhile control SnO2 devices exhibited a PCE of ca. 16%, and small-molecule devices gave PCE values of less than 10%. The devices employing the polymer exhibited improved long-term stability relative to the SnO2 control devices under continuous illumination.

Cross-Linking of Doped Organic Semiconductor Interlayers for Organic Solar Cells: Potential and Challenges

Abstract: Solution-processable interlayers are an important building block for the commercialization of organic electronic devices such as organic solar cells. Here, the potential of cross-linking to provide an insoluble, stable and versatile charge transport layer based on soluble organic semiconductors is studied. For this purpose, a photo-reactive tris-azide cross-linker is synthesized. The capability of the small molecular cross-linker is illustrated by applying it to a p-doped polymer used as a hole transport layer in organic solar cells. High cross-linking efficiency and excellent charge extraction properties of the cross-linked doped hole transport layer are demonstrated. However, at high doping levels in the interlayer, the solar cell efficiency is found to deteriorate. Based on charge extraction measurements and numerical device simulations, it is shown that this is due to diffusion of dopants into the active layer of the solar cell. Thus, in the development of future cross-linker materials, care must be taken to ensure that they immobilize not only the host, but also the dopants.

Benzocyclobutene Polymer as an Additive for a Benzocyclobutene-Fullerene: Application in Stable p-i-n Perovskite Solar Cells

Abstract: A poly(methacrylate) with benzocyclobutene side chains, CL, has been synthesized by radical polymerization for use as a crosslinking additive for a previously reported benzocyclobutene-functionalized fullerene, PCBCB, which can be thermally insolubilized following solution processing. Films of PCBCB incorporating CL and n-doped with (IrCp*Cp)2 exhibit in-plane electrical conductivities around ten times higher than those of n-doped films without CL, while the use of CL also reduces leaching of dopant ions from the film upon washing. The performance and stability of perovskite solar cells using insolubilized PCBCB:CL, insolubilized PCBCB, or PCBM as top electron-extraction layers are compared; cells with undoped PCBCB:CL extraction layers exhibit higher average and maximum power conversion efficiencies (16 and 18.5%, respectively) than their PCBM and PCBCB counterparts. Devices with undoped PCBCB:CL extraction layers also showed excellent thermal stability, retaining 92% of their stabilized power output after aging for 3000 h at 85 °C in the dark in a nitrogen atmosphere.

Ion-exchange doped polymers at the degenerate limit: what limits conductivity at 100% doping efficiency?

Abstract: Doping of semiconducting polymers has seen a surge in research interest driven by emerging applications in sensing, bioelectronics and thermoelectrics. A recent breakthrough was a doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. The improved microstructural control this process allows enables us for the first time to systematically address a longstanding but still poorly understood question: what limits the electrical conductivity at high doping levels? Is it the formation of charge carrier traps in the Coulomb potentials of the counterions, or is it the structural disorder in the polymer lattice? Here, we apply ion-exchange doping to several classes of high mobility conjugated polymers and identify experimental conditions that achieve near 100% doping efficiency under degenerate conditions with nearly 1 charge per monomer. We demonstrate very high conductivities up to 1200 S/cm in semicrystalline polymer systems, and show that in this regime conductivity is poorly correlated with ionic size, but strongly correlated with paracrystalline disorder. This observation, backed by a detailed electronic structure model that incorporates ion-hole and hole-hole interactions and a carefully parameterized model of disorder, indicates that trapping by dopant ions is negligible, and that maximizing crystalline order is critical to improving conductivity.

Nonlinear photocarrier dynamics and the role of shallow traps in mixed-halide mixed-cation hybrid perovskites

Abstract: We examine the role of surface passivation on carrier trapping and nonlinear recombination dynamics in hybrid metal-halide perovskites by means of excitation correlation photoluminescence (ECPL) spectroscopy. We find that carrier trapping occurs on sub-nanosecond timescales in both control (unpassivated) and passivated samples, which is consistent within a shallow-trap model. However, the impact of passivation has a direct effect on both shallow and deep traps. Our results reveal that the effect of passivation of deep traps is responsible for the increase of the carrier lifetimes, while the passivation of shallow traps reduces the excitation density required for shallow-trap saturation. Our work demonstrates how ECPL provides details about the passivation of shallow traps beyond those available via conventional time-resolved photoluminescence techniques.

Disentangling Bulk and Interface Phenomena in a Molecularly Doped Polymer Semiconductor

Abstract: Molecular electrical doping is of central technological relevance for organic (opto-) electronics since it allows control of charge carrier density and Fermi level position in organic semiconductors (OSCs). Here, we chose to investigate the doping capability of the n-dopant 1,2,3,4,1′,2′,3′,4′-octaphenylrhodocene (OPR). Using the bulky, strongly reducing metallocene to dope the electron-transport polymer poly{[N,N-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} [P(NDI2OD-T2)] leads to an increased bulk conductivity and decreased contact resistance. While the former is due to low-level n-doping of the polymer, trap filling and concomitant charge carrier mobility increase, the latter is caused by an accumulation of OPR at an indium tin oxide (ITO) substrate.

A polymeric bis(di-p-anisylamino)fluorene hole-transport material for stable n-i-p perovskite solar cells

Abstract: A norbornene homopolymer with hole-transporting 2,7-bis(di-p-anisylamino)fluorene side chains is compared to the widely used spiro-OMeTAD as a hole-transport material (HTM) in negative-intrinsic-positive (n-i-p) perovskite solar cells (PSCs). PSCs fabricated using p-doped homopolymer achieve a power conversion efficiency of 15.5%, comparable to that of cells incorporating p-doped spiro-OMeTAD as the HTM. However, half-devices made with the polymer exhibit improved light and heat stability in comparison to those incorporating spiro-OMeTAD.

Synthesis, structures, and reactivity of isomers of [RuCp*(1,4-(Me2N)2C6H4)]2.

Abstract: [RuCp*(1,3,5-R3C6H3)]2 {Cp* = η5-pentamethylcyclopentadienyl, R = Me, Et} have previously been found to be moderately air stable, yet highly reducing, with estimated D+/0.5D2 (where D2 and D+ represent the dimer and the corresponding monomeric cation, respectively) redox potentials of ca. −2.0 V vs. FeCp2+/0. These properties have led to their use as n-dopants for organic semiconductors. Use of arenes substituted with π-electron donors is anticipated to lead to even more strongly reducing dimers. [RuCp*(1-(Me2N)-3,5-Me2C6H3)]+PF6− and [RuCp*(1,4-(Me2N)2C6H4)]+PF6− have been synthesized and electrochemically and crystallographically characterized; both exhibit D+/D potentials slightly more cathodic than [RuCp*(1,3,5-R3C6H3)]+. Reduction of [RuCp*(1,4-(Me2N)2C6H4)]+PF6− using silica-supported sodium–potassium alloy leads to a mixture of isomers of [RuCp*(1,4-(Me2N)2C6H4)]2, two of which have been crystallographically characterized. One of these isomers has a similar molecular structure to [RuCp*(1,3,5-Et3C6H3)]2; the central C–C bond is exo,exo, i.e., on the opposite face of both six-membered rings from the metals. A D+/0.5D2 potential of −2.4 V is estimated for this exo,exo dimer, more reducing than that of [RuCp*(1,3,5-R3C6H3)]2 (−2.0 V). This isomer reacts much more rapidly with both air and electron acceptors than [RuCp*(1,3,5-R3C6H3)]2 due to a much more cathodic D2˙+/D2 potential. The other isomer to be crystallographically characterized, along with a third isomer, are both dimerized in an exo,endo fashion, representing the first examples of such dimers. Density functional theory calculations and reactivity studies indicate that the central bonds of these two isomers are weaker than those of the exo,exo isomer, or of [RuCp*(1,3,5-R3C6H3)]2, leading to estimated D+/0.5D2 potentials of −2.5 and −2.6 V vs. FeCp2+/0. At the same time the D2˙+/D2 potentials for the exo,endo dimers are anodically shifted relative to those of [RuCp*(1,3,5-R3C6H3)]2, resulting in much greater air stability than for the exo,exo isomer.