Conjugated Polyelectrolytes as Efficient Hole Transport Layers in Perovskite Light-Emitting Diodes.

Perovskite-based optoelectronic devices have been rapidly developing in the past 5 years. Since the first report, the external quantum efficiency (EQE) of perovskite light-emitting diodes (PeLEDs) has increased rapidly through the control of morphology and structure from 0.1% to more than 11%. Here, we report the use of various conjugated polyelectrolytes (CPEs) as the hole injection layer in PeLEDs. In particular, we find that poly[2,6-(4,4-bis-potassium butanylsulfonate)-4 H-cyclopenta-[2,1- b;3,4- b']-dithiophene)] (PCPDT-K) transfers holes effectively, blocks electron transport from the perovskite to the underlying ITO layer, and reduces luminescence quenching at the perovskite/PCPDT-K interface. Our optimized PeLEDs with PCPDT-K show enhanced EQE by a factor of approximately 4 compared to control PeLEDs with PEDOT:PSS, reaching EQE values of 5.66%, and exhibit improved device stability.

Anaerobic Respiration on Self-Doped Conjugated Polyelectrolytes: Impact of Chemical Structure.

We probe anaerobic respiration of bacteria in the presence of conjugated polyelectrolytes (CPEs). Three different CPEs were used to probe how structural variations impact biocurrent generation from Shewanella oneidensis MR-1. For the self-doped anionic CPE only, absorption spectroscopy shows that the addition of S. oneidensis MR-1 leads to the disappearance of the polaron (radical cation) band at >900 nm and an increase in the band at 735 nm due to the neutral species, consistent with electron transfer from microbe to polymer. Microbial three-electrode electrochemical cells (M3Cs) show an increase in the current generated by S. oneidensis MR-1 with addition of the self-doped CPE relative to other CPEs and controls. These experiments combined with in situ cyclic voltammetry suggest that the doped CPE facilitates electron transport to electrodes and reveal structure-function relationships relevant to developing materials for biotic/abiotic interfaces.

One-pot synthesis of semicrystalline/amorphous multiblock copolymers via divinyl-terminated telechelic polyolefins.

A simple, one-pot approach to synthesize random semicrystalline/amorphous multiblock copolymers (12-17 blocks per chain on average) is demonstrated that takes advantage of acyclic diene metathesis (ADMET) polymerization of α,ω-divinyl-terminated telechelic polyolefins. This synthetic approach offers a generic, viable and economical route to polyolefin-based multiblock copolymers and may be extendable to broader families of multiblock materials.

Effect of chiral 2-ethylhexyl side chains on chiroptical properties of the narrow bandgap conjugated polymers PCPDTBT and PCDTPT.

Two narrow bandgap conjugated polymers containing chiral 2-ethylhexyl side chains were synthesized: poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PCPDTBT*) and poly[(4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b0]dithiophene)-2,6-diyl-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT*). The presence of a chiral substituent provides a handle to study the geometry of interchain aggregates and/or the secondary structure of these conjugated polymers in solution and in thin films via circular dichroism (CD) spectroscopy, provided that the asymmetry in the side-chain is translated to the optically active conjugated backbone. CD signals are observed for PCPDTBT* and PCDTPT* in poor solvent systems, which indicate the presence of chiral ordering in the aggregates. PCPDTBT* shows greater chiral order than PCDTPT* based on their relative anisotropy factors. Additionally, GIWAXS analysis reveals that PCPDTBT* films are more ordered than what is observed for the same polymer containing racemic 2-ethylhexyl chains. Upon solution deposition, the chiral ordering is found to translate to the solid-state microstructure for PCPDTBT* but not PCDTPT*. The presence of a pyridyl nitrogen on the thiadiazolo[3,4-c]pyridine ring of PCDTPT* favors a planar conformation for the backbone such that it has a higher rotational barrier compared to PCPDTBT*. This larger rotational barrier appears to limit the ability of PCDTPT* to adopt a helical structure or relevant chain distortions for achieving chiral aggregates.

Tethered tertiary amines as solid-state n-type dopants for solution-processable organic semiconductors.

A scarcity of stable n-type doping strategies compatible with facile processing has been a major impediment to the advancement of organic electronic devices. Localizing dopants near the cores of conductive molecules can lead to improved efficacy of doping. We and others recently showed the effectiveness of tethering dopants covalently to an electron-deficient aromatic molecule using trimethylammonium functionalization with hydroxide counterions linked to a perylene diimide core by alkyl spacers. In this work, we demonstrate that, contrary to previous hypotheses, the main driver responsible for the highly effective doping observed in thin films is the formation of tethered tertiary amine moieties during thin film processing. Furthermore, we demonstrate that tethered tertiary amine groups are powerful and general n-doping motifs for the successful generation of free electron carriers in the solid-state, not only when coupled to the perylene diimide molecular core, but also when linked with other small molecule systems including naphthalene diimide, diketopyrrolopyrrole, and fullerene derivatives. Our findings help expand a promising molecular design strategy for future enhancements of n-type organic electronic materials.

Electrical properties of doped conjugated polyelectrolytes with modulated density of the ionic functionalities.

We report the synthesis of a series of water-soluble anionic narrow band-gap conjugated polyelectrolytes with a varied density of the ionic functional groups. The charge density is modulated by incorporating the structural units with tetraethylene glycol (TEG) monomethyl ether side chains. These polymers are readily p-doped during dialysis in water. CPEs with TEG side chains exhibit tighter intermolecular packing in the solid state and higher electrical conductivity.

Conjugated polyelectrolyte hole transport layer for inverted-type perovskite solar cells.

Organic-inorganic hybrid perovskite materials offer the potential for realization of low-cost and flexible next-generation solar cells fabricated by low-temperature solution processing. Although efficiencies of perovskite solar cells have dramatically improved up to 19% within the past 5 years, there is still considerable room for further improvement in device efficiency and stability through development of novel materials and device architectures. Here we demonstrate that inverted-type perovskite solar cells with pH-neutral and low-temperature solution-processable conjugated polyelectrolyte as the hole transport layer (instead of acidic PEDOT: PSS) exhibit a device efficiency of over 12% and improved device stability in air. As an alternative to PEDOT: PSS, this work is the first report on the use of an organic hole transport material that enables the formation of uniform perovskite films with complete surface coverage and the demonstration of efficient, stable perovskite/fullerene planar heterojunction solar cells.

Polymer homo-tandem solar cells with best efficiency of 11.3%.

Rational materials design and interface engineering are both essential to realize a high performance for tandem cells. Two identical bulk heterojunctions are connected in series using novel interconnection layers combining pH-neutral conjugated polyelectrolytes and a thin film of ZnO nanoparticles by a solution process. The best performing tandem cells achieve a power conversion efficiency of 11.3%, with 25% enhancement in efficiency compared with single cells, which arises primarily from the increased light absorption.

Narrow band gap conjugated polyelectrolytes for photothermal killing of bacteria.

We report the demonstration of antimicrobial conjugated polyelectrolytes (CPEs) with high NIR absorbance for selective and efficient photothermal killing of bacteria over mammalian cells. The antimicrobial CPE possessing quaternary ammonium (QA) terminated side chains (P1) shows higher binding preference and increased dark toxicity towards Gram-positive and Gram-negative bacteria over mammalian cells. Bestowed by π-conjugated backbones, P1 exhibits a high molar absorptivity of 39.8 L g-1 cm-1 at 808 nm with an efficient photothermal conversion efficiency of 33 ± 1%. Upon 808 nm laser irradiation, P1 shows enhanced bactericidal effects, but not to mammalian cells. Although the anionic CPE counterpart with the same polymer backbone but sulfonate terminated side chains (P2) possesses a similar photothermal conversion ability, it exhibits much lower antibacterial effects due to its low binding affinity. This study thus reveals that bacteria-CPE electrostatic interactions play a major role in bacterial recognition, although hydrophobic interactions also contribute.

Solution-processed pH-neutral conjugated polyelectrolyte improves interfacial contact in organic solar cells.

The intrinsic acidic nature of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transporting layer (HTL) induces interfacial protonation and limits the device performance in organic solar cells based on basic pyridylthiadiazole units. By utilizing a pH neutral, water/alcohol soluble conjugated polyelectrolyte CPE-K as the HTL in p-DTS(PTTh2)2:PC71BM solar cells, a 60% enhancement in PCE has been obtained with an increased V(bi), reduced R(s), and improved charge extraction. These effects originate from the elimination of interfacial protonation and energy barrier compared with the PEDOT:PSS HTL.

High thermal stability solution-processable narrow-band gap molecular semiconductors.

A series of narrow-band gap conjugated molecules with specific fluorine substitution patterns has been synthesized in order to study the effect of fluorination on bulk thermal stability. As the number of fluorine substituents on the backbone increase, one finds more thermally robust bulk structures both under inert and ambient conditions as well as an increase in phase transition temperatures in the solid state. When integrated into field-effect transistor devices, the molecule with the highest degree of fluorination shows a hole mobility of 0.15 cm(2)/V·s and a device thermal stability of >300 °C. Generally, the enhancement in thermal robustness of bulk organization and device performance correlates with the level of C-H for C-F substitution. These findings are relevant for the design of molecular semiconductors that can be introduced into optoelectronic devices to be operated under a wide range of conditions.

Side-chain effects on the conductivity, morphology, and thermoelectric properties of self-doped narrow-band-gap conjugated polyelectrolytes.

This contribution reports a series of anionic narrow-band-gap self-doped conjugated polyelectrolytes (CPEs) with π-conjugated cyclopenta-[2,1-b;3,4-b']-dithiophene-alt-4,7-(2,1,3-benzothiadiazole) backbones, but with different counterions (Na(+), K(+), vs tetrabutylammonium) and lengths of alkyl chains (C4 vs C3). These materials were doped to provide air-stable, water-soluble conductive materials. Solid-state electrical conductivity, thermopower, and thermal conductivity were measured and compared. CPEs with smaller counterions and shorter side chains exhibit higher doping levels and form more ordered films. The smallest countercation (Na(+)) provides thin films with higher electrical conductivity, but a comparable thermopower, compared to those with larger counterions, thereby leading to a higher power factor. Chemical modifications of the pendant side chains do not influence out of plane thermal conductivity. These studies introduce a novel approach to understand thermoelectric performance by structural modifications.

Electronic properties of conjugated polyelectrolyte/single-walled carbon nanotube composites.

Two narrow-bandgap conjugated polyelectrolytes (CPEs) of identical backbone structure but different pendant charges are used to disperse single-walled carbon nanotubes (SWNTs) in MeOH. Films of the resulting CPE:SWNT composites have electrical conductivity dependent on the SWNT loading, which can be increased with acid vapor treatment. The anionic CPE gives higher electrical conductivity for the composite immediately after deposition, whereas a more-significant increase is observed for the cationic counterpart after acid treatment.

Conductive conjugated polyelectrolyte as hole-transporting layer for organic bulk heterojunction solar cells.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been extensively used as the hole-transporting layer (HTL) in bulk heterojunction (BHJ) solar cells, however, its anisotropic electrical conduction and intrinsic acidic nature generally limit the device performance. Here we demonstrate the application of a water/alcohol soluble CPE (CPE-K) as HTLs in BHJ solar cells, achieving a PCE up to 8.2%. The more superior and uniform vertical electrical conductivity found in CPE-K reduces the series resistance and provides efficient hole extraction.

Facile doping of anionic narrow-band-gap conjugated polyelectrolytes during dialysis.

PCPDTBTSO3 K, an anionic, narrow-band-gap conjugated polyelectrolyte, was found to be doped after dialysis. The proposed doping mechanism involves protonation of the polymer backbone, followed by electron transfer from a neutral chain, to generate radical cations, which are stabilized by the pendant sulfonate anions. Formation of polarons is supported by spectroscopy and electrical-conductivity measurements.

Study on the thermal reactions of [60]fullerene with amino acids and amino acid esters.

Thermal reactions of [60]fullerene with a series of amino acids and amino acid esters under aerobic and dark conditions have been investigated. Fulleropyrrolidines can be obtained from these reactions although an aldehyde is not added purposely. Possible reaction mechanisms involving uncommon C-N bond cleavages have been proposed to generate aldehydes, which then react with amino acids and amino acid esters to provide azomethine ylides, followed by 1,3-dipolar cycloaddition to [60]fullerene affording fulleropyrrolidines. Control experiments support our proposed mechanisms, and elucidate the innate nature of C-N bond cleavages of amino acids and amino acid esters.

Alpha-arylation of 3-aryloxindoles.

A versatile method for the synthesis of 3,3-diaryloxindoles via Pd-catalyzed alpha-arylations or an S(N)Ar reaction is described. The reaction proceeds using mild base, is tolerant of a variety of functional groups, and is capable of preparing hindered all-carbon quaternary centers.