Non-conjugated Polyelectrolytes Research Articles

Nonconjugated Polymers Enabled Solar Water Oxidation.

Wholly distinct from conjugated polymers which are featured by generic charge transfer capability stemming from a conjugated molecular structure, solid nonconjugated polymers mediated charge transport has long been deemed as theoretically impossible because of the deficiency of π electrons along the molecular skeleton, thereby retarding their widespread applications in solar energy conversion. Herein, we first conceptually unveil that intact encapsulation of metal oxides (e.g., TiO2, WO3, Fe2O3, and ZnO) with an ultrathin nonconjugated polyelectrolyte of branched polyethylenimine (BPEI) can unexpectedly accelerate the unidirectional charge transfer to the active sites and foster the defect generation, which contributes to the boosted charge separation and prolonged charge lifetime, ultimately resulting in considerably improved photoelectrochemical (PEC) water oxidation activities. The interfacial charge transport origins endowed by BPEI adornment are elucidated, which include acting as a hole-withdrawing mediator, promoting vacancy generation, and stimulating the directional charge flow route. We additionally ascertain that such charge transport characteristics of BPEI are universal. This work would unlock the charge transfer capability of nonconjugated polymers for solar water oxidation. The nonconjugated insulating polymer was utilized as a charge transport mediator for boosting charge migration and separation over metal oxides toward solar water oxidation.

Enhanced performance of organic and perovskite solar cells using simple linear polyethyleneimine with reduced chemical reactivity

Interface engineering based on amine-containing polyelectrolytes has been proven to be useful for high-performance organic and perovskite solar cells by improving charge collection at the interfaces. Here, we demonstrate efficient organic and perovskite solar cells by introducing non-conjugated polyelectrolyte, linear polyethyleneimine (LPEI), as a cathode interfacial layer (CIL). Notably, such a polyelectrolyte contains secondary amine groups in its molecular structure, which could provide chemical stability with most of the state-of-the-art non-fullerene acceptors (NFAs), and concurrently form dipole layer to endow underlying materials with work function tunability. As a result of favorable interfacial contact, organic solar cells with LPEI CIL exhibited a high power conversion efficiency (PCE) of 14.8%. Furthermore, it was found that the LPEI is compatible with conventional metal oxides such as zinc oxide (ZnO) and tin oxide (SnO2) as a bilayer configuration, leading to an increase of PCE from 14.7% to 16% for organic solar cells and from 17.5% to 20% for perovskite solar cells, respectively.

Influence of Backbone Regioregularity on the Optoelectronic and Mechanical Response of Conjugated Polyelectrolyte-Based Hydrogels

The ability to form robust, optoelectronically responsive, and mechanically tunable hydrogels using facile processing is desirable for sensing, biomedical, and light-harvesting applications. We demonstrate that such a hydrogel can be formed using aqueous complexation between one conjugated and one nonconjugated polyelectrolyte. We show that the rheological properties of the hydrogel can be tuned using the regioregularity of the conjugated polyelectrolyte (CPE) backbone, leading to significantly different mesoscale gel morphologies. We also find that the exciton dynamics in the long-time limit reflect differences in the underlying electronic connectivity of the hydrogels as a function CPE regioregularity. The influence of excess small ions on the hydrogel structure and the exciton dynamics similarly depends on the regioregularity in a significant way. Finally, electrical impedance measurements lead us to infer that these hydrogels can act as mixed ionic/electronic conductors. We believe that such gels possess an attractive combination of physical-chemical properties that can be leveraged in multiple applications.

Excitonically Coupled Simple Coacervates via Liquid/Liquid Phase Separation.

Viscoelastic liquid coacervate phases that are highly enriched in nonconjugated polyelectrolytes are currently the subject of highly active research from biological and soft-materials perspectives. However, formation of a liquid, electronically active coacervate has proved highly elusive, since extended π-electron interactions strongly favor the solid state. Herein we show that a conjugated polyelectrolyte can be rationally designed to undergo aqueous liquid/liquid phase separation to form a liquid coacervate phase. This result is significant both because it adds to the fundamental understanding of liquid/liquid phase separation but also because it opens intriguing applications in light harvesting and beyond. We find that the semiconducting coacervate is intrinsically excitonically coupled, allowing for long-range exciton diffusion in a strongly correlated, fluctuating environment. The emergent excitonic states are comprised of both excimers and H-aggregates.

Efficient and Stable Perovskite Solar Cells with a High Open-Circuit Voltage Over 1.2V Achieved by a Dual-Side Passivation Layer.

Suppressing nonradiative recombination at the interface between the organometal halide perovskite (PVK) and the charge-transport layer (CTL) is crucial for improving the efficiency and stability of PVK-based solar cells (PSCs). Here, a new bathocuproine (BCP)-based nonconjugated polyelectrolyte (poly-BCP) is synthesized and this is introduced as a "dual-side passivation layer" between the tin oxide (SnO2 ) CTL and the PVK absorber. Poly-BCP significantly suppresses both bulk and interfacial nonradiative recombination by passivating oxygen-vacancy defects from the SnO2 side and simultaneously scavenges ionic defects from the other (PVK) side. Therefore, PSCs with poly-BCP exhibits a high power conversion efficiency (PCE) of 24.4% and a high open-circuit voltage of 1.21V with a reduced voltage loss (PVK bandgap of 1.56eV). The non-encapsulated PSCs also show excellent long-term stability by retaining 93% of the initial PCE after 700 h under continuous 1-sun irradiation in nitrogen atmosphere conditions.

Accelerating charge transfer via nonconjugated polyelectrolyte interlayers toward efficient versatile photoredox catalysis

One of the challenges for high-efficiency single-component-based photoredox catalysts is the low charge transfer and extraction due to the high recombination rate. Here, we demonstrate a strategy to precisely control the charge separation and transport efficiency of the catalytic host by introducing electron or hole extraction interlayers to improve the catalytic efficiency. We use simple and easily available non-conjugated polyelectrolytes (NCPs) (i.e., polyethyleneimine, PEI; poly(allylamine hydrochloride), PAH) to form interlayers, wherein such NCPs consist of the nonconjugated backbone with charge transporting functional groups. Taking CdS as examples, it is shown that although PEI and PAH are insulators and therefore do not have the ability to conduct electricity, they can form good electron or hole transport extraction layers due to the higher charge-transfer kinetics of pendant groups along the backbones, thereby greatly improving the charge transfer capability of CdS. Consequently, the resultant PEI-/PAH-functionalized nanocomposites exhibit significantly enhanced and versatile photoredox catalysis.

Anionic nonconjugated polyelectrolyte as an anode interfacial layer for polymer solar cells

Since the most high-performing donor polymers in polymer solar cells (PSCs) possessed the deep highest occupied molecular orbital (HOMO) level, interfacial engineering on anode contact is becoming increasingly important. Herein, we demonstrated efficient PSCs using an anionic poly(styrene sulfonate) (PSS) as an anode interfacial layer (AIL). With the formation of the dipole layer, the effective work function (WF) of indium tin oxide (ITO) electrode is significantly increased from 4.8 to 5.3 eV, providing favorable energetic alignment to the quasi-Fermi level of various donor polymers. Moreover, by incorporating cationic polyelectrolytes as a cathode interfacial layer, a pair of electric dipole layers induces a strong built-in electric field across the photoactive layer to drive efficient sweep-out of photogenerated charges. Consequently, the device with PSS AIL exhibited high power conversion efficiencies of 9.2 and 14.8% in PTB7-Th:PC71BM- and PM6:Y6-based PSCs, respectively, both of which are higher than those of the devices with PEDOT:PSS.

Cationic polyelectrolytes as convenient electron extraction layers in perovskite solar cells

The potential of hybrid organic-inorganic perovskite solar cells (PSCs) as a next-generation photovoltaic technology has attracted enormous research interest due to their remarkably low cost and outstanding photo-physical properties. Increasing their power conversion efficiency (PCE) using materials which are similarly low-cost as the perovskite layer remains a critical issue for their commercialization; interlayers play important roles in elevating the PCEs of PSCs and the commonly used interlayers used in PSCs are typically orders of magnitude more costly than the perovskite layer itself. Herein, we report the effect of polyelectrolytes on the energy band structure and device characteristics when they are incorporated as electron extraction layers (EELs) of photovoltaics with both N–I–P and P–I–N geometries. To tune and improve device performance, we used cationic nonconjugated polyelectrolytes (NPEs) based on the poly(ethyleneimine) (PEI) backbone with different anions including bromide (Br−), iodide (I−), and tetrakis (imidazole) borate (BIm4-). This series of NPEs formed electric dipoles at the NPE/metal electrode interfaces; consequently allowing tuning of the energy levels, and work functions (WFs) of the electrodes. Notably, the PCE could be improved from 8.11 to 14.71% in the case of the NIP geometry, or from 7.40 to 13.79% in the case of the P–I–N geometry using a PEIH+BIm4− interlayer. Ultraviolet photoelectron spectroscope (UPS) results reveal that all of the polyelectrolytes, and particularly PEIH+BIm4− effectively decreased the WFs of the metal electrodes. Interestingly, a tunable dipole was achieved on the conducting surfaces (e.g. both cathode and anode electrodes) modified with electrolytes by simply varying the identity of counterions, as verified by the significantly reduced effective WF. These n-Type electrolytes created Ohmic contacts between the electrodes and the perovskite layer. These findings demonstrate that NPEs are effective EELs for high efficiency PSCs.

Ionic moieties in organic and hybrid semiconducting devices: influence on energy band structures and functions

This review summarizes the effects of ions on organic and hybrid semiconductors, with a focus on non-conjugated polyelectrolytes.

General bis(fluorophenyl azide) photo-crosslinkers for conjugated and non-conjugated polyelectrolytes

Photo-patterning and crosslinking of polymer semiconductors, charge injection and/or transport layers are essential to improve the performance and/or functionality of organic semiconductor devices.

Nearly 100% Photocrosslinking Efficiency in Ultrahigh Work Function Hole-Doped Conjugated Polymers Using Bis(fluorophenyl azide) Additives.

Self-compensated (SC) hole-doped conjugated polyelectrolytes with high work functions can provide efficient hole-injection and -collection layers for organic and other semiconductor devices. If these films can be photocrosslinked, the semiconductor overlayer can be deposited from a wider range of solvents, enabling flexibility in device design and fabrication. However, a generic photocrosslinking methodology for these materials is not yet available. Here, we demonstrate that sFPA82-TfO, the recently developed bis(fluorophenyl azide) photocrosslinker that is also i-line compatible, can surprisingly give 100% efficient photocrosslinking for SC hole-doped conjugated polyelectrolytes, i.e., one crosslink per reactive moiety, using mTFF-C2F5SIS-Na, a triarylamine-fluorene copolymer, as the model polyelectrolyte, without degrading its ultrahigh work function of 5.75 eV. The photocrosslinking efficiency is much higher than in the corresponding undoped polyelectrolyte and nonconjugated polyelectrolyte films, where the efficiency is only 20%. We attribute this improvement to the formation of smaller ion multiplet clusters in the hole-doped polyelectrolyte, as suggested by molecular dynamics simulations and infrared spectroscopy, which prevents occlusion of the ionic crosslinker. Photocrosslinking of the SC hole-doped mTFF-C2F5SIS-Na film used as a hole-injection layer in 100 nm-thick PFOP diodes suppresses the leakage current by over 3 orders of magnitude compared to those without crosslinking, to below 30 nA cm-2 at ±2 V. Photocrosslinking of the same film used as the hole-collection layer in PBDTTPD:PC61BM solar cells produces a higher photocurrent density, fill factor, and power conversion efficiency.

Photoluminescence in non-conjugated polyelectrolyte films containing 7-hydroxy-flavylium cation

Chitosan–carboxymethylcellulose/flavylium salt (Ch–CMC/FS) films were obtained at different flavylium salt (FS) concentrations under acidic conditions in order to maintain de benzopyrylium form of the flavylium organic cation. Films were characterized by Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, UV–Vis diffuse reflectance (DRUV) and emission spectroscopy. Thermal properties were also recorded by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. FTIR and Raman spectra showed shifting of the carbonyl vibrations after addition of flavylium salt compound. Thermal stability prevailed even after addition of FS as was determined by TGA and DSC analysis. Ch–CMC/FS showed strong absorption in the visible part of the electromagnetic spectrum centred around λ = 450 nm. Luminescence profile after excitation at λ = 450 nm showed an emission centred around λ = 507 nm. FS appears to be chemically stabilized by the interaction with polyelectrolyte chains.

Effect of counter-ions on the properties and performance of non-conjugated polyelectrolyte interlayers in solar cell and transistor devices.

We have investigated a series of non-conjugated polyelectrolytes (NPEs) which are based on a polyethylenimine (PEI) backbone with various counterions, such as Br− I− and BIm4−, as interfacial layers at the electrodes of solar cells and transistor devices to improve the power conversion efficiency (PCE) and device performance. This new series of NPEs with different counterions are capable of forming electric dipoles at NPE/metal electrode interfaces; as a consequence tuning of the energy levels, and work function (WF) of the electrodes is possible. Using this approach, the PCE of organic solar cells could be improved from 1.05% (without NPEs) to 6.77% (with NPEs) while the charge carrier mobility and on/off ratio of FET devices could be improved, showing the broad utility of this type of material. This study provides a novel approach towards investigating the influence of ions on interfacial dipoles and electrode WFs in solution-processed semiconducting devices.

Polystyrene-block-Poly(ionic liquid) Copolymers as Work Function Modifiers in Inverted Organic Photovoltaic Cells.

Interfacial layers play a critical role in building up the Ohmic contact between electrodes and functional layers in organic photovoltaic (OPV) solar cells. These layers are based on either inorganic oxides (ZnO and TiO2) or water-soluble organic polymers such as poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)] and polyethylenimine ethoxylated (PEIE). In this work, we have developed a series of novel poly(ionic liquid) nonconjugated block copolymers for improving the performance of inverted OPV cells by using them as work function modifiers of the indium tin oxide (ITO) cathode. Four nonconjugated polyelectrolytes (n-CPEs) based on polystyrene and imidazolium poly(ionic liquid) (PSImCl) were synthesized by reversible addition-fragmentation chain transfer polymerization. The ratio of hydrophobic/hydrophilic block copolymers was varied depending on the ratio of polystyrene to the PSImCl block. The ionic density, which controls the work function of the electrode by forming an interfacial dipole between the electrode and the block copolymers, was easily tuned by simply changing the PSImCl molar ratio. The inverted OPV device with the ITO/PS29-b-PSImCl60 cathode achieved the best power conversion efficiency (PCE) of 7.55% among the synthesized block copolymers, exhibiting an even higher PCE than that of the reference OPV device with PEIE (7.30%). Furthermore, the surface properties of theblockcopolymers films were investigated by contact angle measurements to explore the influence of the controlled hydrophobic/hydrophilic characters on the device performances.

Improved Performance in n‐Type Organic Field‐Effect Transistors via Polyelectrolyte‐Mediated Interfacial Doping

AbstractTo enhance electron injection in n‐type organic field‐effect transistors (OFETs), nonconjugated polyelectrolyte (NPE) layers are interposed between a [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer and Au electrodes. A series of NPEs based on an ethoxylated polyethylenimine backbone with various counterions, including Cl−, Br−, and I−, improve electron mobilities up to ≈10−2 cm2 V−1 s−1 and yield on–off ratios (Ion/Ioff) of 105 in PCBM OFETs. Ultraviolet photoelectron spectroscopy reveals that all of the NPEs lead to reduced electron injection barriers (φe) at the NPE/metal interface; this reduction in φe is consistent with dipole formation or n‐type doping at the electrode interface. Absorption measurements of PCBM films treated with NPEs are consistent with n‐doping of the PCBM. Regardless of the type of anion, thick NPE layers lead to high conductivity in the films independent of gate bias, whereas thin NPE layers lead to dramatically improved electron injection and performance. These results demonstrate that thin polyelectrolyte layers can be used to achieve controlled interfacial doping in organic semiconductors. Furthermore, this study provides valuable information about the function of NPEs, which may be exploited to improve device performance and to design new materials for future use in optoelectronic devices.

Cathode modification by the electrostatically self-assembled poly(4-vinylpyridine) derivative for enhancing the performance of polymer solar cells

ABSTRACTA non-conjugated polyelectrolyte, poly(4-vinyl N-ethyl pyridinium bromide) (PVPy-EtBr), is applied to polymer solar cells (PSCs) as a cathode buffer layer either inverted or conventional type polymer solar cells to improve the performances. The Kelvin probe microscopy measurement support the formation of favorable interface dipole at the cathode interface, indicating the reduction of a Schottky barrier for an electron collection from the active layer to the cathode treated with electrostatically self-assembly of PVPy-EtBr. Inverted type PSC with PVPy-EtBr as a cathode buffer layer demonstrate the power conversion efficiency (PCE) of 3.49%, which is better than the device without interlayer (PCE = 2.95%). Most increase in the PCE of the devices with interlayer is resulted from enhancement of the Jsc. This is due to that the reduction of a Schottky barrier at the cathode interface. Conventional type PSC with PVPy-EtBr (3.45%) at the cathode interface also exhibit better the PCE compared to that of the device without interlayer (2.88%).

High Efficiency Inverted Organic Solar Cells with a Neutral Fulleropyrrolidine Electron-Collecting Interlayer.

A novel fulleropyrrolidine derivative, named FPNOH, was designed, synthesized, and utilized as an efficient electron-collecting (EC) layer for inverted organic solar cells (i-OSCs). The grafted diethanolamino-polar moieties can not only trigger its function as an EC interlayer, but also induce orthogonal solubility that guarantees subsequent multilayer processing without interfacial mixing. A higher power conversion efficiency (PCE) value of 8.34% was achieved for i-OSC devices with ITO/FPNOH EC electrode, compared to that of the sol-gel ZnO based reference devices with an optimized PCE value of 7.92%. High efficiency exceeding 7.7% was still achieved even for the devices with a relatively thick FPNOH film (16.9 nm). It is worthwhile to mention that this kind of material exhibits less thickness dependent performance, in contrast to widely utilized p-type conjugated polyelectrolytes (CPEs) as well as the nonconjugated polyelectrolytes (NCPEs). Further investigation on illuminating intensity dependent parameters revealed the role of FPNOH in reducing interfacial trap-induced recombination at the ITO/active layer interface.

Conjugated Polyelectrolyte-Induced Self-Assembly of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complexes.

Water-soluble cationic alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complexes have been demonstrated to undergo supramolecular assembly with anionic polyelectrolytes in aqueous buffer solution. Metal-metal-to-ligand charge transfer (MMLCT) absorptions and triplet MMLCT ((3) MMLCT) emissions have been found in UV/Vis absorption and emission spectra of the electrostatic assembly of the complexes with non-conjugated polyelectrolytes, driven by Pt⋅⋅⋅Pt and π-π interactions among the complex molecules. Interestingly, the two-component ensemble formed by [Pt(bzimpy-Et){CCC6 H4 (CH2 NMe3 -4)}]Cl2 (1) with para-linked conjugated polyelectrolyte (CPE), PPE-SO3 (-) , shows significantly different photophysical properties from that of the ensemble formed by 1 with meta-linked CPE, mPPE-Ala. The helical conformation of mPPE-Ala allows the formation of strong mPPE-Ala-1 aggregates with Pt⋅⋅⋅Pt, electrostatic, and π-π interactions, as revealed by the large Stern-Volmer constant at low concentrations of 1. Together with the reasonably large Förster radius, large HOMO-LUMO gap and high triplet state energy of mPPE-Ala to minimize both photo-induced charge transfer (PCT) and Dexter triplet energy back-transfer (TEBT) quenching of the emission of 1, efficient Förster resonance energy transfer (FRET) from mPPE-Ala to aggregated 1 molecules and strong (3) MMLCT emission have been found, while the less strong PPE-SO3 (-) -1 aggregates and probably more efficient PCT and Dexter TEBT quenching would account for the lack of (3) MMLCT emission in the PPE-SO3 (-) -1 ensemble.

Cathode modification of polymer solar cells by electrostatically self-assembled zwitterionic non-conjugated polyelectrolyte

A non-conjugated zwitterionic polylectrolyte (PVPy-ZW) based on poly(4-vinylpyridine) is applied to polymer solar cells (PSCs) as a cathode buffer layer either inverted or conventional type polymer solar cells. The Kelvin probe microscopy measurement support the formation of favorable interface dipole at the cathode interface, indicating the reduction of a Schottky barrier for an electron collection from the active layer to the cathode. Inverted type PSC with the PVPy-ZW as a cathode buffer layer demonstrate the power conversion efficiency (PCE) of 3.55% (open circuit voltage (Voc)=0.62V, short circuit current (Jsc)=9.49mA/cm2, fill factor (FF)=60.4%), which is better than the device without interlayer (PCE=2.95%, Voc=0.60V, Jsc=8.11mA/cm2, FF=60.6%). This is due to that the reduction of a Schottky barrier at the cathode interface. Conventional type PSC with PVPy-ZW (3.45%) at the cathode interface also exhibit better the PCE compared to that of the device without interlayer (2.88%). Most increase in the PCE of the devices with interlayer is resulted from enhancement of the Jsc.

What conjugated polyelectrolytes tell us about aggregation in polyelectrolyte/surfactant systems

The interaction between conjugated polyelectrolytes (CPEs) and surfactants in aqueous solutions is reviewed, based on results from absorption and fluorescence spectroscopy, NMR, dynamic light scattering (DLS), small angle X-ray (SAXS) and neutron (SANS) scattering, electrical conductivity and molecular dynamics simulations. It is shown that the conjugated polyelectrolytes frequently form aggregates (clusters) in water, but these can be broken up by the addition of co-solvents or surfactants. With oppositely charged surfactants, both enhancement and quenching of fluorescence are observed, depending on the nature of the CPE. These interactions start at very low surfactant concentrations, and, in some cases, associative phase separation is observed. However, there is little evidence for the “pearl necklace” structures observed with surfactants and nonconjugated polyelectrolytes. This may be related to the greater chain rigidity of the CPEs. Associative interactions are also seen with CPEs and non-ionic surfactants, which efficiently break-up the CPE aggregates to form mixed systems with greatly enhanced fluorescence. The relevance of these results to both the nanostructuring of conjugated polyelectrolytes and to our general understanding of polyelectrolyte–surfactant systems will be discussed.