In synthetic metals, free radicals in neutral organic semiconductors are acknowledged as their defects or impurities. However, polycyclic aromatic hydrocarbons (PAHs) with singlet open-shell diradical ground state encourage us to investigate the other potential origin of radicals in neutral organic semiconductors. Herein, for the first time, we observed strong electron spin resonance (ESR) signal in a serials of typical small molecule (SMs). Neutral SMs (NSMs) with strong acceptors including benzothiadiazole, diketopyrrolopyrrole, and naphthalene diimide showed significant ESR signals while they contained strong donors. From the unexpected 1H NMR broadening and increase of ESR signal, we propose TPA-DPP has singlet open-shell ground state and thermally populated triplet species excited via rising temperature, which represent our new viewpoints different from previous reports. The intensity of the ESR signals and singlet–triplet energy gap of the NSMs are related to their electronic delocalization effect and energy band gap. Moreover, significant ESR spectra were also detected in neutral conjugated polymers, e.g., PCDTBT, P3HT, PTB7, and PffBT4T-2OD. We speculate the open-shell quinoid-radical resonance structure may acts as one of the potential origin of the universal ESR signals of organic semiconductors. This study provides a novel perspective to understand the structure–radical–property relationship of organic semiconductors.

The development of high performance hole transport materials (HTMs) without a chemical dopant is critical to achieve long-term device durability. The general design of self-doping materials based on a phenolamine structure with strong electronic spin concentration is reported for the first time. A phenol-enhanced self-doped mechanism is also proposed. Compared to their precursors, dimethylphenolamine derivatives, TBP-OH4, TPD-OH4 and Spiro-OH8, displayed much higher spin concentration in their neutral states. Phenol acts as a hole trap in the traditional concept, however, the films of TBP-OH4, TPD-OH4 and Spiro-OH8 exhibited higher conductivities than those of methoxyl precursors. Meanwhile, phenolamine derivatives have good solublility in polar organic solvents and show good solvent resistance in chlorobenzene. Considering the relatively good band alignment, film-formation and solvent resistance against chlorobenzene, Spiro-OH8 and TPD-OH4 exhibited comparable performance with that of PEDOT:PSS-4083. Most importantly, a new generation of self-doped systems based on a phenolamine structure might provide new insight in developing efficient HTMs for organic electronics.

Inspired by the p-doped PEDOT:PSS, a traditional anode modifier, we proposed to prepare polydopamine:polystyrenesulfonate (PDA:PSS) via the self-polymerization of dopamine in aqueous PSS initially. However, DA and its semiquinone radical were dispersed by PSS to form DA:PSS successfully. Interestingly, a strong electron spin resonance signal was detected in DA:PSS, suggesting the stable semiquinone radical was formed. More importantly, water-soluble DA:PSS exhibited stable and quasi-reversible electrochemical oxidation behavior, and excellent film-formation capability. Consequently, as an indium tin oxide (ITO) anode modifier, solution processed DA:PSS film showed hole injection property in organic light emitting diodes. Our results open a new avenue for the design of semiconductor and organic electronic application inspired by the electron transfer of phenol derivatives such as DA. Phenol-based organic electronic material has showed potential and it should be taken into consideration in the future.Keywords: Hole transport material; Organic electronic; Organic light-emitting diode; PEDOT:PSS; Phenol;

Interface engineering is an important aspect for the improvement of perovskite solar cells (PVSCs). The hole transport layer with good interface contact, transport capability, and matched energy level is indispensable and critical for high-performance photovoltaic devices. Herein, anode interface engineering with an excellent compatible bilayer of poly(3,4-ethylene dioxythiophene):poly(styrenesulfo-nate)/poly(3,4-ethylene dioxythiophene) (PEDOT:PSS/PEDOT) doped with grafted sulfonated-acetone–formaldehyde lignin (PEDOT:GSL) via a low-temperature and water-soluble process is presented. As a water-processable interface material, PEDOT:GSL exhibits higher conductivity, as well as better structural and electronic homogeneities compared with PEDTO:PSS. Consequently, the PEDOT:PSS/PEDOT:GSL bilayer with tuned energy level, optical properties, and the combination of the trap passivation of GSL at the anode/perovskite interface can greatly improve charge extraction ability and reduce the interface recombination. Simultaneously, the homogeneous perovskite film is fabricated through optimizing the annealing process. The device with the power conversion efficiency up to 17.80% is achieved, with 32.6% improvement compared to PEDOT:PSS-only device (13.42%). Our success to achieve high-performance inverted PVSCs provides new understanding of PEDOT:PSS, and also new guidelines for anode interface engineering to further advancement of PVSCs. This promising approach paves the way to realize solution processable highly efficient PVSCs for potential practical applications.

•SAF has great dispersing capability to dope PEDOT.•PEDOT:SAF exhibited high conductivity (3.12 S/cm) and weak acidity (pH ≈ 6).•PEDOT:SAF showed superior excellent waterproofness and UV-absorptivity.•PEDOT:SAF-based PSC achieved highly enhanced device durability and higher PCE.The poor long-term stability of perovskite solar cells (PSCs) tremendously hampers their future commercialization though their superior photovoltaic efficiencies. To enhance the device stability, a new poly(3,4-ethylenedioxythiophene):sulfonated acetone-formaldehyde (PEDOT:SAF) with higher PEDOT content (2:1) is developed considering the excellent dispersing capacity of SAF. PEDOT:SAF exhibits extremely lower acidity with pH value of 6 and higher conductivity of 3.12 S/cm comparing with the former reported sample with lower PEDOT content. Moreover, PEDOT:SAF film shows superior ultraviolet (UV) absorptivity originated from the fluorescence effect of SAF and unexceptionable film waterproofness on account of the high PEDOT content. As a result, the PSC incorporating PEDOT:SAF as the hole extraction layer (HEL) achieves higher power conversion efficiency (PCE) and highly enhanced device stability than the traditional PEDOT:PSS-based device. After 28 days of storage time, our device retains 83.2% from its original PCE, while almost half-degradation is experienced in the PEDOT:PSS controlled device. In addition, SAF is renewable with more simple and inexpensive preparation than that of PSS. Undoubtedly, this new PEDOT:SAF provides us a scaffold for designing stable PSC, and this platform is also shared in other photovoltaic technologies.Download high-res image (343KB)Download full-size image

Sulfonated-acetone-formaldehyde (SAF) was grafted with alkali lignin (AL) to prepare grafted sulfonated-acetone-formaldehyde lignin (GSL). Considering the rich phenolic hydroxyl groups in GSL, we detected a hole mobility of 2.27 × 10–6 cm2 V–1 s–1 with GSL as a hole transport material by space-charge-limited current model. Compared with nonconjugated poly(styrene sulfonic acid), GSL was applied as p-type semiconductive dopant for PEDOT to prepare water-dispersed PEDOT:GSL. PEDOT:GSL shows enhanced conductivity compared with that of PEDOT:PSS. Simultaneously, the enhanced open-circuit voltage, short-circuit current density, and fill factor are achieved using PEDOT:GSL as a hole extract layer (HEL) in sandwich-structure inverted perovskite solar cells. The power conversion efficiency is increased to 14.94% compared with 12.6% of PEDOT:PSS-based devices. Our results show that amorphous GSL is a good candidate as dopant of PEDOT, and we provide a novel prospective for the design of HEL based on lignin, a renewable biomass and phenol derivatives.Keywords: hole transport material interface engineering; lignosulfonate; organic electronic; PEDOT:PSS; phenol radical

Aggregation-induced emission (AIE) characteristics of lignin, which has the intrinsic aggregation behavior, are detected and studied for the first time. A positive correlation between the growth multiple of fluorescence intensity (l1:9/l10:0) and the sulfonation degree was found in the water–ethanol system. The AIE phenomenon and mechanism were further studied by the addition of cetyltrimethyl ammonium bromide (CTAB) owing to the electrostatic interaction with lignosulfonate. It is well known that lignin contains carbonyl groups, stilbene (Ar–CαCβ) and α-carbonyl (Ar–CαO) building blocks. We deduce that cluster of the carbonyl groups and restriction of intramolecular rotation (RIR) effects together contribute to the AIE activity of lignin as lignin does not exhibit blue emission based on its limited conjugated structure. Our results provide a new prospective to understand the fluorescence in lignin and explore novel potential application for the AIE activity of lignin.

Water-soluble alkyl chain sulfobutylated lignosulfonate (ASLS) doped PEDOT was prepared with lignin as raw material. Water processable PEDOT:ASLS was applied as hole injection layer (HIL) to modify ITO. Blue phosphorescent organic light-emitting diode plays a key role for full color display and are very challenging. With PEDOT:ASLS as HIL, a highly enhanced current efficiency of 37.65 cd/A was achieved. Considering our device structure, the result is even better than that of the control device using PEDOT:PSS as HIL. Compared with PSS with regular structure, strong aggregation and oxidation behavior of ASLS contribute to the hole injection capability of PEDOT:ASLS. Considering that ASLS is of disordered and amorphous structure, which is very different from poly(styrene sulfonic acid), it is exciting that ASLS might be of promising potential as a sustainable dopant of PEDOT. More importantly, this work will guide the design of dopant of PEDOT.Keywords: Dopant; Hole transport material; Interface engineering; Organic electronic; PEDOT:PSS; Phenol radical; Solar cell

A water-soluble, ratiometric fluorescent pH probe, L-SRhB, was synthesized via grafting spirolactam Rhodamine B (SRhB) to lignosulfonate (LS). As the ring-opening product of L-SRhB, FL-SRhB was also prepared. The pH-response experiment indicated that L-SRhB showed a rapid response to pH changes from 4.60 to 6.20 with a pKa of 5.35, which indicated that L-SRhB has the potential for pH detection of acidic organelle. In addition, the two probes were internalized successfully by living cells through the endocytosis pathway and could distinguish normal cells from cancer cells by different cell staining rates. In addition, L-SRhB showed obvious cytotoxicity to cancer cells, whereas it was nontoxic to normal cells in the same condition. L-SRhB might have potential in cancer therapy. L-SRhB might be a promising ratiometric fluorescent pH sensor and bioimaging dye for the recognition of cancer cells. The results also provided a new perspective to the high-value utilization of lignin.Keywords: biomass; cancer sensing; FRET; ratiometric sensor; Rhodamine B;

The fluorescence of sulfonated acetone–formaldehyde condensate (SAF) with a nonconventional chromophore is reported for the firsttime. The fluorescence intensity of SAF can be enhanced by the introduction of phenols to obtain sulfonated phenol–acetone–formaldehyde (SPAF) and the emission color can be changed. Both of them show aggregation-enhanced emission (AEE) properties.

The unconventional photoluminescence of sulfonated acetone–formaldehyde (SAF) and acetone–formaldehyde (AF) condensates are uncovered and studied in this work. Their fluorescence had been ignored although they were developed and commercialized as water reducing agents in the concrete industry decades ago. More importantly, based on the traditional mechanism of the Mannich reaction, sulfonated ethylenediamine–acetone–formaldehyde (SEAF) was successfully synthesized by introducing imino groups to SAF. It shows highly enhanced fluorescence both in solution and the solid state. The emission mechanism of these polymers is proposed to be via the cluster of carbonyl groups within the long linear nonconjugated chain. Hydroxy, sulfonic and amino groups in SEAF can produce strong ionic and hydrogen bonds, which contribute to its fluorescent enhancement. Furthermore, it is more interesting that SAF and SEAF possess an aggregation-enhanced emission (AEE) effect, while AF shows aggregation-caused quenching (ACQ). This result confirms the previous report that hydrogen bonds can induce AEE effects. Our study provides a novel perspective for the design of water-soluble luminescent materials with green photoluminescence and AEE property.

Inspired by the electron transfer process during the oxidation of electron-rich phenol derivatives and the serious aggregation properties of lignin, we studied the hole transporting properties of hole-only devices using water soluble lignosulfonate (SL) and an alkyl chain cross-linked lignosulfonate polymer (ASL) as active layers for the first time. SL with a higher phenolic group content shows higher hole mobility than ASL with a lower phenolic group content, which further suggests that phenolic groups are conducive to the hole transporting property. The maximum value of the hole mobility was achieved at 3.75 × 10−6 cm2 V−1 s−1 for SL. This unexpected hole mobility provides a novel perspective for the application of SL as a potential polymeric p-type semiconductor. A water soluble and solution-processable PEDOT:SL material was prepared from the oxidation of EDOT dispersed in SL. As a hole-transport material in OPV with a PCE of 5.79%, PEDOT:SL showed comparable performance to that of the conventional PEDOT:PSS with a device structure of ITO/HTM/PTB7:PC71BM/Al. In principle, amorphous lignosulfonate, which has a complex chemical structure, should show a worse hole transport property as a dopant for PEDOT than PSS, which has a regular molecular structure. In contrast, we found that PEDOT:SL showed comparable performance with that of PEDOT:PSS. The performance of PEDOT:SL is not very dependent on the mass ratio of PEDOT in PEDOT:SL. This also confirmed our proposal and results on the hole transport property of lignosulfonate. Our results provide a promising scaffold and concept for the design and feasible synthesis of HTMs based on the interconversion between phenol and benzoquinone for organic electronics. Moreover, our result also opens an application approach for lignin in the future.

We present an investigation of deep-blue fluorescent polymer light-emitting diodes (PLEDs) with a novel functional 1,3,5-triazine core material (HQTZ) sandwiched between poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) layer and poly(vinylcarbazole) layer as a hole injection layer (HIL) without interface intermixing. Ultraviolet photoemission spectroscopy and Kelvin probe measurements were carried out to determine the change of anode work function influenced by the HQTZ modifier. The thin HQTZ layer can efficiently maximize the charge injection from anode to blue emitter and simultaneously enhance the hole mobility of HILs. The deep-blue device performance is remarkably improved with the maximum luminous efficiency of 4.50 cd/A enhanced by 80% and the maximum quantum efficiency of 4.93%, which is 1.8-fold higher than that of the conventional device without HQTZ layer, including a lower turn-on voltage of 3.7 V and comparable Commission Internationale de L’Eclairage coordinates of (0.16, 0.09). It is the highest efficiency ever reported to date for solution-processed deep-blue PLEDs based on the device structure of ITO/HILs/poly(9,9-dialkoxyphenyl-2,7-silafluorene)/CsF/AL. The results indicate that HQTZ based on 1,3,5-triazine core can be a promising candidate of interfacial materials for deep-blue fluorescent PLEDs.Keywords: 1,3,5-triazine core; charge balance; deep-blue; fluorescent polymer light-emitting diodes; hole injection layer

A new family of highly water-soluble alkyl chain cross-linked sulfobutylated lignosulfonates (AASLSs) with a three-dimensional network structure and naked alkyl sulfonic acid groups were readily prepared by using 1,4-butane sultone (BS) and C6H12Br2 by a one step reaction in water, which simultaneously improved the sulfonation degree and molecular weight. GPC, 1H NMR, FTIR and functional group content tests confirmed their cross-linked chemical structure and efficient nucleophilic substitution reaction mechanism. Furthermore, the dispersion properties of AASLSs in a carbendazim suspension concentrate (SC) system were investigated. AASLS4 with high molecular weights (Mw) and moderate sulfonation degrees showed suspensibility of 99% in 45% carbendazim SC after hot storage at 50 °C for 14 days. Meanwhile, AASLS4 also showed smaller SC particle size and better rheological performance than commercial lignosulfonate. The adsorption isotherms and ζ-potential of AASLSs on carbendazim SC particles were studied to reveal the dispersion mechanism. The alkyl chain cross-linked structure and long alkyl chain-containing sulfonic acid groups contribute to the excellent dispersion properties on carbendazim SC. Our modification approach for lignin might provide a novel concept and prospective avenue for the design of efficient dispersants.Keywords: 1,4-Butane sultone; Alkali lignin; Carbendazim suspension concentrate; Dispersion; Sulfonation

Using lignosulfonate (LS) and alkyl chain-coupled lignosulfonate-based polymer (ALS) as the raw materials, the aggregation behavior of LS and ALS was investigated, and they both showed a unique aggregation behavior to form a block-like self-assembly for the first time. The aggregation behavior and mechanism of LS and ALS were investigated by SEM, TEM and DLS. The block-like aggregates prepared from ALS (micron size) were larger than that of LS (nano size). The unique aggregates were also further confirmed by XPS, meanwhile, SAXS was applied to explore the regular intrinsic characteristics of the block-like aggregates. Inspired by the aggregation behavior of LS and ALS, the electron transfer properties of LS and ALS were also studied including the electrochemical properties and hole mobility measurements. The oxidation peaks at 1.2 V and 1.4 V were observed at the LS and ALS modified electrode, respectively. We studied the hole transport properties of LS and ALS using the space-charge-limited current method (SCLC). Average hole mobilities of 2.95 × 10−6 cm2 V−1 s−1 and 3.18 × 10−7 cm2 V−1 s−1 were estimated for LS and ALS, respectively. The above results indicated that LS and ALS are potential water soluble polymeric p-type semiconductors, and the hole transport property of LS is better than that of ALS. Based on the unique aggregation behavior and hole mobility property described above which will facilitate charge transport, water soluble PEDOT:LS and PEDOT:ALS were prepared and applied as the hole extraction layer (HEL) in polymer solar cells. The PCE decreased with a decrease of the phenolic hydroxyl group content (–OH), which suggested that –OH is important for the strength of the PCE. The application properties were consistent with the results of the aggregation behavior and electron transfer properties. The power conversion efficiency (PCE) of 5.19% from PEDOT:LS-1:1 as the HTL was achieved with a device structure of ITO/HEL/PTB7:PC71BM/Al in our study. Our results showed that the phenolic hydroxyl group content and conjugation structure of amorphous LS contribute to its promising potential as a dopant of semiconductors, such as PEDOT in organic electronics. Our results provide a novel perspective for the design of dopants for semiconductive polymers. In summary, the phenolic hydroxyl group of the polymer will provide hole transport capability due to its oxidation during device operation.

Using a novel and facile method, we synthesized a family of ultrahigh molecular weight, lignosulfonate-based polymers (ALSs) via alkyl chain coupling polymerization. Gel permeation chromatography (GPC) showed a significant increase in weight-average molecular weights (Mws), from 42800 Da of ALS1 to 251000 Da of ALS5—one of the highest Mws among reported lignosulfonates (LSs) to date. Functional group content measurements, FTIR and 1H-NMR confirmed the efficient polymerization by nucleophilic substitution coupling mechanism and suggested a straightforward relationship between the polymerization of lignosulfonate (LS) and consumption of phenolic hydroxyl groups. Moreover, hollow nanospheres were obtained via self-assembly of water-soluble ALS and were investigated by DLS, SEM, TEM and AFM. The hollow sphere structure, with a hydrophilic core and a hydrophobic shell, was confirmed by XPS and elemental analysis. Stable, quasi-solid nanospheres were obtained from ALS by the addition of cetyl trimethyl ammonium bromide (CTAB). Furthermore, ALS2, with its relatively high molecular weight, showed unexpectedly better dispersion properties than the raw material LS and naphthalene sulfonate formaldehyde condensate (NSF) for coal–water slurry. The effective polymerization route to improving Mw and the self-assembly from polymer-only ALS provide novel avenues for high-value application of lignin, a sustainable and abundant bioresource.

•A PCE of 16.97% was obtained with a dopant-free HTM in PVSC.•A very low hole mobility and electron blocking will also ensure high PCE.•Phenol and its derivatives show great potential as building block for HTMs.•Device stability is enhanced with extremely cheap HTMs.With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PVSCs), device lifetime has become one of the extensive research interests and concerns. To enhance the device durability, developing high performance dopant-free hole transport materials (HTMs) is a promising strategy. Herein, two new C3-symmetric HTMs with phenol core, TCP-OH and TCP-OC8 are readily prepared and show ultra-wide energy band-gap and excellent film-formation property. PCEs of 16.97% and 15.28% are achieved with pristine TCP-OH and TCP-OC8 film as HTMs, respectively, even though their hole mobilities are as low as 10−6 cm2 V−1 s−1. Phenol acts as hole trap in traditional concept, however, TCP-OH shows higher hole mobility than that of TCP-OC8. Moreover, TCP-OH shows higher glass transition temperature and better matching band alignment than those of TCP-OC8. Phenol shows great potential as building block for HTMs as it is beneficial to enhance hole mobility of HTMs. Moreover, our study demonstrates an interesting viewpoint to design HTMs with the balance of hole mobility and electron blocking effect.Dopant-free hole transport material with phenol core was readily synthesized showed enhanced stability of PVSCs with PCE of 16.97%.

The development of high performance hole transport materials (HTMs) without a chemical dopant is critical to achieve long-term device durability. The general design of self-doping materials based on a phenolamine structure with strong electronic spin concentration is reported for the first time. A phenol-enhanced self-doped mechanism is also proposed. Compared to their precursors, dimethylphenolamine derivatives, TBP-OH4, TPD-OH4 and Spiro-OH8, displayed much higher spin concentration in their neutral states. Phenol acts as a hole trap in the traditional concept, however, the films of TBP-OH4, TPD-OH4 and Spiro-OH8 exhibited higher conductivities than those of methoxyl precursors. Meanwhile, phenolamine derivatives have good solublility in polar organic solvents and show good solvent resistance in chlorobenzene. Considering the relatively good band alignment, film-formation and solvent resistance against chlorobenzene, Spiro-OH8 and TPD-OH4 exhibited comparable performance with that of PEDOT:PSS-4083. Most importantly, a new generation of self-doped systems based on a phenolamine structure might provide new insight in developing efficient HTMs for organic electronics.

Inspired by the electron transfer process during the oxidation of electron-rich phenol derivatives and the serious aggregation properties of lignin, we studied the hole transporting properties of hole-only devices using water soluble lignosulfonate (SL) and an alkyl chain cross-linked lignosulfonate polymer (ASL) as active layers for the first time. SL with a higher phenolic group content shows higher hole mobility than ASL with a lower phenolic group content, which further suggests that phenolic groups are conducive to the hole transporting property. The maximum value of the hole mobility was achieved at 3.75 × 10−6 cm2 V−1 s−1 for SL. This unexpected hole mobility provides a novel perspective for the application of SL as a potential polymeric p-type semiconductor. A water soluble and solution-processable PEDOT:SL material was prepared from the oxidation of EDOT dispersed in SL. As a hole-transport material in OPV with a PCE of 5.79%, PEDOT:SL showed comparable performance to that of the conventional PEDOT:PSS with a device structure of ITO/HTM/PTB7:PC71BM/Al. In principle, amorphous lignosulfonate, which has a complex chemical structure, should show a worse hole transport property as a dopant for PEDOT than PSS, which has a regular molecular structure. In contrast, we found that PEDOT:SL showed comparable performance with that of PEDOT:PSS. The performance of PEDOT:SL is not very dependent on the mass ratio of PEDOT in PEDOT:SL. This also confirmed our proposal and results on the hole transport property of lignosulfonate. Our results provide a promising scaffold and concept for the design and feasible synthesis of HTMs based on the interconversion between phenol and benzoquinone for organic electronics. Moreover, our result also opens an application approach for lignin in the future.