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For an ideal electron interlayer, both electron injection and hole‐blocking properties are important to achieve better polymer light‐emitting devices (PLEDs) performance. Conjugated polyelectrolytes (CPEs) are applied widely in PLEDs to enhance charge injection. Understanding the role of backbone structures and energetic matching between the CPEs and emitters can benefit charge injection and balance. Herein, a postpolymerization approach to introduce varying amounts of alkyl sulfonate groups onto the backbone of a copolymer of 5‐fluoro‐2,1,3‐benzothiadiazole and 9,9′‐dioctylfluorene is utilized. This study finds that device performance is dependent on the percentage of sulfonate groups incorporated, with the optimal copolymer (CPE‐50%) maintaining efficient ohmic electron injection and gaining enhanced hole‐blocking properties, thereby achieving the most balanced hole/electron current. Therefore, the PLED with CPE‐50% interlayer exhibits the highest efficiency (20.3 cd A−1, 20.2 lm W−1) and the fastest response time (4.3 µs), which is the highest efficiency among conventional thin (70 nm) F8BT PLEDs with CPEs. These results highlight the importance of balanced charge carrier density in CPEs and highlight that postpolymerization modification is a useful method for fine‐tuning ionic content. [ABSTRACT FROM AUTHOR]

Copper(I) thiocyanate (CuSCN) is an effective interlayer material for hole injection and transport in organic electronic devices but its solution processing has conventionally utilized undesirable di‐n‐alkyl sulfide solvents such as diethyl‐ (DES) and dipropyl‐sulfide (DPS). Herein, this paper reports on the use of N,N‐dimethylformamide (DMF) and 1‐methyl‐2‐pyrrolidinone (NMP) as alternative solvents for CuSCN interlayers and performs a detailed comparison of the resulting properties relative to films processed from DES and DPS and two other recent alternatives, dimethyl sulfoxide (DMSO) and ammonium hydroxide. The surface roughness, polymorphism, and surface chemistry of the resulting CuSCN layers are reported. The interlayer surface energy and ionization potential that are key to the overlying semiconductor microstructure and interfacial energy barrier, and hence to charge transport and injection, are also discussed. Finally, systematic device tests using well‐known organic semiconductors in light‐emitting diode, solar cell and field‐effect transistor structures demonstrate the overall suitability of DMSO and DMF as solvents for CuSCN interlayer deposition to achieve better device performance. This study broadens the applicability of CuSCN as a highly efficient hole injection/transport material for organic semiconductor devices by expanding the documented range of suitable CuSCN solvents. [ABSTRACT FROM AUTHOR]

Organic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. Herein, a small molecule donor, DRCN5T, is studied using a variety of morphology and spectroscopy techniques, and blended with both fullerene and non‐fullerene acceptors. Specifically, grazing incidence wide‐angle X‐ray scattering and transient absorption, Raman, and electron paramagnetic resonance spectroscopies are focused on. It is shown that despite DRCN5T's ability to achieve OPV efficiencies of over 10%, it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet‐charge annihilation. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form. Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet‐charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non‐fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA. [ABSTRACT FROM AUTHOR]

Electric-field driven chemical doping modulation in a blend of solution-processed organic semiconductors (OSCs) and solid-state ionic liquids (SSILs) in response to volatile organic compounds (VOCs) provides a new exciting opportunity to facilitate printable and low-power chemical gas sensors (chemiresistors). In order to fully exploit this opportunity, a fundamental understanding of the molecular-level interactions among the OSCs, SSILs, and VOC components during the device operation is urgently needed. Herein, we demonstrate a highly sensitive and selective VOC gas sensor using π-conjugated polymer (here, P3HT as a model homopolymer) and SSIL blends. A newly developed SSIL forms a semi-crystalline solid at room temperature. P3HT with high molecular weight and regioregularity allows an extremely well-interconnected network in blends desirable for efficient charge transport. In P3HT:SSIL blends, we identify electric-field driven strong chemical interactions between π-CP and SSIL to tune the electrical conductivity of the π-CP. The enlarged interfacial areas in blends and the solid-state nature of the SSIL ensure highly tunable electrochemical interactions between them, efficiently modulating the electrical conductivity of the π-CP further upon exposure to different polar and non-polar VOCs. Our results demonstrate the π-conjugated polymer/SSIL complex as a new highly sensitive and selective gas sensor and provide a key scientific understanding of its molecular-level operational mechanism critical for developing molecular sensors towards next generation noninvasive diagnostics. [ABSTRACT FROM AUTHOR]

Printable organic sensors fabricated from solution-processed π-conjugated polymers (π-CPs) are promising candidates to detect volatile organic compounds (VOCs) due to the intriguing physical, chemical and electronic properties of π-CPs. These devices, often termed organic chemiresistors, require good sensing capabilities to transduce stimuli from specific VOCs at low concentrations into analytical electric signals. However, discriminating such VOCs using organic chemiresistors has proven very challenging. Herein, we report that the molecular-level electrochemical doping of π-CPs with solid-state ionic liquids (SILs) significantly improves their electrical conductivity (∼10−1 S cm−1) and selective VOC interactions, which can be manipulated through different π-CPs:SIL blend ratios. These characteristics enable the fine discrimination of VOCs at concentrations in the parts-per-billion (ppb) range under low power consumption (<0.1 μW) and room temperature conditions. Our result provides a new opportunity for developing highly sensitive and selective VOC monitoring platform technologies that are printable on large-area, wearable, flexible and transparent substrates. [ABSTRACT FROM AUTHOR]

The use of photon energy up-converted luminescence driven by triplet-exciton annihilation reactions (TTA-UC) is increasingly gaining attention for developing next-generation light-management, and wavelength-shifting technologies. Here we present a spectroscopic study for elucidating the photophysical mechanism that operates in an unusual TTA-UC model system comprising the blue-light emitting poly(fluorene-2-octyl) (PFO) activator mixed with the green-light absorbing (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) metalo-organic complex. The unconventional character of the PFO:PtOEP composite manifests in the fact that no exothermic triplet energy transfer (TET) is possible between triplet-excited PtOEP and PFO. Yet green-to-blue TTA-UC luminescence of PFO is obtained even when PtOEP is selectively photoexcited by pulsed laser intensities as low as 2.5 mW cm−2. Continuous-wave photo-induced absorption spectroscopy verifies that no energy transfer from triplet-excited PtOEP to the triplet level of PFO takes place, pointing to triplet–triplet annihilation (TTA) events in the PtOEP phase as the origin of the observed TTA-UC PL signal. In the PFO:PtOEP composite, the PtOEP component holds a dual role of annihilator/sensitizer; photon energy storage in PtOEP is enabled via TTA when triplet exciton diffusion coefficient values of DPtOEP = 4.1 × 10−9 cm2 s−1 are reached. With a simple yet powerful solution processing protocol, and by combining Raman and time-gate photoluminescence (PL) spectroscopy we demonstrate that the brightness of the produced TTA-UC luminescence depends on the molecular conformation of the PFO activator. A four-fold increase in the TTA-UC luminescence intensity is registered in the time-integrated and time-gated PL spectra, when the PFO matrix is arrested in its planar β-phase molecular conformation. Further enhancement of the TTA-UC PL signal is achieved when temperature lowers from 290 K down to 100 K. These results stimulate the development of a theoretical model for the microscopic description of triplet exciton migration in disordered photon up-converting solids. Efficient harvesting of photon energy, which is stored in annihilator/sensitizer moieties via TTA events, can be enabled when the molecular conformation of the activator species is properly tuned. [ABSTRACT FROM AUTHOR]

Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (VJ = 11.6 V from 20.0 V and VL = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on VJ and VL, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies. [ABSTRACT FROM AUTHOR]

Here, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance. [ABSTRACT FROM AUTHOR]

Controlling initial bulk‐heterojunction (BHJ) morphology of photoactive layer is critical for device efficiency of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long‐term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in situ monitoring of devices at a molecular level. Here, a thermal annealing preconditioning step is utilized to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in situ degradation pathways. Direct spectroscopic evidence is reported for molecular‐scale phase‐segregation temperature (TPS) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well‐mixed blend morphology (no annealing) shows interface instability related to the hole‐extracting layer via de‐doping. Likewise, initially phase‐segregated morphology at a molecular level (annealed above TPS) shows instability in the photoactive layer via continuous phase‐segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. The results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below TPS is critical for improved operational stability while maintaining high efficiency. Operational stability of polymer:fullerene based organic solar cells depends on their initial bulk‐heterojunction morphology. Both extremes (well‐mixed vs phase‐segregated) show instability; the former caused by interlayer degradation and the latter by further macroscale phase‐segregation coupled with fullerene photodegradation. Thermal annealing below molecular‐scale phase‐segregation temperature is critical to maintain well‐mixed morphology with stable interface for optimized efficiency and operational stability. [ABSTRACT FROM AUTHOR]