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Volatile solid additives have emerged as a promising strategy for enhancing film morphology and promoting the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, a series of novel polycyclic aromatic additives with analogous chemical structures, including fluorene (FL), dibenzothiophene (DBT), and dibenzofuran (DBF) derived from crude oils, are presented and incorporated into OSCs. All these additives exhibit strong interactions with the electron-deficient terminal groups of L8-BO within the bulk-heterojunction OSCs. Moreover, they demonstrate significant sublimation during thermal annealing, leading to increase free volumes for the rearrangement and recrystallization of L8-BO. This phenomenon leads to an improved film morphology and an elevated glass-transition temperature of the photoactive layers. Consequently, the PCE of the PM6:L8-BO blend has been boosted from 16.60% to 18.60% with 40 wt% DBF additives, with a champion PCE of 19.11% achieved for ternary PM6:L8-BO:BTP-eC9 OSCs. Furthermore, the prolonged shelf and thermal stability have been observed in OSCs with these additives. This study emphasizes the synergic effect of volatile solid additives on the performance and thermal stability of OSCs, highlighting their potential for advancing the field of photovoltaics., (© 2024 Wiley‐VCH GmbH.)

The efficiency of an organic solar cell is highly dependent on the complex, interpenetrating morphology, and molecular order within the composite phases of the bulk heterojunction (BHJ) blend. Both these microstructural aspects are strongly influenced by the processing conditions and chemical design of donor/acceptor materials. To establish improved structure-function relationships, it is vital to visualize the local microstructural order to provide specific local information about donor/acceptor interfaces and crystalline texture in BHJ blend films. The visualization of nanocrystallites, however, is difficult due to the complex semi-crystalline structure with few characterization techniques capable of visualizing the molecular ordering of soft materials at the nanoscale. Here, it is demonstrated how cryo-electron microscopy can be utilized to visualize local nanoscale order. This method is used to understand the distribution/orientation of crystallites in a BHJ blend. Long-range (>300 nm) texturing of IEICO-4F crystallites oriented in an edge-on fashion is observed, which has not previously been observed for spin-coated materials. This approach provides a wealth of quantitative information about the texture and size of nanocrystallites, which can be utilized to understand charge generation and transport in organic film. This study guides tailoring the material design and processing conditions for high-performance organic optoelectronic devices., (© 2024 The Authors. Small Methods published by Wiley‐VCH GmbH.)

Conjugated polymers are promising materials for thermoelectric applications, however, at present few effective and well-understood strategies exist to further advance their thermoelectric performance. Here a new model system is reported for a better understanding of the key factors governing their thermoelectric properties: aligned, ribbon-phase poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) doped by ion-exchange doping. Using a range of microstructural and spectroscopic methods, the effect of controlled incorporation of tie-chains between the crystalline domains is studied through blending of high and low molecular weight chains. The tie chains provide efficient transport pathways between crystalline domains and lead to significantly enhanced electrical conductivity of 4810 S cm -1 , which is not accompanied by a reduction in Seebeck coefficient or a large increase in thermal conductivity. Respectable power factors of 173 µW m -1 K -2 are demonstrated in this model system. The approach is generally applicable to a wide range of semicrystalline conjugated polymers and could provide an effective pathway for further enhancing their thermoelectric properties and overcome traditional trade-offs in optimization of thermoelectric performance., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)

Ion migration is one of the most critical challenges that affects the stability of metal-halide perovskite solar cells (PSCs). However, the current arsenal of available strategies for solving this issue is limited. Here, novel perovskite active layers following the concept of ordered structures with functional units (OSFU) to intrinsically suppress ion migration, in which a three-dimensional (3D) perovskite layer is deposited by vapor deposition for light absorption and a 2D layer is deposited by solution process for ion inhibition, are constructed. As a promising result, the activation energy of ion migration increases from 0.36 eV for the conventional perovskite to 0.54 eV for the OSFU perovskite. These devices exhibit substantially enhanced operational stability in comparison with the conventional ones, retaining >85% of their initial efficiencies after 1200 h under ISOS-L-1. Moreover, the OSFU devices show negligible fatigue behavior with a robust performance under light/dark cycling aging test (ISOS-LC-1 protocol), which demonstrates the promising application of functional motif theory in this field., (© 2024 Wiley‐VCH GmbH.)

The aggregation-induced emission (AIE) properties of a systematic series of naphthalene diimides (NDIs) varying the chain length at the imide positions have been studied. A solvophobic collapse of NDI units through the flash injection of THF NDI solutions in sonicating water triggers the formation of stable suspensions with enhanced fluorescence emissions. Shorter chains favor the π-π stacking of NDI units through H-aggregation producing a strong AIE effect showing remarkably high quantum yields that have not been observed for non core-substitued NDIs previously. On the other hand, NDIs functionalized with longer chains lead to more disordered domains where π-π stacking between NDI units is mainly given by J-aggregation unfavoring the AIE effect., (© 2024 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH.)

The dramatic improvement of the PCE (power conversion efficiency) of organic photovoltaic devices in the past few years has been driven by the development of new polymer donor materials and non-fullerene acceptors (NFAs). In the design of such materials synthetic scalability is often not considered, and hence complicated synthetic protocols are typical for high-performing materials. Here we report an approach to readily introduce a variety of solubilizing groups into a benzo[ c ][1,2,5]thiadiazole acceptor comonomer. This allowed for the ready preparation of a library of eleven donor polymers of varying side chains and comonomers, which facilitated a rapid screening of properties and photovoltaic device performance. Donor FO6-T emerged as the optimal material, exhibiting good solubility in chlorinated and non-chlorinated solvents and achieving 15.4% PCE with L8BO as the acceptor (15.2% with Y6) and good device stability. FO6-T was readily prepared on the gram scale, and synthetic complexity (SC) analysis highlighted FO6-T as an attractive donor polymer for potential large scale applications.

The invention of near-infrared pedant-based double-cable conjugated polymers has demonstrated remarkable efficacy in single-component organic solar cells (SCOSCs). This work focuses on the innovative double-cable conjugated polymers aimed at attaining good absorption and suitable energy levels. Specifically, in the aromatic side units, the electron-donating (D) part is designed using a thieno[3,4- c ]pyrrole-4,6-dione (TPD) as a core unit, flanked by two cyclopentadithiophene groups on either side. The electron-deficient (A) terminal groups consist of 2-(3-oxo-2,3-dihydro-1 H -cyclopenta[ b ]naphthalen-1-ylidene) malononitrile (NC), which can be further modified through fluorination to modulate the physical properties and packing modes of the acceptor material. The resulting double-cable conjugated polymers exhibit broad absorption spectra spanning 500-850 nm and possess lowered Frontier energy levels when incorporating fluorine elements, providing decreased voltage losses in SCOSCs. Therefore, SCOSCs fabricated using these polymers have demonstrated power conversion efficiencies ranging from 7.6 to 10.2%, in which fluorine-containing double-cable conjugated polymers showed higher PCEs due to more favorable crystalline packing, enhanced exciton dissociation probability, and charge-transporting ability.

Bisthienoazepinedione (BTA) has been reported for constructing high-performing p-type conjugated polymers in organic electronics, but the ring extended version of BTA is not well explored. In this work, we report a new synthesis of a key building block to the ring expanded electron-deficient pentacyclic azepinedione (BTTA). Three copolymers of BTAA with benzodithiophene substituted by different side chains are prepared. These polymers exhibit similar energy levels and optical absorption in solution and solid state, while significant differences are revealed in their film morphologies and behavior in transistor and photovoltaic devices. The best-performing polymers in transistor devices contained alkylthienyl side chains on the BDT unit ( pBDT-BTTA-2 and pBDT-BTTA-3 ) and demonstrated maximum saturation hole mobilities of 0.027 and 0.017 cm 2 V -1 s -1 . Blends of these polymers with PC 71 BM exhibited a best photovoltaic efficiency of 6.78% for pBDT-BTTA-3 -based devices. Changing to a low band gap non-fullerene acceptor (BTP-eC9) resulted in improved efficiency of up to 13.5%. Our results are among the best device performances for BTA and BTTA-based p-type polymers and highlight the versatile applications of this electron-deficient BTTA unit., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Azulene has aroused widespread interest for constructing optoelectronic materials. However, controlling the dipole orientation of 2,6-azulene units in the conjugated polymer backbone is a significant challenge so far. Herein, by C-H activation strategy, three 2,6-azulene-TPD-based conjugated copolymers with different dipole arrangements were synthesized, where TPD = thieno[3,4- c ]pyrrole-4,6-dione. The dipole arrangements of 2,6-azulene units were random for P(AzTPD-1) , head-to-head/tail-to-tail for P(AzTPD-2) , and head-to-tail for P(AzTPD-3) . These polymers exhibited unipolar n-type semiconductor characteristics in organic field effect transistors. Moreover, regioregular polymer P(AzTPD-3) displayed the best device performance with an electron mobility of up to 0.33 cm 2 V -1 s -1 , which makes P(AzTPD-3) a high-performance n-type polymeric semiconductor. These results demonstrate that incorporation of 2,6-azulene units into the polymeric backbone together with the regulation of the dipole orientation of 2,6-azulene units is an effective strategy for obtaining high-performance organic optoelectronic materials.

Conjugated polymer field-effect transistors are emerging as an enabling technology for flexible electronics due to their excellent mechanical properties combined with sufficiently high charge-carrier mobilities and compatibility with large-area, low-temperature processing. However, their electrical stability remains a concern. ON-state (accumulation mode) bias-stress instabilities in organic semiconductors have been widely studied, and multiple mitigation strategies have been suggested. In contrast, OFF-state (depletion mode) bias-stress instabilities remain poorly understood despite being crucial for many applications in which the transistors are held in their OFF-state for most of the time. Here, a simple method of using an antisolvent treatment is presented to achieve significant improvements in OFF-state bias-stress and environmental stability as well as general device performance for one of the best performing polymers, solution-processable indacenodithiophene-co-benzothiadiazole (IDT-BT). IDT-BT is weakly crystalline, and the notable improvements to an antisolvent-induced, increased degree of crystallinity, resulting in a lower probability of electron trapping and the removal of charge traps is attributed. The work highlights the importance of the microstructure in weakly crystalline polymer films and offers a simple processing strategy for achieving the reliability required for applications in flexible electronics., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

We report the observation of a third crystalline polymorph, "form III", of the well-studied electron-transporting conjugated polymer P(NDI2OD-T2) that exhibits end-on texture. This third polymorph of P(NDI2OD-T2) is distinguished from other polymorphs by having two monomer units incorporated along the backbone-stacking direction, resulting in a doubling of the c axis of the unit cell. Form III crystallites are realized by melt-annealing a thin film followed by slow cooling. The distinct packing of this third polymorph is established through the application of grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements combined with peak simulation of candidate unit cells. The discovery of a third polymorph of P(NDI2OD-T2) provides a fresh opportunity for studying structure/function relationships of this important semiconducting polymer.

Understanding the solution-phase behaviour of organic semiconducting polymers is important for systematically improving the performance of devices based on solution-processed thin films of these molecules. Conventional polymer theory predicts that polymer conformations become more compact as solvent quality decreases, but recent experiments have shown the high-performance organic-semiconducting polymer P(NDI2OD-T2) to form extended rod-like aggregates much larger than a single chain in poor solvents, with the formation of these extended aggregates correlated with enhanced electron mobility in films deposited from these solutions. We explain the unexpected formation of extended aggregates using a novel coarse-grained simulation model of P(NDI2OD-T2) that we have developed to study the effect of solvent quality on its solution-phase behaviour. In poor solvents, we find that aggregation through only a few monomers gives effectively inseparable chains, leading to the formation of extended structures of partially overlapping chains via non-equilibrium assembly. This behaviour requires that multi-chain aggregation occurs faster than chain folding, which we show is the case for the chain lengths and concentrations shown experimentally to form rod-like aggregates. This kinetically controlled process introduces a dependence of aggregate structure on concentration, chain length, and chain flexibility, which we show is able to reconcile experimental findings and is generalisable to the solution-phase assembly of other semiflexible polymers.

Organic solar cells (OSCs) based on polymer donor and non-fullerene acceptor achieve power conversion efficiency (PCE) more than 19% but their poor absorption below 550 nm restricts the harvesting of high-energy photons. In contrast, wide bandgap all-inorganic perovskites limit the absorption of low-energy photons and cause serious below bandgap loss. Therefore, a 2-terminal (2T) monolithic perovskite/organic tandem solar cell (TSC) incorporating wide bandgap CsPbI 2 Br is demonstrated as front cell absorber and organic PM6:Y6 blend as rear cell absorber, to extend the absorption of OSCs into high-energy photon region. The perovskite sub-cell, featuring a sol-gel prepared ZnO/SnO 2 bilayer electron transporting layer, renders a high open-circuit voltage (V OC ). The V OC is further enhanced by employing thermal annealing (TA)-free process in the fabrication of rear sub-cell, demonstrating a record high V OC of 2.116 V. The TA-free Ag/PFN-Br interface in organic sub-cell facilitates charge transport and restrains nonradiative recombination. Consequently, a remarkable PCE of 20.6% is achieved in monolithic 2T-TSCs configuration, which is higher than that of both reported single junction and tandem OSCs, demonstrating that tandem with wide bandgap all-inorganic perovskite is a promising strategy to improve the efficiency of OSCs., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Double-cable conjugated polymers with near-infrared (NIR) electron acceptors are synthesized for use in single-component organic solar cells (SCOSCs). Through the development of a judicious synthetic pathway, the highly sensitive nature of the 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC)-based electron acceptors in basic and protonic solvents is overcome. In addition, an asymmetric design motif is adopted to optimize the packing of donor and acceptor segments, enhancing charge separation efficiency. As such, the new double-cable polymers are successfully applied in SCOSCs, providing an efficiency of over 10 % with a broad photo response from 300 to 850 nm and exhibiting excellent thermal/light stability. These results demonstrate the powerful design of NIR-acceptor-based double-cable polymers and will enable SCOSCs to enter a new stage., (© 2022 Wiley-VCH GmbH.)

The way in which conjugated polymers pack in the solid state strongly affects the performance of polymer-based optoelectronic devices. However, even for the most crystalline conjugated polymers the precise packing of chains within the unit cell is not well established. Here we show that by performing resonant X-ray diffraction experiments at the sulfur K-edge we are able to resolve the tilting of the planar backbones of crystalline poly(3-hexylthiophene) (P3HT) within the unit cell. This approach exploits the anisotropic nature of the X-ray optical properties of conjugated polymers, enabling us to discern between different proposed crystal structures. By comparing our data with simulations based on different orientations, a tilting of the planar conjugated backbone with respect to the side chain stacking direction of 30 ± 5° is determined.

We developed a series of highly crystalline double-cable conjugated polymers for application in single-component organic solar cells (SCOSCs). These polymers contain conjugated backbones as electron donor and pendant perylene bisimide units (PBIs) as electron acceptor. PBIs are connected to the backbone via alkyl units varying from hexyl (C 6 H 12 ) to eicosyl (C 20 H 40 ) as flexible linkers. For double-cable polymers with short linkers, the PBIs tend to stack in a head-to-head fashion, resulting in large d-spacings (e.g. 64 Å for the polymer P12 with C 12 H 24 linker) along the lamellar stacking direction. When the length of the linker groups is longer than a certain length, the PBIs instead adopt a more ordered packing likely via H-aggregation, resulting in short d-spacings (e.g. 50 Å for the polymer P16 with C 16 H 32 linker). This work highlights the importance of linker length on the molecular packing of the acceptor units and the influences on the photovoltaic performance of SCOSCs., (© 2021 Wiley-VCH GmbH.)

We investigate the charge transport physics of a previously unidentified class of electron-deficient conjugated polymers that do not contain any single bonds linking monomer units along the backbone but only double-bond linkages. Such polymers would be expected to behave as rigid rods, but little is known about their actual chain conformations and electronic structure. Here, we present a detailed study of the structural and charge transport properties of a family of four such polymers. By adopting a copolymer design, we achieve high electron mobilities up to 0.5 cm 2 V -1 s -1 Field-induced electron spin resonance measurements of charge dynamics provide evidence for relatively slow hopping over, however, long distances. Our work provides important insights into the factors that limit charge transport in this unique class of polymers and allows us to identify molecular design strategies for achieving even higher levels of performance., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

The planar, aromatic nature of the backbone of conjugated polymers endows them with anisotropic properties. Here we show that the resonant X-ray diffraction of a sulfur-containing semicrystalline conjugated polymer at the sulfur K-edge is highly anisotropic, with strong modulation of diffracted intensity depending upon the relative orientation of the polarization of the incident beam with respect to the diffracting crystal planes. Through determination of the anisotropic resonant scattering factors, we can spectroscopically reproduce the observed resonant anisotropic scattering effects based on a proposed unit cell geometry for the polymer.

We present the design and performance of a polarized resonant soft x-ray scattering (RSoXS) station for soft matter characterization built by the national institute of standards and technology at the national synchrotron light source-II (NSLS-II). The RSoXS station is located within the spectroscopy soft and tender beamline suite at NSLS-II located in Brookhaven national laboratory, New York. Numerous elements of the RSoXS station were designed for optimal performance for measurements on soft matter systems, where it is of critical importance to minimize beam damage and maximize collection efficiency of polarized x-rays. These elements include a novel optical design, sample manipulator and sample environments, as well as detector setups. Finally, we will report the performance of the measurement station, including energy resolution, higher harmonic content and suppression methods, the extent and mitigation of the carbon absorption dip on optics, and the range of polarizations available from the elliptically polarized undulator source., (© 2021 IOP Publishing Ltd.)

The performance of optoelectronic devices based on conjugated polymers is critically dependent upon molecular packing; however, the paracrystalline nature of these materials limits the amount of information that can be extracted from conventional X-ray diffraction. Resonant diffraction (also known as anomalous diffraction) occurs when the X-ray energy used coincides with an X-ray absorption edge in one of the constituent elements in the sample. The rapid changes in diffraction intensity that occur as the X-ray energy is varied across an absorption edge provide additional information that is lost in a conventional nonresonant experiment. Taking advantage of the fact that many conjugated polymers contain sulfur as heteroatoms, this work reveals pronounced resonant diffraction effects at the sulfur K-edge with a particular focus on the well-studied electron transporting 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). The observed behavior is found to be consistent with the theory of resonant diffraction, and by simulating the energy-dependent peak intensity based on proposed crystal structures for P(NDI2OD-T2), we find that resonant diffraction can discriminate between different crystalline packing structures. The utilization of resonant diffraction opens up a new way to unlock important microstructural information about conjugated polymers for which only a handful of diffraction peaks are typically available.

The extensive use of halomethanes (CH 3 X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH 3 I) as a model system, it is shown that cesium lead bromide (CsPbBr 3 ) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH 3 I and release the iodide species. The extent of the halide exchange is directly dependent on the CH 3 I concentration, with the photoluminescence emission of the CsPbBr 3 nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH 3 I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.

Three n-type fused lactam semiconducting polymers were synthesized for thermoelectric and transistor applications via a cheap, highly atom-efficient, and nontoxic transition-metal free aldol polycondensation. Energy level analysis of the three polymers demonstrated that reducing the central acene core size from two anthracenes ( A-A ), to mixed naphthalene-anthracene ( A-N ), and two naphthalene cores ( N-N ) resulted in progressively larger electron affinities, thereby suggesting an increasingly more favorable and efficient solution doping process when employing 4-(2,3-dihydro-1,3-dimethyl-1 H -benzimidazol-2-yl)- N , N -dimethylbenzenamine (N-DMBI) as the dopant. Meanwhile, organic field effect transistor (OFET) mobility data showed the N-N and A-N polymers to feature the highest charge carrier mobilities, further highlighting the benefits of aryl core contraction to the electronic performance of the materials. Ultimately, the combination of these two factors resulted in N-N , A-N , and A-A to display power factors (PFs) of 3.2 μW m -1 K -2 , 1.6 μW m -1 K -2 , and 0.3 μW m -1 K -2 , respectively, when doped with N-DMBI, whereby the PFs recorded for N-N and A-N are among the highest reported in the literature for n-type polymers. Importantly, the results reported in this study highlight that modulating the size of the central acene ring is a highly effective molecular design strategy to optimize the thermoelectric performance of conjugated polymers, thus also providing new insights into the molecular design guidelines for the next generation of high-performance n-type materials for thermoelectric applications.

The crystallization and aggregation behaviors of semiconducting polymers play a critical role in determining the ultimate performance of optoelectronic devices based on these materials. Due to the soft nature of polymers, crystallite imperfection exists ubiquitously. To this aspect, crystallinity is often used to represent the degree of crystallite imperfection in a reciprocal relation. Despite of the importance, the discussion on crystallinity is still on the phenomenological level and ambiguous in many cases. As two major contributors to crystallite imperfection, crystallite size and paracrystallinity are highly intertwined and hardly separated, hindering more accurate and trustworthy structural analysis. Herein, with the aid of synchrotron-based X-ray diffraction, combined with environmentally controlled heating capability, the evolution of crystallite size and paracrystallinity of two prototypical polythiophene-based thin films have been successfully measured. Strikingly, the paracrystallinity of poly(3-hexylthiophene-2,5-diyl) (P3HT) crystallites remains unchanged with annealing, while the paracrystallinity of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2- b ]thiophene] (PBTTT) becomes diminished with crystallite growth. This work delivers a promising gesture to semiconducting polymers community, confirming that it is possible to experimentally separate crystallite size and paracrystallinity, both of which are highly intertwined. With this progress, investigation on the correlation between further detailed microstructural parameters and device performance can be achieved.

Compared to organic solar cells based on narrow-bandgap nonfullerene small-molecule acceptors, the performance of all-polymer solar cells (all-PSCs) lags much behind due to the lack of high-performance n-type polymers, which should have low-aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near-infrared region. Herein, two novel polymer acceptors, DCNBT-TPC and DCNBT-TPIC, are synthesized with ultranarrow bandgaps (ultra-NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm 2 V -1 s -1 obtained for DCNBT-TPC. Blended with a polymer donor, PBDTTT-E-T, the resultant all-PSCs based on DCNBT-TPC and DCNBT-TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short-circuit currents up to 19.44 and 22.52 mA cm -2 , respectively. This is the first example that a PCE of over 10% can be achieved using ultra-NBG polymer acceptors with a photoresponse reaching 950 nm in all-PSCs. These results demonstrate that ultra-NBG polymer acceptors, in line with nonfullerene small-molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all-PSCs., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two fused ladder-type nonfullerene acceptors, DTCCIC and DTCCIC-4F, based on an electron-donating alkylated dithienocyclopentacarbazole core flanked by electron-withdrawing nonfluorinated or fluorinated 1,1-dicyanomethylene-3-indanone (IC or IC-4F), are prepared and utilized in organic solar cells (OSCs). The two new molecules reveal planar structures and strong aggregation behavior, and fluorination is shown to red-shift the optical band gap and downshift energy levels. OSCs based on DTCCIC-4F exhibit a power conversion efficiency of 12.6%, much higher than that of DTCCIC-based devices (6.2%). Microstructural studies reveal that while both acceptors are highly crystalline, bulk heterojunction blends based on the nonfluorinated DTCCIC result in overly coarse domains, while blends based on the fluorinated DTCCIC-4F exhibit a more optimal nanoscale morphology. These results highlight the importance of end group fluorination in controlling molecular aggregation and miscibility.

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs), whereas undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which causes serious hysteresis of PSCs after aging. This issue is, however, rarely studied. The stability study for 3D/2D hybrid systems to date is all under the one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long-chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, a large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in an ambient environment, even though the reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations, K + and Rb + , are introduced into the 2D perovskite to exquisitely modulate the crystal formation, which gives rise to a higher crystallinity of 2D perovskite and a better film morphology with fewer defects. We achieved PCE beyond 21% due to the preferable charge transfer process and reduced nonradiative recombination losses. The structural features also bring about impressive moisture stability, which results in the corresponding PSCs retaining 93% of its initial PCE and negligible hysteresis after aging in an ambient atmosphere for 1200 h.

Interface engineering, especially the realization of Ohmic contacts at the interface between organic semiconductors and metal contacts, is one of the essential preconditions to achieve high-efficiency organic electronic devices. Here, the interface structures of polymer/fullerene blends are correlated with the charge extraction/injection properties of working organic solar cells. The model system-poly(3-hexylthiophene) (P3HT):phenyl-C 61 -butyric acid methyl ester (PCBM)-is fabricated using two different degrees of P3HT regioregularity to alter the blend interchain order and molecular packing, resulting in different device performances. Investigations by electroabsorption spectroscopy on these devices indicate a significant reduction (≈1 V) in the built-in potential with an increase in the P3HT regioregularity. This observation is also supported by a change in the work function (WF) of high regioregular polymer blends from photoelectron spectroscopy measurements. These results confirm the presence of a strong dipole layer acting as a δ-hole-transporting layer at the polymer/MoO 3 /Ag electrode interface. Unipolar hole-only devices show an increase in the magnitude of the hole current in high regioregular P3HT devices, suggesting an increase in the hole injection/extraction efficiency inside the device with a δ-hole-transporting layer. Microscopically, near-edge X-ray absorption fine structure spectroscopy was conducted to probe the surface microstructure in these blends, finding a highly edge-on orientation of P3HT chains in blends made with high regioregular P3HT. This edge-on orientation of P3HT chains at the interface results in a layer of oriented alkyl side chains capping the surface, which favors the formation of a dipole layer at the polymer/MoO 3 interface. The increase in the charge extraction efficiency due to the formation of a δ-hole-transporting layer thus results in higher short circuit currents and fill factor values, eventually increasing the device efficiency in high regioregular P3HT devices despite a slight decrease in cell open circuit voltage. These findings emphasize the significance of WF control as a tool for improved device performance and pave the way toward interfacial optimization based on the modulation of fundamental polymer properties, such as polymer regioregularity.

Development of high-performance organic thermoelectric (TE) materials is of vital importance for flexible power generation and solid-cooling applications. Demonstrated here is the significant enhancement in TE performance of selenium-substituted diketopyrrolopyrrole (DPP) derivatives. Along with strong intermolecular interactions and high Hall mobilities of 1.0-2.3 cm 2  V -1  s -1 in doping-states for polymers, PDPPSe-12 exhibits a maximum power factor and ZT of up to 364 μW m -1  K -2 and 0.25, respectively. The performance is more than twice that of the sulfur-based DPP derivative and represents the highest value for p-type organic thermoelectric materials based on high-mobility polymers. These results reveal that selenium substitution can serve as a powerful strategy towards rationally designed thermoelectric polymers with state-of-the-art performances., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Hybrid organic-inorganic perovskites with appealing optoelectronic properties have attracted significant interest for photovoltaic application. The use of chloride (Cl - )-containing species to induce improved perovskite thin-film microstructures and improved optoelectronic properties is well-established. However, the mechanism for the formation of perovskite films with highly textured, micron-sized grains in the presence of Cl - is not well established. Using synchrotron-based in situ two-dimensional grazing incidence wide-angle X-ray scattering complemented by scanning electron microscopy imaging, we present an oriented attachment mechanism via mineral bridge formation for the microstructural evolution of perovskite films post-treated with methylammonium chloride. We have identified the crucial role of the chlorine-containing intermediate phase as the mineral bridge, which enables the reorientation of primary, nanoscale perovskite grains followed by fusion into uniaxial oriented quasi-single crystal grains. The resulting perovskite films exhibit micron-sized grains with preferential orientation of the tetragonal (110) direction perpendicular to the substrate, resulting in improved solar cell efficiency attributed to improved charge collection. Our findings help to understand the fundamental mechanisms of microstructure evolution via soft processing in hybrid perovskite films.

Parent isoquinoline diimide (IQDI) and phthalazine diimide (PTDI), as two new heterocyclic analogues of naphthalene diimides (NDIs), have been synthesized through an oxidative strategy in 35-79 % yield. X-ray crystallography has been used to support the formation of IQDI, which also show fluorescence quantum yields of 3.5 %. The electrochemical and electrical properties of these molecules have been studied. The electrochemical results show an interesting trend in first reduction potential PTDIg ) leading to the trend IQDI

We demonstrate that the δ to α phase transition temperature of formamidinium-based perovskites is reduced by ∼50 °C through the incorporation of ∼2 wt% γ-butyrolactone (GBL) into the crystal lattice. The intercalation of GBL is found to expand the unit cell of the δ-phase, reducing the energy barrier for thermal conversion.

We report two newly synthesized naphthalene diimide (NDI)-based conjugated polymers, poly[( E )-2,7-bis(2-decyltetradecyl)benzo[ lmn ][3,8]phenanthroline-1,3,6,8(2 H ,7 H )-tetraone-vinylene-thiophene-vinylene] (PNDI-VTV) and poly[( E )-2,7-bis(2-decyltetradecyl)benzo[ lmn ][3,8]phenanthroline-1,3,6,8(2 H ,7 H )-tetraone-vinylene-selenophene-vinylene] (PNDI-VSV) with different donor units as electron-transporting organic semiconductors for organic field-effect transistors (OFETs). Furthermore, we study the effect of vinylene position on electron transport in the NDI polymers by using two similar polymers but with thiophene-vinylene-thiophene (PNDI-TVT) instead of vinylene-thiophene-vinylene or selenophene-vinylene-selenophene (PNDI-SVS) instead of vinylene-selenophene-vinylene. By incorporating vinylene between thiophene (or selenophene) units, the resulting NDI-based polymers PNDI-VTV and PNDI-VSV show larger backbone planarity than PNDI-TVT and PNDI-SVS. The polymers with a shorter acceptor monomer unit (PNDI-VTV and PNDI-VSV) show a strong face-on orientation, whereas those with a longer monomer unit (PNDI-TVT and SVS) exhibit a mixed face-on and edge-on orientation by two-dimensional grazing incidence X-ray diffraction. Optimized PNDI-VTV and PNDI-VSV OFETs exhibit electron mobilities of 0.043 and 0.7 cm 2 /(V·s), which is quite lower than those of PNDI-TVT and PNDI-SVS. In addition, the activation energies for electron transport of PNDI-VTV and PNDI-VSV were larger than those of PNDI-TVT and PNDI-SVS. Overall, this research provides the insight that the molecular alignment on the substrate can be controlled by the sequence of rigid acceptor monomer molecules for improving the electron transport of NDI polymers.

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process.

Chirality, as a fundamental symmetry property, plays an important role in molecular assembly in the solid state, impacting upon the properties and performance of organic materials. Here, heterochiral assembly was observed upon a binaphthol-based axially chiral material in the thin film state, where the heterochiral assemblies of racemic mixtures exhibit superior crystallization behavior and film morphologies than their homochiral counterparts. Additionally, a dramatic increase (nearly 2 orders of magnitudes) in electronic mobility was obtained upon switching the active layers of organic thin-film transistors from homochiral assemblies to heterochiral assemblies. This work not only gives insights into the structure-aggregation property relationships of axially chiral self-assemblies but also offers new opportunities for novel organic soft materials.