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This study is jointly supported by MOST (2017YFA0204702, 2018YFA0208504,) and NSFC (52073016, 51773207, 51973031, 21905018, 21905158) of China. This work was further supported by the Fundamental Research Funds for the Central Universities (buctrc201828, XK1802-2), Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm202043), Jiangxi Provincial Department of Science and Technology (No. 20192ACB20009) and Hong Kong Scholars Program (Grant No. XJ2020051). The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). A.C.Y.L. acknowledges support from the Australian Research Council (FT180100594). The authors acknowledge the use of instruments and assistance at the Monash Ramaciotti Centre for Cryo-Electron Microscopy, a Node of Microscopy Australia, including the Titan Krios (Australian Research Council LE120100090). This work was performed in part at the SAXS/WAXS beamtline at the Australian Synchrotron, part of ANSTO. Z. T. acknowledges the Shanghai Pujiang Program (Grant No. 19PJ1400500).

As a new class of lithium rich cathodes, disordered rock-salt cathodes have been of primary interest, because of their ability to deliver a promisingly high capacity up to 300 mA h/g. Nevertheless, some fundamental issues are yet to be fully understood and a comprehensive mastering of their solid-state chemistry, kinetics and thermal stability is required. Here, we select a high capacity cation-disordered positive electrode- Li1.2Ni0.4Nb0.4O2 as a model compound to study intrinsic reaction mechanism, including charge compensation mechanism, kinetics, thermal stability, and structural evolution. By combining soft and hard X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) with operando and ex-situ differential scanning calorimetry (DSC), galvanostatic intermittent titration technique (GITT), cyclic voltammetry (CV), and X-ray diffraction (XRD), we present holistic information on disordered rock-salt cathode. This work provides beneficial insights into designing and tailoring new positive electrodes with disordered rock-salt structure.

The molecular characteristics, stacking structure, and magnetic properties of Tb-phthalocyanine (TbPc2) with and without the absorption of CH2Cl2 molecules were systematically investigated. The fir...

Typically, conjugated polymers are composed of conjugated backbones and alkyl side chains. In this contribution, a cost-effective strategy of tailoring the length of alkyl side chain is utilized to design high-performing thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors PBDT-BiTPD(Cχ) (χ = 48, 52, 56), in which χ represents the alkyl side chain length in term of the total carbon number. A combination of light absorption, device, and morphology examinations make clear that the shorter alkyl side chains yield (i) higher crystallinity and more predominant face-on crystallite orientation in their neat and BHJ blend films, (ii) higher charge mobilities (6.7 × 10−4 cm2 V−1 s−1 for C48 vs. 3.2 × 10−4 cm2 V−1 s−1 for C56), and negligible charge recombination, consequently, (iii) significantly improved fill-factor (FF) and short current (JSC), while almost the same open circuit voltage (VOC) of ca. 0.82 V in their corresponding BHJ devices. In parallel, as alkyl side chain lengths decrease from C56 to C48, power conversion efficiencies (PCEs) increased from 7.8% for C56 to 11.1% for C52, and further to 14.1% for C48 in their BHJ solar cells made with a narrow bandgap non-fullerene acceptor Y6. This systematic study declares that shortening the side chain, if providing appropriate solubility in device solution processing solvents, is of essential significance for developing high-performing polymer donors and further improving device photovoltaic performance.

Molecular template growth is an efficient approach to enhance the crystallinity of organic semiconductor materials deposited by physical vapor deposition methods. Lattice matching, large grain size...

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.

Summary The development of low-cost and efficient hole transport materials (HTMs) is important for the commercialization of perovskite solar cells (PSCs). Comparing with the widely studied D-A-D and D-π-D linear-type small molecule HTMs, DTB-FL with a D-A-π-A-D molecular design is proposed, featuring facile synthesis and excellent optoelectronic properties. Moreover, the HTM with efficient surface passivation effects and proper energy level alignment at the hole extraction interface effectively inhibits recombination loss and improves the charge collection property. As a result, the champion efficiencies of 21.5% and 19.6% for active areas of 0.09 and 1.0 cm2, respectively, with superior operational stability are achieved by using DTB-FL HTM. In addition, DTB-FL can also be used as efficient HTM for all-inorganic PSCs, producing an impressive PCE of 17.0% with a high Voc of 1.30 V. These results underscore the promising potential of the D-A-π-A-D molecular design in preparing low-cost dopant-free HTMs toward stable and efficient PSCs.

Funder: China Scholarship Council; Id: http://dx.doi.org/10.13039/501100004543, Funder: Royal Society Newton International Fellowship, Heavy heteroatom substitution of the backbone is an effective strategy to improve molecular packing and charge delocalization in polymer semiconductors. Such a backbone modification also facilitates oxidative doping as a result of reduced ionization potential (IP). Here, the effect of single‐atom selenium substitution on doping and charge transport properties of a class of polythiophene copolymers is explored. The room temperature (RT) conductivities of the doped polymers are significantly enhanced by the selenium substitution for both molecular doping and ion exchange doping. The enhanced conduction is rationalized by the better crystallinity of the selenium‐containing system, which can be reinforced by a chain‐extended ribbon‐phase morphology induced by thermal annealing, which is robust toward doping. The resulting increase in the charge delocalization of the doped selenium‐containing system is evidenced by temperature‐dependent conductivities. In ion exchange doped films the maximum conductivity of ≈700 S cm−1 and a high thermoelectric (TE) power factor (PF) of 46.5 μW m−1 K−2 is achieved for the doped selenophene polymer and signatures of a metal‐insulator (M–I) transition are observed that are characteristics for heterogeneous conduction systems. The results show that single‐atom selenium substitution is an effective molecular design approach for improving the charge transport and TE properties of conjugated polymers.

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-1H-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.

The nanomorphology of bulk heterojunction (BHJ) blends based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediy...

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π–π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset., Y6, as a non-fullerene acceptor for organic solar cells, has attracted intensive attention because of the low voltage loss and high charge generation efficiency. Here, Zhang et al. find that the delocalization of exciton and electron wavefunction due to strong π-π packing of Y6 is the key for the high performance.

The field of all-polymer solar cells (all-PSCs) has experienced rapid development during the past few years, mainly driven by the development of efficient polymer acceptors. However, the power conversion efficiencies (PCEs) of the all-PSCs are still limited by insufficient light absorption of the donor/acceptor blend and large energy loss in devices. We herein designed a polymer acceptor PYT1 constructed n-type molecular acceptor Y5-C20 as the key building block and blended it with a polymer donor PM6 to obtain an all-polymer photoactive layer. The optimized PM6:PYT1 all-PSCs achieved a record higher PCE of 13.43% with a very low energy loss of 0.47 eV and a photoresponse of up to 900 nm compared with the Y5-C20 based device with a best PCE of 9.42%. Furthermore, the PCEs of the PM6:PYT1 all-PSCs are relatively insensitive to the 1-chloronaphthalene (CN) additive contents and active layer thickness. Our results also highlight the effect of CN additive on PM6:PYT1 morphology, i.e. , charge generation, and transport find an optimized balance, and radiative and non-radiative loss is simultaneously reduced in the blend. This work promotes the development of high-performance polymer acceptors and heralds a brighter future of all-PSCs for commercial applications.

The vertical component distribution of bulk heterojunction (BHJ) active film shows a significant impact on determining the device performance in polymer solar cells (PSCs). Processing solvent additives are well known for regulating the BHJ active layer morphology; however, there are few reports regarding the quantitative evaluation of the effect. Herein, a study of the quantitative determination of the vertical segregation in combination of molecular ordering of PBDB-T/ITIC blend films with various 1,8-diiodooctane (DIO) contents is provided. A 0.5% (volume ratio) DIO-added blend film achieves the highest power conversion efficiency of 10.75%. The reduced performance of the PSCs resulted from the excessive vertical component segregation and overcrystallization investigated by various techniques. X-ray photoelectron spectroscopy indicates that DIO aggravates the PBDB-T enrichment region at the air side. Neutron reflectivity further quantitatively figures out the phase separation effect. Although increased crystallinity of ITIC and a higher face-on ratio of PBDB-T in active layer were obtained with increased DIO content approved by grazing-incidence wide-angle X-ray scattering (GIWAXS), the enhanced vertical distribution along with the enhanced crystal size of ITIC leads to the reduced performance of the PSCs due to the reduced carrier transportation paths between donor and acceptor.

The emergence of the latest generation of small-molecule acceptor (SMA) materials, with Y6 as a typical example, accounts for the surge in device performance for organic solar cells (OSCs). This study proposes two new acceptors named Y6-C2 and Y6-C3, from judicious alteration of alkyl-chains branching positions away from the Y6 backbone. Compared to the Y6, the Y6-C2 exhibits similar optical and electrochemical properties, but better molecular packing and enhanced crystallinity. In contrast, the Y6-C3 shows a significant blue-shift absorption in the solid state relative to the Y6 and Y6-C2. The as-cast PM6:Y6-C2-based OSC yields a higher power conversion efficiency (PCE) of 15.89% than those based on the Y6 (15.24%) and Y6-C3 (13.76%), representing the highest known value for as-cast nonfullerene OSCs. Prominently, the Y6-C2 displays a good compatibility with the PC71BM. Therefore, a ternary OSC device based on PM6:Y6-C2:PC71BM (1.0:1.0:0.2) was produced, and it exhibits an outstanding PCE of 17.06% and an impressive fill factor (FF) of 0.772. Our results improve understanding of the structure-property relationship for state-of-the-art SMAs and demonstrate that modulating the structure of SMAs via fine-tuning of alkyl-chains branching positions is an effective method to enhance their performance.

State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer-fullerene BHJ solar cells in terms of efficiency, thermal-, and photostability. Considering the strong light absorption in the near-infrared region (600-1000 nm) for most of the efficient acceptors, the exploration of high-performing large band gap (LBG) polymer donors with complementary optical absorption ranging from 400 to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (-F) and unsaturated nitrogen (-N) substituents along the polymer backbones is used to develop the LBG polymer donor PB[N][F]. Results show that the F- and N-substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with an efficiency of only 3.6% and PB[N] with an efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport, and reduce bimolecular recombination and trap-assisted recombination in the PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F- and N-substituted conjugated polymers are promising electron-donor materials for solution-processed non-fullerene BHJ solar cells.

With recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) based on novel donor and non-fullerene acceptor (NFAs), improving the stability of these systems has become the most important issue for their practical applications. Herein, an efficient and highly stable OSC, containing a novel polymer donor and a non-fullerene acceptor system, is reported. The OSC is based on an inverted device structure that utilizes a self-assembled fullerene monolayer (C60-SAM) as the cathode modification layer, and an efficient and highly stable OSC composes of a polymer donor of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-3-fluorothie-no[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and a non-fullerene acceptor of (2,2′-((2Z,2′Z)-(((4,4,9,9-Tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene −2,1-diylidene))dimalononitrile) (IEICO-4F) is presented, showing a PCE of 10%. It further achieves an extrapolated T80 lifetime (the time required to reach 80% of initial performance) of 34,000 h, operating under one sun illumination equivalent. Based on an estimated solar irradiance of 1500 kWh/(m2 year) for China, a potential lifetime of 22 years is inferred for the OSC. Further investigation reveals that the reported C60-SAM modification stabilizes the OSC active layer morphology by lowering the surface energy of the underlying ZnO electron transport layer and suppressing trap-assisted recombination, thereby improving photostability. The results of this work establish important guidelines for the development of non-fullerene based OSCs with enhanced stability and pave the way for the commercialization of OSC technology.

Solution-processed nonfullerene bulk-heterojunction (BHJ) polymer solar cells (PSCs), which are composed of polymer donors and organic acceptors, are proven to manifest promising performance and lo...

Two indenothiophene-based non-fullerene small molecule acceptors (NFSMAs), FTBT and FTTBT, were designed and synthesized to investigate the photovoltaic effect of fusing a thiophene into the core of NFSMAs. Compared with the none-fused FTBT, the thiophene-fused FTTBT achieves a much higher power conversion efficiency (PCE) of 9.79% with a Voc of 0.934 V, a Jsc of 16.01 mA cm−2 and an FF of 65.49%, when it was blended with PM6 polymer donor to fabricate bulk-heterojunction solar cells. Combined photophysical, electrochemical, photovoltaic property and morphology analysis indicates that the boosted device performance mainly lies in two reasons: (i) the incorporation of an electron-donating thiophene ring narrows the optical bandgap by extending π-conjugation, which contributes to a large short-circuit current; (ii) the incorporation of a single thiophene ring transforms the axisymmetrical molecular configuration into a centrosymmetrical one, which decreases the crystallinity and optimizes the packing feature in the blend. It results in a transport-favorable blending morphology and contributes to a high fill factor. The work clarifies an effective molecular design strategy for performance enhancement of organic solar cells.

Summary Flexible organic solar cells (OSCs) with high power conversion efficiency (PCE) and excellent mechanical properties are considered a promising power source for wearable electronic devices. However, simultaneously achieving high efficiency and robust mechanical stability is still challenging because highly crystalline or aggregated microstructures that are thought to be critical for enabling efficient device operation render the active layer brittle. In this study, we demonstrate 3-μm-thick ultraflexible OSCs by utilizing a mixed fullerene/non-fullerene acceptor that can achieve an efficiency of 13% (certified value of 12.3%) with 97% retention in the PCE after 1,000 bending cycles (bending radius of 0.5 mm). In addition, although ultraflexible OSCs cannot survive under the intrinsic tensile test with a large strain, they exhibit excellent mechanical behavior under the cyclic compression-stretching test via the formation of a buckling device structure, yielding an 89% retention in the PCE after 1,000 cycles (45% compression and bending radius of 10 μm). A facile approach introducing a small amount of high-electron-mobility fullerene acceptor into a non-fullerene binary blend enhances charge transport, improves exciton separation, and optimizes the blend morphology with more amorphous regions, thus producing a more efficient and mechanically robust device.

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.

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(2H,7H)-tetraone-vinylene-thiophene-viny...

In recent years, solution processed bulk heterojunction (BHJ) organic solar cells (OSCs) have been intensively studied. The widely used fullerene derivatives as acceptors, (e.g. [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM)), widely used, display a limited visible light absorption and unsatisfactory thermal stability in combination with amounts of donor materials. It was found that rational design rules for novel donors and introduced analysis of non-fullerene acceptors (NFAs) are employed to develop novel systems overcoming these limitations. Here we reported a new small molecule donor, W-CN, containing dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) donor unit and 2-ethylhexyl-cyanoacetate (CN) acceptor unit with thiophene-thieno[3,2-b]thiophene (T-TT) bridge. We chose 2,2'-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC), 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis (4-hexylphenyl)-dithieno[2,3-d:2′, 3′-d']-s-indaceno[1,2-b:5,6-b']dithiophene) (ITIC) and PC70BM as the acceptors. Furthermore, we demonstrated all-small molecule solar cells (all-SMSCs) based on W-CN:IDIC and W-CN:ITIC composites with decent performance with the power conversion efficiencies (PCEs) of 5.26% and 3.89%, respectively. In contrast, W-CN:PC70BM solar cell shows a slightly higher PCE of 5.94% as compared to the investigated NFA systems. Nevertheless, W-CN:IDIC and W-CN:ITIC systems exhibit better thermal stability as compared to the W-CN:PC70BM system. Notably, solar cells based on W-CN:IDIC system maintain almost 96% initial performance after baking at 100 °C for 24 h under inert atmosphere. The highly promising findings demonstrated the importance of novel structure-property relationships for the rational design strategies of photovoltaic materials and suitable selection of acceptors for high performance OSCs.

The determination of the onset temperature for molecular reorganization in conjugated polymer thin films is highly relevant for understanding microstructural evolution and device optimization. In s...

Polymeric semiconductors exhibit exceptionally long spin lifetimes, and recently observed micrometre spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin–orbit and hyperfine interactions lie at the origin of their long spin lifetimes, but the coupling mechanism of a spin to its environment remains elusive. Here, we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges’ hopping motion at low temperatures, whereas an Elliott–Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime, charge, spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains. The long spin lifetimes observed in polymeric semiconductors hold promise for potential applications. A careful study untangles the main mechanism behind them.

Summary Molecular Kekule diradicals have been demonstrated to show unique optoelectronic properties as a function of their diradical character. A series of thienoquinoidal oligothiophenes from dimer to pentamer and substituted with an odd and even number of pyrrolo-dione groups have been prepared and proven to be n-dopable materials showing outstanding ambient stability and excellent electrical and thermoelectric behavior. Going from dimer to pentamer, a progressive change in the diradical character and aggregation mode is observed, with the tetramer showing an optimal diradical character that allows favorable intermolecular contacts with π−π multi-bonding features, while the presence of the two dione groups promotes a cholesteric-like π−π stacking. Both features synergistically contribute to form a material with exceptional ambient stability for an n-doped system exhibiting high electrical conductivities and thermoelectric performance.

Layer-by-layer (LbL) solution processing is a cost-effective technology for the large-scale fabrication of organic solar cells (OSCs). In this work, LbL OSCs were fabricated using PTQ10/J71 as donors and ITC6-IC/IDIC/MeIC/ITCPTC/ITIC as non-fullerene acceptors (NFAs) without using orthogonal solvents and appropriate co-solvents. Compared with traditional bulk heterojunction (BHJ) OSCs, the corresponding solution-processed LbL devices exhibited higher or comparable power conversion efficiencies (PCEs), which had the advantages of reduced energy loss, stronger absorption spectra, better vertical phase separation, partially increased charge transport property and charge collection efficiency. Furthermore, taking the J71/ITC6-IC and PTQ10/IDIC LbL systems as examples, we fabricated large-area LbL OSCs using the doctor-blading process, which is closer to the roll-to-roll (R2R) technology. Importantly, both OSCs based on J71/ITC6-IC and PTQ10/IDIC LbL with an active area of 1.00 cm2 demonstrated encouraging PCEs of over 10%, which is the record efficiency for large-area LbL OSCs reported in the literature to date. Our work indicates that the solution-processed LbL approach not only presents good generality and high device performance, but also is a superior alternative to the BHJ method for the initial evaluation of photovoltaic materials and the industrial production of R2R OSCs.

Although great advances have been achieved in the field of organic solar cells (OSCs) with the rapid development of nonfullerene acceptors (NFAs), devices with simultaneous high power conversion efficiency (PCE) and thermal stability remain a big challenge, particular small molecule NFA-based devices. In this paper, we designed and synthesized a class of pyrene-fused perylene diimides (PDIs) for fabricating ternary OSCs, which exhibited much better device performance than those of the parent PDI counterpart based devices due to the better miscibility and crystallization of the ternary films. Excitingly, Th-PYPDI and O-PYPDI based ternary OSCs showed improved thermal stability, as a result of the better morphology stability under the same annealing conditions. Notably, the O-PYPDI based ternary device exhibited the best PCE of 10.96% and good thermal stability with the PCE remaining at 82% of the initial value after thermal annealing at 80 °C for 72 h.

Realizing the triple functions of simultaneously improved open circuit voltage (Voc), short circuit current density (Jsc) and fill factor (FF) enabled by material design is a vital challenge for achieving efficient and stable polymer solar cells (PSCs). Herein, we developed a new wide-bandgap donor–acceptor (D–A) copolymer, J101, with a down-shifted highest occupied molecular orbital (HOMO) level by chlorine substitution on its 2-alkyl-benzo[d][1,2,3]triazole (BTz) unit. The PSCs fabricated by combining the J101 donor with the indenoindene-containing fused-ring electron acceptor ZITI demonstrated a remarkable power conversion efficiency (PCE) of 14.43% with a high Voc of 0.937 V, a high Jsc of 21.25 mA cm−2 and a high FF of 72.48%, benefitting from the low HOMO level of the donor, suitable nanoscale morphology, efficient charge transport properties and reduced recombination losses. Furthermore, semitransparent PSCs based on the optimized J101:ZITI blend exhibited the best PCE of 11.04% with an average visible transmittance (AVT) of 21.69%. This work demonstrates that the chlorine substituted BTz unit is an excellent electron-accepting building block for high-performance opaque and semitransparent PSCs.

Naphthalene diimide (NDI) with tertiary amine side chains is used to n-dope a series of NDI derivatives of varying energy levels. We demonstrate a photoinduced, intermolecular redox-doping process in which a dimethylpropyl amine side chain attached to one NDI reduces another NDI derivative to form radical anions. The influence of the aromatic core substituents on energy levels, doping efficacy and radical anion stability is studied by cyclic voltammetry, UV-Vis and electron paramagnetic resonance (EPR) spectroscopy. In general, the HOMO energy level of the NDI is responsible for the doping process and the LUMO for air stability of the resulting radical anion. The most electron deficient NDI derivative having two cyano substituents displays the highest doping yield and yields air stable radical anions for both light- and thermally-induced doping. Thermal doping is further accompanied by morphologic changes that stabilize radical anions in air.

Chiral alkyl chains are widely utilized to ensure the solubility of solution-processed organic semiconductors (OSCs), while intrinsic defects due to the mixture of several stereoisomers in one material are frequently neglected. Herein, through introducing an optically pure pendant into the molecular backbone of a core-expanded naphthalene diimide (NDI-DTYM2), enantiopure semiconductor materials (1-R/1-S) and the corresponding racemate (1-rac) were designed and synthesized to investigate the impact of enantiopure and racemic OSCs on charge transport in organic field-effect transistors (OFETs). Surprisingly, a 2–4 times increase in electron mobility (from 0.15 cm2 V−1 s−1 to 0.6 cm2 V−1 s−1 for as-cast devices and from 0.42 cm2 V−1 s−1 to 0.98 cm2 V−1 s−1 for thermally-annealed devices) was discovered in solution-processed OFETs simply by switching the active layer material from racemic to the enantiopure form. Grazing-incidence wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM) and were utilized to investigate the crystallization, molecular micro-organization and film morphology of 1-R, 1-S and 1-rac, demonstrating that the lower mobility of racemic material (1-rac) might be attributed to interfacial defects among different crystalline domains as well as subtle changes in molecular packing.

Sidechain engineering of the benzodithiophene-based high-performance organic solar cell (OSC) material BTR generated four novel p-type small-molecule semiconducting materials to investigate structure–property relationships in p-type organic semiconductors. Through depth-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) studies, it is found that our best performing materials possess an active-layer morphology comprised of a vertically graded structure with different crystallite orientations. We further establish a correlation between a high solubility in the spin-coating deposition solution and the occurrence of this vertical distribution, whereas materials with lower solubility are found to possess a predominantly edge-on molecular orientation. These results further the understanding of this high-performing class of materials and suggest the importance of solubility considerations to OSC morphology.

Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,1,2,3 as well as fabrication-induced defects.4,5 However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.6,7 In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S8). We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.1,9,10 The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.11 It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage., Comment: The following article has been accepted by APL Materials. After it is published, it will be found at https://doi.org/10.1063/5.0044785

The low dielectric constant of organic semiconductors limits the efficiency of organic solar cells (OSCs). In an attempt to improve the dielectric constant of conjugated polymers, we report the synthesis of three semiconducting copolymers by combining the thiophene-substituted diketopyrrolopyrrole (TDPP) monomer with three different monomeric units with varying electron donating/accepting strengths: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP), and naphthalene diimide (P(gNDI-TDPP)). Among the series, BBT-3TEG-TDPP and P(gNDI-TDPP) exhibited the highest dielectric constants (∼5) at 1 MHz frequency, signifying the contribution of dipolar polarization from TEG side-chains. Furthermore, transient absorption spectroscopic studies performed on these polymers indicated low exciton diffusion length as observed in common organic semiconducting polymers. Our findings suggest that utilizing the polar side-chains enhances the dielectric constant in a frequency regime of megahertz. However, it is not sufficient to reduce the Coulombic interaction between hole and electron in excitonic solar cells.

The charge transport–limiting factors in conjugated polymers without single-bond linkages in the backbone have been identified., 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 cm2 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.

An efficient photogeneration of free charge carriers has long been recognized as the paramount challenge in organic photovoltaic (OPV) devices. The low dielectric constant organic semiconductors fall short to reduce strong Coulombic interaction of tightly bound exciton and hence lead to a loss mechanism in OPVs due to charge-carrier recombination. To circumvent this problem, we adopt a strategy to enhance the dielectric constant of organic semiconductors by incorporating tetraethyleneglycol (TEG) side-chains. We report synthesis of three new semiconducting copolymers by combining thiophene substituted diketopyrrolopyrrole (TDPP) monomer with three other monomeric units with varying electron donating strength: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP) and naphthalene diimide (PNDITEG-TDPP). BBT-3TEG-TDPP and PNDITEG-TDPP showed highest dielectric constants (~ 5) at 1MHz frequency suggesting efficient contribution of dipolar polarization from TEG side-chains. To understand the electronic contribution of the polymer backbone and the polarity of TEG side-chains, and the resulting enhancement of the dielectric constant, we further performed first-principles density functional theory calculations. Single-component organic solar cells (OSC) fabricated utilizing these polymers resulted in poor performance which is attributed to the absence of free charge generation. Furthermore, transient absorption spectroscopy studies show low exciton diffusion length as observed in donor-acceptor type conjugated polymers. Our results suggest that, the strategy of enhancing dielectric constant with polar side-chains is not sufficient to reduce Coulombic interaction between hole and electron in OSCs.

Ternary blending of light-harvesting materials has been proven to be a potential strategy to improve the efficiency of solution-processed organic solar cells (OSCs). However, the optimization of a ternary system is usually more complicated than that of a binary one as the morphology of conventional ternary blend films is very difficult to control, thus undermining the potential of ternary OSCs. Herein, we report a general strategy for better control of the morphology of ternary blend films composed of a polymer donor and two nonfullerene small-molecule acceptors for high-performance OSCs using the sequential layer-by-layer (LbL) deposition method. The resulting LbL films form a bicontinuous interpenetrating network structure with high crystallinity of both the donor and acceptor materials, showing efficient charge generation, transport, and collection properties. In addition, the power conversion efficiencies (PCEs) of the ternary LbL OSCs are less sensitive to the blending ratio of the third component acceptor, providing more room to optimize the device performance. As a result, optimal PCEs of over 11, 13, and 16% were achieved for the LbL OSCs composed of PffBT4T-2OD/IEICO-4F:FBR, PBDB-T-SF/IT-4F:FBR, and PM6/Y6:FBR, respectively. Our work provides useful and general guidelines for the development of more efficient ternary OSCs with better controlled morphology.

Two fused ladder type non-fullerene acceptors, DTCCIC and DTCCIC-4F, based on an electron-donating alkylated dithienocyclopentacarbazole core flanked by electron-withdrawing non-fluorinated 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 down shift 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 non-fluorinated 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.