Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials

Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

Dithieno[3,2-b:2',3'-d]pyrrol-Fused Asymmetrical Electron Acceptors: A Study into the Effects of Nitrogen-Functionalization on Reducing Nonradiative Recombination Loss and Dipole Moment on Morphology

Energy loss (E loss) consisting of radiative recombination loss (ΔE 1 and ΔE 2) and nonradiative recombination loss (ΔE 3) is considered as an important factor for organic solar cells (OSCs). Herein, two N-functionalized asymmetrical small molecule acceptors (SMAs), namely N7IT and N8IT are designed and synthesized, to explore the effect of N on reducing E loss with sulfur (S) as a comparison. N7IT-based OSCs achieve not only a higher PCE (13.8%), but also a much lower E loss (0.57 eV) than those of the analogue (a-IT)-based OSCs (PCE of 11.5% and E loss of 0.72 eV), which are mainly attributed to N7IT's significantly enhanced charge carrier density (promoting J SC) and largely suppressed nonradiative E loss by over 0.1 eV (enhancing V OC). In comparison, N8IT, with an extended π-conjugated length, shows relatively lower photovoltaic performance than N7IT (but higher than a-IT) due to the less favorable morphology caused by the excessively large dipole moment of the asymmetrical molecule. Finally, this work sheds light on the structure-property relationship of the N-functionalization, particularly on its effects on reducing the E loss, which could inspire the community to design and synthesize more N-functionalized SMAs.

Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells

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.

π-Extension, Selenium Incorporation, and Trimerization: "Three in One" for Efficient Perylene Diimide Oligomer-Based Organic Solar Cells

Perylene diimide (PDI) and the vinylene-bridged helical PDI oligomers are versatile building blocks for constructing nonfullerene acceptors (NFAs). In this contribution, a benzene-cored star-shaped NFA, namely, TPDI2-Se, was designed and synthesized for organic solar cells (OSCs). The NFA with smaller π-conjugated blades, namely, TPDI-Se, was synthesized for comparison. Using the commercially available PTB7-Th as the electron donor, the best power conversion efficiency (PCE) of 3.62% was obtained for TPDI-Se-based OSCs. However, a much higher PCE of 8.59% was achieved for TPDI2-Se-based devices owing to the π-extension in the peripheral panels. Moreover, the photovoltaic performance of TPDI2-Se-based OSCs is also superior to those of the parent NFA TPDI2 (PCE of 7.84%)- and the blade moiety PDI2-Se (PCE of 6.61%)- based ones. Additionally, a remarkable short-circuit current (J_sc) value of 17.21 mA/cm² was obtained for TPDI2-Se-based OSCs, which is among the highest J_sc values reported in PDI-based OSCs. These results argue that the so-called "three in one" molecule design strategy of π-extension, selenium incorporation, and trimerization offers a robust approach to constructing high-performance PDI-based NFAs.

Fluorination Effect for Highly Conjugated Alternating Copolymers Involving Thienylenevinylene-Thiophene-Flanked Benzodithiophene and Benzothiadiazole Subunits in Photovoltaic Application

Two two-dimensional (2D) donor-acceptor (D-A) type conjugated polymers (CPs), namely, PBDT-TVT-BT and PBDT-TVT-FBT, in which two ((E)-(4,5-didecylthien-2-yl)vinyl)- 5-thien-2-yl (TVT) side chains were introduced into 4,8-position of benzo[1,2-b:4,5-b']dithiophene (BDT) to synthesize the highly conjugated electron-donating building block BDT-TVT, and benzothiadiazole (BT) and/or 5,6-difluoro-BT as electron-accepting unit, were designed to systematically ascertain the impact of fluorination on thermal stability, optoelectronic property, and photovoltaic performance. Both resultant copolymers exhibited the lower bandgap (1.60 ~ 1.69 eV) and deeper highest occupied molecular orbital energy level (EHOMO, -5.17 ~ -5.37 eV). It was found that the narrowed absorption, deepened EHOMO and weakened aggregation in solid film but had insignificant influence on thermal stability after fluorination in PBDT-TVT-FBT. Accordingly, a PBDT-TVT-FBT-based device yielded 16% increased power conversion efficiency (PCE) from 4.50% to 5.22%, benefited from synergistically elevated VOC, JSC, and FF, which was mainly originated from deepened EHOMO, increased μh, μe, and more balanced μh/μe ratio, higher exciton dissociation probability and improved microstructural morphology of the photoactive layer as a result of incorporating fluorine into the polymer backbone.

High-Performance Ambient-Condition-Processed Polymer Solar Cells and Organic Thin-Film Transistors

Large-scale commercial synthesis of bulk-heterojunction (BHJ) solar cell materials is very challenging and both time and energy consuming. Synthesis of π-conjugated polymers (CPs) with uniform batch-to-batch molecular weight and low dispersity is a key requirement for better reproducibility of high-efficiency polymer solar cells. Herein, a conjugated polymer (CP) PTB7-Th, well known for its high performance, has been synthesized with high molecular weight and low dispersity in a closed microwave reactor. The microwave reaction procedure is known to be more controlled and consumes less energy. The precursors were strategically reacted for different reaction time durations to obtain the optimum molecular weight. All different CPs were well characterized using ¹H NMR, gel permeation chromatography (GPC), UV-vis, photoluminescence (PL), electron spin resonance (ESR), and Raman spectroscopy, whereas the film morphology was extensively studied via atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD) techniques. The effect of molecular weight on a conventional BHJ solar cell with PC₇₁BM acceptor was investigated to derive systematic structure-property relationships. The CP obtained after 35 min of reaction time and integrated into BHJ devices under ambient conditions provided the best performance with a power conversion efficiency (PCE) of 8.09%, which was quite similar to the results of CPs synthesized via a thermal route. An enhanced PCE of 8.47% was obtained for the optimized polymer (35 min microwave reaction product) when device fabrication was carried out inside a glovebox. The organic thin-film transistor (OTFT) device with the microwave-synthesized CP displayed better hole mobility (0.137 cm² V^-1 s^-1) as compared to that with the thermally synthesized CP. This study also proved that the device stability and reproducibility of the microwave-synthesized CP were much better and more consistent than those of the thermally developed CP.

Synthesis of Phenanthro[9,10-c]thiophenes by 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-Promoted Cyclo-Oxidation

A new method to prepare phenanthro[9,10-c]thiophenes has been developed. In the presence of triflic acid, 3,4-diaryl thiophenes undergo 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-promoted cyclo-oxidation. NMR and computational studies indicate that protonation of the thiophene plays a key role in this reaction. The reaction can be used to prepare phenanthro[9,10-c]thiophene, as well as derivatives with alkyl, bromo, and methoxy substituents. However, the yields and selectivity of the reaction depend on both the nature and location of the substituents. Bis(3-methoxyphenyl)thiophene reacts under these conditions to give the desired product in 57 % yield, while bis(4-methoxyphenyl)thiophene gives no product. Bis(3-bromophenyl)thiophene did not react, but cyclo-oxidation of bis(4-bromophenyl)thiophene provides the desired product in 34 % yield.

Efficient Construction of Near-Infrared Absorption Donor-Acceptor Copolymers with and without Pt(II)-Incorporation toward Broadband Nonlinear Optical Materials

Organic nonlinear optical (NLO) materials have attracted immense scientific interest in various fields. Broadband NLO response extending to near-infrared (NIR) region is extremely important and remains challenging. Herein, two diketopyrrolopyrrole (DPP)-based donor-acceptor (D-A)-type π-conjugated copolymers with and without Pt(II) incorporation are rationally designed and synthesized toward broadband NLO response materials. The broad intramolecular charge transfer (ICT) absorption reaching 1000 nm due to the strong D-A interaction is well demonstrated by photophysical characterizations. The NLO properties of copolymers are studied using Z-scan technology. Owing to their extended π-conjugated D-A systems and near-infrared ICT absorption properties, both copolymers exhibit laser-induced NLO response to nanosecond as well as picosecond laser pulses upon the wavelengths of 532 and 1064 nm. Interestingly, introducing Pt(II) into the copolymer backbone can evidently improve the NLO property or unexpectedly switch the NLO response from saturable absorption to reverse saturable absorption. Meanwhile, both copolymers are successfully employed as optical limiting materials and exhibit broadband optical limiting abilities. Therefore, we present an efficient strategy toward broadband NLO materials, which may significantly facilitate the understanding of organic molecular structure-property relationship and promote their practical application.

Reduced Energy Loss in Non-Fullerene Organic Solar Cells with Isomeric Donor Polymers Containing Thiazole π-Spacers

Large energy loss is one of the key factors that limit the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we report reduced energy losses of OSCs via introducing thiazole π-spacers with different orientations to replace the thiophene π-spacers of the prototype polymer PBDB-T. The newly formed thiazole-containing isomeric polymers, PBDBTz-2 and PBDBTz-5, exhibited blue-shifted absorption and deeper lying energy levels compared to PBDB-T. When blended with IT-4F, the two polymers realized PCEs of 10.4% for PBDBTz-2 and 9.6% for PBDBTz-5, respectively, which were higher than that of PBDB-T (PCE = 9.3%). More critically, considerable open-circuit voltage (V_oc) enhancements were achieved by PBDBTz-2 and PBDBTz-5, which were 0.14 and 0.21 V higher than that of PBDB-T. A detailed analysis showed that the reduced energy loss resulted from the lower radiative recombination below the band gap and nonradiative recombination loss. This study demonstrated that the introduction of thiazole π-spacers with different orientations is effective to reduce the energy losses of OSCs, which provided valuable inspirations for the development of new conjugated polymers to the efficiency breakthrough of OSCs in future.

Synthesis and optoelectronic properties of benzodithiophene-based conjugated polymers with hydrogen bonding nucleobase side chain functionality

Nucleobase functionalities in conjugated, alternating copolymers participate in interbase hydrogen bonding, which promotes molecular assembly and organization in thin films and enhances optical and electronic properties.

Facile C–H iodination of electron deficient benzodithiophene-S,S-tetraoxide for the development of n-type polymers

A C–H iodination reaction of the electron-poor building block, benzo[1,2-b:4,5-b0]-dithiophene-1,1,5,5-tetraoxide has been reported. This facile functionalization has led to the development of two novel n-type donor–acceptor conjugated polymers.

Developments of Diketopyrrolopyrrole-Dye-Based Organic Semiconductors for a Wide Range of Applications in Electronics

In recent times, fused aromatic diketopyrrolopyrrole (DPP)-based functional semiconductors have attracted considerable attention in the developing field of organic electronics. Over the past few years, DPP-based semiconductors have demonstrated remarkable improvements in the performance of both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices due to the favorable features of the DPP unit, such as excellent planarity and better electron-withdrawing ability. Driven by this success, DPP-based materials are now being exploited in various other electronic devices including complementary circuits, memory devices, chemical sensors, photodetectors, perovskite solar cells, organic light-emitting diodes, and more. Recent developments in the use of DPP-based materials for a wide range of electronic devices are summarized, focusing on OFET, OPV, and newly developed devices with a discussion of device performance in terms of molecular engineering. Useful guidance for the design of future DPP-based materials and the exploration of more advanced applications is provided.

A mechanistic study of the activation of small molecules (H2 and C2H2) by group 14 analogues of selenophene

In this study, the reactivity influenced by group 14 elements (E = C, Si, Ge, Sn, and Pb), which are used as substituents in heterocyclic five-membered rings, was theoretically examined by using density functional theory (B3PW91/def2-SVP).

Narrow bandgap difluorobenzochalcogenadiazole-based polymers for high-performance organic thin-film transistors and polymer solar cells

A series of difluorobenzochalcogenadiazole-bithiophene copolymers are developed for high-performance organic semiconductors.

New fused conjugated molecules with fused thiophene and pyran units for organic electronic materials

Rigid and planar conjugated molecules have substantial significance due to their potential applications in organic electronics. Herein we report two highly fused ladder type conjugated molecules, TTCTTC and TTTCTTTC, with up to 10 fused rings in which the fused-thiophene rings are fused to the chromeno[6,5,4-def]chromene unit. Both molecules show high HOMO levels and accordingly they can be oxidized into their radical cations with absorptions extending to 1300 nm in the presence of trifluoroacetic acid. Thin films of TTCTTC and TTTCTTTC exhibit p-type semiconductor properties with hole mobilities up to 0.39 cm² V⁻¹ s⁻¹. Moreover, TTCTTC shows a high fluorescence quantum yield of up to 16.5% in the solid state.

Fused conjugated molecules TTCTTC and TTTCTTTC with up to ten heterocycles were constructed by fusing fused-thiophene to the chromeno[6,5,4-def]chromene unit.

Thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers for organic solar cells

Thieno[3,4-c]pyrrole-4,6-dione (TPD) based conjugated polymers as an electron donor, acceptor and single-component for application in organic solar cells in the past ten years have been intensively reviewed in this Feature Article.

Conjugated copolymers bearing 2,7-dithienylphenanthrene-9,10-dialkoxy units: highly soluble and stable deep-blue emissive materials

Excellent yields, high stability and solubility. M_w = 36.5–152.0 kDa and Đ = 2.5–3.0. Deep-blue emission with quantum yields up to 17%.

Indacenodithieno[3,2-b]thiophene-Based Wide Bandgap D-π-A Copolymer for Nonfullerene Organic Solar Cells

Herein, a wide-bandgap (2.02 eV) donor-π-acceptor (D-π-A) polymer PIDTT-DTffBTA, composed of a rigid indacenodithieno[3,2-b]thiophene (IDTT) and fluorinated benzo[d][1,2,3]triazole (ffBTA) units as D and A units, respectively, is synthesized. In comparison with its analogue benzodithiophene-alt-benzotriazole copolymer J52 with classic benzodithiophene (BDT) as the D unit, PIDTT-DTffBTA demonstrates a lower-lying HOMO energy level and higher carrier mobilities when paired with a nonfullerene acceptor (NFA) Y6 based on a ladder-type dithienothiophen[3.2-b]-pyrrolobenzothiadiazole central unit. Thus, PIDTT-DTffBTA:Y6 based organic solar cells (OSCs) exhibit an improved power conversion efficiency (PCE) of 11.05% than that of J52:Y6 (7.15%), which is also the highest value for IDTT-based photovoltaic polymers. This result proves that the IDTT unit is also a promising building block to construct not only NFAs but also p-type photovoltaic polymers.

A Narrow-Bandgap n-Type Polymer Semiconductor Enabling Efficient All-Polymer Solar Cells

Currently, n-type acceptors in high-performance all-polymer solar cells (all-PSCs) are dominated by imide-functionalized polymers, which typically show medium bandgap. Herein, a novel narrow-bandgap polymer, poly(5,6-dicyano-2,1,3-benzothiadiazole-alt-indacenodithiophene) (DCNBT-IDT), based on dicyanobenzothiadiazole without an imide group is reported. The strong electron-withdrawing cyano functionality enables DCNBT-IDT with n-type character and, more importantly, alleviates the steric hindrance associated with typical imide groups. Compared to the benchmark poly(naphthalene diimide-alt-bithiophene) (N2200), DCNBT-IDT shows a narrower bandgap (1.43 eV) with a much higher absorption coefficient (6.15 × 10⁴ cm^-1 ). Such properties are elusive for polymer acceptors to date, eradicating the drawbacks inherited in N2200 and other high-performance polymer acceptors. When blended with a wide-bandgap polymer donor, the DCNBT-IDT-based all-PSCs achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm. Such efficiency greatly outperforms those of N2200 (6.13%) and the naphthalene diimide (NDI)-based analog NDI-IDT (2.19%). This work breaks the long-standing bottlenecks limiting materials innovation of n-type polymers, which paves a new avenue for developing polymer acceptors with improved optoelectronic properties and heralds a brighter future of all-PSCs.

Random Polymer Donor for High-Performance Polymer Solar Cells with Efficiency over 14

Constructing random copolymers has been regarded as an easy and effective approach to design polymer donors for state-of-the-art polymer solar cells (PSCs). In this work, we develop a naphtho[2,3-c]thiophene-4,9-dione-based copolymer PBN-Cl as a donor material for PSCs, and a moderate power conversion efficiency (PCE) of 11.21% is achieved with a relatively low fill factor (FF) of 0.615. We then incorporate a similar acceptor unit benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) into the polymeric backbone of PBN-Cl to tune its photovoltaic performance, and a significantly higher PCE of 14.05% is achieved from the random polymer PBN-Cl-B80 containing 80% BDD unit. The enhanced PCE of the PBN-Cl-B80-based device mainly relies on the higher FF value, resulting from the improved charge mobility properties, reduced bimolecular and trap-assisted recombination, and more appropriate phase separation. The results demonstrate a feasible strategy to tune the photovoltaic performance of polymer donors by constructing a random polymer with a compatible component.

Designing dithienonaphthalene based acceptor materials with promising photovoltaic parameters for organic solar cells

Scientists are focusing on non-fullerene based acceptors due to their efficient photovoltaic properties. Here, we have designed four novel dithienonaphthalene based acceptors with better photovoltaic properties through structural modification of a well-known experimentally synthesized reference compound R. The newly designed molecules have a dithienonaphthalene core attached with different acceptors (end-capped). The acceptor moieties are 2-(5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (H1), 2-(5,6-dicyano-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile (H2), 2-(5-methylene-6-oxo-5,6-dihydrocylopenta[c]thiophe-4-ylidene)-malononitrile (H3) and 2-(3-(dicyanomethylene)-2,3-dihydroinden-1-yliden)malononitrile (H4). The photovoltaic parameters of the designed molecules are discussed in comparison with those of the reference R. All newly designed molecules show a reduced HOMO-LUMO energy gap (2.17 eV to 2.28 eV), compared to the reference R (2.31 eV). Charger transfer from donor to acceptor is confirmed by a frontier molecular orbital (FMO) diagram. All studied molecules show extensive absorption in the visible region and absorption maxima are red-shifted compared to R. All investigated molecules have lower excitation energies which reveal high charge transfer rates, as compared to R. To evaluate the open circuit voltage, the designed acceptor molecules are blended with a well-known donor PBDB-T. The molecule H3 has the highest V oc value (1.88 V). TDM has been performed to show the behaviour of electronic excitation processes and electron hole location between the donor and acceptor unit. The binding energies of all molecules are lower than that of R. The lowest is calculated for H3 (0.24 eV) which reflects the highest charge transfer. The reorganization energy value for both the electrons and holes of H2 is lower than R which is indicative of the highest charge transfer rate.

Synthesis and application of amine-containing conjugated small molecules for the automatic formation of an electron transporting layer via spontaneous phase separation from the bulk-heterojunction layer

Carbazole-based conjugated small molecule electrolytes (CSEs) containing different numbers of amine groups were synthesized and applied to bulk-heterojunction (BHJ) organic solar cells for the formation of a spontaneous self-assembled electron transporting layer (ETL). The active layer was spin-coated with a mixture solution containing the BHJ materials and a small amount of CSE, and a thin layer of CSE was formed underneath the active layer (CSE/BHJ bi-layer) via spontaneous phase separation, which is confirmed by the depth profile of the time of flight secondary ion mass spectroscopy (ToF-SIMS) spectrum. The amino groups in the CSEs form hydrogen-bonds with the surface of indium tin oxide (ITO), which acts as an ETL in BHJ solar cells. Moreover, the formed CSE layer is capable of changing the effective work function (WF) of ITO. An increasing number of amino groups in the CSEs (from Cz1N to Cz3N) provides more reduction of the effective WF of ITO, which results in a lower internal resistance and a higher power conversion efficiency (PCE). Furthermore, the enhanced hydrogen bonding between the amines and ITO with an increased number of amine groups has been studied by XPS. This result suggests that one-step processing provides a reduction of the manufacturing cost, which can provide an attractive design concept for ETL fabrication.

Revealing the Position Effect of an Alkylthio Side Chain in Phenyl-Substituted Benzodithiophene-Based Donor Polymers on the Photovoltaic Performance of Non-Fullerene Organic Solar Cells

In this work, position effects of an alkylthio side chain were investigated by designing and synthesizing two copolymers based on a phenyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDTP) and difluorobenzotriazole (FTAZ). The polymer based on the meta-position-alkylthiolated BDTP, named m-PBDTPS-FTAZ, showed a relatively broader bandgap (2.00 vs 1.96 eV) and lower highest occupied molecular orbital (HOMO) energy level (-5.40 vs -5.32 eV) than its para-positioned structural isomeric analogue polymer (named p-PBDTPS-FTAZ), that is, m- and p-PBDTPS-FTAZ with the side chain structured as ethylhexyl- in the phenyl unit and hexyldecyl- in the FTAZ moiety. When blended with ITIC, m-PBDTPS-FTAZ showed a comparable crystallinity but more uniform morphology compared to that of p-PBDTPS-FTAZ. A high power conversion efficiency of 13.16% was achieved for m-PBDTPS-FTAZ:ITIC devices with a high open circuit voltage (V_OC) of 0.95 V, which is higher than that of p-PBDTPS-FTAZ:ITIC devices (10.86%) with a V_OC of 0.89 V. Therefore, m-BDTPS could be an effective donor unit to construct high-efficiency polymers due to its effectively decreased HOMO energy level of polymers while still maintaining good molecular stacking.

Trisulfide-Bond Acenes for Organic Batteries

The molecular design of organic battery electrodes is a big challenge. Here, we synthesize two metal-free organosulfur acenes and shed insight into battery properties using first-principles calculations. A new zone-melting chemical-vapor-transport (ZM-CVT) apparatus was fabricated to provide a simple, solvent-free, and continuous synthetic protocol, and produce single crystals of tetrathiotetracene (TTT) and hexathiapentacene (HTP) at a large scale. Single crystals of HTP showed better Li-ion battery performance and higher cycling stability than those of TTT. A two-step, three-electron lithiation mechanism instead of the commonly depicted two-electron mechanism is proposed for the HTP Li-ion battery. The superior performance of HTP is linked to unique trisulfide bonding scenarios, which are also responsible for the formation of empty channels along the stacking direction. In-depth theoretical analysis suggests that organosulfur acenes are potential prototypes for organic battery materials with tunable properties, and that the tuning of sulfur bonds is critical in designing these new materials.

Supercritical water assisted preparation of recyclable gold nanoparticles and their catalytic utility in cross-coupling reactions under sustainable conditions

Preparation of gold nanoparticles (AuNPs) in environmentally friendly water without using any reducing agents under supercritical conditions is demonstrated. PXRD, XPS, FE-SEM and HR-TEM analysis confirmed the formation of phase-pure and crystalline AuNPs of the size of ∼10-30 nm. The catalytic potential of AuNPs was manifested through a generalized green procedure that could accommodate both Sonogashira as well as Suzuki coupling under aqueous conditions at low catalytic loading (0.1 mol%). The AuNP catalyst was found to be recuperated after the reaction and reused for up to six catalytic cycles with no leaching out of gold species as confirmed through ICP-OES analysis. With no confinement of AuNP catalysis to cross-coupling reaction, synthetic extension to one-flask preparation of π-conjugated semiconductors (4 examples) and their optoelectronic properties were also investigated. Other significant features of the present work include short reaction time, site-selectivity, wide substrate scope, high conversion, good chemical yields and applicability in gram-scale synthesis. Overall, the results of this paper signify an operationally sustainable supercritical fluid processing method for the synthesis of AuNPs and their catalytic application towards cross-coupling reactions in green media.

Improving optoelectronic and charge transport properties of D-π-D type diketopyrrolopyrrole-pyrene derivatives as multifunctional materials for organic solar cell applications

A series of novel diketopyrrolopyrrole-pyrene-based molecules were designed for small molecule based organic solar cell (SMOSC) applications. Their electronic and charge transfer properties were investigated by applying the PBE0/6-31G(d,p) method. The absorption spectra were simulated using the TD-PBE0/6-31G(d,p) method. The results showed that the frontier molecular orbital (FMO) energy levels, reorganization energy, the energetic driving force, and absorption spectra can be tuned by the introduction of different aromatic heterocyclic groups to the side of diketopyrrolopyrrole fragments' backbones. Additionally, the designed molecules possess suitable FMOs to match those of typical acceptors PC₆₁BM and PC₇₁BM. Meanwhile, the designed molecules can act as good ambipolar charge transport materials in SMOSC applications. Meanwhile, the electron and hole reorganization energies of the designed molecules are smaller than those of the typical electron and hole transport materials, respectively. Moreover, the differences between electron and hole reorganization energies do not exceed 0.046 eV. Our results suggest that the designed molecules can act as promising candidates for donor and ambipolar charge transport materials in SMOSC applications.

A New Small-Molecule Donor Containing Non-Fused Ring π-Bridge Enables Efficient Organic Solar Cells with High Open Circuit Voltage and Low Acceptor Content

To achieve high open-circuit voltage (V_oc ) and low acceptor content, the molecular design of a small-molecule donor with low energy loss (E_loss ) is very important for solution-processable organic solar cells (OSCs). Herein, we designed and synthesized a new coplanar A-D-A structured organic small-molecule semiconductor with non-fused ring structure π-bridge, namely B2TPR, and applied it as donor material in OSCs. Owing to the strong electron-withdrawing effect of the end group and the coplanar π-bridge, B2TPR exhibits a low-lying highest occupied molecular orbital and strong crystallinity. Furthermore, benefiting from the coplanar molecular skeleton, the high hole mobility, balanced charge transport and reduced recombination were achieved, leading to a high fill factor (FF). The OSCs based on B2TPR : PC₇₁ BM blend film (w/w=1 : 0.35) demonstrates a moderate power conversion efficiency (PCE) of 7.10 % with a remarkable V_oc of 0.98 V and FF of 64 %, corresponding to a low fullerene content of 25.9 % and a low E_loss of 0.70 eV. These results demonstrate the great potential of small-molecule with structure of B2TPR for future low-cost organic photovoltaic applications.

Theoretical Estimation of Donor Strength of Common Conjugated Units for Organic Electronics

Donor strength of commonly used conjugated building blocks in organic electronics were investigated with density functional theory. The donor molecules were coupled to thiophene-incorporated acceptor groups, and electronic structure calculations were performed for the energies of frontier orbitals, total charge on donors, and particle probability distribution. A novel approach is also developed to analyze the large set of data on frontier orbitals. The electron donating ability of a donor was determined by comparing the highest occupied molecular orbital energies of the calculated structures. The effect of selected acceptor group and chosen functional method were also investigated to accurately determine the donor strength of each conjugated building block. Ethylenedioxythiophene, propylenedioxythiophene, and triphenylamine derivatives were found to be best donors among the conjugated units investigated. Such a comparative analysis of donor strengths is believed to be useful for researchers in designing novel organic semiconductors for organic electronic applications.