Optimization of solvent swelling for efficient organic solar cells via sequential deposition

Compared to bulk heterojunction (BHJ) organic solar cells (OSCs) prepared by the blend casting in “one step process”, sequential deposition (SD) processed OSCs can realize an ideal profile of vertical component distribution due to the swelling of polymer films. Herein, we did trials on several kinds of second solvents for swelling the polymer layer, and investigated the packing structure and morphology of the swollen films and the performance of the resulting devices. We found that an optimized morphology can be achieved by solvent swelling while using orthodichlorobenzene (o-DCB) as the second layer processing-solvent, with polymer donor PffBT-3 as bottom layer, PC71BM as top layer and bicontinuous networks in the middle. Such solvent swelling process also makes the SD method exempt from thermal annealing treatment. The device based on SD yields a power conversion efficiency (PCE) up to 8.7% without any post-treatment, outperforming those from the devices based on SD using other solvents and that (7.06%) from BHJ device, respectively. We also extended the use of this approach to all-polymer blend system, and successfully improved the efficiency from 4.72% (chloroform) to 9.35% (o-DCB), which is among the highest PCEs in all-polymer-based OSCs fabricated with SD method. The results demonstrate that the swelling of the polymer by the second layer solvent is a necessity for SD, paving the way towards additive-free high-performance OSCs

Drastic Effects of Fluorination on Backbone Conformation of Head-to-Head Bithiophene-Based Polymer Semiconductors

This study shows that the backbone conformation of head-to-head type 3,3′-dialkyl-2,2′-bithiophene can be tuned via fluorination of the neighboring benzothiadiazole (BTz). Without fluorination, the polymer backbone is highly twisted, whereas difluorination of BTz produced a coplanar backbone. Monofluorination of BTz yielded a tunable polymer backbone conformation depending on the film annealing temperature. In organic thin-film transistors, the polymer with the head-to-head linkages showed a remarkable hole mobility of >0.5 cm² V^–1 s^–1 upon attaining a planar backbone. Thus, the head-to-head linkage does not necessarily lead to backbone nonplanarity, and achieving planar conformation of 3,3′-dialkyl-2,2′-bithiophene has profound implications in materials design for organic semiconducting devices, yielding good solubility, reduced materials synthetic steps, and improved opto-electrical properties

Green-Solvent-Processable Low-Cost Fluorinated Hole Contacts with Optimized Buried Interface for Highly Efficient Perovskite Solar Cells

Solution-processed hole contact materials, as an indispensable component in perovskite solar cells (PSCs), have been widely studied with consistent progress achieved. One bottleneck for the commercialization of PSCs is the lack of hole contact materials with high performance, cost-effective preparation, and green-solvent processability. Therefore, the development of versatile hole contact materials is of great significance. Herein, we report two novel donor–acceptor (D–A)-type hole contact molecules (FMPA–BT-CA and 2FMPA–BT-CA) with low cost and alcohol-based processability by utilizing a fluorination strategy. We showed that the fluorine atoms lead to the lowered highest occupied molecular orbital (HOMO) energy levels and larger dipole moments for FMPA–BT-CA and 2FMPA–BT-CA. Moreover, fluorination also improves the buried interfacial interaction between hole contacts and perovskite. As a result, a remarkable power conversion efficiency (PCE) of 22.37% along with good light stability could be achieved for green-solvent-processed FMPA–BT-CA-based inverted PSC devices, demonstrating the great potential of environmentally compatible hole contacts for highly efficient PSCs

Legislative Documents

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Transition metal-catalysed molecular n-doping of organic semiconductors

Electron doping of organic semiconductors is typically inefficient, but here a precursor molecular dopant is used to deliver higher n-doping efficiency in a much shorter doping time. Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices(1-9). N(electron)-doping is fundamentally more challenging than p(hole)-doping and typically achieves a very low doping efficiency (eta) of less than 10%(1,10). An efficient molecular n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability(1,5,6,9,11), which is very challenging. Here we show a general concept of catalysed n-doping of organic semiconductors using air-stable precursor-type molecular dopants. Incorporation of a transition metal (for example, Pt, Au, Pd) as vapour-deposited nanoparticles or solution-processable organometallic complexes (for example, Pd-2(dba)(3)) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling greatly increased eta in a much shorter doping time and high electrical conductivities (above 100 S cm(-1); ref. (12)). This methodology has technological implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, molecular dopants and semiconductors, thus opening new opportunities in n-doping research and applications(12, 13)

Enhancing Polymer Photovoltaic Performance via Optimized Intramolecular Ester-Based Noncovalent Sulfur···Oxygen Interactions

Head-to-head (HH) bithiophenes are typically avoided in polymer semiconductors since they engender undesirable steric repulsions, leading to a twisted backbone. While introducing electron-donating alkoxy chains can lead to intramolecular noncovalent S···O interactions, this comes at the cost of elevating the HOMOs and compromising polymer solar cell (PSC) performance. To address the limitation, a novel HH bithiophene featuring an electron-withdrawing ester functionality, 3-alkoxycarbonyl-3′-alkoxy-2,2′-bithiophene (TETOR), is synthesized. Single crystal diffraction reveals a planar TETOR conformation (versus highly twisted diester bithiophene), showing distinctive advantages of incorporating alkoxy on promoting backbone planarity. Compared to first-generation 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR), TETOR contains an additional planarizing (thienyl)­S···O­(carbonyl) interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly lower-lying FMOs, stronger aggregation, closer π-stacking, and better miscibility with fullerenes versus the TRTOR-based counterpart (TffBT-TRTOR). These characteristics are attributed to the additional S···O interaction and electron-withdrawing ester substituent, which enhances backbone planarity, charge transport, and PSC performance. Thus, TffBT-TETOR-based PSCs exhibit an increased PCE of 10.08%, a larger <i>V</i>_oc of 0.76 V, and a higher <i>J</i>_sc of 18.30 mA cm^–2 than the TffBT-TRTOR-based PSCs. These results demonstrate that optimizing intramolecular noncovalent S···O interactions by incorporating electron-withdrawing ester groups is a powerful strategy for materials invention in organic electronics

Alkynyl-Functionalized Head-to-Head Linkage Containing Bithiophene as a Weak Donor Unit for High-Performance Polymer Semiconductors

Building blocks having a high degree of backbone planarity, good solubilizing characteristics, and well-tailored physicochemical properties are highly desirable for constructing high-performance polymer semiconductors. Due to the detrimental steric hindrance created by alkyl chain substituents at the 3- and 3′-positions of bithiophene, “head-to-head” linkage containing 3,3′-dialkyl-2,2′-bithiophenes (<b>BTR</b>) are typically avoided in materials design. Replacing alkyl chains with less steric demanding alkynyl chains should greatly reduce steric hindrance by eliminating two H atoms at the sp-hybridized carbon center. Here we report the synthesis of a novel electron donor unit, 3,3′-dialkynyl-2,2′-bithiophene (<b>BTRy</b>), and its incorporation into conjugated polymer backbones. The alkynyl-functionalized head-to-head bithiophene linkage yields polymers with good solubility without sacrificing backbone planarity; the <b>BTRy</b>-based polymers show a high degree of conjugation with a narrow bandgap of ∼1.6 eV. When incorporated into organic thin-film transistors, the polymers exhibit substantial hole mobility, up to 0.13 cm² V^–1 s^–1 in top-gated transistors. The electron-withdrawing alkynyl substituents lower the frontier molecular orbitals, imbuing the difluorobenzothiadiazole and difluorobenzoxadiazole copolymers with remarkable ambipolarity: electron mobility > 0.05 cm² V^–1 s^–1 and hole mobility ∼0.01 cm² V^–1 s^–1 in bottom-gated transistors. In bulk-heterojunction solar cells, the <b>BTRy</b>-based polymers show promising power conversion efficiencies approaching 8% with very large <i>V</i>_oc values of 0.91–1.04 V, due to the weak electron-withdrawing alkynyl substituents. In comparison to the tetrathiophene-based polymer analogues based on the unsubstituted π-spacer design, the <b>BTRy</b>-based polymers have comparable light absorption but with 0.14 V larger open-circuit voltage, translating to enhanced optoelectronic properties for this attractive design strategy. Thus, alkynyl groups are versatile semiconductor substituents, offering good solubility, substantial backbone planarity, optimized optoelectronic properties, and film crystallinity, for materials innovation in organic electronics

Materials Design via Optimized Intramolecular Noncovalent Interactions for High-Performance Organic Semiconductors

We report the design, synthesis, and implemention in semiconducting polymers of a novel head-to-head linkage containing the TRTOR (3-alkyl-3′-alkoxy-2,2′-bithiophene) donor subunit having a single strategically optimized, planarizing noncovalent S···O interaction. Diverse complementary thermal, optical, electrochemical, X-ray scattering, electrical, photovoltaic, and electron microscopic characterization techniques are applied to establish structure–property correlations in a TRTOR-based polymer series. In comparison to monomers having double S···O interactions, replacing one alkoxy substituent with a less electron-donating alkyl one yields TRTOR-based polymers with significantly depressed (0.2–0.3 eV) HOMOs. Furthermore, the weaker single S···O interaction and greater TRTOR steric encumberance enhances materials processability without sacrificing backbone planarity. From another perspective, TRTOR has comparable electronic properties to ring-fused 5<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyran (DTP) subunits, but a centrosymmetric geometry which promotes a more compact and ordered structure than bulkier, axisymmetric DTP. Compared to monosubstituted TTOR (3-alkoxy-2,2′-bithiophene), alkylation at the TRTOR bithiophene 3-position enhances conjugation and polymer crystallinity with contracted π–π stacking. Grazing incidence wide-angle X-ray scattering (GIWAXS) data reveal that the greater steric hindrance and the weaker single S···O interaction are not detrimental to close packing and high crystallinity. As a proof of materials design, copolymerizing TRTOR with phthalimides yields copolymers with promising thin-film transistor mobility as high as 0.42 cm²/(V·s) and 6.3% power conversion efficiency in polymer solar cells, the highest of any phthalimide copolymers reported to date. The depressed TRTOR HOMOs imbue these polymers with substantially increased <i>I</i>_on/<i>I</i>_off ratios and <i>V</i>_oc’s versus analogous subunits with multiple electron donating, planarizing alkoxy substituents. Implementing a head-to-head linkage with an alkyl/alkoxy substitution pattern and a single S···O interaction is a promising strategy for organic electronics materials design