Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3HT

Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3H

Electropolymerized Star-Shaped Benzotrithiophenes Yield π‑Conjugated Hierarchical Networks with High Areal Capacitance

High-surface-area π-conjugated polymeric networks have the potential to lend outstanding capacitance to supercapacitors because of the pronounced faradaic processes that take place across the dense intimate interface between active material and electrolytes. In this report, we describe how benzo­[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″]­trithiophene (<b>BTT</b>) and tris­(ethylenedioxythiophene)­benzo­[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″]­trithiophene (<b>TEBTT</b>) can serve as 2D (trivalent) building blocks in the development of electropolymerized hierarchical π-conjugated frameworks with particularly high areal capacitance. In comparing electropolymerized networks of <b>BTT</b>, <b>TEBTT</b>, and their copolymers with EDOT, we show that <b>TEBTT</b>/EDOT-based copolymers, i.e., P­(<b>TEBTT</b>/EDOT), can achieve higher areal capacitance (e.g., as high as 443.8 mF cm^–2 at 1 mA cm^–2) than those achieved by their respective homopolymers (<b>PTEBTT</b> and PEDOT) in the same experimental conditions of electrodeposition (<b>PTEBTT</b>: 271.1 mF cm^–2 (at 1 mA cm^–2) and PEDOT: 12.1 mF cm^–2 (at 1 mA cm^–2)). For example, P­(<b>TEBTT</b>/EDOT)-based frameworks synthesized in a 1:1 monomer-to-comonomer ratio show a ca. 35× capacitance improvement over PEDOT. The high areal capacitance measured for P­(<b>TEBTT</b>/EDOT)-based frameworks can be explained by the open, highly porous hierarchical morphologies formed during the electropolymerization step. With >70% capacitance retention over 1000 cycles (up to 89% achieved), both <b>PTEBTT</b>- and P­(<b>TEBTT</b>/EDOT)-based frameworks are resilient to repeated electrochemical cycling and can be considered promising systems for high life cycle capacitive electrode applications

Highly Transparent and UV-Resistant Superhydrophobic SiO₂‑Coated ZnO Nanorod Arrays

Highly transparent and UV-resistant superhydrophobic arrays of SiO₂-coated ZnO nanorods are prepared in a sequence of low-temperature (<150 °C) steps on both glass and thin sheets of PET (2 × 2 in.²), and the superhydrophobic nanocomposite is shown to have minimal impact on solar cell device performance under AM1.5G illumination. Flexible plastics can serve as front cell and backing materials in the manufacture of flexible displays and solar cells

Electron-Deficient <i>N</i>‑Alkyloyl Derivatives of Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione Yield Efficient Polymer Solar Cells with Open-Circuit Voltages > 1 V

Poly­(benzo­[1,2-b:4,5-b′]­dithiophene–thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione) (PBDTTPD) polymer donors yield some of the highest open-circuit voltages (<i>V</i>_OC, ca. 0.9 V) and fill factors (FF, ca. 70%) in conventional bulk-heterojunction (BHJ) solar cells with PCBM acceptors. Recent work has shown that the incorporation of ring substituents into the side chains of the BDT motifs in PBDTTPD can induce subtle variations in material properties, resulting in an increase of the BHJ device <i>V</i>_OC to ∼1 V. In this contribution, we report on the synthesis of <i>N</i>-alkyloyl-substituted TPD motifs (TPD­(CO)) and show that the electron-deficient motifs can further lower both the polymer LUMO and HOMO levels, yielding device <i>V</i>_OC > 1 V (up to ca. 1.1 V) in BHJ solar cells with PCBM. Despite the high <i>V</i>_OC achieved (i.e., low polymer HOMO), BHJ devices cast from TPD­(CO)-based polymer donors can reach power conversion efficiencies (PCEs) of up to 6.7%, making these promising systems for use in the high-band-gap cell of tandem solar cells

Electron-Deficient <i>N</i>‑Alkyloyl Derivatives of Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione Yield Efficient Polymer Solar Cells with Open-Circuit Voltages > 1 V

Poly­(benzo­[1,2-b:4,5-b′]­dithiophene–thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione) (PBDTTPD) polymer donors yield some of the highest open-circuit voltages (<i>V</i>_OC, ca. 0.9 V) and fill factors (FF, ca. 70%) in conventional bulk-heterojunction (BHJ) solar cells with PCBM acceptors. Recent work has shown that the incorporation of ring substituents into the side chains of the BDT motifs in PBDTTPD can induce subtle variations in material properties, resulting in an increase of the BHJ device <i>V</i>_OC to ∼1 V. In this contribution, we report on the synthesis of <i>N</i>-alkyloyl-substituted TPD motifs (TPD­(CO)) and show that the electron-deficient motifs can further lower both the polymer LUMO and HOMO levels, yielding device <i>V</i>_OC > 1 V (up to ca. 1.1 V) in BHJ solar cells with PCBM. Despite the high <i>V</i>_OC achieved (i.e., low polymer HOMO), BHJ devices cast from TPD­(CO)-based polymer donors can reach power conversion efficiencies (PCEs) of up to 6.7%, making these promising systems for use in the high-band-gap cell of tandem solar cells

Impact of Nonfullerene Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells

Small-molecule “nonfullerene” acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet, the efficiency-limiting processes and their dependence on the acceptor structure are not clearly understood. Here, we investigate the impact of the acceptor core structure (cyclopenta-[2,1-b:3,4-b′]­dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, namely CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer, the IDTT-based acceptor achieves power conversion efficiencies (8.4%) that are higher than those of the CDT-based acceptor (5.6%) because of a concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)­fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage

Side-Chain Tunability of Furan-Containing Low-Band-Gap Polymers Provides Control of Structural Order in Efficient Solar Cells

The solution-processability of conjugated polymers in organic solvents has classically been achieved by modulating the size and branching of alkyl substituents appended to the backbone. However, these substituents impact structural order and charge transport properties in thin-film devices. As a result, a trade-off must be found between material solubility and insulating alkyl content. It was recently shown that the substitution of furan for thiophene in the backbone of the polymer PDPP2FT significantly improves polymer solubility, allowing for the use of shorter branched side chains while maintaining high device efficiency. In this report, we use PDPP2FT to demonstrate that linear alkyl side chains can be used to promote thin-film nanostructural order. In particular, linear side chains are shown to shorten π–π stacking distances between backbones and increase the correlation lengths of both π–π stacking and lamellar spacing, leading to a substantial increase in the efficiency of bulk heterojunction solar cells

Benzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophene–Pyrido[3,4‑<i>b</i>]pyrazine Small-Molecule Donors for Bulk Heterojunction Solar Cells

We report on the synthesis, material properties, and bulk heterojunction (BHJ) solar cell characteristics of a set of π-conjugated small-molecule (SM) donors composed of benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) and pyrido­[3,4-<i>b</i>]­pyrazine (PP) units, examining the perspectives of <i>alkyl-substituted PP acceptor motifs</i> in SM designs. In these systems (<b>SM1</b>–<b>4</b>), both the type of side chains derived from the PP motifs and the presence of ring substituents on BDT critically impact (i) molecular packing, and (ii) thin-film morphologies and charge transport in BHJ solar cells. With the appropriate side-chain pattern, the ring-substituted analogue <b>SM4</b> stands out, achieving efficiencies of ca. 6.5% with PC₇₁BM, and fine-scale morphologies comparable to those obtained with some of the best-performing polymer donors in BHJ solar cells. ¹H–¹H DQ-SQ NMR analyses are used to examine the distinct self-assembly pattern of <b>SM4</b>, expected to factor into the development of the BHJ morphology

Dependence of Crystallite Formation and Preferential Backbone Orientations on the Side Chain Pattern in PBDTTPD Polymers

Alkyl substituents appended to the π-conjugated main chain account for the solution-processability and film-forming properties of most π-conjugated polymers for organic electronic device applications, including field-effect transistors (FETs) and bulk-heterojunction (BHJ) solar cells. Beyond film-forming properties, recent work has emphasized the determining role that side-chain substituents play on polymer self-assembly and thin-film nanostructural order, and, in turn, on device performance. However, the factors that determine polymer crystallite orientation in thin-films, implying preferential backbone orientation relative to the device substrate, are a matter of some debate, and these structural changes remain difficult to anticipate. In this report, we show how systematic changes in the side-chain pattern of poly­(benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene–<i>alt</i>–thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione) (PBDTTPD) polymers can (i) influence the propensity of the polymer to order in the π-stacking direction, and (ii) direct the preferential orientation of the polymer crystallites in thin films (e.g., “face-on” vs “edge-on”). Oriented crystallites, specifically crystallites that are well-ordered in the π-stacking direction, are believed to be a key contributor to improved thin-film device performance in both FETs and BHJ solar cells

Solvent Annealing Effects in Dithieno[3,2‑<i>b</i>:2′,3′‑<i>d</i>]pyrrole–5,6-Difluorobenzo[<i>c</i>][1,2,5]thiadiazole Small Molecule Donors for Bulk-Heterojunction Solar Cells

Low-bandgap small molecule (SM) donors that can be solution-processed with fullerene acceptors (e.g., PC₆₁/₇₁BM) are proving to be particularly promising in bulk-heterojunction (BHJ) solar cells. Compared to their π-conjugated polymer counterparts, SM donors are well-defined (monodisperse) and more synthetically modular, with relatively wide ranges of bandgaps that can be achieved in stepwise couplings of various donor and acceptor motifs. However, the optimization of SM–fullerene morphologies and BHJ device efficiencies relies more specifically on the use of processing additives, postprocessing thermal, or solvent vapor annealing (SVA) approaches, and achieving adequate interpenetrating networks and structural order in BHJ thin films can be challenging. In this report, we examine the correlated effects of molecular structure and postprocessing SVA on the BHJ solar cell performance of a set of π-extended SM donors composed of dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole (DTP) and 5,6-difluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole ([2F]­BT) units. In these systems (<b>SM1–SM3</b>), the introduction of additional alkyl substituents and unsubstituted thiophene rings on the peripheral unit groups critically impacts the effects of SVA steps on BHJ solar cell efficiency. We show that the more π-extended and alkyl-substituted analogue <b>SM3</b> stands out, with BHJ device efficiencies of ∼6% obtained from SVA with CS₂, while SVA-treated <b>SM3</b>-based active layers also show the most favorable ordering and carrier mobility patterns. However, unlike numbers of SM donors reported in recent years, DTP–[2F]­BT SM analogues are in general not prone to dramatic performance variations in BHJ thin films cast with processing additives. Our results indicate that the role of SVA steps is not independent of the molecular structure of the SM donors used in the BHJ solar cells