Dithieno[3,2‐b:2′,3′‐d]pyrrole and Benzothiadiazole‐Based Semicrystalline Copolymer for Photovoltaic Devices with Indene‐C60 Bisadduct

A semicrystalline low‐bandgap polymer (PDTPBT) based on alternating dithienopyrrole and benzothiadiazole moieties as a pair of the indene‐C₆₀ bisadduct (ICBA) for polymeric solar cells is reported. The lowest unoccupied molecular orbital (LUMO) level of PDTPBT is measured to be ?3.47 eV, ensuring sufficient energy offset for photoinduced charge transfer to ICBA. Photovoltaic cells are fabricated with ICBA and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as an acceptor. By replacing PC₇₁BM with ICBA, the open‐circuit voltage is increased by 0.23 V and the resulting power conversion efficiency is improved from 1.17% to 1.71%. To optimize the ICBA‐based devices, crystalline low‐bandgap structures should be designed carefully as a pair of ICBA by considering the energy‐level offset for charge separation and crystalline interchain ordering, for minimizing the intercalated ICBAs inside the polymer domain.

     

Synthesis and Photovoltaic Characterization of Dithieno[3,2‐b:2′,3′‐d]thiophene‐Derived Narrow‐Bandgap Polymers

Three novel dithieno[3,2‐b:2′,3′‐d]thiophene‐based low‐bandgap polymers are synthesized by a Suzuki–Miyaura coupling reaction or by direct arylation polycondensation. The polymers present a high molecular weight (26–32 kDa) and narrow polydiversity (1.3–1.7). With a highest occupied molecular orbital (HOMO) energy level around ?5.20 eV, these polymers exhibit a narrow bandgap of 1.75–1.87 eV. All the polymers display strong absorption in the range of 350–700 nm. Bulk‐heterojunction (BHJ) solar cells are further fabricated by blending the as‐prepared polymer with (6,6)‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) at different weight ratios. The best devices contribute a power conversion efficiency (PCE) of 0.73% under AM 1.5 (100 mW cm^?2).

     

Thermochromic and Photovoltaic Properties of an Alternating Copolymer of Dithieno[3,2‐b:2′,3′‐d]thiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione

The synthesis of a new alternating conjugated polymer, PDTTTPD, based on electron‐donating dithieno[3,2‐b:2′,3′‐d]thiophene (DTT) and electron‐withdrawing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) units is reported. This polymer shows strong thermochromic effect in chlorobenzene with a peak shift up to 170 nm. This phenomenon is studied with fluorescence spectroscopy and related to the steric hindrance along the polymer main chain. Polymer solar cells are fabricated from its blends with PC₇₁BM, and shows modest power conversion efficiency up to 2.1%. The low efficiency is due to the low short‐circuit current, which is also attributed to the steric effect. Based on these results, a general design rule for polymer photovoltaic material with controlled backbone conformation is proposed.     

Synthesis and Photovoltaic Properties of Alternating Conjugated Polymers Derived from Indeno[1,2‐b]fluorene and Bithiophene or Thieno[3,2‐b]thiophene‐Cored Benzothiadiazole

Two narrow bandgap copolymers derived from 6,6′,12,12′‐tetraoctyl‐indeno[1,2‐b]fluorene and bithiophene or thieno[3,2‐b]thiophene‐cored benzothiadiazole are synthesized and characterized. The copolymers show broad absorption in the range 350–700 nm. The application of the copolymers as photovoltaic cells with configurations ITO/PEDOT:PSS/blend/Al and ITO/PEDOT:PSS/blend/interlayer/Al is investigated. A power conversion efficiency (PCE) of approximately 3.0% is achieved under an AM 1.5G solar simulator (80 mW cm^?2) for the cells with ITO/PEDOT:PSS/polymer:PC₇₁BM([6,6]‐phenyl‐C₇₁ butyric acid methyl ester) (1:4)/interlayer/Al.

     

Polyterthiophenes Incorporating 3,4‐Difluorothiophene Units: Application in Organic Field‐Effect Transistors

Two terthiophenes bearing core fluorinated thienyl units have been synthesised as potential semiconductor materials for organic field‐effect transistors. Polymerisation of these compounds has been achieved using conventional iron(III) chloride oxidative coupling methods and by electrochemical oxidation. Characterisation of the fluorinated materials has been achieved by absorption spectroscopy and cyclic voltammetry. A soluble hexyl‐functionalised polymer (poly 8b ) was used in an OFET device; hole mobilities were measured up to 3 × 10^?3 cm² · V^?1 · s^?1, and the device had an on/off ratio of 10⁵ and a turn‐on voltage of +4 V.

     

Benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐terthiophene Copolymers Containing Styryl‐Triphenylamine Side Chains: Synthesis and Photovoltaic Performance Optimization with Fullerene Acceptors

A new two‐dimensional‐conjugated polymer (PBDTT3‐TPA) containing benzodithiophene (BDT) and a side chain isolation comonomer is designed and synthesized. Interestingly, PBDTT3‐TPA is compatible with higher lowest unoccupied molecular level (LUMO) acceptors of indene‐C₆₀ bisadduct (ICBA), and polymer solar cells based on PBDTT3‐TPA/ ICBA show an open‐circuit voltage (V_OC) of ca. 0.80 V and a power conversion efficiency of 2.48% under AM1.5G illumination of at 100 mW cm^?2. Furthermore, the energy loss in the corresponding fullerene acceptor devices is discussed, and the increase in the observed V_OC is explained quantitatively by the up‐shifted LUMO energy of ICBA (0.17 eV) and the reduced saturation current (J_SO) in the blends.

     

Synthesis of Novel Conjugated Polyelectrolytes for Organic Field‐Effect Transistors Gate Dielectric Materials

Conjugated polyelectrolytes and their neutral precursors: aminoalkyl‐substituted polyfluorenes (PFNs) with different 4,7‐bis(N‐methylpyrrol‐2‐yl)‐2,1,3‐benzothiadiazole (PBT) contents were synthesized by Suzuki coupling reaction. Their quaternized ammonium polyelectrolyte derivatives were obtained through a post‐polycondensation treatment on the terminal amino groups. The resulting copolymers PFNBr–PBT are soluble in polar solvents such as methanol, DMF, and DMSO. Using the conjugated polyelectrolytes PFNBr–PBT5 as gate dielectric material, poly(3‐hexylthiophene) (P3HT) ‐based field‐effect transistors (FETs) show a high drain current of almost 4.5 mA within a gate voltage of only ?3 V. The calculated hole mobility of P3HT is about 0.58 cm² · V^?1 · s^?1.

     

Synthesis of Alternating Low‐Bandgap Conjugated Polymers Based on Dithieno[2,3‐d:2′,3′‐d′]naphtho[1,2‐b:3,4‐b′]dithiophene and Enhancement of Photovoltaic Properties with Solvent Additives

Two alternating low‐bandgap conjugated polymers with 10,11‐di(3,7‐dimethyloctyloxy)di‐thieno[2,3‐d:2′,3′‐d′]naphtho[1,2‐b:3,4‐b′]dithiophene (NDT) as electron‐donor moieties and N,N′‐di(2‐hexyldecyl)isoindigo (ID) or bis(thieno‐2‐yl)‐N,N′‐bis(2‐hexyldecyl)‐1,4‐dioxo‐pyrrolo[3,4‐c]pyrrole (DPP) as electron‐acceptor moieties, respectively named as PNDT‐ID and PNDT‐DPP, are firstly synthesized and characterized. The polymers exhibit appropriate energy levels, good solution processabilities and broad light absorption properties; the power conversion efficiencies (PCEs) of 0.16%–0.19% for the photovoltaic solar cells (PVCs) from the blend films of the polymers and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) are very low. The performances of the PVCs from the polymers are remarkably increased when a very small amount of 1,8‐diiodooctance (DIO) or diphenyl sulfide (DPS) is used as solvent additives, and the maximal PCEs of 3.79% and 5.01% are respectively achieved in the PVCs from the blend films of PNDT‐ID/PC₇₁BM (W:W, 1:1.5) and PNDT‐DPP/PC₇₁BM (W:W, 1:1.5), with DPS as solvent additives under 100 mW cm^?2 illumination (AM 1.5G).

     

Synthesis,Characterization and Field‐Effect Transistor Performance of Poly[2,6‐bis(3‐alkylthiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]diselenophene]s

Benzo[1,2‐b:4,5‐b′]diselenophene (BDS) has been incorporated for the first time in a polymer. bis(Stannyl)‐functionalized BDS was copolymerized with 3,3′‐bis(alkyl)‐5,5′‐bithiophenes (dodecyl and tetradecyl side chains) through Stille copolymerization, to yield p‐type polymer semiconductors for organic field‐effect transistor application. The electronic and structural effect of the selenium atoms, compared to sulphur atoms in analogous copolymers, is described. The molecular weight has a decisive influence on the photophysical properties and supramolecular ordering, expressed in field‐effect transistor measurements. Saturation mobilities around 10^?2 cm² · V^?1s^?1 are obtained on standard silicon substrates.

     

Effects of Thermal Annealing and Solvent Annealing on the Morphologies and Properties of Poly(3‐hexylthiophene) Nanowires

In recent years, poly(3‐hexylthiophene) (P3HT) nanowires have attracted great interest because of their unique physical and electronic properties. The annealing effects on the properties and morphologies of P3HT nanowires, however, are still not fully understood. In this work, the effects of thermal annealing and solvent annealing on the morphologies and crystallinities of P3HT nanowires prepared by the whisker method are studied. P3HT is first dissolved in heated p‐xylene, followed by a cooling process to room temperature, and P3HT nanowires are formed by self‐assembly. The crystallinities of the P3HT nanowires are observed to increase by both thermal annealing and solvent annealing, as confirmed by differential scanning calorimetry and X‐ray diffraction. The device performances of organic field‐effect transistors based on the annealed nanowires are also examined. After annealing, an enhancement of the charge mobility by one order is observed.

     

Conjugated Side Chain Tuning Effect of Indacenodithieno[3,2‐b]thiophene and Fluoro‐Benzothiadiazole‐Based Regioregular Copolymers for High‐Performance Organic Field‐Effect Transistors

Two regioregular indacenodithieno[3,2‐b ]thiophene and fluoro‐benzothiadiazole‐based donor–acceptor (D–A) type copolymers with different conjugated side chains of hexyl‐phenyl or hexyl‐thienyl are designed and synthesized to study the effects of the side chain on performance of organic field‐effect transistors (OFETs). The effect of the different conjugated side chains on the energy levels, optoelectronic properties, film morphology, and transistors performance as well as the solvent effect on the performance of both D–A copolymers are thoroughly studied. Top‐gate/bottom‐contact OFETs with a hexyl‐thienyl substituted polymer show higher hole mobility up to 0.211 cm² V⁻¹ s⁻¹ than 0.086 cm² V⁻¹ s⁻¹ observed for the polymer with hexyl‐phenyl side chains. In addition, the hexyl‐thienyl substituted side chain polymer shows relatively better stability under bias stress compared to hexyl‐phenyl conjugated side chain. This result sheds light on the impact of conjugated side chain engineering on the performance and stability of OFETs with D–A copolymers.     

Electrochemical polymerization of 1H,7H-pyrrolo[2′,3′:4,5]-thieno[3,2-b]pyrrole and 4H-dithieno[3,2-b;2′,3′-d]pyrrole

Electrochemical polymerization of 1H,7H-pyrrolo[2′,3′:4,5]thieno[3,2-b]pyrrole and 4H-dithieno[3,2-b;2′,3′-d]pyrrole in CH₃CN + 0,1 M p-toluenesulfonate and tetraethylammonium perchlorate, respectively, yields polymers with a conductivity of ca. 5 S/cm. Electrochemistry and elemental analysis indicate the presence of 0,6-0,7 anions per monomeric unit, while spectroelectrochemistry agrees with a polypyrrole structure for polypyrrolothienopyrrole and a polythiophene structure for polydithienopyrrole. The redox cycle for polypyrrolothienopyrrole (at ?0,53 V vs. Ag/Ag⁺) is close to that of polypyrrole, while that of polydithienopyrrole (at 0,0 V) is intermediate between those of polythiophene and polypyrrole. N-Alkylsubstituted dithienopyrroles produce soluble polymers with higher conductivity (40 S/cm) and the lowest band-gap (1,7 eV) ever reported for a polythiophene-like polymer.     

Copolythiophenes with Hydrophilic and Hydrophobic Side Chains: Synthesis,Characterization, and Performance in Organic Field Effect Transistors

Statistical copolymers of regioregular polythiophene with hydrophobic hexyl side chains (P3HT) and hydrophilic 3,6‐dioxaheptyl side chains (P3DOHT) are synthesized by the GRIM method. A comparable study on the different polymers has been accomplished regarding solubility, thin film structure, and performance in organic field effect transistors (OFETs). DSC and XRD measurements are done to investigate the crystallinity of the copolymers. The novel functionalized fully π‐conjugated copolymers P(3HT‐co‐3DOHT) with feed molar ratios of 1:1, 2:1, and 1:2 show hole mobilities in the range of 10^?2 cm² V^?1 s^?1. Using poly(methyl methacrylate) as the dielectric layer in a top gate device architecture leads to air stable transistors without significant losses in the OFET performance over several months.     

New Alternating Copolymers of 3,6‐Carbazoles and Dithienylbenzothiadiazoles: Synthesis,Characterization, and Application in Photovoltaics

Three new low‐bandgap copolymers containing 3,6‐linked carbazole units are synthesized and characterized using a combination of spectroscopic and electrochemical techniques and DFT calculations. The electron‐withdrawing effect of the BTD units leads to a significant decrease of the optical bandgap from 2.59 eV for PBTC to 1.81 eV for PCDTBT, whereas pendant octyl chains on the thiophene ring lead to an increase of the gap to 2.05 eV for PCDOTBT due to steric hindrance. PBTC:PCBM photovoltaic devices exhibit a PCE of 0.35% with an EQE reaching 35% in the blue region of the solar spectrum whereas PCDTBT:PCBM blends are efficient over a wider spectral range due to the lower bandgap of PCDTBT.

     

Characterization and Properties of Poly[N‐(2‐cyanoethyl)pyrrole]

PNCPy was prepared by anodic polymerization and its properties in both doped and undoped state were characterized. The doping level of the oxidized material has been found to be larger than that of other conducting polymers; the more relevant electrochemical properties of the doped material were retained after undoping. SEM and AFM data are consistent with a lumpy surface and a multidirectional growing of the polymer chains. Finally, PNCPy has been combined with PEDOT to prepare three‐layer systems with enhanced electroactivity and electrostability. Results suggest that PNCPy is a potential candidate for the fabrication of electric circuit components that are able to block the current flow below a given potential.

     

Synthesis and Characterization of [2.2]Paracyclophane‐Containing Conjugated Microporous Polymers

Conjugated microporous polymers (CMPs) are prepared from disubstituted and tetrasubstituted [2.2]paracyclophane monomers by a Sonogashira–Hagihara coupling. The polymers obtained exhibit a type I nitrogen gas adsorption profile and H4‐like hysteresis loops, indicating that the [2.2]paracyclophane‐containing CMPs possess slit‐like mesopores. Their Brunauer–Emmett–Teller surface areas are estimated to be above 500 m² g^?1. The step and stacked structure of the [2.2]paracyclophane unit affects the morphology of the polymers because of the contribution of two‐dimensional expansion of the polymer network.     

Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Polymers for Optoelectronic Applications

Over the last decade, great progress has been made in the field of organic electronics. Advancements in organic syntheses as well as in device engineering enabled preparation of polymer solar cells with power conversion efficiency (PCE) exceeding 8%–9%. In search for new polymers suitable for photovoltaic applications, push–pull polymers containing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) motif as an electron deficient (pull) unit emerged as very promising candidates. This Trend Article summarizes research on TPD‐based polymers with a special emphasis on the structure–property relationships.     

Synthesis,Properties, and Solar Cell Performance of Poly(4‐(p‐alkoxystyryl)thiazole)s

A series of polythiazoles (PvTzs) featuring conjugated styryl sidechains equipped with different solubilizing p‐alkoxy‐groups (? OR, R = n‐octyl, n‐dodecyl, 2‐ethylhexyl, 2‐hexyldecyl) is prepared by Negishi‐coupling polycondensation. Soluble material with number‐average molecular weights of up to M_n = 8.5 kDa (polydispersity (PDI) = 1.3, degree of polymerization (DPn) ≈ 20) is obtained, with a head‐to‐tail content of the PvTzs of ≈77%, as estimated from comparison with reference polymers. The polymers exhibit optical absorption properties similar to their polythiophene analogues, while their electrochemical characterization shows a significant stabilization of their frontier orbital levels. Fluorescence measurements indicate that upon excitation of the electron rich alkoxystyryl side‐chains charge transfer onto the more electron deficient polythiazole backbone occurs. This finding is corroborated by density functional theory (DFT) calculations on oligomeric model systems, which also consistently reproduce the optical properties observed for the polymers. The potentialities of these materials for applications in organic electronics can be demonstrated by their use as donor materials in organic photovoltaic cells, which exhibit higher open circuit voltages (V_OC, up to 0.86 V) than P3HT‐ or analogous polythiophene‐based cells (V_OC = 0.5–0.6 V).     

Synthesis and Photovoltaic Performance of a [1,2,3]Triazolo[4,5‐g]quinoxaline‐Based Low‐Bandgap Polymer

A narrow‐bandgap conjugated polymer, PFDTBTzQ‐2OC1, is prepared by alternating [1,2,3]triazolo[4,5‐g]quinoxaline and 9,9‐didodecyl‐fluorene. With a bandgap of 1.63 eV, this polymer has wide absorption ranging from 300–760 nm in film. Bulk heterojunction solar cells fabricated by blending PFDTBTzQ‐2OC1 with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester exhibit a maximum power conversion efficiency of 1.31%, with a short‐circuit current density of 1.98 mA cm^–2, an open‐circuit voltage of 0.74 V, and a fill factor of 0.47.

     

Structural Analysis of Poly(3‐hexylthiophene) Prepared via Direct Heteroarylation Polymerization

This study reports the synthesis and characterization of poly(3‐hexylthiophene) (P3HT) from a direct heteroarylation polymerization of two isomeric monomers, 2‐bromo‐3‐hexylthiophene (monomer 1 ) and 2‐bromo‐4‐hexylthiophene (monomer 2 ). Using the Herrmann–Beller catalyst along with P(o‐NMe₂Ph)₃, the resulting polymers are obtained in excellent yields and exhibit a good number‐average molecular weight (M_n of 33 and 16 kDa, respectively). Detailed ¹H NMR analyses reveal less than 1% of homocouplings and no evidence of β‐branching. Dehalogenation is identified as the main chain termination step and preferentially occurs on monomer 2 . The melting temperature (237 °C) and hole mobility (up to 0.19 cm² V^?1.s^?1) of the nearly defect‐free P3HT obtained from this simple polymerization of monomer 1 are comparable, if not superior, to those obtained with commercially available GRIM and Rieke samples.