Electron-rich Units Research Articles

Low bandgap small molecules based on 2,2-bithiophene-3,3-dicarboximide for soluble-processed solar cells

Small molecules, SM1 and SM2, with bithiophene-imide as the central electron deficient unit and bridged thiophene and terminal triphenyl amine as electron rich units, have been designed and synthesized. The dithienoazepinedione, bithiophene imide (BTI) group, is an attractive electron deficient unit since it demonstrates strong electron-withdrawing character, a planar architecture, and good solubility. The spectra of SM1 and SM2 in the solid thin films show absorption bands with maximum peaks at 368, 495 and 397, 517nm, and the absorption onsets at 598 and 631nm. The electrochemical band gaps are about 1.45 and 1.43eV, somewhat lower than the optical band gaps. The best device with SM1: PC71BM in chloroform with thermal treatment at 130°C showed VOC of 0.91V, JSC of 3.93mA/cm2, and FF of 0.38, which yielded PCE of 1.36%.

Tuning Redox Chemistry and Photophysics in Core-Substituted Tetraazaperopyrenes (TAPPs)

Core substitution of tetraazaperopyrenes (TAPPs) has been achieved, and with it, considerable variation of their photo- and redox-chemical properties. Through Suzuki cross coupling starting from the fourfold core-brominated tetraazaperopyrene, aryl-substituted TAPPs were synthesized, which displayed very high fluorescence quantum yields (up to 100 %) in solution. Besides the Suzuki reactions, Stille and Sonogashira cross-couplings were also found to be suitable methods for core derivatization, as demonstrated in the syntheses of alkynyl-substituted tetraazaperopyrene congeners. Furthermore, TAPPs incorporating intramolecular donor-acceptor combinations of aromatic units (8, 9) were accessible by coupling the electron-poor peropyrene core with electron-rich aromatic units, which act as strong electron donors. Finally, C-heteroatom coupling (O, S, N) gave rise to novel TAPP derivatives with strongly modified redox-chemical behaviour and photophysics in the solid state as well in solution. In particular, TAPP derivatives displaying red fluorescence were obtained for the first time.

Synthesis and characterization of copolyformals containing electron-rich or electron-poor porphyrin units in the main chain and their use as sensors

In this study two new porphyrin monomers, 5,15-di(pentafluorophenyl)-10,20-di(p-hydroxyphenyl)-porphyrin and 5,15-di(pentabutoxyphenyl)-10,20-di(p-hydroxyphenyl)-porphyrin, were synthesized and characterized by NMR and MALDI-TOF mass spectrometry. The chemical characteristics of these monomers are strongly influenced by their respective electronic distribution which, in turn, is determined by the phenyl ring substituents. By reaction with different amounts of 1,20-di(bisphenoxy-A)-eicosane in presence of dibromomethane, copolyformals containing porphyrin dyes at different percentages were subsequently obtained. The sensing responses of the synthesized copolymers were tested with the following analytes: acetic, trifluoroacetic and hydrochloridic acids and nitrogen dioxide. Our data indicate that porphyrin units interact with analytes in a reversible fashion, it being understood that the decabuthoxy porphyrin derivative has a greater affinity than the other one with both acids and NO2.

Synthesis and photovoltaic properties of an alternating polymer based fluorene and fluorine substituted quinoxaline derivatives

An alternating polymer (PFOFTQx) with 9,9-dioctylfluorene (FO) as electron-rich unit and fluorine substituted quinoxaline (FTQx) as electron-withdrawing unit was synthesized and characterized. PFOFTQx showed similar absorption property with that of the counterpart polymer without fluorine atom (synthesized APFO-15). However, the low-lying highest occupied molecular orbit (HOMO) energy level of PFOFTQx was −5.37eV, about 0.07eV smaller than that of synthesized APFO-15. In order to study the photovoltaic properties of the materials, polymer solar cells (PSCs) were fabricated with PFOFTQx as donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor. The power conversion efficiency (PCE) of PSC was 1.77% with a high open-circuit voltage (Voc) of 0.90V for an optimized PFOFTQx:PC61BM weight ratio of 1:5, in comparison with that of synthesized APFO-15-based device (PCE of 1.60% with Voc of 0.77V). This study indicated that fluorine substituted quinoxaline-based polymers would be promising material with a higher Voc for the application in polymer solar cells.

Role of the Side Chain in the Phase Segregation of Polymer:Fullerene Bulk Heterojunction Composites

The most effi cient class of polymer materials (as donors) follows a push-pull molecular design that utilizes internal charge transfer (ICT) from electron-rich units to electron-defi cient moieties within the fundamental repeating unit of π -conjugated polymers ( π -CPs). [ 4–6 ] However, when push-pull π -CP donors form composites with fullerene acceptors to create BHJ active layers, undesirable phase segregation between the two components is observed on the scale of a few hundred nanometers. Because large-scale phase segregation increases the probability of exciton (electron-hole pair) recombination in the BHJ active layer due to the relatively short diffusion length of the exciton in the polymers (approximately 20 nm), [ 7 ] the PCE is drastically reduced in BHJ solar cells with large-scale phase segregation. [ 8,9 ]

A Push–Pull Macrocycle With Both Linearly Conjugated and Cross-Conjugated Bridges

A series of shape-persistent macrocycles featuring both m-phenylene and 2,5-thiophene linkers has been synthesized, including an example where they bridge electron-rich (veratrole) and electron-poor (phthalimide) units. Charge transfer in this "push-pull macrocycle" has been investigated by UV-vis and fluorescence spectroscopies and DFT calculations. The effect of pairing structurally distinct conjugated bridges is discussed in the context of acyclic and symmetrical macrocyclic analogs.

Electroluminescence at pure blue region from a new anthracene-contained polymer

We report an electroluminescence at pure blue region from a new anthracene-contained polymer. Ter(9,9-diarylfluorene)s of this polymeric unit is capable of ambipolar carriers transportation. The polymer also contains both electron-withdrawing oxadiazole unit and electron-rich carbazole unit. Most of all, the anthracene part of polymer is designed for keeping emission at pure blue region. The light-emitting diode fabricated with the emitter results to a luminous efficiency of 6.91cdA−1. The maximum luminescence is 13,159.8cdm−2. Moreover, the main peak of electroluminescence spectrum is at 446.2nm, which is in the pure blue region.

Ladder-Type Dithienonaphthalene-Based Donor–Acceptor Copolymers for Organic Solar Cells

A novel ladder-type dithienonaphthalene (DTN) was designed and synthesized as an electron-rich unit for constructing donor–acceptor copolymers. Different acceptor moieties, including benzo[c][1,2,5]thiadiazole (BT), 5,6-bis(hexyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TBT), and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPP), were used as electron-deficient units for the target copolymers. These copolymers (PDTNBT, PDTNTBT, and PDTNTDPP) were obtained via the Stille coupling reaction and characterized by 1H NMR spectroscopy, UV–vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography (GPC). Owing to the four solubilizing alkyl chains on the DTN unit, all the three copolymers have good solubility in common solvents. Among these polymers, PDTNTBT exhibits the highest space-charge limit current (SCLC) hole mobility of 2.13 × 10–5 cm2 V–1 s–1, which is beneficial for achieving high performance solar cells. Under the simulated AM 1.5G illumination condition (100 mW/cm2), solar cells based on PDTNTBT:PC71BM (1:3, w/w) exhibit a power conversion efficiency (PCE) of 4.8% with a current density of 10.3 mA cm–2, an open-circuit voltage of 0.86 V, and a fill factor of 54%. With the same device fabrication method, PDTNTDPP:PC71BM (1:3, w/w) and PDTNBT:PC71BM (1:3, w/w) based devices exhibit efficiencies of 1.52% and 2.79%, respectively. Furthermore, inverted solar cells based on these copolymer blends are also fabricated. The inverted devices based on PDTNTDPP:PC71BM (1:2, w/w) and PDTNBT:PC71BM (1:2, w/w) exhibit PCEs of 1.60% and 2.89%, respectively, which are similar to their corresponding conventional devices. And the inverted devices based on PDTNTBT:PC71BM (1:2, w/w) show a higher PCE of 5.0%, and more importantly, they are quite stable as demonstrated by the 4.75% PCE after ambient storage for two months.

Naphtho[1,2-b:5,6-b′]dithiophene-Based Donor–Acceptor Copolymer Semiconductors for High-Mobility Field-Effect Transistors and Efficient Polymer Solar Cells

Two donor–acceptor (D–A) copolymers, PzNDTDTBT and PzNDTDTBO, using 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene as an electron-rich unit and benzodiathiazole (BT) or benzoxadiazole(BO) as an electron-deficient one, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The acceptor units were varied from BT to BO for adjusting the energy levels and optimizing the structures of polymers. Both copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap and exhibit not only high field-effect mobilities but also high photovoltaic properties. The hole mobilities reach 0.43 and 0.34 cm2 V–1 s–1 for PzNDTDTBT and PzNDTDTBO film, respectively. Bulk heterojunction solar cells fabricated by using PzNDTDTBT or PzNDTDTBO as electron donor and PC71BM as acceptor show a power conversion efficiency of 3.22% and 5.07%, respectively, under illumination of AM1.5G, 100 mW cm–2. Both the hole mobilities and PCEs are among the highest values in the current reports based on naphthodithiophene-based polymers, indicating that “zigzag” naphthodithiophene-based D–A copolymers are very promising for application as solution-processable organic semiconductors in optoelectronic devices.

Tetrazine–triphenylamine dyads: Influence of the nature of the linker on their properties

Original new electroactive dyads have been prepared, featuring two different electroactive groups, namely a triphenylamine and a tetrazine, linked through various linkers. We describe here the synthesis, the electrochemical and some photophysical properties of these molecules which are composed of two separate components, one triphenylamine, electron-rich unit, and one tetrazine, electron-poor unit. The electroactivity of both moieties of the molecule is quite apparent in each case and potentials depend upon the type of linker between the groups.

Impact of Regiochemistry and Isoelectronic Bridgehead Substitution on the Molecular Shape and Bulk Organization of Narrow Bandgap Chromophores

A comparison of two classes of small molecules relevant to the field of organic electronics is carried out at the molecular and supramolecular levels. First, two molecules that differ only in the position of a pyridyl N-atom within an acceptor fragment are compared and contrasted. X-ray investigation of single crystals reveals that positioning the pyridyl N-atoms proximal to the molecules center changes the molecular shape by bending the molecule into a banana shape. Second, we demonstrate that the banana shape of the molecule can be controlled by replacing a Si atom within the dithienosilole fragment with a C or Ge atom. Here, utilization of cyclopentadithiophene or dithienogermole as the internal electron-rich unit leads to a decrease or an increase in the bending of the conjugated backbone, respectively. Such molecular shape changes alter intermolecular packing and thus affect bulk properties, leading to large differences in the optical, thermal, and crystallization properties.

Synthesis, characterization, and field-effect transistor performance of naphtho[1,2-b:5,6-b′]dithiophene-based donor–acceptor copolymers

Three donor–acceptor (D–A) copolymers, PzNDT-T-DPP, PzNDT-F-DPP and PzNDTTPD, using 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene (zNDT) as an electron-rich unit and diketopyrrolopyrrole (DPP) or thieno[3,4-c]pyrrole-4,6-dione (TPD) as an electron-deficient one, were designed and synthesized via a Pd-catalyzed Stille-coupling method. All copolymers possess good solubility, high thermal stability, as well as good film-forming ability. Optical and electrochemical characterization of thin films of these polymers reveals band gaps in the range of 1.29–2.04 eV. These polymers exhibit excellent hole mobility when used as the active layer in organic field-effect transistors (OFETs) devices. Among the series, the highest hole mobility of 0.11 cm2 V−1 s−1 is achieved in bottom gate and top-contact OFET devices based on PzNDT-F-DPP. This work demonstrates that zNDT is a promising unit to build D–A copolymers for organic semiconductors with good performance.

Charge transfer relaxation in donor–acceptor type conjugated materials

The development of conjugated materials bearing electron-rich and electron-poor units along their backbone introduces new possibilities to control functionality for organic electronic applications through charge transfer character in ground and excited states. A thorough understanding of intramolecular dipoles and their evolution during excited state relaxation is necessary in order to fully exploit this opportunity. PCDTBT is an alternating donor–acceptor copolymer with high photovoltaic efficiency in bulk heterojunction solar cells. We use time-resolved femtosecond transient absorption spectroscopy in solution and in the solid state to study PCDTBT and the dTBT and CDTBT model compounds, fragments of the polymer chain. Higher solubility and slower relaxation make CDTBT particularly suitable to understand the mechanism of charge transfer relaxation in this class of materials. A progressive increase of charge transfer character from the initially moderately polar excited state is mainly driven by solvent reorganization and some torsional rearrangements. Similar relaxation in solid state CDTBT might ultimately lead to the formation of separate charges.

Adjustment of charge trap number and depth in molecular backbone to achieve tunable multilevel data storage performance

A series of conjugated donor–acceptor (D–A) molecules with the electron-rich unit triphenylamine (TPA) as donor and azobenzene chromophore and/or cyano group as acceptors were successfully synthesized. Acceptors varying in number and electron-withdrawing strength were incorporated into the molecular backbone to investigate the impact on device switching behavior. It is found that the data storage states and memory effects are highly dependent on the number and strength of the electron-deficient groups. The devices based on the D–π–A1–π–A2 molecule with two electron-withdrawing groups exhibited excellent ternary memory behavior, while those based on the D–π–A1 or D–π–A2 molecule containing one electron-accepting moiety showed binary memory characteristics. In addition, the bistable memory effects of TPAVC 2 and TPAAH 3-based data storage devices varied from WORM to flash when the acceptor changed from a cyano group to an azobenzene chromophore. Therefore, tunable multilevel memory device performance has been achieved by adjusting the number and electron-withdrawing strength of acceptor moieties in the molecular backbone, offering guidance for the rational design of superior D–A molecules for high density data storage (HDDS) applications.

4,10‐Dibromoanthanthrone as a New Building Block for p‐Type, n‐Type, and Ambipolar π‐Conjugated Materials

New p-type, n-type, and ambipolar molecules were synthesized from commercially available 4,10-dibromoanthanthrone dye. Substitution at the 4,10- and 6,12-positions with different electron-rich and electron-poor units allowed the modulation of the optoelectronic properties of the molecules. A bis(dicyanovinylene)-functionalized compound was also prepared with a reduction potential as low as -50 mV versus Ag(+) with a crystalline two-dimensional lamellar packing arrangement. These characteristics are important prerequisites for air-stable n-type organic field-effect transistor applications.

Synthesis of a conjugated copolymer with benzodithiophene and benzimidazole units

A new accepter unit, dimethyl-2H-benzimidazole, was prepared and utilized for the synthesis of conjugated polymers containing an electron donor–acceptor pair for organic photovoltaics (OPVs). A dimethyl-2H-benzimidazole unit was designed to replace the benzothiadiazole (BT) unit of PCDTBT. An advantage of dimethyl-2H-benzimidazole compared with the BT moiety of PCDTBT is that the solubility of the polymer is improved while the 1,2-quinoid form is maintained to lead the coplanarity of the backbone. New semiconducting copolymers with dialkoxybenzo[1,2-b:3,4-b′]dithiophene as the electron-rich unit and 2,2-dimethyl-4,7-di(2-thienyl)-2H-benzimidazole as the electron-deficient unit were synthesized by Pd(0)-catalyzed Stille coupling polymerization. The resulting P2:PC71BM device had a open circuit voltage (VOC) value of 0.65 V, a short circuit current density (JSC) value of 1.86 mA cm−2 and a FF value of 0.39, with a final power conversion efficiency (PCE) value of 0.47%. A new accepter unit, dimethyl-2H-benzimidazole, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor–acceptor pair for organic photovoltaics. Dimethyl-2H-benzimidazole unit was designed to substitute the benzothiadiazole (BT) unit of PCDTBT. The advantage of dimethyl-2H-benzimidazole compared with the BT moiety of PCDTBT is to improve the solubility of the polymer while keeping the 1,2-quinoid form to lead coplanarity of the backbone.

Effect of Oligothiophene π-Bridge Length on the Photovoltaic Properties of D–A Copolymers Based on Carbazole and Quinoxalinoporphyrin

A series of low-bandgap donor–acceptor (D–A) copolymers, P(C-T-QP), P(C-BT-QP), P(C-TT-QP), and P(C-TT-QP-Zn), using 2,7-carbazole (C) as an electron-rich unit and quinoxalino[2,3-b′]porphyrins (QP) or quinoxalino[2,3-b′]porphyrinatozinc(QP-Zn) as an electron-deficient unit with different length of oligothiophene π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridge between the C donor unit and the QP acceptor unit is thiophene (T) in P(C-T-QP), bithiophene (BT) in P(C-BT-QP), and terthiophene (TT) in P(C-TT-QP) or P(C-TT-QP-Zn). These copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap ranging from 1.66 to 1.73 eV. The influence of the π-bridge and the central Zn ion on the electronic and photovoltaic properties was investigated and discussed in detail. It was found that the π-bridge played an important role in tuning the effective conjugation length and therefore significantly affected the molecular architecture and opt...

Synthsis of an Electron-Rich Donor: 2,7-Bis(<i>5-Bromothiophen-2-Yl</i>)-9,9'-Spirobi[<i>Fluorene</i>

A electron-rich unit, 2,7-bis(5-bromothiophen-2-yl)-9,9'-spirobi[fluorene] has been prepared in high yield over five steps. The compound with good electronic and optical properties may be a candidate for D-A type organic polymers.

Time-Dependent Density Functional Theory Study on Cyclopentadithiophene-Benzothiadiazole-Based Push-Pull-Type Copolymers for New Design of Donor Materials in Bulk Heterojunction Organic Solar Cells

Push-pull-type copolymers - low-band-gap copolymers of electron-rich fused-ring units (such as cyclopentadithiophene; CPDT) and electron-deficient units (such as benzothiadiazole; BT) - are promising donor materials for organic solar cells. Following a design principles proposed in our previous study, we investigate the electronic structure of a series of new CPDTBT derivatives with various electron-withdrawing groups using the time-dependent density functional theory and predict their power conversion efficiency from a newlydeveloped protocol using the Scharber diagram. Significantly improved efficiencies are expected for derivatives with carbonyl [C=O], carbonothioyl [C=S], dicyano [] and dicyanomethylene [C=] groups, but these polymers with no long alkyl side chain attached to them are likely to be insoluble in most organic solvents and inapplicable to low-cost solution processes. We thus devise several approaches to attach alkyl side chains to these polymers while keeping their high efficiencies.

Alkoxy Side Chains in Low Band-Gap Co-Polymers: Impact on Conjugation and Frontier Energy Levels

The molecular design of electron donor conjugated polymers with accurate optoelectronic properties for efficient bulk heterojunction photovoltaic devices has attracted a lot of attention worldwide. Ideally, the polymer should have i) appropriate photon harvesting properties, ii) frontier orbitals that allow both, efficient exciton dissociation and a high open circuit voltage, iii) high charge carrier mobilities, iv) high solubility and v) high molecular weights. Charge transfer D/A copolymers are promising materials that have allowed significant progress in this field. However, although a lot of attention has been given to the design of their molecular conjugated backbone, only few investigations have considered the influence of their solubilizing side chains. The aim of the present work is to show how the nature and the position of side chains may influence above mentioned polymer properties. For that purpose, our team has designed and characterized a series of donor-acceptor alternating conjugated polymers using 2,1,3- benzothiadiazole as electron-deficient unit, and thieno[3,2-b]thiophene as well as thiophenes moieties as electron-rich units. Various side-chains were introduced onto different positions along the same conjugated backbone and the molecular structure/polymer properties relationships have been investigated systematically.