Salem Wakim

Bulk heterojunction solar cells using thieno[3,4-c]pyrrole-4,6-dione and dithieno[3,2-b:2',3'-d]silole copolymer with a power conversion efficiency of 7.3%.

A new alternating copolymer of dithienosilole and thienopyrrole-4,6-dione (PDTSTPD) possesses both a low optical bandgap (1.73 eV) and a deep highest occupied molecular orbital energy level (5.57 eV). The introduction of branched alkyl chains to the dithienosilole unit was found to be critical for the improvement of the polymer solubility. When blended with PC(71)BM, PDTSTPD exhibited a power conversion efficiency of 7.3% on the photovoltaic devices with an active area of 1 cm(2).

Highly efficient organic solar cells based on a poly(2,7-carbazole) derivative

We have studied the utilization of PCDTBT, an alternating poly(2,7-carbazole) derivative, in organic bulk heterojunction solar cells. The effect of polymer molecular weight, PCDTBT:[60]PCBM ratio, and active layer thickness on the device performance is reported. The best performance was obtained when the number-average molecular weights (Mn) are around 20 kDa with a polydispersity index around 2.2. Both PCDTBT:[60]PCBM ratio and active layer thickness affect the light absorption and the charge transport properties. By optimizing these two parameters, power conversion efficiency (PCE) up to 4.35% was reached under calibrated AM1.5G illumination of 100 mW cm−2. When blended with [70]PCBM, PCDTBT exhibited a PCE up to 4.6%.

Morphology control in polycarbazole based bulk heterojunction solar cells and its impact on device performance

Incremental increase in dimethyl sulfoxide (or dimethyl formamide) in ortho-dichlorobenzene solution of poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) gradually reduces the polymer-solvent interaction, the attraction forces between polymer chains become more dominant, and the polymer chains adopt a tight and contracted conformation with more interchain interactions, resulting in a progressive aggregation in both solutions and films. This was used to fine tune the morphology of PCDTBT/PC71BM based solar cells, leading to improved domain structure and hole mobility in the active layer, and significantly improved photovoltaic performance. The power conversion efficiency increased from 6.0% to 7.1% on devices with an active area of 1.0 cm2.

Highly efficient polycarbazole-based organic photovoltaic devices

We combined experimental and computational approaches to tune the thickness of the films in poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)-based organic solar cells to maximize the solar absorption by the active layer. High power-conversion efficiencies of 5.2% and 5.7% were obtained on PCDTBT-based solar cells when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the electron acceptor, respectively. The cells are designed to have an active area of 1.0 cm2, which is among the largest organic solar cells in the literature, while maintaining a low series resistance of 5 Ω cm2.

Charge Transport, Photovoltaic, and Thermoelectric Properties of Poly(2,7‐Carbazole) and Poly(Indolo[3,2‐b]Carbazole) Derivatives

This review highlights recent advances made with poly(2,7‐carbazole) and poly(indolo[3,2‐b]carbazole) derivatives in organic field‐effect transistors, photovoltaic cells, and thermoelectric devices. For each of these applications, several examples of the structure‐property relationships occurring in these classes of materials are discussed. For instance, this review shows that the excellent hole transporting properties of poly(2,7‐carbazole)s make them highly promising materials for p‐type transistors and solar cells. Moreover, the chemical and physical properties of these N‐containing conjugated polymers can be easily tuned to meet the different requirements to set‐up the so‐called plastic electronics where micro‐electronic devices could be printed onto various substrates.

New indolo[3,2-b]carbazole derivatives for field-effect transistor applications

The synthesis, characterization, and field-effect transistor (FET) properties of new indolo[3,2-b]carbazole (IC) based materials are reported. Instead of adding the long alkyl chains on the nitrogen atoms of the IC backbone like many other IC-based molecules, they were added at both ends of the molecule (octylthiophene, p-octylbenzene). Also, the amine groups on the IC backbone were either free or protected by methyl groups. The impact on the organization and thin-film morphology showed that the molecules stand perpendicular to the surface as demonstrated by XRD and AFM. The highest hole mobility obtained by these new p-type organic semiconductors was 0.22 cm2V−1 s−1 with an on/off ratio of about 105. The best performance was obtained with 3,9-di(p-octylbenzene)-5,11-dihydroxyindolo[3,2-b]carbazole. This performance is one of the best obtained by both IC derivatives and materials containing a secondary amine on the backbone.

New low band gap thieno[3,4-b]thiophene-based polymers with deep HOMO levels for organic solar cells

Two new soluble alternating alkyl-substituted benzo[1,2-b:4,5-b′]dithiophene and ketone-substituted thieno[3,4-b]thiophene copolymers were synthesized and characterized. We found that grafting 3-butyloctyl side chains to the benzo[1,2-b:4,5-b′]dithiophene unit at C4 and C8 afforded the resulting polymer (P1) a high hole mobility (∼10−2 cm2Vs−1) and a low-lying HOMO energy level (5.22 eV). Preliminary experiments in bulk heterojunction solar cells using P1 as the electron donor demonstrated a high power conversion efficiency of 4.8% even with PC61BM as the electron acceptor. The introduction of an electron-withdrawing fluorine atom into the thieno[3,4-b]thiophene unit at the C3 position (P2) lowers the HOMO energy level and consequently improves the open circuit voltage from 0.78 to 0.86 V. These values are about 0.1 V higher than those reported for their analogues based on alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene. This work demonstrates that the replacement of the alkoxy chains on the benzo[1,2-b:4,5-b′]dithiophene unit with less electron-donating alkyl chains is able to lower the HOMO energy levels of this class of polymers without increasing their band gap energy.

Syntheses, Phase Behavior, Supramolecular Chirality, and Field‐Effect Carrier Mobility of Asymmetrically End‐Capped Mesogenic Oligothiophenes

A novel series of asymmetrically end-capped mesogenic oligothiophenes, with various oligothiophene core lengths, alkoxy tail lengths, and molecular polarities through introducing alkylsulfanyl or alkylsulfonyl functionalities as the terminal group, have been synthesized by palladium-catalyzed Suzuki cross-coupling and Kumada cross-coupling reactions as key steps. For the single end-capped oligothiophenes, C(m)O-Ar-OT(4)-H in which m=10, 12, 14, 16, and 18, all of these oligomers exhibited a broad temperature range of highly ordered smectic E and enantiotropic nematic phases, apart from the one with the longest octadecyloxy tail. For the double end-capped series C(10)O-Ar-OT(n)-R, R=Ph-SC(6) or Ph-SO(2)C(6) in which n=1, 2, 3, and 4, oligomers with more than one thiophene ring exhibited smectic A and smectic C phases, various crystal polymorphs and/or unusual low-temperature condensed phases. In the nonpolar, alkylsulfanylphenyl-substituted oligothiophene series, both the crystal/solid melting point and mesogenic clear point increased significantly with an increasing oligothiophene conjugation length. In the polar, alkylsulfonylphenyl-substituted oligothiophene series, all the oligomers showed increased melting points, but decreased mesogenic temperature intervals than those of their corresponding alkylsulfanyl counterparts. Remarkably, two different helical structures showing distinct striated textures or striped patterns were observed with a pitch of several to tens of micrometers under a polarized optical microscope upon cooling from their preceding fluidic smectic phases. The unusual twisted smectic layer structures in the thin solid films exhibiting distinct supramolecular chirality of both handednesses, revealed by circular dichroism measurements, were further confirmed by XRD analyses characterized by a sharp layer reflection together with its higher orders and diffuse wide-angle scatterings. In addition, initial studies showed that the highly ordered smectic phase of the single end-capped oligothiophenes can be utilized to improve field-effect charge mobility. C(10)O-Ar-OT(4)-H showed a hole mobility of 0.07 cm(2) V(-1) s(-1) when deposited on octyltrichlorosilane-treated substrates at 140 degrees C and the on/off current ratios reached 5 x 10(5); on the other hand, its mobility was only 8 x 10(-3) cm(2) V(-1) s(-1) on the same substrate when deposited at room temperature.

Synthesis of oligofluorene modified C60 derivatives for organic solar cell applications

New oligofluorene modified C60 derivatives were designed and synthesized viarhodium complex catalyzed direct coupling between C60 and oligofluorene boronic acids in H2O/1,2-dichlorobenzene (1 : 4). The product was first purified by silica gel column chromatography and then further purified by semipreparative HPLC equipped with a Buckyprep column using toluene as an eluent. The strong deep-blue fluorescence of the oligofluorene bridge was completely quenched after end-capped with C60, indicating an efficient energy and/or charge transfer between the π-conjugated bridge and the C60 cages. Cyclic voltammetric measurements show that the HOMO and LUMO energy levels of the synthesized C60 derivatives are similar to those of widely used PC61BM. This means that the energy levels of the C60 derivatives mainly depend on the π electron system of the C60 cage. However, the substituents have huge impacts on the other physical properties of the resulting C60 derivatives, such as solubility, crystallinity, and electron mobility.

Synthesis of N‐Octyl‐2,7‐dimethoxy‐1,8‐bistrimethylsilyl‐3,6‐dibromocarbazole

Abstract Starting with 2,7‐dimethoxycarbazole, a N‐alkyl‐hexafunctionalized carbazole derivative has been synthesized in only three steps with a good overall yield. N‐Octyl‐1,8‐bistrimethylsilyl‐3,6‐dibromo‐2,7‐dimethoxycarbazole is a new synthon that could be useful for the synthesis of conjugated polymers or bioactive compounds.