Ahmed Najari

A Thieno[3,4-c]pyrrole-4,6-dione-Based Copolymer for Efficient Solar Cells

A new low-band-gap thieno[3,4-c]pyrrole-4,6-dione-based copolymer, PBDTTPD, has been designed and synthesized. PBDTTPD is soluble in chloroform or o-dichlorobenzene upon heating and shows a broad absorption in the visible region. The HOMO and LUMO energy levels were estimated to be at -5.56 and -3.75 eV, respectively. These electrochemical measurements fit well with an optical bandgap of 1.8 eV. When blended with PC(71)BM, this polymer demonstrated a power conversion efficiency of 5.5% in a bulk-heterojunction photovoltaic device having an active area of 1.0 cm(2).

A high mobility DPP-based polymer obtained via direct (hetero)arylation polymerization

Many diketopyrrolopyrrole-based (DPP) polymers have shown remarkable transport properties and to fabricate cost efficient materials, the new direct (hetero)arylation polymerization (DHAP) could become a valuable tool. Although the DHAP offers great promise for more efficient, atom-economic and cheaper aromatic C–C bond formations, this method seems to involve a lack of selectivity. Herein, we report an alternating DPP-based copolymer prepared by DHAP that shows a well-defined structure. A rational side-chain design of the comonomers led to a polymer that exhibits a high molecular weight along with excellent charge transport properties (p-type mobility up to 1.17 cm2 V−1 s−1).

Bulk heterojunction solar cells based on a low-bandgap carbazole-diketopyrrolopyrrole copolymer

Bulk heterojunction (BHJ) solar cells fabricated with a phase separated nanomaterial comprising a carbazole-diketopyrrolopyrrole copolymer (PCBTDPP) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are demonstrated with power conversion efficiency>3.5%. The PCBTDPP:PC70BM BHJ nanomorphology was controlled by changing the length of the alkyl side-chain of the polymer and by utilizing processing additives.

Organic thermoelectrics: Green energy from a blue polymer

Efficient energy harvesting from temperature gradients requires thermoelectric materials with low thermal and high electrical conductivities. A conducting polymer can fulfil these conditions if its doping level is controlled precisely.

Easy and versatile synthesis of new poly(thieno[3,4-d]thiazole)s

New alternating copolymers based on thieno[3,4-d]thiazole (TTz) derivatives were synthesized by Stille, Suzuki or direct (hetero)arylation polycondensation (DHAP) reactions using either benzodithiophene (BDT), dithienosilole (DTS), thieno[3,4-c]pyrrole-4,6-dione (TPD), diketopyrrolopyrrole-1,4-dione (DPP) or isoindigo units as comonomers. In particular, the direct hetero(arylation) polycondensation reaction has been shown to be a very interesting tool for a more efficient and economical access to new conjugated polymers. Modifications of the TTz moiety and of the polymer backbone show that it is possible to efficiently modulate the HOMO and LUMO energy levels of TTz-based copolymers. These conjugated polymers exhibit bandgaps between 1.07 and 1.82 eV with HOMO energy levels ranging from −5.06 to −5.48 eV and LUMO energy levels ranging from −3.61 to −4.02 eV.

Direct heteroarylation of β-protected dithienosilole and dithienogermole monomers with thieno[3,4-c]pyrrole-4,6-dione and furo[3,4-c]pyrrole-4,6-dione

Direct C–H bond arylation reactions between heteroarenes and aryl halides provide an atom-economical and “green” alternative to standard cross-coupling reactions (Stille, Suzuki, etc.). Unfortunately, this reaction is not selective and more than one type of C–H bond may react, which, during polymerization reactions, can lead to cross-linked materials. This paper reports the preparation of PDTSiTPD and PDTGeTPD, which have exhibited high efficiencies in organic solar cells, using direct (hetero)arylation polymerization methodologies. In order to circumvent side reactions leading to cross-linked polymers, a number of new dithieno[3,2-b:2′,3′-d]silole (DTSi) monomers were prepared where the β-positions were blocked with alkyl chains and the alkyl groups on the heteroatom were modified. Co-polymers were synthesized with N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) and the oxygen congener, N-alkylfuro[3,4-c]pyrrole-4,6-dione (FPD). However, the resulting polymers were not planar, and conjugation of the backbone was disrupted. An efficiency of 1.7% was achieved in bulk heterojunction solar cells (BHJ-SCs).

Development of quinoxaline based polymers for photovoltaic applications

Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) structure, i.e. a blend of a p-type conjugated polymer with an n-type semiconductor acceptor, have made rapid progress over the past decade. In comparison with inorganic semiconductor solar cells, PSCs have the advantages of low cost, light weight, solution processability and good mechanical flexibility. In the last few years, various classes of electron-donating polymers have been reported for PSCs. Among them, quinoxaline (Qx) and its derivatives have been widely used as building blocks for optoelectronic applications because they can be easily modified by varying the side chains, such as alkyl chains, conjugated aromatic rings, functional groups, etc. Recently, a power conversion efficiency (PCE) of over 11% was achieved for PSCs with Qx-based polymers. This PCE is among the best for PSCs, and it suggests that Qx-based polymers have great potential for highly efficient PSCs. In this article, we review the recent advances in the design and synthesis of such Qx-based conjugated polymers for photovoltaic applications. Particular attention is paid to the chemical structures of the polymers including flexible chains, conjugated side chains, functional groups, Qx derivatives and the effect of the molecular structure on device performance parameters. We believe that further development of Qx-based polymers will lead to a PCE >12% in the near future.

Synthesis of new pyridazine-based monomers and related polymers for photovoltaic applications.

New aromatic compounds with a pyridazine core have been synthesized. Four electron-withdrawing monomers have been easily prepared from simple condensation reactions and ring closure procedures. Optimized HOMO, LUMO, and bandgap energy levels have been obtained. The resulting conjugated polymers have been tested in organic solar cells. First studies have revealed power conversion efficiencies up to 0.5% for an active area of 1.0 cm(2) .

Donor–acceptor alternating copolymers containing thienopyrroledione electron accepting units: preparation, redox behaviour, and application to photovoltaic cells

Four new donor–acceptor semiconducting alternating copolymers consisting of thienopyrroledione (TPD) electron-accepting sub-units and either 2,7-carbazole or dialkoxy substituted benzodithiophene electron-donating moieties were prepared either by Suzuki or Stille coupling. Analytical size exclusion chromatography (SEC) was used to fractionate the copolymer with 1:1 thienopyrroledione to carbazole ratio into 9 sharp fractions. The effective conjugation length (35 aromatic rings) was determined on the basis of the dependence of λmax of the π–π* band on the degree of polymerization, DPn. All four polymers showed similar supramolecular organization resembling that of poly(3-alkylthiophene)s since the mechanism of the self-assembly was the same in both cases: π-stacking of the π-conjugated polymer backbones and interdigitation of n-alkyl side chains. As shown by cyclic voltammetry studies, three copolymers showed band gaps inferior to 2 eV with HOMO and LUMO levels ranging between −5.62 eV to −5.08 eV and −3.53 eV to −3.13 eV, respectively. UV-vis-NIR spectroelectrochemical investigations confirmed the results obtained by cyclic voltammetry, enabling in addition more precise determination of the HOMO level. Raman spectroelectrochemical studies showed that the polymer with 1:1 thienopyrroledione to carbazole ratio is prone to oxidative degradation, consistent with cyclic voltammetry studies. A bulk-heterojunction solar cell was fabricated from the copolymer consisting of thienopyrroledione and dialkoxy substituted benzodithiophene. A power conversion efficiency of 1.63% was achieved for non-optimized devices.

Random D–A1–D–A2 terpolymers based on benzodithiophene, thiadiazole[3,4-e]isoindole-5,7-dione and thieno[3,4-c]pyrrole-4,6-dione for efficient polymer solar cells

Recent developments on regio-regular and random terpolymers based on a donor/acceptor architecture (D1–A–D2–A or D–A1–D–A2) have shown promising results for their use as donor materials in bulk heterojunction (BHJ) polymer solar cells. New random terpolymers including benzo[1,2-b:4,5-b′]-dithiophene (BDT) as the electron-donor moiety and thiadiazolo[3,4-e]isoindole-5,7-dione (TID) and thieno[3,4-c]pyrrole-4,6-dione (TPD) as electron-accepting moieties were synthesized by Stille cross-coupling polymerization. Incremental addition of TPD (from 0 to 90%) led to the following random terpolymers: P[(BDT-TID)x-(BDT-TPD)y]n. Their electro-optical and photovoltaic properties were investigated. These high molecular weight and highly processable random terpolymers exhibited broad and strong absorption (300–800 nm), moderate bandgaps (1.52 eV to 1.64 eV) and deep-lying HOMO/LUMO energy levels (−5.6 eV and −3.9 eV, respectively). First, the optimized inverted device ITO/ZnO/P1:PC71BM/MoO3/Ag (where P1 is a typical D/A copolymer; TID 100%; TPD 0%) showed a power conversion efficiency up to 6.36% which is higher than the PCE reported in the literature for an analog of P1 (3.42%). Then, devices based on random terpolymers (P2–P10) showed power conversion efficiencies ranging from 5.06% (for P7; 40% TID/60% TPD) up to 6.70% (for P2; 90% TID/10% TPD) when PC61PM is used as the electron acceptor. A PCE as high as 7.30%, a Voc of 0.81 V, a Jsc of 13.86 mA cm−2 and a FF of 65% were achieved for P2 when PC71PM was used. This PCE is among the highest PCE values reported for random terpolymer-based polymer solar cells.