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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.

A new semiconducting polymer based on terthiophene-thienopyrrolodione alternating building blocks with a deep HOMO energy level (5.66 eV) is presented. The polymer is prepared by a direct heteroarylation polycondensation reaction, which is a low-cost and green alternative to the standard Stille coupling reaction and thus avoids any use of toxic stannyl derivatives. Integrating the polymer into bulk heterojunction solar cells with [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM) leads to a PCE of over 6% and a high open-circuit voltage of up to 0.94 V. To obtain these results a unique processing additive, 1-chloronaphthalene, is used, and a relatively low concentration of [70]PCBM is used in the blend solution.

New alternating copolymers derived from thieno[3,4-c]pyrrole-4,6-dione (TPD) and 2,7-carbazole moieties have been synthesized by Suzuki cross-coupling reaction and characterized. These polymers combine interesting properties such as good solubility and excellent thermal and air stability. The present studies indicate that the combination of TPD and 2,7-carbazole building blocks can be a very effective way to lower the HOMO energy level and ultimately to enhance the Voc of polymer solar cells. The Voc reported here (up to 1.07 V) is one of the highest observed for polymer:[60]PCBM bulk heterojunction devices, and preliminary results on the photovoltaic devices (power conversion efficiencies up to 1.8%) indicate that performance could probably be improved by increasing the molecular weights and by fine-tuning the electronic properties and the morphology.

A new class of low-bandgap copolymers based on benzodithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) units is reported. Chemical modifications of the conjugated backbone promote both high molecular weights and processability while allowing for tuning of the electronic properties. Copolymers with substituted thiophene spacers (alkyl chains facing the BDT unit) or unsubstituted thiophene spacers tend to have low power conversion efficiencies (PCE less than 1%) due to a bad morphology of the active layer, whereas copolymers with substituted thiophene spacers (alkyl chains facing TPD unit) show enhanced morphology and PCEs up to of 3.9%. Finally, BDT-TPD copolymers without any thiophene spacers still show the best performances with power conversion efficiencies up to 5.2%.

An efficient route for the synthesis of 1-iodo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione as a key intermediate to build new electron-deficient monomers and related conjugated polymers is reported. Along these lines, two new low bandgap copolymers were synthesized from Stille or Suzuki coupling. These polymers were characterized and their properties compared to those of analogous conjugated polymers.

A new dithienyl benzotriazole-based conjugated polymer was synthesized by Suzuki coupling reaction. The polymer was found to be soluble in common organic solvents, such as chloroform, tetrahydrofuran and chlorobenzene, with excellent film-forming properties. The structure of the polymer was confirmed by 1 H NMR, the molecular weights determined by GPC and the thermal properties investigated by TGA and DSC. The polymer films exhibited an absorption band in the wavelength range 300 to 610 nm. Preliminary photovoltaic cells based on the composite structure of indium tin oxide (ITO)/PEDOT:PSS/PCDTBTz:PC 60 BM (1:2 w/w)/ Al showed an open-circuit voltage of 0.92 V, a power conversion efficiency of 2.2% and a short circuit current of 5.33 mA cm -2 .

The improved synthesis of thieno[3,4-c]pyrrole-4,6-dione (TPD) monomers, including Gewald thiophene ring formation, a Sandmeyer-type reaction, and neat condensation with an amine, is presented. This protocol enables faster, cheaper, and more efficient preparation of TPD units in comparison to traditional methods. Furthermore, a series of TPD homo- and pseudohomopolymers bearing various alkyl chains was synthesized via a direct heteroarylation polymerization (DHAP) procedure. UV-visible absorption and powder X-ray diffraction measurements revealed the relationship between the ratio of branched to linear alkyl chains and the optoelectronic properties of the polymers as well as their packing in the solid state.

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 series of new dithieno[3,2-b:2′,3′-d]germole copolymers have been synthesized and characterized. The dithienogermole unit has been polymerized with different aromatic comonomers such as the benzothiadiazole (PGe1-C8 and PGe1-EH) and thieno[3,4-c]pyrrole-4,6-dione (PGe2). Suzuki and Stille coupling polymerizations under various conditions have been utilized. The polymers were then characterized by size-exclusion chromatography and thermal analyses (TGA, DSC), and their optical and electronic properties were investigated by UV-vis absorption spectroscopy and cyclic voltammetry. These low bandgap polymers (1.3-1.7 eV) have also been tested for photovoltaic applications; the best result was achieved with polymer PGe2, which shows a power conversion efficiency of 4.1%. © 2011 American Chemical Society.

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). © 2013 The Royal Society of Chemistry.

Three new n-type copolymers were synthesized using the isoindigo monomer. 5-Octylthieno[3,4-c]pyrrole-4,6-dione (TPD), 5,5′-dioctyl-1,1′-4H-bithieno[3,4-c]pyrrole-4,4′,6,6′(5H,5′H)-tetrone (BTPD) and 3,6-bis(thiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) were utilized as electron-withdrawing comonomers to obtain reduced or low bandgap n-type copolymers with deep HOMO and LUMO energy levels. The TPD and BTPD copolymers were synthesized using direct arylation polymerization and show bandgaps of 1.72 and 1.75 eV, respectively. Their LUMO and HOMO energy levels are also low at −4.2 and −6.0 eV, respectively. We investigated their electron mobility using thin film transistors and achieved electron mobility as high as 3.0 × 10−4 and 3.5 × 10−3 cm2 s−1 V−1 for the TPD and BTPD copolymers. The DPP copolymer was synthesized using Suzuki conditions and shows a low bandgap of 1.35 eV and a low LUMO energy level of −4.0 eV. The DPP copolymer exhibits an electron mobility of 2.7 × 10−4 cm2 s−1 V−1. All these polymers show interesting properties as potential electron acceptors in all-polymer solar 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.