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25th Anniversary Article: Isoindigo‐Based Polymers and Small Molecules for Bulk Heterojunction Solar Cells and Field Effect Transistors

Driven by the potential advantages and promising applications of organic solar cells, donor-acceptor (D-A) polymers have been intensively investigated in the past years. One of the strong electron-withdrawing groups that were widely used as acceptors for the construction of D-A polymers for applications in polymer solar cells and FETs is isoindigo. The isoindigo-based polymer solar cells have reached efficiencies up to ∼7% and hole mobilities as high as 3.62 cm(2) V(-1) s(-1) have been realized by FETs based on isoindigo polymers. Over one hundred isoindigo-based small molecules and polymers have been developed in only three years. This review is an attempt to summarize the structures and properties of the isoindigo-based polymers and small molecules that have been reported in the literature since their inception in 2010. Focus has been given only to the syntheses and device performances of those polymers and small molecules that were designed for use in solar cells and FETs. Attempt has been made to deduce structure-property relationships that would guide the design of isoindigo-based materials. It is expected that this review will present useful guidelines for the design of efficient isoindigo-based materials for applications in solar cells and FETs.

Substituted polythiophenes designed for optoelectronic devices and conductors

Polythiophenes are a very versatile class of conjugated polymers. Substituted polythiophenes can be tailored for various applications by designing the side groups to give the polymer different desired properties. Our work on preparing polythiophenes designed to have high stability in the doped state is described. We also discuss our efforts on tuning the colour of the emission from polythiophenes for use in polymer light-emitting diodes. Design criteria for the synthesis of polythiophenes with high luminescence efficiency for use in light-emitting diodes and lasers are also described. Finally, the design of polythiophenes for use in photodiodes is discussed.

Fluorine substitution enhanced photovoltaic performance of a D–A1–D–A2 copolymer

A new alternating donor-acceptor (D-A1-D-A2) copolymer containing two electron-deficient moieties, isoindigo and quinoxaline, was synthesized. The photovoltaic performance of this polymer could be improved by incorporating fluorine atoms into the quinoxaline units, resulting in an efficiency of 6.32%. This result highlights the attractive promise of D-A1-D-A2 copolymers for high-performance bulk heterojunction solar cells.

Small band gap polymers synthesized via a modified nitration of 4,7-dibromo-2,1,3-benzothiadiazole.

The nitration of 4,7-dibromo-2,1,3-benzothiadiazole was modified by using CF(3)SO(3)H and HNO(3) as the nitrating agent, and the related yield was improved greatly. On the basis of this improvement, two new small band gap polymers, P1TPQ and P3TPQ, were developed. Bulk heterojunction solar cells based on P3TPQ and [6,6]-phenyl-C(71)-butyric acid methyl ester exhibit interesting results with a power conversion efficiency of 2.1% and photoresponse up to 1.1 μm.

Electrochemical bandgaps of substituted polythiophenesElectronic supplementary information (ESI) available: cyclic voltammograms not given within the article. See http://www.rsc.org/suppdata/jm/b3/b301403g/

The electrochemical bandgaps for different soluble substituted polythiophenes have been measured by cyclic voltammetry. The effect of substituents on the oxidation/reduction potentials is discussed. Bandgaps obtained by cyclic voltammetry have been found to be in general higher than optical bandgaps. Among regioregular polymers substituted with a phenyl group at position 3 of the thiophene ring, examples are found that give very symmetric voltammograms. Rationalization for this behaviour is discussed from a conformational point of view.

Integration of amyloid nanowires in organic solar cells

Amyloid nanowires were incorporated in organic photovoltaic devices in order to enhance the transport properties. Amyloid fibrils act as a template for donor-acceptor materials. The current-voltage characteristics under illumination and in the dark display a maximum for the fill factor and the space charge limit current, respectively, at an amyloid nanowire-donor-acceptor mass ratio of 0.014:1:1, associated to a better charge transport in the donor-acceptor domains. The absorption experiments display a redshift associated to a more planar polymer backbone with increasing concentration of amyloid fibrils. Amyloid nanowires present a significant effect on the donor-acceptor materials organization.

Total synthesis, antiprotozoal and cytotoxicity activities of rhuschalcone VI and analogs

The total synthesis of a potent antiplasmodial natural bichalcone, rhuschalcone VI, is described starting from simple and available resorcinol and 4-hydroxybenzaldehyde. Key steps include the solvent-free Aldol syntheses of chalcones, and the successful application of the Suzuki-Miyaura coupling reaction in the synthesis of bichalcones. The present work constitutes a general method for the rapid syntheses of a number of bichalcones related to rhuschalcone VI. Some of the bichalcones showed moderate antiprotozoal activities against Bodo caudatus, a preliminary screening system for antitrypanosomal activities, most of them with little or no cytotoxicity.

High-photovoltage all-polymer solar cells based on a diketopyrrolopyrrole-isoindigo acceptor polymer

In this work, we synthesized and characterized two new n-type polymers PTDPP-PyDPP and PIID-PyDPP. The former polymer is composed of pyridine-flanked diketopyrrolopyrrole (PyDPP) and thiophene-flanked diketopyrrolopyrrole (TDPP). The latter polymer consists of PyDPP and isoindigo (IID). PIID-PyDPP exhibits a much higher absorption coefficient compared to the widely used naphthalene diimide (NDI)-based acceptor polymers, and its high-lying LUMO level affords it to achieve a high open-circuit voltage (Voc). As a result, an all-polymer solar cell (all-PSC) fabricated from a high band gap polymer PBDTTS-FTAZ as the donor and PIID-PyDPP as the acceptor attained a high Voc of 1.07 V with a power conversion efficiency (PCE) of 4.2%. So far, it has been one of the highest PCEs recorded from all-PSCs using diketopyrrolopyrrole (DPP)-based acceptors. Gratifyingly, no obvious PCE decay was observed in two weeks, unraveling good stability of the all-PSC. This work demonstrates that the electron-withdrawing PyDPP unit can be a promising building block for new acceptor polymers in all-PSCs.

Black Polymers in Bulk-Heterojunction Solar Cells

The active materials in polymer solar cells have a decisive role on the performance of the cells. Polymers with extended absorption, i.e., black polymers with absorption covering the whole visible region are desired in order to capture the important parts of the solar irradiation. Different ways of achieving black active materials are discussed and two new alternating polyfluorene (APFO) copolymers with broad absorption, APFO-Black 1 and APFO-Black 2, using two different design strategies are described. The UV-Vis absorption spectra of the polymers extend to approximately 850 nm, and the polymers were used as donors and [6,6]phenyl-C61-butyric acid methyl ester (PCBM)[60] or PCBM[70] as acceptors in solar cell devices in various mixing ratios. The best combinations yielded an overall power conversion efficiency of 1.2% for APFO-Black 1 and 1.5% for APFO-Black 2.

Investigation of hole-mobility in a polyfluorene copolymer by admittance spectroscopy

Transport of holes in a low band gap polyfluorene, APFO-Green6, was investigated by means of admittance spectroscopy in the modulation frequency range 1–106 Hz. At room temperature, hole mobility of APFO-Green6 is dependent on the applied electric field, as commonly observed in disordered organic materials. The excess capacitance toward low frequencies provides evidence for charge relaxation in trap levels. A dispersion parameter of 0.4 was achieved from the trend of hole transit times with the electric field.