p/n Switching of Ambipolar Bithiazole–Benzothiadiazole-Based Polymers in Photovoltaic Cells

Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low Band Gap Semiconductors by Terminal Functionalization

Organic semiconductors with variable charge carrier polarity are required for optoelectronic applications. Herein, we report the synthesis of three novel diketopyrrolopyrrole (DPP)-based D-A molecules having three different terminal groups (amide, ester, and dicyano) and study their electronic properties. An increase in electron acceptor strength from amide to dicyano leads to a bathochromic shift in absorption. Photoconductivity and field effect transistor (FET) measurements confirmed that a small increase in acceptor strength can result in a large change in the charge transport properties from p-type to n-type. The molecule with an amide group, DPP-amide, exhibited a moderate p-type mobility (1.3 × 10^-2 cm² V^-1 s^-1), whereas good n-type mobilities were observed for molecules with an ester moiety, DPP-ester (1.5 × 10^-2 cm² V^-1 s^-1), and with a dicyano group, DPP-DCV (1 × 10^-2 cm² V^-1 s^-1). The terminal functional group modification approach presented here is a simple and efficient method to alter the charge carrier polarity of organic semiconductors.

Bithiazole: An Intriguing Electron-Deficient Building for Plastic Electronic Applications

The heterocyclic thiazole unit has been extensively used as electron-deficient building block in π-conjugated materials over the last decade. Its incorporation into organic semiconducting materials is particularly interesting due to its structural resemblance to the more commonly used thiophene building block, thus allowing the optoelectronic properties of a material to be tuned without significantly perturbing its molecular structure. Here, we discuss the structural differences between thiazole- and thiophene-based organic semiconductors, and the effects on the physical properties of the materials. An overview of thiazole-based polymers is provided, which have emerged over the past decade for organic electronic applications and it is discussed how the incorporation of thiazole has affected the device performance of organic solar cells and organic field-effect transistors. Finally, in conclusion, an outlook is presented on how thiazole-based polymers can be incorporated into all-electron deficient polymers in order to obtain high-performance acceptor polymers for use in bulk-heterojunction solar cells and as organic field-effect transistors. Computational methods are used to discuss some newly designed acceptor building blocks that have the potential to be polymerized with a fused bithiazole moiety, hence propelling the advancement of air-stable n-type organic semiconductors.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

Benzobisthiadiazole-alt-bithiazole copolymers with deep HOMO levels for good-performance field-effect transistors with air stability and a high on/off ratio

Two benzobisthiadiazole-alt-bithiazole copolymers were designed and synthesized, and the resulting transistors achieved high performance with air stability and a high on/off ratio.

p/n-Polarity of thiophene oligomers in photovoltaic cells: role of molecular vs. supramolecular properties

Molecular and supramolecular properties play key roles in the optoelectronic properties and photovoltaic performances of organic materials. In the present work, we show how small changes in the molecular structure affect such properties, which in turn control the intrinsic and fundamental properties such as the p/n-polarity of organic semiconductors in bulk-heterojunction solar cells. Herein, we designed and synthesized two acceptor-donor-acceptor type semiconducting thiophene oligomers end-functionalized with oxazolone/isoxazolone derivatives (OT1 and OT2 respectively). The HOMO-LUMO energy levels of both derivatives were found to be positioned in such a way that they can act as electron acceptors to P3HT and electron donors to PCBM. However, OT1 functions as a donor (with PCBM) and OT2 as an acceptor (with P3HT) in BHJ photovoltaic cells, and their reverse roles results in either no or poor performance of the cells. Detailed studies using UV-vis absorption and fluorescence spectroscopy, time-correlated single photon counting, UV-photoelectron spectroscopy, density functional theory calculations, X-ray diffraction, and thermal gravimetric analysis proved that both molecular and supramolecular properties contributed equally but in a contrasting manner to the abovementioned observation. The obtained results were further validated by flash-photolysis time-resolved microwave conductivity studies which showed an excellent correlation between the structure, property, and device performances of the materials.

Theoretical study on the torsional potential of alkyl, donor, and acceptor substituted bithiophene: the hidden role of noncovalent interaction and backbone conjugation

The torsional potential of substituted bithiophene is influenced not only by steric repulsion, but also by backbone conjugation and noncovalent interactions.

Rational design of D–A1–D–A2 conjugated polymers with superior spectral coverage

Calculations and experiments elucidate factors governing how D–A₁–D–A₂ polymers offer fundamentally improved spectral coverage via allowed transitions to both acceptor LUMOs.

Influence of the Acceptor Composition on Physical Properties and Solar Cell Performance in Semi-Random Two-Acceptor Copolymers

Five novel semi-random poly(3-hexylthiophene) (P3HT) based donor-acceptor copolymers containing either thienopyrroledione (TPD) or both diketopyrrolopyrrole (DPP) and TPD acceptors were synthesized by Stille copolymerization, and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. Poly(3-hexylthiophene-thiophene-thienopyrroledione) polymers P3HTT-TPD-10% and P3HTT-TPD-15% with either 10% or 15% acceptor content were synthesized as a point of reference. Two-acceptor polymers containing both TPD and DPP were synthesized with varying acceptor ratios to fine-tune electrooptical properties, namely, P3HTT-TPD-DPP (1:1) (7.5% TPD and 7.5% DPP), P3HTT-TPD-DPP (2:1) (10% TPD and 5% DPP), and P3HTT-TPD-DPP (1:2) (5% TPD and 10% DPP). The two-acceptor copolymers have broad and uniformly strong absorption profiles from 350-850 nm with absorption coefficients up to 8 × 10⁴ cm^-1 at ∼700 nm for P3HTT-TPD-DPP (1:2). This is reflected in the photocurrent responses of polymer:fullerene bulk heterojunction solar cells with PC₆₁BM as an acceptor where P3HTT-TPD-DPP (1:1) and P3HTT-TPD-DPP (1:2) have peak external quantum efficiency (EQE) values of 61% and 68% at 680 nm, respectively, and at 800 nm show impressive EQE values of 29% and 40%. Power conversion efficiencies in solar cells of P3HTT-TPD-10% and P3HTT-TPD-15% are moderate (2.08% and 2.22%, respectively), whereas two-acceptor copolymers achieve high efficiencies between 3.94% and 4.93%. The higher efficiencies are due to a combination of very large short-circuit current densities exceeding 16 mA/cm² for P3HTT-TPD-DPP (1:2), which are among the highest published values for polymer solar cells and are considerably higher than those of previously published two-acceptor polymers, as well as fill factors over 0.60. These results indicate that semi-random copolymers containing multiple distinct acceptor monomers are a very promising class of polymers able to achieve large current densities and high efficiencies due to favorable properties such as semicrystallinity, high hole mobility, and importantly broad, uniform, and strong absorption of the solar spectrum.