Yinghui He

New building blocks for π-conjugated polymer semiconductors for organic thin film transistors and photovoltaics

Rapidly growing research interest in the field of printed electronics, especially polymer-based organic thin film transistors (OTFTs) and organic photovoltaics (OPVs), stems from the dramatically improved performance of these devices, which become competitive to the amorphous silicon-based counterparts. The performance enhancement of polymer OTFTs and OPVs is largely attributed to the progress made in the development of polymer semiconductor materials. Therefore exploration of new building blocks for developing high-performance polymer semiconductors has been an area of extensive research. This article provides an overview of new building blocks including 21 electron acceptors and 20 electron donors, which were developed over the past three years for constructing π-conjugated polymers, particularly donor–acceptor (D–A) type polymers. Polymers containing these building blocks have shown very promising performance as active semiconductors in OTFTs and OPVs. Rationales for the structural design and the device performance of the polymers based on these new building blocks are discussed.

Thiophene-S,S-dioxidized Indophenine: A Quinoid-Type Building Block with High Electron Affinity for Constructing n-Type Polymer Semiconductors with Narrow Band Gaps

Three thiophene-S,S-dioxidized indophenine (IDTO) isomers, 3 a (E,E,E), 3 b (Z,E,E), and 3 c (Z,E,Z), were synthesized by oxidation of an indophenine compound. 3 b and 3 c could be converted into the most-stable 3 a by heating at 110 °C. An IDTO-containing conjugated polymer, PIDTOTT, was prepared using 3 a as a comonomer through a Stille coupling reaction, and it possesses a narrow band gap and low energy levels. In organic field effect transistors (OFETs), PIDTOTT exhibited unipolar n-type semiconductor characteristics with unexpectedly high electron mobility (up to 0.14 cm(2)  V(-1)  s(-1)), despite its rather disordered chain packing.

Branched alkyl ester side chains rendering large polycyclic (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione (IBDF) based donor–acceptor polymers solution-processability for organic thin film transistors

We report the development and use of a new type of branched alkyl ester side chain for donor–acceptor polymers. The synthesis of the branched alkyl ester side chain precursors is simple and the side chains branching position and branch length can be adjusted conveniently by choosing the readily available starting materials. (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione (IBDF) based donor–acceptor polymers were previously found to have poor solubility in common organic solvents. Herein, we used this new type of branched alkyl ester side chain for the copolymers of IBDF and bithiophene and explored how the branch length would impact the microstructure and charge transport properties of these polymers. With an optimal branch length, the polymer demonstrated ambipolar charge transporting characteristics with a high electron mobility of up to 0.35 cm2 V−1 s−1 and a hole mobility of up to 0.20 cm2 V−1 s−1 in organic thin film transistors (OTFTs), which is comparable to the one with branched alkyl side chains.

Thiophene-S,S-dioxidized indophenine (IDTO) based donor–acceptor polymers for n-channel organic thin film transistors

Two donor–acceptor (D–A) conjugated polymers, PIDTOBT and PIDTOBTz, based on thiophene-S,S-dioxidized indophenine (IDTO) as the acceptor building block are synthesized for solution processed organic thin-film transistors (OTFTs). The influences of the donor unit on the photophysical, electrochemical and electron-transport properties were investigated. These polymers possess very deep highest-occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels due to the strong electron accepting capability of the IDTO moiety. In OTFT devices, both polymers exhibited unipolar n-type charge transport characteristics with electron mobility up to 0.18 cm2 V−1 s−1.

Pyrimido[4,5-g]quinazoline-4,9-dione as a new building block for constructing polymer semiconductors with high sensitivity to acids and hole transport performance in organic thin film transistors

Pyrimido[4,5-g]quinazoline-4,9-dione (PQ) was used for the first time as a building block for π-conjugated polymer semiconductors. Copolymers of PQ and bithiophene showed dramatic bathochromic shifts in their absorption spectra in the presence of protonic (acetic acid and trifluoroacetic acid) and Lewis (BBr3) acids, resulting from the strong interaction of the basic 1,6-nitrogen atoms in the PQ unit with the acid. These polymers exhibited characteristic p-type semiconductor performance with hole mobilities of up to 6.4 × 10−3 cm2 V−1 s−1 in organic thin-film transistors (OTFTs). The potential bioactivity, high sensitivity to acids, and good field effect transistor performance of these PQ-based polymers will enable their application for bio- and chemo-sensors.

Pyrazino[2,3-g]quinoxaline-2,7-dione based π-conjugated polymers with affinity towards acids and semiconductor performance in organic thin film transistors

Pyrazino[2,3-g]quinoxaline-2,7-dione (PQx) was used as a building block for π-conjugated polymer semiconductors, which demonstrated a strong acid affinity by showing marked bathochromic shifts in their absorption spectra. These polymers exhibited semiconductor performance in organic thin film transistors (OTFTs). Copolymers of PQx and bithiophene exhibited electron-dominant ambipolar transport characteristics with electron mobilities of up to 4.28 × 10−3 cm2 V−1 s−1 and hole mobilities of up to 5.22 × 10−4 cm2 V−1 s−1, while copolymers of PQx and thieno[3,2-b]thiophene exhibited hole-dominant ambipolar transport characteristics with hole mobilities of up to 4.82 × 10−2 cm2 V−1 s−1 and electron mobilities of up to 3.95 × 10−3 cm2 V−1 s−1.

Thiophene-S,S-dioxidized indophenines as high performance n-type organic semiconductors for thin film transistors

The synthesis of three new isomerically pure (E,E,E)-form thiophene-S,S-dioxidized indophenine (IDTO) compounds, (3Z,3′Z)-3,3′-((E)-1,1,1′,1′-tetraoxido-5H,5′H-[2,2′-bithiophenylidene]-5,5′-diylidene)bis(1-dodecyl-indolin-2-one) (4a-S1), (3Z,3′Z)-3,3′-((E)-1,1,1′,1′-tetraoxido-5H,5′H-[2,2′-bithiophenylidene]-5,5′-diylidene)bis(5-bromo-1-dodecyl-indolin-2-one) (4b-S1) and (3Z,3′Z)-3,3′-((E)-1,1,1′,1′-tetraoxido-5H,5′H-[2,2′-bithiophenyldene]-5,5′-diylidene)bis(6-bromo-1-dodecyl-indolin-2-one) (4c-S1), and their use as n-channel semiconductors for organic thin film transistors (OTFTs) are reported. Compared to the non-oxidized parent indophenine compound 3,3′-(5H,5′H-[2,2′-bithiophenylidene]-5,5′-diylidene)bis(1-dodecylindolin-2-one) (3a), 4a-S1 exhibited significantly lower HOMO and LUMO energy levels. Having bromine atoms at the 5,5′- (4b-S1) or 6,6′-positions (4c-S1), the HOMO and LUMO energy levels further decreased. In OTFT devices, these IDTO compounds exhibit unipolar n-type semiconductor behavior due to their significantly deeper LUMO and HOMO energy levels than those of 3a that shows ambipolar charge transport performance. The maximum electron mobilities of 4a-S1, 4b-S1 and 4c-S1 are in the order of 10−2 to 10−1 cm2 V−1 s−1, which are much higher than that of 3a (∼10−3 cm2 V−1 s−1), originating from the lower LUMO energy levels and the high isomeric purities of the former compounds. Among the three IDTO compounds, 4c-S1 shows the highest electron mobility of up to 0.11 cm2 V−1 s−1, which is likely due to its most extended π-electron delocalization on the LUMO wavefunction.

A new n-type polymer based on N,N′-dialkoxynaphthalenediimide (NDIO) for organic thin-film transistors and all-polymer solar cells

The electron acceptor building block N,N′-dialkoxynaphthalenediimide (NDIO) has been, for the first time, used to develop an n-type polymer semiconductor, P(NDIO2OD-T). The polymer exhibits unipolar electron charge transport with an electron mobility of up to 5.4 × 10−3 cm2 V−1 s−1 in organic thin-film transistors (OTFTs) and shows good air-stability. When it is used as the acceptor in all-polymer solar cells (all-PSCs), a PCE of up to 3.25% is achieved. The performances of the OTFTs and all-PSCs based on this polymer have improved compared to its N,N′-dialkyl-substituted analogue.

Synthesis, characterization, and air stability study of pyrimido[4,5-g]quinazoline-4,9-dione-based polymers for organic thin film transistors

We report two novel π-conjugated polymers, PPQ2T-TVT-24 and PPQ2T-TT-24, containing the pyrimido[4,5-g]quinazoline-4,9-dione moiety and thieno[3,2-b]thiophene (TT) and (E)-1,2-bis(thiophen-2-yl)ethene (TVT), respectively, via the Stille cross-coupling reaction. Both polymers displayed good thermal stability and promising field effect transistor performance with hole mobilities of up to 3.08 × 10−3 cm2 V−1 s−1 for PPQ2T-TT-24 and 5.34 × 10−3 cm2 V−1 s−1 for PPQ2T-TVT-24. The ambient air stability of the organic thin film transistors based on these two polymers along with another previously reported PQ polymer containing bithiophene (BT) units, PPQ2T-BT-24, was studied. It was found that the moisture (H2O) in the ambient air is a detrimental component responsible for the degradation of the device performance, while oxygen, in contrast, could enhance the carrier mobility. Our study showed that the electron donor comonomer unit significantly influenced the stability of these polymers towards moisture in the stability order of TT > BT ≫ TVT. It was shown that the interaction between H2O and PPQ2T-TT-24 is physisorption and the device performance could be fully recovered, while the interaction of H2O with two other polymers involved chemical reactions, leading to permanent damages to the polymers and only partially recovered device performance upon removal of moisture.

Transistor Sizing for Bias-Stress Instability Compensation in Inkjet-Printed Organic Complementary Inverters

We present a design approach to maintain a stable voltage transfer characteristic in inkjet-printed complementary organic thin-film transistor logic inverters via device sizing. We use transistor-level design to help achieve stable logic gates, so that performance is less dependent on processing conditions and materials properties that are difficult to control for inkjet-printed electronics. Despite bias-stress instability in the individual p- and n-type transistors, a stable inverter switching threshold is achieved by equalizing the magnitudes of positive and negative threshold voltage shifts. Following a typical sizing approach for complementary logic, a p- to n-transistor transconductance ratio of 0.25 places the inverter switching threshold near the center of the voltage supply range. However, we show through calculations and measured results that a ratio closer to 2.5 prevents rapid shift of the switching threshold, which is equally important for reliable inverter operation. Furthermore, we provide a design approach to size digital logic gates with arbitrary probability of output states.