Jesse Quinn

Recent progress in the development of n-type organic semiconductors for organic field effect transistors

This review highlights recent major progress in the development of organic semiconductors as electron transport n-channel materials in organic field effect transistors (OFETs). Three types of materials are discussed: (1) small molecules, (2) polymers, and (3) n-doped small molecules and polymers. Much effort has been made in the modification of known building blocks, development of novel building blocks, and optimization of materials processing and device structures. These efforts have resulted in the achievement of record high electron mobilities for both small molecules (12.6 cm2 V−1 s−1) and polymers (14.9 cm2 V−1 s−1), which are approaching the highest hole mobilities achieved by p-type small molecules and polymers so far. In addition, n-doping of ambipolar and p-type organic semiconductors has proven to be an efficient approach to obtaining a greater number of n-type organic semiconductors. However, it is found that n-type organic semiconductors, in general, still lag behind p-type organic semiconductors in terms of carrier mobility and air stability. Further exploration of new building blocks for making novel materials and optimization of processing conditions and device structures are needed to improve the performance, particularly air stability.

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.

Synthesis and properties of indigo based donor–acceptor conjugated polymers

Indigo is for the first time used as a building block to construct polymer semiconductors for organic thin film transistors (OTFTs). Two donor–acceptor polymers using indigo as the acceptor and bithiophene as the donor are synthesized via Stille coupling polymerization. Two types of acyl groups, 2-hexldecanoyl (for polymer P1) and 2-octyldodecanoyl (for polymer P2), are utilized as solubilizing side chains. 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 indigo moiety. In OTFT devices, characteristic n-type semiconductor performance with electron mobility of up to ∼10−3 cm2 V−1 s−1 is observed.

Dramatically different charge transport properties of bisthienyl diketopyrrolopyrrole-bithiazole copolymers synthesized via two direct (hetero)arylation polymerization routes

Two bisthienyl diketopyrrolopyrrole (DPP)-bithiazole based copolymers were synthesised via two different direct (hetero)arylation polymerization (DHAP) routes. When a bisthienyl DPP and 5,5′-dibromo-2,2′-bithiazole were used as monomers, the resulting polymer PA-1 showed poor solubility, ill-organized chain ordering and low performance in organic thin film transistors (OTFTs). Surprisingly, the synthetic route using a dibrominated bisthienyl DPP and 2,2′-bithiazole as monomers produced a polymer PB-1 with improved solubility, much higher crystallinity and excellent charge transport performance in OTFTs. The electron and hole mobilities of up to 0.53 cm2 V−1 s−1 and 0.06 cm2 V−1 s−1, respectively, achieved for PB-1 are more than one order of magnitude higher than those of PA-1 and also better than the mobilities reported for a similar polymer synthesized via Stille coupling polymerization. The dramatically different carrier mobilities observed for these two polymers are accounted for by their different amounts of α–β coupling linkages and branched (and lightly cross-linked) structure defects formed in the respective synthetic routes. This work also demonstrated for the first time that 2,2′-bithiazole is a suitable monomer for the construction of conjugated polymers with good electron transport performance via DHAP.

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.

An indigo-based polymer bearing thermocleavable side chains for n-type organic thin film transistors

A new n-type semiconducting polymer based on indigo having thermocleavable tert-butoxycarbonyl (t-Boc) groups was synthesized and used as an active layer in organic thin film transistors (OTFTs). Twisting of the polymer main chain due to the presence of the bulky t-Boc groups renders this indigo-based polymer highly soluble. A post-deposition thermal treatment at a temperature above 170 °C could remove the t-Boc groups to retrieve the highly coplanar geometry of the unsubstituted indigo units. The thermally annealed polymer semiconductor films at 200 °C showed an electron mobility of up to ∼6 × 10−3 cm2 V−1 s−1 in OTFTs, which is a 5-fold increase compared to that of the indigo-based polymers reported previously due to the retrieved high backbone coplanarity.

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.

Regioisomeric control of charge transport polarity for indigo-based polymers

In this work we report the opposite charge transport polarity observed for two regioisomeric polymer semiconductors. Previously, we observed n-type electron transport behaviour for a polymer semiconductor, 6,6′-PIDBDT, which contains 6,6′-indigo units. Electron distributions in the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of indigo calculated by density functional theory (DFT) could explain the electron transport behaviour since electrons can be delocalized along the polymer backbone through the LUMO rather than the HOMO via the 6- and 6′-positions. Serendipitously, we found that the 5- and 5′-positions of indigo are occupied by electrons in the HOMO but empty in the LUMO, opposite to the 6- and 6′-positions. This suggests that 5,5′-PIDBDT containing the 5,5′-indigo units, a regioisomer of 6,6′-PIDBDT, may exhibit the opposite p-type charge transport behaviour. To prove our assumption, we synthesized 5,5′-PIDBDT and found that this polymer indeed showed p-type semiconductor behaviour. Hence we demonstrated a novel approach to control the electron or hole charge transport polarity by simply varying the main chain regiochemical connections.