Ambipolar Copolymers Research Articles

High hysteresis and distinctive optoelectronic memory effect for ambipolar thin-film transistors using a conjugated polymer having donor–acceptor heterojunction

π-Conjugated p-type PBDB-T and n-type N2200 macromolecular units are alternatively bonded to generate ambipolar copolymer (i.e. P(BDBT-co-N2200)) for achieving donor-acceptor (D-A) heterojunction. From the laser confocal microscope photoluminescence (PL) spectra of the P(BDBT-co-N2200) copolymer, PL characteristic peaks of PBDB-T and N2200 are simultaneously observed at 695, 760, and 860 nm. The thin-film transistors (TFTs) using P(BDBT-co-N2200) copolymer show ambipolar transistor characteristics originating from the coexistence of p-type and n-type semiconducting macromolecular units. Interestingly, a high hysteresis is observed in the transfer and output characteristics of the TFTs because of the interface traps and near-interface bulk traps. Under light irradiation, distinctive photocurrents and hysteresis are observed, suggesting the optically mediated charge release and photogating effects caused by trap states. Charge trapping with a high hysteresis and photoconduction with the photogating effect of the P(BDBT-co-N2200)-based ambipolar TFTs induce stable and repeatable writing, reading, and erasing operations. The optoelectronic memory devices using the P(BDBT-co-N2200)-based ambipolar TFTs are realized with the merit of a long charge storage time of 40 s. The π-conjugated copolymer, P(BDBT-co-N2200) exhibiting D–A heterojunction, can be applied to multifunctional devices, such as photoresponsive ambipolar transistors and optoelectronic memory devices, for image sensing and data storage. Optoelectronic memory devices using the ambipolar thin-film transistors of π-conjugated P(BDBT-co-N2200) copolymer having donor-acceptor heterojunction are realized with the merit of a long charge storage time. • π-Conjugated p-type PBDB-T and n-type N2200 macromolecular units were alternatively bonded to generate ambipolar copolymer with donor-acceptor (D-A) heterojunction. • The P(BDBT-co-N2200)-based TFT showed photoresponsive ambipolar transistor characteristics. • The P(BDBT-co-N2200)-based ambipolar TFTs were successfully operated as optoelectronic memory devices with repeatable writing, reading, and erasing processes. • Readout current charges were constant regardless of the variation in waiting times and linearly increased with increasing light exposure time.

Tuning the Ambipolar Character of Copolymers with Substituents: A Density Functional Theory Study.

Ambipolar copolymers play a vital role in reducing the cost and simplifying the fabrication of ambipolar organic field-effect transistors (OFETs). However, the development of them lags behind that of p-type and n-type copolymers. Herein, we aim to obtain ambipolar copolymers by introducing appropriate substituents into DPP-TVT [diketopyrrolopyrrole-(E)-1,2-di(2-thienyl)ethane]. We study the effects of substituents (-NH2, -OCH3, -F, -CF3, and -CN) on backbone planarity, electronic structures, and charge carrier mobility by density functional simulations. Electronic structure analyses show that the CN-substituted DPP-TVT lies in the trap-free energy window and has the narrowest band gap, suggesting promising ambipolar character. This was further confirmed by the results of its charge carrier mobility along both intra- and interchain directions. Besides the -CN group, the -NH2 group also proved to be effective in turning DPP-TVT into the ambipolar copolymer. Our study provides insights into modulating the performances of OFETs by introducing appropriate substituents into the copolymers to achieve ambipolar character.

Ambipolar Conjugated Polymers with Ultrahigh Balanced Hole and Electron Mobility for Printed Organic Complementary Logic via a Two-Step CH Activation Strategy.

High mobility ambipolar conjugated polymers are seriously absent regardless their great potential for flexible and printed plastic devices and circuits. Here, ambipolar polymers with ultrahigh balanced hole and electron mobility are developed via a two-step CH activation strategy. Diketopyrrolopyrrole-benzothiadiazole-diketopyrrolopyrrole (DBD) and its copolymers with thiophene/selenophene units (short as PDBD-T and PDBD-Se) are used as examples. PDBD-Se exhibits highly efficient ambipolar transport with hole and electron mobility up to 8.90 and 7.71 cm2 V-1 s-1 in flexible organic field-effect transistors, presenting a milestone for ambipolar copolymer screening. Based on this performance metrics and good solubility, PDBD-Se is investigated as inkjet-printable semiconductor ink for organic complementary logic circuits. Under ambient processing, maximum hole and electron mobilities reach 6.70 and 4.30 cm2 V-1 s-1 , respectively. Printed complementary inverter and NAND gates with transition voltages near VDD /2 are fabricated, providing an easy-handling, general material for printed electronics and logic.

Ambipolar copolymer of dithienocoronenedi-imide and benzo(bis)thiadiazole with balanced hole and electron mobility

A new copolymer P(DTCBTZ) of dithienocoronenediimide (DTCDI) and benzo(bis)thiadiazole (BTZ) as the active layer of organic field-effect transistors (OFETs) is reported. The combination of DTCDI with long alkyl chains and BTZ without alkyl chains results in reduced steric hindrance between acceptors of different polymer backbones. 2,2′-bithiophene is introduced into P(DTCBTZ), and thus donor–acceptor (D–A) interactions are induced. Absorption spectra of solution and thin films of P(DTCBTZ) reveal the existence of relatively strong aggregations of P(DTCBTZ) copolymers in solid state. The new copolymer of P(DTCBTZ) has a HOMO of −5.54eV and a LUMO of −3.72eV, which are suitable for both hole and electron transport. Bottom-gate and top-contact (BGTC) transistors were prepared and balanced charge transport was observed, with hole mobility of 0.018cm2V−1s−1 and electron mobility of 0.019cm2V−1s−1, indicating potential for application of polymers containing DTCDI in ambipolar semiconductors.

Benzodithiophene–Thiadiazoloquinoxaline as an Acceptor for Ambipolar Copolymers with Deep LUMO Level and Distinct Linkage Pattern

Two new conjugated copolymers, PBDTTQ-1 and PBDTTQ-2, with a distinct linked pattern between benzodithiophene–thiadiazoloquinoxaline (BDTTQ) as acceptor and bithiophene as donor were synthesized and characterized. The difference in the linkage between donor and acceptor exerts great influence on the optoelectronic properties of the two polymers. The optical band gap decreases from 1.18 eV for PBDTTQ-1 to 1.03 eV for PBDTTQ-2, due to the lower LUMO energy level (−4.01 eV) of the latter. Moreover, density functional theory calculations demonstrate that the electron density is mainly confined on the acceptor unit in both HOMO and LUMO of PBDTTQ-1, while the electronic densities almost delocalize along the entire backbone of PBDTTQ-2, which facilitates the charge transport within the polymer chain. In contrast to PBDTTQ-1 missing any field-effect characteristics, PBDTTQ-2 exhibits ambipolar charge transporting behavior with mobilities of 1.2 × 10–3 cm2/(V s) for holes and 6.0 × 10–4 cm2/(V s) for electrons.

Photovoltaic performance of bifunctional low band gap conjugated copolymer

We have investigated the bifunctional properties using ambipolar low band gap conjugated copolymer in organic bulk-heterojunction (BHJ) solar cells. Ambipolar copolymer, poly[3,6-dithien-2-yl-2,5-di(2-decyltetradecanyl)-pyrrolo[3,4- c]pyrrole-1,4-dione-5′,5′’-diyl- alt-benzo-2,1,3-thiadiazol-4,7-diyl] (PDTDPP- alt-BTZ), successfully works as the bifunctional material acting as both donor or acceptor. However, because of fast recombination nature and broken charge balance of hole and electron, solar cell performances were significantly limited. The results indicate that having distinct role of each component might be better for the high performance BHJ solar cells rather than bifunctional property based on good charge transport properties of both hole and electrons.