High Performance of Low Band Gap Polymer-Based Ambipolar Transistor Using Single-Layer Graphene Electrodes

Abstract

Bottom-contact bottom-gate organic field-effect transistors (OFETs) are fabricated using a low band gap pDTTDPP-DT polymer as a channel material and single-layer graphene (SLG) or Au source/drain electrodes. The SLG-based ambipolar OFETs significantly outperform the Au-based ambipolar OFETs, and thermal annealing effectively improves the carrier mobilities of the pDTTDPP-DT films. The difference is attributed to the following facts: (i) the thermally annealed pDTTDPP-DT chains on the SLG assume more crystalline features with an edge-on orientation as compared to the polymer chains on the Au, (ii) the morphological features of the thermally annealed pDTTDPP-DT films on the SLG electrodes are closer to the features of those on the gate dielectric layer, and (iii) the SLG electrode provides a flatter, more hydrophobic surface that is favorable for the polymer crystallization than the Au. In addition, the preferred carrier transport in each electrode-based OFET is associated with the HOMO/LUMO alignment relative to the Fermi level of the employed electrode. All of these experimental results consistently explain why the carrier mobilities of the SLG-based OFET are more than 10 times higher than those of the Au-based OTFT. This work demonstrates the strong dependence of ambipolar carrier transport on the source/drain electrode and annealing temperature.

A new rigid planar low band gap PTTDPP-DT-DTT polymer for organic transistors and performance improvement through the use of a binary solvent system

“…The predicted HOMO level was À4.94 eV and HOMO-LUMO gap was 1.77 eV, which are slightly deeper HOMO level and smaller gap compared to the PTDPP-DT-DTT polymer's model compound, (TDPP-DT-DTT) 3 , (HOMO ¼ À4.72 eV and HOMOeLUMO gap ¼ 1.72 eV). The differences in the energy levels and the HOMOeLUMO gap between model compounds can be attributed to the fact that the (TDPP-DT-DTT) 3 is even more planar than the (TTDPP-DT-DTT) 3 (dihedral angles are nearly zero) [29]. Note that the DFT calculated results are consistent with the experimentally resolved data in trend.…”

“…the intensity ratios of 2nd peak/3rd peak were reduced to be 0.975, 0.930, and 0.922 for as-spun, 100, and 200 C annealed films), indicating that aggregation became more extensive upon thermal annealing of the polymer films. In addition, the optical band gap was estimated to be 1.45 eV based on the edge at 855 nm, which was slightly blue-shifted relative to the optical band gap (1.44 eV, 863 nm) of the structurally similar PTDPP-DT-DTT polymer that we have studied previously [29]. The structures of PTDPP-DT-DTT and PTTDPP-DT-DTT differ only in that thiophene units in the former are replaced by thienothiophenes in the latter.…”

“…Both intensity ratios increased as the solvent additive bp increased. The intensity ratios increased even more with thermal annealing at 200 C. These observations indicated good alignment among the (h00) planes with respect to the substrate, which facilitated charge transport through the polymer chain pep stacking structures [20,23,29,35,36]. Overall, the GIXD analysis of the PTTDPP-DT-DTT films indicated that the crystallinity of the PTTDPP-DT-DTT polymer films improved significantly using a binary solvent system with a high bp solvent additive.…”

Recent Advances in Ambipolar Transistors for Functional Applications

“…Researchers usually flank DPP core unit with two five‐membered heteroaryl groups (thiophene for DBT, furan for DBF, selenophene for DBS, thieno[3,2‐ b ]thiophene for DBTT or thiazole‐flanked DPP) due to their favorable properties such as coplanar backbone and intense intermolecular π–π interactions . Specifically, DBT is more promising for ambipolar performance on account of its planar structure and simple chemical modifications . Guo et al incorporated two DPP isomers into one copolymer backbone and obtained balanced ambipolar carrier mobilities (0.02 cm 2 V −1 s −1 ) by reason of highly ordered and extremely close packing distance of the active layer .…”

Thin film transistors based on two dimensional graphene and graphene/semiconductor heterojunctions

“…However, graphene electrode effectively overcomes the above disadvantages of the metal electrode for FETs. Therefore, graphene transparent conductive lms are widely used as an electrode material for GFETs, [56][57][58][59][60][61][62] with a heterojunction structure between graphene electrode and the inorganic and/or organic semiconductor active layer.…”