Anion-induced N-doping of naphthalenediimide polymer semiconductor in organic thin-film transistors

Abstract

Molecular doping is an important strategy to improve the charge transport properties of organic semiconductors in various electronic devices. Compared to p-type dopants, the development of n-type dopants is especially challenging due to poor dopant stability against atmospheric conditions. In this article, we report the n-doping of the milestone naphthalenediimide-based conjugated polymer P(NDI2OD-T2) in organic thin film transistor devices by soluble anion dopants. The addition of the dopants resulted in the formation of stable radical anions in thin films, as confirmed by EPR spectroscopy. By tuning the dopant concentration via simple solution mixing, the transistor parameters could be readily controlled. Hence the contact resistance between the electrodes and the semiconducting polymer could be significantly reduced, which resulted in the transistor behaviour approaching the desirable gate voltage-independent model. Reduced hysteresis was also observed, thanks to the trap filling by the dopant. Under optimal doping concentrations the channel on-current was increased several fold whilst the on/off ratio was simultaneously increased by around one order of magnitude. Hence doping with soluble organic salts appears to be a promising route to improve the charge transport properties of n-type organic semiconductors.

Addition of the Lewis Acid Zn(C6F5)2 Enables Organic Transistors with a Maximum Hole Mobility in Excess of 20 cm2 V−1 s−1

“…We selected Zn(C 6 F 5 ) 2 for a number of reasons including (i) it is a known Lewis acid, (ii) it is commercially available, and (iii) it has yet to be studied as an additive in OSCs. To begin with, we investigated the possible doping effect of Zn(C 6 F 5 ) 2 using electron paramagnetic resonance (EPR) spectroscopy . EPR has been used successfully to study organic‐free radicals and transition metals, as well as detect Lewis acid‐induced doping in semiconductors .…”

Introducing a Nonvolatile N‐Type Dopant Drastically Improves Electron Transport in Polymer and Small‐Molecule Organic Transistors

“…Here, the LUMO and HOMO levels were calculated via density functional theory (DFT) yielding −0.74 and −3.84 eV, respectively. Figure b,c presents the chemical structure of the OSCs N2200 and NDI3HU‐DTYM2, both renowned for their high field‐effect electron mobility ( µ ), along with their energy levels as predicted by DFT calculations. The HOMO/LUMO levels for N2200 and NDI3HU‐DTYM2 were found −5.80/−3.80 eV and −7.09/−4.80 eV, respectively (Figure S1, Supporting Information) consistent with previous reports (which gave a range of −4.00 to −3.80 eV for the LUMO level of N2200) .…”

“…EPR measurements were carried out in order to assess whether DMBI‐BDZC functions as an n‐type dopant for N2200 and NDI3HU‐DTYM2. The EPR method can directly detect the presence of unpaired or free electrons and it has been used extensively in the field of OSCs to evaluate doping . In brief, a magnetic field was used to promote the transition of unpaired electrons present in optimized (in terms of OTFTs performance) n‐doped layers, as compared to the pristine (0 mol%) semiconductor systems.…”

“…However, the characteristically low doping efficiencies and microstructural disruption of the host semiconductor upon introduction of the often bulky molecular dopants are two effects that have so far inhibited widespread adoption of the technology in OTFTs . A further limitation is the fact that although numerous studies have been performed on p‐type doping of OSCs, research on n‐type dopants is rather limited . This is because doping via the traditional ground‐state integer charge transfer (ICT) process requires the donation of an electron from the highest occupied molecular orbital (HOMO) of the dopant, to the lowest unoccupied lower orbital (LUMO) of the host semiconductor.…”

“…One such example is the small‐molecule benzimidazole−dimethylbenzenamine (N‐DMBI), which has been shown to n‐dope a variety of semiconductors by transferring a hydride atom to the acceptor molecule . Another successful example of nontraditional n‐doping is via the addition of salts such as tetrabutylammonium fluoride and tetrabutylammonium hydroxide, the introduction of which results in the formation of radical anions which in turn dope the host semiconductor . The latter approach has proven efficient in n‐doping of the polymer semiconductor poly[[ N , N ′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)] (P(NDI2OD‐T2)), hereafter abbreviated N2200, and was shown capable of producing OTFTs with a maximum electron mobility value of 0.67 cm 2 V −1 s −1 .…”