P-type doped ambipolar polymer transistors by direct charge transfer from a cationic organic dye Pyronin B ferric chloride Gunel Huseynova a , Yong Xu a , Benjamin Nketia Yawson a , Eul-Yong Shin a , Mi Jung Lee b, ** , Yong-Young Noh a, * a Department of Energy and Materials Engineering, Dongguk University, 30, Pildong-ro 1-gil, Jung-gu, Seoul, 100-715, Republic of Korea b School of Advanced Materials Engineering, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul,136-712, Republic of Korea article info Article history: Received 8 September 2016 Received in revised form 7 October 2016 Accepted 8 October 2016 Available online 14 October 2016 Keywords: Organic field effect transistors Conjugated polymer Doping Cationic dyes Charge carrier mobility abstract We report a facile way to improve organic field effect transistor (OFET) performance based on low concentration doping of diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT) solution by an organic cationic dye, Pyronin B (PyB). DPPT-TT OFETs show significantly high field effect mobilities (up to 3.5 cm 2 V 1 s 1 ) by optimizing the doping ratio and solvent selection. The devices also exhibit better on/ off ratio by suppression of n-channel characteristics. Ultraviolet photoelectron spectroscopy and UVevis absorption spectra revealed efficient p-type doping in PyB doped DPPT-TT films, which was confirmed by the Fermi level shifting toward the highest occupied molecular orbital and red shift of the absorption spectrum. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Organic semiconductors (OSCs) continue to attract great interest because of their excellent optoelectronic and unique electric properties compared to their inorganic counterparts [1,2]. OSCs advantages include their ability to be fabricated on flexible and transparent plastic substrates by solution processes, paving the way for flexible, lightweight, transparent, and ultra-thin electronics [3,4]. In particular, constant performance progress is possible by optimization of the molecular structures using organic chemistry to achieve the desired requirements and functionality. Numerous OSCs have been developed to extend the opportunities to explore new applications [5e7]. However, limited electronic properties and unstable operation in ambient environments are considered as impediment to employ these devices in commercial products [8e10]. Therefore various approaches have been developed to improve the electrical properties and ambient instability. Doping of OSCs by introducing small amount of impurities into functional films is one of these approaches proposed as a simple and effective way to resolve these issues without sacrificing the unique advan- tages of organic devices. Recent progress in the field of organic electronics confirms that molecular doping of OSCs successfully contributes to commercialization of the devices such as organic light emitting diodes (LEDs) by promoting efficient charge injection [11e 13]. However, the doping techniques are rarely applied for organic field effect transistors (OFETs) to improve device perfor- mance and stability due to the concerns regarding increased off- state current. The introduction of impurities into OSC film for doping purposes is different from inorganic semiconductors, where doping is simply a replacement of one of the atoms of the inorganic lattice by the dopant atom, providing an extra electron or hole to the host lattice. Doping in organic electronics does not involve replacement of host material atoms by impurity atoms. Rather, it is a simple charge transfer process between two materials, referred to as “donor” and “acceptor” [14,15]. Depending on the type of doping and energy levels of the materials used, the same dopant can be used as a donor or acceptor for different host materials. As with inorganic electronics, OSC doping also provides the host material with extra electrons (n-type doping) or holes (p-type doping). Since the doping process involves the whole molecule rather than individual atoms, achievement of the desired doping * Corresponding author. ** Corresponding author. E-mail addresses: mijung@kookmin.ac.kr (M.J. Lee), yynoh@dongguk.edu (Y.-Y. Noh). Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel http://dx.doi.org/10.1016/j.orgel.2016.10.012 1566-1199/© 2016 Elsevier B.V. All rights reserved. Organic Electronics 39 (2016) 229e235