Charge transport and recombination in heterostructure organic light emitting transistors Fatemeh Maasoumi a , Mujeeb Ullah a , Paul E. Shaw a , Jun Li b , Paul L. Burn a , Paul Meredith a , Ebinazar B. Namdas a,⇑ a Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Australia b Institute of Materials Research and Engineering (IMRE), Research Link, Singapore article info Article history: Received 31 March 2015 Accepted 31 May 2015 Available online 6 June 2015 Keywords: Charge transport Organic semiconductors Light emitting transistors Activation energy abstract Light-emitting field effect transistors (LEFETs) are a class of organic optoelectronic device capable of simultaneously delivering the electrical switching characteristics of a transistor and the light emission of a diode. We report on the temperature dependence of the charge transport and emissive properties in a model organic heterostructure LEFET system from 300 K to 135 K. We study parameters such as car- rier mobility, brightness, and external quantum efficiency (EQE), and observe clear thermally activated behaviour for transport and injection. Overall, the EQE increases with decreasing temperature and con- versely the brightness decreases. These contrary effects can be explained by a higher recombination effi- ciency occurring at lower temperatures, and this insight delivers new knowledge concerning the optimisation of both the transport and emissive properties in LEFETs. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Organic light-emitting field effect transistors (LEFETs) are dual function devices in that they have the electroluminescence capa- bilities of organic light-emitting diodes (OLEDs) plus the switching capabilities of a field effect transistor (FET) in a single architecture [1–5]. This dual functionality can potentially lead to new applica- tions such as simplified pixels for flat panel displays and poten- tially an electrical driven organic semiconductor laser. Although the emission brightness of LEFETs has improved over the last dec- ade [6–17], their electrical switching ON/OFF ratio and EQE at high brightness are still very much sub-optimal. This is in part due to a lack of knowledge with respect to materials design and the absence of a comprehensive understanding of the charge transport and radiative recombination processes that occur in an LEFET. A common method to probe the charge transport in organic semiconductor films is to measure the optoelectronic characteris- tics of the material as a function of temperature in an OFET or diode configuration [18–24]. Specifically, in the diode configura- tion both transient (e.g., Time-of-Flight [25], photo-Charge Extraction in Linearly Increasing Voltage [photo-CELIV] [26]) and steady-state measurements (e.g., Space Charge Limited Current [SCLC]) have been used to measure mobility and recombination. Recently, Armin et al. reported an adapted injection-CELIV tech- nique called MIS-CELIV which is capable of measuring the mobility of both carrier types in diode architectures relevant to operational devices such as solar cells and photodiodes [27]. However, in a functional OLED, we need to simultaneously probe not only the transport properties, but also the recombination dynamics (radia- tive and non-radiative). Thus, the traditional transport measure- ment methodologies only uncover at best half the pertinent physics. Furthermore, these existing techniques require that the injecting contact must be Ohmic. Such a requirement is hard to achieve in an organic diode configuration. In an OFET architecture, the contact resistance at the organic-metal interface can be com- pletely eliminated by employing four-probes or the transmission line technique [28–31]. An OFET structure can potentially map multiple elements of transport such as charge injection at the organic-metal interface, contact resistance and mobility. However, OFETs are generally not designed to emit light and are thus, like the simple diode, not suitable for studying radiative and non-radiative recombination processes. In this work, we simultaneously probe the mobility and injec- tion of carrier types, contact resistance and radiative recombina- tion all as a function of temperature in a model bilayer LEFET comprised of a light-emitting and a charge transporting polymer. The bilayer LEFETs show decreases in the source–drain current, mobility (both electrons and holes), and brightness with decreas- ing the temperature. However, the external quantum efficiency http://dx.doi.org/10.1016/j.orgel.2015.05.051 1566-1199/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: e.namdas@uq.edu.au (E.B. Namdas). Organic Electronics 25 (2015) 37–43 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel