Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Abstract

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.

Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces

“…The first reported use of oxide buffer layers in OLEDs was by Tokito et al 61 in 1996. Now, transition metal oxides are heavily utilized in OLEDs, 60 OPVs, [92][93][94][95][96][97] and OFETs, [98][99][100][101][102][103][104] and are components in many of the current record-breaking devices reported in the literature. 2,105 Transition metal oxides can possess a wide range of work functions, spanning from extreme low of 3.5 eV for defective ZrO 2 to the extreme high of 7.0 eV for stoichiometric V 2 O 5 .…”

Low‐Temperature Solution‐Processed Electron Transport Layers for Inverted Polymer Solar Cells

“…Diethanolamine is known to reduce work function when doped into ZnO [33] and surface modifiers e.g. polymers containing simple aliphatic amine groups are also reported to substantially reduce work function, ii) zinc acetate decompositionAt 200 -300 °C residual hydroxyl species may be acting as shallow surface donors [18,23,34] owing to partial decomposition of the acetate species which is not complete until 310 °C [35] and iii) ZnO crystallization -rapid decomposition occurs above 300 °C where the oxide becomes more heavily n-doped by intrinsic defects i.e. Zn interstitials and O vacancies, which as temperature is further increased are partially recovered by the improved ZnO crystallinity [25,32,36] .…”

Contactless charge carrier mobility measurement in organic field-effect transistors

“…However, as the channel resistance decreases due to the increase of organic semiconductor mobilities, contact resistances often become the bottleneck for the total device performance [1,9,10]. Many strategies have been reported to decrease the contact resistance for either holes or electrons [1,10,11], using selfassembled monolayers [12,13], doping [14,15], interlayers [16,17], or by changing the device lay-out [9,18]. Contact resistance is an (even) more fundamental problem in ambipolar transistors, where both electrons and holes need to be injected [5,19].…”