<br/>The fabrication of polymer-based organic light-emitting diodes (OLEDs) using the inverted method, where the device is built on top of a metal back electrode has definite advantages over the conventional OLED fabrication methods. For example, the inverted method provides a way to control the structure and morphology of the metal electrode. This, in turn, enables improved light management by control of the photonic and plasmonic behaviour within the OLED devices. Silver is a good option as a back metal electrode in OLEDs because of its low cost compared to gold and its low optical loss, high reflectivity, and high conductivity. However, Ag has a work function (~4.4 eV) that is not compatible with the band gap of many polymeric organic semiconductors. Poly(9,9-dioctylfluorene-<i>alt</i>-benzothiadiazole) (F8BT) is a conjugated polymer commonly used in OLED prototypes that has high electron affinity (∼3.3 eV), large ionization potential (∼5.9 eV), and high quantum yield. Combining F8BT with an Ag back electrode results in a high barrier height for charge injection, which results in poor device performance and a high turn on voltage. To overcome this, here, thiol (1DT) and (±)-α-lipoic acid derivatives were used as self-assembled monolayers (SAMs) as an interface layer between Ag back electrode and F8BT thin films to alter the work function of the Ag electrode. It also acts as a passivating layer to improve the performance of the OLEDs and enables air processing of the device. XPS and Raman spectra reveal the binding properties of the SAMs, while the I-V and UPS measurements show that (±)-α-lipoic acid and (±)-4-phenylbutyl 5-(1,2-dithiolan-3-yl)pentanoate improve the charge injection greatly by changing the work function of the Ag and by altering the morphology of the semiconducting polymer. The results obtained gave us valuable insight into the effect of chain length and functional groups of the SAMs in engineering morphological and electronic properties of the metal-polymer interface.

Improved charge injection in inverted polymer semiconductor thin film devices using dithiolane-based self-assembled monolayers

ACS Spring Meeting 2021

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