Solution-Processed White Organic Light-Emitting Diodes with Enhanced Efficiency by Using Quaternary Ammonium Salt Doped Conjugated Polyelectrolyte

Solution-processed white organic light emitting diodes (WOLEDs) with quaternary ammonium salt doped water/alcohol soluble conjugated polyelectrolyte, poly­[(9,9-bis­(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluoren)] dibromide (PFNBr), as electron transport material has been fabricated. Compared with the undoped devices, the performances of such devices with a doped electron transport layer have been dramatically improved to be nearly twice high in luminous efficiency and nearly one-third in response time when the weight ratio of PFNBr to tetraethylammonium bromide (TEAB) was 10:3. Four kinds of quaternary ammonium salts have been investigated to be dopants in the conjugated polyelectrolyte electron transport layer. It has been shown that both the anions and the cations of quaternary ammonium salts can influence the device performance. The dopant who has both a smaller anion and a smaller cation size can exhibit a better device performance. In addition, ultraviolet photoelectron spectroscopy measurement and single-carrier device testing have been employed to investigate the reason why such quaternary ammonium salt dopants can make an obvious improvement in the device performance of WOLEDs. These findings will be beneficial to the progress in design and fabrication of solution-processed WOLEDs suitable for lighting.