Effect of π-Conjugated Polyelectrolyte on Performance of White Polymer Light-Emitting Diodes Based on Excitons and Exciplexes Having Long Intermolecular Distances

The authors studied white polymer light-emitting diodes (PLEDs) based on blue exciton emission by poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and orange-to-red emission from exciplexes formed at the interface between the PFD and poly(4-methyl-triphenylamine-co-acetoaldehyde) (TPA-AA). It is thought that the orange-to-red light-emitting exciplexes formed at the TPA-AA/PFD interface have intermolecular charge-transfer (CT) characteristics, namely, TPA-AA⁺·PFD^-, in which TPA-AA and PFD act as a donor (D) and an acceptor (A), respectively, and long intermolecular distances. TPA-AA is a nonconjugated polymer with good solubility in chloroform and acts as a hole transport layer (HTL). A bilayer PLED, ITO (indium tin oxide)/TPA-AA/PFD/Al, was fabricated by spin-coating, and white electroluminescence (EL), corresponding to Commission Internationale d'Eclairage (CIE) chromaticity coordinates of (0.36, 0.28) and a brightness of 39.6 cd/m², was achieved at 16 V. In order to decrease the turn-on voltage and enhance brightness, a π-conjugated polyelectrolyte, poly(9,9-bis[6′-(N,N,N,-trimethylammonium)hexyl]fluorine-co-alt-1,4-phenylene) bromide (PFN⁺Br^-), was used as an electron injection layer (EIL). This trilayer PLED, ITO/TPA-AA/PFD/PFN⁺Br^-/Al, showed white EL corresponding to CIE chromaticity coordinates of (0.25, 0.31) and a brightness of 1450 cd/m² at 14 V. In the bilayer PLEDs, orange-to-red exciplex emission at ∼630 nm was dominant at a low applied voltage, while at high voltage, blue exciton emission at ∼430 nm and excimer emission at ∼490 nm were comparable in strength to the exciplex emission. Meanwhile, the trilayer PLEDs showed mainly blue exciton emission at low voltage, and orange-to-red exciplex emission that was comparable in strength to the exciton emission at high voltage. These results can be explained by the different degrees of hole accumulation at the TPA-AA/PFD interface, which affects the electric field distribution in the PFD layer and the EL emission processes. Based on the applied voltage dependence of the emission colors of the bilayer and trilayer PLEDs, two types of models, namely, PLED band diagrams and Marcus-type molecular diagrams that represent hole transfer from TPA-AA to PFD, were proposed to explain the working principles of the devices. The working principles presented in this manuscript are different from the recently proposed working principle based on the Auger mechanism for small-molecule organic light-emitting diodes (OLEDs) based on D/A heterojunctions (He et al., Adv. Mater. 2016, 28, 649-654). Having a detailed understanding of the working principles is critical for designing efficient OLEDs, and our models give a new insight into the working principles of OLEDs based on D/A heterojunctions. Furthermore, these models provide a useful means of realizing efficient white PLEDs based on blue exciton emission and orange-to-red exciplex emission.