High-Performance, Flexible Polymer Light-Emitting Diodes Fabricated by a Continuous Polymer Coating Process

Abstract

Large‐area polymer thin films of adjustable thickness have been prepared by a continuous bar‐coating process (see Figure). The promise of this low‐cost polymeric device production technology is exemplified by the fabrication of MEH‐PPV‐based LEDs (see inside front cover).

Transverse charge transport in inkjet printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

“…In the case, of highly heated substrate, surface tension gradients lead to "Marangoni" flow and the "coffee cup ring" effect [38][39][40][41][42][43][44][45][46], as is presented in Figure 10(b), which also leads to columns of transversely aligned PEDOT grains. At a substrate temperature of 303 K, inkjet printing using the printing system in this study generates a flat top surface profile, as is presented in Figure 7(a) [38], with expected convective circulating flow as illustrated in Figure 10 Addition of DMSO and other co-solvents has been reported to increase the in-plane conductivity of both inkjet printed [8,9] and spin coated PEDOT:PSS [15,19,[47][48][49][50] by acquiring a morphology of more coarsened PEDOT grains and reduction of the charge hopping distance [9], while 1D-M-VRH charge transport also applies to in-plane conduction. As transverse conduction is already controlled by 1D-M-VRH charge transport -18 -in inkjet printed PEDOT:PSS films without any co-solvents in this study, further coarsening of PEDOT grains aligned in the transverse direction is expected by adding DMSO up to 5 wt% in the printing ink [9], which is expected to further raise the transverse conductivity of inkjet printed PEDOT:PSS above the values of the transverse conductivity of spin coated PEDOT:PSS with a co-solvent, where the latter totally lacks any transverse PEDOT grain alignment.…”

Transparent Solar Cells Based on Organic Polymers

“…This efficiency, although two orders of magnitude lower than that of a device made by the regular fabrication process, is almost the same as that of the glass/ITO/PEDOT:PSS/MEH-PPV/Al device fabricated by the conventional bottom-up process [55] and indicates a good contact through the lamination process. The higher turn-on voltage and the lower efficiency of the laminated devices compared with the devices fabricated through the bottom-up process with calcium as the cathode [56] are not due to the electric glue of PEDOT:PSS or the lamination process but to the high work function of the aluminum cathode.…”

Solution Processable Ionic p-i-n Organic Light-Emitting Diodes

“…In contrast, relatively little progress has been made to date in designing a reliable and simple fabrication process that ensures the formation of a flat and uniform EL layer over a large area, which is particularly important for achieving the highly efficient and reliable device performance that is required for OLEDs. During the fabrication of OLEDs, the organic layers used are typically prepared using physical vapor deposition (Tang & VanSlyke, 1987, Baldo et al, 1998, Baldo et al, 1999, Adachi et al, 2002, He et al, 2004 or wet solution-coating processes (Friend et al, 1999, Pardo et al, 2000, Jabbour et al, 2001, Ouyang et al, 2002, de Gans et al, 2004, So et al, 2007. To date, OLEDs manufactured using vapor-deposited organic multi-layers of small molecular materials have the best performance record.…”

“…However, the vapor deposition process is quite complex and expensive. Solution-processed devices made of polymeric or small molecular materials are also of interest, because these techniques make possible a simple production technique that uses a non-vacuum process such as continuous coating, screen printing, and Ink-jet printing (Pardo et al, 2000, Jabbour et al, 2001, Ouyang et al, 2002, de Gans et al, 2004, So et al, 2007. In such solution-processed devices, it is of critical importance to achieve strong light emission from a simple OLED structure.…”