Adopting the emerging technology of printed electronics in manufacturing novel ultrathin flat panel displays attracts both academic and industrial interests because of the challenge in the device physics and the potential of reducing production costs. Here we produce all-solution processed polymer light-emitting diode displays by solution-depositing the cathode and utilizing a multifunctional buffer layer between the cathode and the organic layers. The use of ink-jetted conducting nanoparticles as the cathode yields high-resolution cathode patterns without any mechanical stress on the organic layers. The buffer layer, which offers the functions of solvent-proof electron injection and proper affinity, is fabricated by mixing the water/alcohol-soluble polymer and a curable epoxy adhesive. Our 1.5-inch polymer lightemitting diode displays are fabricated without any dead pixels or dead lines. The all-solution process eliminates the need for high vacuum for thermal evaporation of the cathode, which paves the way to industrial roll-to-roll manufacturing of flat panel displays.

In the application of traditional bulk heterojunction polymer solar cells, to prevent the etching of ITO by the acidic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and thereby improve the device stability, pH-neutral PEDOT:PSS is introduced as the hole transport layer (HTL). After treating the neutral PEDOT:PSS with UV-ozone and with an oxygen plasma, the average power conversion efficiency (PCE) of the device increases from 3.44% to 6.60%. Such surface treatments reduce the energy level offset between the HTL and the active layer, which increases the open circuit voltage and enhances hole transportation, leading to the PCE improvement. Moreover, the devices with the neutral PEDOT:PSS HTL are more stable in air than those with the acidic PEDOT:PSS HTL. The PCE of the devices with the acidic PEDOT:PSS HTL decreases by 20% after 7 days and 45% after 50 days under ambient conditions, whereas the PCE of the devices with the pH-neutral PEDOT:PSS HTL decreases by only 9 and 20% after 7 and 50 days, respectively. X-ray photoelectron spectroscopy shows that the acidic PEDOT:PSS etches the indium from the indium-tin-oxide (ITO) electrode, which is responsible for the degradation of the device. In comparison, the diffusion of the indium is much slower in the devices with the pH-neutral PEDOT:PSS HTL.

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