Fig. 1. Schematic diagrams of device structures of solution-processed OLEDs.

(a) Normal structure; (b) Inverted structure; (c) Tandem structure. Soln and VTE in bracket respectively

represent solution-processed and thermally evaporated under high vacuum^[30-31]

Fig. 2. Schematic diagrams of current balance in hole-dominated OLEDs.

(a) A case of ETL without hole blocking; (b) A case of ETL with hole blocking;

(c) Energy levels and capacitance-voltage-current density characteristics of two hole-dominated OLEDs ^[52]

Fig. 3. (a) Schematic diagram of SPP at metal-dielectric surface; (b) Power dissipation spectra of dipole radiation. TMv, TMh and TEh respectively represent TM mode from vertical dipole, TM mode and TE mode from horizontal dipole^[14]; (c) Optical power modal analysis vs ETL thickness of a conventional OLED. The power is distributed into air (Air), substrate (Sub), waveguide (WV), surface plasmon polariton (SPP) and lossy metal (Loss) modes^[56]

Fig. 4. Schematic diagram of solution-processing technologies. (a) Spin coating process; (b) Ultrasonic spray coating process ^[64]; (c) Blade coating process; (d) Inkjet printing process