Improving the conductivity of hole injection layer by heating PEDOT:PSS

Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonic acid) (PEDOT:PSS) is a common material of hole injection layer used in polymer light emitting diodes (PLEDs) and organic solar cells. It can improve the efficiency of the charge collection at the anode. It has been reported that adding glycerol to PEDOT:PSS could increase the conductivity and improve the efficiency of PLEDs and organic solar cells. However, it is less noticed that the conductivity could be improved when the solution of PEDOT was heated before deposition. Here we experimented different concentrations of glycerol into PEDOT:PSS to make G-PEDOT:PSS solution, and heated the G-PEDOT:PSS solution at different temperatures before deposition. The solutions are then spin-coated on the glass and annealed at 140 °C. The conductivity was then measured and compared. The experiments showed that the conductivity of pure PEDOT:PSS slightly increased for 2-3 times, while the G-PEDOT:PSS increased over two orders of magnitudes. The conductivity increased with the heating temperature before deposition. The enhancement of the conductivity of the G-PEDOT:PSS film was higher than that of the pure PEDOT:PSS film. The overall conductivity increase for over three orders of magnitude. The reason is because the high temperature causes the glycerol and PEDOT:PSS to mix evenly. This is helpful for the swelling and aggregation of colloidal PEDOT-rich particles, forming a highly conductive network. When G-PEDOT:PSS resistance is reduced, it may not only increase the hole collection ability, but also replace ITO as the anode layer due to its advantages of low production cost and high work function.