Nanometric Metal Grids as Transparent Conducting Electrodes For OLEDs.

Organic light emitting diodes are polymer-based devices which promise higher efficiency and lower cost than other lighting devices, and will enable new applications which were not previously possible. An important component of OLEDs is the transparent conducting electrode (TCE), which is commonly indium tin oxide (ITO). It has a low sheet resistance (15 and a high transmission (87% on average in the visible wavelength range). However, it is also brittle, expensive and has issues from indium and oxygen migration into the polymer layers of the OLED. There are many alternatives that have been proposed to ITO, one of which is a nanometre thin metal grid. It has been shown to be flexible and if a cheap metal is used, then it can be a low cost solution. The sheet resistance can reach very low levels (e.g. 1, and the transmission can be above 90%. In this thesis, a thorough study is undertaken to investigate how the grid TCE affects the OLED. The grid TCE was optimised using a computer simulation. Then OLEDs were fabricated on them and characterised to investigate how their performance varies as the grid thickness increases. Surprisingly, it was concluded that the best TCE does not make the best OLED. Several possible reasons for this were considered. Grids with different line spacings were also tested and it was found that if the line spacing was too large, the light emission would not be uniform. To overcome this problem, small spacing grids or hybrid grid TCEs consisting of the metal grid and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) PH1000, or regioregular poly(3-hexythiophene) (P3HT) wrapped single-wall carbon nanotubes, were used. The OLEDs fabricated on the hybrid grid OLEDs had a luminance as high as the ITO OLED. These results demonstrated the feasibility of thin metal grids as an alternative for ITO, and will lead to better grid TCE design and optimisation for use in OLEDs