Organic light emitting diode fabrication

A method of monitoring an OLED production process for making a production process OLED device, the production process OLED device having a layered structure comprising an anode layer and a cathode layer, said anode and cathode layers sandwiching a hole injection layer, a hole transport layer and at least one organic light emitting layer, the method comprising:

• fabricating at least one similar OLED device to said production process OLED device, wherein said at least one similar OLED device has a layered structure corresponding to said production process OLED device and a range of hole injection and/or transport layer thicknesses;
• characterizing a spectral variation of a light output of said at least one similar OLED device with respect to variation in said hole injection and/or transport layer thickness;
• partially fabricating a said production process OLED device using said production process, wherein said partial fabrication comprises depositing one or more layers comprising at least said hole injection and/or transport layer;
• measuring a thickness of said one or more layers comprising at least said hole injection and/or transport layer;
• predicting a light output for said partially fabricated production process OLED device, in a target color space, from said measuring, using said characterized spectral variation;
• and
• adjusting said thickness of said one or more layers comprising at least said hole injection and/or transport layer to adapt said light output to a target light output in said target color space.