Improved out-coupling efficiency of organic light emitting diodes by manipulation of optical cavity length

• Highly efficient green PHOLED devices with n-doped electron transport layer.
• Method to control electron current for optimization of charge balance in OLED.
• Comparison of OLEDs with or without optically and electrically optimized ETL.
• Importance of optimization of charge balance as well as optical pathway.

The relationship between thickness of electron transport layer (ETL) and device performance of organic light-emitting diodes (OLEDs) was investigated. Especially, we prepared various OLEDs by varying the thickness of ETL to investigate the difference of device performance. Very interestingly, the device efficiency of green phosphorescent organic light emitting diodes (PHOLEDs) was significantly improved when the thickness of ETL was optimized even though we did not change any materials for such devices except that we applied highly conductive Li doped ETL. This means that the only one factor which is associated with an improvement of device efficiency could be originated from the constructive optical interference. As a result, the simple modification of PHOLEDs only by changing the optical thickness condition causes a dramatic improvement of current efficiency (up to 82.4 cd/A) as well as external quantum efficiency (EQE, up to 23.8%), respectively. Those values correspond to the much more improved ones (by ∼34.4%) compared to those obtained from the normal devices with thin ETL as a reference.

Figure optionsDownload as PowerPoint slide