Interface Effect on Efficiency Loss in Organic Light Emitting Diodes with Solution Processed Emitting Layers

• Published in: Advanced materials interfaces Vol. 3; no. 19; pp. np - n/a
• Main Authors: Ho, Szuheng, Chen, Ying, Liu, Shuyi, Peng, Cheng, Zhao, Dewei, So, Franky
• Format: Journal Article
• Language: English
• Published: Weinheim Blackwell Publishing Ltd 01-10-2016; John Wiley & Sons, Inc
• Subjects: Devices; Disorders; Efficiency; Evaporation; exciplex; high efficiency; interfaces; Molecular orbitals; Organic light emitting diodes; Photoluminescence; Power efficiency; solution processes
• ISSN: 2196-7350; 2196-7350
• DOI: 10.1002/admi.201600320

The performance of multilayered OLEDs with a solution processed emitting layer (EML) is compared to that of counterparts with an evaporated EML and it is found that the interfacial energy changes at the EML and electron transport layer (ETL) interface is a key factor determining the device efficiency. From the results of exciplex photoluminescence emission at the EML/ETL interface and energetic disorder measurements, it is revealed that there is an energy shift in the solution processed EML along with a band tail broadening compared with the device with an evaporated EML, resulting in inefficient hole blocking at the EML/ETL interface and a decrease in device efficiency. Using an ETL with a deep highest occupied molecular orbital (HOMO) level can ameliorate this problem, resulting in a solution processed OLED with a high external quantum efficiency of 29%.
The factors causing efficiency loss in solution processed organic light emitting diodes are investigated compared to evaporated counterparts. Probed by exciplex photoluminescence and energetic disorder, energy level shift/broadening at the interface is found to be the main reason that impairs hole carrier confinement and thus device efficiency. By using deep highest occupied molecular orbital electron transport layer, the efficiency difference can be effectively reduced.