Novel Light Extraction Enhancements for White Phosphorescent OLEDs (Phase I)
Investigating Organization
Universal Display Corp.
Principal Investigator(s)
Dr. Brian W. D'Andrade
Subcontractor
Princeton University, under the direction of Prof. Stephen R. Forrest
Funding Source
Small Business Innovation Research
Award
DOE Share: $100,000
Contract Period
7/21/03 - 4/20/04
In Phase 1, Universal Display Corp., a developer of OLED technologies for flat panel displays, lighting and other opto-electronic applications, is working to demonstrate innovative techniques to improve OLED power efficiencies, a critical performance attribute for the general lighting industry. Universal Display and its research partners at Princeton University and the University of Southern California are developing several novel approaches for producing highly efficient white light using the Company's phosphorescent OLED (PHOLED™) technology.
In addition to the use of this highly efficient PHOLED technology, better light extraction techniques are required to achieve the power efficiency targets of the general lighting market, as in a conventional OLED only approximately 25% of the generated photons are emitted from the device. In this program, Universal Display and Princeton University demonstrated the feasibility of using specialized designs, such as lens arrays, on an OLED device to enhance the amount of generated light that is captured or extracted from the device as useful light.
Specifically, two key objectives of Phase 1 were: Demonstrate and deliver a white PHOLED light source on a glass substrate, with an attached flattened lens array to provide enhanced optical extraction over a similar device without an attached lens. Characterize the above white light sources and demonstrate > 15 lm/W at 800 nits luminance.
During Phase I of this program, the team accomplished several key goals: developed a process for producing different microlens silicon molds, demonstrated device performance characteristics that met expectations, and explored new outcoupling schemes based on models of outcoupling enhancement using aperiodic gratings. Silicon molds for forming microlenses from poly-di-methyl-siloxane (PDMS), a thermal curable elastomer, were fabricated. The ability to control the dimensions and shape of the silicon mold is important since these factors affect the outcoupling efficiency. Our work demonstrated that we have the capability to optimize the silicon molds to further enhance the outcoupling efficiency by adjusting the period and size of the array elements.
Both sets of microlenses formed from the molds improved the outcoupling efficiency by ~22%. The improvement in efficiency was found by comparing the total forward emission from devices with and without the microlens array attached to the glass substrate. The total forward emission was found by using an integrating sphere, such that all forward emitted light was collected in the sphere. The efficiency of a white device operating at 6.3 V and 20 lm/W having CIE (0.39, 0.40) at 800 cd/m2 was improved such that the same device with microlenses operated at 6.3 V and 24 lm/W at 1000 cd/m2, which met our goals.