The integration of new OLED materials, production technologies, and refined test solutions should accelerate OLED development for use as small-size displays in PDAs, cell phones, and digital cameras. But much work still lies ahead. OLEDs have yet to reach performance and production efficiencies needed to make them suitable for widespread use in large-area displays or high-quality video.
In its evolution from LCDs to new organic and polymer LED (PLEDs), the flat-panel-display (FPD) industry is grappling with test issues in both R&D and production. These issues apply to both small-molecule OLEDs and PLEDs. For new designs, material lifetime is a major issue at the R&D stage, and device reliability is always a concern in production. Effective testing is required to fully characterize materials and devices and rapidly move them from R&D into production.
New production technologies are also being investigated for use with OLEDs and PLEDs. For example, OLEDs based on small-molecule technology are compatible with most semiconductor processing methods. Now, "printable electronics" may alter the way these FPDs are made. Soft lithography such as microcontact printing may make it possible to improve OLED resolution and image definition. This, or a similar technology, could reduce the cost of producing active-matrix FPDs by depositing thin-film transistors (TFTs) on the same substrate as the OLEDs. The TFTs may also be organic-based instead of using the present CMOS technology, which should further simplify production.
New materials, continually shrinking device structures, and revolutionary processes are making testing at all stages of OLED development all the more critical. At the same time, measurements have become increasingly demanding. Due to the disordered nature of organic materials, OLEDs have low carrier mobilities. The resulting formation of space charges produces numerous transient effects, some of which cover multiple orders of magnitude in time. To characterize the electroluminescence and phosphorescence mechanisms, instruments must provide high sensitivity and precision. Electrical measurements at femtoamp and microvolt levels are common.
In production, FPD test systems typically measure junction characteristics of the OLED devices and may perform tests on the TFTs. The instrumentation and test methodology must accommodate the relatively high capacitance of these devices without adding excessive test time. Because OLEDs are active light-emitting devices, it's important to characterize their light-current-voltage (LIV) properties under both dc and pulsed dc operation. With the large number of pixels arranged in rows and columns, a switching matrix is required to speed up FPD testing. For best results and lowest cost, each test system is usually tailored to a specific display technology and physical configuration. By matching test variables as closely as possible to actual operating conditions, accurate modeling and lifetime predictions are possible.