Experimental Common Tools: (4) Lifetime Test

Editor : Sapien

Wishing you to be so near to me
Finding only my loneliness
Waiting for the sun to shine again
Finding that it’s gone to far away
To die, to sleep, may be to dream…

– From “Concerto Grosso No. 1 Adagio”, by New Trolls

 Among many experiments involving time domain analysis, the device lifetime measurement is relatively a long-term test for the practical purpose of estimating product lifetime and of understanding failure mechanism. Generally in long-term tests, devices are tested under continuous or periodic stimulation, and measurements are repeatedly performed until they reach pre-defined termination conditions. From declining tendency of device performance in process of time, suitable modeling and data analysis enables the investigation of degradation phenomena as well as the prediction of device lifetime.

 Methods how to determine device lifetime are different depending on device characteristics. In case of an OLED device, for example, the luminance at constant current driving condition is repeatedly measured until it reaches usually 50% of initial reading. Similarly for a photovoltaic device, the output power or solar efficiency at constant light irradiation is periodically measured. The performance of rechargeable battery is characterized as charge/discharge capacity at constant current rate and Coulomb efficiency at a complete cycle of charging and discharging, which are measured over repeated number of cycles as a long-term test. This concept similarly applies to electrochemical measurement by using cyclic voltammetry for a quick stability evaluation of the material.

 The acceleration test is efficient for device lifetime evaluation, in practice, since the ensured lifetime of commercial product is substantially longer than laboratory testing schedule. In order to accelerate the degradation, devices are usually tested under severe electrical and/or thermal conditions. In this case, the factor scaling appropriate for matching between data obtained by the acceleration test and actual device lifetime at service condition is important. Supplementary test such as optical imaging is effective for more accurate prediction of device lifetime, if possible.

 The performance degradation of organic semiconductor devices such as OLED, OPV and OTFT is very sensitive to the penetration of water vapour and oxygen, and the product lifetime tests are necessary. Especially, the evaluation of encapsulation performance of the flexible electronic device is a recent key issue.