Enabling luminescence decay time-based sensing using integrated organic photodiodes

The use of organic photodiodes (OPDs) for measuring phosphorescent lifetimes of optochemical oxygen sensors is described. Phosphorescent indicators with lifetimes ranging from ～5 to 60 μs have been studied using light-emitting diodes as the excitation source and organic photodiodes integrated into the sensor substrate for detection. A measurement system using an adjusted electronic circuitry to detect photocurrents in the nanoampere range is presented. The response behaviour of the organic photodiodes has been characterized, and it was found that a forward (positive) bias had to be applied in order to decrease the response time of the OPDs to a range suitable for phosphorescence decay time measurements. A modulation cutoff frequency of ～100 kHz has been determined, corresponding to a response time of the organic photodiodes of 1.6 μs. Two sensor dyes have been characterized regarding their lifetimes upon exposure to 0–20 % oxygen, and it was shown that results comparable to literature data and inorganic photodetectors can be achieved.