Structurally integrated luminescence based oxygen sensors with Organic LED/ oxygen sensitive dye and PECVD grown Thin Film Photodetectors
Ghosh, Debju (2008) Structurally integrated luminescence based oxygen sensors with Organic LED/ oxygen sensitive dye and PECVD grown Thin Film Photodetectors. PhD thesis, Iowa State University.
Full text available as:
There have been increasing efforts for the development of compact and miniaturized oxygen sensor for various applications including multianalyte sensor and sensors with a reliable and accurate monitoring of oxygen concentration in controlled environments requiring measurements under low oxygen concentration. This work has been devoted to the development of structurally integrated luminescence quenching based sensors using Organic Light Emitting Diode (OLED) as excitation source, oxygen sensitive dyes as the sensing element and PECVD based thin film photodiodes for detection. The whole structure can be extended towards development also on flexible substrates. The developed sensor is less cumbersome and bulky and easy to use compared to the existing sensors. The commonly used Clark electrode for measuring dissolved oxygen suffers from interference from the ambient, requires frequent maintenance and electrolyte change, and also consumes oxygen during the measurement resulting in a little inaccurate measurement and hence restricts the resolution of the sensor. In addition the whole set-up is very bulky, which further motivates the development of these luminescence-based sensors. Another advantage of these luminescence-based sensors is the ease of tunability of sensitivity for different oxygen levels merely by changing the sensor dye. This work has been devoted mainly to the development of appropriate thin film low temperature grown thin film photodiodes optimized for best possible performance of the whole sensor configuration. In addition the three component integration issues were solved by overcoming all the challenges due to electromagnetic noise generated by the OLED, interfering with the photodiode response and hence limiting the sensitivity. Amorphous silicon/silicon germanium and nanocryatalline silicon thin films were characterized for better performance in photodiodes. The sensitivity spectrums of the photodiodes were engineered to have maximum sensitivity around 640nm and minimum around 535nm, which is the peak emission wavelength of OLED. This process development resulted in enhanced immunity of the photodiode to the background resulting in enhanced sensitivity. Two types of detection modes were used: intensity monitoring and PL lifetime monitoring mode. In the intensity mode, the reduction of the PL intensity in presence of oxygen due to quenching effects was monitored. The oxygen sensitive film partially absorbs the OLED light at 535 nm and then it emits at around 640 nm. Two filters were used, one bandpass on top of OLED to reduce the background tail emission followed by the sensor film and then a longpass filter to stop any unabsorbed OLED light. Sensor dyes films are either platinum or palladium based (PtOEP or PdOEP) respectively. A lock-in amplifier was used, to reduce the noise during detection and hence enhancing the sensitivity. But the major challenges were the OLED tail extending to 650nm, which was solved by coumarene doping of the Alq3 based OLED resulting in narrower spectrum. The photoluminescence lifetime of the sensor dye film also changes with higher oxygen concentration and thus provides an alternate method of oxygen monitoring. For the PL lifetime based technique, the OLED was pulsed and it was off during the measurement. This resulted in the possibility of elimination of the filters and hence further minimizing and simplifying the sensor configuration. At this point the frequency response of the photodetectors becomes very important and their response time becomes the major issue. Proper impedance matching of the detecting circuit is critical for fast functioning of the photodetector. The boron diffusion during growth was observed to dominantly affect the frequency response, which was partially solved by using nip structure instead of pin structure photodiode. Using nanocrystalline and development of quantum dot photodetectors with amorphous layer grain boundary passivation further improved the response speed. Absolute quantum efficiencies were improved for the detectors. The thin film photodiodes were grown using PECVD, using both ECR (Electron Cyclotron Resonance) and VHF (Very High Frequency) PECVD techniques.
Archive Staff Only: edit this record