Page 207 - Organic Electronics in Sensors and Biotechnology

184 Chapter Five

segments separate, the PL of the donor, previously absorbed by the
acceptor, becomes detectable by the PD.
A suitable peptide labeled with a rhodamine-based dye as the
donor and a Molecular Probes QSY7 dark quencher as the acceptor
was synthesized. Preliminary results using green ITO/CuPc/NPD/
Alq /CsF/Al OLEDs yielded a maximal PL intensity increase of
3
~100% at 37°C and ~ 40 μM peptide upon 15 min exposure to 25 nM
LF. This PL increase is low relative to that observed by UV laser exci-
76
tation, pointing to an issue associated with OLED excitation in con-
junction with the specific dye employed, which has a Stokes shift of
only ~20 nm. Indeed, using a green inorganic LED with a 530 nm
bandpass filter, and a 550 nm longpass filter in front of the PMT, the
LF increased the PL six fold following peptide cleavage. 79
To increase the change in the output of the PD resulting from
cleavage of the peptide by LF, other donor-acceptor pairs are being
examined, together with microcavity OLEDs that emit a much nar-
rower EL band. Other approaches to reduce the contribution of the
EL tail, e.g., the use of crossed polaroids between the OLED and the
80
PD, are also under investigation.

5.5 OLED/Sensing Component/Photodetector
Integration
We are currently developing a compact PL-based O sensor to evalu-
2
ate a fully integrated platform, where the PD is a p-i-n or n-i-p struc-
ture based on thin films of hydrogenated amorphous Si (a-Si:H) and
18
related materials, or nanocrystalline Si (nc-Si). Similar to OLEDs,
a-(Si,Ge):H-and nc-Si-based PDs are easily fabricated on glass or
plastic substrates.
The composition of the layers in the PDs was first chosen so as
to minimize their sensitivity at the Alq -based OLED EL (i.e., at
3
~535 nm) and maximize it at the PtOEP and PdOEP emission bands,
i.e., at ~ 640 nm. To this end, a-Si:H and a-(Si,Ge):H PDs were evalu-
ated by measuring their quantum efficiency (QE) vs. wavelength. 18
The a-(Si,Ge) PDs are attractive due to their better match with the
dyes’ PL, and lower response at the EL band. However, they are infe-
rior overall due to their higher dark current and lower speed.
In monitoring the oxygen concentration, using the thin-film PDs,
via the I mode, a lock-in amplifier was used with a pulsed OLED. The
detection sensitivity, however, was relatively low due to issues related
to the PDs’ fabrication, and to an electromagnetic (EM) noise stem-
ming from the pulsed OLED and the device wiring when using the
lock-in detection.
One PD-related issue was its high dark current. It was suspected
that this issue is a result of boron diffusion from the p to the i layer
during growth of p-i-n devices. To minimize this diffusion, a SiC