Page 191 - Organic Electronics in Sensors and Biotechnology

168 Chapter Five

analyte-induced changes in the PL decay time τ can be measured dur-
ing the off period of the OLED to monitor specific analytes; this approach
eliminates the need for such filters. Figure 5.1b shows an array operated
in the “front detection” mode, where the PD array is fabricated on a
separate substrate and placed in front of the analyte. As such, the PD
array can serve as a cover for microfluidic wells or channels containing
surface-immobilized or solutions of recognition elements. In both cases,
compact arrays with flexible designs can be generated to address the
various sensing needs whether in the gas or liquid phase.
In developing the OLED sensing platform, we focused first on
OLED/sensing component integration, to generate the compact sen-
sor module. As detailed below, this module was evaluated for moni-
toring various gas- and liquid-phase analytes, including multiple
analytes in a single sample. The second step was directed toward
advanced integration with additionally a PD.

5.2 Structurally Integrated OLED/Sensing Component
Modules
The OLEDs used for the platform development consist of a transparent
conducting anode, typically indium tin oxide (ITO) coated on a glass or
plastic substrate, organic hole- and electron-transporting layers (HTLs
and ETLs, respectively), an emitting layer, and a metal cathode. They
30
are easily fabricated using thermal evaporation in a low-vacuum (~1 ×
10 torr) glove box. Details on their fabrication and encapsulation, for
–6
enhanced long-term stability, are provided elsewhere. 13–17 The organic
layers consisted of a 5 nm thick copper phthalocyanine (CuPc) hole
injecting layer, which is also believed to reduce the surface roughness
31
of the ~140 nm thick treated ITO, and a 50 nm thick N,N’-diphenyl-
N,N’-bis(1-naphthyl phenyl)-1,1’-biphenyl-4,4’-diamine (NPD) HTL.
For blue OLEDs, with peak emission at ~460 to 470 nm, the 40 nm thick
emitting layer was 4,4’-bis(2,2’-diphenylvinyl)-1,1’-biphenyl (DPVBi),
32
or perylene (Pe)-doped 4,4’-bis(9-carbazolyl) biphenyl (Pe:CBP), typi-
cally followed by a 4 to 10 nm thick tris(quinolinolate) aluminum (Alq )
3
electron transport layer. For green OLEDs with peak emissions at ~535
and ~545 nm, the ~40 nm thick emitting layer and ETL were Alq and
3
rubrene-doped Alq , respectively. In all cases, an 8 to 10 Å CsF or LiF
3
buffer layer was deposited on the organic layers, 33, 34 followed by the
~150 nm thick Al cathode. The total thickness of the OLEDs, excluding
the glass substrate, was thus < 0.5 μm. Under forward bias, electrons
are injected from the low-workfunction cathode into the ETL(s). Simi-
larly, holes are injected from the high-workfunction ITO into the
HTL(s). Due to the applied bias, the electrons and holes drift toward
each other and recombine in the emitting layer. A certain fraction of the
recombination events results in radiative excited states. These states
provide the EL of the device.