The properties of organic semiconductors provide opportunities for low-cost
optical sensors. In the first part of this work, the suitability of organic metal-insulator-semiconductor (MIS) capacitors as elements in optical sensing arrays
and charge-coupled devices (CCD) has been investigated. Of utmost importance
in MIS devices is the interface between the insulator and semiconductor, where
interface traps can lead to device instabilities and poor charge transfer in
CCD-type structures. MIS capacitors with either polysilsesquioxane (PSQ) or
photoresist (SU8) as the insulator were fabricated under ambient conditions, with
poly(3-hexylthiophene) as the organic semiconductor. Electrical characterisation
of devices was performed both in the dark and under illumination with bandgap
light.
Admittance measurements obtained in the dark at temperatures of 125-375 K
revealed both PSQ and SU8 devices had good stability up to ~ 325 K; the
capacitance-voltage (C-V) plots showed consistent behaviour with both PSQ
and SU8 devices exhibiting fairly constant flatband voltage, doping density ~1015
cm-³, and interface state density ~10¹º cm-² eV–¹ . However, the bulk mobility in PSQ and SU8 devices deviated from both Arrhenius and T-² temperature dependencies. Bulk mobility of a further set of SU8 devices, fabricated under nitrogen, exhibited an Arrhenius temperature dependency, indicating that despite undergoing several annealing cycles, atmospheric dopants were still present in devices fabricated in air.
C-V plots of both PSQ and SU8 devices revealed increased depletion layer
capacitance and anti-clockwise hysteresis when irradiated with light of energy
greater than or equal to the HOMO/LUMO energy gap. Up to ~ 10¹² photogenerated electrons were trapped at the interface resulting in large shifts in
flatband voltage. De-trapping of electrons was found to be much slower in
SU8 devices, and followed a stretched exponential decay in both PSQ and SU8
devices. The optical response of MIS capacitors was found to depend on the
intensity and wavelength of the incident light and the bias voltage scan rate. A
basic model accounting for these factors was developed which simulated the C-V
response of MIS capacitors during exposure to light. Transfer of photo-generated
electrons between adjacent MIS capacitors, as in a single charge-injection device,
was demonstrated in both C-V and displacement current measurements. This
result indicates the possible use of organic materials in CCD-like structures.
Chapter 6 reports the successful application of an organic photo-voltaic (OPV)
cell as the near-field detector in a surface plasmon resonance (SPR) bio-sensor.
Photo-current generated by the OPV during specific binding events at the sensor surface correlated well with reflectivity measurements made with a silicon photodiode. Sensitivity was found to be lower than a conventional bio-sensor, but
could be enhanced by improved device structure and surface functionalization.