3.3 Biomimetic Photosensors

౪

77

3.3 Biomimetic Photosensors

There are a lot of photosensors that mimic some or all functions of the human vision

system (eye combined with brain) with artificial systems: light detection, signal trans-

duction to electric signal, amplification of the signal, and analysis of the signal.

There is a large variety of chirooptical switches, where a specific wavelength in-

duces a change in the three-dimensional structure of the molecule, mimicking the

change the retina makes in response to light ([9] and references therein); in this case,

the chemical structure changes to a different enantiomer. This change is reversible

with a different wavelength light and quite fast. These switches are not connected to

any further action or analysis. Instead, they are expected to be implemented as optical

computer memory.

Solution processable organic field-effect transistors (OFETs) can also be used as op-

tical sensors called “organic phototransistors” (OPTs) (Figure 3.5) [10]. In this example,

the ionic hydrogel containing silver nanowires is a transparent, flexible gate electrode.

The pyrrole-thiophene copolymer is a narrow band-gap polymer with high hole mobil-

ity that is photosensitive. Therefore, when light hits the copolymer the holes will move

toward the drain, generating current, with that transducing a light signal into a current

output. Polyimide is an organic semiconductor to complete the transistor. The sensitivity

of this photosensor can be tuned, as well as amplified, by the gate voltage.

Figure 3.5: (a) Example of an organic field-effect transistor (OFET) that can be used as an optical sensor

(“organic phototransistor”, OPT) (adapted from [10]). The sensor is the pyrrole-thiophene copolymer, which

transduces the light signal into a current output (b). The sensitivity of this photosensor can be tuned (and

amplified) by the gate voltage (adapted from [10]).

Another organic phototransistor can react differently to two wavelengths, and thus can

be used as an optical logic gate (Figure 3.6) [11]. Dependent on the wavelength, the tran-

sistor is an “and” or “or” logic gate, while at the same time still transducing the optical

signal to a current output and possibly even amplifying the signal at the same time.

These logic gates mimic some of the functions of the brain, where signal output is ana-

lyzed by a variety of the following neurons.