Dynamic Response of Low-Voltage Thin Film Phototransistors Based on DNTT Organic Semiconductor

We analyzed the dynamic response to the light of organic field-effect transistors in bottom-gate/top-contact configuration. We fabricated Al/Al2O3/SAM/DNTT/Au phototransistors by evaporating thin film layers through shadow masks on flexible PEN (polyethylene naphthalate) substrates. The structure is composed of Al layer as the gate electrode, and Au used both for Source and Drain electrodes. DNTT (Dinaphtho[2,3-b: $2^{\prime }$ , $3^{\prime }$ -f]thieno[3,2-b]thiophene) is the active organic semiconductor layer and Al2O3 is the dielectric material, chosen for the high value of the dielectric constant. SAM (self-assembled monolayer) was used to improve adhesion and interface properties between Al2O3 and DNTT. The transistors, sensitive to blue light, were biased at low-voltage (Vgs and $\mathrm { V_{ds}}$ from 0 to 3.5 V). Devices showed low $\mathrm { I_{gs}}$ leakage currents, of the order of $5x10^{-10}$ A, and a clear electro-optical response to the light. The maximum responsivity value was about 0.21 A/W in the static regime, while the lowest irradiance producing a measurable response in dynamic regime was $13~\mu $ W/cm2. Fast time components of the rise time of the light response for the analyzed phototransistors, of the order of few hundreds of ms, turned out to be among the fastest reported in literature for Al/AlOx/DNTT/Au organic phototransistor. These preliminary results are encouraging for developing organic phototransistors for visible light communication.