Organic thin film phototransistors: Materials and mechanism

Highly photosensitive organic phototransistors (OPTs) with an organic thin film transistors configuration based on a biphenyl end capped fused bithiophene oligomer (BPTT) and copper phthalocyanine (CuPC) were prepared. The measured maximum responsivity and the ratio of photocurrent to dark current (I _Ph/I_Dark) in BPTT and CuPC OPTs were 82 A/W, 2 A/W and 2.0 × 10⁵, 1000 under 365 nm UV light with 1.55 mW/cm², respectively. The prepared OPTs showed a photocurrent response similar to the photo to current conversion efficiency (IPCE) spectrum of BPTT and CuPC. The main mechanisms responsible for photocurrent amplification in the devices were examined by comparing theoretical and measured data. The photovoltaic (turn-on) and photoconductive effect (turn-off) of the OPTs were determined by fitting to theoretical equations. The findings confirmed that the operation of the OPTs followed as photovoltaic (turn-on state) and photo-conductive (turn-off state) behaviors.