Bending strain induced photocurrent crossover from positive to negative in the flexible organic phototransistors

The understanding of bending strain effects on the performances of flexible organic phototransistors (FOPTs) is of great importance. In this manuscript, two kinds of FOPTs-single layer of copper phthalocyanine (CuPc) (SL-FOPTs) and hybrid planar bulk-heterojunction of CuPc/CuPc:PTCDA (HPBHJ-OPTs) with PTCDA denotes 3,4,9,10-perylenete-acarboxylic dianhydride, with polyvinyl alcohol as the gate dielectric but different active layer, were fabricated and their strain dependent performances investigated, respectively. For SL-FOPTs, at a given light intensity, with the bending-strain increasing, the photocurrent increases at first, and then decreases after reaching a maximum. A crossover from positive to negative occurs in this process, and the critical strain at which the crossover occurs increases with the illumination light intensity. Different from the observation in SL-FOPTs, the Idark ~ strain curves in HPBHJ-FOPTs exhibit obtuse small peaks at low gate voltages of |Vg|≦10 V, and minimums at large gate voltages of |Vg|≧ 20 V. At large gate voltages with increasing bending strain, the photocurrent of HPBHJ-OPTs decreases and become negative (from positive) at first, with the decreasing rate nearly proportional to the gate voltage, and then increase after reaching a negative minimum. The physical origin of observed photocurrent crossover from positive to negative with increasing bending strain is discussed.