Enhancing spectral contrast in organic red-light photodetectors based on a light-absorbing and exciton-blocking layered system

We demonstrated a highly sensitive red-light photodetector based on a mixed copper phthalocyanine (CuPc) and fullerene C60 photoactive layer, similar to a so-called bulk heterojunction structure usually used in the field of organic photovoltaics. We incorporated an additional set of organic layers that was composed of two organic p-type semiconductors to reduce the blue-light sensitivities of CuPc- and C60-based organic photodetectors. We used α,ω-diphenyl sexi-thiophene (P6T) and α,ω-bis(biphenyl-4-yl)ter-thiophene (BP3T), which are thiophene-based materials and usually have good hole-transporting properties. A thick (>100 nm) P6T layer absorbed blue light, preventing it from reaching the photoactive layer, and a thin (∼20 nm) BP3T layer whose band gap was larger than that of P6T blocked excitation energy transfer from P6T to CuPc. Thus, we successfully demonstrated a red-light photodetector with high peak sensitivity and whose current-voltage characteristics did not worsen. The optimal device showed a peak incident photon-current conversion efficiency of 51.7% at 620 nm and a specific detectivity of 4.0×1011 cm Hz1/2/W.