Förster resonance energy transfer and energy cascade in broadband photodetectors with ternary polymer bulk heterojunction

With the purpose to extend the photoresponse region of the photodetector based on poly(3-hexylthiophene)/[6,6]-phenyl-C71-butyric acid methyl ester (P3HT/PC₇₀BM) host system, a low band gap polymer poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7) was incorporated to fabricate ternary blend photodetectors. The optimal photodetector shows high on/off ratio of 4.6 × 10⁴ with broadband response region from 380 to 750 nm by adding 10 wt % PTB7 to the host system. The improved performance of the device was attributed to energy cascade and the high efficiency of Förster resonance energy transfer between P3HT and PTB7, which are beneficial to current generation. At the same time, the device performance was correlated with the film morphology and crystalline structure of the active layer in the device. Excessive amount of PTB7 was unfavorable for device performance owing to the incompatibility between the two polymers in the ternary system, which hinders the exciton dissociation and charge extraction.