Light-intensity dependent photoresponse time of organic photodetectors and its molecular origin

Abstract

Organic photodetectors (OPDs) exhibit superior spectral responses but slower photoresponse times compared to inorganic counterparts. Herein, we study the light-intensity-dependent OPD photoresponse time with two small-molecule donors (planar MPTA or twisted NP-SA) co-evaporated with C 60 acceptors. MPTA:C60 exhibits the fastest response time at high-light intensities (>0.5 mW/cm 2), attributed to its planar structure favoring strong intermolecular interactions. However, this blend exhibits the slowest response at low-light intensities, which is correlated with biphasic photocurrent transients indicative of the pr esence of a low density of deep trap states. Optical, structural and en ergetical analyses indicate that MPTA molecular packing is strongly disrupted by C 60, resulting in a larger (370 meV) HOMO level shift. This results in greater energetic inhomogeneity including possible MPTA-C 60 adduct formation, leading to deep trap states which limit the low-light photoresponse time. This work provides important insights into the small molecule design rules critical for low charge-trapping and high-speed OPD applications.