End‐Group Functionalization of Non‐Fullerene Acceptors for High External Quantum Efficiency over 150 000% in Photomultiplication Type Organic Photodetectors

Abstract

Photomultiplication‐type organic photodetectors (PM‐OPDs) with high external quantum efficiency (EQE) of over 100% are attracting increasing attention due to their potential importance in detecting weak incident light. Considering that the gain of PM‐OPD is determined by the ratio of carrier lifetime over carrier transit time, a systematic study on the effect of the end‐functionalization of a new extended aromatic fused‐ring non‐fullerene acceptor (NFA) on the carrier trap/transit time of the PM‐OPD. Photophysical analyses by means of ultraviolet‐visible absorption, ultraviolet photoelectron spectroscopy, and photoluminescence combined with structural analyses through grazing‐incidence wide‐angle X‐ray scattering show that fluorination of the NFA with the deepest lowest unoccupied molecular orbital level and non‐isotropic molecular ordering can yield the longest carrier lifetime. Furthermore, surface energy study show that fluorination of the NFA can also yield the most hydrophobic nature, which can allow the most efficient injection barrier thinning/lowering of the active layer/cathode interface under illumination due to the localized acceptor distribution toward cathode, maximizing the hole injection efficiency from cathode. As a result, an unprecedentedly high EQE of 156 000% is obtained from the optimized PM‐OPD. This work shows the importance of the molecular design of acceptor molecules in fabricating high‐performance PM‐OPDs.

Interfacial electrostatic interaction-enhanced photomultiplication for ultra-high external quantum efficiency of organic photodiodes

“…3. To date, the highest EQE of PM-OPD was ~150,000% and most other works also remained near ~100,000% 3,6,7,8,9,10 . The observed EQE in this work which is at least 10-times higher than previous results should be attributed to the dramatically different electron trapping mechanism.…”

“…10. Another important assumption used for the simulation is that the Schottky contact between ITO/PFN-Br and P3HT can be tuned to a pseudo-Ohmic junction because of the nearby trapped electrons under illumination, as suggested in previous studies 6,7,8 . Therefore, different work function values were used for dark and illumination condition.…”

“…Then one can speculate that employing donor-acceptor combinations with a larger lowest unoccupied molecular orbital (LUMO) offset would result in increased electron trapping efficiency and thus a higher EQE. There have been several reports that introduce various acceptor molecules instead of conventional PC71BM with larger LUMO offset 7,8,9 ; however, a clear correlation between LUMO offset and EQE was not found. In fact, the EQE of high-performance PM-OPDs with various combinations of donoracceptor molecules remained approximately 100,000%, with a maximum value of 150,000% 3,6,7,8,9,10 .…”

“…There have been several reports that introduce various acceptor molecules instead of conventional PC71BM with larger LUMO offset 7,8,9 ; however, a clear correlation between LUMO offset and EQE was not found. In fact, the EQE of high-performance PM-OPDs with various combinations of donoracceptor molecules remained approximately 100,000%, with a maximum value of 150,000% 3,6,7,8,9,10 . One possible explanation for this phenomenon is that trapped electrons in localized PC71BM can find various energetically more favorable escape routes other than LUMO offset, such as structural defects or chemical defects, through which trapped electrons can reach the transport level of the donor polymer.…”

“…Recently, research regarding photomultiplication-type organic photodiodes (PM-OPDs) with external quantum efficiencies (EQEs) exceeding 100% have been actively conducted 1,2,3,4,5,6,7,8,9,10 . Because the main application fields of OPDs are expanding not only to cameras, but also to finger/vein/iris recognition sensors and bio-signal sensors, which are operated primarily by weak light sources, PM-OPD can be a promising candidate for the commercialization of OPDs based on their self-signal amplification behavior.…”