Spatial Confinement of the Optical Sensitizer to Realize a Thin Film Organic Photodetector with High Detectivity and Thermal Stability

Abstract

A thin film planar heterojunction organic photodetector (PHJ-OPD) is demonstrated. Different from a conventional sensitizer-doped photodetector, the limited spatial distribution of sensitizer in a PHJ-OPD enables significantly reduced thickness of the active layer without allowing the formation of unnecessary trap sites and electron percolation pathways. As a result, peak external quantum efficiency (EQE) of 120 700% and detectivity over 10 Jones are demonstrated with thin active layer thickness of 150 nm, which can be a significant benefit for high-resolution image sensor application. Furthermore, the operating voltage can be decreased to -5 V while maintaining high detectivity over 10 Jones. Remarkable thermal stability is also observed with minor change in detectivity for 2 h of continuous operation at 60 °C due to morphological robustness of PHJ. This work opens up a possibility of using a thin film PHJ-OPD as a key unit of high-resolution image sensor.

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.…”

“…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.…”

“…In conventional p-type PM-OPDs, where the photoactive layer primarily comprises an electron donor material, such as poly(3-hexylthiophene-diyl) (P3HT), with a small number of acceptor molecules (typically 1 wt%), such as [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), the trapping efficiency of minority carriers or electrons is a function of the energetics of trapped electrons within isolated acceptor molecules used for PM-OPDs because an energetically deep electron trap can suppress the electron detrapping rate. 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.…”