Dae Sung Chung1
POSTECH1
We suggest a strategy to artificially widen the linear dynamic range (LDR) of an organic photodiode (OPD) by introducing a light-intensity-dependent transition of its operation mode, such that a low saturation photocurrent can be overcome by additional operation mechanism. The active layer of OPD is doped with a strategically designed and synthesized molecular switch, which shows typical OPD performances with an EQE < 100% under low-intensity light illumination and photomultiplication behaviors with an EQE > 100% under high-intensity light illumination, resulting in an artificially extended LDR up to 225 dB. Such unique and reversible transition of the operation mode by light intensity self-recognition of molecular-switch-embedded OPDs can be explained by charge trapping/detrapping behavior of closed/open isomers of the molecular switch as well as the unbalanced quantum yield of the photocyclization/photocycloreversion of the molecular switch. To prove the suggested operation mechanism, various molecular switches with various functional groups are studied in conjunction with various photophysical analyses.<br/>Regarding the suggested mechanism, we show that the fluorinated benzene group in the molecular switch is responsible for the formation of interfacial band bending at the interface between the semiconductor and itself, resulting in exciton quenching and electron trapping. In general, such unprecedented exciton quenching and electron trapping may degrade the performance of organic electrons, but in certain applications, these phenomena can be positively utilized. An example suitable for this application is a photomultiplication-type organic photodiode (PM-OPD). The operating mechanism of PM-OPD can be divided into two stages. First, photogenerated excitons are separated at the donor-receptor interface, electrons are trapped by spatially localized trap centers, and remaining holes are transited to the collecting electrode. Second, trapped electrons near the anode interface induce band bending, resulting in hole injection to generate photomultiplication. In other words, molecular switches embedded in OPD become such trapping centers under high-intensity light illumination and regains its original neutral properties under low- intensity light illumination.<br/> After carefully optimizing dynamics of the light-intensity-dependent transition of operation mode of the suggested molecular-switch-embedded OPDs, these unique OPDs are integrated onto CMOS chip to demonstrate organic image sensors with superior sensitivity against strong light illumination.