Realizing Photostable Detection in Organic Photodiodes with ZnO Electron Transport Layer

Organic photodiodes (OPDs) have demonstrated distinctive mechanical properties, such as flexibility and stretchability, which allow their implementation as skin-conforming photodetectors. These photodetectors provide highly reliable measurements of the pulse rate and vein images.[1] This reliability owes to the high specific detectivity (10¹¹ – 10¹³Jones) of the recent OPDs. Achieving this high detectivity has been facilitated by the suppression of dark current via the insertion of an electron transport layer (ETL). ZnO is widely used as an ETL, and the OPDs using ZnO ETL also exhibited mechanical and air storage stability of dark current characteristics, thereby enabling their usage as ultra-flexible photodetectors for continuous measurement of bio-signals.[2]<br/>However, dark current in OPDs using ZnO ETL increases by several orders of magnitude after light irradiation, thereby notably reducing its specific detectivity.[3] Sol-gel ZnO ETL is widely fabricated with a post-annealing at a maximum of 180 ^oC, whereas ZnO ETL requires a minimum temperature of 280 ^oC to fully synthesize from its precursor solution. The defective formation of ZnO ETL is considered to cause the photoinduced increase of dark current in the OPDs. Additionally, the effects of the annealing temperature of the ZnO ETL on the photostability of the OPDs have not been thoroughly studied.<br/>In this work, we examined the effects of the annealing temperature of the ZnO ETL on the photostability of the OPDs. We employed the inverted OPD structure of “glass/ITO (70 nm)/ZnO (30 nm)/P3HT:PC₆₁BM (250 nm)/MoO<i>_x </i>(10 nm)/Ag (100 nm)” where the ZnO ETL was synthesized as a sol-gel from the zinc acetate dehydrate precursor and fabricated using standard spin-coating followed by post-coating annealing. We increased the commonly adopted annealing temperature of the ZnO ETL from 180 to 350 ^oC and confirmed that the external quantum efficiency (EQE) of the devices was not affected. By this high-temperature annealing, we reduced the increase of dark current after light irradiation by 51 times, thereby suppressing the increase to approximately 2.0 times the original value. Specifically, the dark current density at –2 V in OPDs with ZnO ETL annealed at 180 ^oC increased by 100 times from the initial value of (6.2 ± 0.21) × 10^–8 A cm^–2 to (6.2 ± 0.15) × 10^–6 A cm^–2 after irradiation. On the contrary, those with ZnO ETL annealed at 350 ^oC increased by only 2.0 times from the initial value of (1.9 ± 0.19) × 10^–6 A cm^–2 to (3.9 ± 0.39) × 10^–6 A cm^–2. By suppressing this increase in dark current, the stability of the specific detectivity of the OPDs based on the shot noise was improved by 7.1 times. Specifically, the OPDs with ZnO ETL annealed at 180 ^oC showed an initial specific detectivity of 2.8 × 10¹² Jones but deteriorate to 2.8 × 10¹¹ Jones after light irradiation, thereby only exhibiting 10 % retention. On the other hand, the OPDs with ZnO ETL annealed at 350 ^oC showed an initial detectivity of 5.9 × 10¹¹ Jones and held the detectivity to a sufficiently high value of 4.1 × 10¹¹ Jones, thus showing a 70 % retention. Additionally, we demonstrated that UV is the cause of this photo-instability by showing that the increase in dark current after irradiation with wavelengths longer than 380 nm was lower than and at most 17.6 % of those observed after sub-370-nm irradiations. Furthermore, A better quality of the ZnO ETL was identified to be crucial to improve the photostability of OPDs using ZnO ETL, as confirmed through XPS and XRD measurements. These results underscore the importance of choosing a defect-free ETL synthesis or material to realize next-generation photostable OPDs.<br/><br/>[1] T. Yokota et al., <i>Nat. Electron. </i><b>3</b>, 113 (2020).<br/>[2] S. Shimanoe et al., <i>Adv. Electron. Mater.</i>, 2200651 (2022).<br/>[3] J. Huang et al., <i>ACS Nano</i> <b>15</b>, 1753 (2021).