Photostability Improvement of Organic Photodiodes with ZnO Electron Transport Layer

Abstract

Organic photodiodes (OPDs) have demonstrated unique mechanical properties, such as flexibility and stretchability, and can be used as on-skin conformable photodetectors or imagers, for example, to measure biosignals with high reliability. [1][2][3][4][5] Simultaneously, substantial research has demonstrated OPDs with a high specific detectivity comparable with that of commercial silicon photodiodes (%10 12 -10 13 Jones). [6,7] This most representative metric of OPDs has been improved by both enhancing the photoresponsivity and lowering the dark current. [8][9][10][11][12][13] One simple and effective technique to reduce the dark current regardless of the OPD structure is the careful selection and process optimization of the electron transport layer (ETL). [14][15][16][17] ZnO is a well-known and widely used ETL material for OPDs owing to its low work function, high optical transmittance, and easy processing. [18,19] ZnO ETL can be deposited via sputtering, a method that is adaptable to established commercial OPD fabrication. [20,21] However, its general adoption in OPDs is chiefly promoted by its sol-gel processability at low temperatures. [22] This solution processability enables printability and large-area processing of the ZnO ETL, [23,24] which are important assets of OPDs. Utilizing these materials and process advantages, the ZnO ETL has been widely incorporated into a variety of high-performance OPDs, thereby contributing to lowering the dark current and achieving high detectivity (%10 12 Jones) photodetectors in the ultraviolet (UV), visible, and near-infrared ranges. [25][26][27] In addition to their high optoelectronic performance, the application of OPDs with ZnO ETLs as flexible near-infrared photodetectors and imagers has been reported with demonstrated skin conformability and biosignal measurements. [4,5] Recently, to prepare OPDs for commercialization, their performance stability has become essential, and the stability of the photocurrent and dark current values has become the benchmark in exhibiting such stability. [12,26,[28][29][30][31][32] For instance, a study demonstrated the operation of ultraflexible OPDs with ZnO ETL with air stability for more than 10 days, where the photocurrent changed by less than 1% and the dark current changed by a maximum of 2.5, even without a high-barrier passivation layer. [28] The same OPDs underwent 1000 repetitive bending tests, thereby demonstrating the mechanical stability of the OPDs with only 5% change in the photocurrent and dark current change by a maximum of five times. In addition to storage and mechanical stability, stability with respect to external irradiation is critical, given their wide application as photodetectors. The photocurrent