Tunable Energy-Level Alignment in Multilayers of Carboxylic Acids on Silver

Precise energy-level alignment between a metal electrode and an organic semiconductor is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. Here, we introduce monolayer-thick charge-injection layers (CILs) based on aromatic carboxylic acids that can induce an energy-level shift in subsequent layers by up to 0.8 eV. Through a gradual chemical transformation of the as-deposited molecules, we achieve a highly tunable energy-level shift in the range of 0.5 eV. We reveal that the work function and energy-level positions in the CIL increase linearly with the density of induced dipoles. The energy-level position of subsequent layers follows the changes in the CIL. Our results thus connect the energy-alignment quantities, and the high tunability would allow precise tuning of the active layers deposited on the CIL, which marks a path towards efficient charge-injection layers on metal electrodes.