Nanodiamonds as Charge Extraction Layer in Organic Solar Cells: The Impact of the Nanodiamond Surface Chemistry

Diamond nanoparticles so-called nanodiamonds (NDs) have recently experienced raising scientific interest due to interesting optical and electronic properties, nontoxicity, biocompatibility, and large surface area. Another significant feature of NDs is the versatility of the surface chemistry, where various functional groups can be attached. This provides an excellent platform for adjusting NDs properties and functions for many applications including in photovoltaic devices. Herein, high-pressure high-temperature (HPHT) NDs are tested as charge extraction material in organic solar cells using various surface chemistries: as-received (HPHT ND-ar), oxidized (HPHT ND-O), and hydrogenated (HPHT ND-O-H) NDs. Despite the high work function values (approximate to 5.3 eV) of HPHT ND-ar and HPHT ND-O, which make these materials normally suitable for hole extraction, devices made with them failed. In contrast, the work function decreases upon hydrogenation (approximate to 4.5 eV) of the beforehand oxidized NDs, making them interesting for electron extraction. By employing such HPHT ND-O-H for electron extraction layers, PBDB-T:ITIC-based devices reach 77%, while PM6:Y6-based devices reach even 85% of the performance when process on standard ZnO electron transport layers. Improvement of the film-forming qualities of this new electron extraction material is expected to further improve the performance.