Work Function Adjustment of Nb2CTx Nanoflakes as Hole and Electron Transport Layers in Organic Solar Cells by Controlling Surface Functional Groups

The Nb2CTx nanoflakes as members of the MXene family have attracted a lot of attention because of their unique optoelectronic properties. In this work, we use alkali and annealing treatments to regulate the surface functional groups in Nb2CTx and then adjust its work function (WF). After KOH treatment, the −F group is replaced by an −OH group, leading to a decrease in WF from 4.62 eV (Nb2CO1.2OH0.6F0.2) to 4.32 eV (Nb2CO1.2OH0.8). After annealing, −OH is transformed into −O, resulting in a WF increase to 5.03 eV (Nb2CO1.36OH0.2). Polymer solar cells (PSCs) based on PM6:BTP-eC9:PC71BM with tuned Nb2CTx as the electron transport layer (ETL) and the hole transport layer (HTL) exhibit the highest power conversion efficiency (PCE) of 17.64% and 17.51%, respectively. These efficiencies are comparable to those gained using conventional charge transport layers. This work provides a good method to tune the WF of Nb2CTx and exhibits the Nb2CTx nanoflakes have a great application prospect in PSCs.