Effect of Chemical Modification of Fullerene-Based Self-Assembled Monolayers on the Performance of Inverted Polymer Solar Cells

The interface of electron-selective ZnO in inverted polymer bulk-heterojunction (BHJ) solar cells was modified with a series of fullerene-based self-assembled monolayers (C<sub>60</sub>-SAM) containing different anchoring groups (catechol, carboxylic acid, and phosphonic acid), linkage location, and functionalization. The formation of the C<sub>60</sub>-SAM to the surface of ZnO was investigated by processing the SAM through either a solution immersion technique or a solution spin-coating method. It is found that the C<sub>60</sub>-SAMs with the carboxylic acid and catechol termination can be formed onto the surface of ZnO by simple solution spin-coating process, whereas all three anchoring groups can be formed by solution immersion technique. Heterojunction devices were fabricated under different processing conditions to form SAM leading to 2-fold, 75%, and 30% efficiency improvement with the carboxylic acid, catechol, and phosphonic acid C<sub>60</sub>-SAMs, respectively. The main contribution to the variation of efficiency from different SAMs is due to the open circuit voltage affected by different anchoring groups and functionalization of the C<sub>60</sub>-SAM. The results from BHJ devices show an efficiency enhancement of ∼6−28% compared to devices without SAM modification because of the improved photoinduced charge transfer from polymer to the C<sub>60</sub>-SAM/ZnO. The SAM formation condition influences the device performance. Because of the strong acidic nature of the phosphonic acid anchoring group, immersing the ZnO substrate into a solution containing the C<sub>60</sub>-phosphonic acid SAM for an extended period of time will lead to degradation of the ZnO surface. This in turn, leads to devices without any photovoltaic activity, whereas weaker acids like carboxylic acid and catechol-based C<sub>60</sub>-SAMs can be assembled onto ZnO, leading to devices with average efficiencies of 4.4 and 4.2%, respectively.