Effect of Anode Interfacial Modification by Self-Assembled Monolayers on the Organic Solar Cell Performance

A series of self-assembled monolayer (SAM)-based benzoic acid derivatives such as 4-[5'-phenyl-2,2'-bitien-5-yl] benzoic acid (ZE-Ph), 4-[5'-(4-fluorophenyl)-2,2'-bitien-5-yl]benzoic acid (ZE-1F), and 4-[5'-(3,5-difluorophenyl)-2,2'-bitien-5-yl]benzoic acid (ZE-2F) were synthesized to use an interlayer between an ITO electrode and a MoO₃ thin film layer in an organic solar cell (OSC) having poly-3 hexylthiophene (P3HT): [6,6]-phenyl C₆₁ butyric acid methyl ester (PC₆₁BM) blend. The work function and surface wetting properties of the ITO were tuned by SAM molecules. The power conversion efficiency of fabricated OSC devices was improved compared to that of the control device from 1.93 to 2.20% and 2.22% with ZE-Ph and ZE-1F-modified ITO electrodes, respectively. The short-circuit current density (J_sc) was increased from 6.16 to 7.10 mA/cm² and 6.94 mA/cm² with control, ZE-Ph, and ZE-1F-modified solar cells, respectively. The increase in short-circuit current density (J_sc) shows that the hole-transporting properties between ITO and MoO₃ were improved by the use of ZE-Ph and ZE-1F compared with that of the ITO/MoO₃ electrode configuration. The open-circuit voltage (V_oc) of the SAM-modified ITO-based devices was also improved compared with the V_oc of unmodified ITO-based devices. These results show that using a monolayer as an interlayer in OSCs is an important strategy to improve the performance of OSCs. All the device parameters were characterized by Kelvin probe force microscopy, cyclic voltammetry, contact angle, and I-V measurements.