Plasma treatment of ITO cathode to fabricate free electron selective layer in inverted polymer solar cells

Abstract

With Ar plasma treatment of the indium tin oxide (ITO) cathode, we achieve efficient inverted bulk heterojunction solar cells based on poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester, which do not require electron selective layer. The plasma treatment improves the power conversion efficiency of the device from 1.07% to 3.57% with a fill factor of 66%, open-circuit voltage of 0.60 V, and short-circuit current of 9.03 mA cm⁻². This result is comparable to the regular inverted devices with an additional electron selective layer. The Kelvin probe detects a reduction in the ITO work function of ∼0.45 eV after plasma treatment, which finally leads to an increase in the built-in potential and faster carrier extraction. As a result, good device performance is achieved. Because the electron selective layer becomes unnecessary, our strategy suggests a simple way to achieve efficient inverted organic solar cells.