Indium tin oxide modified with sodium compounds as cathode of inverted polymer solar cells

Abstract

Polymer solar cells (PSCs) with inverted structure can greatly improve the photovoltaic stability. This paper reports surface modification of indium tin oxide (ITO) by spin coating a thin layer of various sodium compounds. ITOs with such a treatment were used as the cathode of the inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor, and MoO₃/Al as the anode. Among these sodium compounds, sodium hydroxide (NaOH) gives rise to high photovoltaic performance: an open-circuit voltage (V_oc) of 0.58 V, short-circuit current density (J_sc) of 10.03 mA cm⁻², fill factor of 0.67, and power conversion efficiency of 3.89% under AM 1.5G illumination (100 mW cm⁻²). The efficiency is significantly higher than that (0.35%) of the control devices with untreated ITO as the cathode and even slightly higher than that (3.61%) of PSCs with normal architecture. The high performance of the inverted PSCs is due to the reduction of the work function of ITO by almost 1 eV by NaOH. The reduction in the work function of ITO was also observed by other sodium compounds, and it is consistent with the association constants of the anions of the sodium compounds with proton. The mechanism for the reduction of the work function is attributed to the dipole formation and alignment of the sodium compounds on the ITO surface.