Design of a simple bifunctional system as a self-assembled monolayer (SAM) for inverted tin-based perovskite solar cells

In this work, we functionalized the ITO substrates with a series of self-assembled monolayer (SAM) molecules to improve the hole extraction ability of the electrodes and retard the charge recombination with the devices in an inverted planar p-i-n configuration. Organic molecules with simple structures, namely 4-aminobenzoic acid (AB), 4-(2-aminomethyl)benzoic acid (AM), 4-(2-aminoethyl)benzoic acid (AE), and 4-nitrobenzoic acid (NB) were chosen to functionalize the ITO substrates via simple immersion method to form hole-selective SAM on ITO. The TPSCs were fabricated according to a two-step sequential deposition approach using a co-solvent system, and the AB device was found to exhibit an attractive efficiency of power conversion (PCE) of 7.6 % while the other SAM-based devices showed poorer performance. The AB device also displayed impressive long-term storage stability by maintaining about 80 % of its initial efficiency for over 3500 h without encapsulation, in addition to a long-term (6 h) light-soaking stability, which is superior to the PEDOT: PSS-based device in ambient conditions. The SAM/perovskite interfacial characteristics were studied using UPS, EIS, and TCSPC to understand the energy levels, charge recombination, and hole-extraction nature, respectively, and to support the outstanding performance and stability of the AB device.