Enhancement of 3D/2D Perovskite Solar Cells Using an F4TCNQ Molecular Additive

Recent developments in hybrid organic–inorganic lead halide perovskite solar cells (PSCs) have shown that use of a 2-dimensional (2D) perovskite capping layer on a 3D perovskite absorber layer (3D/2D PSCs) results in substantially improved environmental stability of the devices. However, in these 3D/2D PSC structures the poor charge transporting properties through the 2D layer hinder device performance, particularly for thicker 2D layers. Here, we report a facile method that overcomes some of the inherent challenges in using such 2D capping layers through the introduction of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as a molecular additive into the 2D layer. It is found that this additive undergoes partial charge transfer with the perovskite layer to provide a passivation effect, which also modifies the crystalline domain sizes of the 2D perovskite. PSCs incorporating a F4TCNQ-modified 2D capping layer grown in situ on top of the 3D Cs/MA/FA mixed-cation perovskite absorber layer display modified interfacial characteristics that yield improved charge extraction and reduced charge recombination compared with 3D/2D PSCs comprising an unmodified 2D capping layer. This modification results in 3D/2D PSCs for which the maximum power conversion efficiency is increased to more than 20% compared with 18.2% for unmodified 2D capping layers.