Doping strategies of organic n-type materials in perovskite solar cells: a chemical perspective

The ultimate aim of research dealing with perovskite solar cells (PSCs) is the commercialization of this novel kind of photovoltaic technology. In order to achieve this goal, the efficiency of the cells and even more their stability have to be optimized. Resorting to additives is a simple but effective technique to improve the performance of perovskite solar cells and enhance their stability. Several doping strategies have been tried during the years, involving either the charge transporting layers, the electrodes, or the perovskite layer. Doping has mostly been investigated within the field of hole transporting materials (HTMs), while less research has been focused on electron transporting layers (ETLs). Herein we summarize for the first time the most notable advances in the n-type doping of organic ETLs in PSCs, highlighting bathocuproines, halogen-ammonium salts, electron-donor molecules, N-DMBI and their derivatives, polymer-based dopants, carbon nanoforms, and other structures that cannot be classified in the previously categories.