Printing High‐Quality Formanidinum Lead Triiodide Films: Understanding the Critical Role of α‐Phase Nucleation Before Thermal Annealing

Abstract Upscaling the coating of formamidinium lead triiodide (FAPbI ₃ ) thin film is essential to realizing full printing and thereby roll‐to‐roll production of perovskite photovoltaics. However, thin‐film FAPbI ₃ processed from antisolvent‐free, printing methods suffer from undesirable degradation to the non‐photoactive phase, particularly in ambient processing conditions. Here, the most critical stage in thin‐film processing is identified as the gas‐quenching treatment of the as‐cast wet film. It is crucial to achieve both the nucleation of α‐phase nanocrystals and their sufficient growth at room temperature, which are indispensable for templating the growth of a highly crystalline and stable α‐FAPbI ₃ film during subsequent thermal annealing. The gas‐quenching‐treated film without these α‐phase nanocrystals can only be converted to meta‐stable α‐FAPbI ₃ by thermal annealing. By precisely identifying the spontaneous doping of cesium ion (Cs ⁺ ) as a key factor in triggering α‐FAPbI ₃ nucleation and the role of chloride ion (Cl ⁻ ) in enhancing nanocrystal growth, a generalized mechanism for additive engineering for the precursor ink is theorized. The identified mechanism translates to a power conversion efficiency of 19.36% for fully printed carbon‐electrode solar cells, 16.23% for carbon‐electrode minimodules, and excellent operational stability, showing the promising potential of this proposed fabrication route in the upscaling of perovskite photovoltaics.