Improving the Electron Transport Performance of TiO2 Film by Regulating TiCl4 Post-Treatment for High-Efficiency Carbon-Based Perovskite Solar Cells.

Titanium oxide (TiO2 ) has been widely used as an electron transport layer (ETL) in perovskite solar cells (PSCs). Typically, TiCl4 post-treatment is indispensable for modifying the surfaces of TiO2 ETL to improve the electron transport performance. However, it is challenging to produce the preferred anatase phase-dominated TiO2 by the TiCl4 post-treatment due to the higher thermodynamic stability of the rutile phase. In this work, a mild continuous pH control strategy for effectively regulating the hydrolysis process of TiCl4 post-treatment is proposed. As the weak organic base, urea has been demonstrated can maintain a moderate pH decrease during the hydrolysis process of TiCl4 while keeping the hydrolysis process relatively mild due to the ultra-weak alkalinity. The improved pH environment is beneficial for the formation of anatase TiO2 . Consequently, a uniform anatase-dominated TiO2 surface layer is formed on the mesoporous TiO2 , resulting in reduced defect density and superior band energy level. The interfacial charge recombination is effectively suppressed, and the charge extraction efficiency is improved simultaneously in the fabricated solar cells. The efficiency of the fabricated carbon electrode-based PSCs (C-PSCs) is improved from 16.63% to 18.08%, which is the highest for C-PSCs based on wide-bandgap perovskites.

Self-Driven Prenucleation-Induced Perovskite Crystallization Enables Efficient Perovskite Solar Cells.

Perovskite film with high crystal quality is fundamental to achieving high-performance solar cells. A fast nucleation process is crucial to improving the crystallization quality. Here, we propose a self-driven prenucleation strategy to achieve fast nucleation. This is realized through rational solvent design. The key characteristics of different solvents are systematically evaluated. Among them, formamide, with ultra-high dielectric constant, low Gutman donor number, and a high boiling point, is selected as the co-solvent. These unique characteristics render formamide a double-face solvent that is a good solvent for formamidinium iodide (FAI) and CsI while a poor solvent for PbI2 . As a result, formamide induces the self-driven prenucleation of PbI2 -DMSO seeding crystals and accelerates the nucleation, improving the crystalline quality of perovskite film. The efficiency of the hole transport layer-free carbon-based perovskite solar cells is boosted beyond 19 % for the first time.