Facile low-energy and open-air synthesis of mixed-cation perovskite quantum dots for high-performance solar cells

Abstract

Cesium-formamidinium (FA) lead triiodide (Cs1−xFAxPbI3) perovskite quantum dots (PQDs) are promising building blocks for photovoltaics. However, the mixed-cation colloidal PQDs are difficult to be synthesized in one pot because of the differences in optimal reaction temperatures of Cs (180 °C) and FA (80 °C) precursors; therefore, they are obtained by cation-exchange reactions between pre-synthesized CsPbI3 and FAPbI3 PQDs. Herein, one-pot synthesis of Cs1−xFAxPbI3 PQDs is developed at 60 °C in open air, dramatically reducing the process time and energy consumption compared to the conventional cation-exchange method. Computational calculations considering temperature effects reveal that the formation of mixed-cation PQDs is thermodynamically favorable even at the low synthesis temperature. The temperature effects dominate the mixing energy over the configurational entropy effect as the temperature increased, and thus Cs crystallizes more easily than FA at the synthesis temperature. This one-pot synthesis also allows fine-tuning of the A-site cations of Cs1−xFAxPbI3 PQDs, enabling the control of crystal structures and optical properties, thus optimizing photovoltaic performance. As a result, the fabricated Cs1−xFAxPbI3 PQD solar cells exhibit power conversion efficiencies (PCEs) up to 11.58 %, which is the highest among the reported solar cells fabricated using QDs synthesized at ≤ 60 °C in open air. The optimized solar cell retains 92 % of its initial PCE value after 1,083 h in dry air (dark and relative humidity < 10 %).