Enhancing Transport in Formamidinium Lead Iodide Quantum Dot Solar Cells by Substitution of Octadecene

Perovskite quantum dots (QDs) solar cells are recently becoming very popular as they currently hold the record power conversion efficiency (PCE) of over 16% with a light absorber film made of stacked perovskite QDs of mixed cation Formamidinium (FA)/Cesium lead (Pb) iodide (I).¹ The most common synthesis method (hot injection) is based on the injection at moderate temperatures (50 to 120 °C) of a cation-oleate compound into a saturated lead iodide solution in Octadecene (ODE) in the presence of Oleic Acid and Oleylamine.² This leads to the formation of thick colloids where particles are capped by long chain ligands. However, for application in solar cells these long insulating ligands need to be exchanged in favor of shorter ones, leading to higher interparticle hopping probabilities for photocarriers. FAPbI₃ QDs themselves have ideal bandgap (~1.5-1.6 eV) for the single junction design and longer photocarrier lifetimes, however a reliable and satisfactory ligand exchange protocol is lacking. To have an efficient ligand exchange recipe that does not compromise the structural integrity of the quantum dots, significant efforts have been made in a plethora of studies by addition of small molecules during the synthesis process, zwitterionic compounds or Pb- FA- and I- containing salts and amino acids during the QDs film formation, nonetheless there is still room for improvement.<br/><br/>In this study we explore a different approach to synthesize FaPbI₃ QDs by changing one of the main solvents used in the synthesis stage. Specifically, we replaced ODE with a shorter chain linear alpha olefin. Despite facing some experimental challenges, we successfully obtained high-quality films of FaPbI₃ QDs using this new method. Furthermore, we correlate the new synthesis method with ODE-based QDs in terms of the purification protocol, synthesis practice, optical properties, and their application in QDs solar cells. The results revealed superior photoluminescence properties for the QDs synthesized with the new solvent. QDs obtained in this way have a narrow photoluminescence emission peak at 1.61 eV and exhibit higher absolute quantum yields ranging from 70% to 80% and only a small blueshift (Δλ=5 nm) compared to the ODE-based QDs. Additionally, we compare the performance of the two systems in prototype solar cells, specifically by using layers of stacked QDs with equal thickness (100 nm). The prototype solar cells incorporating the QDs synthesized with the new solvent showed a PCE of 4.9% with 1.06 V open-circuit voltage (V_OC). In relation to an ODE-based solar cell of the same thickness it represents a two-fold increase in PCE, as well as consistently higher V_OC and short-circuit current density (J_sc). Furthermore, we investigate how the new solvent enhances the performance of FaPbI₃ QDs based solar cells without compromising the stability and optical properties of individual quantum dots.<br/><br/>1. Zhao, Q. <i>et al.</i>, <i>Nat. Commun.</i> <b>10</b>, 1–8 (2019).<br/>2. Protesescu, L. <i>et al.,</i> <i>ACS Nano</i> <b>11</b>, 3119–3134 (2017).