Effect of thermal annealing on CsPbBr3 quantum dot films

All-inorganic CsPbBr3 perovskite quantum dots (QDs) have great potential in semitransparent photovoltaics and tandem solar cells due to their excellent stability to hydrothermal and its wide bandgap (2.32 eV). Usually, the crystallization process of quantum dots films is separated from the film formation process without high temperature annealing owing to the quantum dots have crystallized during the synthesis process. This makes quantum dots more flexible in device applications such as flexible solar cells. However, the thermal annealing plays a potential role in improving quantum dots arrangement to enhance the carrier transport ability. Therefore, we systematically study a series of changes brought about by different annealing temperatures on CsPbBr3 QD films and elucidate that thermal annealing leads to the drop of long-chain insulating ligands on the surface of the quantum dots and the grains growth through Ostwald ripening. More detailed, lower temperature (60 °C) annealing is favorable for quantum dot films that could promote the residual solvent evaporation and reduce the defect density to improve carrier transport. Then the performance of the device decreases as the annealing temperature increases owing to the partial fusion between quantum dots. The surface defect state of quantum dots increases and the carrier transport is hindered. And after annealing at 200 °C, the quantum dots grow large enough to form a bulk film at which carrier transport property improved. Finally, the specific effect of annealing on quantum dot films is revealed, which beneficial for obtaining high-performance solar cells.