Slow Cooling and Transfer Dynamics of Hot Excitons in CsPbBr₃ Perovskite Quantum Dots/g-CN Nanosheet Heterostructures: Implications for Optoelectronic Applications

Slow cooling and long-lived hot carrier (HC) population plays a crucial role in crossing the Shockley–Queisser limit and achieving high-efficiency perovskite photovoltaic devices up to 66%. Herein, we investigated the slow cooling of HCs via charge transfer in in-situ synthesized CsPbBr 3 perovskite quantum dots-decorated graphitic carbon nitride nanosheet (CsPbBr 3 PQDs/g-CN NS) heterostructures using ultrafast transient absorption (UTA) spectroscopy. Halide perovskites showed excitation energy-dependent HC cooling. We examined the HC dynamics at two different excitation energies with varying excitation pump fluences. UTA revealed significantly higher slow cooling and slow bleach recovery in CsPbBr 3 PQDs/g-CN NSs (i.e., 632 fs and 845 ps) compared to pure CsPbBr 3 PQDs (i.e., 512 fs and 371 ps) at an excitation energy of 3.54 eV and a fluence of 250 μW, suggesting that HCs transfer through the heterostructure interface. The observed slow cooling can be explained by the stronger interaction between the organic polymeric frameworks of g-CN NSs and CsPbBr 3 PQDs, which passivates the trap states of CsPbBr 3 PQDs and offers slow electron–hole recombination by slowing down the Auger recombination or the hot-phonon bottleneck effect. Additionally, density functional theory (DFT) calculations were carried out to shed light on band alignment at the interface to reveal the mechanism of efficient charge transfer in CsPbBr 3 PQDs/g-CN NSs and validate the experimental findings. The insight of this study is quite beneficial for designing modified perovskite nanostructures for realization in perovskite solar cells in the near future.