Recently reported colloidal lead halide perovskite quantum dots (QDs) with tunable photoluminescence (PL) wavelengths covering the whole visible spectrum and exceptionally high PL quantum yields (QYs, 50-90%) constitute a new family of functional materials with potential applications in light-harvesting and -emitting devices. By transient absorption spectroscopy, we show that the high PL QYs (∼79%) can be attributed to negligible electron or hole trapping pathways in CsPbBr3 QDs: ∼94% of lowest excitonic states decayed with a single-exponential time constant of 4.5 ± 0.2 ns. Furthermore, excitons in CsPbBr3 QDs can be efficiently dissociated in the presence of electron or hole acceptors. The half-lives of electron transfer (ET) to benzoquinone and subsequent charge recombination are 65 ± 5 ps and 2.6 ± 0.4 ns, respectively. The half-lives for hole transfer (HT) to phenothiazine and the subsequent charge recombination are 49 ± 6 ps and 1.0 ± 0.2 ns, respectively. The lack of electron and hole traps and fast interfacial ET and HT rates are key properties that may enable the development of efficient lead halide perovskite QDs-based light-harvesting and -emitting devices.

Light-driven H generation using semiconductor nanocrystal heterostructures has attracted intense recent interest because of the ability to rationally improve their performance by tailoring their size, composition, and morphology. In zero- and one-dimensional nanomaterials, the lifetime of the photoinduced charge-separated state is still too short for H evolution reaction, limiting the solar-to-H conversion efficiency. Here we report that using two-dimensional (2D) CdS nanoplatelet (NPL)-Pt heterostructures, H generation internal quantum efficiency (IQE) can exceed 40% at pH 8.8-13 and approach unity at pH 14.7. The near unity IQE at pH 14.7 is similar to those reported for 1D nanorods and can be attributed to the irreversible hole removal by OH. At pH < 13, the IQE of 2D NPL-Pt is significantly higher than those in 1D nanorods. Detailed time-resolved spectroscopic studies and modeling of the elementary charge separation and recombination processes show that, compared to 1D nanorods, 2D morphology extends charge-separated state lifetime and may play a dominant role in enhancing the H generation efficiency. This work provides a new approach for designing nanostructures for efficient light-driven H generation.

CsPbI3 perovskite quantum dots (QDs) have shown great potential in light-harvesting and light-emitting applications, which often involve the transfer of charge carriers in and out of these materials. Here, we studied size-dependent charge separation (CS) and charge recombination (CR) between CsPbI3 QDs and rhodamine B (RhB) molecules, using transient absorption spectroscopy. When the average size decreases from 11.8 nm to 6.5 nm, the average intrinsic CS time constant decreases from 872 ± 52 ps to 40.6 ± 4.3 ps and the corresponding charge recombination time constant decreases from 3829 ± 51 ns to 1384 ± 54 ns. The observed trend of size-dependent CS and CR rates can be well explained by Marcus theory using the theoretically calculated CS and CR driving forces (ΔGCS and ΔGCR), molecular reorganization energy (λRhB), and electronic coupling strength between QD and RhB (HCS and HCR). Unlike the extensively studied more strongly quantum confined Cd chalcogenide QDs, the CsPbI3 QDs are in a weak quantum confinement regime in which size-dependent coupling strength plays a dominant role in the size-dependent charge transfer properties.

CsPbI3 perovskite quantum dot (QD) solar cells are a promising device platform for further development due to their improved stability compared to bulk materials. The fabrication of these devices often involves postsynthetic washing of colloidal QDs to remove surface ligands. Herein, we investigate how this postsynthetic treatment affects the charge separation (CS) and charge recombination (CR) processes at the interface of CsPbI3 QDs and the TiO2 electron extraction layer. The CS time constant at QD/TiO2 interface decrease from 457 ± 4 ps for QDs directly deposited on TiO2 to 288 ± 1 ps for QDs that were washed twice by methyl acetate, increasing the ET efficiency from 89.9 ± 0.3% to 98.3 ± 0.01%. Interestingly, this treatment also lengthens the average charge recombination time constants from 346 ± 18 ns to 1180 ± 60 ns, which is also beneficial for reducing charge recombination loss. This result demonstrates the pronounced effect of QD washing by methyl acetate (MeOAc) on interfacial charge transfer processes, suggesting the important effect of postsynthesis modification of colloidal perovskite QDs on solar cell efficiencies.