Charge Injection at the Heterointerface in Perovskite CH₃NH₃PbI₃ Solar Cells Studied by Simultaneous Microscopic Photoluminescence and Photocurrent Imaging Spectroscopy

Charge carrier dynamics in perovskite CH₃NH₃PbI₃ solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH₃NH₃PbI₃ layer into the charge transport layer and photocarrier recombination within the CH₃NH₃PbI₃ layer. Furthermore, we found that the decrease in the carrier injection efficiency under prolonged light illumination leads to a reduction in PC, resulting in light-induced degradation of solar cell devices. Our findings provide important insights for understanding carrier injection at the interface and light-induced degradation in perovskite solar cells

Hot Biexciton Effect on Optical Gain in CsPbI₃ Perovskite Nanocrystals

Combining the superior optical properties of their bulk counterparts with quantum confinement effects, lead halide perovskite nanocrystals are unique laser materials with low-threshold optical gain. In such nonlinear optical regimes, multiple excitons are generated in the nanocrystals and strongly affect the optical gain through many-body interactions. Here, we investigate the exciton–exciton interactions in CsPbI₃ nanocrystals by femtosecond transient absorption spectroscopy. From the analysis of the induced absorption signal observed immediately after the pump excitation, we estimated the binding energy for the hot biexcitons that are composed of an exciton at the band edge and a hot exciton generated by the pump pulse. We found that the exciton–exciton interaction becomes stronger for hot excitons with greater excess energies and that the optical gain can be controlled by changing the excess energy of the hot excitons

Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals

We study the origin of photoluminescence (PL) intermittency in formamidinium lead bromide (FAPbBr₃, FA = HC­(NH₂)₂) nanocrystals and the impact of postsynthetic surface treatments on the PL intermittency. Single-dot spectroscopy revealed the existence of different individual nanocrystals exhibiting either a blinking (binary on–off switching) or flickering (gradual undulation) behavior of the PL intermittency. Although the PL lifetimes of blinking nanocrystals clearly correlate with the individual absorption cross sections, those of flickering nanocrystals show no correlation with the absorption cross sections. This indicates that flickering has an extrinsic origin, which is in contrast to blinking. We demonstrate that the postsynthetic surface treatment with sodium thiocyanate improves the PL quantum yields and completely suppresses the flickering, while it has no significant effect on the blinking behavior. We conclude that the blinking is caused by Auger recombination of charged excitons, and the flickering is due to a temporal drift of the exciton recombination rate induced by surface-trapped electrons

Dynamics of Charged Excitons and Biexcitons in CsPbBr₃ Perovskite Nanocrystals Revealed by Femtosecond Transient-Absorption and Single-Dot Luminescence Spectroscopy

Metal–halide perovskite nanocrystals (NCs) are promising photonic materials for use in solar cells, light-emitting diodes, and lasers. The optoelectronic properties of these devices are determined by the excitons and exciton complexes confined in their NCs. In this study, we determined the relaxation dynamics of charged excitons and biexcitons in CsPbBr₃ NCs using femtosecond transient-absorption (TA), time-resolved photoluminescence (PL), and single-dot second-order photon correlation spectroscopy. Decay times of ∼40 and ∼200 ps were obtained from the TA and PL decay curves for biexcitons and charged excitons, respectively, in NCs with an average edge length of 7.7 nm. The existence of charged excitons even under weak photoexcitation was confirmed by the second-order photon correlation measurements. We found that charged excitons play a dominant role in luminescence processes of CsPbBr₃ NCs. Combining different spectroscopic techniques enabled us to clarify the dynamical behaviors of excitons, charged excitons, and biexcitons

Tuning the Direction of Photoinduced Electron Transfer in Porphyrin-Protected Gold Clusters

The interfacial electron-transfer reaction in ligand-protected gold clusters (AuCs) has been extensively investigated, but there are limited reports on organic chromophore ligands for photoinduced electron-transfer reactions of chromophore-attached AuCs. Here, we focused on porphyrins as chromophore ligands because of their tunable redox properties through the insertion of metal ions. We synthesized 1.3 nm diameter AuCs face-coordinated by free-base porphyrin (H2P) or AuIII porphyrin (AuP+) as photofunctional ligands. The synthesized H2P- and AuP+-protected AuCs (H2P-AuCs and AuP+-AuCs) were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible–near-infrared absorption spectroscopy. Femtosecond transient absorption measurements revealed the photodynamics of H2P-AuCs and AuP+-AuCs. The AuCs in H2P-AuCs and AuP+-AuCs act as electron acceptors and electron donors, respectively, achieving control of the photoinduced electron-transfer direction by inserting the metal ion into the porphyrin ligand. This drastic change is caused by the high electrophilicity of AuP+, indicating that the precise design of the protecting ligand can expand the potential of AuCs as photofunctional materials