Rapid Spin Depolarization in the Layered 2D Ruddlesden-Popper Perovskite (BA)(MA)PbI

We report temperature-dependent spectroscopy on the layered (n = 4) two-dimensional (2D) Ruddlesden-Popper perovskite (BA)(MA)PbI. Helicity-resolved steady-state photoluminescence (PL) reveals no optical degree of polarization. Time-resolved PL shows a photocarrier lifetime on the order of nanoseconds. From simultaneously recorded time-resolved differential reflectivity (TRΔR) and time-resolved Kerr ellipticity (TRKE), a photocarrier lifetime of a few nanoseconds and a spin relaxation time on the order of picoseconds was found. This stark contrast in lifetimes clearly explains the lack of spin polarization in steady-state PL. While we observe clear temperature-dependent effects on the PL dynamics that can be related to structural dynamics, spin relaxation is nearly T-independent. Our results highlight that spin relaxation in 2D (BA)(MA)PbI occurs at time scales faster than the exciton recombination time, which poses a bottleneck for applications aiming to utilize this degree of freedom.

Elucidating the Role of Antisolvents on the Surface Chemistry and Optoelectronic Properties of CsPbBrxI3-x Perovskite Nanocrystals

Colloidal lead-halide perovskite nanocrystals (LHP NCs) have emerged over the past decade as leading candidates for efficient next-generation optoelectronic devices, but their properties and performance critically depend on how they are purified. While antisolvents are widely used for purification, a detailed understanding of how the polarity of the antisolvent influences the surface chemistry and composition of the NCs is missing in the field. Here, we fill this knowledge gap by studying the surface chemistry of purified CsPbBrxI3-x NCs as the model system, which in itself is considered a promising candidate for pure-red light-emitting diodes and top-cells for tandem photovoltaics. Interestingly, we find that as the polarity of the antisolvent increases (from methyl acetate to acetone to butanol), there is a blueshift in the photoluminescence (PL) peak of the NCs along with a decrease in PL quantum yield (PLQY). Through transmission electron microscopy and X-ray photoemission spectroscopy measurements, we find that these changes in PL properties arise from antisolvent-induced iodide removal, which leads to a change in halide composition and, thus, the bandgap. Using detailed nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR) measurements along with density functional theory calculations, we propose that more polar antisolvents favor the detachment of the oleic acid and oleylamine ligands, which undergo amide condensation reactions, leading to the removal of iodide anions from the NC surface bound to these ligands. This work shows that careful selection of low-polarity antisolvents is a critical part of designing the synthesis of NCs to achieve high PLQYs with minimal defect-mediated phase segregation.

Enhancing the Performance of Perovskite Light-Emitting Diodes by Humidity Treatment

While humidity treatment has been employed for enhancing the performance of perovskite solar cells and light-emitting diodes (LEDs), only very limited success has been achieved in quasi-two-dimensional (2D) perovskite LEDs (PeLEDs). Here, for the first time, we demonstrate more than one order of magnitude enhancement of the external quantum efficiency (EQE) and electroluminescence (EL) intensity in blue CsPb(Cl/Br)₃ PeLEDs with an organic cation of 2,2-(ethylenedioxy)bis(ethylammonium) (EDBE). Upon humidity treatment, the crystallinity of the three-dimensional (3D) perovskite phase in the EDBE-based perovskite is improved, contributing to an enhancement of photoluminescence quantum yield (PLQY). This work suggests that elaborately modulating the molecular structure of large cations under humidity treatment can serve as an effective strategy to improve the performance of quasi-2D PeLEDs.

Photon-induced deactivations of multiple traps in CH3NH3PbI3 perovskite films by different photon energies

Photon-induced trap deactivation is commonly observed in organometal halide perovskites. Trap deactivation is characterized by an obvious photoluminescence (PL) enhancement. In this work, the properties of traps in CH3NH3PbI3 perovskite films were studied based on the PL enhancement excited by lasers of different wavelengths (633 nm and 405 nm). Two types of electron traps were identified; one can be deactivated by both 633 nm and 405 nm illuminations, whereas the other one can only be deactivated by 405 nm illumination. The energy levels of both types of traps were beneath the conduction band minimum. The expressions of the PL enhancement kinetics due to the trap deactivations by lasers of different wavelengths were derived. The ratio of the constants of the radiative recombination rate and the initial capture rates for both traps was determined from the PL enhancement. The trap deactivation was a photon-related process rather than a photocarrier-related process, and the deactivation time was inversely proportional to the photon flux density.

Hysteresis phenomena in perovskite solar cells: the many and varied effects of ionic accumulation

The issue of hysteresis in perovskite solar cells has now been convincingly linked to the presence of mobile ions within the perovskite layer. Here we test the limits of the ionic theory by attempting to account for a number of exotic characterization results using a detailed numerical device model that incorporates ionic charge accumulation at the perovskite interfaces. Our experimental observations include a temporary enhancement in open-circuit voltage following prolonged periods of negative bias, dramatically S-shaped current-voltage sweeps, decreased current extraction following positive biasing or "inverted hysteresis", and non-monotonic transient behaviours in the dark and the light. Each one of these phenomena can be reproduced and ultimately explained by our models, providing further evidence for the ionic theory of hysteresis as well as valuable physical insight into the factors that coincide to bring these phenomena about. In particular we find that both interfacial recombination and carrier injection from the selective contacts are heavily affected by ionic accumulation, and are essential to explaining the non-monotonic voltage transients and S-shaped J-V curves. Inverted hysteresis is attributed to the occurrence of "positive" ionic accumulation, which may also be responsible for enhancing the stabilized open-circuit voltage in some perovskite cells.

Multiple-Trapping Model for the Charge Recombination Dynamics in Mesoporous-Structured Perovskite Solar Cells

The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO₂ -dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices.

Reversible, Fast, and Wide-Range Oxygen Sensor Based on Nanostructured Organometal Halide Perovskite

Nanostructured materials characterized by high surface-volume ratio hold the promise to constitute the active materials for next-generation sensors. Solution-processed hybrid organohalide perovskites, which have been extensively used in the last few years for optoelectronic applications, are characterized by a self-assembled nanostructured morphology, which makes them an ideal candidate for gas sensing. Hitherto, detailed studies of the dependence of their electrical characteristics on the environmental atmosphere have not been performed, and even the effect of a ubiquitous gas such as O₂ has been widely overlooked. Here, the electrical response of organohalide perovskites to oxygen is studied. Surprisingly, a colossal increase (3000-fold) in the resistance of perovskite-based lateral devices is found when measured in a full oxygen atmosphere, which is ascribed to a trap healing mechanism originating from an O₂ -mediated iodine vacancies filling. A variation as small as 70 ppm in the oxygen concentration can be detected. The effect is fast (<400 ms) and fully reversible, making organohalide perovskites ideal active materials for oxygen sensing. The effect of oxygen on the electrical characteristics of organohalide perovskites must be taken into deep consideration for the design and optimization of any other perovskite-based (opto-) electronic device working in ambient conditions.

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells

Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.