Charge carrier recombination dynamics in a bi-cationic perovskite solar cell.

The compositional engineering is of great importance to tune the electrical and optical properties of perovskite and improve the photovoltaic performance of perovskite solar cells. The exploration of the corresponding photoelectric conversion processes, especially the carrier recombination dynamics, will contribute to the optimization of the devices. In this work, perovskite with mixed methylammonium (MA) and formamidinium (FA) as organic cations, MA0.4FA0.6PbI3, is fabricated to study the influence of the bi-cation on the charge carrier recombination dynamics. X-ray diffraction analysis indicates the existence of the MAPbI3-FAPbI3 phase segregation in the bi-cationic perovskite crystal. The time-resolved photoluminescence dynamics presents a relatively fast carrier recombination process ascribed to the charge transfer from MAPbI3 to FAPbI3 in the bi-cationic perovskite film. The carrier recombination dynamics investigated by transient photovoltage measurements reveals a biphasic trap-assisted carrier recombination mechanism in the bi-cationic device, which involves carrier recombination in the MAPbI3 phase and FAPbI3 phase, respectively. The ultimate presentation of the carrier recombination process is closely related to the charge transfer between the two perovskite phases.

Characterization of the influences of morphology on the intrinsic properties of perovskite films by temperature-dependent and time-resolved spectroscopies.

Organic-inorganic halide perovskites have attracted enormous attention owing to their promising application in photovoltaic devices. The morphology of the perovskites is the key to driving the performance of perovskite devices, which necessitates a systematic study. In this work, two typical morphologies, i.e., flake and cube, of perovskite films are fabricated, and the temperature-dependent optical absorption and photoluminescence properties of the two types of perovskite film are systematically investigated. From the temperature-dependent spectra, both exciton and phase transition temperatures of the flake film are found to be about 10 K lower than those of the cube one. Meanwhile, the influences of the morphology on the exciton binding energy, optical phonon energy and polaron binding energy are quantitatively characterized. The exciton binding of the flake film is nearly three times smaller than that of the cube one, while the phonon coupling energy and the polaron binding energy of the former are about 5 meV and 2 meV larger than those of the latter. Furthermore, the results of photoluminescence lifetime and charge separation efficiency further reveal that the charge carrier kinetics in the two kinds of perovskite films is significantly different. The current study provides a theoretical framework to understand the fundamental physics of perovskites and to promote the design and enhancement of active materials for improved optoelectronic devices.

The influence of the electron transport layer on charge dynamics and trap-state properties in planar perovskite solar cells.

Despite the outstanding photovoltaic performance of perovskite solar cells, the correlation between the electron transport layer and the mechanism of photoelectric conversion is still not fully understood. In this paper, the relationship between photovoltaic performance and carrier dynamics is systematically studied in both TiO2- and SnO2-based planar perovskite devices. It is found that the different electron transport layers result in distinct forward scan results and charge dynamics. Based on the charge dynamics results, the influence of the electron transport layer on charge carrier transport and charge recombination is revealed. More importantly, the trap-state density is characterized, which is proven to be related to the charge carrier dynamics and the specific hysteresis behaviour in the perovskite solar cells. The present work would provide new insights into the working mechanisms of electron transport layers and their effect on hysteresis.

Modification of NiO x hole transport layer for acceleration of charge extraction in inverted perovskite solar cells.

The modification of the inorganic hole transport layer has been an efficient method for optimizing the performance of inverted perovskite solar cells. In this work, we propose a facile modification of a compact NiO x film with NiO x nanoparticles and explore the effects on the charge carrier dynamic behaviors and photovoltaic performance of inverted perovskite devices. The modification of the NiO x hole transport layer can not only enlarge the surface area and infiltration ability, but also adjust the valence band maximum to well match that of perovskite. The photoluminescence results confirm the acceleration of the charge separation and transport at the NiO x /perovskite interface. The corresponding device possesses better photovoltaic parameters than the device based on control NiO x films. Moreover, the charge carrier transport/recombination dynamics are further systematically investigated by the measurements of time-resolved photoluminescence, transient photovoltage and transient photocurrent. Consequently, the results demonstrate that proper modification of NiO x can significantly enlarge interface area and improve the hole extraction capacity, thus efficiently promoting charge separation and inhibiting charge recombination, which leads to the enhancement of the device performances.