International audienceHybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH3NH3PbI3 from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range

The higher dielectric constant and electron donating properties of PTEG-1 electron extraction layer synergistically helps to eliminate the light soaking effect and enhance device performance.

The temperature dependent behaviour of PbS QD solar cells and thin films was investigated, and guidelines for further improvement of the power conversion efficiency are given.

as unstable output characteristics (light soaking phenomena) as well as the role of the traps and mobile ions, are poorly understood.The light soaking phenomenon has been observed in perovskite solar cells with various device structures, which causes concern about the instability in power output of this kind of solar cell. However, the severeness of the light soaking effect is reported to vary from laboratory to laboratory and has been the subject of intense debate regarding the underlying mechanism. One of the proposed scenarios is the trap-filling/detrapping mechanism, [16,17] in which traps are filled under illumination. However, the long time scale observed in this process causes controversial arguments about the location, properties of the trap states, and the role of photo-generated carriers and mobile ions in the trap-filling process. The second scenario considers a sort of doping mechanism in which the mobile ions are proposed to drift toward the opposite electrode under a photo-generated electric field, causing p and n doping at anode and cathode, respectively. [18] This last mechanism does not take into account the role of traps in the light soaking phenomenon. Moreover, the mobile species and their drift directions are under debate. The third scenario proposed to explain the light soaking effect considers variations in the crystal structure of the perovskite due to the alignment of methylammonium cations under light and bias. [19] However, due to different device structures adopted and perovskite morphology obtained in different research laboratories, it is difficult to draw conclusions from these studies.The planar device structure has attracted intensive interest due to the simple low-temperature processing. The photovoltaic performance of these devices is greatly dependent on the perovskite film processing. [20][21][22] Poor coverage and the microcrystallinity of the perovskite films have been shown to produce low device performance. However, various strategies to improve the perovskite film morphology, such as optimizing the annealing temperature, new deposition methods, use of additives, new precursors, and solvent engineering have been developed. [21][22][23][24][25][26][27] Despite the rich knowledge gained in controlling the

Capping colloidal quantum dots (CQDs) with atomic ligands is a powerful approach to tune their properties and improve the charge carrier transport in CQD solids. Efficient passivation of the CQD surface, which can be achieved with halide ligands, is crucial for application in optoelectronic devices. Heavier halides, i.e., I– and Br–, have been thoroughly studied as capping ligands in the last years, but passivation with fluoride ions has not received sufficient consideration. In this work, effective coating of PbS CQDs with fluoride ligands is demonstrated and compared to the results obtained with other halides. The electron mobility in field-effect transistors of PbS CQDs treated with different halides shows an increase with the size of the atomic ligand (from 3.9 × 10–4 cm2/(V s) for fluoride-treated to 2.1 × 10–2 cm2/(V s) for iodide-treated), whereas the hole mobility remains unchanged in the range between 1 × 10–5 cm2/(V s) and 10–4cm2/(V s). This leads to a relatively more pronounced p-type behavior of the fluoride- and chloride-treated films compared to the iodide-treated ones. Cl–- and F–-capped PbS CQDs solids were then implemented as p-type layer in solar cells; these devices showed similar performance to those prepared with 1,2-ethanedithiol in the same function. The relatively stronger p-type character of the fluoride- and chloride-treated PbS CQD films broadens the utility of such materials in optoelectronic devices.