No abstract

State-of-the-art, high-performance perovskite solar cells (PSCs) contain a large amount of iodine to realize smaller bandgaps. However, the presence of numerous iodine vacancies at the surface of the film formed by their evaporation during the thermal annealing process has been broadly shown to induce deep-level defects, incur nonradiative charge recombination, and induce photocurrent hysteresis, all of which limit the efficiency and stability of PSCs. In this work, modifying the defective surface of perovskite films with cadmium iodide (CdI 2 ) effectively reduces the degree of surface iodine deficiency and stabilizes iodine ions via the formation of strong Cd−I ionic bonds. This largely reduces the interfacial charge recombination loss, yielding a high efficiency of 21.9% for blade-coated PSCs with an open-circuit voltage of 1.20 V, corresponding to a record small voltage deficit of 0.31 V. The CdI 2 surface treatment also improves the operational stability of the PSCs, retaining 92% efficiency after constant illumination at 1 sun intensity for 1000 h. This work provides a promising strategy to optimize the surface/interface optoelectronic properties of perovskites for more efficient and stable solar cells and other optoelectronic devices.

Organic−inorganic halide perovskites have emerged as a promising semiconductor family because of their remarkable performance in optoelectronic devices. On the other hand, the stability of perovskites remains a critical issue. In this work, we report a quantitative and systematic investigation of in situ cleaved MAPbBr 3 single-crystal degradation processes in Xray, N 2 , O 2 , and H 2 O environments. The high-quality crystals were monitored by high-resolution X-ray photoelectron spectroscopy with careful control of the exposure time and pressure. The detailed electronic structure and compositional changes of the crystal were tracked throughout the different exposures, and these studies provided insights into the various degradation mechanisms. We identified that ∼10% of the surface MAPbBr 3 degraded to metallic lead under X-rays in vacuum, while N 2 could protect the sample from the degradation for 9 h under the same condition. Other measurements showed that while the surface was not sensitive to pure O 2 , it was susceptible to H 2 O exposure within the top 0.37 nm and a reaction threshold of ∼10 8 Langmuir was found. Below the threshold, H 2 O acted only as an n-type dopant; above it, the surface began to decompose. These observations highlight possible future directions to improve the material stability by environmental control.

The surface composition of perovskite films is very sensitive to film processing and can deviate from the optimal, which generates unfavorable defects and results in efficiency loss in solar cells and slow response speed in photodetectors. An argon plasma treatment is introduced to modify the surface composition by tuning the ratio of organic and inorganic components as well as defect type before deposition of the passivating layer. It can efficiently enhance the charge collection across the perovskite-electrode interface by suppressing charge recombination. Therefore, perovskite solar cells with argon plasma treatment yield enhanced efficiency to 20.4% and perovskite photodetectors can reach their fastest respond speed, which is solely limited by the carrier mobility.

Single crystal CH3NH3PbBr3 samples are exposed to light illumination, with a light intensity about seven times stronger than the sun while under ultrahigh vacuum (UHV) conditions, in order to investigate their chemical and structural stability from prolonged light illumination. X‐ray photoemission spectroscopy measurements show that within 10 h of illumination, about half of the initial C, N, and Br elemental concentrations leave the surface and about half of the perovskite's Pb is converted into metallic Pb. Light exposures while in the UHV system also significantly roughen the surface, and surprisingly, empty voids form ≈1 to 3 µm down in the light exposed region. A framework based on the Kirkendall effect is put forward to explain the observed void formation. This proposed model may be relevant to the slow degradation of perovskite solar cells, which is sometimes attributed to irreversible chemical reactions from undesired diffusion. These measurements and observations reveal the intrinsic behavior of the CH3NH3PbBr3 single crystals under light illumination while in a UHV system where volatile species are free to leave, in contrast to existing device studies on the photostability of perovskite solar cells.