In this work, we report the benefits of incorporating p h e n e t h y l a m m o n i u m c a t i o n ( P E A + ) i n t o ( H C -(NH 2 ) 2 PbI 3 ) 0.85 (CH 3 NH 3 PbBr 3 ) 0.15 perovskite for the first time. After adding small amounts of PEA cation (<10%), the perovskite film morphology is changed but, most importantly, grain boundaries are passivated. This is supported by Kelvin Probe Force Microscopy (KPFM). The passivation results in the increase in photoluminescence intensity and carrier lifetimes of test structures and open-circuit voltages (V OC ) of the devices as long as the addition of PEA + is ≤4.5%. The presence of higher-band-gap quasi-2D PEA incorporated perovskite is responsible for the grain boundary passivation, and the quasi-2D perovskites are also found to be concentrated near the TiO 2 layer, revealed by PL spectroscopy. Results of moisture exposure tests show that PEA + incorporation is effective in slowing down the degradation of unencapsulated devices compared to the control devices without PEA + . These findings provide insights into the operation of perovskite solar cells when large cations are incorporated.

The sensitivity of organic–inorganic perovskites to environmental factors remains a major barrier for these materials to become commercially viable for photovoltaic applications. In this work, the degradation of formamidinium lead iodide (FAPbI3) perovskite in a moist environment is systematically investigated. It is shown that the level of relative humidity (RH) is important for the onset of degradation processes. Below 30% RH, the black phase of the FAPbI3 perovskite shows excellent phase stability over 90 d. Once the RH reaches 50%, degradation of the FAPbI3 perovskite occurs rapidly. Results from a Kelvin probe force microscopy study reveal that the formation of nonperovskite phases initiates at the grain boundaries and the phase transition proceeds toward the grain interiors. Also, ion migration along the grain boundaries is greatly enhanced upon degradation. A post‐thermal treatment (PTT) that removes chemical residues at the grain boundaries which effectively slows the degradation process is developed. Finally, it is demonstrated that the PTT process improves the performance and stability of the final device.

For the first time, we report large-area (16 cm2) independently certified efficient single perovskite solar cells (PSCs) by overcoming two challenges associated with large-area perovskite solar cells. The first challenge of realizing a homogeneous and densely packed perovskite film over a large area is overcome by using an antisolvent spraying process. The second challenge of removing the series resistance limitation of transparent conductor is overcome by incorporating a metal grid designed using a semidistributed diode model. A 16 cm2 perovskite solar device at the cell level rather than at the module level is demonstrated using the modified solution process in conjunction with the use of a metal grid. The cell is independently certified to be 12.1% efficient. This work paves the way toward highly efficient and large perovskite cells without single-junction perovskite solar cells and silicon–perovskite tandems.

For the fabrication of perovskite solar cells (PSCs) using a solution process, it is essential to understand the characteristics of the perovskite precursor solution to achieve high performance and reproducibility. The colloids (iodoplumbates) in the perovskite precursors under various conditions were investigated by UV–visible absorption, dynamic light scattering, photoluminescence, and total internal reflection fluorescence microscopy techniques. Their local structure was examined by in situ X-ray absorption fine structure studies. Perovskite thin films on a substrate with precursor solutions were characterized by transmission electron microscopy, X-ray diffraction analysis, space-charge-limited current, and Kelvin probe force microscopy. The colloidal properties of the perovskite precursor solutions were found to be directly correlated with the defect concentration and crystallinity of the perovskite film. This work provides guidelines for controlling perovskite films by varying the precursor solution, making it possible to use colloid-engineered lead halide perovskite layers to fabricate efficient PSCs.

Halide perovskites such as methylammonium lead iodide (MAPbI 3 ) currently attract considerable attention because of their excellent optoelectronic properties and performance in solar cell devices. Despite tremendous research efforts to elucidate their fundamental properties, ion migration with the presence of ionic defects is still not fully understood. Here, various types of ionic defects for specific (100) and ( 112) lattice facets in single-crystal MAPbI 3 have been investigated systematically. Our measurements reveal significant anisotropic properties. Photoluminescence (PL) and electrical transport measurements show that the (100) facet has higher PL intensity and over 1 order lower trap density compared to that of the (112) facet. We find that the facet-dependent variations of contact potential difference measured with Kelvin probe force microscopy under different bias voltages and light illuminations provide insights into different types of ionic defects on the surface of MAPbI 3 single crystals. We also observe a completely different ion migration behavior on specific crystal facets through nanoscale scanning probe microscopy investigations. Our results indicate that the (100) facet exhibits an n-type behavior dominated with I − vacancies, whereas the (112) facet exhibits a p-type behavior with MA + or Pb 2+ vacancies. The findings on the facet-dependent configuration of ionic defects provide deeper understanding on facet-dependent optoelectronic properties in single-crystal MAPbI 3 .