Two-inch-sized perovskite crystals, CH3 NH3 PbX3 (X=I, Br, Cl), with high crystalline quality are prepared by a solution-grown strategy. The availability of large perovskite crystals is expected to transform its broad applications in photovoltaics, optoelectronics, lasers, photodetectors, LEDs, etc., just as crystalline silicon has done in revolutionizing the modern electronics and photovoltaic industries.

As the black cesium lead iodide (CsPbI3) tends to transit into a yellow δ-phase at ambient, it is imperative to develop a stabilized black phase for photovoltaic applications. Herein, we report a distorted black CsPbI3 film by exploiting the synergistic effect of hydroiodic acid (HI) and phenylethylammonium iodide (PEAI) additives. It is found that the HI induces formation of hydrogen lead iodide (HPbI3+x), an intermediate to the distorted black phase with appropriate band gap of 1.69 eV; while PEAI provides nucleation for optimized crystallization. More importantly, it stabilizes the distorted black phase by hindering phase transition via its steric effects. Upon optimization, we have attained solar cell efficiency as high as 15.07%. Specifically, the bare cell without any encapsulation shows negligible efficiency loss after 300 h of light soaking. The device keeps 92% of its initial cell efficiency after being stored for 2 months under ambient conditions.

Recently, lead halide‐based perovskites have become one of the hottest topics in photovoltaic research because of their excellent optoelectronic properties. Among them, organic‐inorganic hybrid perovskite solar cells (PSCs) have made very rapid progress with their power conversion efficiency (PCE) now at 23.7 %. However, the intrinsically unstable nature of these materials, particularly to moisture and heat, may be a problem for their long‐term stability. Replacing the fragile organic group with more robust inorganic Cs+ cations forms the cesium lead halide system (CsPbX3, X is halide) as all‐inorganic perovskites which are much more thermally stable and often more stable to other factors. From the first report in 2015 to now, the PCE of CsPbX3‐based PSCs has abruptly increased from 2.9 % to 17.1 % with much enhanced stability. In this Review, we summarize the field up to now, propose solutions in terms of development bottlenecks, and attempt to boost further research in CsPbX3 PSCs.

All‐inorganic CsPbBrI2 perovskite has great advantages in terms of ambient phase stability and suitable band gap (1.91 eV) for photovoltaic applications. However, the typically used structure causes reduced device performance, primarily due to the large recombination at the interface between the perovskite, and the hole‐extraction layer (HEL). In this paper, an efficient CsPbBrI2 perovskite solar cell (PSC) with a dimensionally graded heterojunction is reported, in which the CsPbBrI2 material is distributed within bulk–nanosheet–quantum dots or 3D–2D–0D dimension‐profiled interface structure so that the energy alignment is optimized in between the valence and conduction bands of both CsPbBrI2 and the HEL layers. Specifically, the valence‐/conduction‐band edge is leveraged to bend with synergistic advantages: the graded combination enhances the hole extraction and conduction efficiency with effectively decreased recombination loss during the hole‐transfer process, leading to an enhanced built‐in electric field, hence a high VOC of as much as 1.19 V. The profiled structure induces continuously upshifted energy levels, resulting in a higher JSC of as much as 12.93 mA cm−2 and fill factor as high as 80.5%, and therefore record power conversion efficiency (PCE) of 12.39%. As far as it is known, this is the highest PCE for CsPbBrI2 perovskite‐based PSC.

Here, a high-performance graded bandgap structure-based solar cell was designed and demonstrated, comprising a CsPbI 2 Br bottom cell and a CsPbI 3 QD top cell. Several optimizations were conducted to boost the device performance. As a result, the extended photoresponse, high carrier mobility, and well-matched energy levels afford a record power conversion efficiency of 14.45%, coupled with a high J SC of 15.25 mA/cm 2 . The result shows that optical and energy-band manipulation is an effective approach for improving the performance of inorganic perovskite solar cells.

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Parasitic absorption by window layer, electrode layer, and interface layer in the near ultraviolet (UV) region is no longer negligible for high-efficiency perovskite solar cells. On the other hand, UV-induced degradation is also a big component of cell instability. Herein, CsPbCl 3 :Mn-based quantum dots (QDs) are synthesized and applied onto the front side of the perovskite solar cells as the energy-down-shift (EDS) layer. It is found that with very high quantum yield (∼60%) and larger Stokes shift (>200 nm), the CsPbCl 3 :Mn QDs effectively convert the normally wasted energy in the UV region (300−400 nm) into usable visible light at ∼590 nm for enhanced power conversion efficiency (PCE). Meanwhile, conversion of the UV rays eliminated a significant loss mechanism that deteriorates perovskite stability. As a result, external quantum efficiency in the UV region is significantly increased, leading to an increased short-circuit current (3.77%) and PCE (3.34%). Furthermore, the stability of perovskite solar cells has also been improved from 85% to 97% of their initial efficiency after exposure in the UV region with 5 mW/cm 2 intensity by 100 h. In parallel, the organic and silicon solar cells coated by EDS QDs also both confirm the above conclusion with PCE enhancements of 3.21% and 2.98%, respectively. These results suggest that the CsPbCl 3 :Mn QDs play a significant role in improving the efficiency and stability of photovoltaic devices. To our knowledge, this is the first report about CsPbCl 3 :Mn QD-assisted perovskite solar cells.

All-inorganic CsPbBr perovskite solar cells display outstanding stability toward moisture, light soaking, and thermal stressing, demonstrating great potential in tandem solar cells and toward commercialization. Unfortunately, it is still challenging to prepare high-performance CsPbBr films at moderate temperatures. Herein, a uniform, compact CsPbBr film was fabricated using its quantum dot (QD)-based ink precursor. The film was then treated using thiocyanate ethyl acetate (EA) solution in all-ambient conditions to produce a superior CsPbBr-CsPbBr composite film with a larger grain size and minimal defects. The achievement was attributed to the surface dissolution and recrystallization of the existing SCN and EA. More specifically, the SCN ions were first absorbed on the Pb atoms, leading to the dissolution and stripping of Cs and Br ions from the CsPbBr QDs. On the other hand, the EA solution enhances the diffusion dynamics of surface atoms and the surfactant species. It is found that a small amount of CsPbBr in the composite film gives the best surface passivation, while the Br-rich surface decreases Br vacancies (V) for a prolonged carrier lifetime. As a result, the fabricated device gives a higher solar cell efficiency of 6.81% with an outstanding long-term stability.