The Role of Dimethylammonium Iodide in CsPbI3 Perovskite Fabrication: Additive or Dopant?

The controllable growth of CsPbI3 perovskite thin films with desired crystal phase and morphology is crucial for the development of high efficiency inorganic perovskite solar cells (PSCs). The role of dimethylammonium iodide (DMAI) used in CsPbI3 perovskite fabrication was carefully investigated. We demonstrated that the DMAI is an effective volatile additive to manipulate the crystallization process of CsPbI3 inorganic perovskite films with different crystal phases and morphologies. The thermogravimetric analysis results indicated that the sublimation of DMAI is sensitive to moisture, and a proper atmosphere is helpful for the DMAI removal. The time‐of‐flight secondary ion mass spectrometry and nuclear magnetic resonance results confirmed that the DMAI additive would not alloy into the crystal lattice of CsPbI3 perovskite. Moreover, the DMAI residues in CsPbI3 perovskite can deteriorate the photovoltaic performance and stability. Finally, the PSCs based on phenyltrimethylammonium chloride passivated CsPbI3 inorganic perovskite achieved a record champion efficiency up to 19.03 %.

Inorganic cesium lead iodide perovskite CsPbI3 is attracting great attention as a light absorber for single or multi-junction photovoltaics due to its outstanding thermal stability and proper band gap. However, the device performance of CsPbI3 -based perovskite solar cells (PSCs) is limited by the unsatisfactory crystal quality and thus severe non-radiative recombination. Here, vacuum-assisted thermal annealing (VATA) is demonstrated as an effective approach for controlling the morphology and crystallinity of the CsPbI3 perovskite films formed from the precursors of PbI2 , CsI, and dimethylammonium iodide (DMAI). By this method, a large-area and high-quality CsPbI3 film is obtained, exhibiting a much reduced trap-state density with prolonged charge lifetime. Consequently, the solar cell efficiency is raised from 17.26 to 20.06 %, along with enhanced stability. The VATA would be an effective approach for fabricating high-performance thin-film CsPbI3 perovskite optoelectronics.

All-inorganic CsPbI3 perovskite solar cells (PSCs) are becoming desirable for their excellent photovoltaic ability and adjustable crystal structure distortion. However, the unsatisfactory crystallization of the perovskite phase is unavoidable and leads to challenges on the road to the development of high-quality CsPbI3 perovskite films. Here, we reported the intermediate-phase-modified crystallization (IPMC) method, which introduces pyrrolidine hydroiodide (PI) before the formation of the perovskite phase. The hydrogen bonding, which originates from the interaction between the -NH in PI and the dimethylammonium iodide (DMAI) from the precursor solution, improved the crystallization conditions and further prompted the transition from the DMAPbI3 phase to CsPbI3 perovskite phase. The application of the IPMC method not only decreased the trap density but also changed the energy alignment for better separation of electron-hole pairs. As a result, the devices based on the PI-CsPbI3 perovskite films reached an efficiency of 18.72% and maintained 85% of their initial PCE after 1000 h of being stored in an ambient environment (∼25% RH, 25 °C). This work stimulates inspiration on how to conveniently fabricate high-quality perovskite films in industry.

Composition manipulation boosts the efficiency of carbon-based CsPbI3 perovskite solar cells to beyond 14%

Perovskite CsPbI3 is a promising photovoltaic absorber material, thanks to its ideal bandgap for Si-tandem solar cell applications and its excellent thermochemical stability compared with hybrid organic–inorganic perovskites. However, CsPbI3 has its own stability challenges as its photoactive β- and γ-polymorphs are thermodynamically unstable at room temperature compared with the yellow non-perovskite δ-phase. Stabilizing CsPbI3 has, thus, been the subject of considerable research in recent years. While some approaches, such as alloying with halides and reducing crystalline domain size, have proven effective in improving phase stability, these benefits have, thus far, come at the expense of photovoltaic efficiency compared with the state-of-the-art CsPbI3 solar cells. In this perspective, we discuss the progress and limitations of inorganic perovskite stabilization techniques and look forward at how to achieve inorganic perovskite solar cells with both commercially viable efficiencies and lifetimes.

Inorganic metal halide perovskites such as CsPbI3 are promising for high-performance, reproducible, and robust solar cells. However, inorganic perovskites are sensitive to humidity, which causes the transformation from the black phase to the yellow δ, non-perovskite phase. Such phase instability has been a significant challenge to long-term operational stability. Here, a surface dimensionality reduction strategy is reported, using 2-(4-aminophenyl)ethylamine cation to construct a Dion-Jacobson 2D phase that covers the surface of the 3D inorganic perovskite structure. The Dion-Jacobson layer mainly grows at the grain boundaries of the perovskite, effectively passivating surface defects and providing favourable interfacial charge transfer. The resulting inorganic perovskite films exhibit excellent humidity resistance when submerged in an aqueous solution (isopropanol:water = 4:1 v/v) and exposed to a 50% humidity air atmosphere. The Dion-Jacobson 2D/3D inorganic perovskite solar cell (PSC) achieves a power conversion efficiency (PCE) of 19.5% with a Voc of 1.197eV. It retains 83% of its initial PCE after 1260h of maximum power point tracking under 1.2 sun illumination. The work demonstrates an effective way for stabilizing efficient inorganic perovskite solar cells.

Hygroscopicity risk and organic–inorganic hybrid perovskites easy decomposition in solar cells limit their usefulness. Apart from the hybrid organic–inorganic perovskites, inorganic perovskite solar cells display a better stability toward moisture, light soaking, and thermal stressing. However, most inorganic perovskites are inappropriate for single junction or tandem solar cells due to their large bandgaps (>1.8 eV), which eventually results in light absorption loss. Fortunately, cubic CsPbI3 perovskite (having 1.73 eV bandgap) could potentially serve as top cells in tandem devices with silicon solar cells. Poor phase stability of CsPbI3 is considered a major obstacle to design CsPbI3 perovskite solar cells. This review highlights the most recent studies on the progress in CsPbI3‐based solar cell device field. Moreover, this review also summarizes certain strategies to improve phase stability, such as size reduction to nanocrystal or external cations/anions doping, with the aim to improve the devices design.

Efficient inorganic perovskite solar cells made by drop-coating in ambient air

Inorganic cesium lead iodide perovskite CsPbI3 is attracting great attention as a light absorber for single or multi‐junction photovoltaics due to its outstanding thermal stability and proper band gap. However, the device performance of CsPbI3‐based perovskite solar cells (PSCs) is limited by the unsatisfactory crystal quality and thus severe non‐radiative recombination. Here, vacuum‐assisted thermal annealing (VATA) is demonstrated as an effective approach for controlling the morphology and crystallinity of the CsPbI3 perovskite films formed from the precursors of PbI2, CsI, and dimethylammonium iodide (DMAI). By this method, a large‐area and high‐quality CsPbI3 film is obtained, exhibiting a much reduced trap‐state density with prolonged charge lifetime. Consequently, the solar cell efficiency is raised from 17.26 to 20.06 %, along with enhanced stability. The VATA would be an effective approach for fabricating high‐performance thin‐film CsPbI3 perovskite optoelectronics.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Lead-free tin perovskite solar cells

Inorganic CsSnI3 perovskite solar cells (PSCs) have attracted increasing research interest owing to their excellent optoelectronic properties and thermal stability. However, it is a great challenge to fabricate high-quality CsSnI3 films with low defect density due to the quick crystallization growth rate and high content of Sn (II)-related defects. Here, a cation in situ exchange strategy is employed for the fabrication of CsSnI3 perovskite films with tin iodide (SnI2), cesium formate (CsFa), and dimethylammonium iodide (DMAI) as the precursors, where DMASnI3 first forms and then transforms into black CsSnI3 during the thermal annealing, followed by the removal of dimethylamine and formic acid. The prepared CsSnI3 film exhibits high coverage and improved crystallinity with low defects, and the resultant PSC achieves a power conversion efficiency (PCE) of 12.62%, greater than the value (6.82%) for the traditional device with SnI2 and CsI as precursors. Moreover, the target device exhibits improved stability in an N2 environment, maintaining over 85% of its initial power conversion efficiency after 30 days. This cation exchange strategy paves the way to the realization of Sn-based halide perovskite solar cells with high performance and stability.

The organic–inorganic hybrid perovskite solar cells (PSCs) have demonstrated their unprecedented high efficiency and potential for commercialization. The volatile organic components in the hybrid perovskite crystal structure are still a big challenge for long‐term stabilities. Recently, inorganic CsPbI3 perovskite has attracted much attention because of its superior chemical stability over the prevailing hybrid organic–inorganic perovskite and the most suitable band gap among all‐inorganic perovskites. Nevertheless, CsPbI3 suffers from phase instability and low photovoltaic (PV) performance due to its undesirable tolerant factor. Much research effort has been devoted into stabilization of CsPbI3. In this perspective, we review the recent progress on chemical engineering processes for the stabilization of inorganic CsPbI3 perovskite for high‐efficiency PVs. We also discuss the importance of understanding mechanism behind stabilization of CsPbI3 perovskite film and the development of inorganic CsPbI3‐based highly efficient and stable PSCs.

The ideal charge transport materials should exhibit a proper energy level, high carrier mobility, sufficient conductivity, and excellent charge extraction ability. Here, a novel electron transport material was designed and synthesized by using a simple and facile solvothermal method, which is composed of the core-shell ZnO@SnO2 nanoparticles. Thanks to the good match between the energy level of the SnO2 shell and the high electron mobility of the core ZnO nanoparticles, the PCE of inorganic perovskite solar cells has reached 14.35% (JSC: 16.45 mA cm-2, VOC: 1.11 V, FF: 79%), acting core-shell ZnO@SnO2 nanoparticles as the electron transfer layer. The core-shell ZnO@SnO2 nanoparticles size is 8.1 nm with the SnO2 shell thickness of 3.4 nm, and the electron mobility is seven times more than SnO2 nanoparticles. Meanwhile, the uniform core-shell ZnO@SnO2 nanoparticles is extremely favorable to the growth of inorganic perovskite films. These preliminary results strongly suggest the great potential of this novel electron transfer material in high-efficiency perovskite solar cells.

Moisture is not always bad: H2O accelerates the conversion of DMAPbI3 intermediate to CsPbI3 for boosting the efficiency of carbon-based perovskite solar cells to over 16%