Formamidinium Lead Triiodide Perovskite Research Articles

Suppressed surface lattice vacancies and distortion through lattice anchoring for efficient FAPbI3 perovskite quantum dot solar cells

A facile surface lattice anchoring strategy is reported to stabilize the surface lattice of perovskite quantum dots, which could substantially improve their optoelectronic properties and crystal stabilities for application in solar cells.

Inhibition of defect-induced α-to-δ phase transition for efficient and stable formamidinium perovskite solar cells

Defects passivation is widely devoted to improving the performance of formamidinium lead triiodide perovskite solar cells; however, the effect of various defects on the α-phase stability is still unclear. Here, using density functional theory, we first reveal the degradation pathway of the formamidinium lead triiodide perovskite from α to δ phase and investigate the effect of various defects on the energy barrier of phase transition. The simulation results predict that iodine vacancies are most likely to trigger the degradation, since they obviously reduce the energy barrier of α-to-δ phase transition and have the lowest formation energies at the perovskite surface. A water-insoluble lead oxalate compact layer is introduced on the perovskite surface to largely suppress the α-phase collapse through hindering the iodine migration and volatilization. Furthermore, this strategy largely reduces the interfacial nonradiative recombination and boosts the efficiency of the solar cells to 25.39% (certified 24.92%). Unpackaged device can maintain 92% of its initial efficiency after operation at maximum power point under simulated air mass 1.5 G irradiation for 550 h.

Interfacial engineering from material to solvent: A mechanistic understanding on stabilizing [formula omitted]-formamidinium lead triiodide perovskite photovoltaics

Formamidinium lead triiodide (FAPbI3) has recently been considered as the most promising candidate to achieve highly efficient perovskite solar cells (PSCs). Excitingly, the state-of-the-art highest efficiency of FAPbI3 based PSCs have reached over 25%. However, their device stability still lags behind other compositions of mixed-cation and mixed-halide perovskites. Interfacial engineering is a very powerful method to address this issue and passivation agents have been intensively developed, however there is a lack of in-depth understanding regarding the solvent selection during post-treatment. Here, we employed cyclohexylmethylammonium iodide (CMAI) as passivation agent, which is investigated using either isopropanol (IPA) or chloroform (CF) as carrier mediator to study the solvent influence on the stabilization of FAPbI3. We observed a suppressed-defect perovskite surface toward distinguished composition with 2D CMA2PbI4 domain and CMAI domain induced by IPA and CF, respectively. Remarkably, post-treatment with solution of CMAI in CF creates a strain-free environment on the perovskite surface, leading to an improved efficiency of approaching 24% and concurrently an extraordinarily stable α-phase FAPbI3 PSCs under operation condition, retaining 95% of its initial efficiency after 1050-hour aging. Our resulting device stability is one of the most stable FAPbI3 based PSCs reported in literature.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Fabrication of efficient formamidinium perovskite solar cells under ambient air via intermediate-modulated crystallization

Developing simple methods to synthesize perovskite layers under ambient air can facilitate the industrial production of perovskite solar cells (PSCs). While limited progress has been made for the ambient-air fabrication of formamidinium lead triiodide perovskite (α-FAPbI3) layers due to the coexisting non-perovskite polymorph (δ-FAPbI3). Here, a facile N-methyl pyrrolidone (NMP) additive-based method is proposed to produce FA-perovskite (FAPbI3) layers in ambient air and smooth, highly-crystallized and δ-phase free perovskite films are produced under humidity of ∼40%. The resultant PSC delivers a power conversion efficiency (PCE) of 17.29%, ∼20% enhancement from that of PSC produced from traditional dimethyl sulfoxide (DSMO) additive. It is found that a distinct FAI·PbI2·NMP intermediate phase forms during the film growth, providing an alternative crystallization process different to that of traditional δ-FAPbI3 intermediate route. The fine nucleation of FAI·PbI2·NMP intermediate contributes a smooth and dense morphology to the final film. The rapid detachment of NMP molecule from the intermediate enables a direct formation of α-FAPbI3 film, averting the incomplete δ- to α- phase conversion under ambient air. Due to the high film quality, a PCE of 13.55% is still remained after 30-day storage for unencapsulated device, demonstrating the reliability of our method for FA-perovskite ambient-air fabrication.

2D perovskite stabilized phase-pure formamidinium perovskite solar cells

Compositional engineering has been used to overcome difficulties in fabricating high-quality phase-pure formamidinium perovskite films together with its ambient instability. However, this comes alongside an undesirable increase in bandgap that sacrifices the device photocurrent. Here we report the fabrication of phase-pure formamidinium-lead tri-iodide perovskite films with excellent optoelectronic quality and stability. Incorporation of 1.67 mol% of 2D phenylethylammonium lead iodide into the precursor solution enables the formation of phase-pure formamidinium perovskite with an order of magnitude enhanced photoluminescence lifetime. The 2D perovskite spontaneously forms at grain boundaries to protect the formamidinium perovskite from moisture and suppress ion migration. A stabilized power conversion efficiency (PCE) of 20.64% (certified stabilized PCE of 19.77%) is achieved with a short-circuit current density exceeding 24 mA cm−2 and an open-circuit voltage of 1.130 V, corresponding to a loss-in-potential of 0.35 V, and significantly enhanced operational stability.

First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers.

As an alternative to methylammonium lead triiodide (MAPbI3), formamidinium lead triiodide (FAPbI3) perovskites have recently attracted significant attention because of their higher stability and smaller band gaps. Here, based on first-principles calculations, we investigate systematically the intrinsic defects in FAPbI3. While methylammonium (MA)-related defects MAI and IMA in MAPbI3 have high formation energies, we found that formamidinium (FA)-related defects VFA, FAI and IFA in FAPbI3 have much lower formation energies. Antisites FAI and IFA create deep levels in the band gap, and they can act as recombination centers and result in reduced carrier lifetimes and low open circuit voltages in FAPbI3-based photovoltaic devices. We further demonstrate that through cation mixing of MA and FA in perovskites the formation of these defects can be substantially suppressed.

Enhanced crystallization and performance of formamidinium lead triiodide perovskite solar cells through PbI2-SrCl2 modulation

An innovative strategy using strontium chloride (SrCl2) as an additive to modulate the crystallization of formamidinium lead triiodide perovskite was developed in a sequential two-step deposition. The results from structural, elemental, and morphological analyses reveal that the SrCl2 additive is capable of improving the quality and morphology of the pre-deposited PbI2 layer by forming a PbI2-SrCl2 interphase complex through plausible solid-solid interactions. This modulates the subsequent reaction between PbI2 and formamidinium iodide (FAI) to enhance the crystallinity of the prepared perovskite film with denser and larger grain domains. The concomitantly enhanced optoelectronic properties of the SrCl2-derived perovskite film enables an improved power conversion efficiency (PCE) of 18.44% with good reproducibility, which surpasses the performance (PCE: 15.94%) of the control device without SrCl2. This work demonstrates a useful methodology to modulate the morphology and quality of perovskites through solid-solid interactions for achieving high-performance solar cells.

Improved Phase Stability of Formamidinium Lead Triiodide Perovskite by Strain Relaxation

Though formamidinium lead triiodide (FAPbI3) possesses a suitable band gap and good thermal stability, the phase transition from the pure black perovskite phase (α-phase) to the undesirable yellow nonperovskite polymorph (δ-phase) at room temperature, especially under humid air, hinders its practical application. Here, we investigate the intrinsic instability mechanism of the α-phase at ambient temperature and demonstrate the existence of an anisotropic strained lattice in the (111) plane that drives phase transformation into the δ-phase. Methylammonium bromide (MABr) alloying (or FAPbI3-MABr) was found to cause lattice contraction, thereby balancing the lattice strain. This led to dramatic improvement in the stability of α-FAPbI3. Solar cells fabricated using FAPbI3-MABr demonstrated significantly enhanced stability under the humid air.

Effect of Water Vapor, Temperature, and Rapid Annealing on Formamidinium Lead Triiodide Perovskite Crystallization

Perovskite-based solar cells are one of the emerging candidates for radically lower cost photovoltaics. Herein, we report on the synthesis and crystallization of organic–inorganic formamidinium lead triiodide perovskite films under controlled atmospheric and environmental conditions. Using in situ (scanning) transmission electron microscopy, we make observations of the crystallization process of these materials in nitrogen and oxygen gas with and without the presence of water vapor. Complementary planar samples were also fabricated in the presence of water vapor and characterized by in situ X-ray diffraction. Direct observations of the material structure and final morphology indicate that the exposure to water vapor results in a porous film that is metastable, regardless of the presence of argon, nitrogen, or oxygen. However, the optimal crystallization temperature of 175 °C is unperturbed across conditions. Rapid modulation about the annealing temperature of 175 °C in ±25 °C steps (150–200 °C) promotes crystallization and significantly improves the film morphology by overcoming the presence of impregnated water trapped in the material. Following this processing protocol, we demonstrate substantial growth to micron-size grains via observation inside of an environmentally controlled transmission electron microscope. Adapting this insight from our in situ microscopy, we are able to provide an informed materials protocol to control the structure and morphology of these organic–inorganic semiconductors, which is readily applicable to benchtop device growth strategies.

Intercalation crystallization of phase-pure α-HC(NH₂)₂PbI₃ upon microstructurally engineered PbI₂ thin films for planar perovskite solar cells.

The microstructure of the solid-PbI2 precursor thin film plays an important role in the intercalation crystallization of the formamidinium lead triiodide perovskite (α-HC(NH2)2PbI3). It is shown that microstructurally engineered PbI2 thin films with porosity and low crystallinity are the most favorable for conversion into uniform-coverage, phase-pure α-HC(NH2)2PbI3 perovskite thin films. Planar perovskite solar cells fabricated using these thin films deliver power conversion efficiency (PCE) up to 13.8%.