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The two-dimensional (2D) mixed halide perovskite PEA2Pb(I1-xBrx)4 exhibits high phase stability under illumination as compared to the three-dimensional (3D) counterpart MAPb(I1-xBrx)3. We explain this difference using a thermodynamic theory that considers the sum of a compositional and a photocarrier free energy. Ab initio calculations show that the improved compositional phase stability of the 2D perovskite is caused by a preferred I-Br distribution, leading to a much lower critical temperature for halide segregation in the dark than for the 3D perovskite. Moreover, a smaller increase of the band gap with Br concentration x and a markedly shorter photocarrier lifetime in the 2D perovskite reduce the driving force for phase segregation under illumination, enhancing the photostability.

Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI$_3$, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs forms in a concentration comparable to those of the most prominent point defects, whereas the other compound defects are negligible. However, under nonequilibrium thermal and operating conditions, other complexes also become as important as the point defects. Those are the Cs substituting Pb antisite, and, to a lesser extent, the compound vacancies of PbI$_2$ or CsPbI$_3$ units, and the I substituting Cs antisite. These compound defects only lead to shallow or inactive charge carrier traps, which testifies to the electronic stability of the halide perovskites. Under operating conditions with a quasi Fermi level very close to the valence band, deeper traps can develop.

Given their comparatively narrow band gap, mixed Pb–Sn iodide perovskites are interesting candidates for bottom cells in all-perovskite tandems or single junction solar cells, and their luminescence around 900 nm offers great potential for near-infrared optoelectronics. Here, we investigate mixed FAPb1–xSnxI3 offering the first accurate determination of the crystal structure over a temperature range from 293 to 100 K. We demonstrate that all compositions exhibit a cubic structure at room temperature and undergo at least two transitions to lower symmetry tetragonal phases upon cooling. Using density functional theory (DFT) calculations based on these structures, we subsequently reveal that the main impact on the band gap bowing is the different energy of the s and p orbital levels derived from Pb and Sn. In addition, this energy mismatch results in strongly composition-dependent luminescence characteristics. Whereas neat and Sn-rich compounds exhibit bright and narrow emission with a clean band gap, Sn-poor compounds intrinsically suffer from increased carrier recombination mediated by in-gap states, as evidenced by the appearance of pronounced low-energy photoluminescence upon cooling. This study is the first to link experimentally determined structures of FAPb1–xSnxI3 with the electronic properties, and we demonstrate that optoelectronic applications based on Pb–Sn iodide compounds should employ Sn-rich compositions., ACS Applied Materials & Interfaces, 14 (30), ISSN:1944-8244, ISSN:1944-8252

Despite the rapid progress in perovskite solar cells, their commercialization is still hindered by issues regarding long-term stability, which can be strongly affected by metal oxide-based charge extraction layers next to the perovskite material. With MoO3 being one of the most successful hole transport layers in organic photovoltaics, the disastrous results of its combination with perovskite films came as a surprise but was soon attributed to severe chemical instability at the MoO3/perovskite interface. To discover the atomistic origin of this instability, we combine density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) measurements to investigate the interaction of MoO3 with the perovskite precursors MAI, MABr, FAI, and FABr. From DFT calculations we suggest a scenario that is based upon oxygen vacancies playing a key role in interface degradation reactions. Not only do these vacancies promote decomposition reactions of perovskite precursors, but they also constitute the reaction centers for redox reactions leading to oxidation of the halides and reduction of Mo. Specifically iodides are proposed to be reactive, while bromides do not significantly affect the oxide. XPS measurements reveal a severe reduction of Mo and a loss of the halide species when the oxide is interfaced with I-containing precursors, which is consistent with the proposed scenario. In line with the latter, experimentally observed effects are much less pronounced in case of Br-containing precursors. We further find that the reactivity of the MoO3 substrate can be moderated by reducing the number of oxygen vacancies through a UV/ozone treatment, though it cannot be fully eliminated.

Halide perovskites are a promising class of materials for optoelectronic applications due to their excellent optoelectronic performance. However, they suffer from several dynamical degradation problems, the characterization of which is challenging. Atomic scale simulations can provide valuable insights; however, the high computational cost of first-principles methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present refined GFN1-xTB parameters for the accurate description of structural and dynamical properties of CsPbBr3, CsPbI3, and CsPb(I1-xBrx)3. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, directional movement of the Cs cations increases their distance to the surrounding halides, initiating a transition to the nonperovskite phase. However, once enough thermal energy is available, the increased Cs-halide distance can be compensated by increased halide fluctuations, resulting in a transition to the phase-stable tetragonal or cubic phases. Furthermore, we find the mixing of halides increases halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.

Surface passivation has been demonstrated to be an effective strategy to reduce defects of hybrid halide perovskite films for making efficient and stable perovskite solar cells (PSCs). Especially the strong interaction between the passivation agents and the perovskite films is favorable for achieving a durable passivation effect. Pyridine derivatives with bidentate anchoring groups can interact with the uncoordinated Pb2+and minimize perovskite defects. Herein, in order to rationally design bidentate passivation agents, the passivation effects of pyridine (Py) and its derivatives (Py-X) with different functional groups of amino, carboxyl acid, and aldehyde are compared in carbon-based perovskite solar cells (C-PSCs) for the first time. Py-NH2is found to passivate the perovskite CH3NH3PbI3film the best among all the passivation agents. The N atoms on both the pyridine ring and the amino group with lone pair electrons can combine with the uncoordinated Pb2+, effectively reducing the defect density in the Py-NH2-treated perovskite film. First-principles density functional theory (DFT) calculations reveal that the strong interaction between Py-NH2and CH3NH3PbI3strengthens Pb-I bond and hinders the formation of I vacancies. Carbon-based perovskite solar cells (C-PSCs) passivated by Py-NH2achieve a champion power conversion efficiency (PCE) of 14.75%, compared to 11.55% of the control device. The Py-NH2passivated C-PSCs also exhibit good long-term stability, retaining more than 90% of the initial efficiency after 30 days of storage in air with 35-45% relative humidity.

Organic A′-site ligand structure plays a crucial role in the crystal growth of 2D perovskites, but the underlying mechanism has not been adequately understood. This problem is tackled by studying the influence of two isomeric A′-site ligands, linear-shaped n-butylammonium (n-BA+) and branched iso-butylammonium (iso-BA+), on 2D perovskites from precursor to device, with a combination of in situ grazing-incidence wide-angle X-ray scattering and density functional theory. It is found that branched iso-BA+, due to the lower aggregation enthalpies, tends to form large-size clusters in the precursor solution, which can act as pre-nucleation sites to expedite the crystallization of vertically oriented 2D perovskites. Furthermore, iso-BA+ is less likely to be incorporated into the MAPbI3 lattice than n-BA+, suppressing the formation of unwanted multi-oriented perovskites. These findings well explain the better device performance of 2D perovskite solar cells based on iso-BA+ and elucidate the fundamental mechanism of ligand structural impact on 2D perovskite crystallization.

The migration of defects plays an important role in the stability of halide perovskites. It is challenging to study defect migration with experiments or conventional computer simulations. The former lacks an atomic-scale resolution and the latter suffers from short simulation times or a lack of accuracy. Here, we demonstrate that machine-learned force fields, trained with an on-the-fly active learning scheme against accurate density functional theory calculations, allow us to probe the differences in the dynamical behaviour of halide interstitials and halide vacancies in two closely related compositions CsPbI3 and CsPbBr3. We find that interstitials migrate faster than vacancies, due to the shorter migration paths of interstitials. Both types of defects migrate faster in CsPbI3 than in CsPbBr3. We attribute this to the less compact packing of the ions in CsPbI3, which results in a larger motion of the ions and thus more frequent defect migration jumps.

Perovskite solar cells must overcome the long-term stability problem in order to be put into practical use. Materials science, through the development of synthetic chemistry, materials characterization and device engineering can contribute to improvements in stability and scalability towards enabling large scale production. This Collection presents recent research efforts in stabilizing perovskite solar cells with three interconnected themes: characterizing instability, synthesizing stable perovskites and curing the interfaces.

Ionic liquids (ILs) and deep eutectic solvents (DESs) have been lately the solvents of choice in a number of processes because they offer a valid alternative to conventional solvents. Despite main interactions in ILs differ from those in DESs (e.g., electrostatic-type in the former and H-bond-type in the latter), these two neoteric solvents are more closely related that appeared and can be seen as the two sides, the face and the cross, of the same coin. Herein, we hypothesized about a way for transitioning from one to the other. In particular, we promoted the transition from 1-ethyl-3-methylimidazolium chloride (EMIMCl) to EMIMCl·nAcOH-based DESs by the simple addition of stoichiometric amounts of acetic acid (AcOH) to EMIMCl. 1H NMR spectroscopy and DSC studies confirmed the occurrence of such a transition. Molecular dynamics (MD) simulations revealed the capability of the Cl anion to fully accommodate up to 4 AcOH molecules (e.g., EMIMCl·1AcOH, EMIMCl·2AcOH, EMIMCl·3AcOH, and EMIMCl·4AcOH) without signs of H-bond self-interactions between AcOH molecules. These DESs also exhibited quite different solvent properties, with α and β Kamlet-and-Taft parameters that differed from those of EMIMCl and 1-ethyl-3-methylimidazolium acetate (EMIMOAc). Interestingly, excess molar volume and excess viscosity measurements as well as Brillouin spectroscopic experiments indicated that aqueous dilutions of EMIMCl·AcOH-based DESs deviated from ideality as a consequence of the formation of HBs between water molecules and the anion, as observed by 1H NMR spectroscopy.

Mixed iodine-bromine perovskites used in solar cells undergo below a critical temperature an intrinsic demixing into phases with different iodine-bromine compositions. In addition, under illumination they show nucleation of an iodine-rich phase. We predict from thermodynamic considerations that in mixed iodine-bromine perovskites like MAPb(I$_{1-x}$Br$_x$)$_3$ the interplay of these effects can lead to coexistence of a bromine-rich, iodine-rich, and nearly iodine-pure nucleated phase. This three-phase coexistence occurs in a region in the composition-temperature phase diagram near the critical point for intrinsic demixing. We investigate the hysteresis in the evolution of this coexistence when temperature or illumination intensity are cycled. Depending on the particular way the coexistence is established, nearly iodine-pure nuclei should form either in the iodine-rich phase only or both in the bromine-rich and iodine-rich phases. Experimental verification of this fundamentally novel type of light-tunable three-phase coexistence should be possible by a combination of absorption and photoluminescence experiments., 26 pages, 5 figures

Finding schemes for capturing hot carrier is crucial for designing photovoltaic device that more efficiently converts light into electricity. Currently, utilizing fullerene to enhance the cold electron capture from halide perovskite is remarkably prevalent, however, to date, capturing hot carrier in this system remains unlocked. Here, we demonstrate tuning their electronic coupling drives highly efficient hot carrier capture in inorganic perovskite nanocrystal and fullerene hybrid by transient absorption spectroscopy. The formation of state coupled complexes creates new hot carrier transport channels in their binding sites and reverses the originally forbidden hot carrier capture. Moreover, appropriately controlling phonon bottleneck and Auger heat effects in perovskite nanocrystal is demonstrated to enable more efficient hot carrier capture. Finally, we realizes ~ 84% of maximum hot carrier capture efficiency. The findings advance the modulation of hot carrier dynamics in doner-acceptor heterostructures and are essential for the development of practical hot carrier photovoltaics.

Phase instability is one of the major obstacles to the wide application of formamidinium (FA)-dominated perovskite solar cells (PSCs). An in-depth investigation on relevant phase transitions is urgently needed to explore more effective phase-stabilization strategies. Herein, the reversible phase-transition process of FA1−xCsxPbI3 perovskite between photoactive phase (α phase) and non-photoactive phase (δ phase) under humidity, as well as the reversible healing of degraded devices, is monitored. Moreover, through in situ atomic force microscopy, the kinetic transition between α and δ phase is revealed to be the “nucleation–growth transition” process. Density functional theory calculation implies an enthalpy-driven α-to-δ degradation process during humidity aging and an entropy-driven δ-to-α healing process at high temperatures. The α phase of FA1−xCsxPbI3 can be stabilized at elevated temperature under high humidity due to the increased nucleation barrier, and the resulting non-encapsulated PSCs retain >90% of their initial efficiency after >1000 h at 60 °C and 60% relative humidity. This finding provides a deepened understanding on the phase-transition process of FA1−xCsxPbI3 from both thermodynamics and kinetics points of view, which also presents an effective means to stabilize the α phase of FA-dominated perovskites and devices for practical applications.

The commercialization of perovskite solar cells is hindered by the poor long-term stability of the metal halide perovskite (MHP) light absorbing layer. Solution processing, the common fabrication method for MHPs, produces polycrystalline films with a wide variety of defects, such as point defects, surfaces, and grain boundaries. Although the optoelectronic effects of such defects have been widely studied, the evaluation of their impact on the long-term stability remains challenging. In particular, an understanding of the dynamics of degradation reactions at the atomistic scale is lacking. In this work, using reactive force field (ReaxFF) molecular dynamics simulations, we investigate the effects of defects, in the forms of surfaces, surface defects and grain boundaries, on the stability of the inorganic halide perovskite CsPbI$_{3}$. Our simulations establish a stability trend for a variety of surfaces, which correlates well with the occurrence of these surfaces in experiments. We find that a perovskite surface degrades by progressively changing the local geometry of PbI$_{\mathrm{x}}$ octahedra from corner- to edge- to face-sharing. Importantly, we find that Pb dangling bonds and the lack of steric hindrance of I species are two crucial factors that induce degradation reactions. Finally, we show that the stability of these surfaces can be modulated by adjusting their atomistic details, either by creating additional point defects or merging them to form grain boundaries. While in general additional defects, particularly when clustered, have a negative impact on the material stability, some grain boundaries have a stabilizing effect, primarily because of the additional steric hindrance., 29 pages, 7 figures

An atomistic description of tin deposition on ruthenium and its effect on blistering damage is of great interest in extreme ultraviolet (EUV) lithography. In EUV machines, tin debris from the EUV-emitting tin plasma may be deposited on the mirrors in the optical path. Tin facilitates the formation of hydrogen-filled blisters under the ruthenium top layer of the multi-layer mirrors. We have used Density Functional Theory (DFT) to show that tin deposition on a clean ruthenium surface exhibits a film-plus-islands (Stranski–Krastanov) growth mode, with the first atomic layer bonding strongly to the substrate. We find that a single tin layer allows hydrogen to reach the ruthenium surface and subsurface more easily than on clean ruthenium, but hydrogen penetration through the tin film becomes progressively more difficult when more layers are added. The results indicate that hydrogen penetration and blistering occur when only a thin layer of tin is present., A monolayer of Sn makes the Ru surface more permeable to H, but more layers inhibit H penetration.

Halide perovskites make efficient solar cells but suffer from several stability issues. The characterization of these degradation processes is challenging because of the limited spatiotemporal resolution in experiments and the absence of efficient computational methods to study these reactive processes. Here, we present the first reactive force field for molecular dynamics simulations of the phase instability and the defect-induced degradation in CsPbI3. We find that the phase transitions are driven by the anharmonic fluctuations of the atoms in the perovskite lattice. At low temperatures, the Cs cations tend to move away from their preferential positions, resulting in worse contacts with the surrounding metal halide framework which initiates the conversion to a nonperovskite phase. Moreover, our simulations of defective structures reveal that, although both iodine vacancies and interstitials are mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, leading to the decomposition of perovskites to PbI2.

3D mixed-halide perovskite-based red emitters combine excellent charge-transport characteristics with simple solution processing and good film formation; however, light-emitting diodes (LEDs) based on these emitters cannot yet outperform their nanocrystal counterparts. Here the use of diammonium halides in regulating the formation of mixed bromide–iodide perovskite films is explored. It is found that the diammonium cations preferentially bond to Pb–Br, rather than Pb–I, octahedra, promoting the formation of quasi-2D phases. It is proposed that the perovskite formation is initially dominated by the crystallization of the thermodynamically more favorable 3D phase, but, as the solution gets depleted from the regular A cations, thin shells of amorphous quasi-2D perovskites form. This leads to crystalline perovskite grains with efficiently passivated surfaces and reduced lattice strain. As a result, the diammonium-treated perovskite LEDs demonstrate a record luminance (10745 cd m−2) and half-lifetime among 3D perovskite-based red LEDs.

Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (HI and OH) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs., We thank the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under grant no. SEV-2017-0706 for the postdoctoral contract to H.X. We thank the King Abdulaziz City for Science and Technology (KACST) for the financial support to M.G. and S.M.Z. A.H. is thankful for the financial support from the European Union H2020, ESPResSo project grant agreement 764047, and Swiss National Science Foundation project IZLCZ2_170177. Z.W. and S.F. are thankful for the “China Scholarship Council” fellowship (CSC). H.-S.K., M.G., and S.M.Z. are thankful for the financial support from the GRAPHENE Flagship Core 2 project supported by the European Commission H2020 Programme under contract 785219. P.T. and J.A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ANAPHASE (ENE2017-85087-C3). D.J.K. and L.E. acknowledge support from the Swiss National Science Foundation grant no. 200021_160112. S.T. acknowledges funding from the Computational Sciences for Energy Research tenure track programme of Shell, NWO, and FOM (project no. 15CST04-2), the Netherlands. S.D.S. and M.A. acknowledge funding from the Marie Skłodowska-Curie actions (grant agreement no. 841386) under the European Union’s Horizon 2020 research and innovation programme. S.D.S acknowledges support from the Royal Society and Tata Group (UF150033) and EPSRC (EP/R023980/1). X.S. and F.F. acknowledge the funding from the Swiss Federal Office of Energy (SFOE)-BFE (project no. SI/501805-01). K.G. appreciates support from the Polish Ministry of Science and Higher Education within the Mobilnosc Plus program (grant no. 1603/MOB/V/2017/0). K.J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 804349 (ERC StG Cuhl) and RyC fellowship no. RYC-2017-22330. We give thanks to the Spanish State Research Agency for the grant Self-Power (PID2019-104272RB-C54 / AEI / 10.13039/501100011033) and the OrgEnergy Excelence Network (CTQ2016-81911-REDT), and to the Agència de Gestiód'Ajuts Universitaris i de Recerca (AGAUR) for the support to the consolidated Catalonia research group 2017 SGR 329 and the Xarxa d’R+D+I Energy for Society (XRE4S). Part of this work is under Materials Science Ph.D. Degree for A.M. and P.T. and the Chemistry Ph.D. programme for C.P. of the Universitat Autonoma de Barcelona (UAB, Spain). We thank CONACYT for the scholarship to C.P. We acknowledge Libertad Sole and also the Clean-Room from IMB-CNM for FIB process. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (grant no. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya.

Metal halide perovskites (MHPs) are gaining increasing attention as low-cost, high-performance semiconductors for optoelectronics. In particular, their solution processing is compatible with the large-scale manufacturing of thin-film devices, including solar cells and light-emitting diodes. Understanding the coordination chemistry in precursor-solvent solution and atomistic mechanisms of film formation is of great importance for optimizing the optoelectronic properties of the final films. Using the methylammonium lead triiodide (MAPbI3) as an example, we study the complex evolution of the molecular species from the solution to the initial stage of the crystallization by using a combination of density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations. We focus on the widely employed solvents DMSO and DMF, analyze the structures and energies of the iodoplumbate complexes in the form of simple complex of [PbImLn]2−m and polymeric iodoplumbates of ([PbImLn]2−m)x. Based on the calculated formation enthalpies, we propose reaction schemes of MAPbI3 formation in DMSO, DMF and DMF-DMSO binary solvent and explain the advantages of the binary solvent. We highlight the important role of NH…O hydrogen bonds in the formation of iodoplumbates monomers. Our calculations indicate unbalanced reaction energies at several elementary reaction steps in either DMF (formation of [PbI4Ln]2− being highly favourable) or DMSO (formation of [PbI5Ln]3− being retarded). Mixing a small amount of DMSO in DMF gives rise to a better balance in the energies and, therefore, potentially better equilibria in the overall crystallization process and better quality of the final perovskite films.

Defects in halide perovskites play an essential role in determining the efficiency and stability of the optoelectronic devices based on these materials. We present a systematic study of intrinsic point defects in six primary metal halide perovskites, MAPbI3, MAPbBr3, MAPbCl3, FAPbI3, CsPbI3, and MASnI3 (where MA denotes methylammonium and FA denotes formamidinium), based upon density functional theory calculations. Within a single computational scheme, using the SCAN+rVV10 functional, we compare the impact of changing anions and cations on the defect formation energies and the charge state transition levels in the six compounds, and identify the physical origins underlying the observed trends. Dominant defects in the lead iodide compounds are the A+ cation interstitials (A=Cs, MA, FA), charge-compensated by I- interstitials or lead (2-) vacancies. In the lead bromide and lead chloride compounds, halide interstitials are most prominent, and for MAPbCl3, the chlorine vacancy also becomes important. These trends can be explained in terms of the changes in electrostatic interactions and chemical bonding upon replacing cations and anions. Defect physics in MASnI3 is strongly dominated by tin (2-) vacancies, promoted by the easy oxidation of the tin. Intrinsically, all compounds are mildly p doped, except for MASnI3, which is strongly p doped. All acceptor levels created by defects in the six perovskites are shallow. Some defects, halide vacancies and Pb or Sn interstitials in particular, can create deep donor traps. Although such traps might hamper the electronic behavior of MAPbCl3, in bromine- and iodine-based perovskites their equilibrium concentrations are too small to affect the materials' properties.

Two-step sequential deposition has been shown as an effective method to improve the quality of perovskite films and performance of perovskite solar cells (PSCs). In this work, we have developed a two-step sequential deposition method, incorporating an alkali cation (e.g., Cs+) and a benzylammonium (BA+) cation in formamidinium (FA+)-based perovskite thin films for highly efficient and stable PSCs. By combining experimental characterizations and theoretical calculations, we demonstrate that not only the co-incorporation of alkali cations and BA+ significantly improves crystallinity and orientational growth of perovskite films, but also Cs+ enhances passivation effect of BA+ ions on perovskite surface via enhanced NH…I hydrogen bonding. In particular, inclusion of Cs+ in BA+-passivated perovskites promotes the preferred orientation of defect-free (1 0 0) facets. As a result, we observed remarkable improvements in both open-circuit voltage and fill factor of formamidinium-benzylammonium-cesium (FABACs)-based PSCs, achieving a champion efficiency of 22.5%. More importantly, the FABACs perovskite films demonstrated superior resistance to humidity and photo-thermal stress, and the perovskite-based devices (without encapsulation) retained over 95% of the initial efficiency after exposure to air for 2 months. Manipulating perovskite composition by combining small alkali and large organic cations in two-step sequential deposition provides a new path to highly efficient and stable perovskite devices.

Summary The most important obstacle to widespread use of perovskite solar cells is their poor stability under operational stressors. Here, we systematically monitor the evolution of the photovoltaic performance of perovskite solar cells based on formamidinium-cesium lead iodide (FA0.9Cs0.1PbI3) for 600 h, under a series of controlled operational stressors. Although these devices exhibit reasonable thermal stability, their stability under illumination or stabilized power output (SPO) is far from commercial demands. Synchrotron-based nanoprobe X-ray fluorescence and X-ray-beam-induced current measurements reveal that current-blocking Cs-rich phases segregate during stress tests. The decrease in performance is in line with the increasing density of the Cs-rich clusters in area upon illumination. Theoretical calculations indicate that light-generated carriers provide the thermodynamic driving force for that phase segregation. Our findings correlate device performance to microscopic behavior and atomistic mechanisms and shed light on inhibiting the cation-dependent phase segregation during device operation.

Phase-stable CsSnxPb1−xI3 perovskite quantum dots (QDs) hold great promise for optoelectronic applications owing to their strong response in the near-infrared region. Unfortunately, optimal utilization of their potential is limited by the severe photoluminescence (PL) quenching, leading to extremely low quantum yields (QYs) of approximately 0.3 %. The ultra-low sodium (Na) doping presented herein is found to be effective in improving PL QYs of these alloyed QDs without alerting their favourable electronic structure. X-ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I− and Sn2+ ions upon Na doping, which potentially helps to stabilize Sn2+ and suppresses the formation of I vacancy defects. The optimized PL QY of the Na-doped QDs reaches up to around 28 %, almost two orders of magnitude enhancement compared with the pristine one.

The three-precursors approach has proven to be advantageous for obtaining high-quality metal halide perovskite nanocrystals (PNCs). However, the current halide precursors of choice are mainly limited to those highly toxic organohalides, being unfavorable for large-scale and sustainable use. Moreover, most of the resulting PNCs still suffer from low quality in terms of photoluminescence quantum yield (PLQY). Herein we present all-inorganic germanium salts, GeX4 (X = Cl, Br, I), serving as robust and less hazardous alternatives that are capable of ensuring improved material properties for both Pb-based and Pb-free PNCs. Importantly, unlike most of the other inorganic halide sources, the GeX4 compound does not deliver the Ge element into the final compositions, whereas the PLQY and phase stability of the resulting nanocrystals are significantly improved. Theoretical calculations suggest that Ge halide precursors provide favorable conditions in both dielectric environment and thermodynamics, which jointly contribute to the formation of size-confined defect-suppressed nanoparticles.

Polycrystalline perovskite films fabricated on flexible and textured substrates often are highly defective, leading to poor performance of perovskite devices. Finding substrate-tolerant perovskite fabrication strategies is therefore paramount. Herein, we show that a small amount of Cadmium Acetate (CdAc2) in PbI2 precursor solution results in nano-hole array films and improves the diffusion of organic salts in PbI2, and promotes favorable crystal orientation and suppresses non-radiative recombination. Polycrystalline perovskite films on flexible substrate with ultra-long carrier lifetimes exceeding 6 μs are achieved. Eventually, a record PCE of 22.78% is obtained for single-junction flexible perovskite solar cells (FPSCs). Furthermore, we find this strategy is also demonstrated to be applicable for textured tandem solar cells. Our perovskite/silicon tandem solar cells (TSCs) with CdAc2 showed an efficiency of 29.25% (0.5003 cm2), with a certified value being 23.07% (11.8792 cm2), respectively, the highest value reported for solution-based TSCs. Un-encapsulated TSCs based on CdAc2 maintain 109.78% of their initial efficiency after operation at maximum power point under continuous one-sun illumination for 300 hours at 45 oC in N2 atmosphere. This strategy will provide facile access to high-efficiency perovskite-based solar cells.

Cesium lead iodide (CsPbI3) has gained great attention due to its thermal stability and appropriate bandgap (≈1.73 eV) at black (γ) phase potentially suitable for tandem solar cells. However, it is challenging to obtain CsPbI3 film with desired black phase. Herein, we fabricate kinetically favorable γ-CsPbI3 thin films by stoichiometry modulation, where in-situ 2D GIWAXS measurement was innovatively performed to illustrate the phase transition process of the precursor films, to aid a full picture study on the entire film evolution process. Conceptually different from introducing other extrinsic species, the cogenetic doping by excessive cesium iodide is found to tailor energy barriers for phase transformations during both the film formation and ageing process simultaneously. During film growth, excessive CsI affects the formation of Pb−I complex in the precursor solution, which facilitates the δ to γ phase transformation. Also, the Cs-rich resultant film could suppress γ to δ phase transformation. The corresponding CsPbI3 solar cells deliver a PCE of 16.68% without performance loss at continuous maximum power point output (MPP) for ~175 h under continuous illumination in a N2 glovebox. This work highlights the importance of precursors chemistry and provides guidelines to adjust the phase transformation barrier in CsPbI3 films without any foreign additives.

Quasi-2D (Q-2D) perovskites are promising materials applied in light-emitting diodes (LEDs) due to their high exciton binding energy and quantum confinement effects. However, Q-2D perovskites feature a multiphase structure with abundant grain boundaries and interfaces, leading to nonradiative loss during the energy-transfer process. Here, a more efficient energy transfer in Q-2D perovskites is achieved by manipulating the crystallization kinetics of different-n phases. A series of alkali-metal bromides is utilized to manipulate the nucleation and growth of Q-2D perovskites, which is likely associated with the Coulomb interaction between alkali-metal ions and the negatively charged PbBr64– frames. The incorporation of K+ is found to restrict the nucleation of high-n phases and allows the subsequent growth of low-n phases, contributing to a spatially more homogeneous distribution of different-n phases and promoted energy transfer. As a result, highly efficient green Q-2D perovskites LEDs with a champion EQE of 18.15% and a maximum brightness of 25 800 cd m–2 are achieved. The findings affirm a novel method to optimize the performance of Q-2D perovskite LEDs.

In recent years, metal halide perovskites (MHPs) for optoelectronic applications have attracted the attention of the scientific community due to their outstanding performance. The fundamental understanding of their physicochemical properties is essential for improving their efficiency and stability. Atomistic and molecular simulations have played an essential role in the description of the optoelectronic properties and dynamical behavior of MHPs, respectively. However, the complex interplay of the dynamical and optoelectronic properties in MHPs requires the simultaneous modeling of electrons and ions in relatively large systems, which entails a high computational cost, sometimes not affordable by the standard quantum mechanics methods, such as density functional theory (DFT). Here, we explore the suitability of the recently developed density functional tight binding method, GFN1-xTB, for simulating MHPs with the aim of exploring an efficient alternative to DFT. The performance of GFN1-xTB for computing structural, vibrational, and optoelectronic properties of several MHPs is benchmarked against experiments and DFT calculations. In general, this method produces accurate predictions for many of the properties of the studied MHPs, which are comparable to DFT and experiments. We also identify further challenges in the computation of specific geometries and chemical compositions. Nevertheless, we believe that the tunability of GFN1-xTB offers opportunities to resolve these issues and we propose specific strategies for the further refinement of the parameters, which will turn this method into a powerful computational tool for the study of MHPs and beyond.

Light-induced halide segregation hampers obtaining stable wide-band-gap solar cells based on mixed iodide-bromide perovskites. So far, the effect of prolonged illumination on the performance of mixed-halide perovskite solar cells has not been studied in detail. It is often assumed that halide segregation leads to a loss of open-circuit voltage. By simultaneously recording changes in photoluminescence and solar cell performance under prolonged illumination, we demonstrate that cells instead deteriorate by a loss of short-circuit current density and that the open-circuit voltage is less affected. The concurrent red shift, increased lifetime, and higher quantum yield of photoluminescence point to the formation of relatively emissive iodide-rich domains under illumination. Kinetic Monte Carlo simulations provide an atomistic insight into their formation via exchange of bromide and iodide, mediated by halide vacancies. Localization of photogenerated charge carriers in low-energy iodide-rich domains and subsequent recombination cause reduced photocurrent and red-shifted photoluminescence. The loss in photovoltaic performance is diminished by partially replacing organic cations by cesium ions. Ultrasensitive photocurrent spectroscopy shows that cesium ions result in a lower density of sub-band-gap defects and suppress defect growth under illumination. These defects are expected to play a role in the development and recovery of light-induced compositional changes.

Halide perovskites make efficient solar cells but suffer from several stability issues. The characterization of these degradation processes is challenging because of the limited spatiotemporal resolution in experiments and the absence of efficient computational methods to study these reactive processes. Here, we present the first reactive force field for molecular dynamics simulations of the phase instability and the defect-induced degradation in CsPbI3. We find that the phase transitions are driven by the anharmonic fluctuations of the atoms in the perovskite lattice. At low temperatures, the Cs cations tend to move away from their preferential positions, resulting in worse contacts with the surrounding metal halide framework which initiates the conversion to a nonperovskite phase. Moreover, our simulations of defective structures reveal that, although both iodine vacancies and interstitials are mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, leading to the decomposition of perovskites to PbI2.

Owing to its nice performance, low cost, and simple solution-processing, organic-inorganic hybrid perovskite solar cell (PSC) becomes a promising candidate for next-generation high-efficiency solar cells. The power conversion efficiency (PCE) has boosted from 3.8% to 25.2% over the past ten years. Despite the rapid progress in PCE, the device stability is a key issue that impedes the commercialization of PSCs. Recently, all-inorganic cesium lead halide perovskites have attracted much attention due to their better stability compared with their organic-inorganic counterpart. In this progress report, we summarize the properties of CsPb(IxBr1−x)3 and their applications in solar cells. The current challenges and corresponding solutions are discussed. Finally, we share our perspectives on CsPb(IxBr1−x)3 solar cells and outline possible directions to further improve the device performance.

Defects play an important role in the degradation processes of hybrid halide perovskite absorbers, impeding their application for solar cells. Among all defects, halide anion and organic cation vacancies are ubiquitous, promoting ion diffusion and leading to thin-film decomposition at surfaces and grain boundaries. Here, we employ fluoride to simultaneously passivate both anion and cation vacancies, by taking advantage of the extremely high electronegativity of fluoride. We obtain a power conversion efficiency of 21.46% (and a certified 21.3%-efficient cell) in a device based on the caesium, methylammonium (MA) and formamidinium (FA) triple-cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 treated with sodium fluoride. The device retains 90% of its original power conversion efficiency after 1,000 h of operation at the maximum power point. With the help of first-principles density functional theory calculations, we argue that the fluoride ions suppress the formation of halide anion and organic cation vacancies, through a unique strengthening of the chemical bonds with the surrounding lead and organic cations. Defects and defect migration are detrimental for perovskite solar cell efficiency and long-term stability. Li et al. show that fluoride is able to suppress the formation of halide anion and organic cation vacancy defects by restraining the relative ions via ionic and hydrogen bonds.

Transition metal chalcogenides (TMCs) have gained worldwide interest owing to their outstanding renewable energy conversion capability. However, the poor mechanical flexibility of most existing TMCs limits their practical commercial applications. Herein, triggered by the recent and imperative synthesis of highly ductile α-Ag 2S, an effective approach based on evolutionary algorithm and ab initio total-energy calculations for determining stable, ductile phases of bulk and two-dimensional Ag x Se 1- x and Ag x Te 1- x compounds was implemented. The calculations correctly reproduced the global minimum bulk stoichiometric P2 12 12 1-Ag 8Se 4 and P2 1/ c-Ag 8Te 4 structures. Recently reported metastable AgTe 3 was also revealed but it lacks dynamical stability. Further single-layered screening unveiled two new monolayer P4/ nmm-Ag 4Se 2 and C2-Ag 8Te 4 phases. Orthorhombic Ag 8Se 4 crystalline has a narrow, direct band gap of 0.26 eV that increases to 2.68 eV when transforms to tetragonal Ag 4Se 2 monolayer. Interestingly, metallic P2 1/ c-Ag 8Te 4 changes to semiconductor when thinned down to monolayer, exhibiting a band gap of 1.60 eV. Present findings confirm their strong stability from mechanical and thermodynamic aspects, with reasonable Vickers hardness, bone-like Young's modulus ( E) and high machinability observed in bulk phases. Detailed analysis of the dielectric functions ε(ω), absorption coefficient α(ω), power conversion efficiency (PCE) and refractive index n(ω) of monolayers are reported for the first time. Fine theoretical PCE (SLME method ∼11-28%), relatively high n(0) (1.59-1.93), and sizable α(ω) (10 4-10 5 cm -1) that spans the infrared to visible regions indicate their prospects in optoelectronics and photoluminescence applications. Effective strategies to improve the temperature dependent power factor (PF) and figure of merit (ZT) are illustrated, including optimizing the carrier concentration. With decreasing thickness, ZT of p-doped Ag-Se was found to rise from approximately 0.15-0.90 at 300 K, leading to a record high theoretical conversion efficiency of ∼12.0%. The results presented foreshadow their potential application in a hybrid device that combines the photovoltaic and thermoelectric technologies.

Tin (Sn)-based halide perovskites crystallized in the form of quantum dots (QDs) have attracted considerable attention due to their attractive features that are unique to the quantum realm. However, unlike those of their lead (Pb) counterparts, isolation and purification of the as-synthesized Sn-based QDs remain a challenge, as they undergo rapid decomposition in the most common antisolvents as well as in open air. Herein, we discover that CsSnX3 (X = Cl, Br, and I) QDs prepared under ultra-halide-rich conditions exhibit superior durability against the antisolvents even with high hydrophilicity like methyl acetate and can be thus readily purified under ambient conditions without the need for a strict inert atmosphere. First principles calculations reveal that halide-rich synthesis favors the formation of enhanced chemical bonds between QDs and their surface ligands and reduces ligand mobility. Both help to prevent structural decomposition and to preserve the perovskite composition. The successful synthesis of structurally stable Sn-based perovskite QDs accomplished without the use of any particular antioxidants or additional doping, provides alternative material design strategies for enhancing the stability of these fascinating yet fragile nanomaterials.

Narrow-bandgap CsSnxPb1−xI3 perovskite quantum dots (QDs) show great promise for optoelectronic applications owing to their reduced use of toxic Pb, improved phase stability, and tunable band gaps in the visible and near-infrared range. The use of small ions has been proven beneficial in enhancing the stability and photoluminescence quantum yield (PLQY) of perovskite QDs. The introduction of sodium (Na) has succeeded in boosting the PLQY of CsSn0.6Pb0.4I3 QDs. Unfortunately, the initial PLQY of the Na-doped QDs undergoes a fast degradation after one-day storage in solution, hindering their practical applications. Using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we study the effect of Na ions on the strength of surface bonds, defect formation energies, and the interactions between surface ligands and perovskite QDs. Our results suggest that Na ions enhance the covalent bonding of surface tin–iodine bonds and form strong ionic bonding with the neighboring iodine anions, thus suppressing the formation of I and Sn vacancies. Furthermore, Na ions also enhance the binding strength of the surface ligands with the perovskite QD surface. However, according to our AIMD simulations, the enhanced surface ligand binding is only effective on a selected surface configuration. While the position of Na ions remains intact on a CsI-terminated surface, they diffuse vigorously on an MI2-terminated surface. As a result, the positive effect of Na vanishes with time, explaining the relatively short lifetime of the experimentally obtained high PLQYs. Our results indicate that engineering the surface termination of the QDs could be the next step in maintaining the favorable effect of Na doping for a high and stable PLQY of Sn–Pb QDs., A comprehensive DFT and AIMD study shows that the surface termination engineering of Sn–Pb perovskite QDs plays an important role in maintaining the favorable effect of Na doping for a high and stable PLQY.