The first part of the presentation introduces the phenomenon of mixed ionic-electronic conductivity in solids and discusses defect-chemical implications and measuring techniques.

The second part of the presentation summarizes parts of our work on bulk ion conductivity in halide perovskites. It discusses contributions from halide ions and methylammonium cations, and their relevance for polarization phenomena and stability [1]. The finding that light enhances the ion conductivity in the iodides [2] is of broad interest and has the potential to lead to novel “opto-ionic” devices [3]. The fact that bromides do not show this effect can offer a simple explanation for the light-induced demixing [4].

The third part of the presentation refers to interfaces. It is shown that mobility of ions and the significant fraction of ionic charge carriers gives rise to ionic built-in potentials, a point that had not been addressed in the photovoltaic literature and might lead to a paradigm change in the understanding of photoactive interfaces [5].

The presentation includes conventional hybrid perovskites, but also “hollow perovskites” [6] and 2D perovskites [7].

Further breakthroughs in perovskite solar cells require advances in new compositions and underpinning materials science. Indeed, a deeper understanding of these complex hybrid perovskites requires atomic-scale characterization of their transport, electronic and stability behaviour. This presentation will describe combined atomistic modelling and experimental studies on hybrid perovskites [1-5] in two related areas: (i) pathways and energetics of iodide ion transport and the effect of incorporating different sized A-cations including guanidinium-doped and ‘hollow’ type systems; (ii) structure–electronic property relationships and degradation mechanisms in Pb- and Sn-based iodide perovskites including 2D Ruddlesden-Popper type structures.

[1] L. Lanzetta et al., Nature Commun. 12, 2853 (2021); [2] A. Senocrate et al., Chem. Mater. 33, 719 (2021); [3] N. Zibouche, M.S. Islam, ACS Appl. Mater. Interfaces, 12, 15328 (2020); [4] D.W. Ferdani et al., Energy Env. Sci., 12, 2264 (2019); [5] N. Aristidou et al., Nature Commun., 8, 15218 (2017).

New results on the interplay between structural and optoelectronic properties of multilayered perovskites

The presentation will give an insight into the interplay between structural and optoelectronic properties in 2D multilayered halide perovskites, either in Ruddlesden Popper or Dion Jacobson structure. The results will particularly describe specific mechanisms for the interaction of hot carriers with a 2D lattice, including a discussion on the electron-phonon coupling in layered perovskites and a comparison with previous results on 3D perovskites.  Moreover the funneling of electronic carriers to the 2D crystal edges, important for ionization of electron-hole pairs in 2D perovskite solar cells, is shown to be generalized as well to bromide materials. It is including a relaxation of collective polar ordering observed in the bulk at room temperature. Finally the impact of light soaking on the lattice relaxation and carrier trapping, is shown to have a strong influence on carrier transport in operating devices and performances of photovoltaic devices.

Lead halide perovskites have gained much attention in recent years for their remarkable optoelectronic properties as they can be applied in various devices as highly efficient active semiconductor material e.g., in solar cells, x-ray detectors or LEDs. One obstacle to commercialization is the migration of halide ions, which leads to efficiency losses and degradation. Ion migration via point defects also contributes significantly to the total electrical conductivity of halide perovskites. Thus, defect chemical investigations are a key to understand and engineer the electrical properties of halide perovskites.

In this work, we present a setup to measure the electrical conductivity of the model halide perovskite Methylammonium Lead Iodide over a wide range of iodine partial pressures. We find the electrical conductivity to change with different slopes in dependence of the iodine partial pressure in a double-logarithmic representation. This indicates changes in the perovskite’s conduction mechanism, i.e., changes in point defect concentrations. Considering differences in the mobilities of the various defect species, we discuss the dependence of the total conductivity of the perovskite on the iodine partial pressure. Our work will allow to develop a more fundamental understanding about the electrical properties of halide perovskites.

Photo-induced halide segregation is critical to the stability of multijunction photovoltaic-compatible mixed-halide (iodide-bromide) 3D perovskites. However, the effect is not well understood in mixed-halide lower-dimensional (2D or quasi-2D) perovskites whose superior environmental stability can improve long-term performance. In this work, we study phenethylammonium-based mixed-halide 2D (PEA₂Pb(I_1-xBr_x)₄) and quasi-2D (PEA₂MA_n-1Pb_n(I_1-xBr_x)_3n+1) perovskite thin films and characterize the slow occurrence of halide segregation under illumination and its reversal in the dark. Time-dependent photoluminescence spectroscopy shows that a system’s propensity to undergo halide demixing strongly depends on its structural nature. While a pure-2D (n=1) system is largely immune to the light-induced demixing of halides, other structural phases (n>1) comprised of conjoined lead halide octahedral sheets show a lower tolerance to such stressors and consequently form low-energy iodide-rich traps. In multi-dimensional (nominally n=4) thin film systems, the distribution of these phases (n=1, 2…∞), and consequently the stability, are shown to be regulated through solvent-engineering strategies. Differences in ion migration behaviour between structural phases also influences entropy-driven ionic remixing in the dark, which successfully restores the statistical mixed-halide composition in higher-dimensional phases but not in lower-dimensional analogs. These observations therefore establish perovskite dimensionality as a key determinant to the photo-stability of lower-dimensional perovskite optoelectronic devices.

Given the vast chemical space of layered hybrid perovskites, the systematic analysis of structure-property relationship in this type of materials is of high importance. Understanding the differences between Dion-Jacobson and Ruddlesden-Popper (RP) hybrid halide perovskites is limited. Thus, in this study we analyze compositions based on iodide or bromide for the halide and on phenylenedimethanammonium (PDMA) and benzylammonium (BN) as the spacers, which are structurally comparable. We use hydrostatic pressure as a tool to investigate the structure-property relationship. We probe optical, structural properties in the range of 0-0.35 GPa, which effects might be comparable to the processes occurring in the optoelectronic devices (polaron induced strain, lattice mismatch, chemical strain). At this pressure levels we observe optical bandgap reduction in all the compositions. The most significant shift (-54.9 meV) is recorded in (BN)₂PbBr₄ composition under 0.35 GPa pressure. Structural analysis reveals that lattice of all compositions monotonically shrinks but (BN)₂PbBr₄ experience an abrupt change in the lattice parameters lenght at 0.14 GPa. In addition, though the compression along all axes is comparable, there is consistent trend in all the systems showing slightly larger compression along the c axis with a more pronounced trend in the (BN)₂PbBr₄. Bulk modulus calculations reveal a much softer perovskite lattice in the latter composition, in agreement with the much larger shift of the excitonic peak observed as a function of pressure. Similarly, the other three compositions show a comparable bulk modulus as expected from the trend of the excitonic peak under pressure. Molecular dynamics provide additional insights into structure-property relationship. With this work we extend the knowledge about the layered hybrid perovskites based on aromatic spacers providing important insights into the mechanochromic properties of layered hybrid perovskites. Finally, the unique reversibility of their mechanochromic response in this mild pressure range opens new perspective towards the utility of layered hybrid perovskites as platforms for amphidynamic materials and mechanophores,  which expands the perspectives for their future applications.

The structural tunability of perovskites has paved the way for applications such as solar cells and resistive switching memory. Low-dimensional hybrid perovskites can be combined with 3D hybrid perovskites to make more stable solar cells. Interestingly, some low-dimensional hybrid perovskites also exhibit ferroelectricity, or the formation of electrically-polarized domains, making such materials attractive for resistive switching memory. For optoelectronic device applications, we must understand the behavior of the charge carriers, which in low-dimensional perovskites, is very diverse: free excitons, self-trapped excitons, phonon replicas, etc. In addition, because devices are primarily made from thin films, we must establish how the structural and optoelectronic properties of low dimensional perovskites might change with growth method.

Here, we investigate growth of the ferroelectric perovskite (EA)₄Pb₃Br₁₀. [1] We find that changing the growth method can be used funnel charge carriers into the free exciton or phonon-coupled states. Indeed, strains acquired from spin-coating turn off phonon-coupled emission. However, slower film growth methods preserve this phonon-coupled emission, and also increase domain size. Photothermal deflection spectroscopy shows that strain increases electronic disorder near the free exciton absorbance, which is undesired for solar cell applications. In addition, the challenge of making phase-pure films in low-dimensional Ruddlesden-Popper structure ((A’)2(A)n−1BnX3n+1) is overcome by using a single A/A’-site cation, ethylammonium (EA), whose optimal size also prohibits formation of off-target phases. This suggests that molecular design of the A/A’ sites might be used to favor phase purity in low-dimensional films. These results help extend the utility of 2D perovskites by providing design rules for how to grow films with the targeted phase and optoelectronic properties.

[1] Kennard, R.M., Dahlman, C.J., Chung, J., Cotts, B.L., Mikhailovsky, A.A, Mao, L., DeCrescent, R.A., Stone, K.H., Venkatesan, N.R., Mohtashami, Y., Assadi, S., Salleo, A., Schuller, J.A., Seshadri, R. and Chabinyc, M.L. Growth-Controlled Broad Emission in Phase-Pure Two-Dimensional Hybrid Perovskite Films. Accepted to Chemistry of Materials, 2021.

Ferroelastic twin domains are a crystallography related mechanism that affects the charge carrier Dynamics in methylammonium lead iodide (MAPI) crystals. In this work, we have found a chemical route to manipulate the ferroelastic twin domain structure in MAPI thin films by changing the Pb(Ac)₂ / PbCl₂ ratio in the perovskite precursor solution. The ferroelastic twin domain structure of each sample was monitored via piezoresponse force microscopy (PFM) and x-ray diffraction spectroscopy (XRD). Furthermore, we have shown that the changes in ferroelastic twin domains directly affects the charge carrier lifetimes via time-resolved photoluminescence (TRPL). Therefore, this chemical engineering route we have discovered offers a new way of strain engineering in perovskite research.

Along the past few years, the vacancy-ordered halide double perovskites (VOHDP) have been extensively investigated as promising candidates for various optoelectronic applications [1]. Among these compounds, Cs₂TiBr₆ and Cs₂TiI₆ are VOHDP that have been successfully synthesised, and are reported to be interesting photo-active materials with band-gaps well within the visible range (1.02-2.00 eV) and potentially good carrier transport properties [2–4]. Photovoltaic devices using Cs₂TiBr₆ achieved a 3.3% power-conversion efficiency [3], though their actual potential has been recently questioned [5]. In this work, we perform a thorough computational analysis of the electronic structure, the mechanical and chemical stability, and the optical properties, of the entire family of VOHDP that have the same B’-site electronic valence as Ti⁴⁺ (i.e., d¹⁰). By doing so, we probe the potential tuning limits of their opto-electronic properties, extend the existing understanding on the well-established Ti-based materials, and propose the less-known Zr-based compounds (i.e., Cs₂ZrX₆; X=Br, I) as potential prominent alternatives. In particular, we employ three levels of calculations: DFT-PBE, hybrid functionals, and state-of-the-art GW, to calculate their electronic structure, and perform a complete symmetry analysis of the electronic bands to reveal its consequences on the exhibited band-gaps and charge carrier effective masses. We also show the importance of spin-orbit coupling effects, highlight the limitations when calculating quasi-particle corrections, and demonstrate how a simple crystal field theory can be used to describe the observed electronic structure. Furthermore, we investigate all possible decomposition reaction pathways to competing compounds, and further assess their mechanical stability by means of phonon calculations. Overall, our findings unveil the fundamental electronic and optical properties of Cs₂TiBr₆, establish the stability and tunability limits of electronic band-gaps and effective mass within the family of VOHDP materials with a d¹⁰ electronic configuration, but most importantly propose a novel composition for the next generation of optoelectronic devices, namely Cs₂ZrI₆ that has been practically unexplored to date.

Despite remarkable progress, the performance of lead halide perovskite solar cells fabricated in an inverted structure lags behind that of standard architecture devices. In this talk, I will present several strategies for increasing the performance of inverted architecture devices. First, I will describe a general approach to the fabrication of efficient perovskite solar cells from any antisolvent. Next, I will show that the performance of inverted architecture solar cells can be enhanced by modifying the method of antisolvent deposition. Finally, I will present a novel strategy for modifying both the bottom and top interfaces in inverted architecture perovskite solar cells that leads to a simultaneous enhancement in all the photovoltaic parameters. Using this strategy results in a champion device with an efficiency of 23.7%.
 

Halide perovskite semiconductors are extremely promising materials for next-generation photovoltaics and other devices. However, even in spite of their strong performance in devices, these materials exhibit heterogeneity in their chemical, structural, optoelectronic and morphological properties on multiple length scales -- from the macroscale down to the nanoscale. Here, I will present our group's recent work exploring these different length scales of heterogeneity, and how the heterogeneity impacts performance. In particular, I will present multimodal microscopy measurements in which we simultaneously correlate local strucural and chemical properties with local performance. We identify that local, nanoscale trap clusters are responsible for non-radiative power losses but also are the sites that seed degradation -- and these clusters should be the target of elimination to improve device performance and longevity. We show how the local chemical heterogeneity can provide pathways for carriers to avoid these problematic trap clusters, explaining the strong performance of alloyed perovskite compositions. These results highlight the large impact the nanoscale landscape has on performance and device stability -- and, although far from benign, offers a lever to further control device behaviour. 

The carrier transportation in perovskites materials is complicated by the presence of mobile ions, and the charges also affect ion migration.  Defects in perovskites play a major role in changing the dynamics process of ion conduction, while light can change ion migration which in return introduce defects in perovskites.  I will present our recent progress how defects of different type affect the ion migration as well as carrier transport in metal halide perovskites, including both three dimensional perovskites and low dimensional ones. The application of ion migration in perovskites for synapse devices will be briefed.  Several methods of removing surface defects will be presented to alleviate the issues induced by ion migration for more stable perovskite solar cells.

Relatively large organic cations, which on their own cannot stabilize the 3D corner-sharing perovskite framework, can be incorporated in perovskites by appropriate mixing with smaller 3D perovskite-forming cations. Here we utilize the solvent acidolysis crystallization (SAC) technique to grow mixed dimethylammonium/methylammonium (DMA/MAPbBr₃) lead tribromide crystals with the highest, as of yet, dimethylammonium (DMA) incorporation of 44% while maintaining the 3D cubic phase. We then employ different temperature-dependent measurements to explore the implications of such mixed organic cations composition on the structural and optoelectronic properties at low temperatures. We confirm the suppression of the orthorhombic phase upon the incorporation of DMA and a lower tetragonal-cubic phase transition temperature. The anomalous enhancement in the integrated PL observed in the two crystals is due to the different organic cation dynamics. By fabricating charge extraction layer-free photodetectors, we observed that the mixed crystal devices showed higher detectivity and responsivity compared to MAPbBr₃ crystals due to the observed structural compression and reduced surface trap density in the mixed crystals. Fascinatingly, the mixed crystal photodetectors, at 200 K, exhibit a large improvement in their characteristics compared to room temperature, while MAPbBr₃ crystals demonstrate much lesser enhancement due to their higher degree of distortion at lower temperatures.

Organic-inorganic halide perovskites (OIHPs) has emerged as the most promising light-absorber materials in the photovoltaic community due to their near-ideal bandgaps. However, the low formation energies of OIHPs render them unstable. While significant progress has been made in improving the stability of OIHPs, perovskite solar cells (PSCs) will also need to be mechanically reliable if they are to service satisfactorily for decades. In this context, we study the fracture behavior of PSCs by measuring their cohesion energies (Gc) using double cantilever beam method and report a novel approach to strategically enhance the interfacial adhesion and performance of PSCs using self-assembled monolayers (SAMs), where we find that the perovskite solar cell stability is closely intertwined with its mechanical reliability. This work points to a new route for designing mechanically robust PSCs with long-term durability.

Halide double perovskites (A2B’B”X6) have recently shown broadband emission in a single component and attracted various light-emitting device applications. Generally, in these double perovskites, valance and conduction bands are predominately obtained by mono (B’), trivalent (B”) cations, and halogen(X). In this work, we carried out systematically band engineering by distorted octahedra by substituted with mono (Na+), trivalent (In3+) cations, and halide anion (Cl-) and explored their structural, optical properties behavior of the cubic double perovskites Cs2AgBiBr6. In the undoped parent compound, only very weak photoluminescence is observed, but in substitution of Na, In, Cl samples broadband emission peak centered at around 620nm. In addition to this, the substitution of In3+ (25%) and Cl- (100%) showed increased lifetime compared to host material, and comparison of excitation and absorption spectra indicates that all the doped and undoped materials have shown indirect bandgap materials. 

Spectral conversion tailors the incident solar spectrum such that it is better suited for particular photovoltaic absorber material; in our work, an organometal halide perovskite semiconductor. In double cation perovskite solar cells (bandgap 1.57eV), 41% of the incident spectrum is wasted by sub-bandgap losses. In order to reduce these losses in the future, we research up-conversion (UC) crystal BaF₂: Yb³⁺, Er³⁺ for the annihilation of two low-energy photons to form a high-energy photon. The low phonon energy of BaF₂ crystal (240 cm⁻¹) results in reduced non-radiative losses.¹ Hence, BaF₂: Yb³⁺, Er³⁺ UC crystal exhibits a high UC quantum yield of ~10%, comparable to the best available fluoride materials. When tested under terrestrial sunlight representing one sun above the perovskite’s bandgap and sub-bandgap illumination at 980 nm (via a laser), the BaF₂: Yb³⁺, Er³⁺ crystal emits usable upconverted photons in the spectral range of 520 to 700 nm. In our bifacial PSC with the UC crystal beneath, these upconverted photons contribute to 0.38 mA/cm² enhancement in short-circuit current density at a laser intensity equivalent to 120 suns. We demonstrate that UC scales non-linearly with incident intensity, with an intensity equivalent to 880 suns resulting in a 2.09 mA/cm² enhancement in current. Our study validates that UC is a potentially viable process to extend the response of PSC to a wider spectral range.

Lead-free hybrid perovskites have emerged as an attractive green and efficient alternative to be used in solar cells. In this work, we have prepared bismuth-based hybrid perovskites and studied their dehydration-hydration process. The BzImH[BiI]₄·H₂O structure (IEF-4 RT-phase) consists in one-dimensional iodobismuthates and the benzimidazolium cation (BzlmH)⁺ containing one hydration water molecule per formula unit [1]. The transformation from IEF-4 RT-phase to the dehydrated HT-phase (BzImH[BiI]₄) occurs at around 100 °C, being a fully reversible process. Besides, stable and homogenous IEF-4 films have been successfully obtained by spin-coating method exhibiting an excellent absorption coefficient (1.5·10⁵ cm^-1), similar to that reported for lead perovskite materials. Both their good stability under working conditions (temperature and humidity) and excellent optical properties make them suitable for its use in optoelectronic devices. In addition, we have found a reversible dehydration-hydration process in other Bi-based perovskites.

Referencias

[1] A. A. Babaryk, Y. Pérez, M. Martínez, M. E. G. Mosquera, M. H. Zehender, S. A. Svatek, E. Antolín, P. Horcajada. J. Mater. Chem. C 2021, 9, 11358.

Lead-halide perovskite (LHP) nanocrystals (NCs) have been broadly studied over the past few years as active material for several optoelectronic devices from LEDs to solar cells. Alternative approaches to colloidal synthesis have been actively explored where the absence of ligands can improve charge injection/transport without compromising material stability.

Among them, the direct synthesis of LHC NCs within metaloxide nanoporous matrices [1] has demonstrated to be a route with potential to grow stable NCs with good optoelectronic properties amenable to be incorporated into devices. [2]

In this work we use ligand-free MAPbBr3 NCs grown within SiO2 nanoporous matrices as a test-ground to study the interaction of light with LHP at the nanoscale in the presence of different atmospheres. The optical response of our system is fitted with a model that points to the simultaneous presence of light-induced activation/degradation processes strongly affected by the surrounding atmosphere. [3] The reversibility of both processes and their prevalence at the bulk/surface of the NCs is also discussed.

Metal halide perovskites (MHPs) have demonstrated huge potential to build a rich library of materials for a new disruptive optoelectronic technology. The main strength comes from the possibility of easily tune the semiconductor bandgap in order to integrate it in devices with different functionalities – in principle. In reality, this cannot be achieved yet. In fact, while defect tolerance is claimed for MHPs with a bandgap of about 1.6 eV, the model system object of intense investigations, MHPs with lower and higher bandgaps are far from this claim. They show various forms of instabilities which are mainly driven by a strong defect activity.

Here I will discuss our studies on the nature of defects and their photo-chemistry as a function of the semiconductor bandgap and chemical composition in order to identify  their role in defining the semiconductor electronic properties and stability.

Organic-inorganic halide perovskites are record-breaking materials with a wide range of applications spanning photovoltaics, lighting, and detectors. These hybrid materials combine key properties of traditional inorganic semiconductors such as high carrier mobilities, with properties of organic semiconductors such as facile and low-cost synthesis. Single perovskites with stoichiometry ABX₃ like the prototypical solar cell absorber CH₃NH₃PbI₃ (MAPI) have excellent optoelectronic properties, but are hampered by instability, the toxicity of lead and scarcity of iodine. Furthermore, the ABX₃ structural motif limits functionalization because only very few organic molecules fit into the A site cavity. New hybrid materials with alternative perovskite-like structural motifs vastly extend the chemical and structural diversity of this family of materials resulting in unusual and widely tuneable optoelectronic properties.

In this contribution I will demonstrate that it is the chemical and structural heterogeneity of such hybrid materials that determines their electronic and excited state properties. I will showcase first principles calculations - based on density functional theory and many-body perturbation theory - of several perovskite-type materials with optoelectronic properties significantly deviating from those of MAPI. Our calculations allow us to explore the atomistic origin of the unusual excited state properties of these materials and suggest routes for tuning them for tailored applications.

A number of empirical additive and surface passivation treatments have been reported to improve the performance and stability of halide perovskite semiconductors. However, our mechanistic understanding of how these treatments affect the perovskite structures if often lagging, in part due to the wide range of perovskite formulations and growth conditions in use.  We explore the effects of difference treatments on several different perovskite formulations, from the role of large A-site cations, to Lewis bases, and protonated forms of basic species.  Using hyperspectral photoluminescence and nano-infrared microscopy, we show that compositional heterogeneity is ubiquitous in many perovskite formulations and that this compositional heterogeneity is in turn influenced by multiple surface treatment routes.  Ultimately, we demonstrate surface recombination velocities < 10 cm/s and show that we can preserve high quality interfaces even in contact with electron and hole extraction layers.

Cs2AgBiBr6 (CABB) has been proposed as a less toxic and more stable alternative to lead halide perovskites, and was successfully implemented as photoactive layer in optoelectronic devices. However, power conversion efficiencies of CABB-based devices remain much lower than compared to their lead-based analogues, suggesting poor charge transport. On the other hand, microsecond lifetimes have been reported for CABB. In this talk, I will elaborate on the dynamics of charge transport, localization and recombination in CABB thin films, studied using time-resolved photoconductivity and transient absorption/reflectance measurements. On comparing the temperature-dependent photoconductivity measurements of CABB with lead-based perovskites, we find a similar band-like charge transport mechanism.¹ We also find that replacing Bi with Sb shifts the absorption onset, while preserving the charge transport mechanism. In contrast to state-of-the-art lead-based perovskites however, charges in CABB lose mobility within tens of nanoseconds, so that diffusion lengths remain relatively short (~100 nm). Using microsecond transient absorption measurements, we observe that these non-mobile charges have a lifetime of several microseconds, suggesting a long-lived localized state. In contrast with the nanosecond-lifetime observed for mobile charges, we observe microsecond-long lifetimes in transient absorption measurements. The observation that the diffusion length is similar to the grain size suggests that charge localization occurs at the surfaces. Therefore, improving surface quality could be a strategy to optimize performance of CABB-based optoelectronic devices.

Finally, we performed a toxicity study of methylammonium lead iodide perovskites on A. Thaliana plants, and found that the iodide was actually much more toxic than the lead. Altogether, these results stress the importance of further understanding which perovskites are most harmful to the environment, while optimizing the optoelectronic quality of materials with the lowest toxicity.

Halide perovskites offer a flexible platform for tuning optoelectronic properties through changes in composition, connectivity, and dimensionality. In prior work, we have explored the dramatic consequences of stoichiometric and substoichiometric substitutions at the octahedral metal site in halide double perovskites. Our work on halide double perovskites has prompted us to explore still more complex perovskites. I will present our recent studies on further increasing the accessible electronic structures of this versatile family of materials by synthesizing mixed-metal perovskites with three different octahedral metal sites and on combining inorganic sublattices of different dimensionality in a single material. These new halide perovskites show unprecedented crystal structures and unusual electronic structures.