Bismuth-halide-based semiconductors have gained increasing attention for optoelectronics, owing to their low toxicity, high environmental stability under ambient conditions, and easy processability by a wide range of scalable methods. These materials have also been proposed to replicate key features of the electronic structure of lead-halide perovskites that may give rise to defect tolerance, but without the toxicity limitations of the latter [1]. This talk will examine in detail the case of bismuth oxyiodide (BiOI). Through density functional theory calculations, we show that the most common point defects in BiOI are resonant within the bands, or shallow within the bandgap [2]. We develop an all-inorganic device structure, and devise a route to control the preferred orientation of the vapour-deposited BiOI films to achieve photovoltaics with external quantum efficiencies reaching up to 80% at 450 nm wavelength [2]. Further, we demonstrate the strong potential of BiOI for indoor light harvesting to power Internet of Things electronics [3], as well as for bias-free solar water and CO2 splitting [4]. We finish with a discussion of the key factors that will need to be addressed in order to further improve the performance of this material in optoelectronics, focussing especially on the role of carrier-phonon coupling on charge-carrier transport and dynamics.

The toxicity of lead, the leaching problems, and the current regulations designed to limit or eliminate substances that are dangerous to the environment and to people, impose to move towards lead-free metal halide perovskites (MHPs). In addition, while MHPs have been thoroughly investigated for photovoltaics (PV) applications, it is now evident that their superior optical properties could be advantageously exploited in several other fields ranging from photodetectors to photocatalysis. Replacing lead with tin or germanium in 3D or 2D perovskites brings well-known stability issues, and the exploration of all-inorganic perovskite-derivatives containing bismuth or antimony may lead to problems of precursors solubility in wet-chemistry film deposition. Notwithstanding these possible obstacles, the PV application of tin-based MHPs has impressively increased in terms of efficiencies in the last few years. In order to further extend and exploit more recent applications of lead-free perovskites, a thorough experimental and computational structure-property correlation investigation should be provided with the aim of designing more stable and optimized materials. In this contribution we will present some recent studies devoted to the design, synthesis, and application of lead-free MHPs with a specific focus to fields beyond PV such as photocatalysis or photodetectors. Among other examples, we will show how, through a careful materials chemistry design, we could improve the water stability of tin and germanium-based MHPs perovskites and apply these characteristics to photocatalytic applications. The combined use of experimental and computational tools allowed to gain a significant comprehension on the mechanisms underpinning such features. At the basis of the results there is the possible manipulation of the compositional space provided for example by low-dimensional perovskites which can be beneficially used to modulate not only the optoelectronic properties but also the stability of these materials. Further examples will be focused on the application of physical vapor deposition techniques to afford the preparation of challenging all-inorganic lead-free perovskites and exploit their use in photodetectors.

The outstanding opto-electronic properties of metal-halide perovskites (MHPs) have contributed several breakthroughs in photovoltaics and optoelectronics. These materials would be ideally exploited also in heterogeneous photocatalysis, but the poor MHPs stability in aqueous environment has for long inhibited their application to this field. In the last two years, however, a growing family of water-stable and photoactive MHPs have been reported, thus paving the way towards the development MHP-based photocatalysts.

Surprisingly, tin-halide perovskites (THPs), which are notoriously unstable in photovoltaics, have received a large attention because of their suitable band gap, tunable electronic energy levels, and their low toxicity coupled to a superior stability in water environment compared to their lead counterparts. Here, we present high-level ab initio calculations to unveil the key factors determining the reactivity of THPs towards photocatalytic hydrogen production at the perovskite/water interface. Our results highlight that the occurrence of electron polarons at the surface of THPs is paramount in determining the efficiency of the reaction. The stabilization of localized electrons stems from the energy of the conduction band edge and from the peculiar THP defect chemistry, largely centered on tin. Band edge tuning is governed by the interplay between the A-site cation and nature of the halogen, thus fine-tuning the THPs energy levels can be achieved by varying the chemical composition, providing a successful strategy to boost the photo-reactivity of these materials.

Halide double perovskites are a chemically diverse class of materials featuring a vast range of thermodynamically stable compounds with widely tunable optoelectronic properties [1, 2]. Dimensional reduction of 3D lattices is routinely used for band structure and exciton engineering by exploiting the effects of quantum and dielectric confinement. Excitons in quasi-2D derivatives of Pb-based halide perovskites have been explored extensively in the last years. However, the impact of dimensional reduction on the nature of excitons in halide double perovskite is not yet well-understood.

In this contribution, we discuss the optoelectronic properties of 3D and quasi-2D derivatives of Ag/Bi-based halide double perovskites based on first principles calculations using many-body perturbation theory within the GW approximation [3] and the Bethe-Salpeter equation approach [4]. In the 3D double perovskite Cs₂AgBiBr₆, non-hydrogenic and strongly localized resonant excitons arise due to the chemical heterogeneity of alternating AgBr₆ and BiBr₆ octahedra and lead to anisotropic effective masses and pronounced local field effects [5]. We show that dimensional reduction of Cs₂AgBiBr₆ to mono- and bilayer Ruddlesden-Popper and Dion-Jacobson structures has striking effects on the band structure and excitons of these quasi-2D materials. Our results are in line with the experimentally reported optical properties of these materials [6] and demonstrate that structural distortions, quantum confinement effects, and layer stacking can be used to tune exciton localization and binding energies in chemically complex quasi-2D materials.

Lead-free double halide perovskites AIMIMIIIX6 (A,M – metal cations, X – halide anions) offer unprecedented variability of structures and compositions, with all four positions variable independently resulting in many thousands of possible compounds for the same set of constituent elements. A detailed screening of the properties of these huge families of compounds requires application of a combination of high-throughput synthetic (HTP) approaches with HTP characterization and HTP testing for possible applications related to light conversion and emission.

We have developed a “green” protocol for a HTP synthesis of brightly luminescent lead-free microcrystalline Cs2AgxNa1-xBiyIn1-yCl6 (CANBIC) perovskite phosphors absorbing strongly in UV range and emitting broadband photoluminescence (PL) in the visible range with PL quantum yields reaching 98±2% for specific compositions with x = 0.35-0.40 and y = 0.01-0.02. The CANBIC perovskites revealed high stability of spectral properties during many months of ambient storage, thermal stability at open air till 200-250 oC and photochemical stability under UV illumination, making these compounds highly promising for applications in luminescent solar light concentrators and down-shifters. Our HTP approach to CANBIC perovskites includes robot-assisted automated synthesis with any desirable x and y steps combined with HTP spectral charcterization which includes absorption, PL, PL excitation, and Raman spectroscopy as well as time-resolved PL characterization, allowing many hundreds of samples to be produced and characterized in a single working session.

This HTP approach was expanded to other CANBIC-like perovskites, produced either by substituting InIII with SbIII, or by substituting BiIII with FeIII. In the first case we developed a HTP protocol for the synthesis of Cs2AgxNa1-xBiySb1-yCl6 (CANBSC) perovskites with x and y fractions varied independently from 0 to 1. The CANBSC perovskites can be converted into Cs2AgxNa1-xBiySb1-yBr6 (CANBSB) and Cs3Bi2ySb2(1-y)I9 (CBSI) perovskites by a single-step anionic exchange with NaBr and NaI, respectively. These compounds reveal a band-bowing effect, the bandgaps of mixed perovskites being typically lower than those of corresponding individual BiIII- and SbIII-based perovskites. For CANBSC and CANBSB compounds the band-bowing effect was found to increase with an increase of the silver fraction. The lowest bandgaps reached were 2.47 eV for CANBSB and 2.08 eV for CBSI both at x = y = 0.50.

We found that BiIII in CANBIC perovskite can be partially or completely substituted by FeIII resulting in the latter case in Cs2AgxNa1-xFeyIn1-yCl6 (CANFIC) perovskites showing strong absorbance in the visible spectral range and a high environmental stability. A special feature of CANFIC compounds is a structure-directing influence of In on the formation of perovskite phase. While for y = 1.00 only a multi-phase mixture can be obtained in our conditions, introduction of mere 1% InIII steers the precipitation exclusively to the formation of cubic perovskite phase with no other phases detectable. The lowest bandgap reached so far is ca. 2.0 eV for x = 1.00 and y = 0.95-0.99. Anionic exchange of CANFIC did not result in stable single-phase bromide and iodide compounds. A similar structure-directing effect was also found for BiIII and SbIII introduced instead of InIII. Our preliminary results show the feasibility of introducing simultaneously three MIII metals, for example, BiIII, InIII, and FeIII, into the chloride perovskite while preserving the single-phase solid-solution character of the final products, opening broad possibilities for the compositional design of the optical properties. The unique variability of the MIII site of these double perovskites will be addressed in future using the combined HTP approach to the synthesis and characterization developed earlier for CANBIC perovskite phosphors.

Electrical doping enables meticulous tuning of the electronic properties in novel hybrid perovskite semiconductors, which is critical for their successful impelmentation as optoelectronic applications. Nevertheless, the use of substitutional/interstitial impurities as dopants remains difficult, mainly due to dopant phase segregation and defect compensation. In contrast, molecular doping stands as a promising pathway to modulate charge carrier density via charge transfer without altering the perovskite crystal structure. However, the underlying processes facilitating molecular doping in perovskites (e.g. host-dopant interactions) remain highly underexplored. In this talk, we will present our recent work on the molecualr doping mechanism of p-type methylammonium tin-lead iodide films by using an n-type molecule, namely 4-(1,3-Dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)-N,N-dimethylbenzenamine (n-DMBI-H). By employing a combination of experimental and first principles simulation techniques, we identify preferential dative bonding between amino moieties in n-DMBI-H and Sn atoms in perovskite surfaces as a dominant host-dopant interaction that mediates charge transfer. We will discuss the dopant localization within films and deternime that n-DMBI-H is located at perovskite surfaces and grain boundaries, allowing charge carrier density tuning of nearly one order of magnitude. The talk will conclude with details on the incorporation of n-DMBI-H in p-i-n tin-lead perovskite solar cells, which results in lower charge carrier recombination and higher charge selectivity at perovskite/transport layer interfaces. This work elucidates important insights of perovskite molecular doping that are expected to inspire strategies towards next-generation perovskite optoelectronic devices. 

Over the past few years, mixed Sn-Pb perovskites came to the fore as promising absorbing materials for single- and multi-junction solar cells due to the possibility of manipulating the bandgap by changing the Sn to Pb ratio [1]. However, the opto-electronic properties of tin-containing perovskites are adversely affected by intrinsic and extrinsic factors leading to doping [2][3]. In this work, we produced different Sn-Pb perovskites thin-films of composition Cs_0.25FA_0.75Sn_xPb_1-xI₃ with varying tin content between x=0 and x=0.6 via antisolvent spin-coating. In order to improve the properties of the perovskite layers, 20 mol% SnF₂ was added to the tin precursor solution. We studied the effect of light on the background conductivity by means of microwave conductivity techniques. In particular, we looked at the light-induced carge-carriers dynamics before and after exposing the perovskite thin-films to AM1.5 illumination. For the pristine layers, we found charge-carriers mobilities reaching μ ~ 25 cm²/V·s and lifetimes t > 1 μs, which are only marginally dependent on the Sn to Pb ratio. Exposure to external factors, such as ambient air, leads to a severe increase in background conductivity and shortened charge-carriers lifetimes. Lastly, a potential explanation of the degradation mechanism is proposed on basis of defect chemistry.

The performance of single-junction solar cells in thermal equilibrium is limited by the conversion of excess photon energy to heat through hot-carrier—phonon interactions [1]. However, hot-carrier photovoltaic devices, a future-generation technology for solar power conversion, could overcome this limitation. If the temperature of the charge-carrier gas is elevated compared to that of the crystal lattice, extraction of the photoexcited charge carriers with above-bandgap energies may be possible, boosting the energy conversion efficiency. Metal-halide perovskites, and in particular tin-iodide perovskite semiconductors, have been investigated in recent years as potential candidates for such technology, owing to the long hot-carrier cooling times observed in these materials [2]. Reported nanosecond-long cooling dynamics could allow for efficient extraction of hot carriers high above the bandgap, pushing the efficiency limit up to 66% PCE [3,4].

Unfortunately, tin-iodide perovskites suffer from tin-vacancy formation. These hole-donating point defects lead to significant electrical doping of the materials. The ocean of cooled, dopant holes could lead to rapid lowering of hot-carrier temperatures through cold-carrier—hot-carrier scattering events, hindering the application of these materials in hot carrier devices operating under solar illumination [5].

In this talk, we present the results of our investigation of carrier cooling dynamics in tin-iodide perovskites, performed using a novel pump-push-probe terahertz spectroscopic technique [6]. We show that, when applied to high-mobility materials, this technique has high sensitivity which enables the investigation of hot-carrier dynamics at lower excitation densities (below hot-phonon bottleneck threshold) than alternative spectroscopic methods. Our study, performed at these low hot-carrier densities, reveal ultrafast, sub-picosecond cooling dynamics in doped tin-iodide perovskite semiconductors, originating from cold-carrier—hot-carrier scattering. Our results are directly relevant to the applicability of these materials in hot-carrier devices, for which the fast thermalisation of the photoexcited carriers with cold ocean of dopant holes could hinder the efficiency of above-bandgap extraction, highlighting the need for further doping suppression methods in lead-free perovskites.

In photochemistry light is used to induce redox reactions, such as CO2 reduction or H2 evolution. To effectively harvest the incident photons, the photoactive material, typically a semiconductor, must have a suitable optical bandgap. A common problem of widely-studied photoactive materials, such as TiO2, is the large bandgap energy which results in a poor conversion efficiency. 

An interesting class of materials for photochemistry are elpasolites, also known as double perovskites, due to the tunable bandgap. In particular, Cs2AgBiBr6 shows great promise because of its good photo- and chemical stability and relatively low toxicity. Absorption of visible light is, however, limited because of the indirect nature of the bandgap. The bandgap of such silver-bismuth materials can be manipulated by substituting the Bi3+ cation with another cation. 

In this talk we discuss a mechanochemical synthesis approach to (partly) substitute Bi3+ in the Cs2AgBiBr6 crystal structure. Due to the high force and pressure created in a ball mixer, we are able to synthesize phase-pure Cs2AgBi1-xMxBr6 compositions. By controlling the Bi:M ratios we can manipulate the bandgap energy and thus the optical properties. We show that the incorporation of Fe3+ strongly red-shifts the absorption onset of this material, which increases the photochemical activity. 
 

Halide perovskites are well known for their good light absorption properties, tuneable electronic band structures, and good charge carrier mobility. Cs₃Bi₂Br₉ is a lead-free ternary halide perovskite with the ability to absorb light in the UV and visible ranges with a band structure that allows it to be used for photocatalytic CO₂ reduction. However, the low surface area, poor CO₂ adsorption, and high charge recombination of Cs₃Bi₂Br₉ prevents it from obtaining high production rates of solar fuels. Therefore, the purpose of this work is to design a novel heterojunction between templated g-C₃N₄ and Cs₃Bi₂Br₉ with Pt nanoparticles as a co-catalyst. SiO₂ nanospheres with a diameter of 250 nm were synthesised and used as a template to form homogeneous and well-ordered g-C₃N₄ inverse opal structures. Templated g-C₃N₄ were successfully obtained and proven to show a 52% increase in BET surface area with an improvement in CO₂ adsorption compared to bulk g-C₃N₄. Gas-phase photocatalytic reactions were performed and a selectivity shift towards CO production was observed with an increase from 2.23 to 6.28 µmol CO g^-1 h^-1 using templated g-C₃N₄. The incorporation of 1 wt% Pt nanoparticles induced the evolution of 71.83 µmol H₂ g^-1 h^-1 in addition to 7.21 µmol CO g^-1 h^-1 and 1.57 µmol CH₄ g^-1 h^-1. A preliminary assessment of the effectiveness of Cs₃Bi₂Br₉ perovskite showed a production of 5.5 µmol CO g^-1 h^-1 as well as a good electronic band structure alignment with g-C₃N₄. Current stages of this project entail optimising the band structure alignment of the two materials to synthesise a heterojunction that further boosts solar fuels production as well as exploring the effects of different cocatalysts on the efficiency of the process.

Lead halide perovskite materials now play a pivotal role as an emerging class of semiconductors for photovoltaic applications, with a certified record efficiency of 25.7% in 2022. However, their commercialization has been hindered so far by the toxicity of lead and by their instability. Vacancy-ordered halide perovskites, with general formula A₂B⁺⁴X₆, are one of the alternative structures that have been designed to replace Pb²⁺ with non-toxic tetravalent cations. Ti, Zr, Pd, Sn, Te and Pt-based vacancy-ordered perovskites have already been synthesized but those containing tin and titanium have shown the best performances in solar cells. We used colloidal synthesis as a low-cost, scalable solution method for the preparation of cesium titanium halide and cesium tin halide nanocrystals [1]. Moreover, with this method we prepared mixed titanium-tin cesium iodide perovskites, with different amounts of tin and titanium and bandgaps suitable for solar cell applications. We investigated the structural and optical properties of these materials, their stability in air, and their feasibility for solar cells applications.

Double perovskites have been developed to alternatively replace the divalent cations Pb²⁺ in single perovskite with a combination of a monovalent and trivalent cation, forming structure as A₂BB’X₆. Among them, the compounds Cs₂Na_1-xAg_xIn_1-yBi_yCl₆ can emit warm white light with almost unity quantum efficiency and are thus among the most promising materials for solid-state lighting(1). The emission spectrum is attributed to self-trapped excitons (STEs), the emission efficiency is, however, sensitive to the material composition since it is strictly related to the simultaneous presence of a small percentage of Ag and Bi.

Whether Bi atoms cure extrinsic defects or provide natural recombination centers is still a wide-open question while the addition of Ag has been linked to parity breaking in valence band orbitals, increasing the oscillator strength of the interband optical transitions(2,3).

A comprehensive understanding of the microscopic mechanisms producing the rules for efficient emission is fulfilled by a systematic study of the structural and optical properties as a function of the composition to understand the role of Ag and Bi in the high efficiency emission. Photoluminescence quantum yield (PLQY) measurements are complemented with photoluminescence excitation (PLE) and ultrafast spectroscopy in the form of differential transmission (DT/T) and time-resolved photoluminescence (TRPL). The dynamic of optical excitation is studied to investigate the formation of STEs and their decay times.

Bi and Ag substitutions at 0.1% levels are sufficient to promote the formation of STEs with a nearly radiative-recombination-limited dynamics, while effectively curing STE trapping in long-lived dark states. An Ag-induced self-trapping polaronic behavior is also suggested as a potential mechanism for formation of bright STEs thus achieving emission efficiency near unity with a warm white light spectrum in a wide variety of compositions.

Double perovskites realize therefore an almost ideal platform for solid state lighting and these results provide guidance for rational optimization of such compounds in view of the use as phosphors and active materials for LEDs and displays.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. In a decade of intensive research it has been a huge improvement in the performance of these devices. However, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. The photoconversion performance of perovskite solar cells containing alternative metals to Pb is significantly lower than the reported for devices containing Pb, where Sn-based perovskite solar cells is the alternative reporting higher photovoltaic performance close to 14%. Nevertheless, Sn-based perovskite solar cells exhibit a long term stability lower than their Pb containing counterparts, making stability their main problem. In this talk, we highlight how the use of proper additives and light soaking for defect engineering can increase significantly the stability of formamidinium tin iodide (FASnI3) solar cells, and discuss about the different mechanism affecting this stability, beyond the oxidation of Sn2+, and how they can be countered. Effect in Sn-based perovskite LEDs will be also analyzed. Additives can play an important role in stabilization of both Sn-based solar cells and LEDs.

The versatile characteristics of halide perovskites have led to diverse optoelectronic applications. However, control over their intrinsic charge transport behavior such as electrical conductivity and Seebeck has received limited attention. In addition, doping halide perovskites has been difficult due to their self-compensating nature. In this talk, a wide range of tunability (semiconducting, metallic, p-type, n-type) strategies are demonstrated in halide perovskites leveraging their self-doping behavior and polymorphism. I will show that controlling the A-site cation in tin perovskites offers tuning the electrical conductivity, which is in contrast to general assumption that A-site cation does not play significant role in determining the electronic properties. Finally, I will discuss strategies such as molecular doping and additive engineering in tuning the charge transport in this class of (Sn based) materials. Control over the transport behavior of halide perovskites serves as a viable approach for extending their functionality to other energy conversion applications such as thermoelectrics.

The progress in semiconducting lead-halide perovskite compounds within an unprecedentedly short time period drove their advancement for optoelectronic applications, among them tandem perovskite-based photovoltaic materials, low-dimensional (2D, 0D) quantum confined emitters. With the emergence of lead-halide perovskites, the necessity for a sustainable material and process strategy is arising. Recycling and closed cycle processes gain importance, especially in high priced segments using rare or precious raw materials.

Attempts to develop “green” perovskite compounds are currently concentrating on “lead-free” halide perovskites, with the Sn based perovskites as one leading structure. An alternative approach are double-cation perovskites AIMIMIIIX6, where A is an alkali cation, X is a halide, and a couple of MI+MIII represents an isovalent substitution of two PbII cations in the lead-halide (APbX3)2 perovskite structure. The double lead-free compounds combine the variability of A and X sites typical for APbX3 compounds with new possibilities of independent variation of both MI and MIII sites from corresponding pools of MI = Na+, K+, Ag+, Tl+, Au+, etc. and MIII = In3+, Bi3+, Sb3+, Fe3+, Au3+, etc. Among the double halide perovskites, compounds based on Bi and In with a general formula Cs2AgxNa1-xBiyIn1-yCl6 (or CANBIC by first letters of elements) have started to occupy an outstanding position due to the combination of compositional variability, stability, opportunities for sustainable material combinations and promising photoluminescence quantum yields. Along with variable parameters x and y, both Cs and Cl sites can also be varied and potentially substituted with analogs (for example, Cs for Rb or methylammonium, Cl for Br and I).

Recently, we reported on a relatively “green” single-step synthesis of luminescent CANBIC perovskites in 2-propanol:water mixtures and were able to demonstrate a semi-automatic synthesis on a robot-based pipetting platform. That opens the opportunity to systematically the huge parameter space of lead-free, multinary metal, cation and halide double perovskites is a systematic way. First data libraries are presented and characterized according to their optical properties. The talk will be concluded with an outlook, whether the synthesis platform can be made compatible for a Bayesian optimization process looking for the most promising optoelectronic lead-free perovskites.

A great challenge in today’s research on perovskites is finding stable, lead-free alternatives for photovoltaic applications. One interesting route is the replacement of divalent lead with a combination of monovalent and trivalent cations, as in Cs2AgBiBr6. Despite showing higher stability, the photovoltaic performance of Cs2AgBiBr6 based devices is inferior to lead perovskite based devices. The indirect bandgap, high exciton binding energies, and presence of trap states have been discussed as limiting factors of the power conversion efficiency. This work aims to provide new insight into the interplay of excitons and trap states on the charge carrier dynamics in Cs2AgBiBr6 thin films. To do so, we investigated Cs2AgBiBr6 thin films and bilayers of Cs2AgBiBr6 with a transport layer (TL) by means of time-resolved photoluminescence (TRPL), transient absorption (TA) measurements and time-resolved microwave conductivity (TRMC). In addition, an entirely novel technique i.e. double pulse excitation time-resolved microwave conductivity (DPE-TRMC) has been developed. Basically, in DPE-TRMC the sample gets illuminated by two laser pulses arriving with a short time delay. By comparing the photoconductance traces induced by the second pulse in presence and absence of the first pulse, we are able to examine the effect of the long-lived species on the charge carrier dynamics. For these experiments we used identical excitation wavelengths for both laser pulses but varying intensities and delay times. We modelled the results introducing a comprehensive model, which accounts for the free carrier generation yield, localization of free carriers, electron trapping by color centers, and shallow trap states for holes. The iterative analysis of the DPE-TRMC experiments with different intensities and delay times reveals the presence of a high concentration of both electron (1015 cm-3) and hole (1016 cm-3) trap states. Slow (partially) radiative decay of trapped electrons with their countercharges is in agreement with the TRPL measurements. Holes are rapidly trapped in shallow states, from where they get slowly released explaining the complementary TA measurements. These results are compared to results obtained on alloys of Cs2Ag(Bi1-xSbx)Br6. Finally, we also looked into the charge collection in double layers of Cs2AgBiBr6 and various transport layers including TiO2 and C60. In our opinion, the long-lived charge carriers in Cs2AgBiBr6 as observed in this work can offer suitable, lead-free alternatives for many other applications, including X-ray detectors or photocatalysis.

Replacing toxic Pb²⁺ cations in halide perovskites is one of the major challenges to make this technology viable for its widespread implementation. A lot of effort has been, and still is, focused on finding alternative B²⁺ cations such as Sn²⁺ or combinations of B⁺ and B³⁺ cations such as in Cs₂Ag⁺Bi³⁺X₆ double perovskites.

Nevertheless, there exists another option that has been so far mostly overlooked which consists in ABX₃ compounds that are isostructural to perovskites but with opposite charges: A^-B^2-X⁺₃. These compounds are named antiperovskites and several of them have been already demonstrated experimentally.

Here, I will focus on Ag₃SX (X=I or Br), which can also be written as XSAg₃ to highlight the analogy with perovskites. I will show how these antiperovskites can be synthesized by simple and dry methods based on mechanochemistry and moderate thermal annealing. Based on the nature of the X halide, the optical absorption within the visible range can be tuned which makes these semiconductors especially relevant for photovoltaics.

Thin films have also been deposited by pulsed laser deposition, paving the way to the implementation of these antiperovskites in solar cells.

Pulsed Laser Deposition (PLD) has offered unique options for the development of complex materials thin film growth, allowing stoichiometric transfer and multi-compound deposition independent of the relative volatility of the elements, and ultimate control of interfaces. In the field of complex perovskite oxides, PLD opened the way to high-Tc superconducting films requiring stoichiometric transfer of multiple (4 to 5) cations. In this presentation we discuss the rather unexplored but huge potential of PLD for the controlled formation and study of single and double halide perovskite thin films. More specifically we present the unique single-source deposition of multi-compound halide perovskites in-vacuum.

We demonstrate PLD of a variety of compounds from CsSnI₃, Cs₂SnI₆ and Cs₂AgBiI₆ as well as Pb-based compounds. The critical role of the PLD target quality, formed by mechano-synthesis of the individual precursors and pressed into a single solid pellet will be discussed. Moreover, the control of the PLD parameters to achieve near-stoichiometric transfer from a single source target to high quality films presenting the photoactive phases will be demonstrated. This work represents an important step forward in the development of controlled growth and future scalability of halide perovskites for efficient optoelectronic devices.

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 25%. However, challenges and opportunities remain relating to material microstructure, ionic migration and toxicity. While tin halide perovskites offer lead-free alternatives to the currently best-performing lead halide perovskites, their prevalence towards tin vacancy formation and oxidation makes these materials particularly unstable [1].

We have recently investigated ultrafast charge-carrier dynamics in lead-free silver-bismuth semiconductors[2-4] which promise lower toxicity and potentially higher barriers against ion migration than their more prominent lead-halide counterparts. We examined the evolution of photoexcited charge carriers in the double perovskite Cs₂AgBiBr₆ using a combination of temperature-dependent photoluminescence, absorption and optical pump−terahertz probe spectroscopy.[2] We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm²V⁻¹s⁻¹ at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge. Overall, our observations suggest that strong electron−phonon coupling in this material induces rapid charge-carrier localization which may inhibit the use of this material as an efficient light harvester in photovoltaic devices.

We further demonstrate the novel lead-free semiconductor Cu₂AgBiI₆ which exhibits several advantages over Cs₂AgBiBr₆, namely a low exciton binding energy of ~29 meV and a lower and direct band gap near 2.1 eV,[3,4,5] making it a significantly more attractive lead-free material for photovoltaic applications. However, charge carriers in Cu₂AgBiI₆ are found to exhibit similarly strong charge-lattice coupling strength[3] to that in Cs₂AgBiBr₆, suggesting a link with the presence of AgBi. Tuning such charge-lattice interactions therefore emerges as a serious challenge for this class of materials.

The RSC recently launched Energy Advances, EES Catalysis and RSC Sustainability – three new journals themed around energy & sustainability, as part of our ongoing commitment to support the chemical sciences in facing up to global sustainability challenges. Come along and learn how these journals will accelerate progress to achieving the UN Sustainable Development Goals (SDGs) by 2030.

The RSC recently launched Energy Advances, EES Catalysis and RSC Sustainability – three new journals themed around energy & sustainability, as part of our ongoing commitment to support the chemical sciences in facing up to global sustainability challenges. Come along and learn how these journals will accelerate progress to achieving the UN Sustainable Development Goals (SDGs) by 2030.

The RSC recently launched Energy Advances, EES Catalysis and RSC Sustainability – three new journals themed around energy & sustainability, as part of our ongoing commitment to support the chemical sciences in facing up to global sustainability challenges. Come along and learn how these journals will accelerate progress to achieving the UN Sustainable Development Goals (SDGs) by 2030.

Silver-bismuth double perovskites (Ag-Bi DPs) are promising candidates for less toxic and highly stable metal halide perovskites, but their optoelectronic performances still lag behind those of the lead halide counterpart, due to the indirect nature of the bandgap and the strong electron-phonon coupling.[1] To overcome these limitations and enable use in photovoltaics and in other applications beyond photovoltaics,[2] researchers are investigating a large plethora of structural variations on these compounds, which might improve their properties and even reveal the emergence of unexected ones. 

In this contribution, we will report on our recent efforts addressed at the tuning of the Ag-Bi DP scaffold optical properties, through the doping with lanthanide ions [3] and the variation of the dimensionality into 2D monolayer species.[4,5] We will also show some very recent results on the preparation of highly sustainable photovoltaic devices suitable for indoor applications such as those connected to the powering of the Internet of Things ecosystem,[6] based on Ag-Bi DP photoactive layers and carbon electrodes obtained from the recovery of waste (submitted work).

References

[1] Z. Jin, Z. Zhang, J. Xiu, H. Song, T. Gatti, Z. He "A critical review on Bismuth and Antimony-based halide perovskites for photovoltaics and optoelectronics" J. Mater. Chem. A 8, 16166-16188 (2020).

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