The toxicity and bioavailability of lead (Pb) in halide perovskites motivates the search of Pb-free perovskite-inspired materials (PIMs), which would replicate the great optoelectronic properties of the traditional Pb-based counterparts. PIMs employing pnictogen cations from Group VA of the periodic table (e.g., antimony (III) (Sb³⁺) and bismuth (III) (Bi³⁺)) have recently emerged as low-toxicity alternatives for Pb-based halide perovskites, with applications in photovoltaics and beyond [1]. In particular, Bi-based PIMs are interesting because can lead to shallow traps and high defect tolerance in some compositions, while being eco-friendly. This field of research is, however, still in its early infancy with a growing number of efforts to improve the device efficiencies and lifetimes, with the majority of the studies focusing on the shelf-life rather than the operational stability. 

In this talk, I will summarize key recent examples of bismuth-based PIMs studied by us for a range of applications, from photovoltaics for outdoor and indoor, non-linear optics, photocatalysis. In particular, I will summarize our recent findings on CU₂AgBiI₆ PIM and the compositional engineering efforts to improve its morphology and charge carrier transport [2,3]. I will also highlight novel Bi-based compositions for solar cells and indoor photovoltaics.

Solar cells are among the most developed technologies in the renewable energy sector, not only academically, but also commercially. In this context, thin-film photovoltaics (based on emerging absorbers like perovskites, Sb₂Se₃, CZTS etc.) provide an excellent opportunity to expand the reach of solar energy production, thanks to the promising efficiencies combined with light-weight and flexibility. However, for many of these emerging materials, moderate/high temperature annealing (150-300 °C) is required, and the necessary temperature often limits the usable substrates. Photonic curing (also known as flash annealing) could represent a way to overcome this limitation and really unlock the potential of thin film photovoltaics. In this technique, heat is created by the absorption of strong and ultrarapid light flashes that permits to reach very high temperature on the top layer while keeping cold the bottom of the substrate. While this technique has widely been used in silicon,[1] very few reports are present for these emerging materials (mainly on perovskites), [2-4], and fundamental investigation is still missing.

In this talk I will present the most recent results collected on the use of photonic curing on different thin film absorbers, analyzing the structural, morphological and optoelectronic properties and comparing it with traditional thermal annealing.

Perovskite solar cells in which methylammonium lead iodide (MAPbI₃) is used as a solar absorber material, have reached maturity in the last years owing to a concerted effort to optimize material synthesis, stability, and device architectures and performance. However, the halide perovskite family features thousands of stable members beyond MAPbI₃, with broad compositional tunabilty and a wide range of optoelectronic properties. With this talk, I will provide an overview of our current understanding of the electronic and excited-state structure of several classes of perovskites and perovskite-like structures beyond ABX₃ featuring double perovskites, extended perovskites, and chalcohalide materials. Our first-principles calculations based on the GW and Bethe-Salpeter Equation approaches, allow us to map the complex landscape of electronic properties and excitons, understand the impact of chemical and structural heterogeneity, dimensionality, and temperature effects, and provide chemically intuitive rules for when to trust canonical textbook models for excitons in these materials.

Metal halide semiconductors have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 26%, however, these record efficiencies have all relied on incorporation of lead as the metal. The search for less toxic ingredients has led to the emergence of a plethora of new bismuth-based semiconductors, including bismuth halides and chalcogenides. Power conversion efficiencies around 6% have been realised for such materials, triggering new research efforts to explore and eliminate current limitations to performance.

Here, we show that an ultrafast charge-carrier self-trapping process limits long-range charge-carrier transport in most bismuth-based semiconductors.[1-7] We have 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.[1] 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 self-trapped small polaronic state. Alloying Cs₂AgBiBr₆ with Cs₂AgSbBr₆ on the trivalent metal site interestingly leads to significantly stronger self-localisation,[2] which we attribute to self-localised charge carriers probing the energetic landscape more locally thus turning an alloy’s low-energy sites (here, Sb sites) into traps, which dramatically deteriorates transport properties. We further demonstrate the novel lead-free semiconductor Cu₂AgBiI₆ which exhibits a low exciton binding energy of ~29 meV and a lower and direct band gap near 2.1 eV,[3,4] 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 interactions[4,5]. Further work examining five compositions along the AgBiI₄–CuI solid solution line (stoichiometry Cu_4x(AgBi)_1−xI₄) shows that increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities, as well as partly mitigating the extent of such ultrafast self-localisation.[5] Interestingly, we show that thin films of BiOI lack of such self-trapping, with good charge-carrier mobility maintained over longer time scales, reaching ∼3 cm² V^–1 s^–1 at 295 K and increasing gradually to ∼13 cm² V^–1 s^–1 at 5 K, indicative of prevailing bandlike transport.[6] Finally, we examine thin films of AgBiS₂ nanocrystals as a function of Ag and Bi cation-ordering,[7] which is modified via thermal-annealing. We show that homogeneous cation disorder reduces charge-carrier localization, most likely because cation-disorder engineering flattens the disordered electronic landscape, removing tail states that would otherwise exacerbate Anderson localization of small polaronic states.[7]

Overall, self-trapping of charge carriers therefore emerges as a clear challenge for this class of materials. Our findings explore the parameter space governing such self-localization, highlighting the effects of local energetic disorder as an exacerbating factor that may pose new challenges to alloying strategies. In addition, or findings show that cation-disorder engineering may partly mitigate such effects through flattening of the local energy landscape.

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant and non-toxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth (Bi)-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping). It has been suggested that self-trapping in Cs₂AgBiBr₆ originates from strong lattice deformation potential imparted by bismuth along with the presence of soft silver-halide bonds. Given that self-trapping places a fundamental limit on the performance of materials, investigating the influence of chemical composition on the charge-carrier dynamics is highly topical.

This work aims to understand whether the presence of Bi or Ag-halide bonds is in fact crucial to the emergence of self-trapped states. This was done through a dual study of charge-carrier dynamics in BiOI and (AgI)_x(BiI₃)_y thin films. Our investigation reveals that despite possessing a low and strong electron-phonon coupling, there is no ultrafast localisation in BiOI. Instead, by unravelling the early and long-time charge-carrier dynamics in BiOI, we find that the material performance is limited by the presence of multi-phonon emission mediated non-radiative channels in the material.

On the other hand, ultrafast localisation is persistent across both mixed Ag-Bi iodides and BiI₃. Thus, the presence of Bi and/or Ag-halide bonds alone cannot account for self-trapping in these materials. We find that a delicate interplay between chemical composition and crystal and band structures determine the charge-carrier dynamics in (AgI)_x(BiI₃)_y. Overall, our dual study addresses crucial gaps in understanding the limitations of Bi-based PIMs and educate future material design.  

Halide double perovskites, such as Cs₂AgBiX₆ (X = Br, Cl), are emerging semiconductors for optoelectronic applications due to their lower toxicity.^[1] Although exhibiting an indirect bandgap, colloidal Cs₂AgBiCl₆ nanocrystals (NCs) show bright dual-peak photoluminescence spectra.^[2] However, the origin of both emission bands is still under debate. Here, we show that silver (Ag) plays a crucial role in both emissions of these double perovskite NCs. The trapping of holes in Ag vacancies leads to a spatial localization of the hole wave function on the scale of the lattice constant. This provides k-values for the hole wave function at all boundaries of the Brillouin zone, resulting in localized bound excitons with a red emission at 650 nm with an activated temperature dependence. The blue emission (425 nm) stems from the lecithin ligands of the NCs spectrally overlapping with the plasmon resonance of the surface-attached Ag nanoclusters. This work is of importance for the surface chemistry of Cs₂AgBiCl₆ double perovskites and their optoelectronic applications.

Metal halide perovskites emerged as outstanding materials for optoelectronics due to their excellent optoelectronic properties, such as direct and tunable band gaps, large absorption cross sections and long lifetimes and diffusion paths of the charge carriers.[1] So far, the field has been dominated by lead-based perovskites, but issues concerning lead toxicity and stability have led to explore new possible perovskite candidates. In this context tin and double perovskites of different dimensionalities have attracted increasing interest.[2,3] The optoelectronic features, as well as the efficiencies of these materials in optoelectronic devices, is largely influenced by the nature of the chemical bond, the defects activity and the coupling of photogenerated charges with the lattice. 

In this presentation a theoretical perspective is provided about the influence of chemical composition and dimensionality on the defect activity and the charge carrier photophysics in perovskites beyond the APbX3 composition, by focusing on less-toxic Sn and Bi/Ag double perovskites. The defect chemistry and photophysics of tin and double perovskites will be discussed, by keeping a parallelism between 3D and 2D, i.e. MASnI3 vs PEA2SnI4 and Cs2AgBiBr6 vs (BA)4AgBiBr8 phases, as test cases.[4,5] The analysis aims to highlight the effects of chemical composition and quantum confinement (QC) on several key properties of these materials. The origin of the self p-doping, strongly limiting the efficiency of tin perovskites, will be discussed, as well as computationally designed doping strategies aimed to reduce it. Hence, discussion will move to analyze the charge carrier photophysics in tin and double perovskites, by focusing on the  processes possibly originating the sub-gap emissive features in these materials. Specifically, the competition between the exciton self-trapping and trapping/emission at defect centers vs the chemical composition will be discussed. 

This contribution aims to provide a theoretical framework guiding experimentalists in the design of stable and efficient lead-free perovskite materials. 

In this talk I will discuss recent progress in our group on the development of a new class of thin film solar cells employing Silver Bismuth Sulfide as an emerging absorber for solution processed eco-friendly solar cells. I will first introduce our first report on AgBiS2 colloidal nanocrystal solar cells reporting power conversion efficiency of ~6% [1]. Then I will discuss on the opportunities of tuning the optical properties of this ternary compound via controlling cation disorder homogenization. We demonstrated that by homogenizing cation disorder in this compound we can drastically increase the absorption coefficient of this material as one with the high absorption amongst the semiconductors considered for photovoltaics. This taken together with advances on the device architecture led us to reach power conversion efficiencies of ~9% albeit using an absorber of only 35 nm [2]. In the last part of my talk I will describe our initial efforts on developing AgBiS2 nanocrystal inks and their use with environmentally friendly solvents that led us to achieve efficiencies in excess of 7% [3]. I will conclude my talk with our most recent findings towards an improved passivation strategy of AgBiS2 nanocrystal inks along with the formation of a double heterojunction in the device stack that led to power conversion efficiencies in excess of 10% with Voc of 0.5V, FF of 0.75 and Jsc of 28 mA/cm2.

[1] Solution-processed solar cells based on environmentally friendly AgBiS₂ nanocrystals, M Bernechea, N Cates, G Xercavins, D So, A Stavrinadis, G Konstantatos, Nature Photonics 10 (8), 521-525, 2016

[2] Cation disorder engineering yields AgBiS₂ nanocrystals with enhanced optical absorption for efficient ultrathin solar cells, Y Wang, SR Kavanagh, I Burgués-Ceballos, A Walsh, DO Scanlon, G Konstantatos, Nature Photonics 16 (3), 235-241, 2022

[3] Environmentally Friendly AgBiS₂ Nanocrystal Inks for Efficient Solar Cells Employing Green Solvent Processing Y Wang, L Peng, Z Wang, G Konstantatos, Advanced Energy Materials 12 (21), 2200700, 2022

Non-toxic, all-inorganic perovskite light absorbers received great attention in recent years due to their potential to replace lead-containing perovskite materials in photovoltaics. Regardless of the potential of lead-based halide perovskites, their application on large scale is challenging, mainly due to the toxicity of lead and stability issues. Lead-free absorbers, such as the double perovskite Cs₂AgBiBr₆ as a prominent representative, offer high stability and auspicious optoelectronic properties. Cs₂AgBiBr₆ has been studied intensively in the past few years. Various researchers pre-synthesize this absorber as powder by a synthesis method first suggested by Slavney et al. in 2016 [1] and the resulting absorber powder is further dissolved to render a thin film deposition. Although this method is well reproducible, it is only suitable for the lab-scale synthesis of Cs₂AgBiBr₆, with yields usually in the range of a few grams. Besides this rather small yield, the synthesis requires exceptional safety precautions.

Within this work, we present an alternative synthesis approach for Cs₂AgBiBr₆ powder, namely via spray-drying [2]. During spray-drying, the perovskite phase forms by in-situ crystallization upon drying of the atomized precursor solution. The product is compared to the conventionally synthesized absorber. XRD and Raman analyses confirm the product’s phase-purity, whereas UV-Vis spectroscopy confirms the desired bandgap. Furthermore, absorber films in single-junction perovskite solar cells are successfully produced from both absorber powders, clearly demonstrating the suitability of spray-drying as an alternative synthesis method.

Moreover, we investigated the Bi-substitution by Sb to reduce the perovskite’s original bandgap of ~2.2 eV achieving better suitability as top-cell absorber in perovskite-silicon tandem devices [3]. Spray-drying enabled a fast screening and identification of the substitution limit, and could overcome limitations of the conventional synthesis for the Sb-substituted Cs₂AgBiBr₆.

As a side notice, we can tell that we additionally investigated the lead-free perovskites Cs₃Bi₂Br₉ and Cs₃Bi₂I₉. Besides these, lead-based halide perovskites have been synthesized successfully via spray-drying, too. With the prospect of an industrial scale photovoltaic industry, the demand for high-quality perovskite materials will most likely increase in future. Therefore, spray-drying could pave the way for an easy adaptation to increasing production volumes of perovskite powders in general owing to its suitability as screening method and its capability of producing large amounts of powders.

Thin-film PV is a key technology for low cost and low environmental impact solar energy conversion. Metal halide perovskites are leading among thin film PV technologies, but challenges remain regarding toxicity of Pb and stability. Motivated by the search for low cost, non-toxic and Earth abundant materials solutions for thin film PV, new inorganic semiconductors with complex compositions are being explored. One of the challenges with material discovery and new material compositions, is their fabrication in thin film form. Challenges such as volatility incompatibility or solvent incompatibility, hinders progress in either high quality material demonstration or functionality via device integration.

In this presentation, we will discuss how the combination of mechanochemical synthesis and pulsed laser deposition (PLD), allows the exploration of novel Earth abundant and non-toxic semiconductors materials. We will explore the development, growth and understanding of semiconductor materials such as Pb-free double perovskites (e.g. Cs2AgBiBr6), new photoactive light-absorbers such as Ag3SI and p-type transparent electrodes such as S-doped and Cs-doped CuI.

In summary, the presentation will highlight how controlled synthesis and material design can make a significant contribution to the emerging generation of efficient and sustainable thin film-based solar cell devices.

In the field of perovskite solar cells, explorations of new lead-free all-inorganic perovskite materials are of great interest to address the instability and toxicity issues of lead-based hybrid perovskites. Recently, copper-antimony-based double perovskite materials have been reported with ideal band gaps, which possess great potential as absorbers for photovoltaic applications.^1-2 Here, we synthesize Cs₂CuSbCl₆ double perovskite nanocrystals (DPNCs) at ambient conditions by a facile and fast synthesis method, namely, a modified ligand-assisted reprecipitation (LARP) method. We choose methanol as solvent for precursor salts as it is less toxic and easily removed in contrast to widely-used dimethylformamide. Our computational structure search shows that the Cs₂CuSbCl₆ structure containing alternating [CuCl₆]^5- and [SbCl₆]^3- octahedral units is a metastable phase that is 30 meV/atom higher in energy compared to the ground state structure with [CuCl₃]^2- and [SbCl₆]^3- polyhedra. However, this metastable Cs₂CuSbCl₆ double perovskite structure can be stabilized through the solution-based nanocrystal synthesis. Using an anion-exchange method, Cs₂CuSbBr₆ DPNCs are obtained for the first time, featuring a narrow band gap of 0.9 eV. Finally, taking advantage of the solution processability of DPNCs, smooth and dense Cs₂CuSbCl₆ and Cs₂CuSbBr₆ DPNC films are successfully fabricated.

Ultrafast transient spectroscopy is a vital tool to investigate the photophysical processes taking place in energy harvesting materials. In particular, the findings provide valuable insights into the efficiency limiting processes in solar cells. The focus of this talk will be on solution-processed metal-halide perovskite solar cells as they have received immense attention in the field of photovoltaic research due to their outstanding power conversion efficiency, which has surpassed 26% in a relatively short time. Understanding carrier losses at metal halide perovskite/charge transport layer interfaces is a pre-requisite to bring the efficiency closer to the Shockley-Queisser limit. Interfacial recombination can to some extent be accessed through time resolved techniques, however identifying this is often a challenge due to the complexity associated to the interpretation and modelling of the extracted charge carrier transients.

approach is to utilize complementary transient spectroscopic techniques, namely transient absorption spectroscopy and transient photoluminescence spectroscopy, not only to unravel the processes limiting the solar cell’s short circuit density and the open circuit voltage but also to evaluate different charge recombination channels and extraction. Herein, we limit our focus to examine half-stacks comprised of the photoactive layer and the respective hole transport layers. Several different hole transport materials (PTAA, NiOx,,and 4PACz) are adjacent to the photoactive layer with a bandgap of 1.53 eV. We report on challenges faced during performing complementary spectroscopic techniques and interpretation of the extracted transients. Specifically, differentiation between photophysical processes such as charge extraction and interfacial charge recombination.

BiOI is a nontoxic, stable and polar semiconductor, which shows high conversion efficiencies in photocatalytic water splitting. This is due to an ultrafast and effective charge separation also launching coherent phonons after short laser pulse excitation [1]. The coupling of electronic excitations and phonons should also manifest itself in polaronic effects. To gain unambiguous experimental evidence for polaronic effects we have carried out time-resolved photoemission electron microscopy (TR-PEEM) experiments. This technique offers the unique possibility to learn about the dispersion E(k) of the conduction band (CB) and its occupation with electrons as a function of time after pulsed optical excitation. In order to interpret our data it is not sufficient to describe merely the temporal evolution of the electron distributions in a static CB. We observe that the dispersion of the CB changes itself in time. This is in particular monitored around the Gamma point of the CB. This combined temporal change in band-structure and electron distribution can be explained by the formation of a polaronic excitation.

Two-dimensional transition metal carbides (MXenes) are of great interest for a range
of applications in electronics, including solar cells due to their tunable optoelectronic
characteristics, strong metallic conductivity, and attractive solution processability. In this study,
we used photo-conductive atomic force microscopy (pcAFM) to map the local (nanoscale)
photovoltaic performances of the Ti 3 C 2 T x MXene integrated TiO 2 electron transport layer (ETL)
based perovskite solar cells (PSCs) to determine the treatment's impact on the microscopic
charge flow inside the devices. The nanoscale photovoltaic performances of the devices with
MXene integrated ETLs is first studied by using the pcAFM technique. The underlying PV
mechanisms and their localized dependency on the interfacial modification across the layers
must be understood through the investigation of these photoresponses at the nanoscale. The
morphology and photocurrent maps with different applied voltages have been simultaneously
measured with nanoscale resolution from the top surfaces of the devices without back contacts.
Compared to the as-deposited samples, Ti 3 C 2 T x MXene based PSCs show a more uniform and
improved current flow throughout the film. Average local photocurrent for MXene-induced
PSCs is significantly larger than that of as-deposited PSCs at zero applied bias and steadily drops
as positive bias is raised until it reaches the open circuit voltage. Large variations in short-circuit
current were also observed at different locations across the film that appeared identical in
topography images. Our study reveals that MXene-integrated ETLs have the potential to improve
the polycrystalline photovoltaic devices' performance by enhancing the active layers' intrinsic
properties and nanoscale photoconduction.

Metal halide perovskite (MHPs) solar cells represent a promising newcomer in the front of emerging photovoltaic technologies to address the dramatic energy crisis and climate change that we are facing. The exceptional properties of MHPs derive from their hybrid organic-inorganic nature, which allows also for low-cost and straightforward processing. Solar cells containing MHPs as absorbing layer have already achieved a power conversion efficiency of about 25,7 %, close to the efficiency of silicon-based devices. Nevertheless, a major limitation, still preventing the uptake of the technology, is related to the reduced stability of these materials when exposed to operative conditions, namely temperature, light, and moisture. Herein, an effective defect passivation of MHP surfaces is a key strategy to tackle both the stability and the enhancement of solar cell performances. Although many solution-based approaches ¹ have been tested, we propose here an innovative use of plasma, as a solvent-free, scalable, and non-invasive promising strategy to boost MHP solar cells performances ². As benchmark material we used Methylammonium Lead Iodide perovskite; thus we have exposed the surface of polycrystalline thin films made of this material to different plasma conditions implying the variation of power, gas, and treatment time, both for low-pressure (LP) and atmospheric pressure plasmas (APPs). The impact of Ar,  N₂, H₂ and O₂ LPPs on MAPbI₃ optochemical properties and morphology was correlated to the performance of the photovoltaic devices and rationalized by density functional theory calculations³. An interesting improvement in photoluminescence was observed for the Ar and H₂ treated films, while an improvement in PCE was observed only for the Ar treated device. This result was ascribed to the efficient removal of the superficial organic component, revealed through X-ray photoelectron spectroscopy (XPS), following suitable surface passivation by the electron transporting layer deposition. APP treatments, fed with He gas, were tested on the surface of MAPbI_3, too. A milder morphological modification than LPP treatments was observed in this case, but withstanding good surface passivation, as confirmed through the improved photoluminescence intensity, while the effect in terms of photovoltaic devices is still under investigations. Moreover, starting from these encouraging results, new plasma surface processes are now object of evaluation, such as the plasma-deposition of thiophene-like films and the study of LPP treatments applied to tin-based perovskite solar cells, approaches which already show the great versatility and potential of plasma-based techniques.

Recently, inorganic anti-perovskites with the formula X₃AN (X = Ba, Sr, Ca, Mg; A = As, Sb) have been reported to exhibit excellent optoelectronic properties like small carrier effective masses, suitable direct bandgaps, high optical absorption coefficients as well as allowed optical transitions at band edges. These properties can be tuned depending on the X and A site. Extending the composition to quaternary anti-perovskites (X₆AA′N₂) enables the synthetic possibilities for new materials.[1,2]

Herein we report on the ammonothermal synthesis of EA₅Pn₂(NH)₂. The three newly synthesized compounds Ca₅AsSb(NH)₂, Ca₅AsBi(NH)₂ and Sr₅AsBi(NH)₂ crystallize in the tetragonal space group P4/mmm. Their crystal structure was solved and refined by scXRD. Raman spectroscopy was used to further determine structural elements and verify the presence of imide-groups. Further investigations were carried out using powder X-ray diffraction, UV/Vis-spectroscopy, and density functional theory calculations.

The materials exhibit direct bandgaps in the range between 1.90 eV and 1.14 eV. By composition variation of the A-site elements as well as the X-site elements, the effective band gaps can be tuned. Hybrid density functional theory calculations verify the direct nature of the band gap, indicating large band dispersions through the enhanced covalency of the pnictides, benefiting the carrier transport. In summary, these new Imide-anti-perovskite materials exhibit interesting properties as efficient light harvesting materials for single junction solar cells.