A plethora of new semiconductors have recently emerged as versatile materials for solar cells and photocatalytic applications. Combinatorial analytical probes have played a pivotal role in uncovering the mechanisms underpinning light-harvesting performance even before device optimisation has been attempted. Ultrafast optical probes of photoconductivity dynamics are particularly useful here, uncovering the generation, localisation and ultimate recombination of charge carriers following photon absorption.

Probing charge-carrier motion in highly anisotropic semiconductors poses particular challenges. We show how such charge transport can be probed successfully in layered, two-dimensional metal halide perovskites (2DPs),^[1,2,3,4] whose electronic landscape is moderated through quantum confinement. We examine the effects of the high anisotropy of transport in thin films comprising layers that are highly oriented either parallel or perpendicular to the substrate plane.^[1] We further utilise a powerful technique to assess the degree of transport anisotropy in thin films of 2DPs,^[2] based on time-dependent photon reabsorption effects^[2] and THz conductivity probes ^[3]. We show that in (PEA)₂PbI₄ films, time-dependent diffusion coefficients arise from minute misalignment of 2DPs planes occurring at distances far from the substrate, where efficient in-plane transport consequently overshadows the less efficient out-of-plane transport in the direction perpendicular to the substrate. We extract a low out-of-plane excitation diffusion coefficient of 0.26x10⁻⁴ cm² s⁻¹, consistent with a diffusion anisotropy of about 4 orders of magnitude. We further demonstrate the effect of spacer cation length on the electronic and optical properties of lead-iodide-based 2DPs, using alkylammonium cations of varying chain lengths, revealing pronounced odd-even effects on transport anisotropies.^[3]

We further report on charge-carrier conduction in new bismuth-halide based semiconductors.^[5,6,7] We show that such materials exhibit dynamic transitions from large to small polaronic states that dominate the dynamics of charge carriers, and discuss how such transitions are affected by lattice softness in (AgI)_x(BiI₃)_y Rudorffites.^[5] In addition, we examine the quality of the electronic interfaces formed between Cu₂AgBiI₆ (CABI) and commonly used charge transport layers.^[6] We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO₂, induce the formation of unintended impurity phases within the CuI−AgI−BiI₃ solid solution space, significantly influencing structural and optoelectronic properties.^[6]

The rapid advancement in emerging optoelectronic technologies demands highly efficient, affordable, and ecofriendly materials. In this context, ternary chalcogenides, especially ternary selenides, show early promise as a material class due to their stability and remarkable electronic, optical, and transport properties. Herein, we integrate first-principles-based high-throughput computations with machine learning (ML) techniques to predict the thermodynamic stability and optoelectronic properties of 920 valency-satisfied selenide compounds. Through investigating polymorphism, our study reveals the edge-sharing orthorhombic Pnma phase (NH₄CdCl₃-type) as the most stable structure for most ternary selenides. High-fidelity supervised ML models are trained and tested to accelerate stability and band gap predictions. These data-driven models pin down the most influential features that dominantly control key material characteristics. The multistep high-throughput computations identify the ternary selenides with optimal direct band gaps, light carrier masses, and strong optical absorption edges. The extensive materials screening considering phase stability, toxicity, and defect tolerance, finally identifies the seven most suitable candidates for photovoltaic applications. Two of these final compounds, SrZrSe₃ and SrHfSe₃, have already been synthesized in a single-phase form, with the latter showing an optically suitable band gap, aligning well with our findings. The non-adiabatic molecular dynamics reveal sufficiently long photoexcited charge carrier lifetimes (on the order of nanoseconds) in some of these selected selenide materials, indicating their exciting characteristics. Overall, our study suggests a robust in silico framework that can be extended to screen large datasets of various material classes for identifying promising photoactive candidates.[1]

Quasi two-dimensional (2D) perovskites have garnered significant research attention due to their promising optoelectronic properties such as high emission quantum yields and absorption coefficients. Their structural diversity makes them suitable for an array of optoelectronic applications such as light emitting diodes, field effect transistors, and solar cells. The structure of quasi-2D perovskites feature layers of perovskite octahedra intercalated along the {001} direction by bulky organic spacer cations such as n-butylammonium. These cations increase the hydrophobicity and lattice rigidity of the material, resulting in improved operational stability over their three-dimensional perovskite counterparts - a key consideration for their commercial viability in solar cells. 

Despite these benefits, the layered structure of quasi-2D perovskites leads to reduced out of plane charge mobility, hindering their efficiency in solar cells. The use of additives to improve charge transport has been shown to improve the power conversion efficiency of these solar cells. Typically, thiocyanate (SCN-) additives are thought to induce the vertical orientation of 2D perovskite layers, thereby improving performance by enabling charges to be transported between electrodes along the in-plane perovskite layer, where the mobility is much greater. However, the understanding of this process is limited, and as such we are restricted in the identification of potential additives and their use across different perovskites. 

In this presentation I will discuss the structural effect of ammonium thiocyanate (NH4SCN) in quasi-2D perovskites, focussing on the formation and distribution of 2D and 3D perovskite domains in thin films. I will show our studies correlating spectroscopic and morphological measurements with structural properties, and demonstrating resultant improvements in photovoltaic performance. Here, we demonstrate that rather than inducing vertical orientation, the addition of NH4SCN enables the 2D perovskite to act as a scaffold, templating the growth of highly crystalline MAPbI3. These results expand our understanding of the role of additives in the fabrication of high-performance quasi-2D perovskite solar cells. Finally, I will show that the effect of NH4SCN differs with perovskite composition, in particular highlighting the pivotal role of the spacer cation on the choice of additive. We believe that this work provides important insights for improving charge transport in quasi-2D perovskites, enabling highly stable and efficient perovskite photovoltaics. 

Metal-halide perovskites have emerged as a versatile family of materials for both light-harvesting and light-emission applications. Their hybrid organic–inorganic soft lattices give rise to complex electron–lattice interactions that strongly influence charge transport and recombination. Among these interactions, the formation of self-trapped electronic states—driven by strong electron–phonon coupling and local lattice distortions—plays a critical yet not fully understood role in governing the optoelectronic behavior of many perovskite systems.

In this talk, I will introduce our recent efforts to understand and manipulate self-trapped states across perovskites with dimensionalities ranging from 0D to 3D. By integrating temperature-dependent photoluminescence, transient absorption spectroscopy, optical-pump–THz-probe measurements, and DFT calculations, we reveal that charge carriers in the double perovskite Cs₂AgBiBr₆ are rapidly localized by lattice distortion within ~4 ps, causing a ~70% drop in photoconductivity and fundamentally limiting its photovoltaic potential. We further show that dynamic self-trapped excitons may underlie the unusually high energy gain observed in fluorescence upconversion in the 2D perovskite (PEA)₂PbI₄. Additionally, we demonstrate that the emission energy of a zero-dimensional perovskite, (TBA)Sb₂Cl₇, can be effectively tuned by controlling structural disorder in its glassy phase.

Together, these results highlight the pivotal role of electron–lattice interactions in determining the optoelectronic properties of metal-halide perovskites and point toward new design principles for next-generation halide materials.

Phosphosulfides are an intriguing family of semiconductors with hardly any history in PV or optoelectronics in general. They are obtained by combining phosphorus with sulfur and one or more metals. Unlike the well-known III-V semiconductors, phosphosulfides incorporate phosphorus in the +5 (rather than -3) oxidation state [1].

We have been studying these exotic semiconductors with an integrated experimental/computational work strategy inspired by the FAIR data principles. Backed by a unique suite of combinatorial thin-film deposition setups with access to S and P sources, we have explored the Cu-P-S, Ag-P-S, Sb-P-S, Sn-P-S, and Ba-P-S phase diagrams by high-throughput experiments and we can now report the first thin-film synthesis of various compounds. Most of these films are single-phase, stable in air, and semiconducting. In spite of unpassivated surfaces and lack of process optimization, some of these semiconductors already exhibit carrier lifetimes above 10 ns and photoluminescence quantum yields above 0.001% [2].

In the spirit of a perovskite-oriented symposium, I will also present a new growth route for the emerging BaZrS3 sulfide perovskite absorber, allowing to crystallize the material at device-compatible temperatures while retaining the photoluminescence and photoconductivity properties of films crystallized at much higher temperatures.

To objectively assess the quality of these early-stage PV materials at their current development stage, I will finally discuss a recently proposed figure of merit for solar absorbers [3,4].

The efficiency of Si single-junction photovoltaic (PV) cells has increased steadily in recent years, and they now demonstrate power conversion efficiencies (PCEs) remarkably close to their efficiency limit.[1,2] Metal halide perovskite materials have emerged as a strong candidate for use in next-generation PV cells, due to their tunable bandgap and ability to be processed on top of Si cells, and have displayed a rapid rise in performance.[3,4] These developments have given rise to high-performing perovskite-on-silicon tandem cells and these tandem cells now perform more efficiently than their individual counterparts.[5] Multi-junction architectures enable PV devices which can achieve significantly increased PCEs due to more efficient energy capture, since these architectures are comprised of different ‘sub-cells’ employing complimentary bandgaps.[6–8] As such, progress in the area of tandem cells has inspired advancements in the area of triple-junction PVs.[9,10] However, these advanced multi-junction architectures are yet to take advantage of the infrared (IR) portion of the AM1.5 spectrum, as efforts have focussed on processing on top of the rear silicon sub-cell.[7] Here, we aim to address this shortcoming through the rational design of IR absorbing materials. In this work, we fabricate mixed lead-tin sulfide thin films by sulfurising lead-tin halide films using hydrogen sulfide gas. We observe complete conversion of the neat halide films over a range of compositions and fit the X-ray diffraction data to known Pb-Sn-S phases. The bandgaps over the range of compositions are measured to be < 1 eV and the optoelectronic properties of the sulfide films are characterised.

Zintl phosphides are emerging as an alternative class of inorganic solar absorbers that challenge conventional high-performance paradigms. A key differentiator of these materials is mixed bonding motifs and polyanionic units that depart from conventional II-VI , III-V, and perovskite chemistries. This family has progressively demonstrated unusually long non-radiative carrier lifetimes for inorganic materials, favorable defect physics, and phase stability across multiple synthesis formats. Our initial work on BaCd₂P₂ established nanosecond-scale recombination dynamics and strong thermal stability in powders. We followed our work on BaCd₂P₂ powders with the synthesis of quantum dots exhibiting a 21% photoluminescence quantum yield. Continued efforts produced CaZn₂P₂ thin films at low-temperature with ~30 ns carrier lifetimes (by TRMC), demonstrating translation to device-relevant geometries. Our recent studies on SrZn₂P₂ thin films reveal that halide-assisted post-annealing, similar to CdTe’s CdCl₂ treatment, improves grain structure and PL intensity, likely through grain-boundary passivation.

ZnP₂ represents the most exciting member of this family. Across single-crystal, powder, and thin-film forms, we observe carrier lifetimes in the hundreds-of-nanoseconds range—an exceptional result among inorganic absorbers and an order(s)-of-magnitude improvement over Zn₃P₂ and similar phosphides. These efforts identify polyphosphide motifs and shallow-defect formation as central to the suppressed non-radiative recombination channels and highlight a broader chemical design space for high-performance semiconductors.

Collectively, these results show that Zintl-phase phosphides constitute a new absorber platform with earth-abundant elements, scalable thin-film growth, and inherently long lifetimes without relying on hybrid chemistries. This work expands the conceptual boundaries of inorganic photovoltaic materials and points toward a wider class of polyanionic and mixed-bond semiconductors with tunable electronic structure, strong absorption, and promising optoelectronic response.

Pnictogen-based perovskite-inspired materials (PIMs) incorporating Bi and Sb have emerged as promising low-toxicity alternatives to lead halide perovskites, combining environmental stability with rich structural and electronic tunability. In this talk, I will present how we combine compositional and crystal-chemistry design with thin-film processing and device engineering to realize efficient photovoltaics from this family of compounds, and how the same structural motifs also enable emerging functional responses.

I will first focus on Bi- and Sb-based systems such as vacancy-ordered compositions and Cu₂AgBiI₆, showing how compositional engineering and dimensionality tuning can transform quasi-0D semiconductors into more quasi-2D-like absorbers with improved charge transport and competitive outdoor/indoor PV performance[1–3]. I will then discuss our recent outcome in hybrid Sb–Bi halide architectures, where tailored compositions and interfacial control allow perovskite-inspired devices to surpass the 10% efficiency threshold under 1000 lx[4]. Throughout, I will highlight structure–property relationships that link pnictogen–halide connectivity and local structural distortion (bond-angle variance, octahedral distortion) to performance gains, including improved operational stability[5,6].

In the final part, I will show how the same local-structure degrees of freedom—particularly inversion-symmetry breaking and polar nanoregions driven by cation vacancies and compositional tuning—enable emerging functionalities in Bi PIMs. Using Cu–(Ag)–Bi–I systems as model materials, I will demonstrate how nonlinear optical microscopy reveals robust second-harmonic generation, with a stronger response in the vacancy-rich Cu-based composition[7], and point towards multifunctional optoelectronic behavior in pnictogen-based PIMs beyond conventional ABX₃ halide perovskites.

Suboptimal charge-carrier transport remains a major bottleneck for advancing efficient perovskite-inspired material (PIM) solar cells. Even in defect-tolerant systems such as metal chalcohalides, where deep traps are less detrimental, intrinsic charge-carrier localisation can still strongly limit transport. Understanding this localisation process and learning how to suppress it is therefore key for progress in PIMs. Mixed-metal chalcohalides (A₂BCh₂X₃) have recently emerged as promising candidates, offering enhanced chemical stability alongside favourable defect-tolerant optoelectronic properties, with Sn₂SbS₂I₃ showing the highest recorded PCE for this material class. [1]

We will show how charge-carrier localisation can be mitigated in this material family and explain how these behaviours arise through a structural–optoelectronic relationship. By substituting the M(II) cation (Pb, Sn), we tune the lattice from the lower-symmetry and more electronically confined P2₁/c structure in Pb₂SbS₂I₃ to the higher-symmetry Cmcm structure in Sn₂SbS₂I₃, which supports greater electronic dimensionality. This increase in symmetry has pronounced consequences for charge transport: Pb₂SbS₂I₃ exhibits a higher initial mobility (µ_deloc = 4.7 cm²/Vs, measured by optical-pump terahertz-probe) but undergoes ultrafast localisation within a few picoseconds. In contrast, Sn₂SbS₂I₃, despite its larger static lattice distortions, shows lower initial mobility (µ = 2.51 cm²/Vs) yet sustains photoconductivity on nanosecond timescales.

The higher electronic dimensionality in the Sn analogue, combined with the reported high-Z ns² electronic contribution, mitigates defect-mediated localisation. These results demonstrate that lower symmetry and reduced electronic dimensionality promote rapid localisation, whereas higher symmetry and greater dimensionality can sustain transport. [2] This provides a clear pathway for compositionally tuning and ultimately overcoming charge-carrier localisation in mixed-metal chalcohalide PIMs.

Since early on in the success story of lead halide perovskites, the element of bismuth has been discussed as a benign, non-toxic replacement for lead, as the stable oxidation states of both elements have the same 6s² electronic configuration.[1,2] While this leads to a similar chemical behaviour of the elements and similar properties of their compounds, the different charges of the ions prevent direct substitution within the perovskite structure. The perovskite motif can be preserved using an additional metal ion for charge compensation, leading to the double-perovskite or elpasolite structure, but the resulting compounds so far have seen little success in photovoltaic applications.[3] Beyond the restrictions of the perovskite motif, however, hybrid organic-inorganic halide bismuthates display a huge variety of compounds with a multitude of properties, which often show a similar tunability as lead halide perovskites.[4] When preparing halide bismuthates, the choice of organic cation is crucial, as it steers the formation of the inorganic motif and can also add additional functionality to the product. Many different crystal structures have been reported, but structure-property relationships remain poorly understood, especially for compounds containing heterometals.[5,6] We aim to investigate these relationships to better understand this versatile class of compounds to pave the way for future applications as functional materials. The talk will give an introduction into hybrid organic-inorganic halide bismuthates and highlight recent results regarding the structure-property relationships on chosen examples.