The quest for low cost, non-toxic and Earth abundant materials solutions for thin film PV, has sparked significant interest in new inorganic semiconductors with complex compositions. One of the challenges with material discovery and new material compositions, is their fabrication in thin film form. Challenges such as volatility or solvent incompatibility, hinders progress in either high quality material demonstration or functionality via device integration.

To overcome these barriers, we employ mechanochemical synthesis to identify promising halide and chalco-halide powders as candidate materials. These powders serve as targets for thin-film deposition via pulsed laser deposition (PLD), a technique that uniquely enables near-stoichiometric transfer of complex compositions, regardless of elemental volatility. This capability facilitates the fabrication of high-quality thin films for proof-of-concept solar cells, with critical PLD parameters optimized to tailor the properties of complex halide, chalco-halide, and chalcogenide semiconductors.

The versatility of PLD is further leveraged for material discovery through compositional gradient screening on substrates, enabling rapid evaluation via photoluminescence and compositional mapping. The resulting materials, selected based on their band gaps, are explored as photo-absorbers or contact layers, advancing their integration into thin-film solar cell devices. This presentation will highlight strategies for experimental material discovery, thin-film synthesis, and device integration, contributing to the development of the next generation of optoelectronic materials and devices.

References:

https://pubs.acs.org/doi/10.1021/acsenergylett.4c01466

https://doi.org/10.1002/adfm.202316144

https://doi.org/10.1021/acs.chemmater.1c02054

https://doi.org/10.1021/acs.chemmater.3c01349

https://doi.org/10.1016/j.matt.2023.10.003

Bismuth-based semiconductors, including the double perovskite Cs₂AgBiBr₆ but also perovskite-inspired materials such as bismuth oxyhalides, show great promise for sustainable light-energy conversion due to their low toxicity, abundance, and tunable electronic properties. This presentation will explore strategies to enhance the efficiency, stability, and scalability of these materials in photoelectrocatalytic and photovoltaic applications. Methods like automated film production, surface modifications, and heterojunction formation have been employed to improve the performance of BiOI and BiOBr in water splitting and hydrogen evolution reactions. A continuous automated film production method for BiOI photoelectrodes was introduced, significantly improving the reproducibility and efficiency of large-scale production. Surface modifications and heterojunction formation have been explored to optimize PEC performance, with enhanced water oxidation and hydrogen evolution reactions observed. Additionally, the lead-free double perovskite Cs₂AgBiBr₆ was optimized for use in solar cells with improved efficiency through interface engineering and low-cost carbon-based electrodes. These advancements position bismuth-based semiconductors as viable, eco-friendly alternatives for energy conversion technologies.

Chalcogenide perovskites have much to recommend them for photovoltaics (PV). They absorb light strongly, with direct band gap tunable (at least) over the range 1.4 – 1.9 eV. They have limited polymorphism and are stable in air, water, and at high temperature. They are made of Earth-abundant, and (mostly) non-toxic elements, and have isotropic properties. However, there are substantial challenges facing the development of chalcogenide perovskite PV. Synthesis requires aggressive conditions - oxygen-free sulfurization or selenization at high temperature – that severely constrain thin-film growth. The few published reports of transport properties describe n-type material with high electron concentration, undesirable for thin-film PV. Photoluminescence (PL) has been reported, but the quantum yield is often low, and sample-to-sample variability is high. The defects that limit performance are not yet understood, and even less is known about interface and heterojunction design. Clearly, it will be a long road to chalcogenide perovskite PV technology.

I will motivate why, despite these challenges, research on chalcogenide perovskites for PV is worthwhile and exciting. I will then describe our own efforts, which center on the processing and properties of thin films. We have achieved a number of synthesis milestones, including growing thin films of BaZrS₃ and BaZr(S,Se)₃ alloys with tunable band gap, in epitaxial and polycrystalline forms. Selenium alloying can produce films with band gap suitable for single- and dual-junction PV, but the vast majority of synthesis procedures reported to-date focus on pure sulfides. I will discuss our finding of rapid alloying by post-growth selenization of sulfide thin films. This recalls the sulfurization-after-selenization process in CIGS manufacturing, and may make alloy studies more widely accessible. I will also present findings on how variations in cation composition affect crystallization kinetics. These results bolster evidence for BaS₃-liquid-assisted crystal growth, and may be useful for lowering the temperature of thin film synthesis.

I will then discuss our ongoing studies of photoluminescence (PL) and electronic transport. We have previously reported long excited-state PL lifetimes for BaZrS₃ and Ba₃Zr₂S₇, and others have reported band-edge PL even from powder samples. However, PL emission is highly variable, sample-to-sample, and many samples have no measurable band-edge emission. To understand this variability, we carry out a quantitative comparison of temperature-dependent PL of BaZrS₃ and a prototypical halide perovskite, CsPbBr₃. The halide has PL yield between 100 and 10,000 times larger than the chalcogenide. By comparing the vibrational properties of the chalcogenide and the halide, we suggest why defect-assisted recombination may be faster in the chalcogenide. On the other hand, the variability between chalcogenide samples suggests a substantial upside, if the recombination-active defect(s) can be identified and diminished. Our temperature-dependent Hall transport studies find that mobility at room temperature is limited by electron-phonon scattering, even in highly-doped samples; this may be related to our previous finding that chalcogenide perovskites have exceptional dielectric polarizability. At cryogenic temperature, the role of ionized defect scattering varies sample-to-sample. We also find that photoconductive responsivity varies tremendously from sample-to-sample, apparently due to variations in processing that affect the concentration of extended defects. All films are n-type as grown, but with tremendous variability in electron concentration. Studies of post-growth annealing support the hypothesis that the predominant intrinsic shallow donors are sulfur vacancies; we use this understanding to vary electron concentration by over a million-fold.

I will end by highlighting exciting next-steps including alternative methods of thin film deposition to make thicker films at lower temperature, studies of device semi-fabricates including detailed investigation of Mo/BaZrS₃ interfaces, and controlling carrier concentration.

Chalcogenide perovskites are a new emerging group of Pb-free perovskites featuring high environmental stability, direct band gap with extraordinary absorption coefficient, and good carrier transport properties, that can be suitably tailored for photovoltaic applications. Their constituent elements can vary, with the A and B cations having oxidation +2 and +4, respectively, while the anion is a chalcogen, such as S, Se, and Te, with an oxidation state -2. Currently, the most researched chalcogenide perovskite is BaZrS₃, due to its natural abundance of elements and its bandgap, which is favorable for tandem solar cells.

A prerequisite to adapt BaZrS₃ in device architectures is to understand how the complex surface chemistry affects optoelectronic properties of BaZrS₃ thin films. Our research focuses on the study of the BaZrS₃ surface/interfaces, developing methodologies for investigating the interfaces and the band alignment for devices. In our work, the measurements of the surfaces for varied films are achieved by photoelectron spectroscopy (XPS) performed both at in-house laboratories and at synchrotrons, exploiting the variable X-ray energy and the high flux.

In this presentation, I will report the advances of the BaZrS₃ as studied in our research on BaZrS₃ thin films. First, the bulk quality of our samples will be highlighted correlating the bulk properties to the functionality in terms of XRD, XAS and PL [1] . The surface chemistry and electronic structure, specifically orbitally-resolved valence band characteristics in relation to charge-carrier type, will be described as revealed by XPS measurements, differentiating the perovskite peaks from the secondary phases and performing a depth profile analysis of the top few nanometers from the film surface [2]. Our recent highlight includes in situ high-temperature post-annealing in ultra-high vacuum, allowing us to access the perovskite peaks by soft X-ray XPS from the purest film surface achieved so far. Our work is fundamental for unraveling the interfacial properties and the band alignment that directly impact charge transport. The findings will help identifying optimal design parameters for utilization of chalcogenide perovskites in optoelectronic devices, such as photodiodes and solar cells.

Ternary nitrides represent an emerging class of materials with immense potential in solar energy conversion, thermoelectrics, power electronics, coatings, and superconductivity, combining distinctive bonding properties, defect tolerance, and tunable functionalities. [1], [2] However, challenges in synthesis and metastability have limited their exploration compared to oxides. Recent synthetic and computational advances are now opening pathways for their development as next-generation solar materials.

This talk showcases two visible-light-absorbing nitrides that have recently emerged and offer especially interesting optoelectronic properties for solar energy conversion, copper tantalum nitride (CuTaN₂) [3] and zirconium tantalum nitride (ZrTaN₃) [4], with an emphasis on complex physical interactions that define their electronic structures. CuTaN₂ exhibits highly anharmonic structural dynamics, as displayed by phonon calculations and finite-temperature Raman experiments. Ab initio molecular dynamics is used to reveal the microscopic mechanisms of atomic motion, which are linked to macroscopic properties including its negative thermal expansion and temperature-dependent increase in the bandgap, thus emphasizing the critical role of structural dynamics in defining optoelectronic properties. In a second example, ZrTaN₃ thin films synthesized via reactive magnetron co-sputtering are shown to exhibit strong visible light absorption and significant photoelectrochemical activity. Complementary density functional theory calculations reveal that cation disorder, particularly Wyckoff-site occupancy, significantly modulates the bandgap and orbital hybridization in this ternary compound, underscoring the impact of cation arrangement on optoelectronic properties.

These findings highlight the versatility of ternary nitrides as advanced photoactive materials and offer insights into tailoring their properties through atomic-scale engineering.

Toxicity and instability of lead-based metal halide perovskites (MHP) have fueled explorations of new lead-free all-inorganic materials, especially in the CuI-AgI-BiI₃ phase space. In particular, Cu₂AgBiI₆ shows promising optoelectronic properties such as a high absorption coefficient of ~ 10⁵ cm^-1, a direct band gap of ~ 2 eV, a low exciton binding energy comparable to the thermal energy at room temperature, a modest mobility of 1.7 cm²V^-1s^-1 and a nanosecond charge-carrier lifetime.[1] However, the champion power conversion efficiency (PCE) of the Cu₂AgBiI₆ solar cells was only 2.39 %,[2] significantly lagging behind the PCE of the MHP counterparts. It has been shown that Cu₂AgBiI₆ exhibits ultrafast charge-carrier localization, which imposes the fundamental limits on its PCE.[3] However, such low PCEs cannot be solely attributed to the ultrafast charge-carrier localization, given that Cs₂AgBiBr₆, which also shows ultrafast charge-carrier localization and has an indirect bandgap, has achieved the champion PCE of 6.37 %.[4]

Herein, we aim to understand additional factors limiting the PCE of Cu₂AgBiI₆ beyond intrinsic ultrafast localization by investigating optoelectronic properties of charge transport layer (CTL)/Cu₂AgBiI₆ half stacks using CuI and PTAA as hole transport layers and SnO₂ and PCBM as electron transport layers. By analysing absorption spectra, X-ray diffraction (XRD) patterns and optical-pump terahertz-probe (OPTP) transients, we observe that the formation of quaternary phase Cu₂AgBiI₆ is influenced by the deposition of charge transport layers, especially by CuI and SnO₂, which modify its optoelectronic properties. Materials within the CuI-AgI-BiI3 phase space share similar band gaps and lattice parameters, making it challenging to confidently distinguish Cu₂AgBiI₆ from other impurity phases by using absorption spectra and XRD patterns alone. By extracting THz mobilities from OPTP, we confidently factor out Cu₂AgBiI₆ phase and identify impurity phases induced by the transport layer deposition. We find that deposition of CuI on Cu₂AgBiI₆ induces the formation of copper-rich quaternary Cu_xAgBiI_4+x phases near the interface and deposition of Cu₂AgBiI₆ on SnO₂ hinders the formation of the quaternary phase and instead forms ternary and binary phases, leading to the decrease in the mobility. Overall, we highlight that the deposition of CTLs can significantly affect the formation and optical properties of Cu₂AgBiI₆. Characterization of CTL/Cu₂AgBiI₆ half stacks is therefore critical to improve the device performance. Moreover, further developments are needed to suppress the formation of unwanted impurity phases upon deposition of transport layers.

Lead-free halide double perovskites (HDPs, A₂BB^’X₆) with attractive optical and electronic features are regarded as one of the most promising alternatives to overcome the toxicity and stability issues of lead halide perovskites. They provide a wide range of possible combinations and rich substitutional chemistry with interesting properties for various optoelectronic devices.^[1] However, the performance of state-of-the-art lead-free HDPs is not yet comparable to that of lead halide perovskites, especially in the photovoltaic field.^[2] One of the main reasons for this is that HDPs usually have large and/or indirect bandgaps, which limit their optical and optoelectronic properties in the visible and infrared regions. In this presentation, I will talk about our work on double perovskites, including the fabrication of the first planar double perovskite solar cell devices, and the modification of the bandgap and optical properties of HDPs using metal doping/alloying and crystallization control,^[3],[4] as well as provide detailed understanding of the alloying at the atomic level.

In the search for lead-free alternatives for halide perovskites, a number of different research routes are being pursued. Here we will discuss three different pathways presently under investigation. For example, silver pnictohalides have emerged as perovskite-inspired materials for photovoltaics due to their high stability, low toxicity, and promising early efficiencies, particularly for indoor applications. While most research has focused on silver bismuth iodides (Ag–Bi–I rudorffites), antimony analogs remain underexplored due to difficulties in synthesizing Sb-based thin films. Here, a novel synthesis route using thiourea as a Lewis-base additive enabled the preparation of Ag–Sb–I films, which were further optimized by Cu alloying to improve thin-film morphology and increase power conversion efficiency to 0.7% [1]. Theoretical and optical studies confirmed Cu incorporation into a Cu₁₋ₓAgₓSbI₄ phase without altering bandgap properties. Our studies also identified Ag point defects as traps reducing open-circuit voltage, with minor Bi additions enhancing efficiency and stability.

Heteroatom alloying presents an additional strategy for tuning the optoelectronic properties of lead-free perovskite derivatives. Tin-alloyed layered MA₃Sb₂I₉ thin films were synthesized using a solution-based approach with precursor engineering, blending acetate and halide salts [2]. Increasing tin halide concentrations expanded visible-spectrum absorption and improved stability. Tin incorporation into the MA₃Sb₂I₉ lattice introduced new electronic states in the bandgap, confirmed by theoretical calculations. These features enhanced absorption through intervalence and interband transitions while stabilizing charge transport. This system’s robustness toward mixed oxidation states improved ambient stability, demonstrating its potential for various optoelectronic applications.

Finally, the integration of organic semiconducting materials with inorganic halide perovskites offers promising opportunities for tuning optoelectronic properties. Using stable, nontoxic double perovskites as hosts for electroactive organic cations enables the creation of two-dimensional (2D) hybrid materials that are both functional and lead-free for optoelectronic applications. By incorporating naphthalene and pyrene moieties into Ag–Bi–I and Cu–Bi–I double perovskite lattices, we address intrinsic electronic limitations of double perovskites, and modulate the anisotropic electronic properties of 2D perovskites [3]. Among eight newly developed 2D double perovskites, (POE)₄AgBiI₈ containing pyrene moieties was identified as having a favorable electronic band structure, exhibiting a type IIb multiple quantum well system conducive to out-of-plane conductivity and achieving a photocurrent response ratio of nearly three orders of magnitude under AM1.5G illumination. This material was used to create the first pure n = 1 Ruddlesden–Popper 2D double perovskite solar cell. Concluding, these and additional examples discussed in the presentation illustrate the enormous design space available for the generation and tuning of novel lead-free perovskite-derived absorber materials.

[1] Hooijer, R.; Weis, A.; Kaiser, W.; Biewald, A. ; Dörflinger, P. ; Maheu, C.; Arsatiants, O.; Helminger, D.; Dyakonov, V.; Hartschuh, A.; Mosconi, E.; De Angelis, F.; Bein, T. Cu/Ag–Sb–I Rudorffite Thin Films for Photovoltaic Applications. Chem. Mater. 2023, 35, 23, 9988–10000.

[2] Weis, A.; Ganswindt, P.; Kaiser, W.; Illner, H.; Maheu, C.; Glück, N.; Dörflinger, P.; Armer, M.; Dyakonov, V.; Hofmann, J.P.; Mosconi, E.; De Angelis, F.; Bein, T. Heterovalent Tin Alloying in Layered MA₃Sb₂I₉ Thin Films: Assessing the Origin of Enhanced Absorption and Self-Stabilizing Charge States. J. Phys. Chem. C 2022, 126, 49, 21040–21049.

[3] Hooijer, R.; Wang, S.; Biewald, A.; Eckel, C.; Righetto, M.; Chen, M.; Xu, Z.; Blätte, D.; Han, D.; Ebert, H.; Herz, L.M.; Weitz, R.T.; Hartschuh, A.; Bein, T. Overcoming Intrinsic Quantum Confinement and Ultrafast Self-Trapping in Ag–Bi–I- and Cu–Bi–I-Based 2D Double Perovskites through Electroactive Cations. J. Am. Chem. Soc. 2024, 146, 39, 26694–26706.

Solar cells based on nanocrystals have seen increasing interest in recent years due to the continuous rise in their power conversion efficiency. The most efficient cells are often based on nanocrystals that contain lead, such as lead sulfide (PbS) or metal halide perovskites (e.g., FAPbI3 or CsPbI3). While these nanocrystals lead to high efficiencies, they raise concerns regarding their large-scale applicability due to the environmental hazards they pose. In this talk, I will discuss the synthesis and application in solar cells of lead-free nanocrystals, such as those based on bismuth and antimony. I will present a method to synthesize an array of compositions by cation exchange, a simple, low-temperature process. Finally, I will introduce a facile templated growth of small antimony sulfide (Sb2S3) nanorods that are of great interest for application in photovoltaics.

Inorganic semiconductor photoabsorbers such as colloidal nanocrystals are frequently employed for a wide variety of energy-related applications by themselves or as part of a hybrid nanostructure,^[1] including photocatalysis and photovoltaic devices. However, using them as the photoactive layer in photoelectrochemical (PEC) is less common due to the difficulty in binder-free attachment to transparent conductive oxide (TCO) substrates. On the other hand, polymeric carbon nitrides(CNs) are a family of cheap and highly stable semiconductor materials that can be in situ grown on TCOs. Unfortunately, their successful utilization for PEC has yet to reach the performance of state-of-the-art metal-oxide-based systems, among others, due to insufficient light harvesting and inferior conductivity.^[2,3]

In this talk, I present a simple method based on electrophoretic deposition of ZnSe nanocrystals into a porous modified CN layer as the scaffold to form an efficient hybrid photoactive layer over TCO.4 The merits of this simple yet scalable solution processing method will be discussed, including the crucial step of stripping long-chain alkyl surfactant from the nanocrystals. The resulting hybrid structure achieves an impressive Faradaic efficiency towards the oxygen evolution reaction at ca. 87% and doubles the measured photocurrent and IPCE values relative to samples without ZnSe. Our results show the benefit of such a combination in terms of charge separation, stability, and successful water-splitting without using additional co-catalysts—evolving oxygen while minimizing parasitic self-oxidation.^[4] This method paves the way for the incorporation of other nanocrystals into porous organic hosts.