Journal of Materials Chemistry A themed collection on emerging inorganic materials for solar harvesting

Inorganic semiconductors have long played an important role in light-harvesting technologies, including photovoltaics, photoelectrochemical cells and photocatalysts.^1–8 The holy grail of the community working on these materials for sustainable energy applications has been to simultaneously achieve high performance, stability, and cost-effective processing, using materials comprised of earth-abundant, non-toxic elements.⁹ Historically, chalcogenide thin film semiconductors have featured prominently within this community, including kesterites¹⁰ and antimony chalcogenides.¹¹ Over the past decade, the rapid rise in prominence of lead-halide perovskites (LHPs) for photovoltaics has added a new dimension: the defect tolerance of LHPs enables these materials to achieve efficient performance despite the high concentration of traps that result from processing using simple, low-temperature methods.^12,13 This has shaped the direction taken in efforts to develop novel inorganic light harvesters that could replicate the defect tolerance of LHPs, while simultaneously overcoming their toxicity and stability limitations.^12–14 In addition, over the past decade, new application areas have come to prominence where novel inorganic thin film materials are advantageous over crystalline silicon. These include indoor photovoltaics (IPVs), where the wider bandgap of some of these inorganic materials (e.g., Sb₂S₃,^15,16 Se ¹⁷) is closer to the optimal value for harvesting energy from artificial light sources.^18,19 Another application area is the development of artificial leaves and photocatalysts for the direct conversion of chemicals into fuels, driven solely by sunlight.^20–22 Inorganic semiconductors for solar fuels need to have high photochemical stability and corrosion resistance because they often operate in direct contact with the electrolyte under illumination, where holes and electrons are continuously generated. The recent developments in the field of emerging inorganic materials for light harvesting were discussed in a dedicated symposium at the 2025 International Conference on Materials for Advanced Technologies (ICMAT), held in Singapore, accompanied by a themed issue of the Journal of Materials Chemistry A. The articles in this themed issue come under eight themes that cover the discovery, development, and application of inorganic materials for photovoltaics and solar fuels.

Novel materials

Efforts to discover inorganic materials that replicate the defect tolerance found in LHPs have focused on structural, chemical and electronic analogs. The latter is based on the hypothesis that the defect tolerance in LHPs is linked to the electronic structure at the band extrema, which favors shallow trap formation.¹⁴ This implies that defect tolerance does not necessarily require a perovskite crystal structure, provided that the upper valence band is comprised of hybrid states between filled metal valence s orbitals and anion p orbitals, while the lower conduction band is comprised of an antibonding state formed from the hybridization of metal p orbitals and anion p orbitals. Collectively, these families of materials have been referred to as ‘perovskite-inspired’ materials (or PIMs), or alternatively as ns² compounds,²³ given the importance of the valence s² electrons on forming the electronic structure found in LHPs. There has been particular emphasis on compounds based on Bi³⁺ and Sb³⁺, and the classes of materials explored have heavily featured halide, chalcohalide, and chalcogenide compounds.

Vivo and co-workers reviewed the Ag/Bi PIMs (both perovskite and non-perovskite) that have been explored over the past decade.²⁴ The combination of Ag⁺ and Bi³⁺ cations has featured heavily among PIMs studied, including in halide elpasolites (e.g., Cs₂AgBiBr₆),²⁵ Cu–Ag–Bi–I compounds (e.g., Cu₂AgBiI₆),²⁶ AgBiS₂,²⁷ and mixed anion compounds (e.g., AgBiSCl₂).²⁸ A common theme with these Ag/Bi compounds is cation disorder, due to the similar ionic radii between Ag⁺ and Bi³⁺. As a result, Ag⁺ and Bi³⁺ can occupy the same crystallographic site (e.g., in Ag–Bi–I compounds, as well as in AgBiS₂). The distribution between these two cations may not be uniform, and clustering of Ag or Bi can affect the electronic structure and optical properties; this has been especially well studied in AgBiS₂.²⁷ Another common theme with Ag/Bi compounds is the prevalence of carrier localization, where the radial extent of the charge-carrier wavefunctions reduces to within a unit cell.²⁹ This changes charge-carrier transport from band-like to hopping-based, and is accompanied by reductions in the charge-carrier mobilities and diffusion lengths. This lowers the performance of the solar absorbers in photovoltaics, and severely restricts the thickness of the solar absorber that could be used in photovoltaics. For example, AgBiS₂ has achieved among the highest efficiencies for Ag/Bi solar absorbers in photovoltaics (10.8% power conversion efficiency under 1-sun illumination),³⁰ but the absorber thickness is <50 nm.³⁰ Optical pump terahertz probe measurements of AgBiS₂ indicate a diffusion length of only 50 nm.³¹ Overcoming these carrier localization challenges among Ag/Bi solar absorbers is critical to fully benefit from the advantages these materials have, which include strong light absorption and high stability.

Other novel materials systems explored in this themed issue are pnictogen-based chalcohalides ((Sb,Bi)(S,Se)(Br,I)),³² Cu/Sn chalcohalides (CuSn₂SI₃ and Cu_0.35Sn_5.29S₂I₇),³³ and M₃ACl₃ perovskite-derivatives, where M = Ca²⁺, Sr²⁺, Ba²⁺ and A = N³⁻, P³⁻, As³⁻.³⁴ For the pnictogen-based chalcohalides, Dimitrievska and co-workers found that most of the materials investigated exhibit band-edge photoluminescence.³² The exception is BiSeI, which has the strongest electron–phonon coupling, and readily forms deep traps due to Se vacancies. Otherwise, BiSI and BiSeBr hold potential for further exploration due to comparatively weaker electron–phonon coupling and higher formation energies for defects.³² Prior work on BiSBr has similarly shown this material to hold potential for IPV applications, owing to its stable wide bandgap (1.91 ± 0.01 eV), exhibiting a PL lifetime surpassing the 1 ns threshold for materials to be worth further development.³⁵ However, these materials have indirect bandgaps. By contrast, Rosseinsky and co-workers found that CuSn₂SI₃ and Cu_0.35Sn_5.29S₂I₇ have direct bandgaps, albeit these are slightly wide at 2.1 eV (for CuSn₂SI₃).³³ Narrower bandgaps down to ∼1.7 eV were predicted by Rehman and Lin with Ba-based M₃ACl₃ materials (Ba₃PCl₃ and Ba₃AsCl₃), due to contributions from the 5d orbitals in Ba that enhance d–p interactions.³⁴ However, the effective masses are high, especially for electrons which are close to 1m_e on average.³⁴

Kesterites

Cu₂ZnSn(S,Se)₄ is a well-established thin film material used for solar cells, with a bandgap that is tunable between 1–1.5 eV by adjusting the S/Se ratio.^36,37 The structure type is kesterite, which has a tetragonal unit cell (I4̄ space group), comprised of corner-sharing [Cu(S,Se)₄], [Z(S,Se)₄] and [Sn(S,Se)]₄ tetrahedra.³⁸ These materials are appealing because of their composition of earth-abundant, non-toxic elements, as well as their high stability. However, progress in solar cell efficiency stalled for about a decade, remaining at ∼12.6% between 2013 and 2021.^39,40 During this time, LHPs rapidly surpassed kesterites in efficiency, and interest waned in these chalcogenides. Over the past five years, kesterites have regained interest as the efficiency bottlenecks have started to be surpassed, with the certified record efficiency reaching 16.6% as of 2026.⁴¹ Furthermore, the bandgap tunability makes these materials potential candidates as bottom or top cells for thin film tandem photovoltaics, as well as for IPVs for powering Internet of Things electronics.⁴² Some of the important challenges are to reduce non-radiative recombination arising from deep defects that result in open-circuit voltage (V_OC) deficits,^43,44 to widen the bandgap further from 1.5 to 1.9 eV (optimal for IPVs),^18,19 improving the passivation of the materials, developing cost-effective, nontoxic solution processing routes, as well as exploring flexible device applications.

One promising dopant substitute to passivate Sn-related deep defects is Ge.⁴⁵ Schorr and co-workers investigated the substitution of Sn for a mixture of Ge and Si, i.e., Cu₂Zn(Ge_xSi_1−x)Se₄.³⁸ Using diffuse reflectance measurements, they showed that the bandgap can be tuned from 1.31 eV (x = 1) to 2.22 eV (x = 0), with a bandgap of 1.9 eV obtained using x between 0.2 and 0.3.³⁸ Si-rich crystals adopted a wurtz-kesterite structure, whereas the Ge-rich crystals (x = 0.59–1) have a kesterite structure, and both structures co-exist for x between 0.45 and 0.55.³⁸

Alkali doping has been indispensable for passivating grain boundaries in kesterite thin films for solar cells.^46,47 However, Na-doping kesterites can result in lattice distortions. Mulvaney, Liu and co-workers introduced a combination of Na⁺ and Ag⁺ to Cu₂ZnS(S,Se)₄ (CZTSSe), which both occupied the Cu⁺ site. While Na introduction passivates Cu-related defects, Ag introduction alleviates lattice distortions by Na introduction. By adding both monovalent cations, CZTSSe solar cells achieved 13.2% efficiency.⁴⁶ Wong and co-workers systematically investigated the effect of alkali cation introduction (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) on the optoelectronic properties of Cu₂CdSnS₄ (CCTS).⁴⁸ Replacing Zn with Cd resulted in a stannite structure, rather than a kesterite structure, due to the large size of the Cd²⁺ cation, and the bandgap was reduced to 1.4 eV, which is closer to the ideal value (1.34 eV) for single-junction outdoor solar cells. Alkali introduction was found to increase carrier concentration without introducing recombination centers, along with a reduction in upwards band bending at grain boundaries to enable enhanced charge-carrier transport. Of all alkalis investigated, Na incorporation into CCTS resulted in the largest improvement in device performance, from 7.77% (CCTS) to 8.47% (Na:CCTS).⁴⁸

For large-scale manufacturing of kesterites, solution processing may enable lower capital intensity and higher throughput than vacuum-based processing. However, the solution processing of kesterites involved the use of hazardous hydrazine.¹⁰ Nasyori et al. developed a process for solution processing CZTS in ambient air using dimethylsulfoxide as the solvent,⁴⁹ which is not as hazardous as hydrazine,⁵⁰ but should still be handled with caution because dimethyl sulfoxide can penetrate through nitrile gloves and skin. The kesterite films were sulfurized post-deposition, and optimized devices reached 9.4% efficiency under 1-sun illumination.⁴⁹ Saucedo and co-workers also developed a low-toxicity solution processing method to prepare CZTSSe films using 2-methoxyethanol as the solvent.⁵¹ Flexible devices were prepared by depositing onto Mo foil and incorporating Li and Ag, along with engineering the passivating MoSe₂ layer at the kesterite/Mo interface. The authors achieved 11.2%-efficient outdoor solar cells.⁵¹

Simple chalcogenides

The composition of many elements in kesterites can be regarded as a disadvantage. Some of the limitations include high energetic disorder, with high Urbach energies lowering the attainable open-circuit voltage, as well as challenges with achieving phase-pure materials. Balancing out current efforts with increasing the complexity of kesterites through doping and alloying, groups are also working on simple binary and unary chalcogenides. These include Sb₂S₃, which has a bandgap of 1.7–1.8 eV that is well-suited for indoor light harvesting. IPVs are gaining increasing attention because of their importance in harvesting energy from ambient lighting to power the billions of autonomous devices part of the Internet of Things, as well as their potential to enable edge computing.^18,19,52,53 Recently, Sb₂S₃ IPVs were improved in performance by lowering defect densities to reach ∼20% indoor power conversion efficiency (PCE_i) under white light-emitting diode (WLED) lighting, making them among the most efficient chalcogenide IPV materials.^15,16,54 However, an important limitation of the most efficient Sb₂S₃ devices is their reliance on CdS buffer layers.^15,16 Given that Cd is more toxic than Pb and more strictly regulated, it is critical to develop Cd-free Sb₂S₃ devices that can maintain high PCE. Hussien et al. successfully realized this, achieving 18% PCE_i and 7.5% PCE under 1-sun illumination, using only nontoxic, stable oxides as the electron transport layer.⁵⁵ This was accomplished using ultrasonic spray pyrolysis (USP) to prepare a ZnO interfacial layer on TiO₂, which allowed Sb₂S₃ to cover the electron transport layer when also prepared by USP.⁵⁵

Even simpler, elemental Se is now being re-explored for PV. This was the first photovoltaic material investigated, and interest in this material was re-invigorated with the discovery that crystalline Se can make IPVs with 15.1% PCE_i.⁵⁶ Following optimization, the PCE_i has now been improved to 18%,⁵⁷ with bifacial Se IPVs achieving 26.2% PCE_i.¹⁷ The Se used in these IPV devices has been processed by vacuum-based methods (e.g., thermal evaporation),⁵⁷ as well as vacuum-free solution processing techniques.^58,59 The latter are particularly appealing for potentially lower capital-intensity and higher-throughput manufacturing at scale. However, as in early work on solution-processed kesterites,¹⁰ solution processing of Se involved the use of hazardous hydrazine as the solvent.^58,59 Alfieri et al. developed a safer solvent system using a thiol-amine mixture (i.e., polylamine, and ethanethiol) to prepare a propylammonium poly-seleno-telluride salt that can be dissolved in N,N-dimethylformamide for solution processing of Se_1−xTe_x thin films. The bandgap could be tuned from 1.86 eV (pure Se) to 1.20 eV (Se_0.7Te_0.3), and these films exhibited photovoltaic performances of 2.73% and 2.33% under 1-sun illumination, respectively.⁶⁰ Promisingly, unencapsulated Se devices show no performance degradation after 1 month of storage in air, and the open-circuit voltage achieved (854 mV) exceeds that of hydrazine-processed Se solar cells.^58–60

Light-driven hydrogen and oxygen evolution

This themed collection brings together cutting-edge research on light-driven hydrogen and oxygen evolution, spanning novel photocathode and photoanode materials, morphology control, and mechanistic investigations at the atomic scale. Wang et al. report a method to enhance the photoelectrochemical (PEC) proton reduction performance of Sb₂Se₃ nanorod arrays by controlling their diameter in their study on nanowire morphology control in Sb metal-derived antimony selenide photocathodes for solar water splitting.⁶¹ The enhanced PEC performance of the thinner nanorods is attributed to an improved charge transport path in the shortened [hk0] direction.⁶¹ Ha et al. introduced AgBiS₂ as a promising photocathode material for solar water splitting by simultaneously engineering cation disorder and morphology in molecular-ink-derived AgBiS₂ photocathodes.⁶² The high activity of the devices was attributed to enhanced optical absorption and reduced grain boundary recombination.

Longstanding photoanode materials continue to attract significant research attention, with recent advances further pushing the boundaries of their PEC performance. Min et al. show that high activity nanostructured BiVO₄ films can be fabricated by polyethylene glycol (PEG) assisted metal–organic deposition (MOD). The electrodes deliver photocurrent densities of 5.8 mA cm⁻² (SOR, 1.23 V vs. RHE) and 5.4 mA cm⁻² (OER with NiFeOOH catalyst) under simulated sunlight.⁶³ Another promising photoabsorber for PEC water splitting is Fe₂O₃ (hematite), due to the high absorption coefficient and relatively low bandgap (2.1–2.3 eV).⁶⁴ In their contribution Awulachew and others used density functional theory to investigate seven different OER mechanisms of Fe₂O₃ to determine the mechanism of water oxidation. Their work can inform the development of improved hematite-based photoanodes.⁶⁵ One limitation of hematite is its low minority carrier diffusion length, which is typically overcome by co-doping of Sn, Si, etc, to increase the charge-carrier concentration.^66,67 In their study Anushkkaran and others show that the improved activity is a result of the conductivity increase from Zr/Sm co-doping, and that the NiCo-MOF cocatalyst passivates recombination surface states.⁶⁸

More recently, the ferroelectric effect has been reported to enhance the photocatalytic behaviour of some oxide photoabsorbers.^69,70 In their paper, Gunawan and others present a new approach to tuning photoelectrode performance.⁷¹ They report on the effects of polarization on the surface chemistry, charge dynamics, and PEC performance of BiFeO₃ photoanodes and a possible switch from p-type to n-type behavior. The switch is attributed to a negatively-charged surface, gradient energy modulation, and negatively shifted band energies.

In their report, Tzevelekidis et al. demonstrate photocatalytic H₂ evolution from aqueous methanol using a nanostructured Cu/TiO₂ composite. The photocatalyst displays remarkable stability, maintaining full activity for at least 26 hours of solar irradiation and retaining significant activity after 18 months.⁷²

Light-driven oxidation

Beyond solar water splitting, the photocatalytic oxidation of organic compounds represents another important research direction, to which significant efforts have been devoted to improving the quantum yield and reaction selectivity of TiO₂-based photocatalysts. In their report Khaliq et al. obtained In₂S₃/CdS/TiO₂ nanotube-based photoanodes for methanol and methylene blue oxidation that showed apparent quantum yields of up to 13.6% under 632 nm wavelength illumination.⁷³ Yang and others showed that the industrial pigment quinacridone sensitizes TiO₂ for the aqueous oxidation of glycerol to glyceraldehyde in the presence of a TEMPO co-catalyst. The reaction proceeds under red light (620 nm wavelength) and has 80% selectivity.⁷⁴

Photoelectrochemical CO₂ reduction

The development of novel photoelectrode materials and architectures for solar-driven CO₂ reduction has emerged as a compelling research frontier, with recent efforts focusing on improving faradaic efficiency, photocurrent density, enhancing stability and device integration. Wan et al. demonstrate a Cu₂O/Ga₂O₃/ZnGeO_x/TiO₂/SnO₂ photocathode that reduces CO₂ to formate with a faradaic efficiency of 75.4% at −0.1 V vs. RHE. When combined with a BiVO₄/NiCo LDH photoanode into an integrated device, unassisted, solar-driven CO₂ reduction was observed with simultaneous ethylene glycol (EG) oxidation.⁷⁵ Meanwhile, Quadir and others prepared photoelectrodes with a combinatorial sputtering technique followed by annealing with Te in the study on ZnGa₂Te₄ thin-film absorbers for photoelectrochemical CO₂ reduction to products such as CO and formate. The ZnGa₂Te₄ samples demonstrate promising photoelectrochemical stability and photocurrent densities exceeding that of the widely investigated ZnTe photocathodes.⁷⁶

Photocatalytic H₂O₂ production

The report by Jeon et al. shows that an increase in S vacancy concentration and tetragonal distortion around the Cu substituent can raise hydrogen peroxide production rates when compared to pristine ZnIn₂S₄ and Ni-substituted ZnIn_2−xNi_xS₄ nanosheets.⁷⁷ This was attributed to multiple factors, including an increase in visible-light absorptivity, suppression of electron–hole recombination, and improved charge-transport and O₂ adsorption.

Photocatalytic remediation

Photocatalytic reactions of environmental pollutants have attracted growing research interest, with recent studies focusing on the design of novel photocatalyst materials and heterostructures to enhance light harvesting, charge separation, and degradation efficiency of organic contaminants. In their publication, Verma and others investigate the role of organic ligands from sugar press mud (PM) and chemical surfactants to induce shape anisotropy among iron oxide nanocrystals in an aqueous sol–gel (bottom-up) synthetic approach. This transforms hematite (a-Fe₂O₃) nanocrystals from spherical to sheet-and rod-like morphologies (∼24–44 nm) and controls their photocatalytic activity for rhodamine B degradation.⁷⁸

In their paper, Mahmood and others prepare nanohybrid titanium disilicide/carbon quantum dot (TiSi₂/CQD)-based composite photocatalysts and test them for anionic and cationic dye, degradation. The activity increases strongly after the addition of carbon quantum dots, which act as an electron reservoir and boost light absorption.⁷⁹

In their paper, Li and others use XPS, PL, TRPL, SPV and EIS, and EPR to confirm the charge separation mechanism in their catalyst.⁸⁰

Overall, both the themed issue and symposium within ICMAT emphasize the continued vibrancy and diversity of the inorganic materials community, which combines synthesis with device engineering, computations and advanced characterization. A core overarching theme is the strong interconnection between energy and chemical conversion, in that advances in the photovoltaics field also feed through to benefit the PEC and photocatalysis fields. These are underpinned by novel materials and process development, which are required not only for the light absorbers used in solar cells and solar fuels, but also the charge transport layers⁸¹ and further applications, for example in radiative cooling.⁸² This field of research continues to grow, and we anticipate many further advances in materials and development, as well as in the fundamental understanding of these films and devices.

The full collection can be read following this link: https://pubs.rsc.org/en/journals/articlecollectionlanding?sercode=ta&themeid=194d6124-af8a-4085-abbb-bc2c47cebcfb.

As guest editors for this themed collection, we express our appreciation to all the authors who contributed articles, the peer reviewers, and to those who presented their work at the ICMAT symposium. We also thank the Royal Society of Chemistry editorial staff who put together this themed issue.

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