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.