Compositional Tuning of Cu2AgBiI6 Thin Film Solar Absorber via Alkali Metal Substitution: Synthesis and Characterization

• Grant number: 25/15514-7
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: February 01, 2026
• End date: January 31, 2027
• Field of knowledge: Physical Sciences and Mathematics - Chemistry - Inorganic Chemistry
• Principal Investigator: Rodrigo Szostak
• Grantee: Lucas Duarte Rocha de Oliveira
• Host Institution: Centro Nacional de Pesquisa em Energia e Materiais (CNPEM). Campinas , SP, Brazil

Abstract

Perovskite solar cells (PSCs) have revolutionized photovoltaics by achieving higher efficiencies (at lab scale) and lower cost than the normally utilized monocrystalline silicon solar cells. However, they suffer from toxicity and stability issues due to Pb and organic molecules in the crystal. This motivates the search for alternative non-toxic materials that have similar optoelectronic properties. This research seeks to reduce the intrinsic limitations of the lead-free material Cu¿AgBiI¿ (CABI) by partially substituting its monovalent metal sites with alkali ions (Cs¿, Rb¿, K¿). Although CABI offers great properties for indoor and tandem photovoltaics, such as direct bandgap of approximately 2.06 eV and an exceptionally high absorption coefficient (~10¿ cm¿¹), its unmodified films suffer from defect-mediated nonradiative recombination and consequently low power conversion efficiencies of around 1- 2%. Building on prior successes in halide and Sb alloying, which demonstrated that compositional tuning reduces trap densities and extends charge carrier lifetimes, we hypothesize that incorporating larger alkali cations will both passivate defects and red-shift the bandgap by expanding metal-iodide bond lengths, consequently improving photovoltaic performance. To test this hypothesis, two precursor solutions will be prepared in a DMSO:DMF (3:1) mixture: one containing CuI, AgI and BiI¿, and a second containing CsI, KI or RbI. By varying the ratios of these solutions, spin-coated films will be deposited on glass and then thermically treated. Conventional laboratory characterization, including x-ray diffraction, scanning electron microscopy with energy-dispersive x-ray spectroscopy, UV-Vis absorption and photoluminescence spectroscopy, will first assess phase purity, morphology and optoelectronic quality. Subsequently, advanced nanoscale techniques at the CARNAÚBA beamline (nano x-ray fluorescence, absorption spectroscopy, diffraction and excited optical luminescence) will map elemental distribution, structure and oxidation states, possibly with in situ heating to observe crystallization dynamics. A powder XRD study with Rietveld refinement at the PAINEIRA beamline will expose the exact substitution sites (Cu, Ag or interstitial) and quantify lattice parameter shifts. Photophysical measurements will be conducted at LNNano or at University of Palermo (as a BEPE project, if accepted). Properties such as carrier lifetimes and diffusion lengths will correlate structural modification and device-relevant properties. By systematically investigating how alkali metal incorporation modifies CABI's crystalline network and defect states, this research aims to realize more efficient, stable and sustainable perovskite-inspired solar absorbers. (AU)