Investigation of ultra-wide bandgap chloro‑perovskite materials for blue LED applications: Crystal structure, optical and DFT studies

Ultra-wide band gap (UWBG) in Hybrid organic–inorganic perovskites (HOIPs) materials are characterized by band gaps typically greater than 4 eV. These UWBG materials have gained substantial recognition in recent years for their promising applications in high-power, high-frequency and in optoelectronic systems [[1], [2], [3]]. They serve as crucial components in the production of green and blue LEDs [4], as well as lasers, and are widely utilized in radio frequency applications, including military radar systems [[5], [6], [7], [8], [9], [10]]. The wide-band-gap energy makes it well-suited for absorbing or emitting ultraviolet (UV) light, enabling its use in various optoelectronic devices. (UWBG-HOIPs) materials are characterized by (i) their large band gap allows for effective absorption of high-energy UV photons, making them useful in UV detectors and emitters, (ii) their high band gap can complement lower band gap materials in tandem solar cells, potentially increasing overall efficiency. (iii) their low-cost techniques of synthesis and (iv) their thermal stability compared to traditional perovskites. Our breakthrough design employs l-proline (a chiral carbon-based molecule) as a multifunctional structure-directing agent, simultaneously addressing three fundamental challenges in UWBG-HOIP development:(i) steric control via the pyrrolidine ring’s geometric constraints on the inorganic framework [11], (ii) charge balance through zwitterionic stabilization (L-Pro^±) of the CdCl₃ network [12], and (iii) optical tunability via chiral-center-induced circularly polarized UV emission (210–280 nm) with exceptional quantum yield [13]. This innovation demonstrates superior performance relative to conventional HOIPs [14] and III-Nitrides (GaN/AlN) [15,16], achieving room-temperature synthesis (25 °C) compared to solution/solvothermal (25–120 °C) and MOCVD (>1000–1100 °C) processes respectively. While exhibiting higher thermal stability (185 °C) than conventional HOIPs (80–120 °C) though less than III-Nitrides (>1000 °C), our material’s optimal 4.326 eV bandgap bridges the gap between conventional HOIPs (1.5–3.5 eV) and III-Nitrides (GaN: 3.4 eV; AlN: 6.2 eV). The unprecedented combination of circularly polarized luminescence – absent in both reference material systems – and significantly reduced production costs (<0.50 $/cm²vs.50 $/cm² for GaN and 120$/cm² for AlN) [17] positions this technology as a transformative solution for scalable wide-bandgap optoelectronics, with comprehensive performance comparisons detailed in Table 1. Halide organic-inorganic perovskites (HOIPs) have emerged as highly promising materials for next-generation light-emitting diodes (LEDs), primarily due to their exceptional photoluminescence quantum yield (PLQY), broadly tunable emission wavelengths spanning UV to near-IR through halide composition control, and excellent solution processability via various deposition techniques such as spin-coating, inkjet printing, and blade-coating [18]. HOIPs are also attractive due to their electrical conductivity [19] and diverse applications in Schottky barrier diodes (SBDs) [20,21] and their fluorescence [18].

The optical properties of these materials are fundamentally governed by the electronic structure of their inorganic framework. Notably, pure CdX₂ salts (where X = Cl, Br, I) demonstrate significant self-trapped exciton (STE) luminescence across the 2–3.8 eV range, with specific emission peak positions being highly dependent on halogen identity, temperature, and excitation conditions [[22], [23], [24]]. This distinctive STE behavior arises from strong electron-phonon interactions that cause substantial lattice distortions around photoexcited excitons, leading to their self-trapping. While these materials show great potential, it is important to note that research on hybrid organic-inorganic Cd-based halides remains relatively limited, particularly regarding systematic photoluminescence (PL) studies, as comprehensively outlined in Table S1 [[25], [26], [27], [28], [29], [30], [31], [32]]. In this paper, synthesis, crystal structure, spectroscopic, thermal analyses, optical properties and DFT of {(C₅H₉NO₂)(C₅H₉NO₂)CdCl₃}_n hereafter abbreviated {(L–ProH⁺)(L–Pro^±)CdCl₃}_n are reported and discussed.