The lead-free double perovskite Cs₂AgBiX₆ (X = Br, Cl) has recently demonstrated great potential for applications in solar cells, photodetectors, and X-ray detectors. This material is characterized by a dominant resonant absorption feature in the UV-blue spectral region, which is still under controversial discussion regarding its origin. Here, we uncover an electronic fine structure of this optical feature in Cs₂AgBiBr₆ thin films. A detailed temperature-resolved study combining photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy reveals the existence of three optical transitions, situated approximately 100 meV around the resonance at 2.83 eV. PL measurements under pulsed excitation uncover a short-lived blue emission at the absorption resonance energy that persists up to room temperature and indicates the competition of direct emission from the resonant state and fast relaxation into the red emissive ground state. We derive a comprehensive energy scheme and suggest possible mechanisms leading to the observed fine structure splitting.

中文翻译：

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Cs ₂ AgBiBr ₆双层钙钛矿薄膜中光吸收共振的精细结构

无铅双钙钛矿Cs ₂ AgBiX ₆（X = Br，Cl）最近在太阳能电池，光电探测器和X射线探测器中显示出巨大的潜力。这种材料的特征是在紫外-蓝色光谱区域具有显着的共振吸收特征，有关其起源的讨论仍在争议中。在这里，我们发现了Cs ₂ AgBiBr ₆中这种光学特征的电子精细结构。薄膜。结合光致发光（PL），光致发光激发（PLE）和吸收光谱的详细的温度分辨研究揭示了三个光学跃迁的存在，位于2.83 eV共振周围约100 meV。脉冲激发下的PL测量在吸收共振能量下发现了短暂的蓝色发射，该吸收持续到室温，并且表明了直接发射从共振态到快速弛豫到红色发射基态的竞争。我们得出了一个综合的能源计划，并提出了导致观察到的精细结构分裂的可能机制。