Temperature-Dependent Optical Absorption Study of Exciton Binding Energy in Organolead Halide Perovskites

Organolead halide perovskites have recently garnered significant attention for their potential in solar cell applications, as well as their promise for light-emitting and lasing devices. In this study, we investigate the excitonic and electronic properties of mixed halide organolead perovskite CH<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>NH<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>PbCl<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>-<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub>I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layers through temperature-dependent optical absorption spectroscopy. By analyzing spectral features across the 77–300 K range, we determined the exciton binding energy (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_b</i> ≈ 14 meV) and the bandgap energy. The experimentally derived <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_b</i>, obtained from absorption-edge shifts, is nearly half the theoretical value calculated using the Wannier–Mott model with a high-frequency dielectric constant (ε<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> = 6.55). The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_b</i> values close to the thermal energy at room temperature, confirmed by both the experimental and theoretical approaches, indicate predominantly non-excitonic behavior, with approximately 95% of excitons dissociating into free carriers, which is essential for high photovoltaic efficiency. A discontinuity in the bandgap (from 1.57 eV to 1.72 eV) near 150–160 K reflects the impact of the tetragonal-to-orthorhombic phase transition on the material’s optoelectronic properties. While a more accurate treatment of dielectric screening, for example through in situ studies or by considering effective dielectric constants (ε<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_eff</i> ≈ 10), is needed, our results quantitatively confirm the weakly bound excitonic character of mixed-halide perovskites under operational conditions. These findings provide useful insights for their optimized design in solar energy applications.