Precursor solution temperature dependence of the optical constants, band gap and Urbach tail in organic-inorganic hybrid halide perovskite films

The band gap energy and optical constants are important parameters for solar cell design. Here, organic-inorganic hybrid halide perovskite CH3NH3PbI3 films (about 140nm thick) were prepared by vapor deposition at precursor solution temperatures of 30 degrees C, 50 degrees C, 70 degrees C, and 90 degrees C, respectively. By fitting the ellipsometric spectra in the photon energy range of 0.496-4eV, we can find that the optical constants, absorption coefficients, and dielectric function strongly depend on the precursor temperature. Due to the change of electronic band structures and spin-orbit coupling, the band gap energy can be modulated from 1.634eV to 1.516eV with different precursor temperatures. As band gap energy increases, the Urbach energy decreases with each precursor temperature. The film with a precursor temperature of 70 degrees C shows the lowest Urbach tail energy (107 meV). The Urbach tail effects can be ascribed to the cumulative effect of impurities, the degree of disorder, and electron-phonon interaction. This study is helpful for understanding the intrinsic optical properties of perovskite films fabricated under different precursor temperatures, so as to provide important information for understanding the device physics and fabrication of high performance perovskite solar cells.