We present the results of an investigation of the optical characteristics of pristine and CO2-adsorbed MoSe2 monolayers with (without) an external electric field. The optical parameters of interest are the absorption coefficient (α), reflectance (Rf), refractive index (n), and photoconductivity (σ). The impact of an external electric field (−2 × 108 V/cm) on the optical behavior of the MoSe2 monolayer is systematically investigated. The results show the peaks of the real component ( \(\varepsilon_{1}\) ) of the dielectric function for both pristine and CO2-adsorbed MoSe2 monolayers in the energy range of 2–3 eV. The imaginary part ( \(\varepsilon_{2}\) ) of the dielectric function exhibits a shift toward the visible region from the ultraviolet (UV) region, in which CO2 is adsorbed, and this shift increases toward the visible region with the application of an external electric field. Analysis of the absorption index, refractive index, and reflectance reveals that the peaks are aligned in the visible range for both the pristine MoSe2 and CO2-adsorbed MoSe2 monolayers, with (without) an external electric field. The shifts of these peaks follow a similar trend as the imaginary part of the dielectric constant. Lastly, this study provides additional insight into the photo-detection performance parameters (internal quantum efficiency [IQE], external quantum efficiency [EQE], light extraction efficiency [LEE], and responsivity) for both pristine and CO2-adsorbed MoSe2 monolayers, considering the presence or absence of an external field.
