In Bi2Te3-based materials charge-carrier densities are determined by antisite defects and controlling these defects is a key issue for thermoelectric and topological insulator materials. Bi-Te thin films with high-quality thermoelectric properties are deposited using a nano-alloying approach by molecular beam epitaxy (MBE) and sputtering. The in-plane transport properties are measured at room temperature as a function of charge-carrier density. High-accuracy chemical analysis by wavelength-dispersive X-ray spectrometry (WDX) is applied for the first time to these Bi2Te3-based thin films. The acquisition conditions for WDX spectrometry are established using Monte Carlo simulations for the electron trajectories, which guarantees a high lateral resolution and rules out stray radiation generated in the substrate of the films. In contrast to energy-dispersive X-ray spectrometry (EDX), which is usually applied, WDX offers unprecedented accuracy for measuring antisite defect concentrations and thus has a high impact on improving the quality of thin films. The charge-carrier densities are calculated from the WDX results according to the point-defect model of Miller and Li and the thermopower and electrical conductivity are calculated for different charge-carrier densities by solving the linearized Boltzmann transport equation. A good quantitative agreement is found for the dependence of the thermopower on stoichiometry, whereas the electrical conductivity is sensitively affected by contaminants.