Density functional theory calculations have been performed for structures, stability and electronic properties of C20 fullerene doped by hydrogen, fluorine, and chloride atoms. Doping provides efficient tuning of frontier orbitals and electronic distribution in the pristine C20. Dipole moment, HOMO and LUMO energies as well as energy gaps were calculated and analysed. Hydrogen and chlorine dopants provide the minimal value of the energy gap. Based on presented results, on can chose appropriate radical to achieve the desired value of the band gap, needed for concrete solar cell systems or nanoelectronic devices.