The carbon nanotubes (CNTs) in a zigzag (4, 0) shape before and after substituted with F, Cl, and Br atoms were used as a basic computer model depending on the Quantum Espresso package DFT process (in the present work. This method demonstrated that the electronic structure, bandgap, total energy, FTIR spectrum, Raman spectrum, and depolarization spectrum can all being calculated. The simulated results are discovered that the increase in impurity atom size from F to Br as donor groups on nanotubes reduce the energy gaps from 0.959 eV to 0.674 eV, the ionization potentials from 6.088 eV to 5.729 eV, the electron affinities from 5.129 eV to 5.054 eV, and the firm energies from 5.609 eV to 5.392 eV. As a result, these substituted compounds have a high activity to act as a catalyst with broad absorption bands of the solar spectrum in the following order: Br+CNTs > Cl+CNTs > F+CNTs > CNT. This behavior will provide a better output for the solar cells and photovoltaic devices. The increment in HOMO, LUMO, and total energy magnitudes with increasing the impurity atom size from F to Br is given maximum changes, decreases band gaps, and elevated entropy values because of elevating in random.