Influence of halide composition on the structural, electronic, and optical properties of mixed CH3NH3Pb(I1-xBrx)3 perovskites calculated using the virtual crystal approximation method

Extensive studies have demonstrated the promising capability of the organic-inorganic hybrid halide perovskite CH3NH3PbI3 in solar cells with a high power conversion efficiency exceeding 20%. However, the intrinsic as well as extrinsic instabilities of this material remain the major challenge to the commercialization of perovskite-based solar cells. Mixing halides is expected to resolve this problem. Here, we investigate the effect of chemical substitution in the position of the halogen atom on the structural, electronic, and optical properties of mixed halide perovskites CH3NH3Pb(I1-xBrx)(3) with a pseudocubic phase using the virtual crystal approximation method within density functional theory. With an increase of Br content x from 0.0 to 1.0, the lattice constant decreases in proportion to x with the function of a(x) = 6.420 - 0.333x (angstrom), while the band gap and the exciton binding energy increase with the quadratic function of E-g (x) = 1.542 + 0.374x + 0.185x(2) (eV) and the linear function of E-b (x) = 0.045 + 0.057x (eV), respectively. The photoabsorption coefficients are also calculated, showing a blueshift of the absorption onsets for higher Br contents. We calculate the phase decomposition energy of these materials and analyze the electronic charge density difference to estimate the material stability. Based on the calculated results, we suggest that the best match between efficiency and stability can be achieved at x approximate to 0.2 in CH3NH3Pb(I1-xBrx)(3) perovskites.