First-principles modeling of mixed halide organometal perovskites for photovoltaic applications

We computationally investigate organometal CH3NH3PbX3 and mixed halide CH3NH3PbI2X perovskites (X = Cl, Br, I), which are key materials for high efficiency solid-state solar cells. CH3NH3PbX3 perovskites exhibited the expected absorption blue shift along the I -> Br -> Cl series. The mixed halide systems surprisingly showed the CH3NH3PbI3 and the CH3NH3PbI2Cl (or CH3NH3PbI3-xClx) perovskites to have similar absorption onset at similar to 800 nm wavelength, whereas CH3NH3PbI2Br absorbs light below similar to 700 nm. To provide insight into the structural and electronic properties of these materials, in light of their application as solar cell active layers, we perform periodic DFT calculations on the CH3NH3PbX3 and CH3NH3PbI2X perovskites. We find a good agreement between the calculated band structures and the experimental trend of optical band gaps. For the mixed halide perovskites our calculations show the existence of two different types of structures with different electronic properties, whose relative stability varies by varying the X group. For these systems, the calculated formation energies decrease in the order I > Br > Cl, in line with the observed miscibility of CH3NH3PbI3 and CH3NH3PbBr3 compounds, while suggesting a comparatively smaller chlorine incorporation into CH3NH3Pb(I1-xClx)(3) compounds. We also show that Cl atoms preferentially occupy the apical positions in the PbI4X2 octahedra, while Br atoms may occupy both apical and equatorial positions, consistent with reported lattice parameters. The interplay of the organic and inorganic components of the perovskites, possibly mediated by hydrogen bonding between the ammonium groups and the halides, seems to be the key to the observed structural variability.