  A little addition of Cl to \ce{MAPbI3} has been reported to improve the
material stability as well as light harvesting and carrier conducting
properties of organometal trihalide perovskites, the key component of
perovskite solar cell (PSC). However, the mechanism of performance enhancement
of PSC by Cl addition is still unclear. Here, we apply the efficient virtual
crystal approximation method to revealing the effects of Cl addition on the
structural, electronic, optical properties and material stability of
\ce{MAPb(I_{1-x}Cl_x)3}. Our {\it ab initio} calculations present that as the
increase of Cl content cubic lattice constants and static dielectric constants
decrease linearly, while band gaps and exciton binding energies increase
quadratically. Moreover, we find the minimum of exciton binding energy at the
Cl content of 7\%, at which the chemical decomposition reaction changes
coincidentally to be from exothermic to endothermic. Interactions among
constituents of compound and electronic charge transferring during formation
are carefully discussed. This reveals new prospects for understanding and
designing of stable, high efficiency PSCs.
