The t value >1 implies the A cation is too large to fit in to their interstices and possible structures are hexagonal polytype; t = 0.9-1.0 indicates ideal condition with cubic perovskites; 0.71-0.9 -A cations are too small to fit in to their interstices with several possible structures such as orthorhombic and rhombohedral variants and t< 0.71 indicates A and B ions have similar ionic radii with possible close packed structures. For this purpose, it is assumed that for a stable structure to form the cations, just touch the surrounding anions (Goldschmidt's rule), then: t = (r A + r X )/(r B + r X ) = √2 or, where t is called the tolerance factor, r A is the radius of the cage site cation, r B is the radius of the octahedrally coordinated cation and r X is the radius of the anion. Goldschmidt's proposal was that a stable perovskites structure phase would form if the value of the tolerance factor, t, was close to 1.0. However, this is applicable at room temperature to the empirical ionic radii. The tolerance factor for CuPbI 3 at room temperature is 0.801 nm which could be tetragonal structure. The cubic unit cell edge, a, is equal to twice the B─X bond length: 2(B-X) = a. The width of the cuboctahedral cage site, √2a, is equal to twice the A─X bond length: 2(A-X) = √2a. This means that the ideal structure forms when the ratio of the bond lengths is given by: (A-X)/(B-X) = √2. .Because many perovskites structures have been described, it is now usual to use the measured bond lengths in the crystal rather than ionic radii to give an observed tolerance factor t obs : t obs = (A-X)/√2(B-X), where (A─X) is the average of the measured bond lengths between the A cation and the surrounding 12 anions and (B─X) is the average of the measured bond lengths between the B cation and the surrounding six anions. For CuPbI 3 perovskites, a is 2x3.2025 = 6.405Å, width of the cuboctahedral cage site is 2x4.53 equal to 9.06 Å and the tolerance factor given by the average bond length is 1.00 for all the perovskites materials. The stable structure is cubic structure for this material at high temperature. Error! Reference source not found. shows cubic crystal structures of CuPbX 3 derived by replacing . We find that the lattice constants of ABX 3 increase with increasing the size of X from Cl, to Br and I. respectively. This optical band gap value also agrees with the band gap value calculated for phase change in CH 3 NH 3 PbI 3 by UV-Vis spectroscopy. It is also reported that the presence of methylammonium cation or Cs + does not affect the band gap region and it is the inorganic component of lead halide perovskites that play a dominant role in ascertaining the band gap energy. Similarly, it has been reported that replacement of CH 3 NH 3 + or Cs + by a univalent transition d 10 metal ions, e.g., Cu + did not affect the band gap. Naeem et.al. revealed that the role of Cu + with d 10 configuration is just to balance the charge (Cu acts just as a charge donor here) and does not have any important contribution for the conduction and valence band states except for donating one electron to the Pb-I framework as is reported for CH 3 NH 3 + and Cs + ions in CH 3 NH 3 PbI 3 and CsPbI 3 , respectively. However, our result is opposite to their conclusion that Cu is predictable to directly influence the electronic properties, e.g. Cu + has contributions to the band edges of the CuPbX 3 materials. Figure shows the effect of size on the optical bandgap of ternary lead halide perovskites. We observed that the band gap increase with increasing size of A ions and decreasing size of X ions. These chemical trends are important to understand and optimize this type of solar cell absorbers materials. We explain these trends by the above band structure analysis shown in Figure , Figure The valence and conduction bands of methylammonium cation based perovskites are determined by the PbI 6 inorganic octahedron. The organic cation does not contribute to these bands but modulates the band gap by modifying the lead-halide bond distance; as a result, the shifts produced by substituting the organic cation are less pronounced than those produced by substitution of the halide. The full tunability of the band gap for CH 3 NH 3 PbX 3 has been demonstrated for I-Br compositions and strongly suggested for Br-Cl compositions. The valence and conduction bands of Cu + and Ag + based ternary perovskites are not only determined by the inorganic PbI 6 octahedron, but also by the Cu + and Ag + . The role of Cu + and Ag + is not only to modulate the band gap but also by tuning the band structure. Therefore, we demonstrate the full tunability of CuPbX