Photon absorptions leading to singlet and triplet excitonic states in organic solar cells are presented in this study. Applying Fermi's golden rule, the rates of absorption of singlet and triplet excitons are derived using singlet exciton-photon and triplet exciton-spin-orbit-photon-interaction, respectively, as perturbation operators. The rate of triplet absorption depends on the square of the atomic number and hence heavier atoms play the dominant role. Incorporation of heavy metal atoms in the donor organic material enhances the absorption rate and hence absorption, leading of higher generation of excited charge carriers. This increases the conversion efficiency of organic solar cells. The results are compared with experimental studies.