Theoretical Studies on Understanding the Feasibility of Porphyrin-Sensitized Graphene Quantum Dot Solar Cell

We present results of our theoretical investigation on the electronic structure of porphyrin functionalized graphene quantum dots(GQDs). We put our emphasis on how the electronic energy levels of GQDs are modified because of the functionalization by porphyrin molecules. Our study shows that the HOMO-LUMO gap of the GQD-porphyrin hybrid material is lower as compared to GQD of same size. The possibility of forming type-II nanohybrids is explored by varying either the size of the QD or by attaching different functional groups to the porphyrin moiety. We also analyzed the absorption spectra of one model GQD-porphyrin nanocomposite. We found that GQD-porphyrin hybrid material form type-II band alignment for GQD of smaller size. The hybrid material with larger QDs show type-I band alignment however they can be made type-II by proper attachment of the electron donating group to the porphyrin molecule. The type-II band alignment lowers the chance of electron-hole recombination. The free energy of electron injection, that is, the energy gap between LUMO of porphyrin and LUMO of GQD increases with increasing the size of the GQD. So, on the basis of our theoretical study, we suggest that the GQD-porphyrin nanohybrid material with larger GQD may be a good candidate for the application in solar cell.