Selective synthesis of ZnO nanorods on graphene for solar cell applications

In this study, we achieved the selective growth of spatially ordered ZnO nanorods (NRs) in large scale with varying diameters and lengths on graphene pre-coated glass surfaces, resulting in the manufacture of a third-generation core-shell Cu₂ZnSnS₄(CZTS) solar cell. Using a combination of hydrothermal technique and nanosphere lithography, ZnO NR arrays with different diameters and lengths were synthesized on single and bilayer graphene grown on copper foils and transferred to glass substrates via the PMMA-assisted transfer technique. The Langmuir-Blodgett method facilitated the transfer of polystyrene nanospheres as a single layer onto the ZnO seed layer. It was found that the morphology of the ZnO seed layer, crucial for NR synthesis on graphene, significantly influenced the quality of the mono-layer nanosphere mask. Adjustable nanospheres via O₂ plasma treatment were used to synthesize spatially ordered ZnO NR arrays. To reduce defect density at the core (ZnO NR) and shell (CdS/CZTS) interfaces, ZnO NRs were coated with a thin TiO₂ layer before applying the CZTS absorber layer. The monophase kesterite CZTS absorber layer was successfully applied to ZnO NRs synthesized on graphene through thermal evaporation of polycrystalline CZTS powder. Prototype solar cells (Glass/Graphene/ZnO-NRs/TiO₂/CdS/CZTS/Ag) were constructed to demonstrate the application of selectively grown spatially ordered ZnO NRs on graphene layers in a core-shell architecture. A second solar cell, using pristine ZnO NRs, was also fabricated to compare their performances. For the TiO₂-passivized ZnO NR solar cell, V_oc, J_sc, FF, and efficiency were 0.38 V, 20 mA/cm², 26 %, and 1.9 %, respectively, compared to 0.32 V, 11 mA/cm², 24 %, and 0.84 % for the pristine ZnO NRs based cell, highlighting the significant performance improvement due to TiO₂ passivation.