Hierarchical Construction of ZnO Nanorods on Nanofibers for Hybrid Solar Cells

Hybrid inorganic/organic photovoltaic devices based on semi-conducting inorganic materials as n-type acceptors in conjunction with p-type conducting polymers have generated much research interest. Zinc oxide (ZnO) is known for its high electron mobility, acceptable refractive index, and the ease in process when tuning its size and shape. Particularly, continuous electron pathways in ZnO are essential for the transport of the generated charge upon exciton separation at the ZnO/polymer interface to the electrode. In this work, the surface area of ZnO nanofibers (NFs) substrates was further increased by the deposition of nanorods (NRs) to form hierarchical-structured ZnO substrates. FE-SEM images of the ZnO NRs on NFs were shown in the Figure. Photovoltaic performance of hybrid ZnO NFs/poly(3-hexylthiophene) (P3HT) solar cell is enhanced through the deposition of ZnO NRs. The deposition of NRs on electrospun NFs led to an increase in short-circuit current density (J_SC) from 0.47 to 0.89 mA cm^-2 and open-circuit voltage (V_OC) from 0.08 to 0.27 V. The improvement of V_OC can largely be attributed to the higher J_SC due to the higher surface areas and the suppression of reverse current saturation density (J₀) due to better electrode coverage. These increases led to ten times improvement in power conversion efficiency from 0.01% to 0.10%. The improvements were mainly attributed to the shorter NR lengths decreasing charge injection from the Ag electrode. Further improvements in Vocs can be achieved by improving P3HT coverage or reducing the thickness of the NF mats decreasing the electrospinning time since the decreasing the lengths of the NRs of the hierarchical structure can decrease the minority charge blocking in the FTO/ZnO interface. The hierarchical structure based on NFs anchored on thin-films of ZnO nanoparticles (NPs) with NR deposition can solve the trade-off of surface area and the direct electron pathway of the NRs and NPs morphology.