Germinant ZnO nanorods as a charge-selective layer in organic solar cells

A facile method was introduced and demonstrated to synthesize zinc oxide (ZnO) nanorods (NRs) as an electron transporting layer (ETL) for organic solar cells (OSCs). Hydrothermal synthesis of the NRs showed a constant growth rate of 5.5 nm min⁻¹ from germination to sub-micrometer length. The properties were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), absorption spectrophotometry and so on. Based on these measurements, the germinant growth mechanism and its corresponding orientation characteristics were investigated. As an ETL of the OSCs, ZnO NRs enhance the charge extraction from the active layer due to their increased interfacial surface area, but there is an optimal length because of the shunt path formation and UV absorption of long ZnO NRs. As a result, the OSC with the ZnO NRs as ETL shows power conversion efficiency (PCE) up to 6.2%. The J-V characteristics and incident photon-to-current conversion efficiency (IPCE) measurement also reveal that the efficiency enhancement is an assembly of individual results from optical, physical and electrical characteristic of the ZnO NRs.