Effects of ZnO nanoparticle/nanorod composite films on the performance of organic solar cells

Abstract

<p indent="0mm">In past decades, organic solar cells (OSCs) have attracted much attention due to their advantages such as light weight, flexibility, variable color, adjustable band gap and roll-to-roll (R2R) manufacturing. Recently, the certificated power conversion efficiency (PCE) of single-junction OSCs has exceeded 18%, showing great potential for commercial applications. The outstanding performance of OSCs strongly depends on innovative research on the synthesis of efficient active materials. However, essential interfacial modification is also very critical because it can not only benefit charge extraction and transportation, but also effectively protect the active materials and thus improving the device performance. Therefore, various interfacial materials have been investigated intensively for highly efficient OSCs. Zinc oxide (ZnO) nanomaterials with excellent optoelectronic properties are considered to be one of the most promising candidates for electron transport layers (ETLs) of R2R printed OSCs. However, ETLs of the most common ZnO nanoparticles (NPs) inevitably have plenty of surface defects due to their high surface to volume ratio, which impairs the carrier transportation and collection. To address this concern, in this work, a composite film of ETL is designed for OSCs using the mixture of ZnO nanorods (NRs) and ZnO NPs. ZnO NRs with hexagonal crystal structure are synthesized by the solvothermal method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that the intensity of diffraction peak (002) of ZnO NRs was significantly higher than that of diffraction peak (101) and peak (101), indicating that nanorods grow along the <italic>c</italic>-axis orientation. It is also found that the diameter and length of well-crystallized ZnO NRs range from 5 to <sc>8 nm</sc> and from 15 to <sc>40 nm,</sc> respectively. The ZnO NRs are introduced as the electron transport material for OSCs with the structure of ITO/PEDOT:PSS/PM6:Y7/ZnO NPs/ZnO NPs:NRs/Al. The prepared ZnO NRs are blended with ZnO NPs to achieve an optimal surface morphology of the ETL and enhanced light capture capability of the device. ETLs with different ratios of ZnO NRs to ZnO NPs are prepared. When the mass ratio of ZnO NRs reaches 50%, the ETL becomes dense and uniform without apparent cracks and pinholes, so the resultant OSCs achieve the best performance. The best device shows a short circuit current density (<italic>J</italic>_sc)<italic> </italic>of <sc>24.30 mA cm^–2,</sc> an open circuit voltage (<italic>V</italic>_oc) of <sc>0.85 V,</sc> and a fill factor (FF) of 70.17%, corresponding to a champion PCE of 14.50%, which is higher than that of OSCs without ZnO NRs (PCE of 13.69% with <italic>J</italic>_sc<italic> </italic>of <sc>23.59 mA cm^–2, </sc><italic>V</italic>_oc<italic> </italic>of <sc>0.83 V</sc> and FF of 69.91%). Meanwhile, it is shown that the external quantum efficiency (EQE) curve of the optimal OSCs with ZnO NRs is higher than those of OSCs without ZnO NRs, which is consistent with the experimental results of <italic>J-V</italic> curve. The significant improvements attribute to the enhancement on the electron extraction and the light capture capability by the incorporation of ZnO NRs with excellent transport properties and the improved ETL morphology. Appropriately increased roughness after the incorporation of ZnO NRs also benefits the light absorption of the device. This work could come up with a new idea for the synthesis and application of ZnO-based interfacial materials in thin-film solar cells.