Development of flexible, stable, and efficient inverted organic solar cells harvesting light in all directions

In this work, we designed low-cost, liquid-free, efficient, and highly flexible fiber-shaped inverted structure organic solar cells (FOSCs) over a flexible polyethylene terephthalate (PET) monofilament substrate. We also prepared a graphene–ZnO (G-ZnO) composite, wherein G sheets were compacted into a bunched-up structure through the binding force of Zn atoms with the C atoms of G. This composite was then utilized as a bifunctional layer i.e. electron transport and downconversion spectral in the FOSCs. The FOSCs based on the G-ZnO (D-1) demonstrated a power conversion efficiency (PCE) of 2.13% out of which 4.89% and 5% was retained after 8000 times bending and 120 h storage in ambient environmental conditions, respectively. The non-G-ZnO FOSCs (D-2) demonstrated a PCE of 1.78% and retained 5% and 6% of the initial value after 6000 bends and 48 h of storage in ambient environmental conditions, respectively. This better performance of D-1 compared to that of D-2 is due to the interfacial functionalization of G-sheets and ZnO nanoparticles inside the G-ZnO composite. Because of these interfacial chemical bonds, the G sheets were in close contact with each other and attached firmly through the ZnO molecules. As a result, these compacted G layers could serve as a strong barrier resisting the penetration of water molecules inside the device, thereby leading to an improved lifetime for the device. Additionally, the longitudinal and cross linkage of G-sheets could improve the mechanical properties of the G-ZnO composite, which in turn enhanced the flexibility of D-1. Finally, these interface functionalizations could work as linking bridges, providing an additional pathway for the transportation of free charge carriers. Therefore, D-1 demonstrated a higher Jsc by collecting a greater number of charges at the electrode compared to D-2, because the latter lacked similar functionalization.