ZnO Nanostructure Optimization to Enhance High Efficiency Inverted Organic Solar Cells

Much scientific effort has been directed toward developing organic solar cells (OSCs) because of their significant potential to lower manufacturing costs of large area, lightweight, and highly flexible photovoltaics.  Inverted OSCs have gained considerable attention as they offer increased device stability and processing advantages.  ZnO serves as an effective electron extraction layer in inverted OSCs while efficiently blocking holes.  Two ways to fabricate ZnO films includes using a sol-gel method or by depositing pre-synthesized nanoparticles (NPs) and the morphology and thickness of these films greatly impacts light absorption and charge collection.  In this work, we used transfer matrix optical modeling and tailored film processing techniques to optimize these ZnO films for high efficiency OSCs.  We compared the performance of commercially available ZnO NPs with in-house synthesized ZnO NPs and sol-gel derived ZnO films.  Atomic force microscopy measurements show that controlled static or dynamic baking conditions for the sol-gel ZnO films can yield either unstructured films or nanostructured films with “nanoridges”, respectively.  The optical properties of these films were measured with ellipsometry and used in the optical modeling to analyze and rationally design the devices.  We used a blend of [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) and the low-bandgap semiconducting polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7) to make the bulk heterojunction active layer and MoO3 as the top hole collecting layer with a Ag anode.  When using ITO-coated glass as the cathodic substrate, we showed an optimized PCE as high as 8.4% using statically baked, sol-gel derived ZnO.  The sol-gel derived ZnO was not compatible with flexible PET substrates due to the high baking temperature (150˚C) causing the PET to warp.  When using flexible ITO-coated PET substrates and in-house synthesized ZnO NPs, the optimized device performance reached 6.9% PCE, which is exceptional performance for flexible OSCs.  The optical and electrical properties of these films will be discussed and tied to device performance measurements.  These results show the importance of interfacial layer design to device performance and provide a simple way to optimize ZnO films for high performance inverted OSCs.