Efficiency and Air-Stability Improvement of Flexible Inverted Polymer Solar Cells Using ZnO/Poly(ethylene glycol) Hybrids as Cathode Buffer Layers

The flexible inverted polymer solar cells composed of poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C-61 butyric acid methyl ester (PC61BM) blends on the flexible poly(ethylene terephthalate) (PET) substrates were fabricated, which showed improving device performance by using solution-processed ZnO/poly(ethylene glycol) (PEG) hybrids as cathode buffer layers compared to the devices using the pristine ZnO as cathode buffer layers. It is mainly attributed to the effective passivation of the ZnO surface traps, suppression of the interfacial charge recombination, decrease of the work function and improvement of the energy-level alignment between ZnO and PC61BM. When the PEG was introduced into the ZnO, the large aggregates was dispersed and yielded large ZnO nanoclusters containing less domain boundaries. The performance of devices with ZnO/PEG6000 (with averaged molecular weight of 6000) hybrids exhibited the best power conversion efficiency (PCE) of 3.3% compared to the devices with ZnO/PEG400 (with averaged molecular weight of 400) and ZnO/PEG20000 (with averaged molecular weight of 20000). It was found that the short PEG backbone (e.g., M-w = 400) containing less oxygen could not effectively passivate ZnO surface traps, meanwhile, longer PEG backbone (e.g., M-w = 20000) could lead to the formation of the charge transport barrier because of the insulating nature of PEG. Furthermore, solar cells with the ZnO/PEG buffer also showed better air-stability. The 23% degradation was observed after 14 days, compared to the 45% degradation of devices with the pristine ZnO buffer. In addition, due to the simplicity and low-temperature process, the ZnO/PEG hybrids can be well-suitable as cathode buffer for large area roll-to-roll manufacturing of printed polymer solar cells.