PEIE capped ZnO as cathode buffer layer with enhanced charge transfer ability for high efficiency polymer solar cells

• The device with ZnO/PEIE buffer shows a 3.8% efficiency without thermal annealing.
• The faster charge transfer and less recombination are found for ZnO/PEIE buffer.
• AFM is used to study the reason of better device performance with ZnO/PEIE buffer.

In this work, a cathode buffer layer (CBL) with higher charge transfer ability was fabricated by using polyethylenimine ethoxylated (PEIE) covered ZnO nanoparticles (NPs) (ZnO/PEIE). A high power conversion efficiency of 3.8% was achieved for an inverted polymer solar cell based on poly(3-hexylthiophene) (P3HT):phenyl C61-butryricacid methyl ester (PCBM) by using this ZnO/PEIE CBL, which is much higher than that of other cells with only PEIE or ZnO CBL. Transient photovoltage/photocurrent and electrochemical impedance measurements confirm that device with ZnO/PEIE bilayer has faster charge transfer ability and less interfacial charge recombination than devices with other CBLs. AFM characterization shows that ZnO NPs are uniformly covered by PEIE, which would be not only lower the work function of ZnO, but also be beneficial to reduce defects caused by oxygen adsorption on ZnO.

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