Kelvin probe force microscopic imaging of the energy barrier and energetically favorable offset of interfaces in double-junction organic solar cells

A double-junction polymer solar cell (PSC) has attracted extensive attention as a promising approach to increasing efficiency. Tunneling/ recombination interlayers between subcells play a critical role in double-junction PSCs. Interlayers include electron-transport layers (ETLs) such as Nb₂O₅, ZnO, and TiO_x and hole-transport layers (HTLs) including PEDOT:PSS. These materials have all been used as interlayer materials, but it remains unclear which one is better than the other. Kelvin probe force microscopy (KFM) was used to identify the energy barrier and energetically favorable energy offset at the interfaces of acceptor-ETL (e.g., PCBM-Nb₂O₅, PCBM-ZnO, and PCBM-TiO_x) and donor-HTL (e.g., MDMO-PPV/PEDOT:PSS). Here the interface refers to the junction of two materials, formed by drop-casting one on top of other. The interface is buried and is therefore not accessible to the KFM probe. The energy barrier for electron transport from PCBM to ETL was found at ∼0.20, ∼0.12, and ∼0.012 eV at the PCBM-Nb₂O₅, PCBM-ZnO, and PCBM-TiO_x interfaces, respectively. Hole transport from the donor polymer to PEDOT:PSS was found to be energetically favorable with an energy offset of ∼0.14 eV to facilitate hole transport. The thickness independences of the energy barrier and energetically favorable energy offset at the interfaces of acceptor-ETL and donor-HTL were also observed. This work will provide guidance for researchers to identify and select appropriate materials as interlayers in double-junction PSCs.