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Effect of 8-hydroxy-quinolinato lithium thickness on the power conversion efficiency of polymer photovoltaic cells

We investigated the dependencies of a polymer photovoltaic (PV) cell on the thickness of its 8-hydroxy-quinolinato lithium (Liq) layer. The power conversion efficiency (PCE) of the polymer PV cell shows a trend similar to that of the fill factor (FF), but not to the open-circuit voltage (Voc) and the short-circuit current (Jsc), for various Liq layer thicknesses. This trend is likely associated with the series resistance of the polymer PV cell, which depends on the thickness of the Liq layer. As a result, if the PCE of the polymer PV cells fabricated with a Liq layer is to be enhanced, a specific thickness (in our experiment, ∼1.0 nm) should be used for the Liq layer. In addition, we suggest that the electron injection mechanism of the Liq layer between the small-molecular hole/exciton blocking layer (in our experiment, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)) and the metal cathode (in our experiment, Al) strongly influences the PCE of polymer PV cells.