Enhanced Efficiency of Polymer: Fullerene Bulk Heterojunction Solar Cells with the Insertion of Thin TiO₂ Layer near the LiF/Al Electrode

The insertion layer of TiO₂ between polymer-fullerene blend and LiF/Al electrode is used to enhance the short-circuit current I _sc and fill factor (FF). The solar cell based on the blend of poly2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) and C₆₀ with the modifying layer of TiO₂ (about 20nm) shows the open-circuit V_oc of about 0.62V, short circuit current I_sc of about 2.35mA/cm², filling factor FF of about 0.284, and the power conversion efficiency (PCE) of about 2.4% under monochromatic light (500nm) photoexcitation of about 17mW/cm ². Compared to cells without the TiO₂
layer, the power conversion efficiency increases by about 17.5%. Similar effect is also obtained in cells with the undoped MEH-PPV structure of ITO/PEDOT:PASS/MEH-PPV/(TiO₂)LiF/Al. The improved solar cell performance can be attributed to enhanced carrier extraction efficiency at the active layer/electrode interfaces when TiO₂ is inserted.