Improved performance of MEH-PPV/ZnO solar cells by addition of lithium salt

The paper reports the improved performance by addition of lithium bis(trifluoromethanesulfonyl)amide (LiTFSI) to poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in the hybrid solar cells consisting of MEH-PPV as an electron donor and vertically aligned ZnO nanorod array as an electron acceptor. Results show that, with increasing the weight ratio R of LiTFSI/MEH-PPV, the charge transfer efficiency at MEH-PPV/ZnO interface, the device short circuit current (J_sc) and open circuit voltage (V_oc) get increased for R ? 2/10, but decreased when R > 2/10, resulting in a peak power conversion efficiency of η = 0.48% for R = 2/10 at AM 1.5 illumination (100 mW/cm²). It is revealed that the increased J_sc is due to the improved charge transfer between the MEH-PPV and ZnO as a result of the interaction between LiTFSI and MEH-PPV, while the increased V_oc and the decreased charge recombination are attributed to the increased hole mobility of MEH-PPV; moreover, the decreased J_sc and V_oc at high R values are attributed to the morphology degradation in the active layer due to the high LiTFSI content.