Device Performance Related to Amphiphilic Modification at Charge Separation Interface in Hybrid Solar Cells with Vertically Aligned ZnO Nanorod Arrays

This paper reports the chemical modification effects at charge separation interface on the performance of the hybrid solar cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as an electron donor (D) and vertically aligned ZnO nanorod arrays as an electron acceptor (A). Results show that, with increasing the modification time T-s for grafting dye Z907 onto the ZnO surface from 0 to 8 h, the charge transfer efficiency at the MEH-PPV/ZnO interface keeps increasing, the short circuit current (J(sc)) increases and reaches a peak value at T-s = 8 h, but the open circuit voltage (V-oc) increases within T-s = 1-3 h and reduces with further increasing T-s up to >= 6 h. By controlling the T-s, a peak power conversion efficiency of eta = 0.61% at AM 1.5 illumination (100 mW/cm(2)) is obtained for T-s = 6 h. It is revealed that the Z907 modification mainly contributes to the enhanced J(sc) by increasing the charge separation efficiency as a result of the improved electronic coupling property at the D/A interface rather than the light harvesting; on the other hand, the Z907 modification reduces (T-s = 1-3 h) or increases (T-s >= 6 h) the surface defect concentration of the ZnO nanorods, resulting in the increased or reduced V-oc (or electron lifetime tau(e)). It is demonstrated that trapping electrons by the surface defects may facilitate the charge separation at the D/A interface in the MEH-PPV/ZnO devices, and both V-oc and tau(e) correlate to the occupation of injected electrons in conduction band and surface defects. Further analysis provides the relation between V-oc and tau(e) in those devices.