Insights into Working Mechanism of Alkali Metal Fluorides as Dopants of ZnO Films in Inverted Polymer Solar Cells

Abstract

Alkali metal fluorides (AMFs) have been utilized as dopants of ZnO films in PTB7:PC71BM based inverted polymer solar cells (i-PSCs). As a result, power conversion efficiency (PCE) and device stability were obviously improved in i-PSCs with ZnO:AMF. In particular for the i-PSCs with ZnO:NaF (0.2 mol %), PCE of 8.64% was obtained in the fresh devices and the PCE still remained at 7.56% after i-PSCs were retained in the glovebox for 80 days, which were much better than the only ZnO devices. Results of photocurrent density–effective voltage characteristics, electron mobility, and capacitance–voltage characteristics demonstrated that doping ZnO with AMFs can effectively raise the charge carrier extraction, electron mobility, charge carrier density, and built-in potential in the i-PSCs. X-ray photoemission spectroscopy (XPS) measurements indicated that oxygen defects on the ZnO surface were reduced through doping with AMFs. Atomic force microscopy images showed that adding 0.2 mol % AMFs into ZnO did not lead to the lattice distortion of ZnO films. Scanning electronic microscopy–energy dispersive spectrum mapping and XPS depth profiling suggested that there were more Na+ species at the top of the ZnO:NaF surface than K+ or Cs+ at the top of the ZnO:KF or ZnO:CsF surface. Therefore, Na+ have more chances to meet the defects on the ZnO surface than K+ and Cs+ so that the i-PSCs with ZnO:NaF shows the best PCE. These results reveal that NaF is an effective, competitive, and prospective dopant of ZnO in i-PSCs.