Lithium-Induced Defect Levels in ZnO Nanoparticles to Facilitate Electron Transport in Inverted Organic Photovoltaics

In this work, lithium-doped zinc oxide nanoparticles (LZO NPs) with different Li/Zn molar ratios (Li/Zn = 0, 0.05, 0.2) are successfully prepared to form an electron transporting layer (cathode buffer layer) in the inverted-type P3HT:ICBA organic photovoltaic (OPV) devices. As compared with the undoped ZnO NPs buffer layer, a considerable improvement OPVs from 2.344% to 2.946% is obtained by using 5%-LZO NPs as a buffer layer, which owns J_sc of 7.22 mA/cm², V_oc of 0.86 V, and FF of 47.4%. X-ray absorption near-edge structure (XANES) spectra show the increase of unoccupied O 2p-derived states in 5%-LZO NPs, which leads to better carrier conductance. The energy levels of defects in 5%-LZO NPs analyzed by photoluminescence are found to facilitate electron extraction to the cathode. Impedance measurement results indicate that the carrier lifetime is effectively increased to 2176 μs by applying the 5%-LZO NPs buffer layer, showing the improvement of carrier extraction efficiency and resulting in its progressive performance.