Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells

ZnO layer was modified with the addition of Cationic dyes including Crystal Violet (CV)/Ethyl violet (EV) in sol–gel process for an electron transport layer in inverted polymer solar cells (PSCs). X-ray photoelectron spectra showed the presence of CV/EV at the top of ZnO surface. Besides, oxygen defect was significantly reduced by CV/EV modification due to the chloride occupation. Furthermore, the amount of CV/EV decreased progressively from ZnO surface to bottom, being evidenced by depth profile. With modification, the ZnO surface became smoother and more hydrophobic to improve the contact with active layer. Meanwhile, CV/EV participated in the crystallization which resulted in the larger ZnO crystal grain size. Interface dipole after modification would slightly reduce the work function of ZnO and the energy barrier between ZnO and active layer via Ultraviolet Photoelectron Spectroscopy and External Quantum Efficiency analysis. Accordingly, inverted PSCs possessed better morphology, better electron extraction ability with ZnO modified by CV and EV respectively, rendering the power conversion efficiency up to 8.80% and 9.06% in comparison to the pristine ZnO (7.59%). In conclusion, we demonstrate a facile way to improve morphological and electrical properties of ZnO layer by simply adding CV/EV in sol–gel ZnO to fabricate high performance PSCs.