Metamorphosis of the ZnO buffer layer thicknesses on the performance of inverted organic solar cells

This study investigates the zinc oxide (ZnO) buffer layer thickness in the photovoltaic performance of inverted organic solar cells (OSCs) based on an active layer blend of poly(3-hexylthiophene), (P3HT) and [6, 6]-phenyl-C61 butyric acid methyl ester, (PCBM). The ZnO buffer layer acts as a protective layer to prevent the photoactive layer interface by UV light from oxidation. Besides, it reduces the energy barrier to easily transfer electrons between the collecting electrode and the organic acceptor lowest unoccupied molecular orbital level. The buffer layer block holes in P3HT from recombining with electrons in the collecting electrode. The X-ray diffraction analysis show that the constant orientation of the grains according to the c-axis perpendicular to the substrate surface. The optical measurements indicated that all samples have a transmission higher than 60 % in the visible range. A slight shift of the absorption edge toward the small wavelengths was observed as the thickness increased to over 250 nm. The electrical measurements depended on thickness. The resistivity decreased from 5.45 to 4.98 × 10⁻³ Ω.cm, and the mobility increased from 1.66 to 1.71 × 10⁻¹ cm²/Vs when the thickness increased from 65.6 nm to 107.0 nm. This behavior was explained by the crystallinity pattern. The optimization of the ZnO buffer layer caused the short circuit current density to vary from 0.287 to 1.599 mA/cm² and the fill factor to range between 19.08 and 24.55 %. This result increased the power conversion efficiency from 0.007 to 0.043 %. The photovoltaic performance of inverted structure OSCs is strongly dependent on the ZnO buffer layer thickness.