Strontium Fluoride and Zinc Oxide Stacked Structure as an Interlayer in High-Performance Inverted Polymer Solar Cells

Enhanced power conversion efficiency is reported in inverted polymer solar cells when an ultrathin layer of strontium fluoride (SrF₂) is evaporated on the surface of the solution-processed zinc oxide (ZnO) electron transport layer. The photoactive layer is made up of bulk heterojunction composites of poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5- b']dithiopheneco-3-fluorothieno[3,4- b]-thiophene-2-carboxylate] and [6,6]-phenyl-C₇₁-butyric acid methyl ester. The ZnO film acts as an effective electron transport layer, whereas the ultrathin SrF₂ layer improves the energy level alignment and enhances the built-in potential via the formation of an interfacial dipole layer at the interfaces between the ZnO film and the photoactive layer, resulting in an enhanced electron extraction efficiency and a decreased carrier recombination loss. Furthermore, the SrF₂ layer reduces the inherent incompatibility between the hydrophilic ZnO film and the hydrophobic photoactive layer. As a result, all the photovoltaic performance parameters are remarkably improved, leading to a high efficiency of up to 10.46% (with a fill factor of 71.38%), corresponding to a ca. 21% improvement over the reference device performance (8.64%). The use of a ZnO/SrF₂ stacked interlayer provides a simple, but effective, approach to obtain high-efficiency inverted PSCs.