Improved Stability of Interfacial Energy-Level Alignment in Inverted Planar Perovskite Solar Cells

Even though poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been commonly used as a hole extraction layer (HEL) for p-i-n perovskite solar cells (PSCs), the cells' photovoltaic performance deteriorates because of the low and unstable work functions (WFs) of PEDOT:PSS versus those of a perovskite layer. To overcome this drawback, we synthesized a copolymer (P(SS- co-TFPMA)) ionomer consisting of PSS and tetrafluoropropylmethacrylate (TFPMA) as an alternative to conventional PEDOT:PSS. The PEDOT:P(SS- co-TFPMA) copolymer solution and its film exhibited excellent homogeneity and high phase stability compared with a physical mixture of TFPMA with PEDOT:PSS solution. During spin coating, a self-organized conducting PEDOT:P(SS- co-TFPMA) HEL evolved and the topmost PEDOT:P(SS- co-TFPMA) film showed a hydrophobic surface with a higher WF compared to that of the pristine PEDOT:PSS film because of its chemically bonded electron-withdrawing fluorinated functional groups. Interestingly, the WF of the conventional PEDOT:PSS film dramatically deteriorated after being coated with a perovskite layer, whereas the PEDOT:P(SS- co-TFPMA) film represented a relatively small influence. Because of the superior energy-level alignment between the HEL and a perovskite layer even after the contact, the open-circuit voltage, short-circuit current, and fill factor of the inverted planar p-i-n PSCs (IP-PSCs) with PEDOT:P(SS- co-TFPMA) were improved from 0.92 to 0.98 V, 18.96 to 19.66 mA/cm², and 78.96 to 82.43%, respectively, resulting in a 15% improvement in the power conversion efficiency vs that of IP-PSCs with conventional PEDOT:PSS. Moreover, the IP-PSCs with PEDOT:P(SS- co-TFPMA) layer showed not only improved photovoltaic performance but also enhanced device stability due to hydrophobic surface of PEDOT:P(SS- co-TFPMA) film.