Improved efficiency of inverted planar perovskite solar cells with an ultrahigh work function doped polymer as an alternative hole transport layer

The solar energy field is in need of better inverted planar perovskite solar cells that feature appropriate energy level alignment, efficient carrier transport in the hole transport layers, and a high-quality perovskite film on top. To help meet this need, we have developed a new transparent conductive polymer, based on sulfonated poly(thiophene-3-[2-(2-methoxy-ethoxy) ethoxy]-2,5-diyl) (S-P3MEET), as an alternative hole transport layer for inverted triple-cation lead mixed-halide perovskite. The resulting perovskite solar cell device showed efficiency of 17.25%, an open-circuit voltage (V-OC) of 0.965 V, and a fill factor of 80%, with minimal hysteresis. However, the pristine S-P3MEET also had a work function (WF) of -4.98 eV, which is low and needed improvement to achieve high energy level alignment at the perovskite interface. To overcome this drawback and make an ohmic contact at the perovskite interfaces, we tuned the WF of the pristine S-P3MEET by adding a desired weight ratio of a perfluorinated ionomer (PFI) to the S-P3MEET blend solution. The fluorinated S-P3MEET had a better WF of -5.45 eV, leading to remarkable improvement in the V-OC of the inverted perovskite solar cell device. The resulting device with 10% wt PFI doping ratio showed efficiency of 19.6% with a V-OC of 1.07 V. In contrast, the control device (poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)) had lower efficiency, 14.97%, with higher hysteresis effects. Most importantly, the fluorinated S-P3MEET perovskite device showed long-term stability due to the unique hydrophobicity of the capping layer of rich doping PFI on the S-P3MEET surface.