Lithium Polystyrene Sulfonate as a Hole Transport Material in Inverted Perovskite Solar Cells

Despite the exceptional efficiency of perovskite solar cells (PSCs), further improvements can be made to bring their power conversion efficiencies (PCE) closer to the Shockley-Queisser limit, while the development of cost-effective strategies to produce high-performance devices are needed for them to reach their potential as a widespread energy source. In this context, there is a need to improve existing charge transport layers (CTLs) or introduce new CTLs. In this contribution, we introduced a new polyelectrolyte (lithium poly(styrene sulfonate (PSS))) (Li:PSS) polyelectrolyte as an HTL in inverted PSCs, where Li⁺ can act as a counter ion for the PSS backbone. The negative charge on the PSS backbone can stabilize the presence of p-type carriers and p-doping at the anode. Simple Li:PSS performed poorly due to poor surface coverage and voids existence in perovskite film as well as low conductivity. PEDOT:PSS was added to increase the conductivity to the simple Li:PSS solution before its use which also resulted in lower performance. Furthermore, a bilayer of PEDOT:PSS and Li:PSS was employed, which outperformed simple PEDOT:PSS due to high quality of perovskite film with large grain size also the large electron injection barrier (ϕ_e) impeded back diffusion of electrons towards anode. As a consequence, devices employing PEDOT:PSS / Li:PSS bilayers gave the highest PCE of 18.64%.