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

• Published in: Chemistry, an Asian journal Vol. 16; no. 20; pp. 3151 - 3161
• Main Authors: Ali Khawaja, Kausar, Khan, Yeasin, Park, Yu Jung, Lee, Jin Hee, Kang, Ju Hwan, Kim, Kiwoong, Yi, Yeonjin, Seo, Jung Hwa, Walker, Bright
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 18-10-2021
• Subjects: Anodes; Backbone; Charge transport; Chemistry; Diffusion barriers; Energy conversion efficiency; Grain size; hole transport layer; Lithium; Low conductivity; perovskite; Perovskites; Photovoltaic cells; polyelectrolyte; Polyelectrolytes; Polystyrene resins; solar cell; Solar cells
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.202100803

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 (ϕe) impeded back diffusion of electrons towards anode. As a consequence, devices employing PEDOT:PSS / Li:PSS bilayers gave the highest PCE of 18.64%.
This paper introduces Li:PSS, a new polyelectrolyte HTL in which Li+ acts as an inert counter ion for the insulating, anionically charged PSS backbone, which interacts electrostatically with holes in the perovskite layer, resulting in an increase in hole concentration and a decrease in Fermi energy at the interface, improving hole extraction and flow towards the anode.