Influence of pH-neutral lithium polystyrenesulfonate polyelectrolyte on the energy band structure and performance of organic solar cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is used ubiquitously in organic solar cell (OSC) devices, however, it is not clear how the anionic PSS component by itself affects the band structure of OSCs. In this contribution, we've conducted a detailed investigation of the pH-neutral lithium salt of PSS (Li:PSS) and mixtures of Li:PSS with PEDOT:PSS and their effect on the energy band structure and performance of OSCs. There is currently a need for transparent and pH neutral hole transport layer (HTL) materials in the context of OSCs, and Li:PSS itself is a solution-processable, transparent and pH-neutral polyelectrolyte that was found to function as an HTL in OSCs as a thin interfacial layer. Atomic force microscopy measurements reveal smooth films with reduced root-mean-square roughness compared to PEDOT:PSS. Electronic band structures at the interface between Li:PSS based HTLs and the active layer were determined and compared to PEDOT:PSS, revealing low hole injection barriers and a p-type band in the active layer at the HTL interface. These effects are consistent with the improved current density and high efficiency (up to 8.33%) observed in devices using mixtures of Li:PSS and PEDOT:PSS.