n-Type Doped SWCNT Film using Organophosphorus Compound as Cathode in Inverted Perovskite Solar Cells

Abstract

Single-walled carbon nanotubes (SWCNT) typically possess p-type semiconducting properties when exposed to air and oxygen, making them commonly used as anodes in perovskite solar cell (PSC) devices. In this study, we proposed a facile n-type doping strategy to enable SWCNTs to function as cathodes in inverted (p-i-n) PSC [glass / indium tin oxide / poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) / MAPbI3 / n-doped SWCNT : PCBM]. Organophosphorus compounds, including triphenylphosphine (TPP), 1,2-bis(diphenylphosphino)ethane (DPPE), and 1,1'-bis(diphenylphosphino)ferrocene (DPPF), were utilized as molecular n-type dopants for SWCNT electrodes. The doping mechanisms and their application in PSC were validated through computational and experimental studies. Overcoating doped SWCNTs with PCBM was to be essential to remove uncoordinated dopants, enhancing surface contact with the perovskite layer, and maintaining n-type characteristics. This process mitigated energy mismatches caused by shallow LUMO levels of phosphine-based dopants, thereby improving electron transport and optimizing energy levels for efficient charge transfer. Inverted PSC incorporating DPPE-doped SWCNT electrodes demonstrated significant improvements in device efficiency, increasing from 5.1% to 8.03%. From the carrier lifetime studies, these improvements are attributed to rapid electron transfer and reduced recombination at the interface between the doped SWCNT and the bulk perovskite. Notably, the n-type dopants that induced significant energy level shifts adversely affected the PSC performance. The hydrophobic nature of the doped electrodes contributed to the long-term stability of the devices. Unencapsulated PSC with doped electrodes retained 50% of their initial efficiency after 500 h of storage under controlled humidity (25% RH), temperature (25 ℃), and ambient light conditions (300 lux).