Anion‐Modulated Chemical Doping of Organic Hole Conductor Boosts Efficiency and Stability of Perovskite Solar Cells

Abstract

Chemical doping of organic semiconductors enables significant progress in improving their optoelectronic performance. However, the correlation between doping counter ions and charge-transport mechanism has not been yet well-understood. In this study, it is discovered that the anion-dependent degree of delocalization (DOD) of lithium-based dopants significantly determines the doping kinetics as well as the conductivity of organic hole transport layer (HTL), leading to large variation in solar cell efficiency and device stability. Specifically, the incorporation of bis(pentafluoroethanesulfonyl) imide (PFSI − ) as the anion with a high DOD results in one order of magnitude higher film conductivity and thus an elevated power conversion efficiency (PCE) exceeding 22.1%, much higher than the state-of-the-art lithium bis(trifluoromethane)sulfonimide (LiTFSI) (21.1%) and lithium hexafluorophosphate (LiPF 6 ) (20.0%). Moreover, the dopant LiPF 6 with a smaller DOD produces higher doping yield of HTL accompanied by stronger light-induced PCE fluctuation. Structural analysis reveals anion-modulated ion exchange kinetics determine the hole-transport mechanism and device photostability. To mitigate these detrimental effects, a versatile strategy of Li + solvation is developed to modulate the anion dissociation, enabling simultaneous improvement of device efficiency and stability. This study elucidates an intriguing and generally applicable doping mechanism, and envisages a bright future to further developing efficient and stable organic electronics.

Environmentally friendly anti-solvent engineering for high-efficiency tin-based perovskite solar cells

“…5h). [44][45][46] Then, the operational stability tests of the devices prepared using CB and HAc have been performed under maximum power point tracking (MPPT) conditions (Fig. S15, ESI †).…”

Light Soaking Effects in Perovskite Solar Cells: Mechanism, Impacts, and Elimination

“…Recently, the Zhang group reported the anion modulated chemical doping process of Spiro-OMeTAD also influences the LSE of devices. 98 Specifically, the degree of delocalization (DOD) of the counterions (PF 6 , TFSI, and PFSI) in lithium salts determines the cation exchange between Li + and Spiro-OMeTAD + , which influences the strength and reversibility of LSE. For example, PF 6 with a low DOD resulted in a strong and reversible LSE.…”

“…The results showed that the increase of the diffusion length was related to the photogeneration of carriers, but not to the absolute density of carriers. Recently, the Zhang group reported the anion modulated chemical doping process of Spiro-OMeTAD also influences the LSE of devices . Specifically, the degree of delocalization (DOD) of the counterions (PF 6 , TFSI, and PFSI) in lithium salts determines the cation exchange between Li + and Spiro-OMeTAD + , which influences the strength and reversibility of LSE.…”

Sequential Molecule‐Doped Hole Conductor to Achieve >23% Perovskite Solar Cells with 3000‐Hour Operational Stability

“…[22,23] The involved ionic dopant, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Figure 1A), often instigates moisture absorption and ion migration, which induces compositional variation and leads to the degradation of HTLs. [18,24] Moreover, the escape of volatile species such as 4-tert-butylpyridine (tBP) from the film under heat results in severe phase segregation in the HTLs which is often caused by lithium-salt aggregation. [21,25] Long post-oxidation process is typically required due to the low efficiency of the airactivated chemical doping process of Spiro-OMeTAD, leading to low productivity and reliability of the HTL fabrication.…”