Facile Photo-Mediated Dopant for Efficient Oxidation of Spiro-OMeTAD Hole Transport Layer in Perovskite Solar Cells

The advent of perovskites as a material with enviable optoelectronic and physicochemical properties has fuelled extensive research for its use in photovoltaic technology. To date, the power conversion efficiency (PCE) of halide perovskite solar cells (PSC) have surpassed 26%. Despite this impressive achievement, continuous research on perovskites and its device interlayers remains critical to realise future commercialization of PSCs. Commonly, highly efficient PSCs are fabricated using n-i-p structure where 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD) is used as the organic hole transport layer (HTL).
In its pristine state, spiro-OMeTAD has poor hole conductivity, hence it is conventionally doped with lithium salts, particularly, lithium bis(trifluoromethane)sulfonimide (LiTFSI), in the presence of 4-tert-butylpyridine (tBP), which acts as a morphology controller. The dilemma of using these dopants is the need to oxidise the HTL, post-device fabrication, in a controlled environment. Traditionally, this is done by incubating the devices in a dry environment with an adequate level of oxygen. However, such time-consuming exposure could compromise the quality and stability of PSCs due to the perovskites’ propensity to undergo phase transformation as well as the volatile nature of tBP. To address this, many studies have reported the use of alternative dopants or processes to facilitate the oxidation of spiro-OMeTAD. For instance, Kong et al. proposed a fast oxidation method by introducing CO₂ in spiro-OMeTAD solution under ultraviolet light. While these concepts are promising, it could involve complicated procedures. Hence in this work, the efficacy of an alternative dopant, sodium bis(trifluoromethane)sulfonimide (NaFSI), to accelerate spiro-OMeTAD oxidation via a facile photo-doping effect is first reported. To elicit the mechanism of this phenomenon, the respective effects of its cationic and anionic groups on the optoelectronic properties and device performance of FAPbI₃-based PSCs are investigated.
Mainly, UV-vis absorption studies on LiTFSI, NaFSI, and LiFSI-doped spiro-OMeTAD solutions demonstrated that the conversion to conductive oxidised spiro⁺ species is more efficient in the presence of FSI^--based dopants following light exposure, within minutes. Moreover, by comparing different excitation wavelengths, the oxidation process for NaFSI dopant saturates to a greater extent than LiTFSI and LiFSI, under 525 nm excitation. Whereas, it is posited that the Li-based dopants may require higher excitation energies to reach saturation. Based on these findings, the performances of PSC devices prepared using light-exposed and dark-stored spiro-OMeTAD solutions between the different dopants are compared. Notably, it is found that the photo-doping effect enabled a 19% and 35% improvement in PCE of LiTFSI and NaFSI-doped devices, respectively. Without extensive post-fabrication oxidation in a dry environment, the average (and highest) PCE for LiTFSI and NaFSI-doped devices are 18.29% (19.4%) and 21.24% (23.38%), respectively. Overall, the superior performance of NaFSI-doped devices can be attributed to lower shunt resistance and fill factor, possibly from the enhanced concentration of conductive spiro⁺ species in the HTL. This phenomenon is further supplemented by photoluminescence and electrical characterizations.