Carbon Nanotubes to Outperform Metal Using Dopant Engineering and Hole-Transporting Layer Control in Perovskite Solar Cells
Authors: Jeon Il , Shawky Ahmed , Kauppinen Esko , Matsuo Yutaka , Maruyama Shigeo 
Published: February 2020

Lead halide perovskite solar cells (PSCs) are a promising alternative energy source that has received a considerable attention in recent year. Their certified power conversion efficiencies (PCEs) now exceed 20% and only a few challenges, such as long-term stability and cost of fabrication, remain before commercialization. PSCs in general have a structure in which a photo-active material and two charge-selective materials are sandwiched by a transparent bottom electrode and a metal top electrode. Typically, the metal electrodes are thermally evaporated in vacuum, which incur substantial increase in fabrication costs and material costs. Further, using these metal electrodes induces the ion-migration, which is detrimental to the long-term stability of the perovskite layer. Thus, it is crucial that we find an alternative to the metal electrodes to bring PSCs to the next level.
Having high conductivity and facile processability, aerosol synthesized single-walled carbon nanotubes (CNTs) have showcased promising potential as the top electrode in PSCs. Using CNTs as the top electrode drastically improves the PSC stability by both removing the ion migration and functioning as an effective moisture barrier. Further, CNTs are mechanically and chemically robust which contribute to the durability of the PSCs. Moreover, the fact that CNTs are made up of earth-abundant carbon atoms only and the aerosol-synthesized CNT films are direct-transferable means the low-cost production. The only shortcoming of employing the CNTs as the top electrode in PSCs, this far, has been the limited PCE. This is because CNTs do not reflect light unlike metals and doping the CNT top electrodes without damaging the layers underneath is extremely difficult. Recently, we reported a vapor-assisted ex-situdoping method using trifluoromethanesulfonic acid (TFMS), to dope the CNT top electrode in PSCs with no damage to the materials underneath and reported the record-high PCE of 17.56%. However, the vapor doping was too weak as a long exposure to the TFMS vapor resulted in a reaction between TFMS and 4-tert-butylpyridine (t-BP). Also, it was difficult to control the exposure time exactly everytime. Therefore, it is necessary to find a more controlled and effective doping technology. The key here is to develop a new doping method that leads to the minimal interaction with t-BP while demonstrating the maximum doping effect.
Herein, we report CNT-laminated PSCs in which optimized concentration of TFMS dopant in an apolar solvent was applied by a drop-casting method to increase the conductivity of the CNTs while avoiding the reaction with t-BP and the perovskite layer underneath. The choice of apolar solvent was chlorobenzene which exhibited superior doping effect and stability compared with other solvents. Furthermore, higher concentration of spiro-MeOTAD was used in the CNT-laminated PSCs by exploiting the high mobility of the porous CNT network. The combination of those two led to a PCE of 18.5%, which is comparable to 18.3% of the metal electrode-based PSCs. The CNT-laminated devices demonstrated a stability time of more than 1000 operating hours, which is by far greater than that of the reference devices.

Figure 1