Plasma-Exposure-Induced Mobility Enhancement of LiTFSI-Doped Spiro-OMeTAD Hole Transport Layer in Perovskite Solar Cells and Its Impact on Device Performance

Abstract

2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) film currently prevails as hole transport layer (HTL) employed in perovskite solar cells (PSCs). However, the standard preparation method for spin-coated, Lithium bis(trifluoromethylsulfony) imide (LiTFSI)-doped, spiro-OMeTAD HTL depends on a time-consuming and uncontrolled oxidation process to gain desirable electrical conductivity to favor device operation. Our previous work demonstrated that ~10 s oxygen or oxygen containing gas discharge plasma exposure can oxidize spiro-OMeTAD HTL effectively and make PSCs work well. In this communication, hole-only devices are fabricated and in-situ current density-voltage measurements are performed to investigate the change in hole mobility of LiTFSI-doped spiro-OMeTAD films under plasma exposure. The results reveal that hole mobility values can be increased averagely from ~5.0 × 10−5 cm2V−1s−1 to 7.89 × 10−4 cm2V−1s−1 with 7 s O2 plasma exposure, and 9.33 × 10−4 cm2V−1s−1 with 9 s O2/Ar plasma exposure. The effects on the photovoltaic performance of complete PSC devices are examined, and optical emission spectroscopy (OES) is used for a diagnostic to explain the different exposure effects of O2 and O2/Ar plasma. High efficiency, fine controllability and good compatibility with current plasma surface cleaning techniques may make this method an important step towards the future commercialization of photovoltaic technologies employing spiro-OMeTAD hole transport material.

D-A-π-A-D-type Dopant-free Hole Transport Material for Low-Cost, Efficient, and Stable Perovskite Solar Cells

“…As a result, the hole mobility of the TBP and Li-TFSI doped Spiro was calculated to be 4.28 3 10 À3 cm 2 V À1 s À1 , which is coincided with previous reports. 47,[53][54][55] The undoped DTB-FL layer also exhibits high mobility of 3.94 31 0 À3 cm 2 V À1 s À1 , which is comparable to the p-doped Spiro.…”

Metal Halide Perovskite/Electrode Contacts in Charge‐Transporting‐Layer‐Free Devices

“…But compared with cheap MHP materials, these materials are usually expensive, especially for state-of-the-art HTMs. [59,67] Besides, some HTL and ETL materials may result in physical and chemical instability issues in PSCs devices, for instance, the ion migration through the transport layers to metal electrodes, the diffusion of electrode metal atoms through the transport layers to the MHP layer, chemical reactions between the transport materials and MHP layer, accelerating the degradation of PSC devices. [68,69] Many kinds of ETL/HTL-free PSCs are developed to simplify the complicated multilayer device architecture and further reduce manufacturing costs.…”