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.
Highlights
The power conversion efficiencies (PCE) of photovoltaic cells, solid-state dye-sensitized solar cell (DSSC) [1,2] and recently emergent perovskite solar cell (PSC) [3,4,5,6], exhibited a remarkable increase in the last few years
Because spiro-OMeTAD suffers from low hole mobility and low conductivity in its pristine form, dopant additives, typically Lithium bis(trifluoromethylsulfony)imide (LiTFSI) and tert-butyl pyridine (TBP), become necessary to improve charge carrier density, hole mobility, and energy level alignment at light absorber/hole transport layer (HTL) interface, which may increase the conductivity by 1~2 orders of magnitude, reduce the interfacial charge recombination, and favor a good device operation [7,8,9]
Since LiTFSI-doped spiro-OMeTAD as HTL was developed for use in DSSC devices intentionally, it currently prevails as HTL in high-PCE PSC devices, irrespective of the mesoscopic scaffold or planar heterojunction PSC
Summary
The power conversion efficiencies (PCE) of photovoltaic cells, solid-state dye-sensitized solar cell (DSSC) [1,2] and recently emergent perovskite solar cell (PSC) [3,4,5,6], exhibited a remarkable increase in the last few years. One of the most important improvements contributing to higher PCEs was the introduction of a solid state hole transport layer (HTL). Because spiro-OMeTAD suffers from low hole mobility and low conductivity in its pristine form, dopant additives, typically Lithium bis(trifluoromethylsulfony)imide (LiTFSI) and tert-butyl pyridine (TBP), become necessary to improve charge carrier density, hole mobility, and energy level alignment at light absorber/HTL interface, which may increase the conductivity by 1~2 orders of magnitude, reduce the interfacial charge recombination, and favor a good device operation [7,8,9]. Since LiTFSI-doped spiro-OMeTAD as HTL was developed for use in DSSC devices intentionally, it currently prevails as HTL in high-PCE PSC devices, irrespective of the mesoscopic scaffold or planar heterojunction PSC device architecture [10,11,12,13,14]
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