LiTFSI‐Free Spiro‐OMeTAD‐Based Perovskite Solar Cells with Power Conversion Efficiencies Exceeding 19%

Abstract

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)2, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm2) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]+ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)2‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)2. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.

Stability Issues of Perovskite Solar Cells: A Critical Review

“…Li‐TFSI is notorious for its hydrophilic nature, which in turn attracts water in the device and degrades the perovskite layer. [ 78 ] Different studies have shown that the degradation mechanism of the perovskite layer at high temperature and ambient conditions is initiated from spiro‐OMeTAD. [ 79–81 ] It is also reported in these studies that the mechanism starts from the ingress of iodide ions from the perovskite layer to the HTL.…”

HOMO–HOMO Electron Transfer: An Elegant Strategy for p‐Type Doping of Polymer Semiconductors toward Thermoelectric Applications

“…PDPP-[T] 2 -EDOT (M n = 19 420 g mol −1 ; T m = 334 °C in flash DSC at 200 K s −1 ) and Spiro-OMeTAD(TFSI) 2 were synthesized using procedures known in the literature and described in the Supporting Information. [19][20][21][22] Spiro-OMeTAD(TFSI) 2 was characterized using X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. See Figures S1 and S2 in the Supporting Information for its atomic and chemical composition.…”

Single crystal structure and opto-electronic properties of oxidized Spiro-OMeTAD

“…20 Very recently, work from Bach et al has boosted device PCEs to over 19% in the absence of LiTFSI together with an increased device stability attributed to a more hydrophobic HTM layer. 24 In summary, single crystals of doubly oxidized Spiro-OMeTAD were grown and structurally characterized. Based on the structure information, frontier orbitals and electronic band structures were modelled and compared with neutral Spiro-OMeTAD.…”