Revealing the Mechanism of Doping of spiro-MeOTAD via Zn Complexation in the Absence of Oxygen and Light

In this study, a new mechanism for doping of spiro-MeOTAD by Zn(TFSI)2 is revealed, which is completely different from the mechanism induced by LiTFSI. The oxidation of spiro-MeOTAD is facilitated by complexation between the zinc cation and tert-butylpyridine (tBP) even in the absence of oxygen and light. The lone electron pair of nitrogen in tBP coordinates to the zinc cation, confirmed by the Fourier transform infrared spectroscopy peak at 1637 cm–1 corresponding to a pyridine ring mode coupled with nitrogen coordinating to zinc as the Lewis site. The doping mechanism is also evidenced by computational calculations. The coordination of zinc to tBP and TFSI– provides a driving force and stabilizes the oxidized spiro-MeOTAD species, showing the most favorable reaction by forming a [Zn(tBP)3]+(TFSI–) complex with a ΔE of −1.52 eV. Our discovery of the mechanism of doping by Zn(TFSI)2 provides practical insight for the oxidation of spiro-MeOTAD by avoiding ambiguous aging processes in ambient or dry air at the risk of decomposition of other components.