Direct observation of dramatically enhanced hole formation in a perovskite-solar-cell material spiro-OMeTAD by Li-TFSI doping

Abstract

Electron spin resonance (ESR) spectroscopy of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) thin films and perovskite (CH3NH3PbI3)/spiro-OMeTAD layered films are reported. Clear ESR signals (g = 2.0030) were observed by adding a dopant lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) to the spiro-OMeTAD thin films, which directly showed the spin (hole) formation in spiro-OMeTAD by the Li-TFSI doping. The number of spins in the spiro-OMeTAD thin film has increased by more than two orders of magnitude by the Li-TFSI doping under dark conditions, which demonstrates from a microscopic viewpoint that Li-TFSI has high doping effects for the spiro-OMeTAD thin films. Under simulated solar irradiation, the spin density in the spiro-OMeTAD thin films and the perovskite/spiro-OMeTAD layered films largely increased by the Li-TFST doping due to the formation of long-lived holes in spiro-OMeTAD. The transient responses of the number of photogenerated spins, Nspin, of the layered films upon the light irradiation showed the increase and the decrease in the Nspin due to the hole transfer and recombination at the perovskite/spiro-OMeTAD interface. The states of long-lived holes in the spiro-OMeTAD layers were analyzed using the simulation of the ESR spectra, which reveals the mobile photogenerated holes with a lifetime >10 μs.

Short‐Term Stability of Perovskite Solar Cells Affected by In Situ Interface Modification

“…There are several factors that contributed to the observed performance improvement. One factor is the photo‐doping effect . Light soaking of perovskite and TiO x were also suggested as factors contributing to performance improvements after short light exposure .…”

New Experimental Evidences Regarding Conformational Equilibrium in Ammonium−Bis(trifluoromethanesulfonyl)imide Ionic Liquids

“…[1][2][3][4][5] Moreover they have a better thermal and chemical stability in comparison with pyridinium and imidazolium-based ILs. [10][11][12] It can be found in two different conformational geometries: the transoid one with a C2 symmetry is more stable than the cisoid one, which has a C1 symmetry. TFSI is a well known anion, with large applications in electrochemistry, where it has found use, for instance, as component of several types of battery electrolytes [7][8][9] or as additive in dye-sensitized solar cells.…”

“…TFSI is a well known anion, with large applications in electrochemistry, where it has found use, for instance, as component of several types of battery electrolytes [7][8][9] or as additive in dye-sensitized solar cells. [10][11][12] It can be found in two different conformational geometries: the transoid one with a C2 symmetry is more stable than the cisoid one, which has a C1 symmetry. [13] However, from the calculations the energy difference between these two conformers is as low as 2.2 kJ/mol, and therefore in the liquid state both of them are usually present.…”

Electron Spin Resonance Investigations on Perovskite Solar Cell Materials Deposited on Glass Substrate

“…In the recent past, ESR spectroscopy has been successfully employed to better understand the performance of polymer solar cell materials [9]. To date, there has been very limited work reported that investigate the point defects that arise during the fabrication process of perovskite solar cells using the ESR technique [10,11]. For instance, Shkrobe et al studied [10] the charge trapping process in bulk polycrystals of photovoltaically active perovskites and related halogenoplumbate compounds using ESR spectroscopy.…”

“…They demonstrated that the holes are trapped by organic cations whereas Pb 2+ centres trap electrons. In a more recent work [11], Namatame and co-authors employed room-temperature ESR spectroscopy to observe dramatic enhancement of hole formation in a perovskite solar cell material spiro-OMeTAD by Li-TFSI doping. In addition, they observed photo generated spins upon in-situ light irradiation.…”