Spectrum-Dependent Spiro-OMeTAD Oxidization Mechanism in Perovskite Solar Cells

Abstract

We propose a spectrum-dependent mechanism for the oxidation of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), which is commonly used in perovskite solar cells as the hole transport layer. The perovskite layer plays different roles in the Spiro-OMeTAD oxidization for various spectral ranges. The effect of oxidized Spiro-OMeTAD on the solar cell performance was observed and characterized. With the initial long-wavelength illumination (>450 nm), the charge recombination at the TiO2/Spiro-OMeTAD interface was increased due to the higher amount of the oxidized Spiro-OMeTAD. On the other hand, the increased conductivity of the Spiro-OMeTAD layer and enhanced charge transfer at the Au/Spiro-OMeTAD interface facilitated the solar cell performance.

Relationship between ion migration and interfacial degradation of CH3NH3PbI3 perovskite solar cells under thermal conditions

“…The initial difference in values was due to the different surface morphologies of glass and MAPbI 3 despite the same spin coating condition. Since the thickness of spiro-OMeTAD was about 200–250 nm in this study, the conductivity of 1–3 × 10 −5  Scm −1 for spiro-OMeTAD was similar to previous results 28, 36 . Figure 3a,b show the variation of the conductivity of spiro-OMeTAD with time at 85 °C for each structure.…”

“…As mentioned above, spiro-OMeTAD containing MAPbI 3 was observed to undergo enhanced oxidation under irradiation with >450-nm wavelength light 28 . This indicates that not only external conditions, but also MAPbI 3 , can affect the properties of spiro-OMeTAD.…”

“…2a,b. Spiro-OMeTAD can be oxidized to spiro-OMeTAD + by additives like Li-TFSI and external factors such as oxygen and light 24, 28 . Formation of spiro-OMeTAD + from spiro-OMeTAD is indicated by an absorption peak in a particular wavelength region.…”

“…The relationship between MAPbI 3 and spiro-OMeTAD was evaluated; the results showed that under photo-irradiation with >450-nm wavelength light, MAPbI 3 assisted the oxidation of spiro-OMeTAD 28 . In contrast, introduction of CH 3 NH 3 I (MAI) into spiro-OMeTAD can reduce the oxidized spiro-OMeTAD (spiro-OMeTAD + ) 29 .…”

Role of LiTFSI in high Tgtriphenylamine-based hole transporting material in perovskite solar cell

“…Nevertheless, better photovoltaic performance achieved after addition of additives may be related to change in carrier mobility in BT41. [22][23][24][25] The peak appeared at longer wavelength after oxidation is due to that fact that the destabilized bonding orbital by removal of electron leads probably to decrease the transition energy. In Figure 7a, the BT41 solutions with and without additives are compared.…”