We report an in-depth study focusing on the stability of a benchmark electrolyte composition based on a low-volatile 3-methoxypropionitrile (MPN) solvent employed in dye-sensitized solar cells. In the presence of TiO2, the semi-conductor surface plays a catalytic role in the thermal degradation of the electrolyte, which induces, among other effects, the nucleation and growth of a uniform solid electrolyte interphase (SEI) layer that wraps TiO2. On the basis of our actual understanding, we argue that SEI formation is responsible for triiodide depletion in the electrolyte during ageing and also has a simultaneous impact on TiO2 optoelectronic properties through the onset of a visible-light absorption tail, energy modification of intraband trap states, and the induction of an increase in both electron lifetime and transport time in TiO2. In-depth characterization of this layer by using XPS and ToF-SIMS indicates that the chemical composition of this SEI results from solvent and additive degradation, that is, iodide, sulfur, cyano, nitrogen, carbon, and imidazolium rings. The SEI thickness, its content, and the concentration profile strongly vary depending on the ageing conditions. The outcome of this new finding is discussed in comparison with literature observations and stresses the difficulties in reaching long-term stability at 85 °C by using MPN-based electrolytes unless new interfacial engineering is accomplished to impede pinholes between dye molecules on TiO2.
Solid electrolyte interphase (SEI) layers form on sensitized-TiO2 photo-anodes and platinum counter-electrodes when dye-sensitized solar cells (DSSCs) are subjected to an accelerated ageing protocol (e.g. heating at 85 °C in the dark for 500 hours). To understand how this will impact the device operation, electrochemical impedance spectroscopy study showed that the SEI induces an additional electron transfer process from the TiO2 to the electrolyte. This is materialized by the onset of a new charge transfer semi-circle at higher frequencies, predominantly visible under bias voltage similar and above open circuit voltage. Our results emphasized on the detrimental role of the SEI formation on device performance and lifetime.Additionally, ns-transient absorption spectroscopy shows that SEI formation reduces the rate oxidised dye regeneration. We also show that a proportion of the photogenerated holes on the dyes are transferred to the SEI itself. Prolonged ageing duration leads to the electrode's mesoporosity network entirely clogged by the SEI; thus impeding efficient transport of the electrolyte redox couple, also responsible for a further decline in photovoltaic performances.2
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