Impact of annealing on spiro-OMeTAD and corresponding solid-state dye sensitized solar cells

Annealing is a route that has proven to be successful at enhancing the hole mobility of some hole transporting materials. Under this consideration, we systematically investigated the effect of annealing treatment on the spiro-OMeTAD and its corresponding solar cells by annealing the dye-sensitized TiO2 electrodes at 80 and 140 °C. Upon annealing, the crystallization and oxidation of spiro-OMeTAD enhanced, thereby favoring higher hole transfer and transport, ultimately leading to higher short-circuit current (Jsc). However, the annealed devices showed lower open-circuit voltage, which was due to the downshifting of TiO2 Fermi level as revealed by the transient photovoltage measurement. Results obtained in this study indicate the bis(trifluoromethylsulfonyl)imide (Li-TFSI) is responsible for the spiro-OMeTAD oxidization and TiO2 Fermi level reduction during the annealing. Further to be annealed at 140 °C, the 4-tert-butyl pyridine (tBP) evaporated to further down-shift the TiO2 Fermi level and dramatically reduce the Jsc, hence, the corresponded solar cell revealed the lowest efficiency. Typically, the standard temperature for thermal stress testing of solar cells in commercial applications is 85 °C. Therefore, it is highly imperative to have a better understanding of this phenomenon in order to achieve long-term stability of solid-state dye sensitized solar cells.