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We report the molecular interactions between cobalt tris(bipyridyl) [Co(bpy) 3 ] 2+/3+ and additives of 4-tertbutylpyridine (TBP) in a redox electrolyte and the effects on the performance of dye-sensitized solar cells (DSCs). The interactions and consequent effects led us to the concept of a TBP-shielded [Co(bpy) 3 ] 3+ complex, where the [Co(bpy) 3 ] 3+ ion was surrounded by TBP molecules via weak ion-dipole attractions.The TBP molecules were able to bi-functionally shield both [Co(bpy) 3 ] 3+ ions and the TiO 2 surface from undesirable charge recombination, achieving a greater enhancement of open-circuit voltage (V OC ) of about 200 mV than 140 mV in the iodide (I À /I 3 À) system where TBP passivated the TiO 2 surface only.The V OC -J SC trade-off problem was thereby mitigated. Our discovery of the molecular interactions between additives and redox species would stimulate the engineering of new additives or redox mediators to improve the performance of DSCs.

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We report a simple methodology for the selective deposition of an insulating layer on the nanoparticulate TiO2 (np-TiO2) mesoporous layer of perovskite solar cells. The deposited MgO insulating layer mainly covered the bottom part of the np-TiO2 layer with less coverage at the top. The so-called quasi-top-open structure is introduced to act as an efficient hole-blocking layer to prevent charge recombination at the physical contact of the transparent conducting oxide with the perovskite. This leads to an open-circuit voltage higher than that of the reference cell with a compact TiO2 hole-blocking layer. Moreover, such a quasi-top-open structure can facilitate the electron injection from perovskite into the np-TiO2 mesoporous layer and improve the spectral response at longer wavelength because of the less covered insulating layer at the top. This work provides an alternative way to fabricate perovskite solar cells without the need to use a conventional compact TiO2 layer.