We report a new molecular-level interface engineering strategy using a multifunctional ligand that augments long-term operational and thermal stability by chemically modifying the formamidinium lead iodide rich photoactive layer. The surface derivatized solar cells exhibited high operational stability (maximum powering point tracking at 1 sun) with a stabilized T80 (the time over which the device efficiency reduces to 80% of its initial value of post-burn-in) of ≈5950 h at 40 ºC and stabilized efficiency over 23%. The origin of high device stability and performance is correlated to the nano/sub-nanoscale molecular level interactions between ligand and perovskite layer, which is corroborated by comprehensive multiscale characterization. Chemical analysis of the aged devices showed that interface passivation inhibited ion migration and prevented photoinduced I2 release that irreversibly degrades the perovskite.
Photoelectrochemical measurements have been performed on film electrodes consisting of linked nano-sized TiO2 colloids. The film thickness ranged from 1 - 40 micrometers. The film network was attached to a thin transparent conducting layer of SnO2 allowing for photogenerated electrons to be collected in an outer electrical circuit. By illuminating electrodes of different thicknesses with monochromatic light from either side, it was possible to induce charge separation in different regions of the film network. In this way, it was proved that electrons have different probabilities of reaching the back contact depending on the location in the film where they are created. The results also illustrate the importance of the redox species in the cavities of these porous electrodes. By adding acceptors to the electrolyte it was possible to alter the conditions for charge transfer in the nanocrystalline film. It was shown that electron acceptors such as oxygen or iodine in the solution strongly affects the rate of charge transfer at the particle-electrolyte interface and the transport of electrons throughout the TiO2 film-network. Modification of the semiconductor-electrolyte interface with surface adsorbed pyridine induced major changes in the charge transfer events at the interface. The photocurrent yields were greatly improved by this surface treatment. The effect of pH in solution was also investigated. The rate of charge transfer at the particle-electrolyte interface was changed at high surface density of OH--ions. This was explained due to the change of the surface energy causing different driving forces for redox reactions, but also due to the more negatively electrostatic surface potential of the particles preventing the encounter of negatively charged redox species with the colloid surface. Phototransient measurements indicated a depletion of redox species in the pores of the film. Thereby it was pointed out that the dynamics of the redox species in the confined cavities of the film are a limiting factor for the charge separation efficiency in nanocrystalline film. The photovoltage in anaerobic solutions sustained for very long periods, indicating that the linked particles may work as reservoirs for photoexcited electrons if the access to electron acceptors in the solution is choked. It was concluded that surface processes are favored at the small semiconductor particles used in this study.
Viologen derivatives adsorbed on TiO2 surfaces in nanocrystalline-nanoporous electrodes have been studied with respect to electrochromic phenomena. The transparent nature and the high internal surface area of these electrodes are very beneficial for this purpose. Two different viologens were studied giving rise to blue, green, and violet coloration. For one of the viologens we show the possibility to vary the color for the same viologen/TiO2 electrode from transparent to green or violet depending on the applied potential. Depending on the applied potential different optical densities were obtained. Optical density changes from 0.1 up to 1 and coloration efficiencies above 100 cm2C-1, with a maximum value of 440 cm2C-1, were obtained. The switching times for these electrodes for both coloration and bleaching were faster than 1s for a transmittance change larger than 60%, using 0.5M LiClO4 in acetonitrile as electrolyte. Preliminary stability tests were performed. One electrode has been cycled 654 times without substantial decline in coloration efficiency.
Forward biasing of transparent nanocrystalline TiO2 (anatase) films in lithium ion containing organic electrolytes leads to rapid and reversible coloration due to electron accumulation and Li+ intercalation in the anantase lattice. Absorption of > 90 percent light throughout the visible and near IR can be switched on and off within a few seconds. The nanocrystalline morphology of the film plays a primordial role in enhancing the electrochromic process. Preliminary results from the reduction of a surface attached viologen molecule leading to a blue coloration of the film are reported.
Some recent results from our laboratory on the properties of microporous colloidal TiO2 film electrodes are summarized. The photoelectrochemical properties of the colloidal film electrodes are compared with solid transparent thin films of TiO2. In particular the information that can be derived from analysis of the action spectra for front and back side illumination of the films is given. The mechanism of charge separation in the colloidal film is qualitatively discussed in terms of the kinetics at the electrode-electrolyte interface, and some effects of the presence of O2 in the electrolyte are demonstrated. Finally we also briefly report the solar energy efficiency from measurements on dye-sensitized TiO2 film electrodes in solar cells.
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