Interface properties between a low band gap conjugated polymer and a calcium metal electrode

In Situ Investigations of Al/Perovskite Interfacial Structures

Interfacial properties of perovskite layers and metal electrodes play a crucial role in device performance and long-term stability of perovskite solar cells. In this work, we performed a comprehensive study of the interfacial structures and ion migration at the interface of a CH₃NH₃PbI₃ perovskite layer and an Al electrode using in situ synchrotron radiation photoemission spectroscopy measurements. It was found that the Al electrode can react with the perovskite layers, leading to the formation of aluminum iodide species and the bonding between Al and N, as well as the reduction of Pb²⁺ ions to metallic Pb species at the interface. Moreover, during the Al deposition, iodide ions can migrate from the CH₃NH₃PbI₃ subsurface to the Al electrode, while the reduced Pb remains at the subsurface. The depth profile photoemission measurements, made by varying the photon energies of incident synchrotron radiation X-rays, demonstrate that the reaction occurs at the Al/CH₃NH₃PbI₃ interface at least with a thickness of ∼3.5 nm below the perovskite surface. This study provides an atomic-level fundamental understanding of the Al/CH₃NH₃PbI₃ interfacial structures and insight into the degradation mechanisms of perovskite solar cells when using Al metal as the electrode.

Dynamic Evolution from Chain Disorder to Order of PTB7 Condensed State Structures under External Fields

In this research, the effect of external fields (solvent, temperature, solution concentration, and external force) on dynamic evolution from chain disorder to order of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5- b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophenediyl]] (PTB7) condensed state structures was explored by UV-vis absorption spectra, atomic force microscope, and transmission electron microscopy (TEM). It was found that PTB7 main chains presented amorphous conformations induced by the poor solvent 1,2-dichloroethane. However, the local ordered aggregation appeared in amorphous conformations when the solubility of the poor solvent was again lowered by reducing temperature. It is worth noting that the size of ordered aggregation was further increased with the decrease of solution concentration or increase of external force. It was found that there were two main PTB7 absorption peaks in the UV-vis absorption spectra; we denoted A_0 -0 for the intensity of the lower energy absorption peak and A_0-1 for the intensity of the higher energy absorption peak. The ratio R = A_0-0/ A_0-1 was used to characterize the dynamic evolution from disorder to order of the PTB7 condensed state structures in absorption spectra. It increased from 0.94 for PTB7 amorphous state to 1.25 for PTB7 large-size ordered aggregation. The dynamic evolution from chain disorder to order could also be distinctly observed by TEM. It was inferred that PTB7 condensed state structures (amorphous state, local ordered aggregation, and large-scale ordered aggregation) might exist simultaneously because of the complexity of copolymer conformations. This research is meaningful to establish physical basis for the molecule design and the synthesis of materials to enhance photoelectronic device efficiency based on condensed matter physics of conjugated polymer.