The In-Gap Electronic State Spectrum of Methylammonium Lead Iodide Single-Crystal Perovskites.

The density of trap states within the bandgap of methylammonium lead iodide single crystals is investigated. Defect states close to both the conduction and valence bands are probed. Additionally, a comprehensive electronic characterization of crystals is carried out, including measurements of the electron and hole mobility, and the energy landscape (band diagram) at the surface.

Interface Structure of MoO3 on Organic Semiconductors.

We have systematically studied interface structure formed by vapor-phase deposition of typical transition metal oxide MoO3 on organic semiconductors. Eight organic hole transport materials have been used in this study. Ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy are used to measure the evolution of the physical, chemical and electronic structure of the interfaces at various stages of MoO3 deposition on these organic semiconductor surfaces. For the interface physical structure, it is found that MoO3 diffuses into the underlying organic layer, exhibiting a trend of increasing diffusion with decreasing molecular molar mass. For the interface chemical structure, new carbon and molybdenum core-level states are observed, as a result of interfacial electron transfer from organic semiconductor to MoO3. For the interface electronic structure, energy level alignment is observed in agreement with the universal energy level alignment rule of molecules on metal oxides, despite deposition order inversion.

Polyethylenimine (PEI) As an Effective Dopant To Conveniently Convert Ambipolar and p-Type Polymers into Unipolar n-Type Polymers.

In this study, we added a small amount of polyethylenimine (PEI) into several ambipolar and p-type polymer semiconductors and used these blends as channel materials in organic thin film transistors (OTFTs). It is found that PEI can effectively suppress hole transport characteristics while maintaining or promoting the electron transport performance. Unipolar n-channel OTFTs with electron-only transport behavior is achieved for all the polymer semiconductors chosen with 2-10 wt % PEI. The electron-rich nitrogen atoms in PEI are thought to fill the electron traps, raise the Fermi level and function as trapping sites for holes, leading to promotion of electron transport and suppression of hole transport. This work demonstrates a convenient general approach to transforming ambipolar and p-type polymer semiconductors into unipolar n-type polymer semiconductors that are useful for printed logic circuits and many other applications.