Digital cameras as well as many other electronic devices need light-sensitive sensors. In order to meet increasing demand for optoelectronic components of this kind, industry is searching for new semiconductor materials that cover a broad range of wavelengths and are inexpensive. A hybrid material demonstrated that this metal-organic framework (MOF) can be used as a broadband photodetector. As it does not contain any cost-intensive raw materials, it can be produced inexpensively in bulk.
MOFs are highly porous substances, up to 90 percent of which are composed of empty space. They have largely been used to store gases, for catalysis, or to slowly release drugs in the human body. The new MOF compound comprises an organic material integrated with iron ions. The framework forms superimposed layers with semiconducting properties, which makes it potentially interesting for optoelectronic applications. From 400 to 1,575 nanometers, the semiconductor could detect a broad range of light wavelengths. The spectrum of radiation thus goes from ultraviolet to near infrared.
The spectrum of wavelengths a semiconducting material can cover and transform into electrical signals essentially depends on the bandgap — the energetic distance between the valence band and the conduction band of a solid-state material. In typical semiconductors, the valence band is completely full, so the electrons cannot move around. The conduction band, on the other hand, is largely empty, so the electrons can move around freely and influence the current flow. While the bandgap in insulators is so big that the electrons cannot jump from the valance band to the conduction band, metal conductors have no such gaps. A semiconductor's bandgap is just big enough to raise the electrons to the higher energy level of the conduction band by using the light waves. The smaller the bandgap, the less energy required to excite an electron.
By cooling the detector down to lower temperatures, the performance can be improved yet further because the thermal excitation of the electrons is suppressed. Other improvements include optimizing the component configuration, producing more reliable contacts, and developing the material further. Thanks to their electronic properties and inexpensive manufacturing, MOF layers are promising candidates for a variety of optoelectronic applications.
The next step is to scale the layer thickness; in the study, 1.7-micrometer MOF films were used to build the photodetector. To integrate them into components, they need to be significantly thinner. If possible, the aim is to reduce the superimposed layers to 70 nanometers — 25 times smaller than their size. Down to this layer thickness, the material should exhibit comparable properties.