Unlike photodetectors made from inorganic materials, such as silicon, devices fabricated using organic materials could potentially be cheaper to produce, exhibit mechanical flexibility and provide a route for realization of novel photodetector applications.

Now, a group from South China University of Technology and Peking University1 have demonstrated very efficient photovoltaic properties of an organic molecule self-assembled from solution.

Fig. 1: Photograph of the photodetectors placed on a flexible substrate and being tested in this bent state.

The molecule—a condensed benzothiophene—was synthesized from solution and results in crystalline nanoribbons. “The solution process is a simple, low-temperature and low-cost process compared to the fabrication of conventional semiconductor devices,” says Jian Wang from the group. The nanoribbons were then readily dispersed onto any desirable substrate and contacted electrically, and notably, the use of flexible organic substrates (Fig. 1) did not alter their electronic properties.

The devices exhibited favorable photoconductive properties in the ultraviolet and blue part of the visible spectrum. The so-called “off” current in the dark was extremely low, which ensured low electrical losses and a high signal to noise ratio. The highest on/off signal ratio for the devices reached a respectable 1000. These and other typical device parameters approached that of inorganic nanowires and were better than those of molecules such as carbon nanotubes or other organic nanowires.

These properties are due to the structure of the molecule, which is essentially flat, and consists of several aromatic rings. The pi-orbitals pointing out of this plane interact strongly, enabling fast electron transport. “The chemical structure of the crystallized nanoribbons, the large aromatic core and the highly ordered packing all lead to a high carrier mobility,” says Wang.

Although these properties are important, the gold contact electrodes play another important role. During their evaporation, gold atoms create electronic states at the surface of the nanoribbons that temporarily capture photogenerated electrons and release them later on, thus contributing to the overall gain. Through the control over the density of these states the efficiency of the device could be optimized.

The successful demonstration of photoconductive properties makes these nanoribbons a strong candidate for large-scale photodetector applications and may prove a successful template for similar molecules.