Nanostructured Thermionics for Conversion of Light to Electricity: Simultaneous Extraction of Device Parameters

Thermionic conversion involves the direct conversion of heat, including light-induced heat, from a heat source, e.g., solar energy, to electricity. Although the concept is almost a hundred years old, the progress of thermionic convertors has been limited by issues such as the space-charge effect and availability of materials with desirable mechanical and electrical properties, while maintaining a low work function. Nanotechnology could help address some of the main challenges that thermionic convertors face. However, existing models, which were developed for macroscopic convertors, are not capable of describing all aspects of nanostructured devices. We present a method to evaluate the output characteristics of thermionic convertors with a higher precision than the existing models and the ability to simulate a broader range of parameters, including temperatures, active surface areas, interelectrode distances, and work functions. These features are crucial for the characterization of emergent devices due to the unknowns involved in their internal parameters; the model's high numerical precision and flexibility allows one to solve the reverse problem and to evaluate the internal parameters of the device from a set of simple experimental data. As an experimental case, a carbon nanotube forest was used as the emitter and locally heated to thermionic emission temperatures using a 50-mW-focused laser beam. The current-voltage characteristics were measured and used to solve the reverse problem to obtain the internal parameters of the device, which were shown to be consistent with the values obtained using other methods.