GaN-based InGaN/GaN MQWs solar cells for innovative applications: performance and modeling

Gallium nitride (GaN) has emerged in recent years as promising material for optoelectronics devices, from LEDs, laser to photodetectors and solar cells. In particular, the possibility of GaN of growing alloys containing aluminum (AlGaN) and/or indium (InGaN) allows for the tuning of the wavelength from the UV-C to the green spectral range: by varying the indium concentration it is possible to obtain a tunable wide bandgap, making GaN-based InGaN/GaN Multiple Quantum Wells (MQWs) solar cells suitable as additional components of multijunction (MJ) solar cells. Furthermore, these devices present an high thermal stability and outstanding radiation resistance making them suitable in concentrator solar harvesting systems, for wireless power transfer system and space applications due to their reliability and efficiency in harsh environments. In particular, the MQWs structure avoids problems of poor material quality and defect formation and is currently the best technology for GaN-based solar cells: in these devices, the generation of electron-hole pairs occurs within the quantum wells and the generated carriers must therefore be extracted from the wells by means of numerous conduction processes such as barrier-tunneling and thermionic escape. Despite the problem of extraction from the quantum wells, this type of structure is the most advantageous since it avoids problems of poor material quality and defect formation, and is currently the best technology for InGaN/GaN-based solar cells. Through TCAD Sentaurus suite from Synopsis it was possible to reproduce the non-illuminated current-voltage characteristics of these devices, understanding the influence of their main conductions models. Finally, based on closed-formula equations and material parameters, we developed a model that describes the quantum efficiency of these devices, considering their structure, the quantum confinement effect and the temperature-dependence of the absorption coefficient. As demonstrated by the comparison with experimental data, the model is capable of accurately reproducing the spectral characteristics of the solar cells, allows to understand the influence of the thickness of the different layers and conductive processes on device efficiency.