The use of fossil fuel energy resources has left has impacted the ecosystem of earth in many ways. In spite of this, the world will run out of it in the matter of few decades if continue to be used at its pace . Therefore, a great amount of efforts is spent for developing alternative energy resources. In this regard, hydrogen (H2) is an excellent energy carrier and therefore delivers energy in a very efficient manner. Therefore, it has a great potential to play a key role in next generation energy resources. Hydrogen can be produced way by splitting water with solar light under the process of photoelectrochemical (PEC) . A number of challenges exist to PEC technology including the life-time of hydrogen-producing systems, efficiency, and high cost. It has been reported recently that III-V double-heterojunction semiconductor based hydrogen-producing PEC systems are demonstrated to have improved life-time and efficiency as compared to their earlier counterparts . The dramatic improvements in the systems are made by employing an innovative way of decoupling the interfaces of optical absorbing materials with electrocatalytic parts of PEC systems. The nanoscale characterization of double heterojunctions in the working devices is vital to the performance of PEC systems and should be carried out using an elaborate technique such as transmission electron microscopy (TEM). In this report, TEM-analysis of a working gallium arsenide (GaAs) and indium gallium phosphide (InGaP) based double heterojunction PEC system. TEM specimens of the PEC devices were prepared using focused ion beam (FIB) scanning electron microscope (SEM) of model Helios G4 from Thermo-Fisher Scientific. The analysis of the prepared specimens was then carried out using aberration-corrected TEM of model Titan 80-300 ST equipped with energy-filter of model GIF Quantum 966. Moreover, the analysis was carried out by setting the accelerating voltage to 300 kV aberration corrected scanning TEM (AC-STEM) mode of the microscope.