Study of Charge Transfer at the Inorganic Semi-conductor Surface for Solar Cell Applications

Abstract

Solar energy is one of the affordable and clean energies that allow humans to live a sustainable life on earth. The most efficient way to exploit solar energy is photovoltaics that converts the photon to electricity through various kinds of light-harvesting materials. As more accessible photovoltaic is required with low cost and large-scale production, the photovoltaics have advanced up to 3rd generation in which various kinds of the semiconducting layer are included. Thermodynamically favorable charge transfer is essential for their efficient operation; thus, band alignment needs to be constructed precisely based on the profound insight of the charge transfer state of their organization. Therefore, understanding the charge transfer state is important for device development. Also, the charge transfer can be employed to the oxidation/reduction reaction such as the metal-assisted chemical etching (MACE) that excavates all kinds of silicon. A neutral-colored transparent silicon photovoltaic can be produced inexpensively due to the merits of MACE, compared to the conventional method (reactive ion etching), thus promoting their better dissemination. In Chapter 2, the charge transfer mechanism of Cs2SnI6 as promising lead-free perovskite is demonstrated, where its surface state is the main charge transfer pathway in a redox mediator-rich environment. The surface state of Cs2SnI6 shows redox activity and charge storage properties with iodide ion species, detected by the cyclic voltammetry and Mott-Schottky plot, respectively. The influence of Cs2SnI6 surface state during device operation (i.e., charge transfer of Cs2SnI6) is demonstrated in form of the Cs2SnI6-based charge regenerator (quasi-solid-state) for dye-sensitized solar cells. The charge regeneration is successfully achieved through the surface state-mediated charge transfer of Cs2SnI6 and its activity is greater than conventional liquid electrolyte (I-/I3-), which verified by studying regeneration kinetics. Chapter 3 deals with the MACE to fabricate the neutral-colored transparent silicon photovoltaic. Efficient MACE of crystalline silicon is achieved by controlling i) the etching rate and ii) Ag catalyst morphology. The etching rate is improved ca. 2 times by both increasing the etchant temperature and light illumination during MACE, thus, successfully reducing the MACE time. Also, the low coverage from island type morphology of Ag undergoes improvement to the porous monolith layer by introducing the acetonitrile that acts as the surfactant of Ag precursor and coordinates with Ag ion. As a result, Ag catalyst migrates cooperatively during MACE, thus, preventing the uneven etching from the random movement of Ag catalyst.