Thesis Type: Doctorate
Institution Of The Thesis: Middle East Technical University, Faculty of Arts and Sciences, Department of Chemistry, Turkey
Approval Date: 2015
Thesis Language: English
Student: Fırat Es
Supervisor: RAŞİT TURANAbstract:
Photovoltaic (PV) technology needs higher performance - lower cost materials and structures in order to catch the grid parity and become an everyday use power source. The most commonly used material in PV, crystalline silicon, suffers from low absorption due to its indirect band gap nature. In order to overcome this problem, several light trapping structures have been used that increase the path length of photons inside the absorbing body of the device. However, conventional light trapping schemes cannot be applicable to new device concepts based on thin crystalline or diamond cut wafers. Metal assisted etching (MAE) is promising method to form micro and nano structures on the surface of the silicon wafer for optical management. As a top down method, it allows the control of the type and concentration of the dopants, material quality and crystal structure of the starting material. In addition, as a solution based method, it can be adapted to large scale industrial applications where nanoscale structuring can contribute to the development of high performing devices. A variety of structures vi having desired geometry such as nanowires, nano-cones, micro-rods, etc., ranging from nanometer to micrometer sizes with random or periodic distribution can be obtained via MAE by adjusting process parameters comprised of initial geometry of catalytic metal species, acid concentration, oxidative concentration, and temperature, ambient light, etching duration, substrate resistivity and orientation. In this study, MAE has been used to texture mono and multi crystalline silicon solar cells. Mono and multi crystalline Si solar cells with nano-wire texturing have been fabricated with Ag assisted electroless etching. After observation of poor efficiency despite very good optical performance, MAE with additional nitric acid (HNO3) as an oxidative agent has been optimized to control surface structures and reflection. Applicability of this new chemical at various conditions that simulate industrial production environment was proven. Optimized recipe has been applied to mono and multi crystalline Si solar cells to prove the effectiveness of the method for both wafer types. In the end, together with necessary optimizations of cell production steps, enhancement of jSC and efficiency has been attained for both cases. This concludes that the new MAE method based on a new chemistry offers a promising alternative to conventional texturing methods for today’s and future technology.