High Efficiency Thin Film Crystalline Solar Cells

Thin-film high-efficiency crystalline solar cells are expected to play a significant role as a greener, renewable energy source of the future. Such cells have been extensively studied over the past 10 years. Most of this research focused on developing thinner cells to reduce material usage and improving the optical absorption within the thin absorber. Most recently, the active semiconductor layer can be 10 -- 100 times thinner than conventional solar cells by advanced light trapping. However, an improvement in efficiency in thin-film solar cells had not been previously appreciated and is an equally if not more important enhancement of the cells in addition to materials cost saving. This dissertation presents the device physics of thin-film crystalline solar cells and demonstrates the key design principle to achieve higher efficiency by improving two important parameters: open circuit voltage (Voc) and short circuit current (Jsc). The first part of this thesis focuses on achieving high Voc in thin-film c-Si solar cells and demonstrate the voltage enhancement in thin-film Si solar cells in both theoretical simulation and experimental demonstration. Theoretically, thin cells can significantly increase the carrier concentration by confining photo-carriers into a smaller active region and decrease the recombination by reducing the volume of the active region. This results in higher Voc and efficiency. Experimentally, the first Voc enhancement in thin-film solar cells is demonstrated. The 5 æm thick Si cell achieved a Voc of 649 mV, which is superior to the Voc of any other thin-film (sub-25-æm) Si solar cells reported to date. To further improve efficiency, a carrier selective contact of TiO2/Si was developed to reduce the high carrier recombination at the metal contacts associated with the high carrier centration in thin films. Such a contact demonstrates a contact recombination reduction of 33% and a Voc enhancement of 10 mV compared to a conventional metal contact. The second part mainly discusses high Jsc by applying nanoscale light trapping structures to thin-film c-Si and III-V solar cells. Given the challenges in obtaining low surface recombination and high efficiency in nanostructured solar cells, we demonstrated a nanowindow solar cell design with dielectric or wide bandgap semiconductor material that can overcome these challenges. A SiNx nanostructured dielectric layer can provide both light tapping and surface passivation for Si. A thin Si film with such SiNx layer exhibits