Silicon solar cells are described which operate at energy conversion efficiencies independently measured at 18.7 percent under standard terrestrial test conditions (AM1.5, 100 mW/cm2, 28°C). These are apparently the most efficient silicon cells fabricated to date. The high-efficiency results from a combination of high open-circuit voltage due to the careful attention paid to the passivation of the top surface of the cell; high fill factor due to the high open-circuit voltage and low parasitic resistance losses; and high short-circuit current density due to the use of shallow diffusions, a low grid coverage, and an optimized double layer antireflection coating.

High-efficiency silicon solar cells

................................................................................................................................... ii Dedication................................................................................................................................ iv Acknowledgments.................................................................................................................... v Vita........................................................................................................................................... vii List of Tables............................................................................................................................. xii List of Figures............................................................................................................................ xiv Chapter 1 ‐ Introduction........................................................................................................... 1 1.1 ‐ Microbial fuel cells.................................................................................................. 2 1.2 ‐ Microbial fuel cell applications............................................................................... 4 1.3 ‐ Research problems associated with MFC technology............................................. 5 1.4 ‐ Introduction to this research study......................................................................... 6 Chapter 2 ‐ Effects of Nano‐structure Enhanced Cathodes on Electricity Production of Two‐ Chamber Microbial Fuel Cells 2.1 ‐ Introduction............................................................................................................ 8 2.2 ‐ Materials and Methods........................................................................................... 16 2.2.1 ‐ Electrode construction Graphite bar anodes.......................................................................................... 18 Graphite bar cathodes....................................................................................... 20

In Partial Fulfillment of the Requirements for the Degree of

The efficiency of photovoltaic energy conversion is a decisive factor for low-cost electricity from renewable energies. In recent years, the efficiency of crystalline silicon solar cells in mass production has increased annually by about 0.5–0.6%abs per year. In order to maintain this development speed, new technologies must be developed and transferred to industrial production. After the transition from full area Al back surface field cells to passivated emitter and rear contact cells, passivating contacts are an important step to get as close as possible to the efficiency limit of single junction Si solar cells. The theoretical background and the two prominent technologies for passivating contacts are presented and discussed. After implementing passivating contacts, the fundamental limit of single junction Si solar cells of 29.4% is in reach. Multi-junction solar cells are the most promising option to achieve efficiencies greater than 30%. Tandem technologies based on crystalline silicon as bottom cells have the advantage that they are based on a mature technology established on a gigawatt scale and can partially use the existing production capacity. In addition, silicon has an ideal bandgap for the lower subcell of a tandem solar cell. The two most promising material candidates for the top cell, i.e., III/V and perovskites, will be discussed. The presented technology routes show that silicon is able to maintain its outstanding position in photovoltaics in the coming years.

Passivating contacts and tandem concepts: Approaches for the highest silicon-based solar cell efficiencies

High Efficiency Silicon Solar Cells

The reaction of Si (100) with C2H4 from a molecular beam source has been studied using x‐ray photoelectron spectroscopy, electron‐energy‐loss spectroscopy, and Auger spectroscopy. Using these methods, we have studied the kinetics of SiC formation under conditions where no gas‐phase excitation processes can contribute. At Si (100) temperatures below 940 K, a ‘‘Si‐C alloy’’ forms on the surface; annealing to higher temperatures produces SiC exhibiting electron spectroscopic properties identical to SiC (0001). By studies of the characteristic bulk‐ and surface‐plasmon‐loss features in the SiC thin film, it has been shown that surface aggregation of bulk Si on top of the growing SiC film occurs at T≥940 K. Under optimum SiC growth conditions, C2H4 yields about 2×10−3 SiC units per C2H4 surface collision on Si (100).

Studies of SiC Formation on Si(100) by Chemical Vapor Deposition

Three types of new structure a‐Si solar cells having more than 9% efficiency are presented. The first one has a high optical reflection back electrode metal alloyed with optically transparent n‐type μc‐Si deposited on the conventional glass substrate a‐SiC/a‐Si heterojunction solar cell. The second type structure is an inverted p‐i‐n solar cell having Ag/TiO2/a‐Si metal‐insulator‐semiconductor type back surface electrode which more efficiently collects longer wavelength photocarriers just above the band edge. The third structure demonstrated here has a‐Si/polycrystalline tandem junction to pick up the energy of longer wavelength photons passing through the front side of the a‐Si solar cell. All key technologies proposed here are practical and offer more promised real alternatives for the fabrication of high efficiency a‐Si solar cells.

Kouha Fujimoto

Papers

New types of high efficiency solar cells based on a‐Si