Faculty of Engineering and Natural Sciences
Basic Concepts and Interfacial Aspects of High Efficiency
III-V Multijunction Solar Cells
Dr. Erol Sağol
Hahn-Meitner-Institute, Berlin, Germany
As we are approaching the ultimate oil crisis and experiencing the already observable climate change, solar cells are drawing more and more attention and will become more and more important in our daily lives. Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today’s most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs / GaAsSb interface was achieved, when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.
Erol Sağol was born in 1973 in Adana. After his high school education at the “Tarsus American College” and “Ankara Science High School”, he studied physics at the “Bilkent University”. In his Master of Science thesis in Bilkent, he fabricated and characterized semiconductor laser diodes under the supervision of Ali Serpengüzel. After receiving his MSc degree in 1998, he joined the group of Klaus von Klitzing at the “Max-Planck-Institute” in Stuttgart and worked directly with Georg Nachtwei and Ismet Kaya on the breakdown of the quantum Hall effect. In 2000, following his supervisor Georg Nachtwei, he changed to the “Technical University of Braunschweig”, where he completed his PhD studies in 2003. In the following two and a half years he continued working on two dimensional electron systems in the group of Franz Ahlers at the “Physikalisch-Technische Bundesanstalt” in Braunschweig, which is the National Metrology Institute of Germany. There, he worked on the charge transport in two dimensional electron systems by surface acoustic waves and investigated big eddy currents in the quantum Hall effect regime. In March 2006 he joined the group of Thomas Hannappel at the Hahn-Meitner-Institute in Berlin and since then has been developing solar cells based on III-V compound semiconductors.
January 2, 2008, 14:40, FENS G035