By Prof. Rasit Turan, Middle East Technical University, Ankara, &
Director of the Center for Solar Energy Research and Applications (GÜNAM), Turkey.
Fabrication of a c-Si solar cell without any doping process requires asymmetric and carrier selective hetero-junctions on the two sides of the Si wafer using materials with desirable band alignment. Among several others, TiO2 and MoOx layers have been proposed as electron selective and hole selective layers. In the first part of this work, optical and electrical properties of ALD deposited titanium dioxide (TiO2) contact on n-type and p-type c-Si wafers were studied. The optical, compositional, and diode quality dependence of TiO2 on the ALD deposition temperature were analyzed using spectroscopic ellipsometry, AFM, XPS, GI-XRD, and CV measurements. By optimizing the ALD process parameters, an impressive effective minority carrier lifetime of up to 2.3 milliseconds corresponding to an iVoc of ~700 mV was obtained from wet chemical oxide-SiO2/TiO2 passivation stack layers. Finally, the asymmetry in C-V and J-V measurements betweenTiO2/n-type and TiO2/p-type c-Si heterojunctions were examined and the electron transport selectivity of TiO2 was studied.
In the second part, we investigated the feasibility to integrate sub-stoichiometric molybdenum oxide (MoOx) as hole-collecting rear contact in the production sequence of industrial p-type crystalline silicon (c-Si) solar cells with standard size (156 mm x 156 mm). The standard fabrication process of p-type solar cells is utilized yet the back surface field usually formed by boron-diffused or aluminum-alloyed regions was replaced by an optimized MoOx film thermally evaporated at room temperature. The attained peak efficiency of the fabricated solar cells is 17.65 % with Voc of 626 mV, Jsc of 36.8 mA/cm2, and FF of 76.63 %. This results obtained both for TiO2 and MoOx layers are promising for the development of dopant-free and high-efficiency solar cell at low cost.
Hisham Nasser1, Doguscan Ahiboz1, Ezgi Aygun1, Mona Zolfaghari Borra1, Ozan Akdemir1,2, Alpan Bek1,2, Rasit Turan1,2
(1) The Center for Solar Energy Research and Applications (GÜNAM)
(2) Department of Physics, Middle East Technical University (METU), Dumlupinar Blvrd 1, 06800 Ankara, Turkey
Raşit Turan completed B. Sc. and M. Sc. degrees at the Physics Department of Middle East Technical University (METU), Turkey. He received his Ph.D degree from University of Oslo, Norway in 1990. He worked as Post. Doc. at Linköping University, Sweden. He joined METU Physics Department as faculty member in 1991. He worked as a visiting scientist at the Material Science Department of Toronto University, Canada in 1996. His main research interests have been physics and technology of semiconductor materials and devices including solar cells. He has published more than 160 scientific papers in this field in the internationally recognized journals. He has supervised 7 Ph.D. and about 20 M.Sc. studies.
Rasit Turan has coordinated many national and international projects. Among them, European FP6 projects SEMINANO, and METU-CENTER have been among the largest research and support projects coordinated by Dr. Turan. I 2009, he founded a new research center called Center for Solar Energy Research and Applications (GUNAM) on METU campus. GUNAM has attracted nationwide and international attention. Recently, a national solar energy system development project, called MILGES, been given to the consortium where GÜNAM is a key member for solar cell development. He is one of the partners of the project called “PV Smart Skin” conducted by Texas A&M Doha, and supported by QNRF of Qatar.