Homojunction Cells
5.5 Other Homojunction Cell Concepts
5.5.3 Back-Contacted Cells: IBC Cells (Interdigitated Back Contact)Contact)
In the previous chapter, we saw that recombination losses can be reduced by using passivated contacts. There still remain optical losses on the front side due to shad-owing by the contact layers. These optical losses can be avoided if the contacts are moved to the back. Such cells are calledInterdigitated Back Contact cells(IBC).
Figure5.22shows schematically the structure of an IBC cell. Then-area alternates with thep-area. In order to avoid short circuits between then-region and thep-region, an undoped area between the two regions must be maintained: this area does not contribute to the production of electricity. Thus, the emitter does not cover the entire back surface, resulting in some additional losses.
Some minority carriers have to “travel” a longer path to reach the emitter, resulting in a small additional loss called “electronic shading”. However, the fabrication of
40Another solution is to apply the formal gas annealing process (FGA) using a furnace. However, this process produces H+and not H0. H0is the form of hydrogen most suited for passivation. H0 is electrically neutral, H+has a positive charge and H-has a negative charge.
5 Crystalline Silicon Solar Cells: Homojunction Cells 137 such a cell structure is quite complex. An alternative is to apply the IBC concept to heterojunction cells (see Chap.7). Here, thep-region is next to then-region and the emitter can be enlarged compared to the cell concept described in the present Section. Since the thinp- andn-doped amorphous layers have very low conductivity, they can be placed immediately adjacent to each other. HJT-IBC cells have excellent passivation properties due to the amorphous layer. With such a structure, Kaneka Inc. has achieved 26.7% cell designated-area41 efficiency on 79 cm2 and 26.6%
designated-area efficiency on a larger cell of 180 cm2[15].
With passivated “polycrystalline silicon on oxides” (POLO) contacts [16], the ISFH institute (Institut für Solarenergieforschung in Hameln) achieved in 2018 a designated-area cell efficiency of 26.1%, with an open circuit voltage Voc of 726.6 mV, a short-circuit current densityIscof 42.6 mA/cm2, with a fill factorFFof 84% on a 4 cm2p-type Wafer and an IBC structure. The polycrystallinep-regions and the polycrystallinen-regions lie side by side and are mutually isolated by an intrinsic polycrystalline Si(i)-layer.
References
1. H. Jakob, Reinstsilicium – Werkstoff des Computerzeitalters (Praxis der Naturwissenschaften Chemie, Aulius Verlag Köln Deubner & CO KG, 1988)
2. P. Capper,Bulk Crystal Growth in Electronic, Optical and Optoelectronics Materials(Wiley, The Atrium, Southern Gate, 2005)
3. J.D. Murphy, M. Al-Amin, K. Bothe, M. Olmo, V.V. Voronkov, R.J. Falster; The effect of oxide precipitates on minority carrier lifetime in n-type silicon, published online 7 December 2015.
J. Appl. Phys.118, 215706 (2015)
4. A. Hess, P. Krenckel, T. Trötschler, T. Feherenbach, S. Riepe, Development of high performance multicrystalline silicon with controlled seeding (Fraunhiofer Institut for Solar Energy Systems ISE; publica.fraunhofer.de/dokumente/N-521450.html, 2018)
5. G. Stokkan, A. Song, B. Rynigen,Investigation of the Grain Boundary Character and Disloca-tion Density of Different Types of High Performance Multicrystalline Silicon(Crystal Journal Published by MDPI, 2018)
6. A. Hein, Einflüsse von Verunreinigungen im Silicium und in der Ätzlösung auf das anisotrope Ätzen von Silicium in wäßrigen KOH-Ätzlösungen. Dissertation, TU-Berlin 2000, Berlin 2000 7. A. Luque, S. Hegedus,Handbook of Photovoltaic Science and Engineering. ISBN
13:978-0-471-49196-5(H/B) (Wiley, 2005)
8. S. Gatz, et.al. Analysis of local Al-doped back surface fields for high efficiency screen-printed solar cells. Energy Procedia S. 318–323 (2011).https://doi.org/10.1016/j.egypro.2011.06.143 9. D.A. Neamen,Semiconductor Physics and Devices, Basis Principles(McGraw Hill, 2003) 10. P. Saint-Cast et al., A review of PECVD aluminium oxide for surface, in27th PVSEC, 2012,
European Photovoltaic Solar Energy Conference and Exhibition(Frankfurt, 2012)
11. E. Urrejola Davanzo, Aluminium-silicon contact formation through narrow dielectric openings, Ph.D. thesis, at University Konstanz, Konstanz, 2012
41The “designated area” is the area of the wafer on which the active solar cell is located. Often a large wafer is used for research cells, but the solar cell is only located on a small area of the entire wafer—the “designated area”—The term “designated area” is only used in Research and Development.
138 S. Leu and D. Sontag 12. B. Fischer, Loss analysis of crystalline silicon solar cells using photoconductance and quantum
efficiency measurements, Ph.D. thesis at University Konstanz, Konstanz, 2003
13. J. Benick; TOPCon – „Überwindung der fundamentalen Hindernisse für eine neue Weltrekord-Siliciumsolarzelle“, Fraunhofer Institut für solare Energiesysteme ISE, Oktober,2013https://
www.ise.fraunhofer.de/de/forschungsprojekte/topcon.html
14. F. Feldmann, M. Bivour, C. Reichel, M. Hermle; S.W. Glunz, A passivated rear contact for high-efficiency n-type silicon solar cells enabling high Voc and FF > 82%, in28th European Photovoltaic Solare Energy Conference and Exhibition, 30 Sept. 2013
15. K. Yoshikawa, H. Kawasaki, W. Yoshida, T. Irie, K. Konishi, K. Nakano, T. Uto, D. Adachi, M. Kanematsu, H. Uzu, K. Yamamoto, Silicon heterojunction sola cell with interdigitated back contacts for a photoconversion efficiency over 26%, nature energy, published 20 March 2017, volume 2, article number: 17032
16. C. Klamt, M. Rienäcker, F. Haase, N. Folchert, R. Brendel, R. Peibst, V. Krausse, and J.
Krügener, Intrinsic poly-crystalline silicon region in between the p+and n+POLO contacts of an 26.1%-efficient IBC solar cell, Brussels, Belgium, 26.09.2018, in35th European Photovoltaic Solar Energy Conference and Exhibition, Brussels, 2018
Sylvère Leu was from 2008 to 2017 with the Meyer Burger Group, where he became a member of the executive board, as Chief Innovation Officer and Technology Officer (CIO/CTO).
Since his retirement in 2017, he is active as technology con-sultant, for the Meyer Burger Group. Sylvère graduated from the Swiss Federal Institute of Technology in Zürich (ETHZ). He additionally obtained a Master’s Degree in Business Adminis-tration at the University of St. Gallen (HSG). As a Swiss pio-neer, Sylvère Leu started to work in photovoltaics 30 years ago.
He constructed an industrially relevant laminator and sun sim-ulator in his own company. Beside his job he worked as an associate lecturer at University of St. Gallen (HSG), for several years, in the field of industrial production. At the end of 2005 he was charged with building up an integrated 250 MWp photo-voltaic facility, including wafer, cell and module manufacturing, in Germany.
Detlef Sontagis Senior Technologist for Solar Cells at Meyer Burger (Germany) GmbH. since 2015—one of the world’s lead-ing PV equipment manufacturlead-ing companies. He graduated as a physicist at the University of Konstanz in 2000, and obtained his Ph.D. degree in 2004 working on novel wafer materials for Pho-tovoltaics. In 2004 he joined Deutsche Cell GmbH, a subsidiary of Solar World AG, as quality assurance engineer with insights in the industrial crystalline silicon PV manufacturing processes along the whole value chain from block casting via cell pro-cessing to module assembling. Detlef Sontag joined the equip-ment manufacturer Roth & Rau AG as Senior Principal for cell technology in 2009. There he supported the planning and com-missioning of the Technology centre and worked actively on the development of cell technologies like PERC and HJT including project coordination of corresponding R&D-projects.