RESEARCH ARTICLE Performance comparison of AlGaAs, GaAs and InGaP tunnel junctions for concentrated multijunction solar cells Jeffrey F. Wheeldon 1 * , Christopher E. Valdivia 1 , Alexandre W. Walker 1 , Gitanjali Kolhatkar 1 , Abdelatif Jaouad 2 , Artur Turala 2 , Bruno Riel 3 , Denis Masson 3 , Norbert Puetz 3 , Simon Fafard 3 , Richard Are `s 2 , Vincent Aimez 2 , Trevor J. Hall 1 and Karin Hinzer 1 1 Centre for Research in Photonics, University of Ottawa, Ottawa, ON, Canada 2 Centre de Recherche en Nanofabrication et en Nanocaracte ´ risation CRN 2 , Universite ´ de Sherbrooke, Sherbrooke, QC, Canada 3 Cyrium Technologies Inc., Ottawa, ON, Canada ABSTRACT Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/ AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time-dependent and time-average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000-suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10 4 V cm 2 followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of 5  10 4 V cm 2 within the range of doping concentrations studied. Copyright # 2010 John Wiley & Sons, Ltd. KEYWORDS concentrated photovoltaics; tunnel junctions; AlGaAs; GaAs; InGaP; multijunction solar cell *Correspondence Jeffrey F. Wheeldon, Centre for Research in Photonics, University of Ottawa, Ottawa, ON, Canada. E-mail: wheeldon@site.uottawa.ca Received 19 March 2010; Revised 11 June 2010 1. INTRODUCTION Under high concentration, multijunction (MJ) solar cells are at the forefront of photovoltaic performance, with conversion efficiencies above 40% [1–4] using the AM1.5D spectrum. MJ solar cells are composed of tandem n–p junctions connected electrically in series, where the band gap of each subcell is tailored to a different range of photon energies. Hence, MJ solar cells are able to more efficiently convert the solar spectrum into electrical power than their single junction (1J) counterparts. The GaInP/ InGaAs/Ge triple junction (3J) solar cell represents the most common type of MJ commercial designs, shown schematically in Figure 1. In comparison to 1J solar cells, MJ solar cells are more complex and costly to design and fabricate because they are composed of more expensive III–V semiconductor materials on a group IV substrate, and hence represent a significant increase in the cost compared to traditional devices. For terrestrial applications, this cost challenge is overcome by reducing the device area. This is achieved by coupling solar cells to concentrating optics, such as mirrors or lenses, which typically intensifies sunlight by a factor of 500 in commercial systems. In the GaInP/InGaAs/Ge triple junction solar cell, the subcells are composed of tandem n–p junctions that are monolithically integrated into a multilayered device, such as the one depicted in Figure 1a. The tandem n–p junctions impose the challenge of conducting current between the subcells, which is addressed by introducing tunnel junctions (TJ) that connect the p terminal of one subcell to the n terminal of an adjacent subcell. The TJ itself is a heavily doped p–n junction with doping concentrations typically >10 19 cm 3 . An example AlGaAs/AlGaAs TJ is shown schematically in Figure 1b. Due to the importance PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. (2010) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1056 Copyright ß 2010 John Wiley & Sons, Ltd.