High efficiency multijunction solar cells: Electrical and optical properties of the dilute nitride sub-junctions

Multijunction solar cells with III-V semiconductor sub-junctions have the highest conversion efficiency of all photovoltaic devices [1]. These devices are applied in concentrated photovoltaics, where optical components are used for concentrating light ultimately over thousand time smaller solar cell chip. Another important application area is satellites and other space applications. High power-to-mass ratio and radiation hardness makes multijunction III-V semiconductor solar cells clearly the most applied source for electricity in space. Key parameter in these devices is conversion efficiency. The world record conversion efficiency is currently 46%, when theoretical limit is 86.8% [1,2]. Although theoretical maximum cannot be achiever, there is still plenty of room to improve. In order to achieve very high efficiencies, high-quality materials optimized for absorbing certain parts of the solar spectrum are needed. Dilute nitrides, in the form of GaInNAs(Sb), is particularly interesting material family, because it can be grown lattice-matched on conventional GaAs and Ge substrates with the band-gap of ~1.4 eV‒0.7 eV [3]. For example, GaInP / GaAs / GaInNAs(Sb) (/ Ge) solar cell has realistic potential for achieving a very high conversion efficiency in terrestrial and space applications [4]. One of the main challenges is to be able to grown high-quality dilute nitride with ~1 eV band-gap. We present recent results on electrical and optical properties of ~1 eV band-gap dilute nitride solar cells, grown by molecular beam epitaxy. The influence of materials composition, fabrication parameters, as well as post growth treatments on the material properties and photovoltaic performance are shown [5,6]. To this end, we use deep level transient Fourier spectroscopy (DLTFS), capacitance-voltage spectroscopy, external quantum efficiency measurements, light-current-voltage measurements, and photoluminescence spectroscopy. We show how material composition has a remarkable influence on the deep levels properties and background doping. Broader defect-related DLTFS spectra were recorded from the compounds with more material components (GaInNAs vs. GaNAsSb vs. GaInNAsSb). Sb was found to reduce the unintentional background doping and at the same time increase the effective capture cross section of the dominant deep levels [6]. Furthermore, we show a clear dependency between several critical material parameters and As/group-III flux ratio: as a result increase in flux ratio decreases the dilute nitride solar cell performance at investigated range [5]. The role of Ga vacancies and related point defects on the background doping will be also discussed. The results are also reflected against the operation of high-effiency multijunction solar cells with dilute nitride sub-junctions. 1. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, “Solar cell efficiency tables (version 47)” Progress in Photovoltaics: Research and Applications 24, pp. 3–11, 2015. 2. A. Martí, G. L. Araújo, "Limiting efficiencies for photovoltaic energy conversion in multigap systems," Solar Energy Materials & Solar Cells 43, pp. 203–222, 1996. 3. A. Aho, V. Polojärvi, V. Korpijärvi, J. Salmi, A. Tukiainen, P. Laukkanen, M. Guina, "Composition dependent growth dynamics in molecular beam epitaxy of GaInNAs solar cells", Solar Energy Materials & Solar Cells 124, pp. 150–158, 2014. 4. A. Aho, A. Tukiainen, V. Polojärvi, M. Guina, “Performance assessment of multijunction solar cells incorporating GaInNAsSb”, Nanoscale Research Letters 9, pp. 61:1–61:7. 5. V. Polojärvi, A. Aho, A. Tukiainen, M. Raappana, T. Aho, A. Schramm, M. Guina, “Influence of As/group-III flux ratio on defects formation and photovoltaic performance of GaInNAs solar cells”, Solar Energy Materials & Solar Cells 149, pp. 213–220, 2016. 6. V. Polojärvi, A. Aho, A. Tukiainen, A. Schramm, M. Guina, “Comparative study of defect levels in GaInNAs, GaNAsSb, and GaInNAsSb for high-efficiency solar cells”, Applied Physics Letters 108, pp. 122104:1–122104:5, 2016.