Perovskite-Silicon Tandem Solar Cells

Perovskite-Silicon Tandem Solar Cells

Details

Why silicon/perovskite tandems?
The current global photovoltaics market (nowadays taken for more than 90% by crystalline silicon solar cells) has seen a sustained growth of its production capacity by more than 20% annually. From a longer-term perspective, the market is expected to make a transition towards ultra-high efficiencies (see Fig. 2).  For this, further efficiency improvement s will be needed, beyond the single-junction efficiency limit of silicon (which is 29.4 %) [2].  The most straight-forward way to do so is in the form of a silicon-based tandem solar cell, where a wider-bandgap top cell overlays the silicon bottom cell.

In perovskite/silicon tandem solar cells, the perovskite top cell efficiently harvests the blue part of the solar spectrum, while transmitting the red part, which is absorbed in the silicon bottom cell. In this way, the tandems can overcome the single-junction efficiency limit of silicon solar cells.
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Figure 1. a) Efficiency evolution for single- and multi-junction solar cells by showing the achieved and predicted values [3]. b) AM1.5 solar spectrum (yellow) showing the absorbed wavelength ranges for silicon (red) and perovskite (blue) solar cells. 

Wide bandgap metal-halide perovskites
Realistic industrial applications require upscaling the perovskite top-cell technology, compared to the current lab standard. These top cells should then be monolithically integrated onto standard, textured SHJ cells in so-called two-terminal tandem solar cells. Such monolithic integration implies that the top cell needs to be deposited directly on top of the bottom cell (see Figure 2.b). An important consequence of such a monolithic tandem configuration is the fact that the two subcells must be designed to generate a similar photocurrent under maximum-power-point operation, else the total current density of the tandem device will be limited by lower current density, leading to lower performance. This current matching requirement limits the ideal top cell bandgap to a narrow range of Eg ≈1.7–1.8 eV while silicon has Eg ≈ 1.1 eV (Figure 2.a). From this perspective, perovskite solar cells are attractive top-cell candidates: Essentially, this is a thin-film photovoltaic technology using relatively wide bandgap absorbers and remarkable optoelectronic properties (such a sharp absorption onset and relatively long carrier diffusion lengths). In its single-junction configuration, deposited on glass, efficiencies above 23% have now been reported. [4]  Monolithic tandem devices, using SHJ solar cells have also already been reported, with efficiencies >25%. [5]  Moreover, with compositional engineering, the bandgap of the perovskite absorbers can be opened further, into the range that is ideal for top cell applications.

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 Figure 2. a) The dependency of the efficiency of a silicon-based tandem PV cell for varying top-cell bandgap. [6] b) Schematic illustration of monolithic (2T tandem) and 4-terminal (4T tandem) solar cells. [7]

References
[1] Haegel, Nancy M., et al. "Terawatt-scale photovoltaics: Trajectories and challenges." Science 356.6334 (2017): 141-143.
[2] Richter, A., Hermle, M., & Glunz, S. W. (2013). "Reassessment of the limiting efficiency for crystalline silicon solar cells." IEEE Journal of Photovoltaics, 3(4), 1184-1191.
[3] Albrecht, S. and B. Rech (2017). "Perovskite solar cells: On top of commercial photovoltaics." Nature Energy, 2.1 (2017): 16196.
[4] NREL Efficiency Table. https://www.nrel.gov/pv/assets/images/efficiency-chart-20180716.jpg, NREL (Accessed on July 24th, 2018)
[5]  Sahli, Florent, et al. "Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency." Nature Materials (2018): 1.
[6Yu, Z., Leilaeioun, M., & Holman, Z. (2016). Selecting tandem partners for silicon solar cells. Nature Energy, 1 (September), 16137.
[6Eperon, G. E., Hörantner, M. T., & Snaith, H. J. (2017). Metal halide perovskite tandem and multiple-junction photovoltaics. Nature Reviews Chemistry, 1(12), 0095.