Perovskite/Silicon Tandem Solar Cells: Challenges Towards High-. Efficiency in 4-Terminal ... Abstract â Perovskite and silicon solar cells have recently been shown to be ..... inverted semi-transparent planar perovskite solar cells in substrate ...
Perovskite/Silicon Tandem Solar Cells: Challenges Towards HighEfficiency in 4-Terminal and Monolithic Devices Jérémie Werner,a Florent Sahli,a Brett Kamino,b Davide Sacchetto,b Matthias Bräuninger,a Arnaud Walter,b Soo-Jin Moon,b Loris Barraud,b Bertrand Paviet-Salomon,b Jonas Geissbuehler,b Christophe Allebé,b Raphäel Monnard,a Stefaan De Wolf,a,1 Matthieu Despeisse,b Sylvain Nicolay,b Bjoern Niesen,a,b and Christophe Ballifa,b a
Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT) Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Rue de la Maladière 71b, 2002 Neuchâtel, Switzerland. b 1
CSEM, PV-Center, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland.
Now at King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Thuwai 23955-6900, Saudi Arabia.
Abstract — Perovskite and silicon solar cells have recently been shown to be perfect partners for tandem devices with potentially very high efficiency at low additional costs over standard silicon cells. Here, we present the development of efficient perovskite top cells suitable for 4-terminal and monolithic tandem integration on silicon heterojunction bottom cells. We show a 4-terminal tandem measurement with 25.6% efficiency on small cells and 23.2% on a 1 cm2 fully integrated device. Monolithic tandems with >20% efficiencies were developed on several types of silicon wafers, allowing for a direct optical comparison. We identify parasitic absorption to be the limiting factor for high performance and discuss several practical solutions to reduce them. Index Terms — amorphous silicon, hybrid organic inorganic perovskites, monolithic, multijunction solar cells, silicon heterojunction solar cells.
I. INTRODUCTION Crystalline silicon solar cells are approaching their theoretical Auger limit of 29.4%, with the record value currently at 26.6% [1]. One of the most promising approaches to overcome this efficiency limit relies on reducing thermalization losses by stacking several absorber materials with different bandgaps in a multi-junction device. For example, an established technology such as crystalline silicon can be combined with a low-cost, wide band gap thin-film top cell, which will harvest effectively the high-energy photons while transmitting the lowenergy photons to the silicon cell. With a tunable band gap, low material costs, compatibility with various fabrication techniques and a high performance of up to 22.1% [1], perovskite solar cells represent a very promising top cell candidate for tandem solar cells with >30% efficiency potential when combined with a silicon bottom cell [2]. A perovskite/silicon tandem solar cell can be made with mainly two approaches: as a mechanically-stacked 4-terminal tandem or a monolithically-integrated 2-terminal tandem, where the top cell is directly processed onto the bottom cell. Both 1
configurations have the potential to exceed 30% efficiency and have their own advantages, but also present several challenges, either in complex system integration for the 4-terminal configuration or challenging manufacturing of the sub cells due to the required process compatibility in the case of a monolithic configuration. Several groups have made rapid progress in the last two years, now showing >23% efficiency for monolithic tandems [3], [4] and >26% in the 4-terminal configuration [5]– [8]. Here, we present our recent developments towards >1 cm2 large perovskite/silicon tandem cells in both 4-terminal and monolithic configurations. We compare several perovskite material compositions, which allows us to tune the band gap and improve layer uniformity over larger areas. This led to a fully integrated 4-terminal tandem device with 23.2% total efficiency when both sub cells had an aperture area of 1.03 cm2. We then present monolithic tandem devices with >20% efficiency, discuss their limitations due to parasitic absorption in transport layers and present solutions with more transparent alternative materials. Finally, we optically compare the 4terminal and 2-terminal tandem devices, in order to discuss the role of surface texture in the bottom cell. II. EXPERIMENTAL METHOD Low-temperature planar perovskite solar cells were fabricated on glass/ITO substrates. The electron contact was made of a bilayer of polyethylenimine and C60 or phenyl-C61-butyric acid methyl ester (PCBM). The perovskite absorber was grown with a two-step method consisting of an evaporated PbI 2 layer transformed into the final perovskite phase by spin coating a solution of methylammonium (MA) iodide (and formamidinium (FA) iodide as well as formamidinium bromide for the mixed cation material) and annealing at 100°C for 30 min. The hole transport layer was made of 2,2’,7,7’ tetrakis(N,N-di-4- methoxyphenylamino)-9,9’ spirobi-fluorene (spiro-
Now at King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Thuwai 23955-6900, Saudi Arabia.
III. RESULTS AND DISCUSSION The development of the perovskite/silicon tandems presented here started with fabricating an efficient near-infrared transparent perovskite top cell. To this end, we developed a low-temperature process with a hybrid evaporation/solution processing 2-step method which allowed us to fabricate semitransparent MAPbI3 perovskite cells (Eg = 1.55 eV) with up to 16.4% efficiency [6]. By mechanically stacking this small cell with an aperture area of 0.25 cm2 onto a 4 cm2 silicon bottom cell, a 4-terminal tandem measurement of 25.2% could be demonstrated experimentally, while taking into account the metallization shadowing losses in the silicon cell. The perovskite absorber material was then modified towards slightly higher band gaps by introducing FA as a second cation and bromide as second halide. The band gap thus increased from 1.55 eV to 1.65 eV, which, consequently, provided more current in the silicon bottom cell during the 4-terminal tandem measurement as shown in Figure 1. Since the performance of this modified semitransparent top cell could be maintained at Perovskite top cell, 0.25 cm2 1086 mV 20.0 mA/cm2 77.1 % Bottom cell, filtered 695 mV 17.0 mA/cm2 79.8 % 4-Terminal:
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Fig. 1: EQE measurements of perovskite/silicon 4-terminal tandem cells, comparing two different perovskite compositions: FAMAPbI3-xBrx and MAPbI3
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OMeTAD) and the transparent electrode consisted of a thin MoOx layer and a sputtered IO:H/ITO bilayer. Silicon heterojunction solar cells were fabricated from n-type doped crystalline silicon float-zone wafers. Intrinsic and doped hydrogenated amorphous silicon layers were deposited in a PECVD reactor to passivate the silicon surface and create carrier-selective contacts. The back contact consisted of a sputtered ITO/Ag stack, whereas the front electrode was made of a thin sputtered IZO layer when used as bottom cells for monolithic tandems, or of sputtered ITO for 4-terminal tandem bottom cells. Current-voltage measurements were carried out on a two-lamp (halogen and xenon) class AAA WACOM sun simulator with an AM1.5g irradiance spectrum at 1000W/m2. External quantum efficiency (EQE) spectra were obtained on a custommade spectral response set-up.
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Fig. 2: Integrated mechanically stacked 4-terminal tandem device with 1.03 cm2 aperture area in both sub-cells: A) J-V curves of top and bottom cells and maximum power point tracking curve of the perovskite top cell. During the measurement of each sub cell, the other sub cell was kept at open circuit.
16.2% under maximum power point tracking, the total 4terminal tandem efficiency increased to 25.6%. These 4-terminal tandem measurements were, however, not yet obtained from fully integrated tandem devices, since both sub cells had considerably different sizes (0.25 cm2 for the top cell and 4 cm2 for the bottom cell) and because the sub cells were measured separately as previously reported [9]. We therefore developed a fully integrated 4-terminal tandem device with both sub cells having an aperture area of 1.03 cm2 and being permanently attached to each other. We therefore further adapted the top cell to achieve high performance on 1 cm2 area, which requires high uniformity in the deposition of all cell layers and high film quality, especially for the perovskite absorber. The spin coated PCBM electron transport layer was thus replaced by an evaporated layer of C60 of ~6 nm thickness. The perovskite composition was adapted to CsFAPbI3-xBrx, which, in our case, was key to fabricate high quality perovskite layers on larger areas. With these improvements, a 15.2% semitransparent top cell was fabricated and integrated on a silicon heterojunction bottom cell specifically designed with the same cell geometry and size. This fully integrated mechanically stacked 4-terminal tandem device had an efficiency of 23.2%, as shown in Figure 2. The top cell was limited by a fill factor of ~70% and the bottom cell mainly by the current, which was reduced by shadow losses due to the
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Fig. 3: Comparison of EQE measurements on monolithic and 4terminal tandem devices and silicon heterojunction single junction reference cell
thick evaporated metal fingers used on the top cell back electrode. Those two losses are directly linked and might be drastically reduced by changing the metallization process from evaporation to screen printing. However, this technique requires a temperature-stable cell, which will be the focus of further development. Compared to 4-terminal tandem devices, monolithic integration implies the additional requirement of process compatibility in terms of temperature and surface texture. We therefore first developed monolithic tandem cells on double-side polished silicon wafers, which allowed us to use a spin coated perovskite top cell. With these tandem cells, we could achieve efficiencies of up to 21.2% [3]. However, these cells were severely limited by their optics. In the infrared region, the bottom cell had a low spectral response due to the polished rear side. This could be improved by using rear-side textured wafers, which helped to increase the bottom cell current by 0.77 mA/cm2 [6]. Other optical losses were found in the MoOx protection buffer layer deposited on spiro-OMeTAD. This layer was found to become more oxygen deficient during Ar plasma exposure, thus increasing the parasitic absorption in this layer. WOx was found to be an interesting alternative owing to its larger resilience towards sputtering damage [9]. Moreover, the spiro-OMeTAD molecule used in the hole transport layer strongly absorbs photons at wavelengths 1 cm2,” J. Phys. Chem. Lett., vol. 7, pp. 161–166, 2016. K. A. Bush, A. F. Palmstrom, Z. (Jason) Yu, M. Boccard, R. Cheacharoen, J. P. Mailoa, D. P. McMeekin, R. L. Z. Hoye, C. D. Bailie, T. Leijtens, I. M. Peters, M. C. Minichetti, N. Rolston, R. Prasanna, S. E. Sofia, D. Harwood, W. Ma, F. Moghadam, H. J. Snaith, T. Buonassisi, Z. C. Holman, S. F. Bent, and M. D. McGehee, “23.6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability,” Nat. Energy, no. February, pp. 1–7, 2017. B. Chen, M. Yang, S. Priya, and K. Zhu, “Origin of J-V Hysteresis in Perovskite Solar Cells.,” J. Phys. Chem. Lett., pp. 905–917, Feb. 2016. J. Werner, L. Barraud, A. Walter, M. Bräuninger, F. Sahli, D. Sacchetto, N. Tétreault, B. Paviet-Salomon, S.-J. Moon, C. Allebé, M. Despeisse, S. Nicolay, S. De Wolf, B. Niesen, and
[7]
[8]
[9]
[10]
C. Ballif, “Efficient Near-Infrared-Transparent Perovskite Solar Cells Enabling Direct Comparison of 4-Terminal and Monolithic Perovskite/Silicon Tandem Cells,” ACS Energy Lett., pp. 474–480, 2016. F. Fu, T. Feurer, T. P. Weiss, S. Pisoni, E. Avancini, C. Andres, S. Buecheler, and A. N. Tiwari, “High-efficiency inverted semi-transparent planar perovskite solar cells in substrate configuration,” Nat. Energy, vol. 2, no. 16190, 2016. T. Duong, Y. Wu, H. Shen, J. Peng, X. Fu, D. Jacobs, E. Wang, T. C. Kho, K. C. Fong, M. Stocks, E. Franklin, A. Blakers, N. Zin, K. McIntosh, W. Li, Y. Cheng, T. P. White, K. Weber, and K. Catchpole, “Rubidium Multication Perovskite with Optimized Bandgap for Perovskite‐Silicon Tandem with over 26% Efficiency,” Adv. Energy Mater., vol. 1700228, pp. 1–11, 2017. J. Werner, J. Geissbuehler, A. Dabirian, S. Nicolay, M. Morales Masis, S. De Wolf, B. Niesen, and C. Ballif, “Parasitic absorption reduction in metal oxide-based transparent electrodes: application in perovskite solar cells,” ACS Appl. Mater. Interfaces, 2016. F. Sahli, B. Kamino, J. Werner, M. Bräuninger, B. PavietSalomon, L. Barraud, R. Monnard, J. P. Seif, A. Tomasi, Q. Jeangros, A. Hessler-Wyser, S. De Wolf, M. Despeisse, S. Nicolay, B. Niesen, and C. Ballif, “Improved optics in monolithic perovskite/silicon tandem solar cells by a nanocrystalline silicon recombination layer,” Manuscr. under Rev., 2017.
