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Lukas Kegelmann

Bio: Lukas Kegelmann is an academic researcher from Helmholtz-Zentrum Berlin. The author has contributed to research in topics: Perovskite (structure) & Tandem. The author has an hindex of 13, co-authored 26 publications receiving 1943 citations.

Papers
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Journal ArticleDOI
11 Dec 2020-Science
TL;DR: A monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15% is reported, made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovSKite cell.
Abstract: Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

876 citations

Journal ArticleDOI
TL;DR: In this paper, an 18% efficient monolithic silicon/perovskite tandem solar cells were constructed by combining optical optimization of the device architecture including light trapping approaches. But the design of the tandem cell is limited by the photocurrent generated in the silicon bottom cell that is reduced due to reflectance losses.
Abstract: Tandem solar cells combining silicon and perovskite absorbers have the potential to outperform state-of-the-art high efficiency silicon single junction devices. However, the practical fabrication of monolithic silicon/perovskite tandem solar cells is challenging as material properties and processing requirements such as temperature restrict the device design. Here, we fabricate an 18% efficient monolithic tandem cell formed by a silicon heterojunction bottom- and a perovskite top-cell enabling a very high open circuit voltage of 1.78 V. The monolithic integration was realized via low temperature processing of the semitransparent perovskite sub-cell where an energetically aligned electron selective contact was fabricated by atomic layer deposition of tin oxide. The hole selective, transparent top contact was formed by a stack of the organic hole transport material spiro-OMeTAD, molybdenum oxide and sputtered indium tin oxide. The tandem cell design is currently limited by the photocurrent generated in the silicon bottom cell that is reduced due to reflectance losses. Based on optical modelling and first experiments, we show that these losses can be significantly reduced by combining optical optimization of the device architecture including light trapping approaches.

549 citations

Journal ArticleDOI
TL;DR: In this paper, a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes, were studied.
Abstract: Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (VOC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the VOC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the VOC of the device. Importantly, the VOC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the VOC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the VOC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.

457 citations

Journal ArticleDOI
TL;DR: In this article, the authors compare and highlight trends in the band gap tunability and device performance metrics in reported metal halide perovskite alloys of a wide compositional range from low band gap compounds, such as FA0.75Cs0.25Sn0.5PbBr3 with an absorption onset close to 2.4 eV.
Abstract: Solar cells based on metal-halide perovskite semiconductors inspire high hopes for efficient low cost solar cell technology. This material class exhibits a facile tunability of the band gap making them interesting for multi-junction device technology. We here compare and highlight trends in the band gap tunability and device performance metrics in reported metal halide perovskite alloys of a wide compositional range from low band gap compounds, such as FA0.75Cs0.25Sn0.5Pb0.5I3 with an absorption onset of 1.2 eV, to high bandgap compounds, such as CsPbBr3 with an absorption onset close to 2.4 eV. In between, metal halide perovskites can seemingly be seamlessly tuned by compositional engineering. However, mixed bromide–iodide compounds with band gaps above 1.7 eV often exhibit photo-induced phase segregation inducing domains with lower band gaps that emit photons of low energy. This effect also reduces the photoluminescence quantum yield and hence the maximum open circuit voltage achievable in devices. This highlight summarizes general trends for metal halide perovskites with respect to their absorption onset. Furthermore recent progress as well as possible roadblocks for the band gap tunability of metal halide perovskites are highlighted as this is of particular importance for the development of multifunction solar cell technology.

278 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the triple cation perovskite photovoltaics with inorganic cesium were shown to be thermally more stable, contain less phase impurities and are less sensitive to processing conditions.
Abstract: Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.

3,470 citations

Journal ArticleDOI
08 Jan 2016-Science
TL;DR: It is shown that using cesium ions along with formamidinium cations in lead bromide–iodide cells improved thermal and photostability and lead to high efficiency in single and tandem cells.
Abstract: Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ∼20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2](0.83)Cs(0.17)Pb(I(0.6)Br(0.4))3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm(2) cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.

2,412 citations

Journal ArticleDOI
01 Mar 2019-Nature
TL;DR: A double-layered halide architecture for perovskite solar cells enables the use of dopant-free poly(3-hexylthiophene) as a hole-transport material, forming stable and scalable devices with a certified power conversion efficiency of 22.7 per cent.
Abstract: Perovskite solar cells typically comprise electron- and hole-transport materials deposited on each side of a perovskite active layer. So far, only two organic hole-transport materials have led to state-of-the-art performance in these solar cells1: poly(triarylamine) (PTAA)2–5 and 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)-9,9ʹ-spirobifluorene (spiro-OMeTAD)6,7. However, these materials have several drawbacks in terms of commercialization, including high cost8, the need for hygroscopic dopants that trigger degradation of the perovskite layer9 and limitations in their deposition processes10. Poly(3-hexylthiophene) (P3HT) is an alternative hole-transport material with excellent optoelectronic properties11–13, low cost8,14 and ease of fabrication15–18, but so far the efficiencies of perovskite solar cells using P3HT have reached only around 16 per cent19. Here we propose a device architecture for highly efficient perovskite solar cells that use P3HT as a hole-transport material without any dopants. A thin layer of wide-bandgap halide perovskite is formed on top of the narrow-bandgap light-absorbing layer by an in situ reaction of n-hexyl trimethyl ammonium bromide on the perovskite surface. Our device has a certified power conversion efficiency of 22.7 per cent with hysteresis of ±0.51 per cent; exhibits good stability at 85 per cent relative humidity without encapsulation; and upon encapsulation demonstrates long-term operational stability for 1,370 hours under 1-Sun illumination at room temperature, maintaining 95 per cent of the initial efficiency. We extend our platform to large-area modules (24.97 square centimetres)—which are fabricated using a scalable bar-coating method for the deposition of P3HT—and achieve a power conversion efficiency of 16.0 per cent. Realizing the potential of P3HT as a hole-transport material by using a wide-bandgap halide could be a valuable direction for perovskite solar-cell research. A double-layered halide architecture for perovskite solar cells enables the use of dopant-free poly(3-hexylthiophene) as a hole-transport material, forming stable and scalable devices with a certified power conversion efficiency of 22.7 per cent.

1,681 citations

Journal ArticleDOI
10 Nov 2017-Science
TL;DR: Because photocurrents are near the theoretical maximum, the focus is on efforts to increase open-circuit voltage by means of improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring.
Abstract: The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years' time. At this stage of their development, the key issues concern how to achieve further improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theoretical maximum, our focus is on efforts to increase open-circuit voltage by means of improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges associated with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and determination of the best ways to counteract degradation mechanisms.

1,371 citations

Journal ArticleDOI
TL;DR: In this paper, the authors improved the efficiency of monolithic, two-terminal, 1-cm2 perovskite/silicon tandems to 23.6% by combining an infrared-tuned silicon heterojunction bottom cell with the recently developed caesium formamidinium lead halide pervskite.
Abstract: As the record single-junction efficiencies of perovskite solar cells now rival those of copper indium gallium selenide, cadmium telluride and multicrystalline silicon, they are becoming increasingly attractive for use in tandem solar cells due to their wide, tunable bandgap and solution processability. Previously, perovskite/silicon tandems were limited by significant parasitic absorption and poor environmental stability. Here, we improve the efficiency of monolithic, two-terminal, 1-cm2 perovskite/silicon tandems to 23.6% by combining an infrared-tuned silicon heterojunction bottom cell with the recently developed caesium formamidinium lead halide perovskite. This more-stable perovskite tolerates deposition of a tin oxide buffer layer via atomic layer deposition that prevents shunts, has negligible parasitic absorption, and allows for the sputter deposition of a transparent top electrode. Furthermore, the window layer doubles as a diffusion barrier, increasing the thermal and environmental stability to enable perovskite devices that withstand a 1,000-hour damp heat test at 85 ∘C and 85% relative humidity. Perovskite solar cells can complement silicon photovoltaics in multijunction devices. Here, the authors optimize light harvesting in monolithic perovskite-on-silicon devices and fabricate a certified 23.6% efficient, 1 cm2 tandem solar cell with a perovskite device that withstands damp heat tests.

1,163 citations