Complex Refractive Indices of CesiumâFormamidinium-Based Mixed

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Letter

Complex Refractive Indices of Cesium-formamidinium-based Mixed Halide Perovskites with Optical Bandgaps from 1.5 to 1.8 eV Jérémie Werner, Gizem Nogay, Florent Sahli, Chien-Jen Terry Yang, Matthias Bräuninger, Gabriel Christmann, Arnaud Walter, Brett Kamino, Peter Fiala, Philipp Löper, Sylvain Nicolay, Quentin Jeangros, Bjoern Niesen, and Christophe Ballif ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Complex refractive indices of cesiumformamidinium-based mixed halide perovskites with optical bandgaps from 1.5 to 1.8 eV Jérémie Werner,1 * Gizem Nogay,1 Florent Sahli,1 Chien-Jen Terry Yang,1 Matthias Bräuninger,1 Gabriel Christmann,2 Arnaud Walter,2 Brett A. Kamino,2 Peter Fiala,1 Philipp Löper,1 Sylvain Nicolay,2 Quentin Jeangros,1 Bjoern Niesen,1, 2 and Christophe Ballif 1, 2 1

Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT),

Photovoltaics and Thin-Film Electronics Laboratory, Rue de la Maladie`re 71b, 2002 Neuchâtel, Switzerland. 2

CSEM, PV-Center, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland

ABSTRACT Cesium-formamidinium-based mixed-halide perovskite materials with optical bandgaps ranging from 1.5 to 1.8 eV are investigated by variable-angle spectroscopic ellipsometry. The determined complex refractive indices are shown to depend on the fabrication procedure and environmental conditions during processing. This data is complemented by additional optical and structural characterization, as well as the demonstration of efficient perovskite solar cells. Finally, the data is used in optical simulations to provide guidelines for the optimization of perovskite/silicon tandem solar cells.

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Perovskite solar cells have attracted a lot of attention recently, not only for their high photoconversion efficiencies with initial record values >22%,1 but also due to their tunable optoelectronic properties, which are attractive for multijunction solar cell applications. Indeed, the composition of the perovskite absorber, namely the cation (e.g. Cs, formamidinium (FA), methylammonium (MA) and combinations thereof), the metal (e.g. Pb and/or Sn) and the halide (e.g. I, Cl, Br), can be modified to yield materials with absorption edges ranging from 1.2 eV up to >2 eV.2 Perovskite-based tandem solar cells comprise either two perovskite subcells of different bandgaps stacked together3,4 or a wide-bandgap perovskite cell on top of a different type of solar cell with a narrow bandgap, such as crystalline silicon or copper indium gallium selenide.5–7 So far, a complete optoelectronic dataset is only available for the widespread methylammonium lead iodide composition with a bandgap that is not ideal for tandem devices.8– 12

However, a systematic set of complex refractive indices of multi-cation, mixed-halide

perovskite materials is essential to model the optical properties of devices and accelerate the development of single and multijunction solar cells.13–17 In this work, we determine the complex refractive indices of perovskite layers based on CsyFA1-yPb(IxBr1-x)3 materials with absorption onsets from ~1.5 eV to ~1.8 eV using variable-


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angle spectroscopic ellipsometry. These perovskite materials were chosen as they have been shown to be more stable than MA-based compounds18–20 and to exhibit high device performances with both 1-step18 and 2-step processing protocols.21 The ellipsometry data is complemented with an extensive dataset of optical, structural and electronic properties of the perovskite layers. To validate the relevance of these perovskite materials, they are implemented in efficient perovskite solar cells. Finally, optical simulations of perovskite/silicon monolithic tandem solar cells are presented to demonstrate how this measured optical data can be used to define practical optimization pathways. Perovskite layers were fabricated using a sequential 2-step hybrid deposition method, comprising a co-evaporation step of a cesium halide compound and lead iodide (PbI2), followed by spin coating of the FA halide solution. Such a hybrid deposition protocol has already been proven to be effective in the development of tandem solar cells, both in terms of performance and upscaling capability.22 Three different cesium halides were used in this study: cesium iodide (CsI), cesium chloride (CsCl) and cesium bromide (CsBr), all co-evaporated with PbI2. The final cesium content in the perovskite films was controlled by adjusting the evaporation rates of the Cs- and Pb-containing compounds. A mixture of formamidinium iodide (FAI) and formamidinium bromide (FABr) dissolved in ethanol was then spin-coated on this layer and subsequently annealed in air to form the final ~320-nm-thick perovskite layer. Overall, the final perovskite composition and layer morphology were defined by the choice of the evaporated cesium halide compound, its evaporation rate with respect to the one of PbI2, and the composition of the FA-halide solution. These perovskite films were characterized by variable-angle spectroscopic ellipsometry and photospectrometry. The resulting complex refractive indices (n + ik) are shown in Figure 1.


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More details on the ellipsometric models and raw data from these measurements can be found in the Supplementary Information. The characterization and modeling protocols are based on our previous work on methylammonium lead triiodide.8 Figure 1a shows the complex refractive indices of two perovskite materials, which exhibit similar absorption edges but were deposited using either CsI or CsCl in the co-evaporation step. The differences in n and k values are a clear indication that ellipsometry measurement results, and more generally the optical properties of perovskite materials, are highly dependent on the exact composition and hence fabrication procedure. For example, environmental aspects such as the presence of humidity during the annealing step can drastically affect the optical properties, as shown in Figure 1a, and with more details in Figure S1a-c. The real part of the refractive index is found to significantly decrease with increasing ambient humidity levels. Note that the interdiffusion process is known to be accelerated in the presence of humidity during the annealing of the film.23 Indeed, it enhances the diffusivity of organo-halides in the PbI2-contaning layer, which in turn reduces the amount of unconverted PbI2 residues in the final layer. This effect is also clearly observed in the present experiment (Figure S1d), as illustrated by X-ray diffraction (XRD) spectra of CsCl-based perovskite films prepared with different humidity levels, spin speed during the FA-halide solution coating and annealing time. The amount of unconverted lead iodide increases when the amount of spin-coated FA-halides is not sufficient (spin speed too high or solute concentration too low), when the humidity level is too low or when the samples are annealed for too long.


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1.2 CsI15%, 0:1 CsCl8%, 1:3-35%RH CsCl8%, 1:3-50%RH

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Figure 1: Variable-angle spectroscopic ellipsometry measurements: a) Comparison of complex refractive indices of two perovskite compositions yielding a similar absorption edge at ~1.69 eV and effect of humidity level during perovskite annealing with ambient relative humidities of 35%RH or 50%RH; b) Comparison of complex refractive indices of perovskite materials made with CsI, CsCl or CsBr; c) Absorption coefficient (α) spectra of CsBr-based perovskite materials with absorption edges ranging from 1.54 eV to 1.74 eV, measured by photothermal deflection spectroscopy (PDS). The inset shows the corresponding photoluminescence (PL) spectra (additional PL data can be found in Supplementary Information); d) Comparison of the absorption coefficient spectra measured by PDS or

from ellipsometry measurements of k (ߙ = 4ߨ݇/ߣ). The legends indicate the ratio between the Cs halide evaporation rate and the one of PbI2 (percentage) and the mixing ratio of the FAI and FABr in the spincoated solutions. The legend indicates the ratio between the Cs halide evaporation rate and the one of

PbI2 (percentage) and the mixing ratio of the FAI and FABr in the spin-coated solutions.


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Figure 1b shows n and k data corresponding to four perovskite compositions having different absorption onsets between ~1.6 eV and ~1.8 eV. In order to confirm the absorption onsets from ellipsometry data, photothermal deflection spectroscopic (PDS) measurements were carried out on selected samples and the resulting absorption coefficient (α) spectra are shown in Figure 1c. As illustrated in Figure 1d, ellipsometry and PDS measurements are well in agreement. The absorption onsets increase as expected with increasing Cs and/or with increasing concentration of smaller anions (Cl, Br), as also shown in Figure S2 and Figure S10. This bandgap widening is attributed to a decrease of the lattice constant, which is observed by XRD as a shift of the (100) reflection peak around 14° (Figure S3). The initial photoluminescence (PL) peaks of the CsBrbased perovskites are shown in the inset to Figure 1c. They are located at photon energies ~50 meV higher when compared to the optical bandgaps of these materials. A detailed characterization by atomic force microscopy (AFM), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) of the structural properties of these layers is shown in Figure S5. From energy-dispersive X-ray spectroscopy (EDX) chemical maps, the Cs:Pb atomic concentration ratio in the perovskite films could be estimated: 0.16 ± 0.03 with a 15% evaporation rate of CsI to PbI2 and 0.18 ± 0.03 with a 10% evaporation rate of CsBr to PbI2.


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Figure 2: STEM high-angle annular dark-field micrograph and corresponding EDX chemical map and profiles of the cross-section of a CsI perovskite solar cell in the p-i-n configuration. The backgroundsubtracted K or L EDX edges show how the different elements are distributed across the device. Arrowheads indicate background subtraction artifacts due to the low signal-to-noise ratio of the high energy Cs K edge.

To test the performance of these perovskite materials, solar cells with a p-i-n polarity were developed using a 17-nm-thick spiro-TTB24 hole transport layer thermally evaporated onto ITOcoated glass substrates. The perovskite absorber layer was then deposited with the 2-step method discussed above. The electron contact was made of an evaporated bilayer of 20-nm-thick C60 and 5-nm-thick TmPyPB,25 followed by Ag metallization. For tandem applications, the opaque Ag electrode used for these cells would have to be replaced by a transparent electrode, as presented elsewhere.6 Figure 2 shows a STEM micrograph and corresponding energy-dispersive X-ray spectroscopy (EDX) data of one of these cells, in this case based on a perovskite layer made with the CsI precursor deposited at a deposition rate of 15% relative to that of PbI2 and using a pure FABr solution. The perovskite layer exhibits a compact microstructure, without large voids. While its interfaces are slightly richer in I, Br still diffused down to the spiro-TTB interface during the


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annealing step. As shown in Figure S5, a CsBr-based perovskite cell shows a similar microstructure, while a CsCl-based composition features a top surface composed of smaller grains and pores (arrowhead in Figure S5). Despite these structural differences, all three devices perform similarly with efficiencies at maximum power point tracking of 14.6%, 14% and 15% for these materials based on CsI, CsCl and CsBr, respectively (Table S2). Hence, even if they yield comparable device parameters, it is important to note that structural and morphological variations resulting from different compositions, deposition methods and environmental conditions, do affect ellipsometric data (e.g. see Figure 1a) and are at the origin of the large spread in published data.8–12 While absolute values should be assessed with caution, the n and k data retrieved here enables reliable optical simulations and provides valuable insights to optimize devices as discussed later in this article. The EQE measurements shown in Figure 3 demonstrate that a high spectral response can be obtained with all the perovskite compositions investigated in this study. The photogenerated currents calculated from these spectra vary from ~22 mA/cm2 for the narrowest optical bandgap (~ 1.51 eV) to ~15 mA/cm2 for the widest (~ 1.8 eV), following an expected parallel line to the maximum obtainable current for a given bandgap and the AM1.5g spectrum, as shown in Figure S6. Table S1 summarizes the optical bandgaps for all investigated perovskite compositions. This data, extracted from EQE curves, is confirmed by bandgap values measured from Fouriertransform photocurrent spectroscopy (Figure S2), photothermal deflection spectroscopic measurements (Figure 1c), as well as the k values determined from ellipsometry.


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Figure 3: External quantum efficiency spectra of single-junction perovskite cells with various compositions, indicated in the legends by their fabrication conditions, in a) using CsCl and b) CsBr and CsI. The legends indicate the ratio between the Cs halide evaporation rate and the one of PbI2 (percentage) and the mixing ratio of the FAI and FABr in the spin-coated solutions.

Additional current-voltage measurements can be found in the Supplementary Information, showing, amongst others, the increase in open-circuit voltage with optical bandgap. The best cells are found to have bandgaps between 1.6 and 1.65 eV and demonstrate efficiencies of ~15%, whereas the cell with the widest bandgap of ~1.8 eV had an efficiency of ~11% at maximum power point (Table S2).


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Figure 4: Transfer-matrix modeling of perovskite/silicon heterojunction monolithic tandem solar cells with flat interfaces: a) Variation of the perovskite layer composition and bandgap. The inset graph shows the simulated currents for the top and bottom cells when varying the top cell bandgap without changing its thickness (580 nm). b) Perovskite absorber thickness variation with a CsBr10%-1:2 material; The simulated tandem cell layer stack from back to front: Ag/a-Si:p/a-Si:i/c-Si/a-Si:i/aSi:n/nc-Si:n/nc-Si:p/ITO/NiOx/Perovskite/C60/SnO2/IO:H/MgF2 (explained in more details in the Supplementary Information).

Perovskite/silicon monolithic tandem solar cells are complex optical devices, comprising numerous layers for which the thickness and material choice in terms of optical properties have a significant influence on the overall tandem performance. Figure 4 shows transfer matrix simulations of perovskite/silicon monolithic tandem solar cells using an amorphous silicon (aSi)/crystalline silicon (c-Si) heterojunction bottom cell in rear-emitter configuration, a nanocrystalline silicon (nc-Si) tunnel junction22 and a perovskite top cell with NiOx as hole transporting layer and C60/SnO2/IO:H as the front contact stack. This configuration was chosen for its practical feasibility,6,22 its up-scalability and compatibility with future designs using textured surfaces, as it uses materials that can be deposited by physical or chemical vapor deposition techniques. In order to limit the complexity of the simulations presented here, all


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layers were considered to be flat (no surface roughness or texture). Complex refractive indices were either measured in house or taken from literature.8,15,26–30 The current of a monolithic tandem solar cell is limited by the subcell that generates the lowest current and the tandem voltage ideally is the sum of the subcells’ voltages. Therefore, the optimal performance will be obtained when the top cell material is chosen such that it provides the highest voltage possible, while allowing the tandem cell to stay close to a current-matched situation.31 Figure 4a illustrates a situation where the optical bandgap of the top cell is increased without adjusting its thickness, leading quickly to a large current mismatch outside of the optimized region. Figure 4b shows the effect of varying the perovskite layer thickness on the same tandem cell. As expected, this affects the top cell only for wavelengths λ > 500 nm, where the perovskite absorbs in the low-finesse thin-film interference regime.32 Further research will therefore be necessary to develop thick perovskite layers (up to 1 µm) with still high optoelectronic quality. It is also evident from this data that, due to parasitic absorption losses and practical thickness constraints, the highest optical bandgap which still allows for a currentmatched situation is lower (~1.65-1.68 eV) compared to what would be expected from an ideal, loss-free, tandem cell (~1.75 eV).33 Further optical simulations are available in the Supplementary Information, showing how varying the charge transport layer (NiOx and C60) and electrode (front TCO and intermediate contact) thicknesses can help reducing these parasitic absorption losses limiting the available photogenerated current. In summary, we investigated CsFA-based perovskite materials having absorption onsets between 1.5 and 1.8 eV by variable-angle spectroscopic ellipsometry to determine their complex refractive indices. We discussed the fabrication procedure involving a 2-step hybrid deposition method and the effects of process parameters and environment on the measured ellipsometric


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data. This data was supported by an in-depth structural study and by experimental cell data, confirming the good optoelectronic quality of the prepared materials. We finally demonstrated the usefulness of such a broad optical dataset by carrying out optical simulations of perovskite/silicon monolithic tandem solar cells, showing practical optimization pathways for the development of the next-generation tandem devices. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX: Experimental methods for layer and device fabrication and characterization; XRD data for various process conditions; PL data; FTPS data; structural analysis by electron and atome probe microscopy; summary table of bandgap energy values; complete device electrical characterization; TMM simulations with variations on charge transport and electrode layers; ellipsometric raw data. AUTHOR INFORMATION Corresponding Author *E-mail [email protected]. Phone +41 21 69 54258 ACKNOWLEDGMENT This work was funded by the Nano-Tera.ch Synergy project, the Swiss Federal Office of Energy under Grant SI/501072-01, the Swiss National Science Foundation via the Sinergia Episode (CRSII5_171000) and NRP70 Energy Turnaround PV2050 (407040) projects. REFERENCES


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17

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1 . 2 Letters ACS Energy

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2 .1

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Page 19 of 21 Ag

100 nm

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Br K Ag K 300 200 100

ACS Paragon Plus Environment b)

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E Q E (-)

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ACS Energy Letters

Page 20 of 21

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F r a c tio n o f a b s o r b e d lig h t

1 .0 ACS Energy Letters

1 .21 0 of 21 Page

0 .2

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Complex Refractive Indices of CesiumâFormamidinium-Based Mixed

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