Supporting Information
Hole-Transport Material Variation in Fully Vacuum Deposited Perovskite Solar Cells
Lauren E. Polander, Paul Pahner, Martin Schwarze, Matthias Saalfrank, Christian Koerner, Karl Leo*
Institut für Angewandte Photophysik, Technische Universität Dresden, 01069 Dresden, Germany
Contents
Experimental Details
Materials
Page S2
General Device Fabrication
Page S2
Perovskite Layer Deposition
Page S3
Profilometry
Page S3
Grazing Incidence X-Ray Diffraction
Page S4
Ultraviolet Photoelectron Spectroscopy
Page S5
Conductivity Measurements
Page S5
Device Characteristics
Page S6
References
1
Page S2
Page S6
Experimental Details
Materials. All reagents were purchased from Aldrich and used as received unless otherwise stated.
Methylammonium iodide (CH3NH3I) was prepared from methylamine and hydroiodic acid as
reported1 and purified by recrystallization in ethanol and isopropanol. Buckminster fullerene (C60) was
purchased from Bucky, USA, and purified twice by thermal gradient sublimation. 2,2',7,7'Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-MeO-TAD, 1) was purchased
from Feiming Chemical Limited and used as received. N4,N4,N4',N4'-Tetrakis(4-methoxyphenyl)[1,1'-biphenyl]-4,4'-diamine
spirobifluorene
(MeO-TPD,
(Spiro-MeO-TPD,
3),
2),
2,7-bis[N,N-bis(4-methoxy-phenyl)amino]-9,9-
2,2',7,7'-tetrakis-[N,N-di(4-methylphenyl)amino]-9,9'-
spirobifluorene (Spiro-TTB, 4), 2,2',7,7'-tetrakis-(N,N-diphenylamino)-9,9'-spirobifluorene (SpiroTAD, 5), and 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF, 6) were
purchased from Lumtec and purified twice by gradient sublimation. 2,2'-(Perfluoronaphthalene-2,6diylidene)dimalononitrile (F6-TCNNQ) and NDP9 were purchased from Novaled GmbH and used as
received.
General Device Fabrication. Thin film samples were prepared by thermal vapor deposition in ultrahigh vacuum (UHV) with a base pressure at or below 10–7 mbar. The layer thicknesses were
determined during evaporation using quartz crystal monitors calibrated for the respective material
prior to evaporation. The glass/ITO substrates (Corning Eagle XG, 1.1 mm, Thin film devices, USA)
were carefully cleaned with NMP, ethanol, and oxygen plasma before being transferred into the
vacuum chamber. One general stack design was used for all cells as depicted in Figure S1. Layers 1–3
were deposited by thermal evaporation in a UHV system (K.J. Lesker, U.K.). The substrates were
transported in a nitrogen-sealed transfer box to a second UHV chamber for the deposition of layer 4
(CH3NH3PbI3–xClx perovskite deposition detailed below) and again, via nitrogen-filled glovebox, to a
third UHV chamber for the deposition of layers 5 and 6 (CreaPhys GmbH, Germany). The completed
solar cells were encapsulated with a transparent encapsulation glass, fixed by UV-hardened epoxy
glue, in a nitrogen-filled glovebox.
Figure S1. General device architecture for all cells in the study. Perovskite in layer 4 refers to the
methylammonium lead iodide/chloride perovskite of the form CH3NH3PbI3–xClx.
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Perovskite Layer Deposition. The perovskite layer deposition was performed in a custom-made
vacuum system (CreaPhys GmbH, Germany) by thermal co-evaporation of PbCl2 and CH3NH3I at a
base pressure of 10−7 mbar. The deposition ratio and layer thickness were monitored using calibrated
quartz crystal microbalances (QCM, Thickness/Rate Monitor STM-100/MF, Syncon Instrument).
Three QCM sensors are present in the chamber, one monitoring the rate of each evaporation source
and a third at the height of the substrate holder. The PbCl2 source QCM was used to calibrate the
deposition rate of PbCl2 using normal tooling procedures (ρ = 5.85 mg cm–3). However, a fluctuating
signal was observed at each of the three QCMs during heating of the CH3NH3I source and the
deposition rate of this material could not be determined using normal methods. Alternatively, the
amount of CH3NH3I in the chamber was monitored by stabilizing the chamber pressure at ca. 1.0×10–4
mbar. The third QCM placed at substrate level was used to monitor the perovskite layer formation.
Initially, this QCM was calibrated by tooling with PbCl2. However, as the density and exact
composition of the final film is unknown, the thickness of this layer was more accurately determined
using profilometry measurements (Figure S2, details below). After loading the sources, the chamber
was evacuated and allowed to stabilize overnight. To begin evaporation, the CH3NH3I crucible was
heated slowly until a pressure of ca. 1.0×10–4 mbar could be maintained in the chamber (crucible
temperature of 130–160 °C). The PbCl2 source was then heated to achieve a rate of ca. 0.5–0.7 Å s−1
(360–380 °C) at the PbCl2 QCM, which corresponded to a deposition rate at the substrate QCM of ca.
0.8–1.0 Å s−1. The films were dark brown when removed from the vacuum chamber and no annealing
treatments were performed. Formation of a perovskite film was confirmed using grazing incidence Xray diffraction (GIXRD) for a sample deposited on glass (Figure S3, details below).
Figure S2. Profilometer trace of a vacuum deposited perovskite layer on glass with silver overlay.
Profilometry. The perovskite layer thickness calibration was carried out by evaluating the actual
thickness of deposited layers using a profilometer (Dektak 150, Veeco, CA). A mask was used to
prepare a sample with a sharp perovskite layer edge on a glass substrate. Subsequently a thin silver
layer was evaporated on top (without mask) to enable scanning of a more rigid surface and avoid
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direct air exposure of the perovskite. A constant force of 3.0 mg was applied to the stylus (5.0 µm
radius) while measuring the height profile of the step-like surface. Several scans were performed at
different areas on the sample (Figure S2) and it was determined that the average roughness was
approximately 6 nm and the film thickness was ca. 195 nm (corresponding to a reading of 100 nm at
the substrate QCM monitor). This correction factor was used for the thickness calibration throughout
the study.
Grazing Incidence X-Ray Diffraction (GIXRD). The thin-film perovskite sample was measured
using a Bruker D8 Discover diffractometer in GIXRD geometry. For the thin-film measurements, Cu
Kα radiation from an X-ray tube operated at 40kV and 40mA was paralleled by a 3rd generation 60mm
Göbel mirror. The diffracted beam was detected by a scintillation counter after passing a parallel plate
collimator (0.35° angular acceptance). The diffraction pattern of the film was collected by variation of
the 2θ detector angle in the range of 3–90° with an angle step width Δ2θ = 0.1° and 30s sampling
time. The measurements were carried out at two different incidence angles in order to eliminate air
scattering contributions. The first measurement was taken at Ω = 0.3°, slightly above the critical angle
of total external reflection. At this incidence angle, the penetration depth is in the range of the
perovskite film thickness and therefore contributions from the glass substrate are assumed to be
negligible. The second measurement was taken at Ω = 0.07°, well below the critical angle, and was
used as background correction for the first measurement.
Figure S3. Grazing incidence X-ray diffraction of a vacuum deposited perovskite layer on glass.
Diffraction peaks at 14.2°, 28.6°, and 43.3° are assigned to the (110), (220), and (330) planes of a
tetragonal perovskite structure, respectively. The typical tetragonal structure for CH3NH3MX3 hybrid
perovskites (M=Pb, Sn; X=Cl, Br,I) consists of a three-dimensional anionic framework of PbX6 (X =
I, Cl) octahedra with methylammonium cations in the interstitial space.2 Additional peaks are present
in the diffractogram as shown in Figure S3. These are believed to correspond to imperfections in the
perovskite deposition, and particularly to excess PbX2 present in the sample. Although the
evaporation conditions were held constant, slight fluctuations could be seen in the relative PbX 2
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content from sample-to-sample, which presumably also effected the solar cell performance. For this
reason, multiple substrates were fabricated and evaluated for each HTM in the study.
Ultraviolet Photoelectron Spectroscopy (UPS). The organic layers (ca. 10–12 nm) were thermally
evaporated at a pressure of 10–8 mbar on sputter cleaned metal foils or plasma cleaned ITO and
transferred in UHV to the UPS chamber. UPS spectra were acquired from He I (21.22 eV) excitation
lines using a PHOIBOS 100 system (Specs, Berlin, Germany) at base pressure of 10 –11 mbar. All
experiments were calibrated to the Fermi edge of an atomically clean gold surface. UPS traces for
HTMs 2–6 are shown in Figure S4 and an energy level diagram for the full system is shown in Figure
1 of the main text. The values for the perovskite layer and HTM 1 are as reported.3
Figure S4. UPS measurements on HTM 2 (—), 3 (—), 4 (—), 5 (—), and 6 (—). The spectra are
calibrated to vacuum level position and scaled to the same intensity. The IP values are defined as the
onset of the highest occupied molecular orbital as shown in the graph.
Conductivity Measurements. The conductivity measurements of the HTMs were performed in situ
during quartz-crystal controlled co-evaporation of matrix and dopant molecules at a pressure ca. 10–8
mbar. The current increase was measured between the contacts of a meander-like structure (1 nm Cr /
40 nm Au) upon increasing layer thickness of the matrix:dopant films in the channel (b = 500 µm, l =
0.111 m) during deposition. The sign of the applied voltage (10 V) did not alter the magnitude of the
current (Ohmic behavior).
Due to a broad distribution of conductivities among the chosen HTMs, doping techniques were used
to establish matrix layers with comparable conductivities. Each material was initially tested at 2 wt%
doping of either F6-TCNNQ (HTMs 1–5) or NDP9 (HTM 6). In the case of HTM 5 and 6, this doping
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ratio resulted in conductivities well below the range of the other HTMs (σ < 1×10–5 S cm–1).
Therefore, an increased doping ratio of 10 wt% was used for these materials in order to achieve
comparable conductivity values (Figure S5).
Figure S5. Comparison of the conductivity values for HTMs 1–6. The black bars indicate the doping
ratios used for device fabrication. The weight percent and molar ratio of dopant (F 6-TCNNQ) in the
matrix material are noted for HTM 1–5. For HTM 6, only the weight percent is noted due to the use of
a proprietary dopant (NDP9).
Device Characterization. The J–V characteristics were recorded with a Keithley 2400 source
measure unit using a sun simulator 16S-003-300 (Solarlight Company Inc.) that was tuned to
mismatch corrected light intensity, determined by an outdoor reference cell (Fraunhofer Insitute for
Solar Energy Systems, Freiburg, Germany). Mismatch correction was performed using the measured
EQE spectrum, the EQE of the reference diode, and the illumination spectrum of the sun simulator,
which is recorded with every J–V measurement.4 The active area of the solar cells was determined
using optical microscopy to be approximately 6.44 mm2. For the EQE measurement, a homemade
setup based on a zenon arc lamp, a monochromator (Cornerstone 260), and a lock-in amplifier (Signal
Recovery SR 7265) was used. The setup was calibrated with a reference diode (Hamamatsu S1337).
References
(1)
Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338,
643.
(2)
Mitzi, D. B. Prog. Inorg. Chem. 1999, 48, 1.
(3)
Schulz, P.; Edri, E.; Kirmayer, S.; Hodes, G.; Cahen, D.; Kahn, A. Energy Environ. Sci. 2014,
7, 1377.
(4)
Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct. Mater. 2006, 16,
2016.
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