Post on 06-Apr-2020
SUPPLEMENTARY INFORMATIONDOI: 10.1038/NPHOTON.2014.255
NATURE PHOTONICS | www.nature.com/naturephotonics 11
Supplementary Information
A Novel Route to Achieve High Efficient Multiferroic oxides Solar Cells
Riad Nechache1,2, Catalin Harnagea,2 Shun Li,2 Luis Cardenas,2 Wei Huang,2 Joyprokash Chakrabartty2 and Federico Rosei2
1NAST Center & Department of Chemical Science & Technology, University of Rome Tor Vergata Via della Ricerca Sceintifica, 00133 Rome, Italy.
2INRS -Centre Énergie, Matériaux et Télécommunications, Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada.
3Center for Self-Assembled Chemical Structures, McGill University, H3A 2K6 Montreal, Quebec, Canada
1. Ordering and disordering in BFCO double perovskite
BFCO has been theoretically designed to overcome the difficulties related with the coexistence of
ferromagnetism and ferroelectricity in perovskite oxides1. BFCO thin films have been shown to
simultaneously exhibit excellent FE properties with a spontaneous polarization up to 50 C cm-2
and strong magnetic moment at saturation of ~1 B per Fe-Cr pair2. The double perovskite BFCO
system (with A2BB’O6 structure) offers an example of d5-d3 magnetic super-exchange interaction
because both Fe and Cr are ordered and in the +3 ionic states. However, the realization of BFCO
double perovskites with highly ordered phase (Fig S1) is restricted experimentally, since Fe3+ and
Cr3+ ions are chemically and electronically similar (i.e., same valence state and close ionic radii).
The calculated density of states indicates that the Eg of BFCO is defined by the difference between
the Cr 3d–O 2p hybrids valence band and the empty Fe 3d conduction band1. Altering Eg requires
the modification of transition metal (TM)-O bond lengths and their interaction energies, namely
hybridization energy and coulomb repulsion. Considering the inverse dependence of Eg with
respect to the lattice parameter, i.e., the smaller the lattice parameter the larger the bandgap3, one
Bandgap tuning of multiferroic oxide solar cells
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can conclude that tuning Eg involves modifying the lattice parameters of BFCO. In the d5-d3
system, due to the homogeneous distribution of the spins in Fe and Cr degenerated d-orbitals, the
oxygen octahedra surrounding the TM cations are rigid and will be less sensitive to strain4,5. In
this undistorted octahedral coordination, only a single electronic transition occurs.6 Thus the latter
can mainly be accommodated by limited octahedral rotations or tilts and will only result in a small
and very limited change of Eg. A more significant modification of Eg could be achieved by the
presence of Fe4+(d4)Cr2+(d4) or Fe2+(d6) Cr4+(d2) Jahn Teller (JT) pairs distribution7.
Figure S1 a, Bulk phase diagram as a function of differences in FV and ionic radii (ri) (adopted
from Ref. 11). b, Schematic representation of the distribution of ordered domains with D size in
disordered region of BFCO. The corresponding Fe/CrO6 arrangements in ordered and disordered
double perovskite, O-DP and d-DP, respectively is also illustrated.
The JT effect in these TM cations, i.e. MTO6 octahedra deformation, results in strong electron-
lattice coupling in the system. Therefore, the physical properties of this correlated system are
strongly coupled to the shape and rotation of MTO6 octahedra. The Jahn-Teller (J-T) distortion of
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the octahedral symmetry of TMO6 leads change in the d-d transition from octahedral (Fe3+,Cr3+)
to tetragonal (Fe2+,Cr4+). On the other hand, due to the J-T effect, other possible d-d excitations
may occur. This JT distortion of TMO6 octahedra involves displacements of the TM cation from
the centre of the octahedron and the change is the TM-O bond distances. The combination of the
JT distortion and oxygen-octahedron rotations in this situation (i.e. Fe2+, Cr4+) offers the
opportunity for significant band-gap engineering.8 The modification of transition metal – oxygen
(TM-O) bond lengths and their interaction energies, that is, the hybridization energy and the
Coulomb repulsion will alter the bandgap.9,10 In addition, the large difference in the formal valence
(FV) and ionic radii (ri) between Fe2+ and Cr4+ will permit high spontaneous and tunable ordering
of TM elements 11 (Fig. S1). A boundary condition between disordered (with perovskite structure
AB0.5B’0.5O3) and ordered phases in such compounds is suggested by Anderson et al12.
2. X-ray photoemission spectroscopy measurements (XPS).
XPS has been used to investigate the chemical composition and the oxidation states of the Fe and
Cr elements present in the different BFCO layers. For comparison we also performed
measurements on epitaxial BFO and BCO thin films grown in the same conditions from 10% Bi-
rich targets (i.e. 580 C and 10 mTorr of oxygen partial pressure). The observed XPS Fe and Cr 2p
core-level spectra are illustrated in the Fig. S2. For Transition metal ions, the 2p core level splits
into 2p1/2 and 2p3/2 components. The binding energy of Fe 2p3/2 is expected to be 710.7 eV for Fe3+
and 709 eV for Fe2+. In the case of Cr, the expected 2p3/2 values are around 576.3 and 575.2 for
Cr3+ and Cr4+ respectively.
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Figures S2. XPS Fe (top) and Cr (bottom) 2p lines spectra of a. BFCO (R= 0.3%), b. BFCO (R =
0.9%), c. BFO and d. BCO thin films epitaxially grown on SrTiO3 (100) substrates using the same
PLD deposition parameters (i.e. Oxygen partial pressure and substrate temperature). e. Fe2+ and
Cr4+ fractions versus growth temperature of BFCO thin films.
To quantify the fraction of Fe and Cr in each chemical state using XPS, we used the method
described by Aronniemi et al.13,2 Using the traditional Shirley background subtraction, we
deconvoluted the 2p core levels for the different samples. The line shape used to represent the 2p
main peaks was a Gaussian–Lorentzian (GL) product with a constant exponential tail. We imposed
the fitting parameters such that the tail parameters and the GL ratio of the 2p1/2 main peak are equal
to those of 2p3/2 and the satellites are purely Gaussian and without any tail. Table S1 shows the
fraction of Fe and Cr in the different valence states obtained for the 100 thick epitaxial BFCO,
BFO and BCO thin films. The results suggest that the 3+ state is predominant in all films. Highly
ordered BFCO are obtained when the Cr4+ and Fe2+ fractions are significant in the films. This
direct relationship between the cationic ordering and the Cr and Fe components is further
evidenced in Fig. S2e. The fractions of Cr4+ and Fe2+ linearly increase with the substrate
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temperature during the BFCO growth. This indicates that the ordering between Fe and Cr in BFCO
is mainly achieved by the Fe2+ and Cr4+. In contrast to the 3+ valence state, the larger difference
in the valence state between Fe2+ and Cr4+ promotes cationic ordering in BFCO. This is confirmed
by XPS results obtained for highly ordered BFCO films (R = 5.1%) where Fe2+ and Cr4+ valence
states are predominant (cf. Fig. S3).
Table S1. Valence state ratio of Fe and Cr in BFO, BCO and BFO layers.
A substantial Fe2+ fraction (22%) is observed for BFO films. The formation of Fe2+ is attributed to
the presence of oxygen vacancies commonly occurring in the deposition processes of such
perovskite thin films14,15. The existence of oxygen vacancies was promoted by the reducing
conditions (i.e low oxygen partial pressure and low growth rate) used during the PLD growth. To
ensure charge neutrality, Fe with valence 2+ is formed. Higher Fe2+ fractions (Fe2+/Fe3+ ratio up
to 42%) have been also observed in such materials16,17.
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Figure S3. XPS Fe (top) and Cr (bottom) 2p lines spectra of a, highly ordered (o-BFCO) and b,
disordered BFCO (d-BFCO) phases.
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Figure S4. XPS spectra of the O1s signal of BFCO thin films with and without O2- vacancies,
respectively. The deconvolution of the O1s line results in peaks around 530 eV and 531.5 eV,
corresponding to oxygen in the BFCO lattice and presence of oxygen vacancies.18 The BFCO films
[labelled BFCO N] with high concentration of O2- vacancies were obtained when the films were
deposited under Nitrogen atmosphere. From the ratios of the peak intensities [BFCO (O)
spectrum], we estimate that the oxygen vacancies in our films are present at a level of less than
5%.
3. Growth parameters versus cationic ordering characteristics
From Figure S5a and b, we conclude that the effect of the laser repetition rate (f) or growth time
on ordering is more significant at high deposition temperatures (630-710 °C). High ordered BFCO
phase with large domain size D was obtained when the films were grown at 710 °C and at f =2 Hz.
Figure S5. a, Variation of ordered domain size D with the laser repetition rate in the BFCO thin
films directly deposited on NSTO(100) substrates. b, Substrate temperature dependence of R ratio.
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The trend observed in Figure S6b between unit cell volume of the ordered domain and bandgap in
BFCO films may be described in terms of the bandgap being directly proportional to interatomic
separation. The BFCO cell volume was estimated from reciprocal space mapping measurements
(RSM) (cf. Fig. S6) obtained for each grown sample. The RSM measurements were performed
around the (204) reflection of STO substrates. The SRO buffer layer was intentionally not used to
avoid any additional peak contribution which could complicate the interpretation of results.
In all cases, BFCO films exhibit a tetragonal distorted perovskite structure since the out-of-plane
(OP) pseudo-cubic lattice c is larger than the in-plane (IP) parameter a. The reciprocal lattice point
of each layer (the position of the centre of the peak) is located close to that of STO along the Qx
axis for each map, indicating that the in-plane lattice parameter of the heterostructure is very close
to that of the substrate. This reveals highly strained epitaxial layers throughout the whole
heterostructure, the strain originating from the lattice mismatch between the films and the
substrate.
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Figure S6. a, Reciprocal space mapping of BFCO films around (204) STO reflection. From highly
ordered film (left) to disordered film (right). b, Relationship between the observed bandgap and
the volume of ordered domain unit cell in BFCO films prepared at different laser repetition rate f.
The highly ordered (o-BFCO) and disordered (d-BFCO) BFCO phases have a more concentrated
spot (204) with a significant difference in lattice parameters in particular along the c axis. The OP
lattice parameters are 4.01 and 3.95 Å for o- and d-BFCO phases, respectively. Consequently, the
volume cell of o-BFCO is larger than that of the disordered phase. For BFCO films with different
R ratios, the (204) reflection is split in two distinct spots suggesting the coexistence of the o- and
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d-BFCO phases in the films. The main structural parameters, such as pseudo-cubic lattice
parameters of perovskite oxide layers, superstructure/main peaks R ratio (I½½½/I111) and the
ordered domain size (D) of BFCO are summarized in Table S2. The relevant difference between
the deposited BFCO layers is the ordered domain size (D).
Table S2. Main structural parameters obtained for the different BFCO single layer based-
heterostructures. For comparison, the results of highly ordered BFCO (o-BFCO) and disordered
BFCO (d-BFCO) are also shown.
The BFCO L1 layer has a D value of 24.6 nm which is reduced to 9.7 nm in the L4 film (within
experimental error of 3%). The results also highlight that the relationship between the lattice
parameters and R (i.e. degree of B-site ordering) observed in BFCO films can be explained by the
arrangements of distorted CrO6 and FeO6 octahedra. Since Cr4+ and Fe2+ are Jahn-Teller ions and
the CrO6 and FeO6 octahedra are distorted, the alignment of the elongated octahedra increases the
out-of-plane lattice parameter. In films with a random arrangement of Cr and Fe, the directions of
the distorted JT CrO6 and FeO6 octahedra should also increase their randomness. This random
arrangement decreases the elongation in the c direction. The unit cell volume of the disordered
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BFCO region remains almost constant (cf. Table S2) as is also the case for BFO. We believe that
the significant difference between ordered/disordered BFCO and BFO originates in the nature of
their respective bandgap. In ordered BFCO, the band gap is defined by d-d transitions1, sensitive
to structural modification whereas the bandgap in BFO is dominated by a transition between the
Fe and O orbital. However, even in BFO, in a very recent publication19 it was calculated that the
bandgap changes significantly (from 2.6 to 1.4eV) with high compressive stress (max ~28 GPa).
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4. Ferroelectric and piezoelectric atomic force microscopy measurements.
Figure S7. a. (E)2 (E) plots showing the reducing of the BFCO bandgap with R ratio. Typical
ferroelectric hysteresis loops recorded for b. BFCO films with cationic ordering in the range of
low R/small D and high R/large. For BFCO films exhibiting R ≤ 0.1% and small D is previously
reported elsewhere (ref 2).
The absorption properties of 100 nm-thick BFCO films grown directly on NSTO (0.5 wt%)
substrates with different cationic ordering parameters are illustrated in Fig. S7a. Highly ordered
BFCO films (R = 5.1%) were obtained when the film is deposited at f= 2Hz and at substrate
temperature of 700 °C. From the (E)2 (E) plot we estimated the bandgap for this film to be around
1.4 eV. The ferroelectric measurements were performed at frequencies ranging between 1-2 kHz
and at room temperature. The results show the unsaturated hysteresis loop of the highly Fe2+/Cr4+
ordered BFCO films (i.e. R = 5.1%) with low ferroelectric polarization value (Fig. S7b & c).
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Therefore, to extract the differences in ferroelectric domain evolution, we performed PFM
measurements on BFCO film surfaces. Figure S8 shows typical OP PFM images for BFCO films
with different R ratios. The as-grown films show no clear preferential orientation of their
ferroelectric polarization (images not shown here). We decide to only focus on the OP component
BFCO polarization due to the vertical PV device architecture used here. Ferroelectric domains are
visible after applying a DC voltage (± 8V). A comparison of topology with the out-of-plane PFM
images shows no direct relationship between grain microstructure and ferroelectric domains in all
films. Homogeneous contact distribution is obtained in o and d-BFCO films.
The PFM image indicates that the perpendicular component of polarization can be switched
between two stable states. Disordered BFCO exhibit a stronger PFM signal than that recorded for
o-BFCO (cf. Fig. S9a). Furthermore, the 3D illustration of the OP PFM images demonstrate that
a large domain switching is also observed in the d-BFCO case.
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Figure S8. Topography (top) and vertical (bottom) PFM measurements of a, highly ordered
BFCO, b, L1, c, L2, d, L3 and e, highly disordered BFCO films.
A more detailed analysis of the PFM images reveals the presence of two types of domains in the
switched areas, which can be distinguished by the amplitude of their signal response (cf. Fig. 9Sb).
The surface area with a high OP-PFM response (the red or blue regions in figures, representing
either negatively- or positively-poled areas), increases progressively from the L1 to L4 film. The
ratio (Rg) between the PFM amplitude (cf. Fig. 9Sc) of these two types of domains is strongly
dependent on the cationic ordering in BFCO films (cf. Fig. 9Sd). In agreement with the optical
absorption properties, the PFM results suggest the presence of two BFCO phases, a disordered one
with high PFM response and an ordered BFCO phase with low PFM response. We believe that the
JT distortion of TMO6 octahedra occurs to the detriment of that of the cube-octahedron BiO12
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around the Bi cations. As known in such systems1, ferroelectricity is due to the displacement of
Bi3+ ions in the crystal structure and the decrease of this displacement will result in the decrease
of the generated dipole and thus polarization magnitude. In highly ordered Fe2+/Cr4+ cationic
system, the JT distortion might compensate, partly that of the Bi-oxygen environment thereby
lowering the ferroelectricity of BFCO films.
Figure S9. a, PFM signal distribution in a, ordered and disordered BFCO films and b, L1, L2, L3,
L4 films. c, Corresponding PFM amplitude distributions estimated from PFM images of BFCO
films. d, Absorption peak position versus the ratios R and Rg.
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5. Ultraviolet photoelectron spectroscopy results and band energy distribution.
Ultraviolet photoelectron spectroscopy (UPS) allows the determination of the absolute value of
the work function (Fermi level, Ef) and ionization potential (equivalent to valence band edge, Ev)
of semiconductor materials. UPS was carried out using He I (21.22 eV) photon lines from a
discharge lamp. The BFO, BCO and BFCO films were epitaxially grown directly on (100)-oriented
Niobium- (0.5 wt %) doped SrTiO3 (NSTO) (from Crystec Gmbh). The direct bandgap of the
materials was estimated from the absorption measurements (cf. Fig. S10a). The thickness of the
obtained films are all around 100 nm, which eliminates the background signal from the NSTO
substrate.
The full UPS spectra for all films are illustrated in Figure S10b. The inset figure shows the
regions of interest. Ef is extracted by subtracting the cut-off value of the curve from the kinetic
energy of He I (21.22 eV) photon. The Ev is extracted from the cut-off value of the curve and it
represents the energy below the Fermi level of the material. The UPS spectrum collected from the
Au sample was used as reference for experimental data correction. The BFO and NSTO results
here agree well with the theoretical and experimental values reported previously20,21. For
disordered BFCO (d-BFCO) the electronic structure is close to that of BFO and BCO (cf. Fig.
S10b). However a significant difference is observed in the highly ordered BFCO (o-BFCO) case.
New states are visible in the 1-3 eV area of the valence band. These correspond to the two highest
states, X at ca.-1.7 eV and X1 at ca. -3.9 eV. The energy of the highest occupied state measured
by UPS corresponds to the ionization potential (Ev) of the o-BFCO structure and allows us to
distinguish the ordered vs. disordered phases. The ordered phase is characterized by a higher work
function (-4.1 eV) and a lower Ev (-5eV). Moreover, d-BFCO, BFO and BCO are characterized by
a higher energy cutoff, resulting in Ef and Ev values of -4.5 and -6.2 eV, respectively. Figure S10d
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shows the energy level diagrams displaying the conduction and valence energies of each type of
materials, and the corresponding Fermi levels (dashed lines) and band edges.
For structures where BFCO films were prepared at different growth times (i.e. S1-S3), the
schematic energy band diagrams are difficult to establish due to the existence of complex
ordered/disordered BFCO phases in the films. Qualitative analysis could be performed based on
the UPS results obtained from these films (cf. Fig S10c). Figure S10e illustrates the energy band
distribution of the each component material involved in the structures S1, S2 and S3. In those
cases, the BFCO part is represented by empty and filled rectangular shapes related to disordered
and ordered regions respectively, which coexist in the films.
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Figure S10. a. Optical absorption properties of BFO, BCO and BFCO thin films. b. Corresponding
UPS valence band structure of NSTO, o-BFCO/NSTO, d-BFCO/NSTO, BFO/NSTO and
BCO/NSTO heterostructures. c. UPS results obtained for S-series BFCO films. d. and e. energy
level diagram showing the conduction and valence band energies of each film, and the Fermi levels
(dashed lines). Energy-level diagram based on UPS results showing the valence and conduction
energies of each of the component materials involved in the BFCO device.
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Energy level alignment in the heterostructures
To establish the band energy distribution in the different heterostructures we assumed the work
functions () of ITO and NSTO to be 4.8 eV22 and 4.0 eV6, respectively. For the heterostructures
deposited on NSTO substrates, since ITO > BFO/BCOBFCO in all cases (i.e. 4.1 - 4.7 eV), when ITO
is in contact with BFCO under thermal equilibrium, electrons pass from the conduction band of
BFCO into ITO until the Fermi levels equalize (cf. Fig. S 10 c& d). This leaves behind a depletion
region in BFCO with an upward band bending. The region of this contact is highly resistive, called
a barrier layer, and this contact is a Schottky contact. In contrast, an ohmic contact with a
downward band bending will form at the interface between the BFCO and NSTO interface because
of BFO/BCOBFCO > NSTO. The region of this contact is of low resistance, called an anti-barrier layer
and does not affect the conduction behaviour under an applied voltage. The same procedure was
adopted for the heterostructures grown on SRO coated STO substrates.
6. PV measurements:
Detailed PV performance measurement
Light source spectral and total irradiance:
The Sun simulator and IV measurements:
Current-voltage (I-V) characteristics were measured under AM 1.5 100 mW.cm-2 simulated
sunlight (Photo Emission Tech, Inc; http://www.photoemission.com/SS50A.html) with a Keithley
2400 sourcemeter (SM). The used model SS50 AAA has a class as per ASTM E927, AAA with
non-uniformity of Irradiance of 2% or better over 2x2 inch area (Fig. S11). The system for device
characterization was calibrated with a Si reference diode (Fig. S11). The SM was controlled by a
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computer using an application written under TESTPOINT software platform. The procedure to
record IV curves was set as following: starting from 0 to maximum voltage value (Vmax), then
decreasing down to the pre-set minimum value (Vmin) and then increasing to the maximum (to
capture eventual hysteretic effect). The number of measurement points between Vmin and Vmax was
set to 50-100 and with time per step of 0.3 or 0.5 s resulting in cycling frequency of 0.033 to 0.010
Hz.
Figure S11. Picture of our Sun Simulator class AAA and of the Si reference cell used for this study.
The EQE measurement system:
For the external quantum efficiency measurements of our devices we used Oriel IQE200 certified
system (Fig. S12). The system is equipped with 250 W HTQ light source covering the spectral
ranges of 300–1800 nm with spectral resolution of 10 nm. These systems meet the ASTM E1021-
12 standard test method for spectral responsivity measurements of PV devices. The system is
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calibrated using NREL certified Si and Ge detectors. A probe station was used to connect the
device.
Figure S12. Picture of our QE measurement system.
Definition of ’active area’:
The ITO top electrodes were deposited through a shadow mask using PLD. The electrodes were
connected using needles probes (25 m in diameter) attached to xyz micro-positioners (Cascade
microtech). We compared PV measurements performed with and without masking. We used a
stripe mask applied under an optical microscope to correctly cover the area surrounding the
electrode. We found a negligible difference. This can be explained as follows: according to Fig.
4a, the calculated absorption depth is from 60 nm (≤ 650 nm) to 180 nm ( ≥850 nm) (Fig. S13).
This means that the light reaching the active area is originated from a very narrow region around
the top electrode. The current generated from this scattered light is negligible compared to that
generated from the direct light.
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Figure S13. Absorption depth calculated from the Fig 4.a in manuscript (reciprocal of the
absorption coefficient ).
In addition, as shown in Fig. S14, the short-circuit current (Jsc) calculated by integrating the EQE
vs wavelength (20.18 mA/cm2) is very close to that measured from IV characteristics under 1 Sun
illumination (20.51 mA/cm2; Fig 4 d- M1 in MS) within a 2% experimental error.
Figure S14. Jsc Calculated from EQE measurements (Fig 5.c in MS).
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Figure S15. Complete I-V cycle (0 ->Vmax ->Vmin ->Vmax) showing no hysteresis.
Repeatability of PV measurements:
As already mentioned we prepared three samples for each type of device discussed in the MS.
Table S3 summarizes the efficiencies obtained. Since we deposited 2D arrays of ITO electrodes
on top of the devices we analysed at least 3 to 4 areas on each device. We obtained a 25 %
dispersion in efficiency which we consider as the cumulative experimental error (including film
uniformity, top electrode size, uniformity of the incident light, current and voltage errors).
Table S3. Obtained Efficiencies (in %) for different devices fabricated for the study.
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Video clip description
We show a video clip recorded during one of the I-V measurements, illustrating the change of the
curve upon 1.5 A.M. light illumination of a single BFCO-based device. In our data acquisition
software, the x-axis represents the voltage in percent of the maximum applied value for that curve
(0.2 V) (cf. PV-BFCOmovie.mov).
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