bandgap tuning of multiferroic oxide solar cells ...€¦ · a novel route to achieve high...

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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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bandgap tuning of multiferroic oxide solar cells ...€¦ · a novel route to achieve high...

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