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Electrical and Optical Properties of La AFe BO Perovskite Films (with A= Sr and Ca, and B= Co, Ga, Ti): Towards Interlayers for Optoelectronic Applications #

Pierre-Marie Geffroy, Sylvain Vedraine, Frédéric Dumas-Bouchiat, Sudip Kumar Saha, Alexandre Gheno, Fabrice Rossignol, Pascal Marchet, Remi Antony, Johann Bouclé, and Bernard Ratier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09083 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Electrical and Optical Properties of La1-xAxFe1yByO3-δ

Perovskite Films (with A= Sr and Ca, and

B= Co, Ga, Ti): Towards Interlayers for Optoelectronic Applications Pierre-Marie Geffroy,a Sylvain Vedraine,b,* Frédéric Dumas-Bouchiat,a Sudip. K. Saha,b Alexandre Gheno,b Fabrice Rossignol,a Pascal Marchet,a Rémi Antony,b Johann Boucléb and Bernard Ratierb a.

SPCTS - Science des Procédés Céramiques et de Traitements de Surface, 87068 Limoges Cedex, France.

b.

XLIM UMR 7252, Université de Limoges/CNRS, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France. * Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. This paper shows that perovskite oxides can be used as p-type interfacial layers of optoelectronic devices thanks to their optical, electrical properties, morphologies and workfunctions. La1-xAxFe1-yByO3-δ perovskite films (with A= Sr, Ca and B=Co,Ga,Ti) have been synthesized by pulsed layer deposition (PLD). The impacts of A-site and B-site cation substitutions, as well as of oxygen stoichiometry in the perovskite structure on their optical and electrical properties have been studied. The oxygen stoichiometry has a large impact on the electrical conductivity and the absorption spectra of the perovskite films, enabling us to finely tune the material composition or annealing to match most of the requirements associated to their use as selective contact in optoelectronic devices. Finally, we suggest La0.8Sr0.2Fe0.7Ga0.3O3-δ and CaTi0.8Fe0.2O3-δ perovskite films as relevant candidates as p-type interlayers for third generation solar cells or organic and hybrid light-emitting devices. Using an annealing of 500°C, CaTi0.8Fe0.2O3-δ was successfully integrated in a working perovskite light emitting diode (PeLED) based on the methylamonnium lead bromide (CH3NH3PbBr3) hybride perovskite, showing comparable performance to a reference devices based on a conventional PEDOT:PSS layer.

A. Introduction. Recent works show that a large class of perovskites presents potential interesting properties for optoelectronic and photocatalytic applications, with visible band gaps, 1,2 nanosecond recombination lifetimes, and semi-conducting behavior.3 Among this large class of perovskites, the semi-conducting perovskite oxides are of great interest for optoelectronic applications, because these materials are often p-type semi-conductors with visible band gaps.4 Their electrical


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conductivity and band gaps are easily scalable in relation with the oxygen stoichiometry or with the nature of the cation substitution in the perovskite structure.5 Nevertheless, the perovskites often show indirect gaps leading to a low absorbance, except for some oxynitrides and organometallic halides.6 Therefore, only a few of these materials can absorb the maximum amount of solar photons allowed by their band gap. More complex perovskites such as double perovskites can be elaborated in order to adapt their properties for potential solar applications. But, careful investigation of their properties needs to be done. Point defects in the lattice and defect-induced energy states will strongly act on the photo-excited electron-hole pairs if this material is used as an active layer. These materials need to be investigated in order to gauge of their interest for optoelectronic application through: i/ their properties, ii/ their stabilities and iii/ the use of a low-cost and low-temperature deposition. In this article, we propose to investigate the first point: are these materials can be consider for such application. Other points will be investigated in further work. We can note that perovskite oxides such as titanate offer a very good photostability and corrosion resistance in aqueous solution. Many of these materials have been already investigated as photocatalysts for water splitting or CO2 photoreduction.7 The Achille’s heel of the hybrid organometal halide perovskite family can therefore be reduced when combined with a perovskite oxide as protecting layer. Due to the high temperature annealing, the integration of this layer inside a device without degrading the active layer can be done through two solutions: 1/ by depositing the oxide on the FTO (as we did inside the article) or 2/ to use a lamination process.8 Moreover, perovskite oxides can present a variety of properties such as a high transparency, which makes them good candidates as p-type interfacial layer. Currently, the mostly used hole transporting layers in the field of third generation perovskite solar cells are the molecular glass


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Spiro-OMeTAD for standard cell architectures, and the conjugated polymer PEDOT:PSS, mainly used for inverted solar cell architectures or in the field of light emitting diodes. Unfortunately, both of these materials suffer from a poor stability under ambient conditions, due to the necessity to use unstable additives9 or due to an intrinsic acidity which can deteriorate the device active layer. Moreover, low-cost deposition using sol-gel process is already known to work with oxide of perovskite of La1−xSrxFe1−yGayO3−δ.10 Nevertheless, we leave for the moment the effect of temperature for future studies because we believe it is first important to show the functionality of these materials. In this work, we demonstrate for the first time, new promising perovskite materials with interesting optical and electrical properties for optoelectronic applications. We also give a better understanding of the electrical and optical properties of semi-conducting perovskites, considering that such information remain scarce in the literature, and in particular concerning their optical properties. This work focuses on the impacts of A-site and B-site cation substitution and oxygen stoichiometry on the optical and electrical properties of La1-xAxFe1-yByO3-δ perovskite oxide thin films (with A= Sr and Ca, and B= Co, Ga, Ti) prepared by pulsed laser deposition (PLD). A particular attention is paid to the impact of the oxygen stoichiometry, which is likely to largely influence the electrical conductivity and the absorption spectra. Finally, we also discuss the potentialities of these new perovskite oxide films in terms of optoelectronic applications. In this aim, we integrated the most promising candidate (CaTi0.8Fe0.2O3-δ) in a functional perovskite light emitting diode (PeLED), and compare device performance to that of a reference device based on the conventional PEDOT:PSS p-type interlayer. B. Materials and methods


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B.1. Synthesis of La1-xAxFe1-yByO3-δ perovskite targets (with A= Sr and Ca, B= Co, Ga, and Ti). The La1-xAxFe1-yByO3-δ perovskite powders (with A= Ca, and Sr, B= Co, Ga, and Ti) were synthesized using a solid-solid route. High-purity oxide and carbonate precursors, including La2O3 (99.99%, Sigma-Aldrich), CaCO3 (99.99%, Sigma-Aldrich), SrCO3 (99.99%, SigmaAldrich), Fe2O3 (99.5%, Alfa Aesar), Co3O4 (99.7%, Alfa Aesar), Ga2O3 (99.9%, Alfa Aesar), TiO2 (99.9%, Alfa Aesar), were mixed by attrition milling using 800 µm zirconia balls in an ethanol media (600 rpm for 3 hours). All of the obtained powders were calcined at 1000°C for 8 h, and the phase purity was confirmed with X-ray diffraction (Siemens D5000, Cu-Kα). The calcined powders were finally ground by attrition milling (1000 rpm) to obtain a monomodal grain size distribution centered around D50= 1-2 µm. Then, the powders were pressed at 60°C under 100 MPa and sintered to obtain green pellets of 24 mm in diameter and 1 mm in thickness. The sintering conditions of pellets are listed in Table 1. The density of the sintered pellets was measured using the Archimedes’ method. We would like to draw your attention on such temperature which is applied only on the pellet and not on the final sample. Other examples of such high temperature initial material synthesis exist for interfacial layers. For example, TiO2 material can be synthesized at high temperature such as the TiO2 P25 from Degussa which is known to give high efficient solar cells.11,12 Materials

Acronym

Sintering

Density of starting powders

Relative density of

conditions

(pycnometer, g.cm-3)

sintered pellets

La0.8Sr0.2Fe0.7Ga0.3O3-δ

LSFG8273

1350°C, 4 h, air

5.7

> 95%

La0.6Sr0.4Fe0.6Ga0.4O3- δ

LSFG6464

1300°C, 2 h, air

6.0

> 95%

La0.5Sr0.5Fe0.7Co0.3O3- δ

LSFCo5573

1350°C, 4 h, air

6.1

95%

La0.5Ca0.5Fe0.7Co0.3O3- δ

LCFCo5573

1250°C, 4 h, air

5.7

95%

CaTi0.8Fe0.2O3- δ

CTF182

1200°C, 2 h, air

5.8

> 98%

Table 1. Sintering conditions of the dense La1-xAxFe1-yByO3-δ perovskite pellets.


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B.2 Elaboration of La1-xAxFe1-yByO3-δ perovskite films (with A= Sr and Ca, and B= Co, Ga, Ti) by pulsed layer deposition. Perovskite films were prepared by Pulsed Laser Deposition (KrF – 248 nm, 10 Hz) in a Ultra-High vacuum chamber (10-8 mbar). At this UV-wavelength regime, the deposition process is well known to be very congruent. Targets of the desired composition were then sprayed at a fluency of around 4 J.cm-2 under dynamic oxygen pressure (pO2 = 0.3 mbar). During the deposition step, substrates were heating at 700°C and a 15’-500°C post-treatment step was systematically applied during the cooling process. Deposition rates were measured at ≈ 0.20 nm.s-1, value classically observed for oxide materials. Crystallographic Phases were determined by X-ray diffraction (XRD) (INEL CPS 120). The substrate was oriented at low angle incident (< 5°) to avoid the Bragg diffraction peaks of single crystal of sapphire, R-Plane (1 1 0 2), used as a substrate (Fig. 1). It is to be noticed that the Xray diffraction diagrams confirmed the presence of a perovskite phase.13,14 However, the CaTi0.8Fe0.2O3-δ coating obtained by PLD showed a lower crystallinity degree. Complementary XPS chemical characterization has shown the elemental composition perovskite thin film corresponds to the material of the target during the PLD process.


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Figure 1. X-ray diffraction patterns of La1-xAxFe1-yByO3-δ perovskite films (with A= Sr and Ca, and B= Co, Ga, Ti) synthesized by pulsed laser deposition.

Figure 2 shows a quite dense homogenous columnar microstructure of a thin LCFCo5573 film deposited on a sapphire substrate. The PLD process is known as a strong anisotropic deposition process often leading to columnar growth especially for oxide materials. As expected, the columnar microstructure of the LCFCo5573 film leads to an out-of-plane homogenous grain growth along with an axis perpendicular to the substrate surface. Because both microstructure and crystal intrinsic properties should drive the macroscopic layer properties, a control of microstructure appears essential for the development of materials of interest. Each column (width: 50 nm × high: 200 nm) seems to be composed of smaller spherical grains of about 50 nm in diameter. Several SEM images realized on different cross sections of the sample show the same homogeneous 200 nm-thick layer.


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Figure 2: SEM images of a cross-section of a thin LCFCo5573 film synthesized by pulsed laser deposition and b) dense sintered pellets of LCFCo5573 perovskite obtained by conventional route.

B.3. Electrical conductivity measurements. The electrical conductivity characterizations of dense perovskite pellets used as targets for PLD were performed by four-point probe method using rectangular dense bar samples (i.e., the density is higher than 95%). After sintering, the pellets were machined to obtain the bars with approximate dimensions of 25×1×2 mm3. The surface of the bar samples was polished using the P400 and P4000 silicon carbide polishing pads (Buehler). Platinum electrodes were then coated on both ends of the bar sample. Two additional platinum electrodes were placed 4-5 mm from each end of the bar. The platinum electrodes were prepared via platinum ink coating (Pt paste, Ferro, CDS), and the samples were heated up to 1000°C in air to obtain cohesive and porous platinum electrodes. The electrical conductivity characterizations of dense thin films obtained by PLD were performed by four-point probe method using tungsten micro-tips with an interspacing of 1 mm in between each of them. A keithley 224 associated to a current source keithley 616 Digital Electrometer was used for the electrical measurement.


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B.4. Optical characterizations. The optical characterizations were performed with thin perovskite films of 150-200 nm in thickness (except for LSFG8273 film with 90 nm in thickness) on C type sapphire substrate (Neyco/BT Electronics). An AGILENT Cary 300 reflectometer was used for the reflexion and transmission measurements. Absorbance α was deduced from transmission T following the equation = −(⁄) ( ), where d is the thickness of the layer. Absorbance and optical gap could be linked following the Tauc equation16: = ( − ) , where is the incident photon energy, B is a constant corresponding to the band tailing parameter and r corresponds to a specific index: 2 for indirect allowed transitions, 3 for indirect forbidden transitions, 1/2 for direct allowed transitions or 1/3 for direct forbidden transitions. The value of was calculated by extrapolating the linear region of the obtained curves. B.5. Surface potential measurements. HD Kelvin Probe Force Microscopy (HD-KPFM), was used as a tool to measure the local contact potential difference between a conducting atomic force microscopy tip and the sample . Studies were performed with a Nano-Observer AFM from CSI©. RMS (root mean squared) roughness measured on our samples confirms the presence of relatively flat surfaces: 8.5 ± 3.5 nm for LSFC5573, 3.5 ± 0.7 nm for LSFG8273, 0.8 ± 0.2 nm for LCFC5573, 10.9 ± 1.3 nm for LSFG6464 and 3.1 ± 0.7 nm for CTF182. The values of work function were measured first with gold and ITO in order to calibrate the tip. Good agreements were obtained (5.10 eV ± 0.05 eV for gold, 4.89 eV ± 0.05 eV for ITO) comparable with what is reported in the literature. For the characterizations of the perovskite oxides, gold was used as reference for the tip work function ɸ !" extraction. Then, perovskites work functions were deduced from: ɸ"#$! =


9


ɸ !" − % . These work functions were compared to the literature work function of ITO, FTO, PEDOT:PSS, Ag and conduction/valence bands of CH3NH3PbI1-xClx, CH3NH3PbBr3, PCBM, BCP, Al (Figure 3).15, 17,18 These materials are very common for inverted perovskite solar cells or LED.19 We can note that perovskite oxide can be considered to be used as hole transporting layer between FTO and CH3NH3PbI1-xClx or CH3NH3PbBr3. Nevertheless, we can note that it is important to approximate the position of the valence band. Optical measurement

4.4

4.89

4.60 4.82

5.3

-6 -7

electrode

p-type interfacial layers

Ca

BCP 2.9

2.9

Al

3.38 3.75

PCBM

CH3NH3PbBr3

4.87

CH3NH3PbI3-xClx

4.73

PEDOT:PSS

-5

FTO

-4

ITO

-3

CaTi0.8Fe0.2O3- δ

-2

La0.5Ca0.5Fe0.7Co0.3O3- δ

-1

La0.8Sr0.2Fe0.7Ga0.3O3-δ

0

La0.5Sr0.5Fe0.7Co0.3O3- δ

will allow us to investigate this point.

Workfunction (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.9 4.2

5.3

5.68

Active layer

5.9 6.4

n-type

electrode

Figure 3. Workfunctions measured on La1-xAxFe1-yByO3-δ perovskites and compared to PEDOT:PSS, CH3NH3PbI3-xClx, CH3NH3PbBr3, PCBM BCP, Ca and Al B.5. Light emitting diode realization and characterization. A layer of 40 nm of CTF182 has been deposited using PLD on conductive and transparent glass/FTO substrates. During the PLD deposition, substrates were heated at 400°C. DRX shows a low crystalized layer and a postannealing leads to improve the crystallization of perovskite phases (see supporting information). Then, a post-annealing at 500°C during 15 min was systematically applied under air. Such annealing is a typical temperature already used by the literature when a layer of TiO2 is


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deposited. We will show later that such annealing is enough to make a functional device. The glass/FTO/CTF182 electrode was then used as a hole-selective injection contact for hybrid perovskite LED. To do so, an active layer composed of CH3NH3PbBr3 perovskite (50 nm) was deposited by spin-coating from a N,N-dimethylformamide (DMF) solution. Then, an interfacial layer of bathocuproine (BCP, 20nm) was thermally evaporated followed by a top electrode deposition using thermal evaporation method at 10-6 mbar. Calcium (20 nm) and aluminium (100 nm) was deposited as top electrode in order to facilitate electron injection, as usually reported in the literature. Current-voltage characteristics were measured by a Keithley 2440 5A sourcemeasure unit. Luminance data were measured by a calibrated Centronic photodiode (radius of 0.56 cm) connected to a Keithley 2700 multimeter. All the instruments were connected with a computer interfaced through Labview software. The electroluminescence spectrum was measured through a calibrated B&WTek BRC112E CCD array spectrometer. Measurements were performed under inert atmosphere. C. Results and discussion C.1 Electrical characterization. C.1.1. Impact of the nature of cation substitution. The perovskite films show a large range of electrical conductivity in relation with the nature of cation substitution in perovskite structure. As expected, the electrical conductivity depends on the Fe or Co ratio, because the electrical conductivity is mainly governed by the concentration of electronic holes and the mobility of electronic hole through Fe•-O-Fe× and Co•-O-Co× bonds. CoCo• and FeCo• correspond also to an electron hole located on a B site of the perovskite structure.20 However, the concentration of electronic hole and also electrical conductivity depend on


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Sr and Ca ratio on A site o perovskite structure13. The nature of cation on A site has also the impact on the mobility of electronic holes through the perovskite structure

21

, then it

notes large variation of electrical conductivity between LCFCo5573 and LSFCo5573, as reported in table 2. The positive Seebeck coefficients of perovskite films reported in Table 2 confirm the charge of carriers in materials or p type semi-conductivity. The Fe substitution by Co, on site B of the perovskite structure, increases the electronic conductivity due to an increase of electronic carrier concentration in the perovskite structure, according to reactions 1 and 2. × • 2CoCo + VO•• + 12 O2 ( g ) ⇔ 2CoCo + OO×

reaction 1

× • 2FeCo + VO•• + 12 O2 ( g ) ⇔ 2FeCo + OO×

reaction 2

Electrical

LSFCo5573

LSFG8273

LCFCo5573

LSFG6464

CTF182

2.22

0.043

0.03

0.009

Too low

37

187

48

126

conductivity (S.cm-1) Seebeck coefficient (µV.C°-1)

Table 2. Electrical conductivity at room temperature of perovskite films obtained by pulsed laser deposition. The Ga ratio decreases the electrical conductivity, because Ga presents only one oxidation state, i.e. +3. Then, LSFG8273 films exhibit a higher electrical conductivity than that of LSFG6464 films. The La substitution by Sr or Ca in A-site of the perovskite structure leads to the creation of oxygen vacancies in the perovskite structure and probably the increase of the oxygen diffusion coefficient, according to reaction 3.


12

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' 2 BO LaFeO 3 → 2 B La + VO•• + 2OO×

reaction 3

C.1.2. Impact of the oxygen under stoichiometry. Table 3 shows a large discrepancy between the electrical conductivity of thin films obtained by PLD and those of dense pellets. We assume here that the columnar microstructure of the perovskite films, as reported in Figure 2, has a large impact on the electrical conductivity. In case of dense pellets, we have a large and isotropic grain size (close to 1 micron), and the electrical conductivity decreases usually when the density of grain boundaries increases (i.e. grain size decreases). Other assumption concerns grain and the grain boundaries chemical composition. Intrinsic properties of perovskite thin film obtained by PLD can be likely different to that in dense sintered pellets obtained by conventional route. Then, the electrical conductivity of perovskite films is measured in planar direction, where the density of grain boundaries is the highest. For instance, the column of LCFCo5573 film has a characteristic size lower than 50nm, see Figure 2.

LSFCo5573

LSFG8273

LCFCo5573

LSFG6464

CTF182

18 300

8.8

22.2

6.8

0.00009

0.08

0.28

0.18

0.28

0.42

2.22

0.043

0.03

0.009

Too low

9.18

0.03

0.45

0.013

Too low

Electricial Conductivity (S.cm-1) Dense pellets Activation energy (eV) Electrical conductivity (S.cm-1) Films (before annealing) Electrical conductivity (S.cm-1) Films (after annealing at 600°C under air)

Table 3. Electrical conductivities at room temperature of perovskite thin films obtained by pulsed laser deposition in relation with their annealing atmosphere and elaboration process.


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The elaboration process has a very significant impact on the electrical conductivity of the perovskite (Table 3). First, the partial pressure of annealing atmosphere governs the coefficient of oxygen under stoichiometry in the perovskite films, as reported in recent works.20 Figure 4 shows the evolution coefficient of oxygen under stoichiometry in relation with the oxygen partial pressure in surrounding gas. The coefficient of oxygen under stoichiometry decreases with the oxygen partial pressure in the surrounding gas, according to reaction 4.

O×O ← → 12 O2 + VO••

reaction 4

Figure 4. Oxygen under stoichiometry in La1-xSrxFe1-yGayO3-δ perovskite materials at 900°C under a range of pO2 from 21 atm (air) to 10-5 atm (nitrogen), and 10-22 atm. (argon-5% hydrogen) evaluated by TGA and iodometric titration method 20,22.

C.2.

Optical characterization. Figure 5a shows that the LSFG8273 and CTF182 are

highly transparent in the visible region, while the LSFCo5573 and LCFCo5573 present a high absorption over the same wavelength range. CTF182 presents specific oscillations along the spectrum due to internal optical interferences. Distance between each oscillation ' = 2.7 using a thickness of is related to the optical index that we can approximate here to & 200 nm.23 The rather high optical index indicates that high reflection can appear, due to


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the optical index difference between each layer. Measured reflection indicates that between 14% and 35% of the light will be lost in reflection, depending on the interference type (constructive or destructive) in the layer. Indeed, at one interface, intensity of the reflexion coefficient is governed by the equation ( = [(& − & )⁄(& + & )]² where R≈0 if & ≈& . In our case, we can deduct from the approximated (non-dispersive) optical index that the CTF182/air interface lead to an average of 20% of reflection. Nevertheless, a CTF182/active layer interface should present a lower reflection due to a diminution of the index difference between the perovskite oxide (& ) and its juxtaposed layer (& ). ' of 2.7 is closer to the real optical index of the active layer, such as CH3NH3PbI3 Indeed, & than optical index of air (1) on the visible range.

(αhυ)2.1011 (eV2 cm-2)

100

(a)

90 80

0,6

(b)

1,6

LSFG8273 (r=2)

1,2

LSFG8273 (r=0.5)

0,5 0,4 0,3

0,8

70

0,2

0,4

60

LSFG8273 LSFCo5573 CTF182 LCFCo5573

40 30

10 0 400

500

600

700

Wavelength (nm)

800

2,5

hυ (eV) 3,5

4,5

1,2

20

300

0 1,5

900

(αhυ)2.1012 (eV2 cm-2)

50

0,1

0

(αhυ)0.5.103 (eV2 cm-2)

2

1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

(c)

1

CTF182 (r=2)

0,8

CTF182 (r=0.5)

0,6 0,4 0,2 0 1,5

2,5

hυ (eV) 3,5

(αhυ)0.5.103 (eV2 cm-2)

24 ,

Transmission (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4,5

Figure 5. Optical properties of La0.8Sr0.2Fe0.7Ga0.3O3-δ (LSFG8273), La0.6Sr0.4Fe0.7Co0.3O3-δ (LSFG6464),

La0.5Sr0.5Fe0.7Co0.3O3-δ

(LSFCo5573),

CaTi0.8Fe0.2O3-δ

(CTF182),

La0.5Ca0.5Fe0.7Co0.3O3- δ (LCFCo5573) perovskite films fabricated by pulsed laser deposition: a) transmission spectra, b) and c) Tauc Fit of LSFG8273 and CTF182 respectively.


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A transmission higher than 70 % for 200 nm-thick CTF182 and LSFG8273 layers is obtained for wavelengths higher than 500 nm. Nevertheless, the thickness can be reduced in a realistic optoelectronic device. The interfacial layer thickness needs to be sufficiently thick in order to bend the energy bands, but sufficiently thin in order to be transparent. Transmission of the same perovskite material but for thinner films (40 nm) deposited on FTO was also measured and led to a value of almost 85% for a wavelength of 550 nm (see supporting information). Absorption spectrum proves that a very small absorption is obtained in the visible part of the spectrum for a 200 nm-thick layer. Optical band gap approximated using Tauc plots (Fig. 5b, c) using r of about 2 (for indirect allowed transitions). Direct band gap appears at about 4 eV for the CTF182 films and 3.6 eV for the LSFG8273 films. Using r of ½, we obtained an indirect gap of 3.3 eV and 2.8 eV for CTF182 and LSFG8273 respectively. Figure 5.c shows the interferences due to the thin layer of CTF182 below the indirect gap. In the case of the Figure 5.b., a tail appears down to 1.8 eV and a mobility gap characterization is therefore necessary. Indirect gap is not limiting if these materials are used as an interfacial layer, where intra-band transitions are not require, except is to the gap is too low to make the material an electron blocking layer, and an hole conductive layer in same time. These layer can be able to protect the active layer from UV-light, while allowing visible light passing through it. It must be noticed that the tail appears close to the bandgap. Origin of this tail is still under investigation but it can be considered in a first approach to be linked to sub-bandgap states, possibly associated to shallow traps.25 This tail presents an exponential shape and depends on the purity and the thermal history of the sample.


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LSFCo5573 and LCFCo5573 show intense IR-visible absorptions. Band gaps of these materials are rather low (lower than 0.5 eV, estimated by ellipsometry) indicating a metallic behaviour. The presence of cobalt leads to a smaller optical gap unlike titanium, gallium or iron. The workfunctions and the optical gaps of perovskite materials are listed in Table 4. Materials

Acronym

Work function

Gap (eV)

La0.8Sr0.2Fe0.7Ga0.3O3-

LSFG8273

4.82

2.8 (indirect)

(eV)

La0.5Sr0.5Fe0.7Co0.3O3-

LSFCo5573

4.60

>0.5

La0.5Ca0.5Fe0.7Co0.3O3-

LCFCo5573

4.73

>0.5

CaTi0.8Fe0.2O3-

CTF182

4.87

3.3 (indirect)

Table 4. Workfunctions measured on La1-xAxFe1-yByO3-δ perovskite films.

Beyond the electrical properties, an annealing of 600°C under air strongly impacts the optical properties of perovskite films (Figure 6). LCFG5573, LSFG6464 and LSFCo5573 absorption decrease together with an increase of their conductivities. On the contrary, LSFG8273 shows an increase of its absorption in the visible region together with a decrease of its conductivity. After an annealing under a mixture of argon with 5% of H2 at 650°C for 1 hour, the electrical conductivity of perovskite films increases while the absorption coefficient of perovskite films decreases. This dependence of resistivity and absorption with the oxygen partial pressure of the annealing atmosphere could be attributed to a release of oxygen or the creation of oxygen vacancy concentration in perovskite phase.26 For instance, this impact of annealing atmosphere at high temperature on oxygen vacancy concentration has been confirmed by XRD and TGA methods on the pellets of LSFG8273 perovskite 27,28.


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Let us mention that properties of CTF182 do not largely change after the argon/H2 annealing, likely due to a low content of iron and a low variation of oxygen under stoichiometry with oxygen partial pressure of annealing treatment. Tauc plot presents a slight variation of the optical direct gap, which decreases after an annealing under argon/H2. Besides, shape of the tails changes after an annealing under argon/H2 (5%). A shoulder of the tauc plot appears at about 3 eV for CTF182 and 3.2 eV for LSFG8273. This variation of optical properties with an annealing atmosphere is linked to the increase of oxygen vacancy concentration in perovskite phase when the pO2 of annealing atmosphere increases.

400

500

600

700

800

(αhυ)2 .1010 (eV2 cm-2)

Wavelength (nm) 9 8 7 6 5 4 3 2 1 0

300

400

500

CTF182 (Air)

2

2,5

3

hυ (eV)

600

700

3,5

4

Before annealing Annealing under Air Annealing under Argon + 5%H2

100 90 80 70 60 50 40 30 20 10 0

800

Wavelength (nm)

(d) CTF182 (Argon + 5%H2)

1,5

(c) LSFG6464

Before annealing Annealing under Air Annealing under Argon + 5%H2

(αhυ)2 .1010 (eV2 cm-2)

300

(b) LSFG8273

100 90 80 70 60 50 40 30 20 10 0

Before Annealing Annealing under Air Annealing under Argon + 5%H2

Absorption (%)

(a) CTF182

100 90 80 70 60 50 40 30 20 10 0

Absorption (%)

Absorption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300

400

500

600

700

800

Wavelength (nm)

(e)

7 LSFG8273 (Argon + 5%H2)

6

LSFG8273 (Air)

5 4 3 2 1 0 1,5

2

2,5

3

3,5

4

hυ (eV)

Electrical and Optical Properties of La1âxAxFe1 ... - ACS Publications

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