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Mar 2, 2015 - Films are deposited from a metal–organic precursor complex solution followed by annealing in air at 673 K for 2 h. .... M Lorenz , M S...
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Solution Processed Bismuth Ferrite Thin Films for All-Oxide Solar Photovoltaics Devendra Tiwari,*,† David J. Fermin,‡ T. K. Chaudhuri,† and Arabinda Ray§ †

Dr. K. C. Patel Research and Development Centre and §P. D. Patel Institute of Applied Sciences, Charotar University of Science and Technology, Changa, Gujarat India 388421 ‡ School of Chemistry, University of Bristol, Cantock’s Close, Bristol, United Kingdom BS8 1TS ABSTRACT: The present work delivers the first assessment of BiFeO3 (BFO) thin films as an absorber for sustainable all-oxide photovoltaic devices. Films are deposited from a metal−organic precursor complex solution followed by annealing in air at 673 K for 2 h. Xray diffraction, complemented by quantitative analysis, indicated formation of pure BFO with rhombohedral structure (R3C). Atomic force microscopy suggests deposition of compact and smooth films with spherical particles of sizes ∼150 nm. A direct band gap of 2.2 eV is ascertained from UV−vis−NIR spectroscopy. Mechanistic aspects of the BFO formation are discussed based on thermograveminetric analysis, differential scanning calorimetry, and infrared spectroscopy of the precursor complex. A proof-of-concept BFO/ ZnO heterojunction based solar cell fabricated by solution processing delivered a photoconversion efficiency of 3.98% with open-circuit voltage (Voc), short-circuit current density, and fill factor of 642 mV, 12.47 mA/cm2, and 50.4%, respectively. The device exhibits a maximum external quantum efficiency of nearly 70%. These parameters are among the highest values reported for all oxide PV. Analysis of the Voc, series resistance, and conversion efficiency as a function of temperature revealed valuable information about recombination processes.

1. INTRODUCTION Thin film solar cell (TFSC) technologies, CdTe and CIGS, currently occupy above 10% share of the total PV installed capacity. Considering the exponential growth of the PV market, TFSC market experiences a formidable challenge in terms of materials availability and processing cost.1,2 This can be mitigated by choosing new sustainable materials and nonvacuum liquid processing techniques. Conventionally, TFSC are being manufactured by vacuum deposition methods. In contrast to vacuum processing methods, solution-based approaches require very low capital and offer high material utilization and throughput.3 In terms of material replacement, low band gap metal oxides have been considered in terms of stability and process ability. Laboratory test cells employing CuO, Cu2O, Fe2O3, and Co3O4 in a heterojunction or Schottky configuration have been examined, with Cu2O/ZnO junction showing the maximum conversion efficiency of 3.8%.4 Recently, BiFeO3, bismuth ferrite (BFO), has been shown to possess potential for photovoltaic devices. Ramesh and coworkers have demonstrated a novel mechanism of charge separation of light-generated electron−hole pair at magnetic domain walls of BFO leading to above band gap open circuit voltages.5 Other rectifying architectures such as ITO/BFO/ metal (Au or Pt) or graphene/BFO/Pt and n-ZnO/p-BFO/ N719 dye (sensitizer)/CuSCN (hole transport layer) have also been investigated.6−9 However, the conversion efficiencies of these devices still remain below 1%. Also, literature indicates that a simple BFO-based PV device utilizing solely a heterojunction is still missing, and essentially this forms the scope of present work. © XXXX American Chemical Society

BFO is a perovskite (rhombohedral symmetry) featuring a direct band gap between 2.2 and 2.7 eV, exhibiting piezoelectric and room-temperature multiferroic properties.9,10 Studies have linked BFO to n-type conductivity, but other reports have provided evidence of p-type conductivity.11,12 DFT calculations have shown that the generation of p-type BFO is more likely than n-type.13 Such multifunctional behavior have opened opportunities to exploit BFO for potential application in fields of optoelectronics, spintronics, sensors, and photocatalysis.10 It has been understood that the cause for its great variety of materialistic properties lies in the delicate nature of its structure. The smaller cation Fe3+ is located at center of the octahedron formed by six O2− anions at the corners. The larger Bi3+ is at interstices surrounded by 8 Fe3+ cations and 12 anions.10,14,15 Thus, each of the octahedrons shares a terminus with other. A slight strain in structure causing tilting of these octahedrons by external stimulus or internal chemical pressure (introduced by doping or strain) will result in noticeable change in its properties.16 Different kinds of methods has been investigated for depositing thin films of BFO, including both physical and chemical techniques. Physical deposition route include pulsed laser deposition, molecular beam epitaxy, and RF sputtering on single crystalline ceramic substrates such as Sr(Ru or Ti)O3 with or without an inert metallic layer (Ti or Pt) underneath to manipulate the crystal orientation the final BFO films.17−21 On Received: December 23, 2014 Revised: February 25, 2015

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This cycle is repeated 20 times, and the final stack of precursor layers is then converted to BFO by heating at 673 K for 2 h in air to get a thickness of ∼1.5 μm. The top contacts are made by painting circular pads (area 0.35 cm2) with colloidal graphite (Ted Pella). The J−V characteristics of the cells are measured in dark and under illumination using a solar simulator (Newport-Stratfort 150W, 96000) with simulated AM 1.5 spectrum and power density of 100 mW/cm2. Temperaturedependent cell characterization is done using a cryostat in the temperature range of 140−320 K.

the other hand, chemical methods comprise sol−gel and chemical vapor deposition on ITO, Si, or single crystalline ceramic layer (SrTiO3 or LaNiO3) using metal nitrates or organometallic compound as a precursor. Often these processes employed high temperature (>773 K) for synthesis and annealing.22−25 The present work examines for the first time the properties of highly phase pure BFO, prepared by a solution-based method, as an absorber layer in a thin-film solar cell configuration. Bismuth ferrite films are deposited on glass using a metal−organic precursor complex solution, yielding phase pure, adherent films with a direct band gap of 2.2 eV. An absorber with this band gap is capable of utilizing not more than 40% of terrestrial solar photons. A solar cell made using BFO/ZnO heterojunction yielded nearly 4% conversion efficiency under AM1.5 irradiation, one of the highest reported for an all-oxide all-solution based devices. Fundamental aspects associated with recombination losses at the device level are established from analysis of the Voc, series resistance, and conversion efficiency as a function of temperature.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Bismuth Ferrite: Analysis of the Precursor Complex. Figure 1 contrasts the infrared spectra of

2. EXPERIMENTAL SECTION 2.1. Deposition of BFO Films. BFO films are deposited onto a soda lime glass (SLG) substrates from a metal−organic complex precursor solution. The precursor solution consisted of metal salts, i.e., ferric trichloride (0.1 M), bismuth trichloride (0.1 M), and thiourea (0.3 M), as an organic ligand, dissolved in methanol. Soda lime glass substrates (2.54 × 2.54 cm2) are sequentially cleaned with chromic acid, deionized water, and neutral pH detergent and again rinsed with deionized water, followed by rinsing with methanol and drying by jet of dry air. The precursor complex is spin-coated onto the cleaned substrates at 1200 rpm. The precursor film is preheated at 473 K for 10 min and subsequently annealed at 673 K for 2 h in air, promoting the thermolysis of the precursor films to BFO. The temperature for heating is decided on the basis of thermogravimetric analysis of powder obtained from scraping the precursor films. The chemicals used are of analytical grade supplied by Merck Limited, India. 2.2. Characterization of Films. Films were characterized by X-ray diffraction (XRD) from a diffractometer (Bruker, D2 Phaser) employing Ni-filtered Cu Kα radiation. The elemental analysis of the films was performed by inductively coupled plasma-optical emission spectroscopy (ICP-OES; PerkinElmer, Optima 3300RL). The sample was digested in a nitrogen environment in a glovebox preventing the contamination by excess atmospheric oxygen. The precursor complex was investigated by FTIR spectroscopy (Thermo, Nicolet 6700), while thermolysis of precursor was investigated by TGA and DSC conducted at heating rate of 10 K/min (Mettler-Toledo, TGA/DSC 1). Film topography is examined using AFM (Nanosurf, Easyscan 2), and its optical properties are probed by UV−vis−NIR spectroscopy in transmittance and reflectance mode (Shimadzu 3600). 2.3. Device Fabrication and Characterization. Thin film solar cells are deposited upon indium doped tin oxide (ITO) coated soda lime glass (SLG) substrates with device structure graphite/BFO/ZnO/ITO/SLG. ITO substrate is first coated with ZnO using sol−gel spin-coating route described elsewhere, with a final annealing step at 673 K in air for 2 h.26 The BFO layer is deposited layer-by-layer spin-coating the precursor solution via the procedure described above. After each step of spin-coating the films are heated at 473 K for 10 min in air.

Figure 1. FTIR spectra of pure thiourea and the metal−organic precursor complex.

pure thiourea and the air-dried metal−organic precursor complex (MOPC) containing FeCl3, BiCl3, and thiourea. The spectrum of pure thiourea features the CS bond vibration at 735 cm−1, coupled C−N stretching and N−H bending at 1470 and 1600 cm−1, and symmetric and asymmetric N−H stretching at 3180 and 3280 cm−1, respectively. These vibrational responses are altered upon complexation with metal ions in the MOPC; in particular, the CS stretching frequency undergoes a considerable bathochromic shift from 735 to 700 cm−1. On the other hand, hypsochromic shifts are observed in the coupled C−N stretching (1470−1600 cm−1) and N−H bending (1490−1625 cm−1). Shifts to higher wavenumbers are also observed for the N−H stretching frequencies in the MOPC from 3180 to 3280 cm−1 and 3210 to 3300 cm−1. These observations can be explained by assuming a strong interaction between thiourea and the metal ions via the S atom.27 Such interaction leads to a decrease in the double-bond character of CS bond, increase bond order of C−N bond, and development of partial positive charge on the nitrogen atoms. Thermogravimetiric analysis (TGA) and differential scanning calorimetry (DSC) data associated with the thermolysis of MOPC are shown in Figure 2. The TGA curve shows two major weight loss steps, with the first one occurring between 393 and 493 K. This step corresponds to the thermolysis of MOPC to form a sulfide, which may include the loss of HCl and H2O. This step is endothermic as revealed by the corresponding DSC feature. The second step is broad, starting at 493 K and continuing until 873 K, corresponding to the oxidation of the sulfide intermediate to BFO. The DSC curve marks the beginning of this step by a sharp exothermic B

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4a (2-dimensional view) and Figure 4b (3-dimesional view). AFM images show the presence of spherical particles with sizes

Figure 4. AFM of BiFeO3 film on glass in (a) 2D and (b) 3D views.

of the order of 150 nm. The films are homogeneous and compact devoid of any crack or pinhole. The root-mean-square roughness of films over a 2 μm × 2 μm area is approximately 15 nm. Transmittance and reflectance spectra of BFO films (500 nm thick) are shown in Figure 5a. Both the spectra show interference patterns characteristic of a highly homogeneous film thickness. The strong absorption in the wavelength range of 625 and 500 nm can be attributed to the fundamental bandto-band electronic transition. The absorption coefficient (α) is calculated from transmittance (T) and reflectance (R) data using

Figure 2. TGA and DSC analysis of the MOPC thermolysis.

behavior, further suggesting a higher thermodynamic stability of the BFO with respect to the sulfide intermediate. 3.2. Characterization of Films. X-ray diffraction (XRD) of the BFO thin film along with quantitative structural fitting profile is shown in Figure 3. All of the experimental XRD peaks

α=

⎤ 1 ⎡ T ln⎢ ⎥ t ⎣ (1 − R )2 ⎦

(1)

where t is the mean film thickness. Based on this relation, the absorption coefficient of BFO is larger than 104 cm−1 for energies above the band gap. As shown in Figure 5b, the BFO band gap is 2.2 eV as calculated from the Tauc expression, which is consistent with previous studies.9,10 A material with such a band gap is expected to utilize less than 40% of the total incident photon energy. Room temperature conductivity of the films obtained by the two-probe method is determined to be 0.2 S/cm. On the other hand, the thermoelectric power (TEP) measured by the hotprobe point method at a temperature gradient of 3 K is +285 μV/K. The majority carrier concentration (p) is estimated from

Figure 3. X-ray diffraction pattern of BiFeO3 films along with quantitative fitting profile.

match the standard JCPDS file (86-1518) for BiFeO3 with rhombohedral structure (space group: R3C), with the various plane assignments shown in the figure. The average crystallite size is approximately 55 nm, as calculated from the peak broadening of (012) peak using the Scherrer relation. Quantitative structural analysis is done by Rietveld refinement of the XRD patterns using Topas 4.2 software. Peak fitting has been performed using the SPV-II function which is a composite of Gaussian and Lorentzian functions. The Rp and Rwp values for fitting are 5.69 and 7.35, respectively, confirming an excellent fit to the R3C space group. The lattice parameters as determined from the refinement are a = 0.5578 nm and b = 1.3858 nm, and total cell volume is 0.37352 nm3. The analysis also shows two different Fe−O bond lengths (0.2122 and 0.1908 nm) as well as two Bi−O bond lengths (0.2551 and 0.2251), while the Bi−Fe bond length is 0.3143 nm. The Fe− O−Fe and O−Bi−O bond angles are around 155.6° and 72.8°, respectively. The composition of films as analyzed using ICPOES is found to be Bi:Fe:O 1:1:3.28; i.e., the films are metal deficient and oxygen excess stoichiometry. The surface topography of a typical BFO film on glass as probed by atomic force microscopy (AFM) is shown in Figure

⎛ ⎛ TEP ⎞⎞ p = Nv ⎜⎜A − ⎜ ⎟⎟⎟ ⎝ k B / e ⎠⎠ ⎝

(2)

where A is a constant associated with carrier scattering (assuming ionized impurities, A = 4), with Nv is the effective density of state for valence band (∼2.5 × 1017 cm−3); p is estimated to be 5 × 1017 cm−3, which suggest moderate p-type doping. From the values of conductivity and carrier concentration, the hole mobility is found to be 2.5 cm2 V−1 s−1. 3.3. Characterization of Graphite/BFO/ZnO/ITO/SLG Solar Cell. A cross-sectional scanning electron micrograph of a typical BFO/ZnO/ITO/SLG stack is illustrated in Figure 6a. The nanostructured BFO films obtained by sequential spincoating steps is rather compact, forming a strong adherent contact to the ZnO layer. The current−voltage characteristics of graphite/BFO/ZnO/ITO/SLG cell under dark and AM1.5 illumination (power density 100 mW cm−2) at 300 K are shown in Figure 6b. The active area of the device is 0.35 cm2. The photoconversion efficiency of the device is found to be 3.98% with short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) of 12.47 mA/cm2, 642 mV, and C

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Figure 5. (a) Transmittance and reflectance spectra of BiFeO3 film and (b) Tauc plot for determination of band gap.

Figure 6. (a) Cross-sectional SEM and (b) room temperature J−V characteristics of BFO all-oxide and all-solution based solar cell.

Figure 7. (a) External quantum efficiency of BiFeO3 solar cell and (b) time-resolved photoluminescence (PL) of the device. Inset shows the room temperature PL spectrum.

Figure 8. (a) Device efficiency and series resistance as well as (b) open-circuit voltage as a function of temperature.

50.4%, respectively. These figures of merits are among the highest for all-oxide all-solution based thin film solar cells. The external quantum efficiency (EQE) of the cell is illustrated in Figure 7a. The EQE shows little wavelength dependence between 350 and 560 nm, with a maximum value of 70%. For wavelength shorter than 350 nm, the EQE drops

due to absorption by the ZnO layer. The EQE also show a long tail at wavelengths longer than 600 nm, which could be linked to either high recombination rates at the back contact or short minority carrier lifetimes.28 The latter parameter was estimated from time-resolved photoluminescence (TRPL) responses as shown in Figure 7b. The TRPL data are collected at the D

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4. CONCLUSIONS BiFeO3 (BFO) films are deposited on glass substrates using the metal−organic precursor complex (MOPC). Analysis of the MOPC thermolysis by TGA and DSC shows that the formation of a sulfide intermediate at temperatures below 493 K, while a stable BFO phase is reached at 673 K. XRD and AFM reveal the formation of phase pure and smooth films characterized by grains in the range of 15 nm, while transmittance and reflectance data indicate an absorption coefficient >104 cm−1 and a direct band gap of 2.2 eV. The method has been also used to deposit an all-oxide BFO/ZnO heterojunction based solar cell with photoconversion efficiency of nearly 4% and quantum efficiency of about 70% at room temperature. These figures are among highest reported for all oxide PV cells. Detailed analysis of the solar cells indicates recombination takes place primarily at the BFO/ZnO interface. Preliminary results are affirmative and suggest the potential of BFO as absorber for sustainable all-oxide thin film solar cells.

photoluminescence peak located at 565 nm (inset Figure 7b) corresponding to band-to-band transitions. Fitting the TRPL transient to a first-order decay yields a minority carrier lifetime of 16 ns. This further confirms that the tailing in the QE spectrum is due to such short lifetimes of minority carriers. 3.4. Temperature Dependence of Graphite/BFO/ZnO/ ITO/SLG Cell Performance. The device performance is also studied as a function of temperature in the range of 140−320 K. The dependence of device efficiency and series resistance (RS) with temperature are shown in Figure 8a, while the behavior of Voc is illustrated in Figure 8b. As the temperature is decreased, Rs displays an exponential increase while Voc linearly increases. The device efficiency achieves a maximum of 4.88% at 240 K, followed by a sharp decrease at lower temperatures due to the strong increase in Rs. The series resistance of a device should be ideally ohmic and should rise linearly with temperature. However, frequently series resistance shows an exponential dependence on temperature, implying a non-ohmic behavior due to semiconductor resistance.28 Such observation can be described by the presence of back-contact barrier following thermionic emission. Thus, the series resistance of a device can be expressed as a linear contribution of the ohmic (Ro) and non-ohmic component in form of28,29 ⎛ϕ ⎞ C exp⎜ B ⎟ R s(T ) = R o + T ⎝ kBT ⎠

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Corresponding Author

*E-mail [email protected]; Tel +44 (0) 7778391976 (D.T.). Present Address

D.T.: School of Chemistry, University of Bristol, Cantock’s Close, Bristol, United Kingdom BS8 1TS. Notes

(3)

The authors declare no competing financial interest.

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where C is linked to the effective Richardson’s constant and ϕB is the back-contact barrier height. A plot of Rs versus 1/T yield values for ϕB and Ro of 82 meV and 4.6 ohm cm2, respectively. It can be thus recognized that the large series resistance arises not only from the existence of back-contact barrier but also from the ohmic part. The value of the latter can be attributed to high resistivity of the ZnO layer and a thin transparent conducting oxide substrate (thickness: 50 nm; sheet resistance: 40 ohm cm2). The dependence of Voc on temperature can provide information on the activation energy for recombination (EA) based on28,29 Voc(T ) =

J nk T EA − B ln 00 e e JL

AUTHOR INFORMATION

ACKNOWLEDGMENTS D.T., T.K.C., and A.R. are grateful to the support of Charotar University of Science and Technology, India. Also, D.T. and D.J.F. acknowledge the financial support by the Engineering and Physical Research Council, UK, through the PVTEAM Consortium (EP/L017792/1).

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where n, J00, and JL are the device ideality factor, reverse saturation current prefactor, and the photocurrent, respectively. From the data shown in Figure 8b, EA is estimated to be 0.78 eV. Considering EA < Eg and the ideality factor (extracted from the J−V characteristics) is larger than 2, it could be postulated that recombination losses occur predominantly at the BFO/ ZnO junction.29 A parameter that could have a substantial effect in the device performance is the thickness of the buffer layer, i.e., ZnO. EA and n have complex interplay with the buffer layer thickness as described in previous studies.28 In particular, increasing buffer layer thickness causes the Fermi level at absorber/buffer interface to move away from the conduction band edge lowering the Voc. This behavior also affects the interfacial concentration of charge carriers which manifest itself in the device ideality factor. E

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