How Transparent Oxides Gain Some Color: Discovery of a CeNiO3

Research Article Cite This: ACS Comb. Sci. XXXX, XXX, XXX−XXX

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How Transparent Oxides Gain Some Color: Discovery of a CeNiO3 Reduced Bandgap Phase As an Absorber for Photovoltaics Hannah-Noa Barad,* David A. Keller, Kevin J. Rietwyk,‡ Adam Ginsburg, Shay Tirosh, Simcha Meir, Assaf Y. Anderson, and Arie Zaban Department of Chemistry, Center for Nanotechnology & Advanced Materials, Bar Ilan University, 5290002 Ramat Gan, Israel S Supporting Information *

ABSTRACT: In this work, we describe the formation of a reduced bandgap CeNiO3 phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO3 phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of CexNi1−xOy is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce− Ni−O materials. The library displays a reduced bandgap throughout, calculated to be 1.48−1.77 eV, compared to the starting materials, CeO2 and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO3 phase is realized. The CeNiO3 reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured by surface photovoltage spectroscopy, which shows that the source of the photovoltaic activity is the reduced bandgap CeNiO3 phase, making it a viable material for solar energy. KEYWORDS: combinatorial materials science, photovoltaics, metal oxides, high-throughput, cerium oxide

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etc.,23−25 making it a potentially interesting candidate for PV as well. There has been one study that utilized a thin film of CeO2 as the electron transport layer in hybrid perovskite based solar cells, achieving efficiencies of ∼13%.26 In this case, the CeO2 did not play the role of the absorber material in the solar cell, but still helped improve the efficiency. In a different study,27 CeO2 self-assembled nanoparticles (∼5 nm) doped with either La or Zr were implemented as the photoactive material in a solar cell structure containing an I2/I3− electrolyte (similar to dye sensitized solar cells). The doped CeO2 nanoparticles had a photoresponse up to 500 nm, and the solar cells, under AM1.5G illumination, gave an efficiency of 0.9%. However, a photoresponse up to 500 nm (corresponding to a bandgap of ∼2.5 eV) is still not enough to collect light from the whole visible region of the solar spectrum. Attempts have been made to tune the CeO2 bandgap or change its electronic properties by inducing defect states in the CeO2 lattice or by doping CeO2 with transition metals, which may also improve the CeO2 electronic properties. One study by Khan21 promoted the formation of Ce3+ and oxygen vacancies in CeO2 nanostructures using electron beam irradiation. The electron beam irradiation reduced the CeO2 bandgap from 3.36 eV down to 3.12 eV and the photocatalytic properties of the irradiated CeO2 were improved. Another study by Elias et al.24

INTRODUCTION Metal oxide (MO) based solar cells have recently been emerging as a source of renewable energy1−3 because of their high stability, low production costs, and their environmental compatibility. Although MOs are vastly used in the field of solar energy as the electron or hole contacts,4−8 as well as the transparent conductive oxide (TCO) layer,9,10 they are not commonly utilized as the absorber layer material. The discrepancy arises from the fact that most MOs have a wide bandgap (>3 eV), which makes them unsuitable as light absorbing materials yet appropriate for the transparent components of solar cells. However, there is a small number of MOs that have been examined as the absorber layer in photovoltaic (PV) devices. The MOs studied as PV absorbers include mainly Cu2O,11−13 BiFeO3,14,15 Co3O4,16−19 and Bi2O3.20 Because there are so few MO absorbers, new MOs with bandgaps in the visible region of the solar spectrum need to be discovered. Realizing new MOs as PV absorbers can be done either by doping existing MOs and tuning their bandgaps or forming previously unreported MO phases, however there have also been many investigations on existing MOs for PV applications as well. A potential candidate for bandgap tuning is CeO2, the most abundant rare earth MO, which is a wide bandgap (3.4 eV) semiconductor material with high thermal stability and catalytic properties.21,22 Accordingly, CeO2 has been used for many energy related applications such as photocatalysis, fuel cells, oxygen storage capacitors, greenhouse gas conversion, sensors, © XXXX American Chemical Society

Received: February 28, 2018 Revised: April 9, 2018

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DOI: 10.1021/acscombsci.8b00031 ACS Comb. Sci. XXXX, XXX, XXX−XXX

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ACS Combinatorial Science

bandgap that can be used as a new metal oxide absorber material for PV, making MOs more advantageous for the field of solar energy.

used transition metals, such as Mn, Cu, and Ni to substitute Ce4+ in CeO2 nanoparticles and showed improved catalytic abilities for CO oxidation, yet did not mention any changes in the substituted CeO2 bandgap. Ni or NiO have been widely used to dope CeO2 and improve its electronic properties, especially its redox abilities for catalysis. The enhancement of the Ni-doped CeO2 properties arises from structural changes in the crystal lattice, mainly the substitution of Ce4+ with Ni2+. The substitution can also lead to oxygen vacancies in the CeO2 lattice,22 and moreover, the catalytic enhancement may also originate from interfacial modification of the CeO2 with Ni nanostructures excluding the doping effect of Ni on CeO2.28 Incorporation of Ni into the CeO2 lattice can be done using many different methods like wet chemical impregnation of nanoparticles,23 electrospinning,29 wet chemical synthesis,28,30,31 coprecipitation and gel-coprecipitation,32−36 sol−gel,37,38 combustion,22,39 and reverse microemulsions.40 Many reports claim that it is impossible to incorporate more than 20% Ni into the CeO2 lattice.40 Nonetheless, there are some reports that show up to 30% Ni in the CeO2 lattice, although there also may be some phase segregation forming Ni or NiO within the CeO2.28,30,41 The insertion of Ni into the CeO2 results in improved catalytic and electronic properties of the doped CeO2, though none of the past work mentions changes in the Ni-doped CeO2 optical properties, that is, no bandgap reduction. The lack of changes in the optical properties would mean that the Ni-alloyed CeO2 is unsuitable as an absorber for PV. However, the absence of change in the optical properties might be due to the deposition techniques used to form the Ni-doped CeO2. The synthetic routes, generally a wet chemical synthesis of a sort, have mostly been performed at room temperature (or low temperatures) and the resulting product usually needs to be calcined. A different deposition method or a deposition done at higher temperatures may lead to the required optical changes or formation of new Ce−Ni−O phases. In this work, we prepared Ce−Ni−O thin films by pulsed laser deposition (PLD), which is a nonequilibrium42 physical vapor deposition process that has not been utilized previously to fabricate this combination of materials. The deposition was performed at 600 °C and the resulting thin film did not require any post annealing procedures, making PLD appealing as a onestep deposition technique. We used combinatorial materials science and high-throughput analysis methods to form a continuous compositional spread (CCS)1,43 of CexNi1−xOy on one large area substrate, which is essentially a library of different compositions. The library contained both the pure phases CeO2 and NiO, as well as all the varying compositions of CexNi1−xOy in between them. The PLD deposition and combinatorial materials science approach allowed us to fabricate a Ce0.5Ni0.5Oy phase. Using optical and structural characterization methods we were able to determine that the phase was CeNiO3, which had a significantly reduced bandgap, compared to the starting materials (CeO2 and NiO). As a result of the lower bandgap, we decided to use the CexNi1−xOy film as the absorber layer in a solar cell combinatorial library. Current voltage (I−V), surface photovoltage spectroscopy (SPS), and incident photon to voltage efficiency (IPVE) measurements were employed to analyze the CeNiO3 PV activity. The results showed a material with PV behavior, arising from the lower bandgaps, achieving photovoltages above 500 mV and maximum powers of 4 μW cm−2. This work shows for the first time CeNiO3 with a reduced

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EXPERIMENTAL SECTION Combinatorial Library Preparation. Synthesis of CexNi1−xOy CCS Films. The CexNi1−xOy CCS films were deposited by pulsed laser deposition (PLD). A glass substrate (72 × 72 mm2, purchased from Hartford Glass Co., Inc.) was washed in deionized water then cleaned in a sonication bath with soap, then rinsed with dry ethanol and washed again with deionized water. After the wet wash, the glass substrate was treated using Ar Plasma (PLASMA-PREEN II-862, Plasmatic Systems, Inc.) for 5 min. The substrate was then placed in the PLD system (Neocera) together with the targets of the starting materials Ce2O3 (99.99% pure, Testbourne Ltd.) and NiO (99.9% pure, Kurt J. Lesker Company). The PLD vacuum chamber was pumped down to a base pressure of 7 × 10−5 Torr prior to deposition. The target substrate distance was kept at 50 mm, the temperature was set to 600 °C, and oxygen was flowed into the chamber reaching a deposition pressure of 2.6 × 10−3 Torr. To form the CCS film each target was sequentially ablated with 20 pulses by a KrF excimer laser (248 nm, CompexPro, Coherent). First, the CeO2 was ablated with 20 laser pulses on one side of the target; then the substrate was rotated by 180°, and the targets were exchanged, after which 20 pulses of the NiO target were ablated across from the CeO2 deposition position. This target exchange and substrate rotation was repeated for a total of 750 cycles, leading to 15,000 pulses per target. The laser energy density was tuned to 1.5 J cm−2, with a beam spot size of 0.06 cm2, and the repetition rate was 8 Hz. The CeO2 and NiO reference libraries were deposited using the same setup, parameters, and glass substrate, except the substrates were static during the deposition of the corresponding individual target. Each target was ablated separately for 15000 pulses, on two different substrates, to form single material films as the CeO2 and NiO reference libraries. To deposit the CexNi1−xOy CCS as PV libraries the same parameters mentioned above were used, but the substrate was a 72 × 72 mm2 fluorine-doped SnO2 (FTO) coated glass covered with the TiO2 electron transport layer (synthesis will be described further on in this section). Synthesis of TiO2 Layer. The TiO2 thin film layer was prepared by a home-built spray pyrolysis system44 on an FTO coated glass substrate, TEC 15 (72 mm × 72 mm, Hartford Glass Co., Inc.). The glass substrate was cleaned and treated using the same procedure mentioned above. The substrate was placed on a hot plate (Harry Gestigkeit GmbH) and heated to 450 °C. The TiO2 precursor solution was made by mixing 7.5 mL of titaniumtetraisopropoxide (TTiP, Acros Organics) and 5 mL of acetylacetone (Acros Organics) in 240 mL of ethanol (Carlo Erba Reagents), at a pH of 6.8. The precursor carrier gas used was cleaned and filtered dry air, and the flow rate was set to 69 mL hr−1. The spray pyrolysis system setup is comprised of a 3-axis CNC robot (EAS GmbH), a Sono-tek 120 kHz ultrasonic spraying nozzle, and a syringe pump (New Era Pump Systems, Inc.) for the precursor solution. The solution was sprayed using a program that was set to form a linear thickness gradient of the TiO2 layer, with thicknesses varying from 180 to 350 nm. Deposition of Au Back Contacts. To complete the CexNi1−xOy CCS PV libraries 169 Au back contacts were sputtered on the CexNi1−xOy layer (which was already B


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Axis HS spectrometer (England) equipped with a monochromatic Al Kα X-ray source (photon energy 1486.6 eV). The measurements were carried out under UHV conditions, at a base pressure of 5 × 10−10 Torr (and no higher than 3 × 10−9 Torr). Survey and high-resolution spectra were acquired at a pass energy of 80 and 40 eV, respectively. The source power was normally 75 or 150 W. The binding energies of all of the elements were recalibrated by setting the CC/CH component of the C 1s peak to 285 eV. Quantitative surface chemical analysis was performed using high-resolution core-level spectra after the removal of a nonlinear Shirley background. Solar Cell Device Characterization. Current Density− Voltage (J−V) Measurements. The J−V characteristics of 169 solar cells in the CexNi1−xOy CCS PV library were measured with a home-built automated scanning J−V system described in depth in previous reports.11,45 To make an electrical contact to the FTO on the library substrate, a metal frame was ultrasonically soldered around the sample edges (ultrasonic soldering system, USS-9200, MBR ELECTRONICS GmbH). The library was illuminated using a laser driven light source (LDLS, EQ-99FC, ENERGETIQ) xenon lamp, which was calibrated to the AM1.5G solar spectrum. The source meter for the J−V electrical measurements was a Keithley 2400 source measurement unit (SMU). The J−V curve was measured twice, in ascending and descending scan directions, for all the points. The ascending and descending measurements were performed to obsereve the possible existence of hysteresis due to capacitance in the cells. Solar cells that had less than a 15% difference between the ascending and descending curves were defined as photovoltaic, and the rest of the points were removed from the analysis. Kelvin Probe Measurements. The surface photovoltage spectroscopy (SPS), work function, and the ionization energy of the CexNi1−xOy CCS library were measured using a scanning Kelvin probe microscope, combined with an air photoemission system and monochromated quartz tungsten halogen lighting module (ASKP150200, KP Technology Ltd.).51 The measurements were done under ambient conditions. A stainless-steel tip (2 mm diameter) was calibrated against a gold reference sample; the work function of the tip was 4.22 eV. For the air photoemission measurements, the sample was exposed to monochromatic UV light and the photon energy was varied from 5.0 to 6.8 eV. The onset of the photocurrent was analyzed to determine the ionization energy and work function of the CexNi1−xOy CCS film. SPS measurements were performed by monitoring changes in the CPD under illumination by monochromatic light that ranged from 1000 to 400 nm. IPVE and IPCE Measurements. The incident photon to voltage/current efficiency (IPVE/IPCE) for the CexNi1−xOy CCS PV library was measured in a home-built scanner similar to the J−V scanner. The IPVE/IPCE setup includes an x−y−z scanning stage connected to a Keithley SMU and an optical fiber connected to an LDLS coupled to a filter wheel. The technique involves measuring the open circuit voltage or short circuit current as a function of the photon energy of a monochromatic light source over a range from 400 to 950 nm, with 50 nm band-pass filter intervals. For the IPVE the library was kept at open circuit voltage, and for the IPCE the library was kept at short circuit current. A Keithley 2450 was used to force the current to be 0.0 nA (or the voltage to be 0.0 nV) across the device and the open circuit voltage (or short circuit current) was thus measured.

deposited on to the TiO2 layer), to form 169 individual solar cells. The Au contacts were made by RF magnetron sputtering (AJA International Inc.), using a shadow-mask with an array of 13 × 13 round holes (hole diameter 1.8 mm), which was placed on the substrate with the TiO2 and CexNi1−xOy layers. The sputtering system was pumped down to a base pressure of 1.4 × 10−7 Torr. The Au was deposited from a 2″ Au target (99.99%, Testbourne, Ltd.) under Ar gas at a flow rate of 30 sccm, and the total pressure in the chamber was set to 2 mTorr. The substrate temperature was 23 °C, and the target power was set to 100 W. The deposition time was 8 min and 10 s, which corresponded to an Au contact thickness of 100 nm, calibrated by a quartz microbalance. Materials Characterization. Material Composition. The CexNi1−xOy CCS film composition was measured using energy dispersive X-ray spectroscopy (EDS). The EDS spectra were obtained by an 80 mm2 X-max detector (Oxford Instruments) that was coupled to a High-resolution scanning electron microscope (HRSEM, Magellan 400L, FEI). In the library, 169 points, corresponding to the positions of the Au back contacts (that were only deposited on the CexNi1−xOy CCS PV library) were measured and then analyzed using the Aztec software (Oxford Instruments). Optical Measurements. Scanning optical spectroscopy measurements were performed on 169 points (corresponding to the positions of the Au back contacts that were deposited on the CexNi1−xOy CCS PV library) in the CexNi1−xOy CCS library. Total transmission (TT), total reflection (TR), and specular reflection (SR) were measured (in a spectral range of 320−1000 nm) under ambient conditions, with a home-built scanner reported elsewhere.45 The scanner is an optical fiber based system that consists of a CCD array spectrometer (USB4000, OceanOptics) and two integrating spheres. The measurement points were circular with a diameter of 3 mm. The absorptance was calculated based on the total transmission and total reflection measurements as A = 1 − TT − TR. The bandgaps were calculated using Tauc plots,46 based on the absorptance calculation and a calculation of the absorption coefficient.47 The optical measurements were also used to calculate the thickness of the different layers of the CexNi1−xOy CCS PV library. Using the TT and SR with a commercially available optical modeling software (CODE), which fits simulated reflection and transmission spectra to the measured ones,48 the thickness of the layers was calculated. The wavelengths used ranged from 380 to 1000 nm for the analysis, and the simulation was based on the OJL interband transition model.49 Structural Analysis. The CexNi1−xOy CCS library was measured by X-ray diffraction (XRD) to determine the crystalline structures that were formed in the library. The measurements were performed down the central line of the library as well as for two lines on the right and left sides of the library, using a Rigaku Smartlab workstation, with a θ−2θ scan range of 10−90°. The data was analyzed using CrystalMaker software and the database used to determine the crystal structures of the CeO2 and NiO was the ICDD database with cards ICDD 00-023-1048 and ICDD 00-044-1159. To determine the new phase that was observed we used the Materials Project open accesses database material ID mp77702450 together with the CrystalDiffract CrystalMaker analysis tools. Chemical and Electrical Analysis. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos C


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Figure 1. CexNi1−xOy CCS libraries. (a) Side view of CexNi1−xOy CCS film deposited on a glass substrate, depicting the thickness gradient formed by the static PLD deposition. A total of 750 cycles of deposition were performed in order to achieve the CexNi1−xOy CCS film. On the left is a top view of the material positions compared to each other on the substrate. (b) Side view of CexNi1−xOy CCS PV library, with an electron conducting layer of TiO2 (deposited with a thickness gradient) and 169 Au back contacts, representing 169 solar cells in the PV library. (c) Actual image of the PV library with the CexNi1−xOy CCS film as the absorber layer, showing brown coloring throughout the library. (d) %Ni atomic composition map of the CexNi1−xOy CCS film library. The top side contains 99% Ce and the bottom side has 92% Ni spanning a large variation in film composition. Each point in the map corresponds to the positions of the 169 back contacts in the PV library.

Figure 2. Optical analysis of the CexNi1−xOy CCS library. (a) Absorptance spectra of different Ni concentrations down the central line of the library (Figure 1d) compared to the CeO2 and NiO reference spectra. A large change in the absorptance onset can be seen between the CexNi1−xOy CCS film and the references. (b) Enlargement of the area marked with the black dashed rectangle in panel a. The changes in the absorptance can be seen as an increase in absorptance up to 25% Ni and then a decrease from 48% Ni until 90% Ni, showing that the red shift in absorptance changes for the different Ni concentrations. (c) CexNi1−xOy CCS library bandgap map showing a bandgap narrowing of the CexNi1−xOy CCS film from the starting materials (3.3 eV) down to a range of 1.48−1.77 eV. Each point in the map corresponds to the positions of the 169 back contacts in the PV library.

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RESULTS AND DISCUSSION

and NiO layers were deposited sequentially for 750 cycles, forming the CexNi1−xOy CCS films by interdiffusion of the layers during the deposition. To form the PV device, a layer of TiO2 (used as the electron transport layer) was deposited with a thickness gradient onto a conductive substrate, on top of which the CexNi1−xOy CCS film was deposited with back contacts to close the circuit (Figure 1b). A total of 169 Au back contacts were deposited, representing 169 solar cells with different TiO2 thicknesses and different CexNi1−xOy compositions.

The CexNi1−xOy CCS films were prepared by PLD using the combinatorial materials science approach. Figure 1 shows a schematic depiction of two different libraries that were prepared: a Ce−Ni−O combinatorial library as well as the PV library structure. The CexNi1−xOy CCS films were deposited on glass for physical and structural characterization (CexNi1−xOy CCS library) and later deposited as the absorber layer in a separate PV structured library (CexNi1−xOy CCS PV), to examine the CexNi1−xOy CCS film PV activity. The CeO2 D


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Figure 3. XRD measurements of the CexNi1−xOy CCS library. (a) XRD spectra of the central line in the library with different Ni concentrations, indicating the formation of CeO2 and NiO in some areas of the library as well as a new phase formed at 10−48% Ni concentrations. (b and c) The (112) and (224) planes detected for the new phase formed in the CexNi1−xOy CCS library. Very large peak shifting can be seen compared to the CeO2 reference library. (d) Detection of the CeNiO3 reduced bandgap phase by using the “Materials Project” computed database.50 The calculated phase matched almost perfectly with the measurements and the unit cell volumes were very similar, 239.4 Å3 for the measured phase and 233.389 Å3 for the computed phase from the Materials Project.

To determine the CexNi1−xOy CCS film composition, energy dispersive X-ray spectroscopy (EDS) measurements were performed on the whole library. The results showed a wide dispersion of the materials’ atomic composition throughout the CCS layer from ∼99% Ce on the top side of the library (Figure 1d) to 92% Ni on the bottom side of the library. The wide dispersion allowed for a thorough study of the effect of the different compositions on the materials and their properties. An actual image of the PV library is given in Figure 1c, where a brown color can be seen clearly throughout the library (the thickness of the CexNi1−xOy CCS film is given in Figure S1c). The color change is an indication of the optical alterations in the CexNi1−xOy CCS film compared to the transparent CeO2 and NiO reference libraries (Figure S1). To characterize the visually observed optical changes in the CexNi1−xOy CCS film, the library as well as the CeO2 and NiO reference libraries, were measured using a high-throughput optical spectroscopy scanner. The measurements included total transmission (TT) and total reflection (TR), used to calculate the absorptance of the CexNi1−xOy CCS film.45 The CeO2 and NiO reference absorptance onsets were both at 375 nm (Figure 2a), indicating materials that are transparent to the visible region of the solar spectrum. The bandgaps of both reference material libraries were calculated using Tauc plots1,46 and the results showed an average calculated bandgap of 3.3 eV (3.36

and 3.32 eV for CeO2 and NiO, respectively) for both reference libraries, corresponding to reports in the literature.21,52 The full library bandgap results (calculated for both direct and indirect bandgaps) can be seen in the bandgap color maps (Figure S2). The absorptance of the CexNi1−xOy CCS library, as well as the two reference spectra, are given in Figure 2. Seven different Ni concentrations from the center line in the library are given (center line marked with arrows in Figure 1d). Both reference spectra exhibited absorptance from 375 nm, explained above, however, the absorptance of the CexNi1−xOy CCS film changed substantially compared to the reference libraries. The absorptance onsets for all Ni concentrations were considerably shifted to higher wavelengths, absorbing light at much lower energies than the reference materials. The shift occurred at all Ni concentrations, although the magnitude of the shifting varied between the different Ni concentrations. Figure 2b, an enlargement of the rectangular area marked by the dashed black line in Figure 2a, shows the change in absorptance shifting for the examined Ni concentrations. The enlarged area shows that there is an increase in absorptance starting from 5% Ni to 25% Ni. At 48% Ni, which is similar in absorptance to 10% Ni, a decrease in absorptance is seen until 90% Ni, which is comparable to 5% Ni in absorptance. The changes in absorptance may be related to changes in the crystal structure of the CexNi1−xOy CCS film, originating from new phases or E


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cell volume, determined from our XRD measurements, was 239.4 Å3, which correlated well with the unit cell volume calculated by the Materials Project, 233.389 Å3. The unit cell volume that we measured for the CeNiO3 was far from the unit cell volumes of either CeO2 or NiO (158.42 and 54.66 Å3, respectively) measured crystalline phases. The XRD results together with the absorptance and bandgap measurements for the CexNi1−xOy CCS library indicate that we were able to form a CeNiO3 phase with a much smaller bandgap than both starting materials. The CeNiO3 reduced bandgap state was probably formed because we used PLD as the deposition technique. Since PLD is a kinetically driven method, it allows for the formation of kinetically stable phases that may not be achievable by other depositions techniques. Thus, in our case, we were able to form the CeNiO3 reduced bandgap phase, which has not, to the best of our knowledge, been previously reported experimentally. The CeNiO3 also appears for 48% Ni concentration and may also be present in the other Ni concentrations but in a more amorphous state, since all the concentrations have a large change in the bandgap compared to the starting materials. At 71% Ni and above, peaks for NiO start to form, mainly the (012) plane at 37.06°, and the main peak (111) of the CeO2 still appears at 71% and 85% Ni. To further confirm that the changes in absorptance and new crystal structure do not arise from oxygen vacancies, we performed X-ray photoelectron spectroscopy (XPS) measurements. When oxygen vacancies are formed in CeO2 some of the Ce4+ ions are reduced to Ce3+,21 resulting in their peak formation in XPS. Figure S4 shows the results for the XPS measurements performed for 5%, 25%, 48%, and 90% Ni concentrations. The results show the peak formation for the Ce4+ 3d states28,54,55 for the Ni concentrations of 5%, 25%, and 48%, but no peaks appear for the Ce3+ states. Only at the 90% Ni concentration does a small Ce3+ peak start to form (marked with an arrow), indicating formation of oxygen vacancies at 90% Ni. As for the Ni2+ states, the peaks form in all Ni atomic concentrations, however, at 90% Ni concentration the Ni3+ state appears as well, indicating the formation of defects.56 Since the Ce3+ was not detected for the 25% and 48% Ni concentrations (within the resolution of the XPS system used), we determined that the source of the changes in bandgap and phases did not arise from oxygen vacancies. From the XRD and XPS data we were able to conclude that a new reduced-bandgap phase, CeNiO3, was formed in the CexNi1−xOy CCS library, although still containing CeO2 and NiO impurities. However, the reduced bandgap throughout the CexNi1−xOy CCS library in combination with the CeNiO3 phase led us to believe that this new material may be utilized as an absorber material for PV. The next step was to integrate the CexNi1−xOy CCS library, including the reduced bandgap CeNiO3 phase, into a working PV device. The device was formed as a library of different layers with the CexNi1−xOy layer containing the varying compositions as the absorber (Figure 1b). The electron transport layer was a thin film of TiO2 deposited with a thickness gradient (180−350 nm), which is perpendicular to the deposition direction of the CexNi1−xOy layer. The TiO2 was deposited with a thickness gradient in order to increase the combinatorial PV library diversity and to determine whether there is a dependence of the PV performance on the electron transport layer thickness. On top of the TiO2 the CexNi1−xOy CCS layer was deposited and on top of that 169 Au back contacts were deposited through a static shadow mask, to form 169 solar cells with varying TiO2

stress and strain in the crystal lattice. Another source of changes in optical properties may arise from oxygen vacancies formed in the CexNi1−xOy CCS film. Changes in optical properties have been reported for nanostructured CeO2 containing oxygen vacancies, yet the recorded narrowed bandgaps reached only 3.12 eV (starting from 3.36 eV for the nondefected sample).21 In our case (Figure 2c) the bandgap calculations show a narrowing to a range between 1.48 to 1.77 eV (compared to 3.36 and 3.32 eV for the CeO2 and NiO reference libraries, respectively), much lower than the bandgap reduction that has been reportedly caused by oxygen vacancies. It is important to note that we deposited the CexNi1−xOy CCS layer at higher oxygen pressures (for PLD, up to 330 mTorr) and we consistently achieved a reduced bandgap throughout the CexNi1−xOy library, signifying that the bandgap reduction does not occur because of oxygen vacancies. Consequentially, the source of the changes in the absorptance and bandgap of the CexNi1−xOy CCS film probably mostly originate from the formation of a new crystalline phase. The subsequent analysis performed was X-ray diffraction (XRD) to determine the crystalline phases that compose the CexNi1−xOy CCS library, which may also shed light on the observed optical changes. XRD measurements are given for the Ni concentrations that correspond to the central line in the library (Figure 1d), and measurements were also taken on the CeO2 and NiO reference libraries. The reference measurements (Figure S3) show that the cubic CeO2 (ICDD 00-023-1048) phase was obtained with peaks at 28.45° (111), 32.92° (200), 47.15° (220), 55.95° (311), 58.91° (222), 69.14° (400), 78.65° (420), and 87.93° (422), as well as the trigonal NiO (ICDD 00-044-1159) phase with peaks at 36.85° (101), 43.18° (012), 74.86° (113), and 78.51° (202). Figure 3a shows the diffraction spectra of the different Ni concentrations in the CexNi1−xOy CCS library. At 5% Ni concentration the only phase that appears is the CeO2, albeit with peaks slightly shifted (maximum shift of 0.3 degrees), indicating that the 5% Ni adds strain to the CeO2 lattice and acts as a dopant. At 10% Ni concentration the same CeO2 phase appears; however, there are changes in peak intensities that might point to larger lattice distortions.37 Aside from a small amount of the CeO2 phase, for 25% Ni concentration there is a tremendous increase in peak intensities for only two peaks as well as a very large shift from the CeO2 original peak positions. The shifted peaks are at 32.36° and 67.85° both moved to lower 2theta angles by more than 0.55 degrees as compared to the reference CeO2 phase, which can be seen in Figure 3b and c. Such large shifts and changes in peak intensities can demonstrate the formation of new crystalline phases. Since some of the CeO2 phase still exists for 25% Ni it is reasonable to assume that X, in CexNi1−xOy, can have a range of values, 0 ≤ x ≤ 0.5. To determine the new phase from the XRD results, we searched for any crystalline phase that could match the spectra we obtained with x values between 0 and 0.5. No matches were found in any of the known XRD databases so we decided to use the “Materials Project”, which is an open access web-based computed materials (or predicted materials) database.53 The material that matched our XRD measurements for the 25% Ni concentration, including peak positions and peak intensities, was CeNiO3,50 as can be seen in Figure 3d. The two peaks at 32.36° and 67.85° that appear in our measurements correspond to the (112) and (224) planes of the CeNiO3, respectively, and are in almost perfect agreement with the computational calculations from the Materials Project. Moreover, the unit F


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Figure 4. PV characterization of the CexNi1−xOy CCS solar cell library. (a) Voc, (b) Jsc, (c) FF, and (d) Pmax maps of the 169 solar cells in the CexNi1−xOy CCS PV library. The results show high photovoltages, up to ∼550 mV throughout the library, while the photocurrents and Pmax remain low. This may be due to the CeO2 impurity in the CexNi1−xOy CCS layer, interfering with the PV activity of the CeNiO3, which is confirmed by the low FF in the library. The white areas in the maps are points that did not show PV activity. (e) J−V curves of three different Ni concentrations, which show rectifying PV activity thus establishing that the CeNiO3 reduced bandgap phase can be used as an absorber material for PV systems.

Figure 5. SPS measurements for (a) central line in the CexNi1−xOy CCS PV library with different Ni concentrations, showing the changes of the surface photovoltage for the different Ni concentrations. (b) Comparison of the CPD of the 25% Ni concentration to the CeO2 and NiO references, as well as to TiO2. The results show that the photovoltage is very different in the CexNi1−xOy CCS PV library, and is due to the CeNiO3 reduced bandgap phase and not to the other materials that are present in the library or the solar cell structure. (c) Photovoltage measurements for the central line in the CexNi1−xOy CCS PV library with different Ni concentrations. The results correlate with the same trends seen for the SPS measurements, further implying that the source of the PV activity is indeed the CeNiO3 reduced bandgap phase.

the FF values that reach an average of 35%. The FF is affected by the series and shunt resistance of the solar cell, meaning that the solar cells have low shunt resistance and high series resistance, which can be the result of an insulating phase (CeO2) within the solar cell library that prevents good charge transport and forms recombination centers in the layer, reducing PV performance. Moreover, the slight hysteresis seen in the J−V curves (Figure 4e), can also be a result of the CeO2 phase present in the CCS film. Since the CeO2 is a highly resistive phase, it may cause capacitance to occur within the CCS film, leading to the J−V curves’ hysteresis that is observed, lowering the PV performance. Indeed, the Pmax achieved for the CexNi1−xOy CCS PV library had a maximum of 4 μW cm−2,

thicknesses and varying CexNi1−xOy thicknesses and compositions. Figure 4 shows the PV parameter maps (averaged from the ascending and descending J−V scan) of the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and maximum power (Pmax) for the 169 solar cells in the CexNi1−xOy CCS PV library. Most of the library displayed PV activity, reaching a Voc of ∼550 mV, which is quite a high photovoltage for a solid state all-oxide solar cell. The Jsc reached ∼20 μA cm−2, which is reasonable considering that the CexNi1−xOy CCS layer contains CeO2 that is more of an insulating material, which may inhibit charge transport along the solar cell. The interference caused by the CeO2 phase in the CexNi1−xOy CCS layer is confirmed by G


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Figure 6. Energy band positions for (a) TiO2 as a highly n-type material, which is to be expected for the electron conducting layer. (b) CeNiO3 reduced bandgap phase, showing a slight p-type nature. Where CBM is the conduction band minimum, VBM is the valence band maximum, EF is the Fermi level, VAC is the vacuum level, and Φ is the work function. (c) The TiO2|CeNiO3 junction, indicating that band-bending occurs at the interface between the materials. The band-bending favors photoelectron injection from the CeNiO3 absorber material to the TiO2 electron transport layer, promoting PV activity. These results support the J-V results that we observed for the PV library.

bandgap phase, and the IPVE results have the same trend of increase and decrease in photovoltage as the CPD, with a dependence on the Ni concentrations. When the amount of Ni in the library rises, the photovoltage begins to drop since more of the NiO phase appears in the sample. When comparing the reference libraries with the 25% Ni concentration (Figure 5b) a large change can be seen between the shapes of the difference spectra. First, the references do not reach the same surface photovoltages that the CexNi1−xOy CCS PV library achieves. Second, the peak of the 25% Ni is shifted to higher wavelengths as a result of the reduced bandgap. Third, the 25% Ni spectrum has much higher surface photovoltages for all the wavelengths that were measured compared to the references. These variations in CPD further support that the photovoltage arises from the CeNiO3 reduced bandgap phase. The CPD of the 25% Ni concentration is also compared to a reference TiO2 library to show that the TiO2 electron conducting layer in the CexNi1−xOy CCS PV library is not the source of the photovoltage. The results indicate that the TiO2, which is only active around 400 nm, is not the source of the photovoltage, as it does not have a surface photovoltage response in the regions of high wavelengths as the CexNi1−xOy CCS library. Comparison between the references and the CexNi1−xOy CCS PV library for the IPVE and incident photon to current efficiency (IPCE) for the PV library, are given in Figure S5, and also show the same behavior as the SPS (higher photovoltage for the combinatorial library and current onset at 500 nm). The IPVE is able to separate the components of the system to better indicate the source of the photovoltage. For the CeO2 and NiO the onset of the IPVE is only seen at the bandgap of the materials, meaning that the PV activity seen in the CCS library is due to the CeNiO3 reduced bandgap phase. The IPVE also shows that the CeO2 trap states are not the source of the photovoltage, since the IPVE onset is at the CeO2 bandgap and the trap states do not provide any current that is useful for PV activity. It is important to note that the photovoltages gained in both the SPS and IPVE measurements were not the same as the J-V measurements since the light sources were not calibrated to 1 sun as in the J-V analysis.

which is not very high, however, these results are comparable to previously reported performances of all-oxide PV devices.18,19,57 From the maps it is also possible to see that the varying thickness of the TiO2 electron conducting layer did not affect the PV performance, and there was no preference for a certain thickness. The lack of dependence of the PV performance on the TiO2 layer thickness is promising, since it means that the electron conducting layer thickness is not a limiting factor in the PV performance, thus less device optimization is required. J−V curves of 10%, 25%, and 48% Ni, given in Figure 4e, show a rectifying behavior with minimal hysteresis between the ascending and descending scans. The results indicate proper PV activity even though it is low. However, our goal was not to achieve solar cells with high efficiencies but to check if the CeNiO3 reduced bandgap phase is suitable as an absorber material for solar cells, which we were successfully able to show. To further investigate the source of the PV activity in the solar cells Kelvin probe measurements and incident photon to voltage efficiency (IPVE) measurements were performed. Figure 5a shows the surface photovoltage spectroscopy (SPS) measurements for different Ni concentrations down the central line of the CexNi1−xOy CCS PV library. The results show the contact potential difference (CPD), which is correlated to the surface photovoltage for the different Ni concentrations. For the 5% and 10% Ni the results show that the surface photovoltage is similar to that of the CeO2 reference (Figure 5b), with a broad peak at higher wavelengths. The broad peak probably results from trap states in the CeO2 phase, which is also present in the 5% and 10% Ni concentrations (determined by XRD). As the Ni concentration increases there is an increase in the surface photovoltage and the broad peak at the higher wavelengths disappears. The onset of the surface photovoltage is between 700 and 750 nm, which corresponds very well with the bandgaps of the CexNi1−xOy CCS film. The correlation of the surface photovoltage with the bandgap indicates that the photovoltage in the CexNi1−xOy CCS PV library is due to the CeNiO3 reduced bandgap phase. The IPVE measurements, which are equivalent to external quantum efficiency analysis but measured on voltage, (Figure 5c) also show that the photovoltage onset correlates with the CeNiO3 reduced H


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of the TiO2 and the CeNiO3 and showed that when a junction between the materials was formed, the resulting band-bending favored the injection of photoexcited electrons from the CeNiO3 into the TiO2, which lead to charge separation and PV activity. Our work shows that PLD can be used to form new phases with very different properties from the starting materials. Due to the change in properties, these new materials can be used in applications, such as absorbers in solar cells, that their parent materials could not be used for, opening a new avenue for finding more materials for solar energy applications.

The Kelvin probe was also used to measure the work function and valence band maximum (VBM) energy (ionization energy) positions for the TiO2 and CeNiO3 reduced bandgap phase, to see whether the solar cell structure favors electron injection. The work function (Fermi energy, EF) of the CeNiO3 was measured to be 4.66 eV and the VBM position was determined to be placed 0.6 eV lower than the EF. Together with the calculated bandgap (Eg), of 1.6 eV for the CeNiO3 and 3.28 eV for the TiO2, we were able to determine the energy positions of the materials, as can be seen in Figure 6a and b. The band positions show that the TiO2 is a highly ntype material,58 which is to be expected for TiO2 as the electron transport layer. The CeNiO3 seems to be slightly p-type, which is beneficial for the solar cell activity as it will form a p-n heterojunction. On the basis of the energy band positions, when TiO2 and CeNiO3 are coupled (Figure 6c) to form a junction, the energy alignment shows a small upward bandbending induced in the TiO2 and a slight downward bandbending formed in the CeNiO3. The observed band-bending promotes photogenerated electrons to be injected from the absorber material, CeNiO3, to the electron conductor, TiO2, generating charge separation and PV activity. Although the PV activity is, in general, not very high, it could be that the PV activity of the CeNiO3 reduced bandgap phase is potentially much better than what we achieved. The improvement of the PV activity could be attained by removing the CeO2 and NiO impurities that exist within the CeNiO3 phase, thus reducing charge transport inhibiting factors in the PV device. However, even though the impurities exist we were still able to form a solar cell with a new CeNiO3 reduced bandgap phase as the absorber material, changing the nature of the starting materials, CeO2 and NiO, thus adding a new metal oxide absorber material to the solar energy family.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscombsci.8b00031. Reference library images, thickness maps, full bandgap maps for the reference libraries, XRD measurements for the reference libraries, XPS measurements, IPVE measurements for the reference libraries, and IPCE measurements of the full library (PDF) Crystallographic information file for the CeNiO 3 structure (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hannah-Noa Barad: 0000-0003-0764-6421 Kevin J. Rietwyk: 0000-0002-2266-2713 Assaf Y. Anderson: 0000-0003-1657-4415

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Present Address ‡

K.J.R.: Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia.

CONCLUSION In this research we used PLD, previously unused for the chosen material, to form a CexNi1−xOy CCS film to examine the effect of incorporating widespread compositions of Ni into CeO2. The CexNi1−xOy CCS film library exhibited a large change in absorptance compared to the starting materials. The optical characterization showed a bandgap reduction (from 3.3 eV) throughout the library to a range of 1.48−1.77 eV, depending on the Ni concentration. To understand the source of the optical changes in the library the CexNi1−xOy CCS film was characterized by XRD and a different phase was observed. By using the Materials Project open access computational calculations database we were able to determine that the phase is a new CeNiO3 reduced bandgap phase. The CeNiO3 reduced bandgap phase is formed by the PLD’s kinetically governed deposition process, which allows for phases to be discovered, which cannot otherwise be formed by conventional synthetic routes. Since the CeNiO3 absorbs in the visible region of the solar spectrum it was utilized as the absorber layer in a PV library, with TiO2 as the electron conducting layer and 169 Au back contacts, which define 169 solar cells in the PV library. The CexNi1−xOy CCS PV library gave a Voc of ∼550 mV, yet low currents, and as a result, low efficiencies, which is probably due to the CeO2 phase that is still present in the layer and can hinder charge carrier diffusion. The source of the PV activity was determined using SPS and IPVE measurements, which showed that the photovoltage onset correlated with the bandgaps achieved for the CexNi1−xOy CCS layer. Furthermore, Kelvin probe measurements resolved the energy band positions

Author Contributions

All authors have given approval to the final version of this manuscript. Funding

This work has received funding from the Israel Science Foundation (grant 1729/15) and the Israeli National Nanotechnology Initiative (INNI, FTA project). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Prof. Ilya Grinberg for the fruitful discussions and ideas, and Dr. Michal Ejgenberg, from the Department of Chemistry at Bar Ilan University, for her assistance with the XPS measurements. H.N.B. and D.A.K. would like to thank the Israeli Ministry of Science, Technology, and Space for their financial support.

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REFERENCES

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Electron-Based Solid State TiO2|Ag Solar Cells. Adv. Mater. Interfaces 2016, 3, 1500789.

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How Transparent Oxides Gain Some Color: Discovery of a CeNiO3

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