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Electron-Hybridization Induced Enhancement of Photo Activity in Indium-Doped CoO 3

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Koushik Majhi, Vijay Singh, Kevin James Rietwyk, David A. Keller, Hannah Noa Barad, Adam Ginsburg, Zhi Yan, Assaf Y. Anderson, Arie Zaban, and Dan Thomas Major J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10673 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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The Journal of Physical Chemistry

Electron-Hybridization Induced Enhancement of Photo Activity in Indium-Doped Co3O4 Koushik Majhi†a, Vijay Singh†b, Kevin James Rietwyka, David A. Kellera, Hannah-Noa Barada, Adam Ginsburga, Zhi Yana, Assaf Y. Andersona, Arie Zaban*a and Dan Thomas Major*b a

Department of Chemistry, Institute for Nanotechnology & Advanced Materials, Bar Ilan University, Ramat Gan 52900, Israel. b

Department of Chemistry and the Lise Meitner-Minerva Center of

Computational Quantum Chemistry and the Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel

† These authors contributed equally. E-mails: [email protected], [email protected]

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Abstract We investigate the significance of indium (In) doping of Co3O4 in the operation of TiO2│Co-In-O│RuO2 all-oxide solar cells employing combinatorial experiments and density functional theory (DFT) calculations. We observed an increase in the open circuit voltage Voc of more than 240 mV with a factor of 4 in enhancement in the short-circuit current Jsc in the low-doping range. This constitutes a maximum power that is five times greater than pure Co3O4–based photovoltaic (PV) devices. Surprisingly, a concurrent marginal change in the bandgap and a decrease in the optical absorption coefficient as a function of indium concentration was observed, contrary to what has been assumed previously. Using DFT in conjunction with joint density of states calculations, we show that with increasing amounts of In, there is a reduction in the low energy photon absorption due to disallowed electronic transitions. Moreover, we show that emergence of In 5s states results in a freeelectron-like-band in the conduction band. We propose that this might reduce the rate of carrier recombination (reflected in higher open circuit voltage) and enhance the electron diffusion lengths (reflected in higher short circuit current), leading to improved PV activity. We expect that our results will advance the understanding and development of novel metal oxide semiconductors for low-cost PV applications.

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1 Introduction In order to meet projected energy demands, it is essential to pursue alternative renewable energy sources. Solar energy is a concrete alternative for a sustainable and environment-friendly global energy supply. Recently, a new direction in photovoltaic (PV) research has emerged, focusing on solar cells that are based entirely on metal oxide (MO) semiconductors, i.e. all-oxide PV.1 The all-oxide PV approach is very attractive due to the chemical stability, nontoxicity, and abundance of many metal oxides that can be fabricated as thin films under ambient conditions. MOs are already widely utilized as components in PV cells, such as transparent conducting front electrodes or electron-transport layers, while only very few MOs were used as light absorbers.2-5 Among several transition metal oxides, Co3O4 exhibits intriguing chemical and catalytic properties, and is a potential light absorbing PV material with two optical bandgaps (1.50 and 2.20 eV) in the visible region.6 Co3O4 forms a spinel-type crystal structure that contains two cation-coordinating micro-environments. This spinelmaterial has a four-fold tetrahedral site, where Co is in a +2 oxidation state and a sixfold octahedral site, where Co is in a +3 oxidation state.7-8 For Co3O4, the band edges are composed of 3d states originating from the octahedral and tetrahedral cobalt sites.7 Recently, our computed one-electron energy level diagrams have revealed that strong Co-O anti-bonding states are present at the top of the valence band for Co3O4, hinting at a defect tolerant capacity in Co3O4.9 However, despite the photon absorption in the visible region of the solar spectrum, Co3O4 absorbs low energy photons as well, hence hindering PV activity.6-7 One solution to such photon loss is material doping or alloying, which remains a focal point of oxides research.8, 10-13

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Recently, several studies have suggested that isovalent substitution of the spinel cations sites can be used to modulate both the band edge character and the magnitude of the electronic band gaps, and hence the optical absorption.8,

10-16

For instance,

Woodhouse et al.13 suggested that the substitution of Fe and Al for Co in a Co3O4 spinel framework with a nominal stoichiometry of Co3-x-yAlxFeyO4 (x and y are ca. 0.18 and 0.30, respectively), is an ideal candidate for an efficient single photoelectrode for water photoelectrolysis. The authors also found that photocurrent is limited by the slow kinetics of hydrogen evolution. To improve the photocurrent in Co3O4, which is an important ingredient in PV performance, Walsh et al.8 substituted group 13 cations (Al, Ga or In) into Co3O4 and analyzed the systems using first principles DFT calculations and experiments. They found

that

the

intrinsic

charge

transport

properties

resulted

in

poor

photoelectrochemical (PEC) performance. They ascribed this to inefficient electronhole extraction rates and high resistivity associated with small polaron carriers in Co3O4. We note that in this previous study, the In-doped material was not synthesized, due to difficulties in synthesizing a homogeneous CoIn2O4 film, and hence only characterized computationally, but not experimentally. Moreover, only a single doped material, with complete replacement of Co3+ (i.e. CoIn2O4) was constructed in-silico, and therefore the system contained only Co2+ cobalt ions. We note that previous studies have shown that Co3+ ions contribute significantly to both valence and conduction bands, and thus could be of importance for PV properties.6-7 One solution to the poor charge transport problem was suggested by Feng et al.10 In order to achieve appropriate bandgap, electron mobility, and optical absorption, Co3O4 may be alloyed into a composite material simultaneously possessing Al, Ga, and In. The authors found that by changing the different Al:Ga:In ratios in Co3O4, 4


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such as 2:1:1, 1:2:1, and 1:1:2, the bandgap at the Γ point can be tuned to 1.98, 1.64 and 1.30 eV, respectively. Along these lines, Newhouse et al.12 performed combinatorial optimization of spinel Co3-xMxO4 (M = Al, Ga, and In) alloy thin films prepared by inkjet printing. The prepared film contains simultaneously several combinations of Al, Ga, and In ratios. They found that the highest photocurrent and greatest improvement of the dark currents, relative to pristine Co3O4 films, were measured in films with Al:Ga:In ratios of 1.5:1:1.9 with x = 0.40 in Co3-xMxO4. Surprisingly, they also found that with an increasing indium mole fraction, the structure becomes disordered and the photocurrent diminishes. Furthermore, Lee et al.15 synthesized and evaluated the photocatalytic activities of spinel oxides Co(Al1xGax)2O4

using the photodegradation of methyl orange and phenol. The photoactivity

of the Co(Al0.5Ga0.5)2O4 sample was dependent on both pH and substrate. In spite of the significant achievements obtained in the above-mentioned works, a theoretical understanding of the effect of doping was not presented, hampering further improvements. In spite of improved properties of In-doped and alloyed Co3O4 obtained in previous work, these materials are not sufficiently optimized as potential metal oxide semiconductors with an appropriate energy bandgap, optical absorption, and good electrical conductivity. Indeed, CoIn2O4 is not a good energy harvesting material due to its low energy band-gap10 and due to inefficient electron-hole extraction rates and high resistivity associated with small polaron carriers8. In the present work, we study a range of indium doped Co3O4 materials for all-oxide PV applications, using both combinatorial experiments and first principles DFT calculations. In spite of a marginal change in the bandgap and a decrease in the optical absorption at low levels of alloying, we observe an enhancement in the PV response. Using DFT calculations,

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we show that one plausible solution to the above-mentioned poor charge transport problem is partial substitution of Co+3 by the iso-valent In+3 (in the low doping limit), which results in increased charge transport due to the appearance of a dispersed, lowenergy band. However, we also suggest an additional crucial PV performance enhancement factor, namely a reduction in low-energy photon absorption due to disallowed transitions, which reduces recombination rates.

Experimental and Computational Methods 1.1 Experimental details In the following we will briefly describe the fabrication and characterization techniques employed in the current work. Further details may be found in our previous work.

1, 17-18

A brief discussion of the limitations of the experimental

techniques employed in this work, may be found in the SI. Fabrication: Three Co-In-O thin-film libraries (a grid of 13 × 13 cells) were fabricated and characterized to determine the correlations between material properties and solar cell performance. The substrate used for the solar cell libraries was a commercial glass (with dimensions of 71.3 mm × 71.3 mm) covered with a transparent conducting oxide film, fluorine-doped SnO2 (FTO, TEC15 from Hartford Glass Co.), with a sheet resistance of 15 Ω/square. The substrates were washed with soap, ethanol, and thoroughly rinsed with de-ionized water. An ethanol precursor of TiO2 was made from 0.20 M titanium tetra isopropoxide and 0.40 M acetylacetone.19 Subsequently, the precursor was sprayed through a spray nozzle (Spraying Systems Co.) in conjunction with a CNC x-y-z scanner to produce a linear thickness gradient

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along the x-axis (ranging from 140 to 290 nm) via multiple spray cycles on an FTO substrate placed on a 450 °C hot-plate. A commercial radio frequency (RF)-sputtering system (AJA International Inc.) was employed to deposit combinational oxide films. Two commercial Co3O4 and In2O3 targets (Kurt J. Lesker Company, 2 inch diameter, 99.9% pure) were placed 180° from each other and directed at the sample. A vacuum base pressure of 1.0 × 10-7 Torr was achieved in the chamber. An RF power of 50 and 100 W was applied to the In2O3 and Co3O4 targets, respectively. The deposition pressure was 3 × 10-3 Torr, at room temperature with a distance of 13 cm between the targets and substrate, at a target vertical angle of ~51°. The deposition time was 2 hours. To avoid changes in oxidation states, no oxygen was flowed into the chamber during the deposition. The Co-In-O compound layer (~300 nm thickness) was obtained with a spatially varying chemical composition along the y-axis. A grid of 13 × 13 (169) round RuO2 back contacts (~100 nm thickness), each with a contact area of 2.6 mm2, were deposited by sputtering. The sputtering deposition parameters for the back contacts were a base vacuum pressure of 1.0×10-7 Torr, an Ar inert gas flow of 30 sccm, and an RF power of 100 W was applied to a RuO2 target (Kurt J. Lesker Company, 2 inch diameter, 99.9% pure). A deposition pressure of 2 × 10-3 Torr was obtained, at room temperature with a distance of 13 cm between target and substrate, and target vertical angle of ~51°. The back contacts were deposited for 648 seconds. All the fabrication parameters are listed in Table 1.

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Table 1. Deposition parameters for Co-In-O based solar cells.

Base Material Processing

pressure [torr]

Work

Ar

pressure

flow

flow

[torr]

[sccm]

[sccm]

ambient

N/A

Temperature [C]

Coating Power time [W]

Spray TiO2

O2/Air

[s]

35 ambient

450

pyrolysis

N/A

300

[lit/min]

CoCo3O4

10-7

25

310-3

30

0

100

7200

10-7

25

310-3

30

0

50

7200

10-7

25

210-3

20

10

150

648

Sputtering CoIn2O3 Sputtering RuO2

Sputtering

Characterization: Characterization of the libraries was carried out via highthroughput methodologies. Optical transmission and reflection were measured with an optical-fiber compatible CCD array spectrometer (USB4000, Ocean Optics) to obtain optical absorbance over the visible range. These measurements were used to calculate the film thickness and optical bandgap using Beer-lambert’s law and Tauc plots, as described elsewhere.20 Scanning energy-dispersive X-ray spectroscopy (EDS) was carried out to obtain the spatial distribution of the chemical composition of the library.21 EDS in the HRSEM was performed by an 80 mm2 X-max detector (Oxford Instruments). A fiber-coupled laser-driven light source (LDLS), EQ-99FC from ENERGETIQ (a xenon lamp), was attenuated with an AM1.5G filter and neutral density filters to produce an appropriate intensity for AM1.5 solar spectrum in the

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region of 380 - 950 nm. The sample was mounted on an automated X-Y scanning table (Maerzhaeuser Wetzlar), allowing a gold spring-loaded probe from top to make smooth contact with each of the gold contacts. Current density - voltage (i.e. J - V) curves measured under illumination were recorded with a Keithley 2400. The voltage scan range was from -0.20 to +0.60 V, with 20 mV steps, for both ascending and descending measurements, with a delay of 0.4 seconds between each step. To study crystallinity, symmetry, and inversion of the spinel structure of Co-In-O materials, Micro Raman spectroscopy (LabRAM HR, Horiba Jobin Yvon Corporation; laser wavelength of 532 nm, magnification of ×100, and a beam diameter of 2 µm) measurements were taken at 7 equally spaced points across the library. Finally, material and structural properties of the 169 Glass|FTO|TiO2|Co-In-O|RuO2 solar cells were investigated, and the data were plotted as 2D maps to reveal any spatial correlations of properties between the cells. The work function (φ) and the ionization energy (IE) of the sample were measured under ambient conditions using a scanning Kelvin Probe microscope (SKPM) combined with an air photoemission system (ASKP150200, KP Technology Ltd.) with a 2 mm diameter stainless-steel tip. The tip was calibrated against a gold reference sample. For the Kelvin Probe measurements, the Co-In-O film was deposited onto an FTO coated glass under identical conditions. Using Kelvin probe, we measured the work function difference between the tip and the sample in thermal equilibrium, and we measured the electrical potential when the sample was illuminated. Microstructural aspects were studied using cross section scanning electron microscopy (Inspect, FEI ) images produced by focused ion beam (FIB).

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The IPCE (incident photon-to-current efficiency) measurements were performed using a home-made high-throughput IPCE scanning system. The system consisted of a broad spectrum light source (Laser Driven Light Source, LDLS EQ-99 by energetics) which is coupled with optical fibers to a filters' wheel loaded with a set of 12 different bandpass filters, where each filter has a bandwidth of 50 nm. This design guarantees a much stronger photon flux, compared to commercial IPCE systems, leading to an excellent signal-to-noise ratio. However, this comes with the cost of much lower spectral resolution, compared to commercial IPCE systems. The output photocurrent is measured using a Keithley 2450. APCE (absolute photon-to-current efficiency) was calculated from the IPCE as APCE()=IPCE ()/A(), where A() is the optical absorption. APCE is more useful parameter when evaluating recombination with the semiconductor.

2.2 Computational details All calculations were performed using the Vienna Ab Initio Simulation Package (VASP), which is a plane-wave implementation of DFT.22-27 The valence electrons were described in terms of Kohn–Sham (KS) single-electron orbitals. These orbitals were expanded in a plane-wave basis with an energy cutoff of 600 eV. Core electrons were defined within the PAW methodology. We have considered only specific compositions for the theoretical calculations, in particular, x = 0.17, 0.33, 0.50, and 0.67 in Co3-xInxO4. We have used Γ-point-centered k-point meshes for all calculations. The k-point grid was 8 × 8 × 8 for PBE calculations, while for calculations with the HSE06 hybrid functional we employed a Γ-point-centered 4 × 4 × 4 k-point mesh because of the significant increase in the 10


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computational cost for computations with exact exchange.27 BZ integration was done by the tetrahedron method with Blöchl corrections.28 For the hybrid HSE06 calculations, the BZ was integrated using Gaussian smearing with a smearing width of 0.01 eV. All of the atoms were allowed to relax until the net force per atom was less than 0.01 eV/Å. Due to the high cost of hybrid functionals, only the low-doping cases of indium (x = 0.17 and 0.33) were considered with 5% of exact, nonlocal Hartree– Fock (HF) exchange.29 The choice of HF exchange, α = 5%, was based on our previous study7 where we have shown that the pure PBE functional, the hybrid HSE06 (α = 5% HF exchange), and the Sc-GW0 method (full-self consistency over G) are the most reliable methods for the electronic and optical properties in Co 3O4. We did not include the PBE + U functional in this study, as we and others have previously shown that this functional is not appropriate for Co3O4.7 6 To investigate the discrepancy between the fundamental band gap and the optical band gap, optical properties were also calculated in the PAW framework using the method of Gajdoš et al.30 The imaginary part of the macroscopic dielectric tensor Im(ε) is directly related to the optical absorption spectrum of any material.31-33 The expression for the imaginary part of the dielectric tensor is as follows: 30

2 𝜀𝛼𝛽 (𝑞,𝜔) =

4𝜋 2𝑒 2ћ4 𝛺

lim 1 ∑ 2𝑤𝑘 𝛿 (𝜀𝑐𝑘+𝑞 − 𝜀𝑣𝑘 − 𝜔) 𝑞2 𝑐 𝑣 𝑘

𝑞 →0

, ,

× ⟨𝑢𝑐𝑘 |𝑖 ∇α − 𝑘𝛼 |𝑢𝑣𝑘 ⟩⟨𝑢𝑐𝑘 |𝑖 ∇β − 𝑘𝛽 |𝑢𝑣𝑘 ⟩

∗

(1)

→

The symbols α (β) is one Cartesian component of the unit vector 𝑞̂ = 𝑞𝑞 . The q stands for the Bloch vector of the incident wave. The indices c and v refer to conduction and

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valence band states, respectively, and 𝑢𝑐𝑘 is the cell periodic part of the orbitals at the k-point k. The electron mass is denoted by m, ћω is the energy of the incoming light, 𝛺 is the volume of the primitive cell. The k-point weights wk are defined such that they sum up to 1. We consider the Fermi weights, f, equal to 1 for occupied and 0 for unoccupied states. The factor 2 before the weights accounts for the fact that we consider a spin degenerate system. The expression 𝛿(𝜀𝑐𝑘+𝑞 − 𝜀𝑣𝑘 − 𝜔), is the Dirac delta function, and ⟨𝑢𝑐𝑘 |𝑖∇α − 𝑘𝛼|𝑢𝑣𝑘 ⟩ is the matrix elements of the momentum operator. We have also computed the joint density of states (JDOS) using the optic code of Furthmüller.34-35 The feature of JDOS is strongly depended on the k-point mesh and smearing parameter.36 Therefore, we also checked different sets of the smearing and k-points, but only show the result for a fixed smearing of 0.30 eV with a 1000 k-point mesh.

2 Results and discussion To investigate the significance of indium doping of Co3O4 on PV performance, we have fabricated Co-In-O solar cell library and its schematic representation is shown in Figure 1a. For the study of the Co-In-O crystal structure, composition, optical properties and electrical resistance properties, an additional library was fabricated, where Co-In-O was deposited on bare glass (with the same dimensions), using the same sputtering procedure described earlier. The library composition map in Figure 1b depicts the relative percentage of In to In and Co content present throughout the library. This is the In2O3 “doping percentage” or “mixing ratio” in Co3O4 that shows a wide compositional range, with values ranging between 14.82% to 33.26% In. This 12


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mixing range was chosen based on the peak Jsc vales. A plot of the photocurrent density as a function of the In atomic % shows a peak value of 88 µA/cm2 at 18.50% of In doped Co3O4 (Figure S2). Below 18.50% Jsc values started decreasing, and above 23.42% of In doped Co3O4, we did not observe any PV activity.

Figure 1. (a) Schematic representation of a TiO2│Co-In-O│RuO2 combinatorial PV device library. TiO2 has a linear thickness gradient and the absorber layer (Co-In-O) has a linear chemical composition gradient from RF-(co)sputtering. (b) Composition map presented by the indium percent of the total Co and In content. In order to enable comparison of the performance of cells with and without the In 2O3 alloying, J-V characteristics of the TiO2│Co3O4│RuO2 and TiO2│Co-In-O│RuO2 device libraries were measured and are shown in Figure 2. The first noticeable difference between the two libraries is that the open circuit voltage Voc is significantly enhanced, by 47%, upon addition of In2O3, from 450 to 660 mV (note the change in color scale of the voltage). Secondly, the maximum short circuit current, Jsc, is ~20 µA for pure Co3O4 (Figure 2), whereas it improves to 88 µA, which is more than 4 times greater, for the case of the Co-In-O system. Finally, the maximum power output Pmax increased by more than 5 times compared to the pure Co3O4 library and the internal quantum efficiency (IQE) follows a similar trend as for Jsc and Pmax. We note that the Co-In-O library becomes non-PV at high In atomic percentages (> 22%), seen

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as the white area towards the bottom of the device each library, where the In concentration is highest (Figure 1b and 2).

Figure 2. Maps of the Voc, Jsc, Fill factor (FF), Pmax and IQE as a function of cell position in the library. Each map represents cells with RuO2 back contacts. Right hand side column is the maps of PV parameters for pure Co3O4 deposited at the same conditions as the Co-In-O library for comparison. White spaces in the library indicate inactive photovoltaic cells.

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To investigate role of pinholes in the photovoltaic device we have employed FIB measurements. We did not observe any pinholes/shortcuts in the regions where the In concentration is highest (images from SEM and cross section FIB are given in the SI, Figure S1). The PV inactivity for higher In concentration could be due to many reasons, such as increase in strain due to lattice mismatch, an increase in lattice structure disorder,12 or the development of metallic characteristics (i.e. finite density of states on the Fermi level). To understand the enhanced PV performance in the low doping regime, we have measured the absorption spectra for the TiO2│Co-In-O device library, scanning from the Co-rich side to the In-rich side (Figure 3).

Figure 3. Optical absorption spectra for the Co-In-O system, as In content increases, the spectra exhibit a reduction in the absorption coefficient. For comparison we also show pure Co3O4..

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Each point in the PV library (i.e. different % of In) exhibits similar spectral features, yet with a systematic blue-shift. In contrast to the PV results, the optical absorption spectra exhibit a reduction in the absorption coefficient as the In content increases. These finding are in accord with those of Newhouse and Parkinson.12 Based on the above results, we conclude that even though the absorption intensities decrease, the PV activity is enhanced. To understand how In doping influences the optical and electronic properties of the parent compound Co3O4, we use first principles DFT as a tool for further analysis. One of the prerequisites for performing DFT calculations, is the existence of a reliable structure of the material at hand, and knowledge regarding the phase purity of the system. Thus, Raman spectroscopy has also been used to gain information regarding the chemical properties of the spinel structures and changes in lattice parameters of the Co-In-O library. The Raman spectra were recorded for the Co-In-O system on glass (Figure 4).

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Figure 4. (a) Raman spectra of Co-In-O systems. Compared to Co3O4, the Raman peaks of the Co-In-O systems shift to lower frequencies. We observed that Raman spectra of the Co-In-O materials, recorded at different locations on the library, show similar features to pure Co3O4. For comparison, we have included the Raman spectrum of pure Co3O4 in SI, Figure S3, which contains five vibrational modes.4 It is important to note that the features between 300-400 cm-1 in the Raman spectra originate from the glass substrate,37 and not from pure In2O3 phase segregations. This noise is also present in the pure Co3O4 Raman spectra (Figure S3 in SI). Thus we conclude that we have a pure cubic spinel structure without any additional phases (in spite of slight spectral broadening and shifts). These results are similar to those reported in the literature. The shifts might indicate lattice expansion in the Co-In-O system, relative to pure Co3O4, which may be attributed to the larger ionic radii of In3+ (0.94 pm) compared to Co3+ (0.55 pm). Based on these results, we employed a spinel-type crystal structure for both Co3O4 and Co-In-O for the DFT calculations. To investigate changes in the electronic structure of the Co-In-O materials, we initially computed total and partial densities of states (Figure 5). The Co3O4 energy band-gap obtained using the PBE functional changes marginally from ~0.34 to ~0.40 eV by alloying with In (Figure S4).

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Figure 5. Calculated total and partial densities of states for Co3-xInxO4 for x = 0.00 (a), x = 0.17 (b), x = 0.33 (c), and x = 0.50 (d) using the PBE functional.

Since the PBE functional underestimates the energy band-gap in solid state materials,38 we augmented the computed estimate with hybrid DFT calculations using the HSE06 functional with 5% of exact exchange7 for the x = 0.17 and 0.33 compositions. Again, we find a marginal variation, ca. 0.10 eV, in the energy band gap (Figure S5). Specifically, we find a slight increase in the band gap for x = 0.17 and a slight decrease for the x = 0.33 composition. Our computed partial density-of-states for the parent compound (Co3O4) clearly reveals the identity of Co2+ states (in the range 0.40 – 1.00 eV) and Co3+ states (in the range 1.00 – 2.00 eV) in the conduction band (Figure 5a). In contrast, alloying with In changes the nature of the conduction band significantly, and we find that all Co-3d, In-5s and O-2p states mix, yielding delocalized states in both spin channels (Figure 5b-d). However, we cannot completely rule out the possibility that the low-energy

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transition in the Co-In-O system is of a d-d nature. Further experimental assessment, such as neutron scattering, would be necessary to determine whether d-d transitions are involved, as was done for related cobalt based oxides 39-40. In order to determine the energy band diagram for the Co-In-O systems, SKPM and air photoemission measurements were executed at three different points in the PV library. These points correspond to 18.60, 21.50, and 27.80% of In concentration in the Co3O4 matrix. The work functions (φ) were found to be 4.88, 4.93, and 4.89 eV, respectively, as summarized in a band diagram in Figure 6.

IE

IE

IE

Figure 6. Schematics of band diagrams of the Co-In-O systems in SKPM measurements for three different positions in the PV library. IP is ionization potential, VL is the vacuum level, ECB is the conduction band minima, EVB is the valence band maxima, EF is the work function and Eg is the optical band gap corresponding to the ~2.00 eV absorption peak. Here, points 1, 2, and 3 correspond to 18.60, 21.50, and 27.80% In doped Co3O4. The IE were found to be 5.47, 5.47 and 5.49 eV, respectively. It is noticeable that the IE are constant and there are only minor changes in the Fermi level, which is located near the valence band, maintaining the p-type nature of Co3O4. There is no significant change in the electronic structure based on the SKPM measurements that can explain the observed enhancement in Voc. 19



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To observe low-energy photon absorption in these materials, which was not accessible with our current experimental set up, we have computed the imaginary part of the dielectric constant (Figure 7a), which can directly be compared with the experimentally obtained absorption spectra. Interestingly, we find a decreasing trend in the absorption intensities, which is in agreement with the experimental observation (Figure 3).

Figure 7. (a) Calculated imaginary part of the macroscopic dielectric constant of Co3-xInxO4 (solid circles) for different compositions. (b) Calculated joint DOS (JDOS), with respect to the energy (eV) using the PBE functional. For JDOS we have used a smearing of 0.3 eV with a 1000 k-point mesh.

Based on our DFT calculations, we also find that low-energy photon absorption (i.e. < 0.50 eV) is avoided, as these photons are essentially filtered out, even for small amounts of In-alloying. Filtering of such states might help removing carrier recombination centers, and hence increase the effective concentration of e-h pairs. Thus, the beneficial effect of In alloying on both Voc and overall efficiency, may be ascribed to the crucial role of recombination kinetics.41-42 This is in contrast to what has been assumed by Feng et al., where the authors find that the increase in optical

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absorption in lower photon energies is even more substantial because the CBM is now mainly an s-like state. Next, we have attempted to understand the factor of 5 increase in Jsc due to Inalloying, despite a reduction in the absorption intensities. This can possibly be attributed to a change in the topology of the electron bands.43 In Figure 8, we compare the computed total and orbital projected band-structure for the parent, as well as doped (x = 0.17), systems. We note that in the region of ca. 2.00 eV above the Fermi level, we may discern that In-alloying has pushed down a free electron-like parabolic band (bandwidth 2.00 eV) in the region of PV interest (a bar line in Figure 8a). We find that this band has strong contribution from O-2p and In-5s (Figure 8b, c) states. Therefore, strong hybridization of In-5s and O-2p states likely pushes the O-2p states down and generates a free-electron-like band in the conduction band, i.e. the region of PV interest. This might increase the electron diffusion length, and consequently, a large number of photogenerated charges can reach the electron collecting TiO2-contact before they are lost due to the thermalization or other loss processes, and this might explain the higher photocurrent observed for the TiO2|CoIn-O|RuO2 devices. Thus, the combined effects of In-doping in Co3O4 are reduction in the low-energy photon absorption and generation of a free-electron-like band in the conduction band in the region of PV interest. These factors are plausible reasons for the enhanced operation of TiO2|Co-In-O|RuO2 PV cells. To confirm the above theoretical results we also performed APCE measurements. The result of the APCE measurements for pure Co3O4 and a Co-In-O system (cell with 17.8% indium) are shown in the Figure 8d. We find that when we excite electrons using photons of the energy 2.95 eV (420 nm), the APCE increases from 2% to 4%.

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Our results clearly reveal that if we do not include electronic absorption in the In-5s band (Fig. 8c), the observed photocurrent does

Figure 8. Calculated total (black line) and partial band structure for Co3-xInxO4 for x = 0.00 (a) and x = 0.17 (b) using the PBE functional. The arrows in a-c represent the region of interest for PV activity. Experimentally observed APCE spectra is shown for x = 0.00, i.e. Co3O4, and x = 0.17, i.e. Co2.83In0.17O4, compositions (d) not change significantly. On the other hand, when we allow electronic absorption in the In-5s band, the observed absolute photon-to-current conversion rate increases significantly. Therefore, it is not the deeper penetration (that is possible due to the observed lower absorption coefficient by In-doping) that is causing the higher photocurrent; rather, this is attained by a free-electron-like band structure created by the In 5s electrons. It is important to note that we also observed a similar parabolic band structure for higher In-concentrations i.e. for x = 0.33 and 0.50 (Figure S7a-d).

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However, for x = 0.67 we observe a weak metallic nature at the Г-point of BZ (Figure S7e), possibly destroying the PV activity of the material. Furthermore, the observed decrease in absorption intensities can be understood by investigating the number of joint density of states (JDOS) available for absorbing photons. Indeed, these can be quantified by computing the optical JDOS. The JDOS was calculated by means of Eq. 1, but setting all matrix elements to unity. Integration of JDOS measures the effective number of electrons taking part in transitions up to the frequency ω. The JDOS results are shown in Figure 7b. Here we observe a substantial decrease in the intensities of the JDOS due to In-doping in the region of PV interest. Thus, the number of electrons participating in transitions in the region of PV interest for the parent compound Co3O4 is greater than for the doped system. In contrast, the low energy JDOS (below 1.00 eV) changes marginally with In content. Therefore, in the low frequency region, where JDOS remains invariant (Figure 7b), the observed decrease in intensities in the absorption spectra (Figures 3, 7a) may be explained by disallowed transitions due to selection rules. In view of above, we plot the orbital projected band structure of In-5s states, and find that In-5s states appear both in the region of PV interest (mainly), as well as at lower energies, close to the Fermi level (Figure 8c). These In-5s states might limit the photon absorption, due to the dipole selection rule, i.e. Δl = ±1, where Co|𝑑⟩ - In|𝑠⟩ transitions are optically disallowed. Thus, the observed decrease in absorption intensities and the beneficial effect of indium alloying on both Voc and Jsc was rationalized using DFT calculations.

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3 Conclusions In the present work, we adopted a rational-design driven combinatorial methodology, where we combined high throughput characterization and DFT calculations to study the effect of In-doping on Co3O4 PV-properties. In spite of a marginal change in the bandgap and a decrease in the optical absorption at low levels of alloying, we observed an enhancement in the PV response. Specifically, we observed a factor of 5 increase in the PV performance (Pmax) at 17.80% indium doping. Using Raman spectroscopy, we revealed that the crystal structure of Co3O4 does not change by In doping, and the doped material remains in a cubic spinel form. Based on DFT calculations, we proposed that with increasing amounts of In, there is a reduction in the low energy photon absorption due to disallowed electronic transitions. Moreover, we showed that emergence of In 5s states results in a free-electron-like-band in the conduction band. We propose that these features may reduce the rate of carrier recombination (reflected in higher open circuit voltage) and enhance the electron diffusion lengths (reflected in higher short circuit current), leading to improved PV activity. This study might help the development of novel metal oxide semiconductors.

Supporting Information FIB Cross sectional images taken at four different locations of the TiO2│Co-InO│RuO2 device library on FTO coated glass; plot showing variation of photocurrent density for the TiO2│Co-In-O│RuO2 system as a function of In atomic %.; Raman spectra of pure Co3O4 on the glass substrate; bar diagram showing the variation of the

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energy band gap as a function of indium concentration using the PBE and HSE06 functional with a 5% exact exchange; total and partial density of states for Co 3-xInxO4 using the HSE06 functional with a 5% short-range HF exchange; and total and partial band structure for Co3-xInxO4 using the PBE functional. This material is available free of charge via the Internet at http://pubs.acs.org.

Funding Information The authors acknowledge financial support from the Israeli National Nanotechnology Initiative (INNI) (FTA Project).

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