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Parasiticabsorptionreductioninmetaloxide-basedtransparentelectrodes:applicationinperovskitesolarcells

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Parasitic Absorption Reduction in Metal Oxide-Based TransparentElectrodes: Application in Perovskite Solar CellsJeremie Werner,*,† Jonas Geissbuhler,†,‡ Ali Dabirian,† Sylvain Nicolay,‡ Monica Morales-Masis,†

Stefaan De Wolf,† Bjoern Niesen,†,‡ and Christophe Ballif†,‡

†Photovoltaics and Thin-Film Electronics Laboratory, Institute of Microengineering (IMT), Ecole Polytechnique Federale deLausanne (EPFL), Rue de la Maladiere 71b, 2002 Neuchatel, Switzerland‡CSEM, PV-Center, Jaquet-Droz 1, 2002 Neuchatel, Switzerland

*S Supporting Information

ABSTRACT: Transition metal oxides (TMOs) are commonly used in a widespectrum of device applications, thanks to their interesting electronic,photochromic, and electrochromic properties. Their environmental sensitivity,exploited for gas and chemical sensors, is however undesirable for application inoptoelectronic devices, where TMOs are used as charge injection or extractionlayers. In this work, we first study the coloration of molybdenum and tungstenoxide layers, induced by thermal annealing, Ar plasma exposure, or transparentconducting oxide overlayer deposition, typically used in solar cell fabrication. Wethen propose a discoloration method based on an oxidizing CO2 plasmatreatment, which allows for a complete bleaching of colored TMO films andprevents any subsequent recoloration during following cell processing steps.Then, we show that tungsten oxide is intrinsically more resilient to damageinduced by Ar plasma exposure as compared to the commonly used molybdenumoxide. Finally, we show that parasitic absorption in TMO-based transparentelectrodes, as used for semitransparent perovskite solar cells, silicon heterojunction solar cells, or perovskite/silicon tandem solarcells, can be drastically reduced by replacing molybdenum oxide with tungsten oxide and by applying a CO2 plasma pretreatmentprior to the transparent conductive oxide overlayer deposition.

KEYWORDS: molybdenum oxide, tungsten oxide, CO2, plasma treatment, perovskite, silicon heterojunction, tandem, solar cell

1. INTRODUCTION

Transition metal oxides (TMOs) have attracted considerableattention for use in gas and chemical sensors1,2 and, morerecently, for thin-film transistors and optoelectronic devices,such as light-emitting diodes and solar cells, including organic,perovskite, and silicon heterojunction solar cells.3−5 Their highwork function, large bandgap, efficient carrier selectivity, andhigh transparency make them viable and cost-effectivecandidates for these applications, where they are employed asprotective buffer layers,6−10 optical spacers,11−13 or chargetransport layers.14−21

TMOs are susceptible materials which are sensitive to theirenvironment, such as air or oxygen exposure,22 temper-ature,23−26 UV-light,27 UV-ozone,28 or plasma treatments.29−33

This is because many TMOs readily undergo redox reactions.Evaporated metal oxides are especially sensitive, as they areoften substoichiometric when as-deposited, due to an oxygendeficiency.3,5 The presence of oxygen vacancies createspositively charged structural defect states in the bandgap,which enables the attractive hole injection properties of TMOsdespite their n-type semiconductor character.3 Some of thedefect states also act as localized color centers, resulting inabsorption in the visible/near-infrared spectrum, spread at

different wavelengths around 800 nm depending on theiroxidation states. This sensitivity and the associated colorationare desirable for some applications such as gas and chemicalsensing. TMOs were for these reasons widely studiedspecifically for their photochromic34−36 and electrochromic37,38

properties. However, for optoelectronic applications, wherehigh transparency is required in charge transport layers, anycolor change could be detrimental and result in performancereduction.In particular, it was recently pointed out that the optical

properties of TMO layers are strongly affected during sputterdeposition of a transparent conducting oxide (TCO) overlayer,resulting in a more pronounced absorptance of the TMO/TCOstack than the expected sum of their individual absorptancevalues.6,18 If such a stack is used as a front window electrode ina solar cell, this increased absorptance directly translates into adecreased photocurrent and, as a result, reduced deviceperformance.

Received: April 13, 2016Accepted: June 24, 2016

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.6b04425ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

It would therefore be highly interesting to find either apretreatment method to prevent the appearance of thisabsorptance increase or a post-treatment to recover sputterdamage (discoloration). Inspiration for this can be taken froman early study on photochromism of amorphous transitionmetal oxides by Colton et al.34 They showed that TMO filmscan be bleached (decolored) by thermal annealing at 300 °C inan oxidizing atmosphere, preventing simultaneously any further(post)coloration. However, high temperature annealing is notdesirable for many applications due to the temperaturesensitivity of underlying films, such as for perovskite solarcells,39 silicon heterojunction solar cells,40 or other organicoptoelectronic devices.5 To date though, an alternativebleaching method at low temperature has yet to be reported.Here, we study the coloration of TMOs, induced by

temperature, Ar plasma exposure, and TCO overlayer sputterdeposition, and demonstrate a low-temperature bleachingtreatment based on CO2 plasma exposure. We show how thistreatment can prevent and recover TMO layer coloration whenapplied respectively before or after Ar plasma exposure. Toillustrate this method, we investigate and compare twocommonly used substoichiometric metal oxides: molybdenumoxide (MoOx, x < 3) and tungsten oxide (WOx, x < 3). Theirchemical and optical properties are characterized by UV−visspectrophotometry and X-ray photoelectron spectroscopy(XPS), showing a larger resilience for WOx to sputter damage.Finally, we apply the knowledge gained from these findings insemitransparent perovskite solar cells, showing how parasiticabsorption can be strongly reduced in the transparentelectrode.

2. EXPERIMENTAL SECTIONMetal Oxide Thin Film Deposition and Characterization.

MoOx, WOx, and vanadium oxide (V2Ox) were deposited by thermalevaporation on SCHOTT AF32 glass substrates at a rate of 0.4 Å/s,using stoichiometric powder (MoO3 Alfa Aesar Puratronic, 99.9995%,WO3, Sigma-Aldrich, 99.995%, and V2O5, Sigma-Aldrich, 99.99%) inMo and W closed boats. The thickness was fixed at ∼10 nm, unlessstated otherwise. The base pressure was (3−5) × 10−6 mbar. Allplasma treatments were carried out in a custom-made parallel platePECVD system with RF excitation frequency fixed at 81.36 MHz (25°C, 0.95 mbar, 100 sccm, 88 mW/cm2). The Ar plasma treatment wascarried out for 10 min and the CO2 plasma treatment for 20 min. UVlight irradiation tests were made using an ELC 500 light exposuresystem.Spectrophotometric measurements were conducted with a

PerkinElmer Lambda 900 spectrophotometer using an integratingsphere. XPS data were obtained on a Kratos AXSI Ultra with an energyresolution of 100 meV, for which the metal oxide layers weredeposited on AF32 glass substrates. XPS spectra were fitted with theGaussian−Lorentzian product using the Multipak software package.Binding energies were calibrated according to the carbon peak C 1s(284.6 eV), and all XPS curves were normalized for better comparison.The Raman shift of the films was measured in a confocal Ramanmicroscope with an excitation wavelength of 514 nm (Monovista CRS+, S&I GmbH).Device Fabrication and Characterization. Perovskite solar cells

were fabricated with the same method described in our previouswork.9 These cells are based on a low-temperature, planar devicearchitecture, using a fullerene-based electron transport layer and aperovskite absorber formed by sequentially depositing PbI2 by thermalevaporation and spin coating of methylammonium iodide dissolved in2-propanol, followed by thermal annealing at 100 °C. After thedeposition of a 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorenes (spiro-OMeTAD) hole-transport layer, a 10 nm thickTMO buffer layer was deposited by thermal evaporation at a

deposition rate of 0.4 A/s. Finally, the transparent electrode wasdeposited by radio-frequency sputtering of a 100 nm thick IO:H/ITOlayer stack. Illuminated current density−voltage cell measurementswere carried out on a two-lamp class AAA WACOM sun simulatorwith an AM1.5g irradiance spectrum at 1000 W m−2, using laser-cutaperture masks to define an illuminated area of 0.25 cm2. Externalquantum efficiency (EQE) spectra were measured on a custom-madespectral response setup equipped with a xenon lamp, a gratingmonochromator, and lock-in amplifiers.

3. RESULTS AND DISCUSSIONFigure 1 shows the absorptance and optical bandgap energiescalculated from Tauc plots for three thermally evaporatedTMOs: MoOx, WOx, and V2Ox. These TMO layers had adevice-relevant thickness (∼8−12 nm), while keeping in mindthat their coloration is a bulk effect and thus scales withthickness.35 For application as a front window layer in solarcells, V2Ox is not suitable because of its narrower opticalbandgap, causing a blue cutoff, and was therefore not furtherconsidered in this study. We chose to use MoOx for mostexperiments presented here, as it is the most studied and widelyused TMO and was already demonstrated to work well inseveral types of photovoltaic devices, including perovskite andsilicon heterojunction solar cells.6,7,18

Figure 2a shows the main subject investigated in this paper:the synergistic absorptance effect observed in sequentiallydeposited TMO/TCO layer stacks, illustrated by the exampleof MoOx. To demonstrate that this effect is not limited toMoOx, we tested other TMOs, namely WOx and V2Ox, andobserved a similar synergistic absorptance increase (Figure S1).By testing several types of TCO overlayers such as ITO, IZO,IO:H, and ZnO, we found that their chemical composition doesnot seem to affect the observed absorptance effect (Figure S2).Moreover, when depositing the TMO onto the TCO layer, thiseffect was not observed (Figure S1). These findings confirmthat the synergistic absorptance increase is exclusively due to amodification of the TMO layer during the TCO sputteringprocess. Comparing the observed color changes to opticalspectra reported in the literature indicates that the TMO layeris reduced during TCO sputtering, resulting in the formation ofadditional oxygen vacancies, as evidenced by a broad sub-bandgap absorption peak centered at a wavelength of ∼800nm.5 During TCO deposition by sputtering, the samples are in

Figure 1. Absorptance spectra and Tauc plots of ∼10 nm thickevaporated MoOx, WOx, and V2Ox layers on glass.

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contact with a plasma, which will slightly heat the sample (<100°C) and lead to exposure with UV light as well as ionbombardment. In order to gain a better understanding of theTMO modifications, we assessed each of these factorsindividually.The effect of thermal annealing on the absorption of a MoOx

film is shown in Figures 2b and 2c. When annealed in air, theMoOx layer absorptance increases up to ∼200 °C and thenstarts to bleach again at higher temperatures (Figure 2b). Thistrend can be explained by the competition between a reduction(Mo6+ to Mo5+) reaction, dominant at low temperatures, andan oxidation (Mo5+ to Mo6+) process at high temperatures inthe oxygen-rich atmosphere, confirming the observationsreported by Colton et al.34 As expected, in an inert atmosphere,such as nitrogen, the same annealing treatment results in asimilar coloration at low temperatures (<200 °C) (Figure 2c).Leftheriotis et al. showed similar coloration of metal oxide filmswhen annealed in a vacuum.26 They also demonstrated thatbleaching the films by annealing in an oxygen-rich atmosphereat high temperature led to a polycrystalline film, losing its as-deposited amorphous nature. High-temperature reoxidation cantherefore be excluded as a bleaching solution for temperature-sensitive optoelectronic applications.Figures 2d and 2e show the effect of full Ar plasma or only of

UV-light irradiation on the optical absorptance of a MoOx

layer. We observe that the coloration scales with the power ofthe plasma and therefore with bombardment energy.29 Theinfluence of Ar ion bombardment was already widely reportedand known to reduce oxides to lower stoichiometry due to thepreferential sputtering of oxygen.33 Also, when removing theeffect of ion bombardment by protecting the sample with aquartz plate, thus exposing the sample only to plasmaluminescence, the layer is still affected. This effect is notspecific for transition metal oxide; similar behavior was alsoobserved for thin amorphous silicon passivation layers used insilicon heterojunction technology.41 UV light can also result incoloration, as shown in Figure 2e and reported elsewhere.27

After identifying that elevated temperature, UV light, and Arion bombardment all contribute to the coloration of TMOlayers during TCO overlayer deposition, we assessed severalapproaches to prevent coloration or bleach colored TMO layersat low temperatures.First, we explored oxygen plasma30,42,43 and UV-ozone28,44

treatments, which are known to modify the work function andchange the stoichiometry of metal oxide films. We thereforeexposed TMO layers to UV-ozone and could completely bleachthe colored films (Figure S3). However, when re-exposed to anAr plasma, the coloration recovered to a large extent the initialintensity. O2 plasma exposure was found to be inefficient as anoxidizing method (Figure 2f) due to the presence of ionbombardment having a reduction effect similar, but to a lowerextent, to the one of an Ar plasma treatment.43 This findingalso suggests that introducing oxygen in the TCO sputteringgas mixture would not help to lower the reducing effect of theplasma.To the best of our knowledge, the use of a CO2 plasma has

not yet been proposed to engineer the optical properties ofTMOs. Figures 3a and 3c show the absorptance spectra forMoOx and WOx, when treated with CO2 and Ar plasmas. TheAr plasma was used here to “mimic” the effect of a TCOdeposition by sputtering on a TMO layer, without actual filmdeposition. For comparison, Figures 3b and 3d show the closer-to-device cases with the full TMO/TCO stacks, with or withoutthe CO2 plasma treatment before TCO sputtering. FromFigure 3, three main effects can be observed: (1) CO2 plasmapretreating of as-deposited MoOx or WOx films considerablyreduces the damage caused by a subsequent Ar plasma and thuscan prevent, at least partially, the coloration observed in TMO/TCO stacks; (2) a CO2 plasma can be used to effectively bleacha colored film, which has been treated with an Ar plasma, andthus recover the as-deposited optical properties; (3) comparedto MoOx, WOx is inherently less affected by Ar plasmaexposure and ion bombardment and can be completelybleached by CO2 plasma exposure.

Figure 2. Absorptance spectra of MoOx films: (a) as a bare film, in a stack with an indium zinc oxide (IZO) overlayer, and compared to theabsorptance of a bare IZO layer; (b) annealed in air at temperatures ranging from 100 to 500 °C; (c) annealed in air and N2 with and without IZOoverlayer; (d) Ar plasma treated for 5 min; (e) irradiated with UV light for 10 min; (f) O2 and N2 plasma treated.

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In order to better understand the chemical modificationsinduced to the MoOx and WOx films by the plasma treatmentsdiscussed above, XPS measurements were carried out oncotreated samples. The full set of XPS spectra are given inFigures S4 and S5. We only focus on the Mo and W core levelsand carbon peaks to draw conclusions on changes in thechemical states of these elements due to the plasma treatments.

Figure 4 shows the core levels of MoOx and WOx, theirrespective carbon peaks, and the fraction of their oxidationstates measured by fitting the core level curves after the plasmatreatments. Both Mo and W are known to have several stableoxidation states,5 and it is widely accepted that they can beidentified from their binding energies. In addition, the impact ofAr plasma exposure on MoO3 and WO3 oxides (with Mo6+ and

Figure 3. Absorptance spectra of MoOx and WOx thin films after plasma treatments (a, c) and with a sputtered IO:H/ITO TCO bilayer depositedon top of the TMOs (b, d). The Ar and CO2 plasma treatments were carried out for 10 and 20 min, respectively.

Figure 4. XPS spectra of MoOx and WOx before and after several plasma treatments: for MoOx, (a) Mo 3d, (b) C 1s, and (c) Mo oxidation states;and for WOx (d) W 4f, (e) C 1s, and (f) W oxidation states. Panels c and f show the oxidation state distribution of Mo and W present in the TMOlayers and are derived from the fitting of the curves in panels a and d. Details on the fits are given in the Supporting Information in Figure S6. TheTMO layers for these XPS measurements were coevaporated with the layers used for spectrophotometry shown in Figure 3.

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W6+ oxidation states) has been studied in the past, and it hasbeen confirmed by XPS measurements that it leads to theappearance of lower oxidation states of the metal atoms.33

Figure 4a shows the two characteristic core level peaks ofMo, at 232.7 eV for Mo 3d5/2 and at 235.8 eV for Mo 3d3/2, inthe as-deposited MoOx film, in good agreement with theliterature.27 This as-deposited MoOx is largely composed Mo inthe 6+ oxidation state, and the fraction of Mo6+ even increasesafter CO2 plasma treatment, proving the oxidizing effect of thistreatment. The Ar plasma treatment carried out on as-depositedMoOx dramatically reduces its oxygen content and causes thefilms to be largely composed of 5+ and 4+ oxidation states,which are at the origin of the large absorptance of this film, asshown in Figure 3a. Conversely, the CO2 plasma-treated MoOxis quite resilient to Ar plasma exposure and still contains 88 at.% Mo6+ oxidation states after going through the same Arplasma treatment, resulting in the lower optical absorptancecompared to the sample without CO2 plasma pretreatment.The as-deposited WOx has a large content (∼25 at. %) of

W5+ which is significantly larger than the Mo5+ content in as-deposited MoOx layers. The effects of CO2 and Ar plasmaexposure follow the same trend as for the MoOx layers: Thecontent of Mo6+ increases after CO2 plasma treatment, and thefilm becomes then resilient to subsequent Ar plasma treatment,resulting in a final W5+ content <10 at. %. This confirms thespectrophotometric observations in Figure 3c that a WOx filmcan be totally bleached by a CO2 plasma and that a CO2 plasmapretreatment prevents further coloration. This pretreatmentalso prevents a valence band shift and the appearance of Mo d-states in the bandgap observed for the untreated samples afterAr plasma exposure, as shown in Figure S5c for MoOx andFigure S5g for WOx.Further investigations will be necessary to determine the

origin of this resilience after CO2 plasma treatment, as observedin our current set of measurements. We carried out apreliminary Raman spectroscopy measurement of MoOx layerson glass to determine their structural properties, as shown inFigure S7. It clearly indicates that the as-deposited layer areamorphous, with very broad peaks around the expectedposition of the α-phase peaks, as shown in Figure S7a. TheRaman spectrum is not affected by the CO2 plasma treatment,which indicates that the observed resilience dos not originatefrom structural changes. One possible explanation in the case ofMoOx could be the formation of Mo−O−C bonds that appearsas a shoulder in the C 1s peak after the CO2 plasma treatment.In fact, such a shoulder emerges in the C 1s core-level spectraof Ar plasma-treated samples relative to the as-deposed MoOx(Figure 4b). However, the same effect of the CO2 plasma is notvisible in WOx XPS spectra.Furthermore, it is evident from the spectrophotometric

measurements that WOx is intrinsically more resilient to ionbombardment than MoOx. A possible explanation for thisdifference was formulated by Meyer et al.:8 WOx could formnanocrystalline clusters (WO3)n, which offer large cross sectionfor incoming particles such as Ar+ ions. Another possibleexplanation involves the comparison of the standard reductionpotentials of MoO3 and WO3, +0.075E°/V and −0.090E°/V,respectively.45 The more positive the reduction potential of amaterial, the more readily it can be reduced. Therefore, MoO3is intrinsically more prone to reduction compared to WO3.Application in Solar Cells. As mentioned above, the

coloration of TMO layers can induce optical losses when usedfor transparent electrodes of optoelectronic devices and could

therefore be detrimental to device performance. On the basis ofour present findings, we implemented different TMO/TCOstacksuntreated and exposed to CO2 plasmain solar cells,where they have recently found widespread application.Specifically for semitransparent perovskite solar cells forbuilding integration or tandem applications, TMOs are usedunderneath the transparent top electrode to avoid sputterdamage to the sensitive charge transport and perovskite layersduring TCO sputtering.6 Until now, MoOx has typically beenused for these applications.7,9,46 Duong et al. recently pointedout that a significant part of the sub-bandgap absorptance oftheir mesoscopic semitransparent perovskite solar cellsoriginates from the MoOx/ITO electrode.46 Based on theresults shown in Figure 3, it would be beneficial, at least interms of optical properties, to replace MoOx by WOx andpossibly to introduce a CO2 plasma treatment between theTMO evaporation and the TCO sputtering processes. Wetherefore implemented this absorption mitigation strategy tosemitransparent perovskite solar cells, with a device architectureas shown in the inset to Figure 5, and obtained similar electrical

performances with WOx and MoOx layers, with or without CO2plasma treatment (Figure S8), showing steady efficienciesbetween 11 and 13% during maximum power-point tracking.WOx-based cells showed slightly increased series resistances,which we attribute to a nonoptimized thickness. Further workwill be necessary to optimize the electrical performance of thesecells.The optical gains of these cells due to the CO2 plasma

treatment and the use of WOx are illustrated in Figures 5 and 6.Figure 5 shows the external quantum efficiency (EQE)measurements of semitransparent perovskite cells with WOxand MoOx layers. Because of the reduced parasitic absorptionlosses, the WOx-based cell has a higher spectral responsecompared to the MoOx-based cell, when illuminated throughthe transparent electrode comprising the TMO/TCO stack.This configuration is particularly important for monolithicperovskite-based tandem cells (such as perovskite on silicon orperovskite on CIGS), where the perovskite top cell has to beilluminated through this electrode. When illuminated from theother side (glass substrate side), the two cells show similar

Figure 5. External quantum efficiency of semitransparent perovskitesolar cells with either MoOx or WOx as protective buffer layer.Illuminated from either hole-collecting (HTL) or electron-collecting(ETL) side, as defined in the schematic of the cell.

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EQE. This case is relevant for single-junction devices or for topcells in mechanically stacked four-terminal tandems. However, again can be observed in the sub-bandgap spectral region, asillustrated by Figure 6. Low values in this graph indicate hightransmittance through the perovskite top cell. Previouslyreported semitransparent perovskite solar cells had over 20%sub-bandgap absorptance due to the use of FTO-coated glasssubstrates and suboptimal layer stacks.6,7 After changing thesubstrates to ITO, the sub-bandgap absorptance could bereduced to 10−12%,9,46 corresponding to the MoOx as-deposited case in Figure 6. Replacing this MoOx layer by aWOx layer with a CO2 plasma treatment helped to furtherreduce these sub-bandgap parasitic absorptions by 2−3%abs. Ina tandem configuration, where this near-infrared light istransmitted to a low-bandgap bottom cell (e.g., a wafer-basedsilicon solar cell), this would therefore lead to an improvedbottom cell photocurrent. The implications of these results formonolithic perovskite/silicon heterojunction tandem solar cellsare illustrated by the EQE curves shown in Figure S9. Currentgains in both top (+0.2 mA/cm2) and bottom (+0.7 mA/cm2)cells were observed when replacing MoOx by WOx,demonstrating the benefit of using WOx in such tandem cells.The remaining parasitic absorptance is then mostly due to theabsorptance in the electron and hole transporting layers. Thesemeasurements clearly demonstrate that replacing MoOx byWOx and applying a CO2 plasma pretreatment prior to TCOdeposition can strongly reduce the parasitic absorption lossesand increase photocurrent in both the top cell and the bottomcell of tandem devices.Silicon heterojunction solar cells could also profit from these

findings. Indeed, MoOx was recently demonstrated to be anefficient hole collector for this class of devices,18 opening theroad toward fully doping-free silicon photovoltaics.20 The CO2plasma treatment may however be difficult to apply withoutdamaging the passivation quality of the underlying intrinsicamorphous silicon layer, as illustrated by the lifetime measure-ments shown in Figure S10. Also, it was shown that for thisapplication a certain substoichiometry is beneficial if notnecessary to ensure an appropriate band line-up, needed for alow-resistivity contact to the silicon layers.14,47 As we haveshown, the CO2 plasma improves the stoichiometry of theTMO films by filling the oxygen vacancies, reducing the

number of defect states that are responsible for the efficienthole extraction. Nevertheless, the replacement of MoOx byWOx might already provide significant optical gains to justifyfurther investigation.21 It was also recently shown that WOxrequires a nearly perfect stoichiometry to achieve similar bandbending as obtained with p-type amorphous silicon.48 A CO2plasma, or another similar technique, could therefore beinteresting to control and adjust the stoichiometry ofsubstoichiometric TMO layers deposited by thermal evapo-ration. MoOx/WOx stacks could also be explored as they couldprovide the better electronic properties of MoOx and at thesame time the resilience toward plasma damages of WOx.

4. CONCLUSIONS

We have shown an effective and low-temperature method basedon CO2 plasma exposure to bleach a colored TMO film andprevent further reduction by subsequent Ar plasma exposure.XPS analysis showed that this treatment strongly oxidizes theTMO layers. By comparing MoOx and WOx, we have shownthat WOx is inherently more resilient to sputtering damage, andby pretreating it with a CO2 plasma, any further colorationduring Ar plasma exposure can be completely avoided. Thesefindings are particularly important in solar cells where parasiticabsorption has a detrimental impact on device performance.We therefore compared WOx and MoOx in semitransparentperovskite solar cells and showed that the use of WOx leads tooptical gains. These findings are important for the developmentof highly transparent electrodes based on metal oxides andTCOs, as those used for perovskite solar cells, perovskite/silicon tandem solar cells, and silicon heterojunction solar cells,and are expected to enable device efficiency improvements.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b04425.

Complementary UV−vis spectrophotometric and XPSmeasurements with fitting details; J−V and EQEmeasurements of semitransparent perovskite cells withand without CO2 plasma pretreatment and with MoOx

or WOx (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail jeremie.werner@epfl.ch; Ph +41 21 69 54258 (J.W.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We gratefully acknowledge Sylvain Dunand for his support inthe deposition of the MoOx thin films, Arnaud Walter for hishelp in perovskite cell development, Dr. Franz-Josef Haug forRaman measurements, and Pierre Mettraux from EPFLMHMC for XPS measurements. The project comprising thiswork is evaluated by the Swiss National Science Foundationand funded by Nano-Tera.ch with Swiss Confederationfinancing, by the Swiss Federal Office of Energy, under GrantSI/501072-01, by the DOE under the FPaceII project (No. DE-EE0006335), and by the Competence Center Energy andMobility (CCEM) under the project CONNECT-PV.

Figure 6. Sub-bandgap absorptance of semitransparent planarperovskite cells with MoOx or WOx (CO2 plasma treated or withouttreatment) as buffer layer in the rear transparent electrode.

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■ REFERENCES(1) Sunu, S. S.; Prabhu, E.; Jayaraman, V.; Gnanasekar, K. I.;Seshagiri, T. K.; Gnanasekaran, T. Electrical Conductivity and GasSensing Properties of MoO3. Sens. Actuators, B 2004, 101 (1−2), 161−174.(2) Tesfamichael, T.; Arita, M.; Bostrom, T.; Bell, J. Thin FilmDeposition and Characterization of Pure and Iron-Doped Electron-Beam Evaporated Tungsten Oxide for Gas Sensors. Thin Solid Films2010, 518 (17), 4791−4797.(3) Elumalai, N. K.; Vijila, C.; Jose, R.; Uddin, A.; Ramakrishna, S.Metal Oxide Semiconducting Interfacial Layers for Photovoltaic andPhotocatalytic Applications. Mater. Renew. Sustain. Energy 2015, 4 (3),11.(4) Wang, C.; Irfan, I.; Liu, X.; Gao, Y. Role of Molybdenum Oxidefor Organic Electronics: Surface Analytical Studies. J. Vac. Sci. Technol.B 2014, 32 (4), 040801.(5) Meyer, J.; Hamwi, S.; Kroger, M.; Kowalsky, W.; Riedl, T.; Kahn,A. Transition Metal Oxides for Organic Electronics: Energetics, DevicePhysics and Applications. Adv. Mater. 2012, 24 (40), 5408−27.(6) Werner, J.; Dubuis, G.; Walter, A.; Loper, P.; Moon, S.-J.;Nicolay, S.; Morales-Masis, M.; De Wolf, S.; Niesen, B.; Ballif, C.Sputtered Rear Electrode with Broadband Transparency for PerovskiteSolar Cells. Sol. Energy Mater. Sol. Cells 2015, 141, 407−413.(7) Fu, F.; Feurer, T.; Jager, T.; Avancini, E.; Bissig, B.; Yoon, S.;Buecheler, S.; Tiwari, A. N. Low-Temperature-Processed EfficientSemi-transparent Planar Perovskite Solar Cells for Bifacial andTandem Applications. Nat. Commun. 2015, 6, 8932.(8) Meyer, J.; Winkler, T.; Hamwi, S.; Schmale, S.; Johannes, H.-H.;Weimann, T.; Hinze, P.; Kowalsky, W.; Riedl, T. Transparent InvertedOrganic Light-Emitting Diodes with a Tungsten Oxide Buffer Layer.Adv. Mater. 2008, 20 (20), 3839−3843.(9) Werner, J.; Weng, C.-H.; Walter, A.; Fesquet, L.; Seif, J. P.; DeWolf, S.; Niesen, B.; Ballif, C. Efficient Monolithic Perovskite/SiliconTandem Solar Cell with Cell Area > 1 cm2. J. Phys. Chem. Lett. 2016, 7,161−166.(10) Qiu, M.; Zhu, D.; Bao, X.; Wang, J.; Wang, X.; Yang, R. WO3with Surface Oxygen Vacancies as Anode Buffer Layer for HighPerformance Polymer Solar Cells. J. Mater. Chem. A 2015, 4, 1−7.(11) New, E.; Howells, T.; Sullivan, P.; Jones, T. S. Small MoleculeTandem Organic Photovoltaic Cells Incorporating an a-NPD OpticalSpacer Layer. Org. Electron. 2013, 14 (9), 2353−2359.(12) Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.;Heeger, J. New Architecture for High-Efficiency Polymer PhotovoltaicCells Using Solution-Based Titanium Oxide as an Optical Spacer. Adv.Mater. 2006, 18 (5), 572−576.(13) Roy, A.; Park, S. H.; Cowan, S.; Tong, M. H.; Cho, S.; Lee, K.;Heeger, A. J. Titanium Suboxide as an Optical Spacer in Polymer SolarCells. Appl. Phys. Lett. 2009, 95 (1), 013302.(14) Battaglia, C.; Yin, X.; Zheng, M.; Sharp, I. D.; Chen, T.;Mcdonnell, S.; Azcatl, A.; Carraro, C.; Ma, B.; Maboudian, R.; Wallace,R. M.; Javey, A. Hole Selective MoOx Contact for Silicon Solar Cells.Nano Lett. 2014, 14, 967−971.(15) Gretener, C.; Perrenoud, J.; Kranz, L.; Baechler, C.; Yoon, S.;Romanyuk, Y. E.; Buecheler, S.; Tiwari, A. N. Development of MoOxThin Films as Back Contact Buffer for CdTe Solar Cells in SubstrateConfiguration. Thin Solid Films 2013, 535, 193−197.(16) Gao, Y. Metal-Oxides Hole Injection/Extraction layer inOrganic Semiconductor Devices. ECS Trans. 2011, 35 (19), 11−22.(17) Vasilopoulou, M.; Douvas, A. M.; Georgiadou, D. G.; Palilis, L.C.; Kennou, S.; Sygellou, L.; Soultati, A.; Kostis, I.; Papadimitropoulos,G.; Davazoglou, D.; Argitis, P. The Influence of Hydrogenation andOxygen Vacancies on Molybdenum Oxides Work Function and GapStates for Application in Organic Optoelectronics. J. Am. Chem. Soc.2012, 134 (39), 16178−16187.(18) Geissbuhler, J.; Werner, J.; Martin de Nicolas, S.; Barraud, L.;Hessler-Wyser, A.; Despeisse, M.; Nicolay, S.; Tomasi, A.; Niesen, B.;De Wolf, S.; Ballif, C. 22.5% Efficient Silicon Heterojunction Solar CellWith Molybdenum Oxide Hole Collector. Appl. Phys. Lett. 2015, 107(8), 081601.

(19) Gerling, L.; Mahato, S.; Voz, C.; Alcubilla, R.; Puigdollers, J.Characterization of Transition Metal Oxide/Silicon Heterojunctionsfor Solar Cell Applications. Appl. Sci. 2015, 5 (4), 695−705.(20) Bullock, J.; Hettick, M.; Geissbuhler, J.; Ong, A. J.; Allen, T.;Sutter-Fella, C. M.; Chen, T.; Ota, H.; Schaler, E. W.; De Wolf, S.;Ballif, C.; Cuevas, A.; Javey, A. Efficient Silicon Solar Cells withDopant-Free Asymmetric Heterocontacts. Nat. Energy, no. January2016, 1, 15031.(21) Bivour, M.; Temmler, J.; Steinkemper, H.; Hermle, M.Molybdenum and Tungsten Oxide: High Work Function WideBand Gap Contact Materials for Hole Selective Contacts of SiliconSolar Cells. Sol. Energy Mater. Sol. Cells 2015, 142, 34−41.(22) Irfan, I.; Gao, Y. Effects of Exposure and Air Annealing onMoOx Thin Films. J. Photonics Energy 2012, 2 (1), 021213.(23) Zhang, Z.; Xiao, Y.; Wei, H. X.; Ma, G. F.; Duhm, S.; Li, Y. Q.;Tang, J. X. Impact of Oxygen Vacancy on Energy-level Alignment atMoOx/Organic Interfaces. Appl. Phys. Express 2013, 6, 095701.(24) Lin, S.-Y.; Chen, Y.-C.; Wang, C.-M.; Hsieh, P.-T.; Shih, S.-C.Post-annealing Effect Upon Optical Properties of Electron BeamEvaporated Molybdenum Oxide Thin Films. Appl. Surf. Sci. 2009, 255(6), 3868−3874.(25) Siokou, A.; Leftheriotis, G.; Papaefthimiou, S.; Yianoulis, P.Effect of the Tungsten and Molybdenum Oxidation States on theThermal Coloration of Amorphous WO3 and MoO3 Films. Surf. Sci.2001, 482−485, 294−299.(26) Leftheriotis, G.; Papaefthimiou, S.; Yianoulis, P.; Siokou, A.Effect of the Tungsten Oxidation States in the Thermal Colorationand Bleaching of Amorphous WO3 films. Thin Solid Films 2001, 384,298−306.(27) Fleisch, T. H. An XPS Study of the UV Reduction andPhotochromism of MoO3 and WO3. J. Chem. Phys. 1982, 76 (2), 780.(28) Shi, X. B.; Xu, M. F.; Zhou, D. Y.; Wang, Z. K.; Liao, L. S.Improved Cation Valence State in Molybdenum Oxides by Ultraviolet-Ozone Treatments and its Applications in Organic Light-EmittingDiodes. Appl. Phys. Lett. 2013, 102, 233304.(29) Kim, J. H.; Lee, Y. J.; Jang, Y. S.; Jang, J. N.; Kim, D. H.; Song, B.C.; Lee, D. H.; Kwon, S. N.; Hong, M. The Effect of Ar PlasmaBombardment upon Physical Property of Tungsten Oxide Thin Filmin Inverted Top-emitting Organic Light-Emitting Diodes. Org.Electron. 2011, 12 (2), 285−290.(30) Liu, X.; Yi, S.; Wang, C.; Irfan, I.; Gao, Y. Effect of OxygenPlasma Treatment on Air Exposed MoOx Thin Film. Org. Electron.2014, 15 (5), 977−983.(31) Tate, T. J.; Garcia-Parajo, M.; Green. Mi. lon ImplantationEffects in Polycrystalline W03 Thin Films. J. Appl. Phys. 1991, 70 (7),3509−3511.(32) Xie, F. Y.; Gong, L.; Liu, X.; Tao, Y. T.; Zhang, W. H.; Chen, S.H.; Meng, H.; Chen, J. XPS Studies on Surface Reduction of TungstenOxide Nanowire Film by Ar + Bombardment. J. Electron Spectrosc.Relat. Phenom. 2012, 185 (3−4), 112−118.(33) Driscoll, T. J.; McCormick, L. D.; Lederer, W. C. Altered LayerFormation and Sputtering Yields for 5 keV Ar+ Bombardment ofMoO3 and WO3. Surf. Sci. 1987, 187 (2−3), 539−558.(34) Colton, R. J.; Guzman, A. M.; Rabalais, J. W. Photochromismand Electrochromism in Amorphous Transition Metal Oxide Films.Acc. Chem. Res. 1978, 11 (4), 170−176.(35) He, T.; Yao, J. Photochromism of Molybdenum Oxide. J.Photochem. Photobiol., C 2003, 4 (2), 125−143.(36) Scarminio, J.; Lourenco, A.; Gorenstein, A. Electrochromismand Photochromism in Amorphous Molybdenum Oxide Films. ThinSolid Films 1997, 302, 66−70.(37) Dasgupta, B.; Ren, Y.; Wong, L. M.; Kong, L.; Tok, E. S.; Chim,W. K.; Chiam, S. Y. Detrimental Effects of Oxygen Vacancies inElectrochromic Molybdenum Oxide. J. Phys. Chem. C 2015, 119,10592−10601.(38) Meenakshi, M.; Gowthami, V.; Perumal, P.; Sivakumar, R.;Sanjeeviraja, C. Influence of Dopant Concentration on the Electro-chromic Properties of Tungsten Oxide Thin Films. Electrochim. Acta2015, 174, 302−314.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b04425ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

G

(39) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.;D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.;Mosconi, E.; De Angelis, F.; Boyen, H.-G. Intrinsic Thermal Instabilityof Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater.2015, 5, 1500477.(40) Battaglia, C.; Cuevas, A.; De Wolf, S. High-efficiency CrystallineSilicon Solar Cells: Status and Perspectives. Energy Environ. Sci. 2016,9, 1552.(41) Demaurex, B.; De Wolf, S.; Descoeudres, A.; Charles Holman,Z.; Ballif, C. Damage at Hydrogenated Amorphous/Crystalline SiliconInterfaces by Indium Tin Oxide Overlayer Sputtering. Appl. Phys. Lett.2012, 101 (17), 171604.(42) Bao, X.; Zhu, Q.; Wang, T.; Guo, J.; Yang, C.; Yu, D.; Wang, N.;Chen, W.; Yang, R. Simple O2 Plasma Processed V2O5 as an AnodeBuffer Layer for High-Performance Polymer Solar Cells. ACS Appl.Mater. Interfaces 2015, 7 (14), 7613−7618.(43) Lopez-Carreno, L. D.; Benitez, G.; Viscido, L.; Heras, J. M.;Yubero, F.; Espinos, J. P.; Gonzalez-Elipe, R. Oxidation ofMolybdenum Surfaces by Reactive Oxygen Plasma and O2(+)Bombardment: an Auger and XPS Study. Surf. Interface Anal. 1998,26 (4), 235−241.(44) Kim, J.; Suman, C. K.; Noh, S.; Lee, C. Surface Treatment ofMolybdenum Oxide for Performance Improvement of Organic LightEmitting Diodes. Displays 2010, 31 (3), 139−142.(45) Vanysek, P. Electrochemical Series. In CRC Handbook ofChemistry and Physics, 91st ed.; Haynes, M., Ed.; CRC Press: 2010.(46) Duong, T.; Lal, N.; Grant, D.; Jacobs, D.; Zheng, P.; Rahman,S.; Shen, H.; Stocks, M.; Blakers, A.; Weber, K.; White, T. P.;Catchpole, K. R. Semitransparent Perovskite Solar Cell With SputteredFront and Rear Electrodes for a Four-Terminal Tandem. IEEE J.Photovoltaics 2016, 6 (3), 679−687.(47) Battaglia, C.; De Nicolas, S. M.; De Wolf, S.; Yin, X.; Zheng, M.;Ballif, C.; Javey, A. Silicon Heterojunction Solar Cell with PassivatedHole Selective MoOx Contact. Appl. Phys. Lett. 2014, 104, 113902.(48) Mews, M.; Korte, L.; Rech, B. Oxygen Vacancies in TungstenOxide and Their Influence on Tungsten Oxide/Silicon HeterojunctionSolar Cells. Sol. Energy Mater. Sol. Cells 2016, DOI: 10.1016/j.solmat.2016.05.042.

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DOI: 10.1021/acsami.6b04425ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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