Hydrogenation of Phosphorus-Doped Polycrystalline Silicon Filmsfor Passivating Contact Solar CellsThien N. Truong,*,† Di Yan,† Christian Samundsett,† Rabin Basnet,† Mike Tebyetekerwa,† Li Li,‡

Felipe Kremer,§ Andres Cuevas,† Daniel Macdonald,*,† and Hieu T. Nguyen*,†

†Research School of Engineering, College of Engineering and Computer Science, ‡Department of Electronic Materials Engineering,Research School of Physics and Engineering, and §Centre for Advanced Microscopy, The Australian National University, Canberra,ACT 2601, Australia

*S Supporting Information

ABSTRACT: We characterize and discuss the impact of hydrogenationon the performance of phosphorus-doped polycrystalline silicon (poly-Si)films for passivating contact solar cells. Combining various character-ization techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, low-temperature photoluminescence spec-troscopy, quasi-steady-state photoconductance, and Fourier-transforminfrared spectroscopy, we demonstrate that the hydrogen content insidethe doped poly-Si layers can be manipulated to improve the quality of thepassivating contact structures. After the hydrogenation process of poly-Silayers fabricated under different conditions, the effective lifetime and theimplied open circuit voltage are improved for all investigated samples (upto 4.75 ms and 728 mV on 1 Ω cm n-type Si substrates). Notably, sampleswith very low initial passivation qualities show a dramatic improvementfrom 350 μs to 2.7 ms and from 668 to 722 mV.

KEYWORDS: hydrogenation, doped polycrystalline silicon, amorphous silicon, passivating contacts, photoluminescence

■ INTRODUCTION

For decades, stacks consisting of a doped polycrystalline silicon(poly-Si) film and an ultrathin silicon oxide (SiOx) layer havebeen employed in microelectronic devices. They act as activelayers in thin-film transistors because of benefits of gainamplification,1−3 emitter efficiency enhancement,4 resistancereduction,5 and so forth. When a doped poly-Si/SiOx stack isapplied to a crystalline Si (c-Si) solar cell, it forms a carrierselective junction and, at the same time, provides excellentsurface passivation.6−10 These advantages have prompted thedevelopment of novel solar cell structures featuring dopedpoly-Si/SiOx stacks as the carrier selective contacts,11−15

enabling the achievement of very high open circuit voltages(Voc) of up to 733 mV and a light-to-electricity powerconversion efficiency of 26.1%.13

Doped poly-Si passivating contacts can be fabricated byvarious methods: in situ depositing doped poly-Si layers usinga low-pressure chemical-vapor deposition (LPCVD)16 techni-que at intermediate temperatures (∼630 °C), implantingdopants into such LPCVD layers,13 implanting dopants intohydrogenated amorphous silicon (a-Si:H) deposited by aplasma-enhanced chemical-vapor deposition (PECVD) techni-que17,18 or adding dopants in situ during the PECVDprocess,8,19,20 depositing doped a-Si:H films by atmospheric-pressure chemical-vapor deposition,21 or sputtering both thesilicon and the dopant.22 In all of the above cases, a high-temperature annealing step is required to recrystallize the Si

film and activate the dopants; alternatively, a thermal dopantdiffusion process into an intrinsic a-Si:H or poly-Si film can beperformed.23 In general, the doped poly-Si films often containa high density of defects, as recently reported by Nguyen etal.24 These defects can potentially affect the quality of thepassivating-contact structures and thus the overall performanceof the solar cells.On the other hand, the benefits of hydrogenation processes

are well-known for c-Si wafers and solar cells.25−30 Hydro-genation via a forming gas annealing (FGA)31−33 step or viathe deposition of hydrogen-rich thin films such as SiNx:H

34−37

or Al2O3/H30,38,39 has been shown to lead to significant

improvements in the electrical properties of silicon solar cellsby deactivating recombination centers associated with variousimpurities and defects. Moreover, the use of FGA is also well-known for improving the interface between the c-Si substrateand the poly-Si/SiOx stack in passivating contact solarcells.12,30

In principle, hydrogenation techniques could also be used topassivate defects within the doped poly-Si films themselves, ina similar manner to the c-Si substrates, and thus, they couldimprove the overall performance of doped poly-Si/SiOxpassivating contacts. In this work, we explore such possibilities.

Received: November 13, 2018Accepted: January 17, 2019Published: January 17, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 5554−5560

© 2019 American Chemical Society 5554 DOI: 10.1021/acsami.8b19989ACS Appl. Mater. Interfaces 2019, 11, 5554−5560

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First, we investigate the microscopic structure of the dopedpoly-Si/SiOx/c-Si stacks and demonstrate that the doped poly-Si layer contains both amorphous and crystalline siliconphases. We then utilize the distinct luminescence properties ofthose amorphous and crystalline phases to unambiguouslydemonstrate bulk hydrogenation of the doped poly-Si film.After that, we assess the effects of hydrogenation on the film’squality and on its ability to passivate the c-Si substrate, usingvarious independent techniques, including low-temperaturephotoluminescence (PL) spectroscopy, quasi-steady-statephotoconductance (QSSPC), and Fourier-transform infraredspectroscopy (FTIR). The results demonstrate that thehydrogen atoms can passivate nonradiative defects inside thedoped poly-Si layer and thus significantly improve the qualityof the overall passivating-contact structure.Experimental Details. For this study, 1 Ω cm, ⟨100⟩-

oriented float-zone (FZ) n-type c-Si wafers with an initialthickness of ∼275 μm were used. After removing possiblesurface damage in a tetramethylammonium hydroxide etchingsolution, ultrathin layers of SiOx (∼1.3 nm) were grownchemically on both sides. Subsequently, the samples werecoated with 100 nm layers of PECVD a-Si:H on both sides andsubjected to phosphorus diffusion processes at two differenttemperatures (830 and 860 °C). The resultant phosphorussilicate glass layers were removed by means of a dilutedhydrofluoric solution (stage 1). Note that during this high-temperature step a large fraction of the a-Si becomespolycrystalline, which means that, essentially, the poly-Si/SiOx selective contact structure is already formed. Sub-sequently, different hydrogenation treatments were performed.In the first of them, the samples were annealed at 500 °C for30 min in a forming gas mixture consisting of argon andhydrogen (stage 2). They were then coated with 80 nm layersof PECVD SiNx:H on both sides, followed by the same FGAprocess at 500 °C for 30 min (stage 3). Finally, the SiNx layerswere removed by means of a concentrated HF solution and thesamples were annealed again in nitrogen at 600 °C for 2 h(stage 4). It is expected that after this step (stage 4), allhydrogen should have effused out of the doped poly-Si layers,thus permitting to investigate their performance in the absenceof hydrogen. Figure 1 summarizes the process steps mentionedabove. All measurements (PL, QSSPC, and FTIR) were donefor each stage (1−4), after the SiNx layers were removed (ifpresent) by a concentrated HF solution.Transmission electron microscopy (TEM) images were

captured using a JEOL JEM-2100F instrument. TEM lamellaewere prepared by a focused-ion beam (FIB) milling technique.Element maps were obtained by energy-dispersive X-rayspectroscopy (EDS) accessories in the TEM system. Steady-state micro-PL (μ-PL) spectroscopy measurements wereperformed using a HORIBA LabRAM system equipped witha confocal microscope. The incident laser beam was focused to

a 5 μm diameter spot onto the sample surface through a 50×objective lens, and emitted PL signals were directed to amonochromator. A liquid-nitrogen-cooled InGaAs arraydetector with a detection range between 750 and 1600 nmwas used to collect steady-state PL signals. The excitationsource for the PL measurements was a 405 nm solid-state laser.All PL measurements were performed at 80 K using aTHMS600 Linkam stage. Minority-carrier lifetime and impliedopen circuit voltage (iVoc) measurements were performed witha Sinton Instruments WCT-120 contactless photoconductancetester. FTIR experiments were taken in the range of 320−4000cm−1 with a resolution of 4 cm−1.

■ RESULTS AND DISCUSSIONS

First, we demonstrate that the a-Si:H film deposited by thePECVD technique is not fully recrystallized after thephosphorus diffusion process. Figure 2a shows a HAADFimage of the 100 nm n-doped poly-Si/1.3 nm SiOx/c-Sistructure. Figure 2b represents the corresponding elementalmapping of silicon (orange color) and oxygen (yellow color),clearly showing the position of the intermediate oxide layer.Such a position can also be observed in the EDS line scanprofiles of O and Si (Figure 2c), with very clear intensity peaks.Figure 2d shows a high-resolution TEM image of the interface.The interfacial oxide film can be observed as a thin stripe ofamorphous structure between the poly-Si and c-Si, as noted inthe figure. The doped poly-Si layer shows both crystalline andamorphous phases, whereas as expected, the c-Si substrateshows a very uniform crystalline structure. To further confirmthe coexistence of crystalline and amorphous phases in thedoped poly-Si layer, X-ray DPs from both the c-Si substrateand the poly-Si layer are also given in Figure 2d. Thediffraction pattern from the doped poly-Si layer contains bothamorphous and crystalline signatures, revealed by a series ofconcentric rings (for the amorphous phase) and discrete brightspots (for the crystalline phase) (see Figure 2d, right corner),compared with the only bright-spot pattern from the c-Sisubstrate (Figure 2d, left corner).40

In addition, Figure 2e shows a PL spectrum captured fromthe 100 nm n + poly-Si/1.3 nm SiOx/c-Si sample at 80 K,excited with the 405 nm laser. The sharp peak located at∼1125 nm is the band-to-band emission from the c-Sisubstrate.41 The spectrum also contains a very broad peak inthe range 1300−1400 nm. This broad peak originates from theradiative defect states inside the doped poly-Si layer, asrecently reported by Nguyen et al.24 However, there is no PLemission from the amorphous Si phase in the doped poly-Sifilm, which is at stage 1 and not yet hydrogenated.Numerous works have reported a strong PL emission from

a-Si:H films deposited on both c-Si and glass substrates at lowtemperatures. The characteristic a-Si:H peak is much broader

Figure 1. Flow chart of the experimental processes. The SiNx layers were removed before all measurements if present.

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than the c-Si peak and its position varies between 850 and 950nm depending on deposition conditions42−47 (see Figure S1,the Supporting Information). Without hydrogen, amorphousSi films contain an extremely high density of defect states,which likely suppress the broad a-Si:H PL peak. To confirmthis hypothesis, we capture the PL spectra from an a-Si:H filmdeposited by PECVD at 300 °C on top of a SiOx/c-Si samplebefore and after an annealing step at 600 °C in N2 for 2 h. Atthis temperature, the amorphous Si film is not recrystallizedyet, but hydrogen atoms can be expected to escape out of it.The results in Figure 3 show that this is indeed the casebecause the a-Si:H PL peak located at ∼850 nm is completelyquenched by the 600 °C N2 annealing step. Therefore, it issensible to assume that after the even higher temperature(>800 °C) used for dopant diffusion, there will be essentiallyno hydrogen left in the bulk of the poly-Si layer, which explainsthe absence of an a-Si:H PL peak in Figure 2e.The fact, confirmed by TEM and X-ray diffraction, that there

are both amorphous and crystalline phases in the doped poly-Si layer, but the amorphous phase does not emit the PL signalcommonly attributed to a-Si:H, provides us with a uniqueopportunity to evaluate the effects of hydrogenation on the

doped poly-Si layer. PL spectra captured from a doped poly-Sisample at various stages (stages 1−4 in Figure 1) are given inFigure 4a. The spectra at stage 1 (after dopant diffusion) andstage 2 (after FGA) are nearly identical, suggesting thatannealing in forming gas at 500 °C does not affect significantlythe bulk properties of the doped poly-Si layer and the poly-Si/SiOx interface, although it is known to improve the SiOx/c-Siinterface.12,30 However, the spectrum at stage 3 (annealed inthe presence of a hydrogen-rich SiNx film) shows a very clearpeak located at ∼900 nm, which matches the characteristicemission from a-Si:H in previous studies.47 This is strongevidence that the amorphous Si phase has been hydrogenated.Also, the intensity of the peak corresponding to luminescencefrom the poly-Si layer increases after the hydrogenation step,indicating that nonradiative defects inside the poly-Si film havealso been passivated. In addition, the intensity of the c-Si peakalso increases, which indicates an improvement of surfacepassivation, leading to a higher excess carrier concentration inthe c-Si. Although the recombination level at room temper-ature should be different from that at 80 K, the c-Si PL signalfrom the hydrogenated sample should be still higher than thatfrom the nonhydrogenated sample at room temperature, asevidenced by the higher effective carrier lifetime in Figure 4d.To consolidate these conclusions, we bring the sample to

stage 4 (annealing in nitrogen at 600 °C for 2 h). As shownabove, this annealing process removes most of hydrogen fromthe doped poly-Si layer (from both the crystalline andamorphous phases). As can be seen in Figure 4a, the emittedPL spectrum goes back to the shape it had at stages 1 and 2, inwhich the a-Si:H peak is absent and only the doped poly-Sipeak is present. Figure 4b,c shows the integrated PL intensitiesfrom the a-Si:H peak, taken from 750 to 1000 nm to avoidoverlapping with the c-Si peak, and the doped poly-Si peak,taken from 1250 to 1550 nm after subtracting the contributionfrom the a-Si:H peak in this wavelength range (see Figure S2,the Supporting Information), at the different experimentalstages. It can be seen that the effects of hydrogenation are veryclear for both PL peaks and at different diffusion temperatures.It is also noticeable that at the higher diffusion temperature ahigher fraction of the amorphous phase has been recrystallized,thus yielding lower a-Si:H PL intensities after hydrogenation.

Figure 2. (a) High-angle annular dark-field (HAADF) image of thephosphorous-doped poly-Si/SiOx/c-Si structure. The sample wasprepared by FIB milling with a capped platinum layer (bright Ptstrip). The scale bar is 20 nm. (b) TEM−EDS elemental mapping ofthe sample (Si + O + Pt overlayed, Si: orange; O: yellow; Pt: green).The scale bar is 20 nm. (c) TEM−EDS line scan profiles of Si and Owith clear peaks representing the SiOx interface. (d) High-resolutionTEM image of the selected area, with corresponding diffractionpatterns (DPs) from the poly-Si film and c-Si substrate. The scale baris 2 nm. (e) PL spectra from the phosphorous-diffused poly-Si samplecaptured at 80 K using the excitation wavelength of 405 nm with aspot size of 5 μm in diameter. The diffusion process is at 860 °C.

Figure 3. PL spectra from an a-Si/H/SiOx/c-Si sample before (asdeposited by PECVD) and after annealing in nitrogen at 600 °C for 2h. The PL spectra are captured at 80 K using a 405 nm excitationlaser. The c-Si PL peak at around 1125 nm is not observed due to thefact that the 405 nm laser light is mostly absorbed in the 100 nm layerof amorphous silicon film.

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However, Figure 4c shows that the doped poly-Si PL intensityat 860 °C is still lower than that at 830 °C. There are, in fact,two competing effects of radiative and nonradiative defects.The doped poly-Si PL peak originates from the radiativedefects. The fact that this PL peak is smaller for the higherdiffusion temperature (860 °C) could indicate a lower overalldefect concentration inside the 860 °C doped poly-Si film,compared with the 830 °C case. However, within the samefilm, after hydrogenation, the nonradiative defects should behydrogenated, yielding the higher intensity of the doped poly-Si PL peak.

Next, we investigate the impact of hydrogenation on theeffective minority-carrier lifetime (at a minority-carrier densityof 1 × 1015 cm−3) and the implied open circuit voltage (iVoc)(at 1-sun intensity) of silicon wafers coated on both sides witha poly-Si/SiOx passivating contact. The results are shown inFigure 4d. After FGA (stage 2), the passivation qualityincreases for both phosphorus diffusion temperatures, eventhough there is little change in the PL spectrum from thedoped poly-Si layer itself, as shown in Figure 4a. These twopieces of evidence indicate that only the SiOx/c-Si interface isimproved by the FGA step at stage 2. After stage 3, however,hydrogen atoms from the SiNx:H have been introduced into

Figure 4. (a) PL spectra from phosphorus-diffused poly-Si samples at various stages, captured at 80 K using the excitation laser of 405 nm. Thephosphorus diffusion temperature is 860 °C. (b) Integrated PL intensity from the hydrogenated amorphous silicon peak and (c) the dopedpolycrystalline silicon peak. The integrated intensity of the a-Si:H peak is taken from 750 to 1000 nm, whereas that of the doped poly-Si peak istaken from 1250 to 1550 nm after subtracting the contribution of the a-Si:H peak in this wavelength range. (d) Minority-carrier lifetime andimplied open circuit voltage of the samples at various stages. The minority-carrier density is 1 × 1015 cm−3.

Figure 5. FTIR spectra of the samples phosphorus diffused at (a) 830 and (b) 860 °C with and without hydrogenation. The negative baselines aredue to the different backgrounds between the calibration c-Si wafer and the investigated samples.

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both the poly-Si film and the SiOx/c-Si interface, thus resultingin a further improvement of the passivation quality. Theimprovement is particularly important for the sample preparedat 830 °C diffusion temperature because its initial effectivelifetime and iVoc were very low (340 μs and 668 mV,respectively). After hydrogenation, both the lifetime and iVocincrease significantly (>2.5 ms and >720 mV, Figure 4d).These observations were also made for samples where the FGAstep of stage 2 was skipped, that is, for samples that wentdirectly from the formation of the doped poly-Si film (stage 1)to the SiNx-assisted hydrogenation process (stage 3) (seeFigure S3, the Supporting Information).It is interesting to note that the performance of passivating

contacts formed at the higher temperature of 860 °C is evenbetter than that of layers formed at 830 °C, both before andafter hydrogenation. This shows that there is a complexinterplay between levels of crystallinity, doping, and hydro-genation. It is clear, however, that the presence of hydrogen inthe poly-Si film is correlated with achieving high qualitypassivation. After stage 4 (annealed again in N2 at 600 °C for 2h), the lifetimes and iVoc drop dramatically because of theabsence of hydrogen inside the films. However, the effectivecarrier lifetime at stage 4 is less than that at stage 1. Thissuggests that there should be other unknown mechanisms thatcould influence the overall carrier lifetime. For example, afterbeing annealed at 600 °C, the FZ sample may experience the“dead zone” thermal activating defects according to Grant etal.48

Finally, we also verify the presence of Si−H bonds in thepoly films using FTIR measurements, as shown in Figure 5.Comparing the absorbance spectra of the doped poly-Si/SiOx/c-Si stacks with and without hydrogenation, the sample withhydrogenation clearly shows the stretching mode of Si−Hbonds at around 2100−2200 cm−1, consistent with thereported value in the literature.49−52 This observation is validfor all of the hydrogenated samples, prepared at various dopantdiffusion temperatures.

■ CONCLUSIONIn summary, we have identified significant hydrogenationeffects in the bulk of phosphorus-doped polycrystalline siliconlayers and their application for improving solar cell passivatingcontacts. The silicon films were found to contain bothamorphous and crystalline phases, each yielding a distinctluminescence peak at low measurement temperatures. We havethen employed various independent characterization techni-ques to verify the presence of hydrogen inside the films afterthe hydrogenation process. The results have demonstrated thathydrogen can be introduced into the doped silicon films toimprove the quality of the poly-Si/SiOx passivating contacts.We have corroborated the technique on various initial low-quality doped polycrystalline silicon films, all of which havebeen improved significantly after the hydrogenation process.These characterization and hydrogenation techniques open anexciting avenue for optimizing passivating-contact structures.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b19989.

PL spectra from a-Si:H films deposited directly onquartz substrates with various substrate temperatures

and frequency modes; integrated PL intensities between1250−1550 nm from the doped poly-Si peak before andafter subtracting the contribution of the a-Si:H PL peak;and minority-carrier lifetime and implied open circuitvoltage iVoc of various phosphorus-doped poly-Sipassivating-contact samples before and after hydro-genation (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: thien.truong@anu.edu.au (T.N.T.).*E-mail: daniel.macdonald@anu.edu.au (D.M.).*E-mail: hieu.nguyen@anu.edu.au (H.T.N.).ORCIDThien N. Truong: 0000-0002-4082-7999Mike Tebyetekerwa: 0000-0002-4243-6043Hieu T. Nguyen: 0000-0003-1667-1135Author ContributionsH.T.N. conceived the idea, designed the overall experiments,and supervised the project. D.M. co-supervised the project.T.N.T. fabricated samples and performed measurements. D.Y.,C.S., R.B., M.T., L.L., and F.K. contributed to the materialfabrication, characterization, and experimental setup. A.C. andD. M. contributed to the data analysis. T.N.T. and H.T.N.analyzed the data and wrote the manuscript. All authorscontributed to the discussion of the results and reviewed themanuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work has been supported by the Australian RenewableEnergy Agency (ARENA) through Research grant RND017and the Australian Centre for Advanced Photovoltaics (ACAP)Collaboration grant. The authors acknowledge the facility andtechnical support from the Australian National FabricationFacility (ANFF), ACT Node and the Australian Microscopy &Microanalysis Research Facility at the Centre of AdvancedMicroscopy, The Australian National University. The authorsthank Dr Ziv Hameiri at the University of New South Walesfor performing FTIR measurements. H.T.N. acknowledges thefellowship support from the ACAP. M.T. acknowledges theresearch support from the Australian Government ResearchTraining Program (RTP) Scholarship.

■ REFERENCES(1) Ning, T. H.; Isaac, R. D. Effect of Emitter Contact on CurrentGain of Silicon Bipolar Devices. IEEE Trans. Electron. Dev. 1980, 27,2051−2055.(2) Van Halen, P.; Pulfrey, D. L. High-Gain Bipolar Transistors withPolysilicon Tunnel Junction Emitter Contacts. IEEE Trans. Electron.Dev. 1985, 32, 1307−1313.(3) Post, I. R. C.; Ashburn, P.; Wolstenholme, G. R. PolysiliconEmitters for Bipolar Transistors: A Review and Re-Evaluation ofTheory and Experiment. IEEE Trans. Electron. Dev. 1992, 39, 1717−1731.(4) de Graaff, H. C.; de Groot, J. G. The SIS Tunnel Emitter: ATheory for Emitters with Thin Interface Layers. IEEE Trans. Electron.Dev. 1979, 26, 1771−1776.(5) Hamel, J. S.; Roulston, D. J.; Selvakumar, C. R.; Booker, G. R.Two-Dimensional Analysis of Emitter Resistance in the Presence ofInterfacial Oxide Breakup in Polysilicon Emitter Bipolar Transistors.IEEE Trans. Electron. Dev. 1992, 39, 2139−2146.

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DOI: 10.1021/acsami.8b19989ACS Appl. Mater. Interfaces 2019, 11, 5554−5560

5558

(6) Kwark, Y. H.; Swanson, R. M. N-Type SIPOS and Poly-SiliconEmitters. Solid-State Electron. 1987, 30, 1121−1125.(7) Gan, J. Y.; Swanson, R. M. Polysilicon Emitters for SiliconConcentrator Solar Cells. In IEEE Conference on PhotovoltaicSpecialists; IEEE, 1990; pp 245−250.(8) Feldmann, F.; Bivour, M.; Reichel, C.; Hermle, M.; Glunz, S. W.Passivated Rear Contacts for High-Efficiency n-Type Si Solar CellsProviding High Interface Passivation Quality and Excellent TransportCharacteristics. Sol. Energy Mater. Sol. Cells 2014, 120, 270−274.(9) Romer, U.; Peibst, R.; Ohrdes, T.; Lim, B.; Krugener, J.; Bugiel,E.; Wietler, T.; Brendel, R. Recombination Behavior and ContactResistance of N+and P+poly-Crystalline Si/Mono-Crystalline SiJunctions. Sol. Energy Mater. Sol. Cells 2014, 131, 85−91.(10) Peibst, R.; Romer, U.; Larionova, Y.; Schulte-Huxel, H.;Ohrdes, T.; Haberle, M.; Lim, B.; Krugener, J.; Stichtenoth, D.;Wutherich, T.; et al. Building Blocks for Back-Junction Back-Contacted Cells and Modules with Ion-Implanted Poly-Si Junctions.2014 IEEE 40th Photovoltaic Specialist Conference PVSC, 2014; pp852−856.(11) Glunz, S. W.; Feldmann, F.; Richter, A.; Bivour, M.; Reichel,C.; Steinkemper, H.; Benick, J.; Hermle, M. The Irresistible Charm ofa Simple Current Flow Pattern25% with a Solar Cell Featuring aFull-Area Back Contact. 31st European Photovoltaic Solar EnergyConference and Exhibition: Hamburg, 2015; pp 259−263.(12) Stodolny, M. K.; Lenes, M.; Wu, Y.; Janssen, G. J. M.; Romijn,I. G.; Luchies, J. R. M.; Geerligs, L. J. N-Type Polysilicon PassivatingContact for Industrial Bifacial n-Type Solar Cells. Sol. Energy Mater.Sol. Cells 2016, 158, 24−28.(13) Haase, F.; Hollemann, C.; Schafer, S.; Merkle, A.; Rienacker,M.; Krugener, J.; Brendel, R.; Peibst, R. Laser Contact Openings forLocal Poly-Si-Metal Contacts Enabling 26.1%-Efficient POLO-IBCSolar Cells. Sol. Energy Mater. Sol. Cells 2018, 186, 184−193.(14) Richter, A.; Benick, J.; Muller, R.; Feldmann, F.; Reichel, C.;Hermle, M.; Glunz, S. W. Tunnel Oxide Passivating ElectronContacts as Full-Area Rear Emitter of High-Efficiency p-Type SiliconSolar Cells. Prog. Photovoltaics Res. Appl. 2017, 26, 579−586.(15) Merkle, A.; Seren, S.; Knauss, H.; Min, B.; Steffens, J.;Terheiden, B.; Brendel, R.; Peibst, R. Atmospheric Pressure ChemicalVapor Deposition of In-Situ Doped Amorphous Silicon Layers forPassivating Contacts. In 35th European Photovoltaic Solar EnergyConference and Exhibition, 2018; pp 785−791.(16) Tarr, N. G. A polysilicon emitter solar cell. IEEE Electron DeviceLett. 1985, 6, 655−658.(17) Feldmann, F.; Muller, R.; Reichel, C.; Hermle, M. IonImplantation into Amorphous Si Layers to Form Carrier-SelectiveContacts for Si Solar Cells. Phys. Status Solidi RRL 2014, 08, 767−770.(18) Reichel, C.; Feldmann, F.; Muller, R.; Reedy, R. C.; Lee, B. G.;Young, D. L.; Stradins, P.; Hermle, M.; Glunz, S. W. Tunnel OxidePassivated Contacts Formed by Ion Implantation for Applications inSilicon Solar Cells. J. Appl. Phys. 2015, 118, 205701.(19) Feldmann, F.; Reichel, C.; Muller, R.; Hermle, M. TheApplication of Poly-Si/SiOxcontacts as Passivated Top/Rear Contactsin Si Solar Cells. Sol. Energy Mater. Sol. Cells 2017, 159, 265−271.(20) Nogay, G.; Stuckelberger, J.; Wyss, P.; Rucavado, E.; Allebe, C.;Koida, T.; Morales-Masis, M.; Despeisse, M.; Haug, F.-J.; Loper, P.;et al. Interplay of Annealing Temperature and Doping in HoleSelective Rear Contacts Based on Silicon-Rich Silicon-Carbide ThinFilms. Sol. Energy Mater. Sol. Cells 2017, 173, 18−24.(21) Borden, P.; Xu, L.; McDougall, B.; Chang, C. P.; Pysch, D.;Voisin, P.; Polysilicon, S. W. G. Tunnel Junctions as Alternates toDiffused Junctions 23rd European Photovoltaic Solar Energy Conferenceand Exhibition: Valencia, Spain, 2008; pp 1149−1152.(22) Yan, D.; Cuevas, A.; Phang, S. P.; Wan, Y.; Macdonald, D. 23%Efficient P-Type Crystalline Silicon Solar Cells With Hole-SelectivePassivating Contacts Based on Physical Vapor Deposition of DopedSilicon Films. Appl. Phys. Lett. 2018, 113, 61603.

(23) Yan, D.; Cuevas, A.; Bullock, J.; Wan, Y.; Samundsett, C.Phosphorus-Diffused Polysilicon Contacts for Solar Cells. Sol. EnergyMater. Sol. Cells 2015, 142, 75−82.(24) Nguyen, H. T.; Liu, A.; Yan, D.; Guthrey, H. L.; Truong, T. N.;Li, Z. Z.; Li, Z. Z.; Al-jassim, M. M.; Cuevas, A.; Tebyetekerwa, M.;et al. Sub-Bandgap Luminescence from Doped Poly and AmorphousSilicon Films and Its Application to Understanding Passivating-Contact Solar Cells. ACS Appl. Energy Mater. 2018, 1, 6619−6625.(25) Wu, I.-W.; Huang, T.-Y.; Jackson, W. B.; Lewis, A. G.; Chiang,A. Passivation Kinetics of Two Types of Defects in Polysilicon TFTby Plasma Hydrogenation. IEEE Electron Device Lett. 1991, 12, 181−183.(26) Mews, M.; Schulze, T. F.; Mingirulli, N.; Korte, L. HydrogenPlasma Treatments for Passivation of Amorphous-Crystalline Silicon-Heterojunctions on Surfaces Promoting Epitaxy. Appl. Phys. Lett.2013, 102, 122106.(27) Sio, H. C.; Phang, S. P.; Trupke, T.; Macdonald, D. Impact ofPhosphorous Gettering and Hydrogenation on the Surface Recombi-nation Velocity of Grain Boundaries in P-Type MulticrystallineSilicon. IEEE J. Photovoltaics 2015, 5, 1357−1365.(28) Hallam, B. J.; Hamer, P. G.; Wang, S.; Song, L.; Nampalli, N.;Abbott, M. D.; Chan, C. E.; Lu, D.; Wenham, A. M.; Mai, L.; et al.Advanced Hydrogenation of Dislocation Clusters and Boron-OxygenDefects in Silicon Solar Cells. Energy Procedia 2015, 77, 799−809.(29) Hamer, P.; Bourret-Sicotte, G.; Martins, G.; Wenham, A.;Bonilla, R. S.; Wilshaw, P. A Novel Source of Atomic Hydrogen forPassivation of Defects in Silicon. Phys. Status Solidi RRL 2017, 11,1600448.(30) Schnabel, M.; van de Loo, B. W. H.; Nemeth, W.; Macco, B.;Stradins, P.; Kessels, W. M. M.; Young, D. L. Hydrogen Passivation ofPoly-Si/SiO x Contacts for Si Solar Cells Using Al 2 O 3 Studied withDeuterium. Appl. Phys. Lett. 2018, 112, 203901.(31) Hallam, B. J.; Hamer, P. G.; Wenham, S. R.; Abbott, M. D.;Sugianto, A.; Wenham, A. M.; Chan, C. E.; Xu, G.; Kraiem, J.;Degoulange, J.; et al. Advanced Bulk Defect Passivation for SiliconSolar Cells. IEEE J. Photovoltaics 2014, 4, 88−95.(32) Cuevas, A.; Allen, T.; Bullock, J. Skin Care for Healthy SiliconSolar Cells. 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC)2015, No. 1; pp. 1−6.(33) Yan, D.; Cuevas, A.; Wan, Y.; Bullock, J. Passivating Contactsfor Silicon Solar Cells Based on Boron-Diffused RecrystallizedAmorphous Silicon and Thin Dielectric Interlayers. Sol. EnergyMater. Sol. Cells 2016, 152, 73−79.(34) Cuevas, A.; Kerr, M. M. J.; Schmidt, J. Passivation of CrystallineSilicon Using Silicon Nitride. 3rd World Conference on PhotovoltaicEnergy Conversion; Osaka, Japan, 2003; pp 913−918.(35) De Wolf, S.; Beaucarne, G. Surface Passivation Properties ofBoron-Doped Plasma-Enhanced Chemical Vapor Deposited Hydro-genated Amorphous Silicon Films on p-Type Crystalline SiSubstrates. Appl. Phys. Lett. 2006, 88, 022104.(36) Wan, Y.; Yan, D.; Bullock, J.; Zhang, X.; Cuevas, A. Passivationof C-Si Surfaces by Sub-Nm Amorphous Silicon Capped with SiliconNitride. Appl. Phys. Lett. 2015, 107, 231606.(37) Yan, D.; Cuevas, A.; Wan, Y.; Bullock, J. Silicon Nitride/SiliconOxide Interlayers for Solar Cell Passivating Contacts Based onPECVD Amorphous Silicon. Phys. Status Solidi RRL 2015, 9, 617−621.(38) Hoex, B.; van de Sanden, M. C. M.; Schmidt, J.; Brendel, R.;Kessels, W. M. M. Surface Passivation of Phosphorus-Diffused N+-Type Emitters by Plasma-Assisted Atomic-Layer Deposited Al2O3.Phys. Status Solidi RRL 2011, 6, 4−6.(39) Bullock, J.; Yan, D.; Cuevas, A. Passivation of Aluminium-N+Silicon Contacts for Solar Cells by Ultrathin Al2O3 and SiO2Dielectric Layers. Phys. Status Solidi RRL 2013, 7, 946−949.(40) Williams, D. B.; Carter, C. B. The Transmission ElectronMicroscope. Transmission Electron Microscopy; Springer US: Boston,MA, 2009; pp 3−22.(41) Nguyen, H. T.; Rougieux, F. E.; Mitchell, B.; Macdonald, D.Temperature Dependence of the Band-Band Absorption Coefficient

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b19989ACS Appl. Mater. Interfaces 2019, 11, 5554−5560

5559

in Crystalline Silicon from Photoluminescence. J. Appl. Phys. 2014,115, 043710.(42) Pankove, J. I.; Carlson, D. E. Electroluminescence inAmorphous Silicon. Appl. Phys. Lett. 1976, 29, 620−622.(43) Engemann, D.; Fischer, R. Photoluminescence in AmorphousSilicon. Phys. Status Solidi 1977, 79, 195−202.(44) Alvarez, F.; Williams, F. Photoluminescence of HydrogenatedAmorphous Silicon. J. Non-Cryst. Solids 1982, 50, 139−148.(45) Rauscher, B.; Pfleiderer, H.; Bullemer, B. Tail-Tail Lumi-nesccnce in Amorphous Silicon. Phys. Status Solidi 1983, 78, 623−632.(46) Murayama, K.; Sagawa, R.; Monji, K.; Tsushima, K.; Deki, H.Luminescence Gap in Hydrogenated Amorphous Silicon. J. Non-Cryst.Solids 2012, 358, 2027−2030.(47) Nguyen, H. T.; Rougieux, F. E.; Yan, D.; Wan, Y.; Mokkapati,S.; de Nicolas, S. M.; Seif, J. P.; De Wolf, S.; Macdonald, D.Characterizing Amorphous Silicon, Silicon Nitride, and DiffusedLayers in Crystalline Silicon Solar Cells Using Micro-Photo-luminescence Spectroscopy. Sol. Energy Mater. Sol. Cells 2016, 145,403−411.(48) Grant, N. E.; Markevich, V. P.; Mullins, J.; Peaker, A. R.;Rougieux, F.; Macdonald, D. Thermal Activation and Deactivation ofGrown-in Defects Limiting the Lifetime of Float-Zone Silicon. Phys.Status Solidi RRL 2016, 10, 443−447.(49) Anutgan, M.; Anutgan, T.; Atilgan, I.; Katircioglu, B.Photoluminescence Analyses of Hydrogenated Amorphous SiliconNitride Thin Films. J. Lumin. 2011, 131, 1305−1311.(50) Hallam, B.; Tjahjono, B.; Wenham, S. Effect of PECVD SiliconOxynitride Film Composition on the Surface Passivation of SiliconWafers. Sol. Energy Mater. Sol. Cells 2012, 96, 173−179.(51) Debieu, O.; Nalini, R. P.; Cardin, J.; Portier, X.; Perriere, J.;Gourbilleau, F. Structural and Optical Characterization of Pure Si-Rich Nitride Thin Films. Nanoscale Res. Lett. 2013, 8, 31.(52) Lozac’h, M.; Nunomura, S.; Sai, H.; Matsubara, K. PassivationProperty of Ultrathin SiOx:H/a-Si:H Stack Layers for Solar CellApplications. Sol. Energy Mater. Sol. Cells 2018, 185, 8−15.

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DOI: 10.1021/acsami.8b19989ACS Appl. Mater. Interfaces 2019, 11, 5554−5560

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