Selective Contact for Crystalline Silicon Solar Cells

Full paper Silicon Solar Cells

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Tantalum Nitride Electron-Selective Contact for Crystalline Silicon Solar Cells Xinbo Yang,* Erkan Aydin, Hang Xu, Jingxuan Kang, Mohamed Hedhili, Wenzhu Liu, Yimao Wan, Jun Peng, Christian Samundsett, Andres Cuevas, and Stefaan De Wolf* the bandgap of silicon, resulting in large recombination losses (up to 50%) in highefficiency c-Si solar cells.[1] Because of this, the mainstream industrial solar cells (i.e., the so-called screen-printed Al-back surface field cell), have a practical efficiency limit of only ≈19.5% with currently available passivation and metallization technologies.[2] The concept of “passivated emitter and rear locally diffused cell” (PERL) partially resolves this issue by minimizing the contact fraction and heavily doping the silicon surface underneath the metal contact. With complex photolithography patterning steps, this concept led to a champion PCE of up to 25%.[3] However, the industrial production of PERL cells has limitations in terms of performance and requires elaborate and cost-intensive processing. A simpler approach that does not rely on any doping/patterning is to use dopant-free carrier-selective contacts (DF-CSCs), sometimes referred to as passivating contacts, which significantly reduce carrier recombination in both contact and noncontact regions. Using DF-CSCs enable high operating voltages and hence a high PCE.[4] Initially, single ultrathin films (e.g., SiO2, Al2O3, and a-Si: H), which were inserted between the doped/undoped silicon surface and the metal contact, have been investigated as simple passivatingcontact layers for c-Si solar cells.[5–7] However, the ultrathin films suffer from poor stability during metallization, and no obvious efficiency improvement has been achieved on a device level. A series of thin films with extremely low or high work function and/or suitable band offsets with silicon (Figure 1) were recently investigated as potential DF-CSCs for c-Si solar cells.[8–26] For example, thin films with either a high work function (e.g., MoOx) or a small valence band offset with silicon (e.g., CuO) (see Figure 1, left side) were developed as holeselective contacts. Thin films with a low work function (e.g., LiF, Mg) or a small conduction band offset with silicon (e.g., TiO2) were also developed as electron-selective contacts. Using a high-performance hole-selective MoOx contact or an electronselective TiO2 contact,[11,19] c-Si solar cells with efficiencies more than 22% were achieved. By combining MoOx and LiF/Al contacts, a dopant-free asymmetric heterocontact c-Si solar cell with a PCE close to 20% was achieved with a simple fabrication flow.[27] Many organic-layer based DF-CSCs, such as poly(3,4ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS),

Minimizing carrier recombination at contact regions by using carrier-selective contact materials, instead of heavily doping the silicon, has attracted considerable attention for high-efficiency, low-cost crystalline silicon (c-Si) solar cells. A novel electron-selective, passivating contact for c-Si solar cells is presented. Tantalum nitride (TaNx) thin films deposited by atomic layer deposition are demonstrated to provide excellent electron-transporting and hole-blocking properties to the silicon surface, due to their small conduction band offset and large valence band offset. Thin TaNx interlayers provide moderate passivation of the silicon surfaces while simultaneously allowing a low contact resistivity to n-type silicon. A power conversion efficiency (PCE) of over 20% is demonstrated with c-Si solar cells featuring a simple full-area electron-selective TaNx contact, which significantly improves the fill factor and the open circuit voltage (Voc) and hence provides the higher PCE. The work opens up the possibility of using metal nitrides, instead of metal oxides, as carrier-selective contacts or electron transport layers for photovoltaic devices.

1. Introduction With the increasing availability of high-quality crystalline silicon (c-Si) wafers and excellent dielectric surface passivation schemes, minimizing carrier recombination at contact regions becomes increasingly important for a reaching high power conversion efficiency (PCE) of c-Si solar cells. When the metal is in direct contact with the silicon surfaces, very large densities of electronic states at the interface can be introduced into Dr. X. Yang, Dr. E. Aydin, H. Xu, J. Kang, Dr. W. Liu, Prof. S. De Wolf KAUST Solar Center King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900, Saudi Arabia E-mail: [email protected]; [email protected] Dr. M. Hedhili KAUST Core Lab King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900, Saudi Arabia Dr. Y. Wan, J. Peng, C. Samundsett, Prof. A. Cuevas Research School of Engineering Australian National University Canberra, ACT 2601, Australia The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201800608.

DOI: 10.1002/aenm.201800608

Adv. Energy Mater. 2018, 1800608

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hole-blocking properties, due to their small conduction band offset and large valence band offset, as measured at the TaNx/n-Si interface. Thin TaNx interlayers are demonstrated to provide a moderate passivation to silicon surfaces while simultaneously reducing the contact resistivity on n-type silicon surface. By implementing a hole-blocking TaNx/Si heterocontact, an efficiency over 20% is achieved on n-type silicon solar cells.

2. Results and Discussion

Figure 1. Band alignment diagram of dopant-free carrier selective contacts for c-Si solar cells. The dashed green lines are the conduction and valence bands of silicon, and the red line is the work function.

8-hydroxyquinolinolato-lithium (Liq), perylene diimide, have also been developed.[28–32] Metal-oxide, fluoride, and polymerbased DF-CSCs are easy to deposit at an acceptable cost (e.g., thermal evaporation, spin coating, or atomic layer deposition, ALD); however, most of them suffer from a poor stability, either chemically or thermally,[11,16,18,23,24,33] which limits their industrial applications. In this work, we present a novel transition metal nitride electron-selective contact for c-Si solar cells. Tantalum nitride (TaNx) has been widely used as a copper diffusion barrier for microelectronics, and as a photoanode for photo-electrochemical water splitting, due to its high conductivity and chemical stability.[34–37] Han et al. have reported that ALD-deposited TaN thin films are capable of suppressing copper diffusion effectively up to an annealing temperature of 600 °C.[34] We show that TaNx/Si heterojunctions exhibit excellent electron-transporting,

TaNx thin films were deposited by ALD at 250 °C with a mixed N2/H2 reactant gas. The chemical composition of the TaNx film was analyzed by using high-resolution X-ray photoelectron spectroscopy (XPS). The XPS survey spectra of the TaNx thin films in the binding energy range 0–1200 eV, presented in Figure S1 in the Supporting Information, show the presence of Ta, O, N, and C signals. The presence of C is considered to be a peculiar characteristic of the XPS analysis, mainly because of the ambient atmosphere in the analysis chamber. Figure 2 shows the XPS core-level spectra of Ta 4f and N 1s for the TaNx films. The Ta 4f core level (Figure 2a) is fitted with three doublets (Ta 4f7/2–Ta 4f5/2) with a fixed area ratio of 4:3 and a doublet separation of 1.9 eV. The Ta 4f7/2 components are located at the bonding energy of 24.7, 25.5, and 26.2 eV. The main Ta 4f7/2 peak at the low binding energy of 24.7 eV is associated with TaN bonding, and the Ta 4f7/2 subcomponents centered at 25.5 and 26.2 eV are attributed to the tantalum oxynitride (TaOxNy) and tantalum oxide (TaOx) species, respectively.[38–41] The calculated Ta atomic percentages of TaNx, TaOxNy, and TaOx are 76.0%, 14.2%, and 9.8%, respectively. The N 1s core level (Figure 2b) overlaps with the broad Ta 4p3/2 component located at 403.6 eV. The dominant N 1s peak at the bonding energy of 396.3 eV further confirms the existence of TaN bonding in the film. The subcomponent N 1s peaks located at 397.1 eV can be attributed to the nitrogen in TaOxNy, while the peaks at 398.0 and 399.6 eV are possibly due to nitrogen bound to hydrocarbons.[39] The existence of TaOxNy and TaOx phases might be mainly attributed to the reaction of the Ta precursor with residual oxygen in the ALD chamber. In summary, XPS measurements demonstrate the presence of a dominant TaNx

Figure 2. XPS core-level spectra of a) Ta 4f and b) N 1s for TaNx films deposited by ALD.


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Figure 3. a) Dark J–V curves of Al/Si Schottky structures (inset) with and without a TaNx interlayer. The voltage is applied to the top Al electrode and positive current flows from Al to Si. b) Dependence of contact resistivity of TaNx/n-Si heterocontacts on the TaNx film thickness.

phase with small amounts of TaOxNy and TaOx impurities in the deposited films. Figure 3a shows the dark J–V curves of Al/Si Schottky structures (Figure 3a inset), with and without a 2.5 nm thick TaNx interlayer between the front Al circular contact and the silicon substrate. On the n-Si substrate, the diode without TaNx displays a nonohmic behavior, due to the presence of a large Schottky barrier height (ΦB) between n-Si and Al.[17,42] In contrast, on the p-type silicon (p-Si) substrate, the dark J–V curve shows a ohmic behavior, due to the low barrier height between p-Si and Al.[42] By inserting a thin TaNx interlayer between n-Si and Al, the J–V curve exhibits a perfectly linear, ohmic contact characteristic. However, on a p-Si substrate where holes carry most of the current, the J–V behavior indicates a very high resistance under both negative and positive applied bias, blocking the flow of the current. These results demonstrate that a TaNx/Si heterocontact can block holes effectively. Figure 3b shows the contact resistivity (ρc) of TaNx/n-Si heterocontacts as a function of TaNx thickness. By inserting a TaNx interlayer of only 1 nm, ρc at the Al/n-Si interface is significantly reduced to 42 mΩ cm2. Increasing the TaNx thickness from 1.0 to 4.0 nm increases ρc from 42 to over 200 mΩ cm2. With an ultrathin TaNx interlayer (1.0 and 2.5 nm), all the dark J–V curves with different diameters of front contacts show a perfect ohmic behavior (e.g., see Figure S2, Supporting Information), which enables the extraction of the ρc value accurately. However, some of the dark J–V curves (e.g., 0.5, 1.0, and 1.5 mm front contact diameters) are nonlinear when the TaNx thickness increases to 4.0 nm, which causes a large error bar for the extracted ρc value. Agrawal et al. reported that carrier extraction for optoelectronic devices can be improved by introducing a thin interlayer between the semiconductor and the metal, which might be attributed to Fermi-level depinning resulting from the reduced density of metal-induced gap states in the semiconductor bandgap.[43] We must point out that the extracted ρc value includes the resistance at the TaNx/n-Si, TaNx/Al, and Al/n-Si interfaces as well as the bulk resistance of the TaNx film and n-Si substrate. Therefore, it can be considered as the upper-limit contact resistance at the TaNx/n-Si heterocontact. The ρc value of the TaNx/Si heterocontact is comparable to that of the electron-selective TiO2 and MgF2 Adv. Energy Mater. 2018, 1800608

contacts,[17,21] which indicates that the TaNx contact can be suitable for the fabrication of high-efficiency c-Si solar cells. To elucidate the mechanisms underlying the electron-selective transport properties of the TaNx/n-Si heterocontact, we investigate the band alignment at the TaNx/n-Si interface. TaNx films exhibit an optical bandgap of ≈2.9 ± 0.05 eV, which we show by fitting the absorption spectrum measured by a UV–vis spectrometer with an integrating sphere (see Figure S3, Supporting Information). Figure 4a shows the UV photoelectron spectroscopy (UPS) spectrum of TaNx thin film using a He-I excitation (21.22 eV). The work function of TaNx, 4.3 ± 0.05 eV, is calculated by taking into consideration the onset (17.9 ± 0.05 eV) in the UPS spectrum at high binding energy. The gap between the Fermi level and the maximum of valence band (Ef −EV) in TaNx, obtained from the cutoff in the UPS spectrum, is 2.7 ± 0.05 eV. Similarly, the work function and (Ef −EV) values of the Si substrate are 4.4 and 1.0 eV, respectively, obtained by also measuring the UPS spectrum (see Figure S4, Supporting Information). If we assume that there is no band bending in the thin TaNx film and that the silicon bands bend down by 0.1 eV, we obtain the band alignment diagram at the TaNx/n-Si interface shown in Figure 4b. The result is that a small conduction band offset (ΔEc ≈ 0.15 eV) that allows electrons to pass through the TaNx layer and a large valence band offset (ΔEv ≈ 1.6 eV) that can block the holes effectively. Such electron transport and hole blocking characteristics are consistent with the observed dark J–V behaviors shown in Figure 3a. A high-performance carrier-selective contact requires not only a low contact resistivity at the contact interface but also a good surface passivation that enables a low carrier recombination velocity at the interface. The quality of passivation of the TaNx films on silicon surface is quantified in terms of effective carrier lifetime (τeff) using the quasi-steady state photoconductance (QSSPC) technique.[44] Figure 5 shows the τeff versus the excess carrier density (Δn) for a n-Si substrate passivated by TaNx films, and the upper limits of effective surface recombination velocity (Seff) calculated at the Δn of 1 × 1015 cm−3. An ultrathin TaNx interlayer (1.0 nm) can reduce Seff from ≈106–107 (Si/Al direct contact) to ≈390 cm s−1. The measured τeff, at the Δn of 1 × 1015 cm−3, increases as the TaNx film thickness increases from 1.0 to 4.0 nm, whereas the corresponding Seff

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Figure 4. a) The UPS spectrum of TaNx film using He-I excitation; b) Band alignment at TaNx/n-Si interface, indicating a very small ΔEc of 0.15 eV.

decreases from 390 to 99 cm s−1. The passivation mechanism of the TaNx film might be attributed to the hydrogen present in the films during the N2/H2 plasma deposition, which saturates some of the dangling bonds at the silicon surface. These results demonstrate that thin TaNx films can provide moderate passivation to silicon surfaces, which likely is sufficient to unpin the Fermi-level, giving thereby our contact its desired carrier selectivity. However, the surface passivation of these TaNx layers is poorer than the passivation obtained by TiO2 and TaOx films[17,25] but may be improved by a further optimization of the deposition process. N-type silicon solar cells featuring a full-area electronselective TaNx contact (Figure 6a) are fabricated to verify the performance of the TaNx/n-Si heterocontact on the device. The textured front side with random pyramids has a borondiffused p+ region, which was passivated by Al2O3 and then capped with a double-layer antireflection SiNx/MgF2 stack. A full-area heterocontact consisting of ≈2.5 nm TaNx capped with

Figure 5. Effective lifetime curves of n-Si substrate passivated by thin TaNx films. The Seff calculated at the Δn of 1 × 1015 cm−3 are shown together.


thermally evaporated Al, as shown in the high-resolution transmission electron microscopy (HRTEM) cross-sectional image of Figure 6b, is applied at the rear. This full-area contact with no doping/patterning allows a simple fabrication sequence and a low-resistance 1D electron flow to the rear terminal. A control cell with no TaNx interlayer was made in the same batch. The light J–V curves of silicon solar cells, at AM1.5 conditions, with and without a TaNx interlayer, are shown in Figure 6c. Corresponding photovoltaic parameters, series resistance and shunt resistance are also included. Due to the high carrier recombination velocity and the contact resistivity at the Al/n-Si surface (Figure 3a), the control cell exhibits low values of Voc (583 mV) and fill factor (FF) (73.1%) and a high series resistance (1.42 Ω.cm2), and hence a low PCE of 16.1%. By inserting a thin TaNx interlayer between n-Si and Al, both FF and Voc are significantly improved, from 73.1% to 81.8% and from 583 to 632 mV, respectively. This, in turn, significantly improves the PCE up to 20.1%, representing 4.0% absolute efficiency enhancement over the control device. The improved FF and Voc can be directly attributed to the presence of the TaNx interlayer, which simultaneously reduces the contact resistivity (Figure 3b) and the carrier recombination velocity (Figure 5) at the rear surface. The series resistance dramatically reduced to 0.33 Ω.cm2. Figure 6d shows the external quantum efficiency (EQE) spectra of the devices, with and without a TaNx interlayer. Both devices display a very high EQE in the short wavelength range (