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Wet Chemical Preparation of Silicon Tunnel Oxides for Transparent Passivated Contacts in Crystalline Silicon Solar Cells Malte Koehler, Manuel Pomaska, Florian Lentz, Friedhelm Finger, Uwe Rau, and Kaining Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02002 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

Wet Chemical Preparation of Silicon Tunnel Oxides for Transparent Passivated Contacts in Crystalline Silicon Solar Cells Malte Köhler*†, Manuel Pomaska†, Florian Lentz†‡, Friedhelm Finger†, Uwe Rau† and Kaining Ding†

†

IEK-5 Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

‡

Now with: Helmholtz Nano Facility, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

Keywords: Transparent passivated contact (TPC); silicon oxide (SiO2); tunnel oxide; passivated contact; selective contact; silicon surface passivation; silicon carbide (SiC); wet-chemical oxidation ABSTRACT

Transparent Passivated Contacts (TPC) using a wide band gap microcrystalline silicon carbide (µc-SiC:H(n)), silicon tunnel oxide (SiO2) stack are an alternative to amorphous silicon based contacts for the front side of silicon heterojunction solar cells. In a systematic study of the µc-SiC:H(n)/SiO2/c-Si contact, we investigated selected wet-chemical oxidation methods for the formation of ultra-thin SiO2, in order to passivate the silicon surface while ensuring a low contact resistivity. By tuning the SiO2 properties, implied open circuit voltages of 714 mV and contact

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resistivities of 32 mΩcm were achieved using µc-SiC:H(n)/SiO2/c-Si as Transparent Passivated Contacts.

Recent developments in crystalline silicon solar cell technology show a high potential for solar cells featuring fully passivated contacts. As compared to crystalline silicon solar cells that utilize local direct metal-silicon contact, the fully passivated contact gives rise to higher open circuit voltages (Voc) and consequently higher cell efficiencies. One prominent representative of such passivated contact solar cells is the silicon heterojunction (SHJ) solar cells, using amorphous silicon (a-Si) as a passivation layer on both the front and the back side of the cell.1 However, as a drawback, the a-Si parasitically absorbs parts of the incident light within the solar spectrum range before it can reach the actual absorber.2 Another representative is the recently developed TOPCon3 / POLO4 concept where passivation and contacting are realized by a stack of a silicon tunnel oxide (SiO2) and a polycrystalline silicon (poly-Si) on the rear side of the solar cell. This concept holds the current world record efficiency of 25.7% for double side contacted silicon solar cells.5 If used on the front side of the cell, the poly-Si would also introduce a significant parasitic absorption loss. In order to overcome the limitation of parasitic absorption, we introduce a new Transparent Passivated Contact on the front side of solar cells. It consists of an n-type doped hydrogenated microcrystalline silicon carbide (µc-SiC:H(n)) layer and an ultra-thin silicon tunnel oxide (SiO2). The advantage of the µc-SiC:H(n) is its wide optical band gap of 2.8 – 3.2 eV.6 In comparison, the optical band gap of a-Si:H and poly-Si are 1.7 eV and 1.12 eV, respectively.7,8 To achieve a high transparency a high crystallinity of the µc-SiC:H(n) is necessary.9 Therefore, a high density of hydrogen radicals needs to be present during the hot wire chemical vapor


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deposition (HWCVD). These hydrogen radicals are known to damage the crystalline silicon (c-Si) surface which deteriorates the passivation quality of the µc-SiC:H(n)/c-Si interface.6,10,11 Therefore, a capping layer on the c-Si surface is necessary which ensures a high passivation quality while maintaining a sufficient electrical conductivity to the front contact of the solar cell. One promising representative of such a capping layer is a thin silicon tunnel oxide (SiO2).12 The thickness of the SiO2 has to be optimized, since it must be thick enough to protect the c-Si surface. However SiO2 should be as thin as possible (< 2 nm)13, since the electrical current generated in the solar cell must pass the isolating SiO2. The current can either pass by direct conduction through pinholes in the SiO2 or by quantum mechanical tunneling through the potential barrier. 4,14 These SiO2 tunnel oxides also attracted a lot of attention e.g. in the transistor community to enhance the electrical properties.15,16 We performed intensive studies on the tunnel oxide formation using selected wet-chemical oxidation agents. For the oxidation experiments we used double side polished n-type float zone silicon wafer. The wafers were pre-cleaned by standard RCA (Radio Corporation of America) cleaning. The initial oxide was removed in hydrofluoric acid (HF). To form the tunnel oxide we used: (i) DI-water (H2O) as a reference, since all wafers were rinsed after the HF dip, (ii) nitric acid (HNO3), (iii) piranha solution (H2O2:H2SO4), (iv) Standard Clean 2 (SC-2, H2O:H2O2:HCl) and (v) hydrochloric acid (HCl). All wet chemical processes were conducted in the cleanroom of the Helmholtz Nano Facility (HNF)17 to prevent particle contamination. After oxidation, the µc-SiC:H(n) films were deposited by HWCVD on both sides of the oxidized wafer resulting in symmetric samples. For the deposition, Monomethylsilane (H3Si-CH3, MMS), Hydrogen (H2) and Nitrogen (N2) where used as precursor gases. The resulting µc-SiC:H(n) layers had a thickness of 35-40 nm. We used two different flow rates of MMS resulting in high


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and low crystallinity layers with higher and lower electrical conductivity, respectively.10 More experimental details can be found in the supporting information S1. The thickness of the tunnel oxide was measured by spectral ellipsometry (SE) directly after the growth of the oxide. The contact resistivity was derived from transfer length measurements (TLM). The passivation quality in terms of implied Voc (iVoc) and saturation current density (J0) was derived by the quasi steady-state photoconductive decay method (QSSPC). The iVoc was extracted at an illumination of one sun whereas J0 was calculated by the method of Kane and Swanson 18 at an excess carrier density of 1x1015 cm-3. More measurement details can be found in the supporting information S2. The results of the oxide thickness measurements by SE (Figure 1a) show that the treatment with different oxidation agents results in different oxide thicknesses (dox). By varying the oxidation time (tox, indicated above and below the bars in Figure 1a) the oxide thickness can be tuned in a range specific for each oxidation agent. Using nitric acid, dox ranged from 1.0 - 1.5 nm for 15 s to 10 min of tox, respectively. This is in good agreement with recent reports in literature.19,20 Therefore, we assume that our SE provide reasonable results for dox >1 nm. For dox below that, the measurement might not be as accurate. Nevertheless, it is useful to compare dox relatively. The oxides produced by piranha solution, SC-2 and HCl are resulting in a thickness of 0.9 - 1 nm (15 sec – 10 min), 0.6 - 0.9 nm (10 – 30 min) and 0.5 - 0.9 nm (10 – 40 min), respectively. The oxidation reaction in HNO3 is faster than oxidation in e.g. SC-2 and piranha solution resulting in the thickest oxide. Since the wet-chemical oxidation process is self-limiting, dox cannot be increased above a specific limit by longer tox. After removing the RCA oxide using hydrofluoric acid the wafers were rinsed in DI-Water. During this rinsing process, very thin oxide of ~0.5 nm grows (Figure 1, H2O). Since the quality of this H2O-SiO2 is not sufficient to achieve a good


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passivation, the rinsing time should be kept as short as possible to prevent unintentionally growth of an oxide. A schematic band diagram of the c-Si/SiO2/µc-SiC:H(n) structure is shown in Figure 1b. The electrons can tunnel through the SiO2 while the holes are repelled since the potential barrier is higher for holes than for electrons. A thickness of the tunnel oxide as small as possible is favorable for extracting the photo-generated current of the solar cell. Since the tunnel probability of the excess charge carriers exponentially increases with decreasing thickness of the SiO2 the current also increases exponentially for pure tunnel transport.

Figure 1. a) Oxide thickness (dox) depending on the oxidation agent. The minimum and maximum oxidation time is indicated above and below the bars. b) Schematic band diagram of the c-Si/SiO2/µc-SiC:H(n) with conduction band (EC), valence band (EV) and Fermi level (EF). A very thin oxide is therefore required to allow most of the charge carriers to pass the oxide which results in a low contact resistivity of the front side. Hence, the contact resistivity (ρc) of the Ag/µc-SiC:H(n)/SiO2/c-Si layer stack is an important parameter for the Transparent


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Passivation Contact. A low contact resistivity results in a low series resistance, which is essential for a good fill factor and therefore a high efficiency of a solar cell. The results of the contact resistivity measurements by TLM are shown in Figure 2. The contact resistivity is highest for the thickest oxide fabricated by HNO3 and lowest for the SC-2 oxide with dox of 0.6 nm. The exponential decay of the contact resistivity with decreasing oxide thickness changes the slope at an oxide thickness of ~1 nm.

Figure 2. Contact resistivity (ρc) derived from TLM measurements as a function of the oxide thickness (dox). Open symbol and closed symbol results were obtained using a high and low conductive µc-SiC:H(n), respectively. Error bars display relative errors on the right log(ρc) scale. A dashed line is included as a guide to the eye. The contact resistivity for oxide thicknesses 1 nm the main transport mechanism of the charge carriers seems to be quantum mechanical tunneling since the slope


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increase exponentially with increasing oxide thickness. The tunnel probability T(E) of charge carriers depends exponentially on the width of the potential barrier which in this case can be attributed to the thickness of the oxide dox: ∝ − with 2 = Φ ℏ where mt is the tunnel mass of the particle, Φ is the height of the potential barrier of the SiO2 on c-Si and ℏ is the reduced Planck constant.4 For thin oxides, inhomogeneities in the thickness of the SiO2 can lead to a direct contact between µc-SiC:H(n) and c-Si often described as pinholes. These pinholes would create an alternative current path through the oxide. With a decreasing oxide thickness more pinholes should occur and the pinhole phenomenon should become more dominant, resulting in a lower contact resistivity. Since contact resistivity saturates for small oxide thickness, pinholes seem not to be present in our oxides. Pinholes are often described after high temperature annealing which might cause a shrinking of the oxide. Peibst et al.4 have shown that an area pinhole density (radius of the pinhole: 2 nm) of 108 cm-2 leads to ρc 0.5 nm) is required to maintain a good passivation quality in the range of 700 mV iVoc. The low passivation quality for the high conductive µc-SiC:H(n) on very thin oxides indicates that the c-Si/SiO2 interface is impaired during the HWCVD deposition. Similar deterioration effects are reported in literature associated with hydrogen etching of SiO2.10,21,22 These radicals originate from the precursor gasses for the deposition, which are diluted in molecular hydrogen. These precursor molecules decompose near the hot wires and form a high density of hydrogen radicals. Very thin oxides are likely to be removed completely before the growing µc-SiC:H(n) covers and protects the c-Si surface. For high hydrogen radical densities and thin oxide thicknesses the SiO2 might be


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completely removed. Therefore, the radicals can directly damage the silicon surface and introduce additional dangling bonds.6 This results in a low surface passivation quality. The difference in passivation quality of different oxides at the same thickness might be attributed to the surface roughness and the density of the oxide. Yamashita et al.23 have shown that an oxidation in HNO3 results in a higher c-Si surface roughness while oxidation in SC-2 and Piranha solution results in smoother surfaces. This can lead to a higher defect density for HNO3 oxides and therefore a higher interface recombination. The SC-2 oxide has shown the lowest defect density in the study but the atomic density of the SC-2 oxide was found to be lower than for HNO3 and Piranha oxide.23 This might lead to a faster deterioration of the SC-2 oxide and a lower passivation quality. The fluctuations of several millivolts in iVoc might result from different process conditions. The time after deposition where the sample was still heated in the HWCVD chamber was not tracked and therefore not constant. This heating might lead to an out-diffusion of hydrogen which would otherwise passivate additional dangling bonds and results in a changing passivation quality. A direct influence of the conductivity of the µc-SiC:H(n) on the iVoc cannot be seen for dox above 0.5 nm. This finding indicates that the passivation mechanism is independent from the conductivity of the µc-SiC:H(n).


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voltage iVoc [mV]

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implied open circuit


720

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Piranha SC-2

700 680 HNO3

660

H 2O

640

HCl

0.5

1.0

1.5

oxide thickness dox [nm] Figure 3. Implied open circuit voltage (iVoc) obtained by QSSPC measurements as a function of the oxide thickness (dox). Open symbol and closed symbol results were obtained using a high and low conductive µc-SiC:H(n), respectively. Dashed lines are included as a guide to the eye representing the upper and lower limit of our data. To compare our obtained results for µc-SiC:H(n)/SiO2 to results from literature for poly-Si/SiO2 and TOPCon contacts, the saturation current density (J0) is plotted as a function of the contact resistivity (ρc) in Figure 4. J0 is an expression of the passivation, since it is a measure for the surface recombination current density of the device which should be as low as possible. The figure was modified from the publication by Peibst et al.4 Figure 4 reveals that our contact resistivities are higher than results reported for poly-Si and TOPCon samples. The lowest achieved ρc was 32 mΩcm² for the SC-2 oxide. Nevertheless, a low J0 of 2.2 fAcm-2 could be realized by using piranha solution. Our results follow the trend of decreasing J0 while increasing ρc like shown in the previously published results by Feldman et


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al.24, Yan et al.25 and Gan and Swanson26. The results obtained by Peibst et al.4 reveal both, a lower ρc and lower J0.

102

saturation current density J0 [fA/cm²]

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n TopCon Feldmann n poly-Si Yan n poly-Si Gan n poly-Si Peibst HNO3 HCl H2O Piranha SC-2

1

100 10-2 10-1 100 101 102 103

contact resistivity ρc [mΩcm²] Figure 4. Saturation current density (J0) derived from QSSPC as function of the contact resistivity (ρc). Oxide thicknesses are varied as shown in Figure 1. Open symbol and closed, colored symbol results were obtained using a high and low conductive µc-SiC:H(n), respectively. Data reported from Refs. 4,8,19,25–30. Originally published in Peibst et al.4 The higher contact resistivity of our samples might originate from a more compact SiO2 compared to the SiO2 after the crystallization of poly-Si. The crystallization process from amorphous silicon to poly-Si is usually done at 800 °C to 1050 °C.29 These high temperature processes are known to introduce pinholes by which the current can flow directly into the passivated contact.31,32 This additional current path leads to a lower contact resistivity. During this crystallization process the doping atoms in the doped a-Si might diffuse through the pinholes into the c-Si, which leads to a low passivation quality at these interfaces. Since Peibst et al.4 have


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crystallized intrinsic amorphous silicon and then doped the poly-Si by ion implanting, no doping atoms diffuse into the c-Si. Therefore, larger pinholes are tolerable for a good passivation quality while reducing the contact resistivity even further. All our processes are low temperature processes (~ 250 °C) which might lead to less or no pinholes, a higher contact resistivity and due to the closed oxide layer possibly a better passivation quality.

We presented a new approach for a high Transparent Passivated Contact for the front side of crystalline silicon solar cells. We used a stack of thin silicon tunnel oxide SiO2 and µc-SiC:H(n) to passivate the contacts. Selected wet-chemical oxidation agents were investigated to minimize the oxide thickness of the tunnel oxide. As a result, the contact resistivity was reduced to 32 mΩcm², implied open circuit voltages up to 714 mV and saturation current densities down to 2.2 fAcm-2 were demonstrated. The minimum oxide thickness while maintaining a high passivation quality and low contact resistivity was found to be in the range of 0.5 nm to 0.9 nm using SC-2 for wet-chemical oxidation. These results hold great promise for our transparent passivated contact to become an attractive alternative to conventional TOPCon or POLO contacts.


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SUPPORTING INFORMATION The supporting information contains experimental details on cleaning and oxidizing procedures of the wafers as well as detailed description of the deposition parameters for the µc-SiC:H(n) process. It also contains details on the measurement of the contact resistivity and the passivation quality.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT The authors gratefully acknowledge the funding of the German Federal Ministry of Economic Affairs and Energy in the framework of the TUKAN project (grant: 0324198D).

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Wietler, T. F.; Tetzlaff, D.; Krügener, J.; Rienäcker, M.; Haase, F.; Larionova, Y.; Brendel, R.; Peibst, R. Pinhole Density and Contact Resistivity of Carrier Selective Junctions with Polycrystalline Silicon on Oxide. Appl. Phys. Lett. 2017, 110 (25), 253902.


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a) Oxide thickness (dox) depending on the oxidation agent. The minimum and maximum oxidation time is indicated above and below the bars. b) Schematic band diagram of the c-Si/SiO2/µc-SiC:H(n) with conduction band (EC), valence band (EV) and Fermi level (EF). 149x66mm (300 x 300 DPI)


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Contact resistivity (ρc) derived from TLM measurements as a function of the oxide thickness (dox). Open symbol and closed symbol results were obtained using a high and low conductive µc-SiC:H(n), respectively. Error bars display relative errors on the right log(ρc) scale. A dashed line is included as a guide to the eye. 289x202mm (300 x 300 DPI)



Implied open circuit voltage (iVoc) obtained by QSSPC measurements as a function of the oxide thickness (dox). Open symbol and closed symbol results were obtained using a high and low conductive µc-SiC:H(n), respectively. 287x201mm (300 x 300 DPI)


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Saturation current density (J0) derived from QSSPC as function of the contact resistivity (ρc). Data reported from Refs. [4], [7], [16], [22]–[28]. Originally published in [4]. 304x201mm (300 x 300 DPI)


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