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© 2018 IEEE. A fully dry and hydrofluoric-free low-temperature process has been developed to passivate n-type crystalline silicon (c-Si) surfaces. Particularly, the use of a hydrogen (H2) plasma treatment followed by in situ intrinsic hydrogenated amorphous silicon (a-Si:H) deposition has been investigated. The impact of H2 gas flow rate and H2 plasma processing time on the a-Si:H/c-Si interface passivation quality is studied. Optimal H2 plasma processing conditions result in the best effective minority carrier lifetime of up to 2.5 ms at an injection level of 1 × 1015 cm-3, equivalent to the best effective surface recombination velocity of 4 cm/s. The reasons that enable such superior passivation quality are discussed in this paper based on the characterization of the a-Si:H/c-Si interface and c-Si substrate using transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, and deep-level transient spectroscopy. ispartof: IEEE JOURNAL OF PHOTOVOLTAICS vol:8 issue:6 pages:1539-1545 status: published

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiOx) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiOx film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells. This work was supported by the European Union’s Horizon2020 Programme for research, technological development, and demonstration [Grant 857793] and by the Kuwait Foundation for the Advancement of Sciences [Grant CN18- 15EE-01].

Poly-Si/SiOx passivating contacts enable the manufacturing of highly-efficient Si solar cells, but their fabrication commonly relies on an extra high-temperature process such as dopant diffusion or thermal annealing for achieving excellent passivation and contacting properties. This extra process is eliminated in the fired passivating contact (FPC) approach used for simplified fabrication of poly-Si/SiOx passivating contacts. Instead, FPCs rely on the thermal budget of the fast/short and high-temperature firing process used for metallization of solar cells to achieve similar final properties. Despite this, compatibility of FPCs with industrially viable metallization techniques has not been demonstrated yet, which is studied in this work for fire-through Ag screen-printing and Ni/Ag plating. With screen-printing, low recombination current density (J(0)) down to 4.9 fA/cm(2), low contact resistivity between the Ag contacts and the FPC (rho(c,m)) down to 7.2 m Omega.cm(2), and Ohmic transport through the FPC including the SiOx film were achieved using wet-chemically grown SiOx. Nevertheless, J(0) of metallized regions (J(0,m)) exceeded 1000 fA/cm(2). Reducing J(0,m) was attempted by mitigating the blistering observed in FPCs, but J(0,m) remained high. With Ni/Ag plating, excellent surface passivation with J(0) down to 2.7 fA/cm(2) and very low J(0,m) < 50 fA/cm(2) were achieved, but no Ohmic contacts could be obtained. Integration of screen-printed FPCs in large-area n-TOPCon solar cells was also demonstrated, yielding average efficiencies of 18.4%, limited mainly by the high J(0,) (m) and series resistance of the FPCs. The results presented reveal the challenges for the industrialization of FPCs and provide valuable insights for tackling these. The authors would like to acknowledge Patrick Choulat from Imec and Rajiv Sharma from KU Leuven for the valuable discussions and their help with the sample fabrication, Thomas Nuytten and Stefanie Sergeant from Imec for the Raman spectroscopy measurements, and Bastien Douhard and Mustafa Ayyad from Imec for the SIMS measurements. This work was supported by the European Union’s Horizon 2020 Programme for research, technological development, and demonstration [grant number 857793]; and by the Kuwait Foundation for the Advancement of Sciences [grant number CN18-15EE-01].

Metal silicide is a well-known material for contact layers; however, it has not been tested in the context of doping-free carrier selective contacts. Thin film deposition of an appropriate metal with mild annealing treatment is an interesting alternative to the more complex depositions of other compound materials. Reaction of Yb deposited on top an i-a-Si:H passivation layer results in the formation of YbSix on top of a remnant i-a-Si:H, following a low-temperature annealing below 200 degrees C. Such a contact is an interesting candidate as a doping-free electron-selective contact. Detailed investigation of the i-a-Si/YbSix contact shows that Yb thickness, i-a-Si:H thickness and silicidation annealing conditions play a significant role in determining the recombination current density (J(0,metal)) and the contact resistivity (rho(c)). Low J(0,metal) of 5 fA/cm(2) and low rho(c) below 0.1 Omega.cm were independently demonstrated for such i-a-Si:H/YbSix contacts. We also demonstrate that lowtemperature silicidation can be combined with contact sintering (160 degrees C/25 min) or module lamination (160 degrees C/20 min), which are potential pathways for process simplification. Combining the optimized i-a-SEFI/YbSix electron contact with MoOx based hole contact in the MolYSili doping-free cell (i-a-SEH/MoOx + i-a-SEFI/YbSix), we achieved 16.7% in average efficiency and 17.0% for the champion cell. Furthermore, the YbSc contact stability was evaluated at module level and excellent thermal stability of the MolYSili laminate was demonstrated using the damp-heat test method (humidity 85%, 85 degrees C, 1000 h), where the laminated MolYSili cell did not show any degradation in the cell efficiency. This is the first proof-of-concept demonstration of a stable silicide-based contact for low-temperature processed doping-free solar cells. The authors thank Pieter Lagrain, Olivier Richard, and Hugo Bender for TEM measurement. Moreover, the authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. The work in this paper was partially funded by the Kuwait Foundation for the Advancement of Sciences under project number CN18-15EE01. imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy systems. Cho, J (corresponding author), Katholieke Univ Leuven, ESAT Dept, B-3001 Leuven, Belgium. jinyoun.cho@eu.umicore.be

Metal oxides are interesting materials for use as carrier-selective contacts for the fabrication of doping-free silicon solar cells. In particular, MoOx and TiOx have been successfully used as hole and electron selective contacts in silicon solar cells, respectively. However, it is of paramount importance that good thermal stability is achieved in such contacts. In our work, we combined i-a-Si:H/MoOx based hole contacts with electron contacts featuring i-a-Si:H/TiOx/low work function metal (ATOM) to fabricate doping-free cells, termed MoIyATOM cells. We found that the thermal stability of the ATOM contact was improved when the i-a-Si:H was annealed (300 degrees C for 20 min in N-2) before depositing TiOx (Le. pre-TiOx annealing), which reduces the hydrogen content in i-a-Si:H by about 27 Vorei, and thereby the H-related degradation of the ATOM contact characteristics. Moreover, it was found that reducing the thickness of the low-work function metal on top of the TiOx enhanced the thermal stability of the ATOM contact. With these adaptations, the MoIyATOM cell efficiency was improved by 3.5 %(abs), with the highest efficiency of 17.6%. Moreover, the cells show improved thermal stability after the above-mentioned pre-TiOx annealing, which is confirmed by annealing tests at cell level as well as damp-heat tests at module level. The insights of this study could be used to tailor other metal-oxide based electron or hole contacts. The authors thank Praveen Dara and Johan Meersschaut for ERD measurement. Moreover, the authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. The work in this paper was partially funded by the Kuwait Foundation for the Advancement of Sciences under project number CN18-15EE-01. Imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy systems. Cho, JY (reprint author), Katholieke Univ Leuven, ESAT Dept, Kasteelpk Arenberg 10, B-3001 Leuven, Belgium; Umicore, Watertorenstr 33, B-2250 Olen, Belgium. Radhakrishnan, HS (reprint author), IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. jinyoun.cho@eu.umicore.be

The potential of passivating contacts incorporating in situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films grown by low pressure chemical vapor deposition (LPCVD) is demonstrated in this work by integrating these layers at the rear side of large-area (241.3 cm(2)) bifacial n-type Tunnel Oxide Passivated Contact (nTOPCon) solar cells with diffused front emitter and screen-printed contacts. In situ doped poly-Si films are studied as their use could simplify the production of industrial n-TOPCon solar cells compared to the common approach relying on ex situ doping of intrinsic LPCVD poly-Si films. The developed poly-Si passivating contacts exhibited excellent characteristics with low recombination current densities in passivated and screen-printing metallized regions down to 2.3 fA/cm(2) and 65.8 fA/cm(2), respectively, and a low contact resistivity of 2.0 m Omega.cm(2). For reaching the best passivating contact characteristics and high solar cell efficiencies, a poly-Si film thickness of 150-200 nm was found to be optimal while a polished rear surface morphology was found to be beneficial. The best solar cell reached a certified power conversion efficiency of 23.01% along with a high open circuit voltage of 691.7 mV, enabled by the passivating contacts with the in situ doped poly-Si films. 1-cell glass-glass laminates were also fabricated with the developed solar cells, which showed no loss in their power output both upon 400 thermal cycles and after 1000 h of damp heat testing. Lastly, a roadmap is presented, indicating strategies to achieve efficiencies up to 25.5% with n-TOPCon solar cells incorporating the in situ P-doped LPCVD poly-Si films. The authors would like to acknowledge Sukhvinder Singh from Imec and Rajiv Sharma from KU Leuven for the valuable discussions and their help with the sample fabrication. This work was supported by the European Union’s Horizon2020 Programme for research, technological development, and demonstration [grant number 857793]; and by the Kuwait Foundation for the Advancement of Sciences [grant number CN18-15 EE-01].

In early 2017, the world record efficiency for single-junction crystalline silicon (c-Si) solar cells was achieved by merging amorphous silicon (a-Si:H)/c-Si heterojunction technology and back-contact architecture. However, to fabricate such silicon heterojunction interdigitated back-contact (SHJ-IBC) solar cells, complex a-Si:H patterning steps are required to form the interdigitated a-Si:H strips at the back side of the devices. This fabrication complexity raises concerns about the commercial potential of such devices. In this work, a novel process scheme for a-Si:H patterning using damage-free laser ablation is presented, leading to a fast, simple and photolithography-free emitter patterning approach for SHJ-IBC solar cells. To prevent laser-induced damage to the a-Si:H/c-Si heterocontact, an a-Si:H laser-absorbing layer and a dielectric mask are deposited on top of the a-Si:H/c-Si. Laser ablation only removes the top a-Si:H layer, reducing laser damage to the bottom a-Si:H/c-Si heterocontact under the dielectric mask. This dielectric mask is a distributed Bragg reflector (DBR), resulting in a high reflectance of 80% at the laser wavelength and thus providing additional protection to the a-Si:H/c-Si heterocontact. Using such simple a-Si:H patterning method, a proof-of concept 4-cm(2) SHJ-IBC solar cell with an efficiency of up to 22.5% is achieved. The authors gratefully acknowledge the financial support of IMEC's Industrial Affiliation Program for Si-PV. This project has also received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no. 727523 (NextBase).

Previously, IMEC proposed the i(2)-module concept which allows to process silicon heterojunction interdigitated back-contact (SHJ-IBC) cells on thin ( < 50 mu m) Si wafers at module level. This concept includes the bonding of the thin wafer early on to the module cover glass, which delivers mechanical support to the wafer and thus significantly improves the production yield. In this work, we test silicone and ethylene vinyl acetate bonding agents and prove them to be resistant to all rear side processes, including wet and plasma processes. Moreover, a lift-off process using a sacrificial SiOx layer has been developed for emitter patterning to replace conventional lithography. The optimized process steps are demonstrated by the fabrication of SHJ-IBC cells on 6-inch 190 mu m-thick wafers. Efficiencies up to 22.6% have been achieved on reference freestanding wafers. Excellent V-oc of 734 my and J(sc) of 40.8 mA/cm(2) lead to an efficiency of 21.7% on silicone-bonded cells, where the high V-oc indicates the process compatibility of the bonding agent. The developments that enabled such achievements and the key factors that limit the device performance are discussed in this paper. The authors would like to thank the financial support of imec's industrial affiliation program for Si-PV. This work was also partially funded by Kuwait Foundation for the Advancement of Sciences under project code: P115-15EE-01. The research has received funding from the European Union's Seventh Programme for research, technological development and demonstration under grant agreements no. 609788 (Cheetah), and also funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270. The authors gratefully acknowledge Joachim John for the spectral response measurements and Yiming Yang for the SEM measurements.

Utilisation of expensive silicon (Si) material in crystalline Si modules has come down to 4 g Si per watt-peak in 2018, mainly as a result of reduction in wafer thickness and kerf losses as well as increase in module efficiencies. With continued progress in conventional multi-wire sawing of ingots, wafers as thin as 100 mu m could eventually be produced. Beyond this, kerfless lift-off technologies are being investigated which enable wafer thicknesses well below 100 mu m with negligible Si kerf waste. Such thin Si wafers and foils would be much lighter in weight than today's standard 165-180 mu m-thick wafers and would exhibit considerable flexibility and fragility. This necessitates a rethink about how to handle and process thin Si into solar devices in a manufacturing line with high mechanical yield and high throughput. This paper gives a broad overview of the different approaches for fabricating solar cells on thin Si foils. In particular, three routes are discussed in detail, namely (1) freestanding processing of thin Si, (2) processing of thin Si supported mechanically on a conductive low-cost Si substrate ("wafer-equivalent" approach) and (3) processing of thin Si bonded to a transparent glass superstrate. In each case, the main challenges are explained and the recent progress in addressing them are summarised. Kerfless 50 mu m-thick epitaxial Si foils lifted-off using porous Si and thinned-down Si wafers (below 70 mu m) are used as model substrates for this work. The authors gratefully acknowledge the efforts of several people who were involved in this work over the last few years, namely Jonathan Govaerts, Stefano Granata, Menglei Xu, Ergi Donercark, Ivan Sharlandshiev, Shashi Kiran Jonnak, Shruti Jambaldinni, Robert Roozeman, Jarkko Heildcinen, Thomas Kaden, Zuzana Kovacova, Erich Neubauer, Michael Grimm and Kaori Nagaoka. The authors also acknowledge the funding received for this work from the European Commission for the H2020 project CABRISS under grant agreement No. 641972. Imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy systems. Radhakrishnan, HS (reprint author), Imec Vzw Partner EnergyVille, Kapeldreef 75, B-3001 Leuven, Belgium. sivarama@imec.be

We present a novel process sequence to simplify the rear-side patterning of the silicon heterojunction interdigitated back contact (HJ IBC) cells. In this approach, interdigitated strips of a-Si:H (i/p(+)) hole contact and a-Si:H (i/n(+)) electron contact are achieved by partially etching a blanket a-Si:H (i/p(+)) stack through an SiOx hard mask to remove only the p(+) a-Si:H layer and replace it with an n(+) a-Si:H layer, thereby switching from a hole contact to an electron contact in situ, without having to remove the entire passivation. This eliminates the ex situ wet clean after dry etching and also prevents re-exposure of the crystalline silicon surface during rear-side processing. Using a well-controlled process, high-quality passivation is maintained throughout the rear-side process sequence leading to high open-circuit voltages (V-OC). A slightly higher contact resistance at the electron contact leads to a slightly higher fill factor (FF) loss due to series resistance for cells from the partial etch route, but the FF loss due to J(02)-type recombination is lower, compared with reference cells. As a result, the best cell from the partial etch route has an efficiency of 22.9% and a V-OC of 729 mV, nearly identical to the best reference cell, demonstrating that the developed partial etch process can be successfully implemented to achieve cell performance comparable with reference, but with a simpler, cheaper, and faster process sequence. European Commission's Horizon 2020Framework Programme, Grant/Award Num-ber: 727523; IIAP‐SiPV; Kuwait Foundationfor the Advancement of Sciences, Grant/Award Number: P115‐15EE‐01

We present a novel approach to simplify the rearside a-Si:H patterning of silicon heterojunction interdigitated back-contact solar cells. In our current process, laser ablation and lift-off are used. Since lift-off is not industrially-viable, we propose to replace it with a selective deposition process which ensures a-Si: H is realised only on c-Si surface and not on the SiOx mask, at the end of the process, based on cycles of a-Si: H deposition and etching. While neither the deposition nor the etching is truly selective, this method relies on the difference of a-Si: H etch rates on c-Si and SiOx surfaces to achieve selectivity as the net end-result. The main challenge is addressing the trade-off between selectivity and c-Si surface passivation. The authors gratefully acknowledge the financial support of imec’s industrial affiliation program for Si-PV. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 727523 (NextBase).

This paper compares the electrically active damage in dry-etched n-type float-zone silicon, using NF3/Ar or H2-plasma exposure and assessed by deep-level transient spectroscopy (DLTS) and recombination lifetime analysis. It is shown that the NF3/Ar-plasma damage consists of at least four different types of electron traps in the upper half of the band gap, which can be associated with vacancy- and vacancy-impurity-related complexes. In the case of H2-plasma damage, it is believed that the accumulation of point defects results in a gradual disordering of the near-surface layer. These defect levels also act as recombination centers, judged by the fact that they degrade the minority carrier lifetime. It is finally shown that lifetime measurements are more sensitive to the etching-induced damage than DLTS.This paper compares the electrically active damage in dry-etched n-type float-zone silicon, using NF3/Ar or H2-plasma exposure and assessed by deep-level transient spectroscopy (DLTS) and recombination lifetime analysis. It is shown that the NF3/Ar-plasma damage consists of at least four different types of electron traps in the upper half of the band gap, which can be associated with vacancy- and vacancy-impurity-related complexes. In the case of H2-plasma damage, it is believed that the accumulation of point defects results in a gradual disordering of the near-surface layer. These defect levels also act as recombination centers, judged by the fact that they degrade the minority carrier lifetime. It is finally shown that lifetime measurements are more sensitive to the etching-induced damage than DLTS.