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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

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.

One of the promising ways to increase the efficiency of silicon heterojunction (SHJ) cells is to replace the doped hydrogenated amorphous silicon (a-Si:H) contact layer by doped hydrogenated microcrystalline silicon (mu c-Si:H) which has better suited opto-electrical properties. In this work, we report on the development of a plasma-enhanced chemical vapor deposition process for mu c-Si:H. We demonstrate an n-type mu c-Si:H layer with high crystalline fraction and low parasitic absorption. The developed layer is implemented as the front electron contact of a SHJ cell. A significant improvement in J(sc) of 0.9 mA/cm(2) is achieved on device level while maintaining high V-OC values (> 725 mV), leading to an efficiency of 20.6% for the best cell. Cell efficiency is limited by a decreased FF which is attributed to the increased sensitivity of mu c-Si:H layers to ITO sputtering damage. The authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. Part of this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270.

Eliminating a doping process could be an effective way to reduce the production cost of c-Si cells. However, in absence of highly doped Si, the formation of a high quality contact is not straightforward. The lack of field-effect passivation from a lowly doped region can lead to a high recombination current density at the contacts (J(0,metal)) and moreover, contact resistivity (rho(c)) typically increases when doping level is decreasing. In this work we focus on reducing the contact resistivity of an electron-selective contact for doping-free cells. Although the effect of low work function metals (LWMs) in combination with an i-a-Si:H layer has already been reported, the synergy effect of a LWM and a MIS (Metal-Insulator-Semiconductor) contact structure on top of the ia-Si:H has not been reported yet. Here, we demonstrate a new ATOM (i-a-Si:H / TiOX / low workfunction metal) contact structure as an electron-selective contact using an i-a-Si:H layer, a TiOX interfacial layer and Ca (Phi 2.9eV) without requiring an additional n(+) doping process. The addition of TiOX in between the i-a-Si:H layer and the Ca decreases the pc by about 2 orders of magnitude. Despite of increased J(0,metal) due to e-beam processing of TiOX, the Ca based ATOM contact increases the potential max efficiency up to 25 %. To the best of our knowledge, this is the first demonstration of an electron-selective contact comprising a low work function metal, an interfacial TiOX and an i-a-Si:H passivation layer. This type of contact could be a promising route for the optimization of doping-free cells (C) 2017 The Authors. Published by Elsevier Ltd. The authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. And part of this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270.

We present a novel process scheme for a-Si:H patterning using damage-free laser ablation, resulting in a simple, fast, and photolithography-free emitter patterning of silicon heterojunction interdigitated back-contact (SHJ-IBC) solar cells. An a-Si:H laser-absorbing layer and a stack of sacrificial dielectric layers are deposited on top of a-Si:H/c-Si heterocontact to prevent laser damage. Laser ablation only removes the top aSi: H layer, which limits laser damage to the surface of dielectric layers. These dielectric layers form a distributed Bragg reflector with a high reflectance of 80% at the laser wavelength which results in additional protection of the bottom a-Si:H/c-Si heterocontact. The significant reduction of laser damage is confirmed by atomic-force microscopy and photo-luminescence measurements. Such damage-free laser ablation process was successfully incorporated in a SHJ-IBC process flow and a best efficiency of 21.8% was achieved. The authors gratefully acknowledge the financial support of imec’s industrial affiliation program for Si-PV. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 727523.