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The EU crystalline silicon (c-Si) PV manufacturing industry has faced strong foreign competition in the last decade. To strive in this competitive environment and differentiate itself from the competition, the EU c-Si PV manufacturing industry needs to (1) focus on highly performing c-Si PV technologies, (2) include sustainability by design, and (3) develop differentiated PV module designs for a broad range of PV applications to tap into rapidly growing existing and new markets. This is precisely the aim of the 3.5 years long H2020 funded HighLite project, which started in October 2019 under the work program LC-SC3-RES-15-2019: Increase the competitiveness of the EU PV manufacturing industry. To achieve this goal, the HighLite project focuses on bringing two advanced PV module designs and the related manufacturing solutions to higher technology readiness levels (TRL). The first module design aims to combine the benefits of n-type silicon heterojunction (SHJ) cells (high efficiency and bifaciality potential, improved sustainability, rapidly growing supply chain in the EU) with the ones of shingle assembly (higher packing density, improved modularity, and excellent aesthetics). The second module design is based on the assembly of low-cost industrial interdigitated back-contact (IBC) cells cut in half or smaller, which is interesting to improve module efficiencies and increase modularity (key for application in buildings, vehicles, etc.). This contribution provides an overview of the key results achieved so far by the HighLite project partners and discusses their relevance to help raise the EU PV industries' competitiveness. We report on promising high-efficiency industrial cell results (24.1% SHJ cell with a shingle layout and 23.9% IBC cell with passivated contacts), novel approaches for high-throughput laser cutting and edge re-passivation, module designs for BAPV, BIPV, and VIPV applications passing extended testing, and first 1-year outdoor monitoring results compared with benchmark products. This project has received funding from the European Union's Horizon 2020 Programme for research, technological development, and demonstration under Grant Agreement no. 857793.

Contactless plating with electroless solutions can provide self-aligned high-efficiency contacts with very low silver content, using simple and inexpensive equipment. With prior surface activation, it can even be used to metallize both sides of bifacial silicon solar cells simultaneously. However, we observe in such a coplating process with nickel, where surface activation is achieved by immersion plating and thickening by electroless plating, that V oc and fill factor can sometimes significantly decrease with immersion-plating time. To understand the reason for this electrical degradation, we studied the impact of immersion plating on the microstructure of the plated silicon surface. The evolution of the Si-Ni interface was studied by scanning and transmission electron microscopies, energy-dispersive X-ray analysis, secondary ion mass spectrometry, and scanning spreading resistance microscopy. Our attention focused on metal in-diffusion, silicon roughening and etching as the origin for increased recombination. Etching was found to have a significant impact on V oc. The thickness of N + Si etched during Ni deposition can in fact suppress in a few locations most of the field-effect passivation underneath the contacts. This means that a thicker surface field with doping beyond 1019/cm3 must be foreseen under the plated areas, or that the amount of Si lost in the reaction must be reduced. Our observations also confirm that immersion plating can hinder silicide formation and allow Ni in-diffusion, which may be a concern for reliability.

Perovskite/crystalline-silicon (c-Si) tandem solar cells offer a viable roadmap for reaching power conversion efficiencies beyond 30%. In this configuration, however, the silicon cell now mainly receives an infrared rich illumination spectrum where the absorption coefficient of silicon is poor. To boost the light absorption in this wavelength interval, transmitted through the top cell, a solution is to introduce a dedicated nanoscale texture on the front side of the silicon bottom cell and tweaking the optical elements in its design. These optical elements are manifold, with tunable geometrical dimensions, layer thicknesses, and refractive indices. For optically optimizing nanostructured silicon solar cells, electromagnetic wave solving methods, such as the rigorous coupled-wave analysis (RCWA), are normally used but they become computationally infeasible when several design variables are involved in an optimization problem. In this work, a natural selection algorithm, known as the genetic algorithm, is coupled with RCWA for extracting the optimal values of various design parameters in four-terminal perovskite/c-Si tandem devices in parallel. Both two-side-contacted and interdigitated back-contacted silicon heterojunction cell structures, featuring an inverse nanopyramid grating texture on the front, are optimized for five interdependent variables using a genetic algorithm for the bottom cell application and benchmarked against their random pyramid textured analogs. Our study shows that an optimized inverse nanopyramid grating texture can outperform the standard random pyramid texture when considering incidence angle variations. More importantly, the study illustrates how a genetic algorithm can support modeling complex solar cell structures with numerous degrees of freedom.

Carefully tailored periodic nanostructures on the light wavelength scale, such as diffraction gratings, benefit from wave optics for efficiently trapping the weakly absorbing infrared photons in crystalline-silicon (c-Si) absorbers. In contrast with the conventional random pyramid texture, diffraction gratings can be designed to target specific wavelength ranges by the selection of the grating pitch. Absorption enhancement at infrared wavelengths in a silicon solar cell is especially desired when it operates below a perovskite top cell in a tandem device. In this article, inverse nanopyramid gratings of 800 nm pitch are proposed as an alternative front-surface texture to random pyramids in silicon heterojunction devices with interdigitated back contacts that are to be used as bottom cells in four-terminal perovskite/c-Si tandem devices. By doing so, we report a short-circuit current density gain of 0.53 mA/cm 2 with respect to the random pyramid texturing for the bottom c-Si cell. The rationale to substitute random pyramids by inverse nanopyramid gratings is, however, not justified in single-junction operation despite achieving the power conversion efficiency of 22.3% since the degraded optical performance at shorter wavelengths offsets the absorption enhancement at longer wavelengths, resulting in similar levels of short-circuit current densities for both texture types. ispartof: Ieee Journal Of Photovoltaics vol:10 issue:3 pages:740-748 status: Published online

© Copyright 2019 American Chemical Society. Low contact resistivity ( c ) and low recombination current density at the metallized area (J 0,metal ) are the key parameters for an electron-selective contact in solar cells, and an i-a-Si:H/TiO x /low work function metal (ATOM) structure could satisfy these criteria. In this work, to achieve strong downward band bending, an Yb ( = 2.5-2.6 eV)/Ag stack is used. Moreover, the impact of (1) substrate topography (flat or textured), (2) TiO x thickness, and (3) Ti precursor (TTIP vs TDMAT) on the ATOM contact performance is investigated. The results show that the ATOM contact is relatively insensitive to the surface topography and to the Ti precursors (TTIP or TDMAT) used for the atomic layer deposition (ALD) of TiO x . However, the TiO x thickness has a significant impact on the c and marginally on the J 0,metal of the ATOM contact. For all topography cases and Ti precursors, 1 nm thick TiO x results in the lowest c value, most likely due to E F,metal depinning. In the silicon heterojunction solar cell, this ATOM contact (i-a-Si:H/TiO x /Yb/Ag) yields a solar cell efficiency of 19.2% with a high V OC of 723 mV without the need of a doped n-a-Si:H layer. Concerning the thermal stability of these contacts, TEM images confirm that Yb does not diffuse into the i-a-Si:H layer after an annealing at 180 °C for 30 min thanks to the TiO x layer behaving as a diffusion barrier. 98% of the initial solar cell efficiency is preserved even after successive annealing treatments at 150 and 175 °C, which are values in the same temperature range used in the module lamination and the sintering of the printed Ag. These results in combination with the demonstrated efficiency underline that the ATOM contact is a promising route to realize high-efficiency solar cells. ispartof: ACS APPLIED ENERGY MATERIALS vol:2 issue:2 pages:1393-1404 status: published

Integrating and operating PV in a system-efficient way is the key to increasing penetration levels to evolve toward a more sustainable and clean energy system. Reliable generation forecasting is the solution to tackle fluctuations in PV power generation and its negative effects on power systems and energy markets. Clouds play a key role in solar power, so cloud detection is important in PV power generation forecast. A color-based cloud detection algorithm is proposed in this paper. It considers 12 different color channels together with the K-means algorithm to detect clouds in sky images from an All-Sky Camera. The result of the proposed method has been compared to both classic and state-of-the-art methods. The experimental results show that the proposed approach has better performance in comparison to state-of-the-art methods.

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

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).

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.

One of the advantages of an epitaxially grown emitter is that its doping profile is not limited by the diffusion process and that almost any arbitrary profile shape can be realized. In this simulation study we make use of the epitaxial doping profile’s flexibility to optimize the emitter of an nPERT solar cell. Initially an optical and electrical model is set up in Sentaurus TCAD and calibrated based on measurements of an actual device with an efficiency of 22.5%. Next, simulations for a box shaped profile with varying thicknesses and doping concentrations are carried out. Additionally, a two-step emitter consisting of a combination of two box profiles – one located at the surface and another deep profile underneath the former one – is also simulated. The results clearly identify the optimal doping concentrations and thicknesses which result in the highest efficiencies. Outside of these optimal regions the most significant loss mechanisms are pointed out. To be highlighted is that the switch from a the single box profile emitter to an optimal two-step profile emitter realizes a gain of 0.4% absolute resulting in an efficiency of 22.9%. This demonstrates that the freedom in the shape of the epitaxial doping profile can be employed to further increase the solar cell efficiencies.

In this work, we improved the performance of the MWT-PERT solar cells focusing on increasing their V-oc by combining the MWT concept with n-PERT technology. The impact of different post-laser treatments on the via surface morphology and the via passivation was investigated. KOH texturing can partially remove the laser damage in the via and reduce the via SRV to 1000 cm/s. With optimized post-laser treatment and via passivation, an average V-oc of 685mV was achieved for our large-area n-type MWT-PERT cells. Front Ni/Cu plating and rear Ag and Al screen-printing were used for the metallisation, the compatibility of this hybrid metallisation scheme was studied. Using industrial solder-through interconnection technology, the cell was integrated in a laminate reaching a one-cell module efficiency at 20%. The reliability of the one-cell modules was preliminarily investigated in an extended reliability test. (C) 2017 The Authors. Published by Elsevier Ltd. The authors gratefully acknowledge the financial support of the PV4FACADES project (Solar-Era.net) and Imec's industrial affiliation program for Photovoltaics (IIAP-PV).

This work focuses on analytical modelling as an alternative to numerical simulations for determining via-related recombination losses in MWT silicon solar cells. Two new analytical models are presented in this framework. The first model deals with calculation of the local via recombination velocity S via from photoconductance lifetime measurements. The second model relates S via to loss in V OC by solving the diffusion equation in two-dimensions. From short-loop tests, we show that these analytical models are valid over a wide range of via densities. The results also show that alkaline etching treatment after the laser via drilling step reduces S via by an order of magnitude and thus allows for good via passivation. We then make use of these models for estimating V OC loss in our MWT solar cells as a result of via recombination. We find that via recombination losses can be substantial at high via densities and so by limiting the number of vias on large area cells (239 cm 2 ) and with good via passivation, the V OC loss can be limited to ~ 1 mV. This finding is in agreement with our cell integration results.

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

A possible research path to increase the photo-generated current in silicon heterojunction (SHJ) solar cells is to replace doped layers on the front-side of the cell, which result in significant parasitic light absorption losses. MoOx is one candidate to replace the p-doped a-Si:H layer in such devices, although it is claimed to be relatively unstable to thermal treatments. We found that a MoOx film with a thickness of 6 nm is sufficient to achieve a JSC of 36 mA/cm2, which is 0.5 mA/cm2 on average higher than that of our classical SHJ reference cell. We also established a contact sintering condition for printed Ag at 160 °C after MoOx deposition, without degrading the cell performance. The champion MoOx-contacted cell yielded VOC of 724 mV and FF of 74.1%, resulting in an efficiency of 19.3%. From a detailed analysis of the interfaces of the hole contact, an interfacial a-SiOx of 1.6–2 nm was observed between a-Si:H and MoOx irrespective of the MoOx thickness (6–10 nm) before and after contact sinter annealing at 160 °C. We postulate that this a-SiOx layer acts as an interfacial dipole layer and also increases the contact resistivity at this contact. The intrinsic stability of the optimised MoOx-contacted cell is studied using a one-cell mini-module under standard damp-heat testing (85 °C/85% humidity/1000 h). More than 97 %rel of the original efficiency is maintained after 1010 h of testing, which is comparable to the behavior observed in a classical SHJ reference one-cell mini-module that was similarly tested.

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 (VOC). 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 J02‐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 VOC 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.

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 have studied the effect of different cleanings at device level for nPERT (passivated Emitter, Rear Totally diffused) cells with a rear epitaxial emitter. Cleanings based on sulfuric acid and peroxide mixtures (SPM), which are effective but also expensive, were used as reference. We found that simplified cleanings such as those based on sulfuric acid and ozone mixtures (SOM), and those based on water and ozone (IPV), are giving similar results at device level as the expensive SPM cleaning, leading to similar J0e, J0, FSF, and lifetime values. The implied VOC values of corresponding test devices fabricated using the simplified cleanings are at the same high level as the test devices using more expensive SPM cleanings. Actual nPERT devices with moderately doped epitaxially-grown emitters with a constant doping profile (box profile) showed average efficiencies above 22.0% for both SOM and IPV cleanings. Very high VOC values of up to 695 mV and implied VOC values of up to 730 mV were shown for cells with a flat rear surface. Finally, the best nPERT cells with rear epitaxial emitter and SOM cleaning showed efficiencies up to 22.5%, as externally confirmed by ISE CalLab.

The influence of the plasma ignition condition during PECVD deposition from a silane/hydrogen mixture on the amorphous silicon passivation of crystalline silicon surface is investigated. The changes in this process step mainly consist in varying the power density for very brief durations in between 1 s and 3 s. We find that the ignition phase contributes significantly in the film growth, especially in the a-Si:H/c-Si interface formation. In particular, the deposition rate increases with ignition power density. TEM cross-section inspection presents a rougher a-Si:H/c-Si interface with higher plasma power and thus, a tendency for nano-clusters formation caused by the crystalline nature of the substrate. In-situ plasma diagnostics reveal the gradual raise up of I Ha*/ I SiH* with the power density leading to worse SiH* abstraction to the surface. Whereas, time-resolved optical emission spectroscopy explains the possible recombination mechanism in the plasma due to higher-silane related reactive species (HSRS) formation via polymerization reactions. Our results point out that the ignition conditions with a rather low power for longer time give the best passivation, resulting an effective lifetime up to 9 ms.

In order to reduce the material cost for silicon solar cells, several research groups are investigating methods to minimize the silicon consumption for making monocrystalline silicon wafers. One promising approach is deposition of an epitaxial layer on porous silicon, followed by detachment of the layer. This contribution discusses improvements in the epitaxial wafer fabrication by optimization of the porosification process. The introduction of a layered porous silicon structure allows to independently improve both epitaxial layer quality and detachment yield. In this way, we have managed to obtain 100µm thick silicon wafers with effective lifetimes up to 1.3ms, and 40µm thick wafers with effective lifetimes up to 700µs. We will also review the current status of the process development for solar cells made on thin wafers. Two approaches are presented. In the first approach, heterojunction solar cells are fabricated on freestanding epitaxial wafers of 40µm thickness. In the second approach, high efficiency (21%) heterojunction back-contacted cells are fabricated on wafers that are bonded to a glass superstrate. Challenges for device processing and limitations in cell performance are discussed.

In this work, we have investigated the impact of interstitial Fe contamination on the effective minority carrier lifetime of n-type Cz silicon bulk material for high efficiency solar cells. The study covers a Fe concentration in the silicon bulk from 3.5 × 1012 cm-3 to 2.7 × 1014cm-3. We have added 5 different concentrations (30, 100, 300, 1000 and 3000 ppb) of Fe intentionally to a wet chemical process tank and measured the transfer to the silicon wafer surface mimicking a possible contamination during wet chemical processing. In order to fabricate carrier lifetime test vehicles, the silicon wafer is then passivated with thermal silicon oxide from both sides. The surface contamination is driven into the bulk by mimicking a high temperature process during solar cell manufacturing. Effective minority carrier lifetime is measured at injection levels from 1 × 1013 cm-3 to 3 × 1015cm-3. We have fitted the theoretical curve for interstitial Fe derived from the SRH theory to the measured values and extracted the Fe contamination concentration. This value is comparable to the calculated value extracted from the surface contamination measurement. For low level injection (LLI), we extracted the capture cross section for interstitial Fe to be 6.45 × 10-17 cm/s ± 2.23 × 10-17 cm/s. The measured Fe contamination levels are used for the conversion efficiency fitting of a n-type bifacial silicon solar cell using QUOKKA simulations. The simulations show that very low Fe contamination concentrations of [Fe]bulk = 3.5 × 1012 cm-3 ([Fe]surf = 6 × 1010cm-2) already degrade the solar cell efficiency by 10% relative.

Copyright © 2018 John Wiley & Sons, Ltd. In this work, the ATOM (intrinsic a-Si:H/TiO x /low work function metal) structure is investigated to realize high-performance passivating electron-selective contacts for crystalline silicon solar cells. The absence of a highly doped Si region in this contact structure is meant to reduce the optoelectrical losses. We show that a low contact resistivity (ρ c ) can be obtained by the combined effect of a low work function metal, such as calcium (Φ 2.9 eV), and Fermi-level depinning in the metal-insulator-semiconductor contact structure (where in our case TiO x acts as the insulator on the intrinsic a-Si:H passivating layer). TiO x grown by ALD is effective to achieve not only a low ρ c but also good passivation properties. As an electron contact in silicon heterojunction solar cells, inserting interfacial TiO x at the i-a-Si:H/Ca interface significantly enhances the solar cell conversion efficiency. Consequently, the champion solar cell with the ATOM contact achieves a V OC of 711 mV, FF of 72.9%, J SC of 35.1 mA/cm 2 , and an efficiency of 18.2%. The achievement of a high V OC and reasonable FF without the need for a highly doped Si layer serves as a valuable proof of concept for future developments on passivating electron-selective contacts using this structure. ispartof: PROGRESS IN PHOTOVOLTAICS vol:26 issue:10 pages:835-845 status: published

The heterojunction interdigitated back-contact (HJ IBC) cell technology enables remarkably high cell efficiencies, but requirescomplex processing for the rear-side patterning of the interdigitated a-Si:H electron and hole hetero-contacts. Therefore, the HJ IBC process flow must be simplified into a sequence that is cost-effective and industrially-compatible. Towards this goal, a litho-free, all-dry process sequence is being developed. As part of this simplified process flow, a novel in situ dry clean process called “nano-envelope” clean is developed to replace the wet cleaning after dry etching of a-Si:H. Surface contaminationanalysis and passivation studies prove that the developed dry clean is as effective as the standard wet clean. Since the “nano-envelope” clean can bedone in the same PECVD tool, the sequence from dry etching to repassivation can be done fully in situ.

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.

A TiOx layer is well known as an electron-selective contact material because of its asymmetric band offsets with respect to c-Si. When applying TiOx layers as passivating electron-selective contacts, forming sub-stoichiometric TiOx is important to obtain a low contact resistivity because oxygen vacancies increase the conductivity of TiOx and provide n-type doping effects. In this work, oxygen vacancies at SiOx/TiOx interfaces are investigated by atomic depth profiling of XPS measurements. Three kinds of TiOx layers are studied grown by either e-beam evaporation, atomic layer deposition or sputtering on c-Si. In all three TiOx samples, a resulting stack of c-Si/SiOx/TiOx could be noticed XPS measurements that show SiOx peaks near the c-Si/TiOx interface. Moreover, clear TiO2 peaks, which can be measured at the surface of all three TiOx layer types, gradually change to Ti or TiSi2 peaks near the SiOx/TiOx interface. This indicates that many oxygen vacancies seem to exist at the SiOx/TiOx interface. This TiOx reduction may contribute to the formation of a dipole and increased downward band bending resulting in a lower contact resistivity in the electron-selective contacts. As a result, hetero-junction solar cells with i-a-Si:H/TiOx/Ca/Al contacts exhibit a significant series resistance reduction of about 40 % compared to solar cells with i-a-Si:H/Ca/Al contacts.

Dry plasma etching, commonly used by the Photonics community as the etching technique for the fabrication of photonic nanostructures, could be a source of device performance limitations when used in the frame of silicon photovoltaics. So far, the lack of silicon solar cells with state-of-the-art efficiencies utilizing nanophotonic concepts shows how challenging their integration is, owing to the trade-off between optical and electrical properties. In this study we show that dry plasma etching results in the degradation of the silicon material quality due to (i) a high density of dangling bonds and (ii) the presence of sub-surface defects, resulting in high surface recombination velocities and low minority carrier lifetimes. On the contrary, wet chemical anisotropic etching used as an alternative, leads to the formation of inverted nanopyramids that result in low surface recombination velocity and low density of dangling bonds. The proposed inverted nanopyramids could enable high efficiency photonic assisted solar cells by offering the potential to achieve higher short-circuit current without degrading the open circuit voltage. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

In the frame of the development of thin crystalline silicon solar cell technologies, surface nanopatterning of silicon is gaining importance. Its impact on the material quality is, however, not yet fully controlled.We investigate here the influence of surface nanotexturing on the series resistance of a contacting scheme relevant for thin-film crystalline silicon heterojunction solar cells. Two dimensional periodic nanotextures are fabricated using a combination of nanoimprint lithography and either dry or wet etching, while random pyramid texturing is used for benchmarking. We compare these texturing techniques in terms of their effect on the series resistance of a solar cell through a study of the sheet resistance (Rsh ) and contact resistance (Rc) of its front layers, i.e., a sputtered transparent conductive oxide and evaporated metal contacts. We have found by four-point probe and the transfer length methods that dry-etched nanopatterns render the highest Rsh and Rc values. Wet-etched nanopatterns, on the other hand, have less impact on Rc and render Rsh similar to that obtained from the nontextured case., Authors' post-print version in Abdo et al., IEEE Journal of Photovoltaics, July 2015

We recently proposed a novel test structure for the extraction of recombination characteristics of metal-contacted areas on silicon wafers using quasi-steady state photoconductance measurements. In this test structure, photoconductance measurements are performed on several wafer areas with different contact fractions. Each area consists of a lattice of point contacts on an otherwise passivated wafer. The requirement of constant excess carrier densities throughout the quasi-neutral wafer bulk is absolutely essential for the simple extraction of figures of merit for contact recombination. In this study, we first discuss the requirements for constant injection levels during photoconductance measurements, without reference to a particular geometry. Then, we use a simple 1-D model to elucidate how injection levels vary in the test structure plane. We use the model to investigate limiting cases for which the requirement of constant injection levels is satisfied. In addition, we investigate the breakdown of the assumption of constant injection levels. We discuss the influence of nonconstant excess carrier concentrations on photoconductance measurements in the context of our test structure. Finally, we validate our model by comparison with experiments. Our analysis is particularly useful in the context of understanding the design rules for contact size and pitch in our test structure.

Argon (Ar) and oxygen (O2) plasmas are performed on silicone adhesives to eliminate the negative influence of silicone on amorphous silicon (a-Si:H) surface passivation of wafers bonded to glass. Both the Ar and O2 plasmas lead to oxidation of the silicone surface, consisting in an increase of oxygen/carbon ratio, of degree of crosslinking, and of material density. The oxidized silicone is more resilient than pristine and does not interact with the a-Si:H passivation process, allowing for state-of-the-art surface passivation of wafers bonded to glass. Similarities between the modifications induced by the Ar and O2 plasmas on the silicone indicate the secondary role of the O2 radicals in the oxidation process. Moreover, amorphous/crystalline heterojunction interdigitated back contact solar cells (a-Si:H/c-Si HJ i-BC) are fabricated on freestanding and bonded wafers treated with Ar plasma. The devices show comparable open-circuit voltages of up to 675 mV, confirming at device level the efficacy of the treatment.

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin ( 15 cm −3 which corresponds to a minimum minority carrier diffusion length of ~670 µm (> 16 times the silicon thickness). With such high quality epitaxial foils combined with high detachment yield, very high efficiency solar devices on thin silicon substrates would be a reality in the near future.

In the future solar devices will be manufactured onto thin mono-crystalline silicon wafers (foils) potentially bonded to substrate carriers. In this study, the influence of the substrate carrier potential (grounded or floating) on the amorphous silicon a-Si:H surface passivation of the bonded foil is discussed for the cases of conductive and insulating substrate carriers. A conductive carrier leads to an increase in deposition rate and ion bombardment energy, which is detrimental for sample lifetime. An insulating carrier, e.g., a glass substrate, inverts the trend and improves the a-Si:H surface passivation.

We have investigated fine grained polycrystalline silicon thin films grown by direct chemical vapor deposition on oxidized silicon substrates. More specifically, we analyze the influence of the doping type on the properties of this model polycrystalline silicon material. This includes an investigation of defect passivation and benchmarking of minority carrier properties. In our investigation, we use a variety of characterization techniques to probe the properties of the investigated polycrystalline silicon thin films, including Fourier Transform Photoelectron Spectroscopy, Electron Spin Resonance, Conductivity Activation, and Suns-Voc measurements. Amphoteric silicon dangling bond defects are identified as the most prominent defect type present in these layers. They are the primary recombination center in the relatively lowly doped polysilicon thin films at the heart of the current investigation. In contrast with the case of solar cells based on Czochralski silicon or multicrystalline silicon wafers, we conclude that no benefit is found to be associated with the use of n-type dopants over p-type dopants in the active absorber of the investigated polycrystalline silicon thin-film solar cells.

The influence of the cleaning process on the amorphous silicon passivation of homojunction emitters is investigated. A significant variation in the passivation quality following different cleaning sequences is not observed, even though differences in cleaning performance are evident. These results point out the effectiveness of our cleaning treatment and provide a hydrogen termination for intrinsic amorphous silicon passivation. A post-deposition treatment improves the passivation level yielding emitter saturation currents determined by Auger recombination in the order of 70 fA/cm2 and below.

A detailed investigation of the laser damage to amorphous silicon (a-Si:H) layers patterned by laser ablation (LA) and wet chemical etching is presented. This approach can be applied to pattern the rear side of silicon heterojunction interdigitated back-contact solar cells. Only the top sacrificial a-Si:H laser-absorbing layer of an a-Si:H/SiOx/a-Si:H/c-Si stack is ablated. Laser damage in the bottom a-Si:H layer and a-Si:H/c-Si interface is analyzed by both scanning electron microscopy and transmission electron microscopy. We show that the a-Si:H/c-Si passivation is degraded by laser damage and that this degradation can be diminished by increasing laser processing speed. This is attributed to a decrease of laser-irradiated area, and particularly smaller overlapping zones of adjacent laser pulses. The re-passivation quality after LA and wet etching is similar to that of as-passivated samples. This indicates that laser damage is not present in the bulk c-Si substrate but only in the a-Si:H passivation layer, which is removed during subsequent wet etching, thus allowing high quality repassivation., Marie Sklodowska-Curie Action; Epitaxial silicon foil solar cells with interdigitated back contacts 657270 — EpiSil-IBC published in physica status solidi (RRL) - Rapid Research Letters doi:10.1002/pssr.201700125

Texturing of crystalline silicon at the light wavelength scale is a promising approach for light management in ever thinner cells. However, this approach requires adapting the device design and process, which has so far revealed to be a challenging task. Indeed, successful integration requires compromising the excellent optical properties for preserving the electrical ones. We briefly summarize what has been learned so far in the community and illustrate some of these learnings with the fabrication and improvement of 1-μ m-thin monocrystalline nanotextured silicon solar cells.

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.

A parametric study of post-deposition hydrogen plasma treatment of intrinsic a:Si:H films is performed. We demonstrate a significant improvement in passivation of c-Si(100) promoting epitaxy after an in-situ hydrogen plasma treatment depending mainly on the pressure and slightly on the power. Plasma diagnostic indicates an increase of Hα* signal with high power and low pressure. However, our analysis reveals a better hydrogen incorporation with high pressure and a slight increase in monohydride with high power. Longer H2 plasma duration up to 50 s shows no detrimental effect on the passivation quality. Optimizing the in-situ H2 plasma treatment, high minority carrier lifetime over 15 ms was achieved after short thermal annealing. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

The August 2014 Special Issue of Solar Energy Materials & Solar Cells is dedicated to the 2nd workshop on Silicon Materials held in Rome, Italy, during October 7-8, 2013. During the workshop several discussions were held on the main question, 'Which are the challenges for Si material research'. The discussion focused on the results of the online survey and on the outstanding contributions of experts and presenting authors. One of the most debated points was how to select technologies that would enable cost reduction. A few possible scenarios to reduce costs were proposed. No consensus was reached with respect to the best scenario enabling the strongest cost reduction. During the workshop 27 contributions in addition to four personal statements from invited experts were given by industry, research institutes and universities.

The passivation characteristics of thermally grown silicon dioxide (SiO 2 ) and hydrogenated amorphous silicon (a-Si:H) layers are investigated, using a combination of photoluminescence and capacitance-voltage analysis techniques. Key findings are the significant passivation degradation of SiO 2 and a-Si:H layers induced by metallization through electron beam evaporation. The degradation correlates with an increase in silicon dangling bond defect density at the interface with silicon (for both SiO 2 and a-Si:H) or in the passivation layer (a-Si:H). Performing the metallization by thermal evaporation is an effective method to avoid such process-induced damage, as is forming gas annealing at 450°C, which effectively recovers the interface characteristics of SiO 2 layers. Deposition of amorphous silicon on a thermal SiO 2 layer induces bulk and interface defects in the SiO 2 layer-but in this case, a 450°C forming gas anneal is not possible due to the thermal budget limitations of a-Si:H, thereby posing problems for solar cell structures which rely on a combination of PECVD a-Si:H and thermal SiO 2 passivation layers.