91128 PALAISEAU CEDEX ... - École Polytechnique

91128 PA L A I S E AU C ED E Xwww.lpicm.poly technique.fr

Introduction .......................................................................................................................................................................................................... 3

I. Collaborative projects ............................................................................................................................................................................ 7

A. NATHISOL (ANR, October 2012 to September 2016) ............................................................................................... 7

B. IMPETUS (ANR, November 2013 to October 2017) ..................................................................................................... 8

C. PhotoNVoltaics (EU-FP7, November 2012 to October 2015) ............................................................................. 9

D. IPVF project E (IPVF, March 2015 to December 2019) ............................................................................................ 9

E. IPVF project A: New processes for high efficiency silicon based solar cells(IPVF, November 2014 to December 2019) ...................................................................................................................... 10

II. Independent projects .............................................................................................................................................................................. 11

III. Building scientific community ...................................................................................................................................................... 13

A. International Workshop on New materials and Innovative Solar Cell Architectures ................. 13

B. PVSiXT Workshop Series with Martin Green (Invited Professor) .................................................................. 14

C. Weekly Seminar Series ........................................................................................................................................................................ 14

IV. Update on personnel ............................................................................................................................................................................ 15

A. Soumyadeep MISRA ............................................................................................................................................................................... 15

B. Dennis LANGE ............................................................................................................................................................................................... 16

C. Bastien BRUNEAU ...................................................................................................................................................................................... 17

D. Zheng FAN ......................................................................................................................................................................................................... 18

E. Alienor TOGONAL ........................................................................................................................................................................................ 19

F. Paul NARCHI ...................................................................................................................................................................................................... 20

G. Jean-Maxime ORLACH .......................................................................................................................................................................... 20

H. Farah HADDAD .............................................................................................................................................................................................. 21

I. Jian TANG ............................................................................................................................................................................................................. 21

J. Alice DEFRESNE ........................................................................................................................................................................................... 22

K. Zuzana MRAZKOVA ................................................................................................................................................................................... 22

L. Fatme JARDALI ............................................................................................................................................................................................. 23

M. Ronan LEAL ..................................................................................................................................................................................................... 23

Table of contents

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N. Rasha KHOURY ............................................................................................................................................................................................ 24

O. Junkang WANG ............................................................................................................................................................................................. 24

P. Fabien Lebreton ............................................................................................................................................................................................ 25

Q. Gwenaelle HAMON .................................................................................................................................................................................... 25

R. Guillaume Fischer ...................................................................................................................................................................................... 26

S. Mutaz Al-Ghzaiwat ...................................................................................................................................................................................... 26

T. Laurent Guin ..................................................................................................................................................................................................... 27

V. Update on research personnel ................................................................................................................................................. 29

A. Movement of Personnel and Alumni ....................................................................................................................................... 32

VI. Update of tool platform ...................................................................................................................................................................... 33

VII. Outlook .................................................................................................................................................................................................................... 35

VIII. Publications ......................................................................................................................................................................................................... 37

a. Publications in peer-reviewed journals ................................................................................................................................... 37

b. Invited international conferences ................................................................................................................................................ 39

c. Conference proceedings ...................................................................................................................................................................... 40

d. Oral conference presentations ...................................................................................................................................................... 40

e. Book Chapters ............................................................................................................................................................................................... 41

f. Patents .................................................................................................................................................................................................................... 41

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Pere Roca i Cabarrocas

LPICM director

2015 has been an “excellent cru” as we say for wine. This report proves it and I encourage you to taste it (to read it all). The scientific production of PVSiXT was again outstanding in terms of publications in high impact journals, invited oral presentations at international and national conferences, and patent applications.

In 2015 PVSiXT firmly consolidated its position and reached an efficient steady-state operation with weekly seminars, regular project meetings, and workshops. Moreover we have enjoyed a constant improvement in the infrastructure and equipment thanks to the support of the BEER team, and global functioning thanks to the support from the electronics, the informatics and the administration teams.

Tailored voltage waveforms, low temperature plasma epitaxy, tandem radial junction solar cells, silicon on III-V, in situ photo-luminescence, nanoscale characte-rization… are a few keywords which distinguish PVSiXT from other research groups and position us in the very competitive worldwide PV field. They were recognized by various awards at international conferences. It is this background and these developments which are at the basis of our involvement in various IPVF projects (A, H, J, K, F) that we will continue to develop together: resear-chers, engineers, postdocs, and PhD candidates. I like to think that the strength of PVSiXT is mainly due to the uniqueness and origins of each one of its members. This reflects on the pluridisciplinarity of research topics covering plasma processes, thin film deposition and characterization techniques, optics and photonics, new solar cell architectures, and modeling at all levels of the value chain (plasmas, materials and devices).

2015 has also been a year of strong interaction with our industrial partner SunPower (judging from the frequent DHL boxes I have seen going in and out). I take this as a proof of the high quality research being carried out at PVSiXT.

Looking forward, 2016 will be a year to prepare the transition, with the IPVF building and projects gaining momentum and the new building ready to host PVSiXT in 2017. Let me finish by saying THANK YOU to all PVSiXT members, thank you for making it so successful and thanks to Erik and Fabrice for taking the leadership. Enjoy reading the report!

Introduction

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Lars OBERBECK,

Head of Solar, R&D Department,

TOTAL New Energies

It is with great pleasure that I introduce the second edi-tion of the PVSiXT Annual Report. 2015 was a rich year for the joint team, with significant scientific achieve-ments, as demonstrated by the high number of publi-cations in renowned journals and the strong conference contributions.

Such excellent scientific work was enabled by having an excellent, motivated and multicultural team and also by improvements in the laboratory environment: our clean-room which houses the cluster tool received an air-conditioning system and simultaneously the clean-liness was visibly enhanced, providing the team with a much improved work environment.

Moreover, 2015 has seen the start of the IPVF projects which cover a broad range from silicon solar cells, CIGS/CZTS and III-V on silicon tandem cells to advanced

characterization and modeling. With the first IPVF PhD student being housed in our offices and the first IPVF equipment installed in our laboratories, IPVF is now no longer a paper exercise, but has become reality!

The team is strongly involved in about half of the IPVF projects and in particular, of course, in project A on various building blocks for thin, low-cost silicon solar cells. Initial results have been achieved here, including the demonstration of the first bifacial n-PERT solar cell.

The new institute – the building will be available in the second half of 2017 – will need time to grow and to become visible in the scientific community, and it needs contributions from all of us to make IPVF suc-cessful, building on the strengths of PVSiXT: plasma deposition/epitaxy of silicon films, Tailored Voltage Waveform plasma processes, advanced characteriza-tion…

I also acknowledge that based on the team’s strengths we have been able to start another collaborative project that is of strong interest to our affiliate SunPower. My hope and expectation is that in the future we can even more work together on such projects, creating value for both PVSiXT and SunPower.

I expect that 2016 will be again rich of challenges for all of us, but also of rewards. We will see more equipment in our laboratories and more IPVF scientists, project collaborations with our numerous partners will further intensify, and we will need to prepare our move into the IPVF building in 2017. I thank all of you for your great work and in particular Pere, Erik and Fabrice for making our collaboration so successful and providing scientific guidance. Merci beaucoup and keep up the great level of work!

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In 2015, PVSiXT has been involved in several scientific projects funded by the ANR (Nathisol, Impetus) and the FP7 program of EEC (PhotoNVoltaics).

Most importantly, PVSiXT has contributed to the defi-nition and start of several IPVF projects. IPVF (Institut Photovoltaique d’Ile de France) is labeled an ITE (Insti-tut pour la Transition Energétique) by the Commissariat Général à l’Investissement and has been awarded an 18 M€ grant. CNRS, Ecole Polytechnique and Total are amongst the founding members (see also www.ipvf.fr). In this context, PVSiXT has contributed to more than half of the ten projects, all started in 2015.

The project A “New processes for high efficiency silicon based solar cells” fits well with existing PVSiXT back-ground and scientific objectives. Both Total and LPICM are heavily involved in the project, together with our IPVF partners Air Liquide and EDF.

Both partners are also involved in projects H “New tools for electrical and optoelectronic characterization”. “ and J “Theory and modeling”.

A new project F on perovskites has also been started in 2015 with a leading contribution by LPICM.

Both teams are also strongly involved in the combina-tion of silicon and III-V alloys to target a photovoltaic tandem cell reaching 30% at a competitive cost. Two projects are being pursued: IPVF project E “Develop-ment of III-V cells for high efficiency solar cells” and ANR project Impetus with two different technical approaches.

Finally PVSiXT contributes through module and tandem expertise to IPVF project B “Innovative CIGS modules for light-weight applications” and C “CZTS materials with only abundant & non toxic elements”.

I. Collaborative projects

Leader

A New processes for high efficiency silicon based solar cells J.Penaud

B Innovative CIGS modules for light-weight applications N. Naghavi

C CZTS materials with only abundant & non toxic elements C. Guerard

D New concepts for ultrahigh PV conversion efficiencies JF Guillemoles

E Development of III-V cells for high efficiency solar cells P. Aing

G Nano-GPS for virtual coupling of new instruments O Acher

H New electrical and optometric characterization J.P. Kleider

I Advanced chemical characterization of PV devices A. Etcheberry

J Theory and modeling S. Laribi

K Life cycle analysis of new PV technologies D. Jahan

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A. NATHISOL (ANR, October 2012 to September 2016)

Partners: INL, École Centrale Lyon, LPICM, LPN, and TOTAL.

The goal of the Nathisol project is to fabricate high performance thin film devices based on epitaxial c-Si films produced by low-temperature PECVD, which are transferred to foreign substrates and nanostructured to maximize the light absorption.

While the main contribution of LPICM and Total was in the fabrication of the materials and solar cells, surface passivation and economical evaluation, researchers from PVSiXT team contributed also to the optimization of the nanopatterning, improvements of the transfer techniques and the light trapping analysis.

The high performance thin film devices can be fabricated only through a close collaboration between all partners with INL providing the optical design, LPN using their transfer, nanopatterning and contacting processes. A schematic description of the process to fabricate a 3 µm thick solar cell with Jsc of 25 mA/cm2 measured by EQE is shown in Fig. 1.

The fabrication process for c-Si solar cells on glass is as follows: A) epitaxial growth of c-Si film by PECVD; B) deposition of (N)-passivation/ZnO:Al/Ag/Al, anodic bon-ding on glass and lift-off; C) scanning electron micros-copy image of the nanopattern; D) structuring of the top surface of the epilayer with inverted nanopyramids; E) deposition of the (N)-passivation/ITO and of the front contacts.

B. IMPETUS (ANR, November 2013 to October 2017)

Partners: LPICM, III-V Lab, GeePS, Total.

In the IMPETUS project, an innovative approach for III-V/Si multijunction solar cells is studied, aiming at high conversion efficiencies.

The targeted device is a tandem junction composed of a III-V top cell (AlGaAs) and a group IV bottom cell (Si1-xGex). The choice of AlyGa1-yAs as the top material is justified because it provides the optimum bandgap combination with Si1-xGex, 1.63 eV/0.96 eV, able to reach conversion efficiencies in excess of 42% in a tan-dem configuration.

In our inverted metamorphic approach, we first use MOVPE to grow the AlGaAs top cell on a lattice matched substrate, and then perform low temperature (LT-) PECVD heteroepitaxial growth of SiGe for the bottom cell, as shown in Figure 1. The tandem cell is then trans-ferred to a low cost carrier and the GaAs substrate is reclaimed (see figure below).

In 2015, we have shown the first structural and electrical characterizations of Si(PECVD)/III-V(MOVPE) interfaces. Furthermore, the growth of highly doped crystalline Si by low-temperature PECVD enables us to fabricate hybrid tunnel junctions with low resistivity and a high current, suitable to interconnect the two subcells of the tandem III-V/Si solar cell.

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C. PhotoNVoltaics (EU-FP7, November 2012 to October 2015)

Partners: LPICM, TOTAL, IMEC, INL, UNamur, Obducat, and Chalmers.

Since November 2012, seven European teams from diverse fields (photovoltaics, photonics and nanolitho-graphy) have worked enthusiastically on the FP7 project PhotoNVoltaics.

They identified the conditions for a disruptive solar cell generation that integrates advanced light trapping sche-mes into thin-film crystalline silicon (c-Si) solar cells. In tight interaction loops between models and experi-ments, they learned how to capture the light efficiently, which photonic nanostructures work best as front-side textures, and how can these structures be efficiently integrated into the c-Si solar cell.

Now that the project has come to an end, the project partners have set forth all the guidelines of how to make these highly efficient thin-film c-Si solar cells, and identified routes for low-cost integration.

D. IPVF project E (IPVF, March 2015 to December 2019)

Partners: IPVF, Riber, GeePS, LPN, TOTAL, EDF.

III/V elements represent an ideal material for the fabri-cation of high efficiency solar cells due to their wide bandgap range, with a current record of 28.8 % for single junction GaAs and a theoretical limit up to 44.7% for multijunction cell structures.

A major part of the high cost of such III-V based solar cells (in comparison to other PV technologies: organic, Si, perovskites…) is the substrates/bottom cells (Ge or GaAs). Therefore an important cost reduction is expec-ted if low cost substrate materials could be used ins-tead. Hence, a tandem cell using active Si substrate is a solution to reduce the cell cost and increase the efficiency above the SJ Shockley-Queisser limit.

In this project, AlGaAs based structures were epitaxially grown on GaAs substrate. These structures were first optimized using simulation of cell designs based on an extensive literature survey. The cell was then grown using a molecular beam epitaxy (MBE), an effective tool for research with high vacuum and low contamination characteristics. Analysis of the MBE’s doping profile and growth conditions (temperature, pressure…) show pro-mising innovation in the field. Process steps (selective etching, metallization, wafer removal, and passivation) and characterization techniques have been optimized to obtain a cell with an efficiency of 12.5% for a bandgap of 1.78eV.

MBE growth III-V top cell

Si bottom cell

Epoxy

Reusable substrate

Lift-off

4-terminal structure

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E. IPVF project A: New processes for high efficiency silicon cells (IPVF, November 2014 to December 2019)

Partners: LPICM, TOTAL , EDF, AirLiquide.

This project aims to develop new processes for the next generation of high efficiency thin crystalline silicon (c-Si) solar cells. One of the first objectives is to reduce the device thickness without losing in efficiency in order to reduce the cost of PV electricity. This project aims at exploring key technologies which could possibly be integrated in industry to improve cells and therefore the competitiveness of PV systems.

Several building blocks have been identified to contri-bute to the next generation of c-Si solar cell both decrea-sing cost and increasing efficiency:

1. In order to increase efficiency, improved light trap-ping is investigated by dry processes. Further-more, these dry processes are also thought to be used for cleaning steps.

2. Surface passivation with new and advanced mate-rials compatible with nanostructured surfaces obtained by dry processes is needed.

3. Plasma epitaxial growth of doped surfaces is inves-tigated as a low temperature process to replace dopant diffusion process.

4. Advanced metallization based on plating of copper and nickel will allow high current densities due to reduced shadowing.

The integration of these different building blocks is led in parallel with optimization of the needed device archi-tecture and modification of the process flow in order to be compatible with thin substrates (<100 µm) leading to solar cells with an efficiency >20% with a reduced number of process steps.

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The second collaboration contract between LPICM and TOTAL (2014-2016) has been augmented by the addi-tion of several project types. Beyond the common col-laboration framework, there is the possibility to carry out independent projects by Total using the PVSiXT common platform. Three projects have been defined on silicon, cells, and modules. It allows to push for research at higher TRL’s (Technology Readiness Level) in the proper environment.

Second, bilateral collaborative projects have been designed to allow a close and direct cooperation between TOTAL affiliates and LPICM experts to solve key technical issues closer to the development stage.

II. Independent projects

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PVSiXT has contributed in building a scientific commu-nity on photovoltaics in the Paris area (Ile de France).

A major contribution arises from the start of IPVF (see section I). Starting the IPVF projects has helped the researchers from Total, EDF, Air Liquide, Horiba, Riber, Ecole Polytechnique and the laboratories of the CNRS PV federation to get to know each other better. PVSiXT has also contributed to the IPVF scientific days on April

14th at Ecole Nationale de Chimie and on the 5th of October at Total.

In addition to these actions associated with the IPVF, several other activities have been organized to connect with the broader scientific community and build future partnerships.

III. Building scientific community

A. International Workshop on New materials and Innovative Solar Cell Architectures

The workshop was held on the 13th of October in École polytechnique. The objective was to focus on the ways to increase the energy we can get out of an absorber material (silicon, III-V, perovskite…) by using diffe-rent contact materials (doped layers, metals, oxides, graphene, organics…):

– What are the limitations of standard doped layers? – Do we need doped layers?– Which materials can be used to achieved efficient

electron and hole collection?– What can we learn from modeling about contacts?– Are bifacial cells the way to go?

It has gathered renowned international experts to dis-cuss informally during a day and a half. Let us cite:

– Pietro Altermatt (Leibniz University of Hannover) “Modeling solar cells: the continuous transition from ordinary metal contacts, to passivated contacts, to hetero-emitter contacts”

– Antonio Marti (Universidad Polytechnica de Madrid) “Three Terminal Heterojunction Bipolar Transistor Solar Cells (3T-HBTSC): A new Approach for Contac-ting Multijunction Solar Cells”

– Martin Green (University of New South Wales) “Perovskite Solar Cells: Can They Boost Si Cell Per-formance?”

– Stefaan De Wolf (EPFL) “Towards dopant-free silicon solar cells”

– Radovan Kopecek (ISC Konstanz) “Bifaciality: One small step for technology one giant leap for KWh cost reduction”

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B. PVSiXT Workshop Series with Martin Green (Invited Professor)

Professor Martin Green of the UNSW was a part-time guest at the PVSiXT laboratory from October 1 to 21st, financed by Total through the Chaire Polytechnique, and a series of workshop sessions were organized to best benefit from his presence in the lab.

Working group sessions were organized on perovskites, PV market overview, c-Si technology, characterization and point contacts, nanowires, and epitaxy/multijunc-tions. These workshops were designed to allow resear-chers and students in the lab and from Total to present their work and views on these subjects, and then to have a free exchange with a world-renowned expert on these subjects. This format was quite intense, and allowed some very constructive discussions on the way forward on a number of research topics in the lab, and to build personal links between PVSiXT and the research community.

C. Weekly Seminar Series

A weekly seminar series (organized in 2015 by PVSiXT researchers Martin Foldyna and Jean-Luc Maurice) has been held at the LPICM. These seminars consist of presentations on topics of broad interest to the group, both in areas directly and peripherally linked to PV. The speakers may be new arrivals to the LPICM or PVSiXT, or invited speakers (often present for collaboration or a thesis defence).

Seminars of note in 2015 were given by Prof Z. Donko (Budapest), Prof. V. Dalal (Iowa SU), Prof. M. Konagai (Tokyo IT), Dr. D. Lausch (Fraunhofer CSP).

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IV. Self introduction of PhD work Graduated in 2015

A. Soumyadeep MISRA

Defended September 23, 2015

Supervisor: Pere Roca i Cabarrocas

Funding: École polytechnique

Doctoral School Scholarship (EDX)

Single and tandem radial junction silicon thin film solar cells based on PECVD grown crystalline silicon nanowire arrays

Owing to their enhanced light trapping and anti-reflection properties, silicon nanowires (SiNWs) provide an effective research platform for developing a new generation of low-cost and high efficiency solar cells. By decoupling light absorption and carrier collection directions, SiNWs facili-tate the use of very thin intrinsic layers in PIN radial junc-tion solar cells. An efficient approach to grow SiNWs on low cost substrates is to use plasma-assisted vapor-liquid-so-lid (VLS) method combined with low surface tension and low melting point metals.

This thesis is dedicated to understanding the growth mechanism of such SiNW arrays and optimizing the perfor-mance of radial junction solar cells. We have carried out a series of experiments, studying the axial and radial growth of SiNWs using tin (Sn), and found that the experimental observations are consistent with the presence of a thin sidewall wetting Sn layer. We have proposed a phenome-nological model to show that this wetting layer stabilizes the metal droplet on the top of nanowire and even pro-motes its growth when the droplet is exhausted. Using a thin (~100 nm) hydrogenated amorphous silicon (a-Si:H) intrinsic layer in the radial junctions results in a high built-in field and hence a good collection of photo-generated car-riers. By optimizing the number of radial junctions per unit area we have managed to achieve an initial efficiency ~8%.

We have also shown that such ultra-thin absorbers limit the light-induced degradation to 6%, compared to 15-20% for planar junctions. Moreover, replacing the top layer by a more transparent n-type hydrogenated microcrystalline silicon oxide (µc-SiOx:H) enhances the response of the cell in the blue spectral region and improves the efficiency up to 9.7% with an open-circuit voltage (Voc) of 0.82 V. In a separate approach, by adding a thin p-type a-Si:H layer at the p-type crystalline SiNW/ intrinsic a-Si:H interface a Voc as high as 0.9 V has been achieved. We have also demons-trated radial junction solar cells incorporating lower band gap intrinsic layers (µc-Si:H and a-SiGe:H) to take the advan-tage of a broader solar spectrum. Finally, we have success-fully applied them for the realization of tandem radial junc-tion (PIN/PIN) solar cells [Fig. 1], which to our knowledge is the first demonstration of such kind of devices.

Fig. 1. Dark field STEM image of transversal cross-section of tan-dem radial junction solar cell showing a-Si:H, n-type and p-type µc-SiOx:H and intrinsic pm-Si:H layers. The inset schematically shows the approximate position of the cross-section.

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B. Dennis LANGE

Defended September 2015

Supervisors: Nicolas Triantafyllidis

(LMS), Pere Roca i Cabarrocas (LPICM)

Funding: École polytechnique Monge

Scholarship

Piezoresistivity of thin film semiconductors under mechanical strain with application to solar cells

The aim of this PhD thesis was to characterize and model the influence of strain on thin film solar cells in order to evaluate if strain might enhance solar cell effi-ciency.

The influence of mechanical strain on the resistivity ρ of intrinsic and doped hydrogenated amorphous and microcrystalline silicon (a-Si:H and µ-Si:H) thin films, as well as indium tin oxide and aluminum doped zinc oxide, was examined via uniaxial tension and compres-sion tests and a simultaneous measurement of the resistivity both parallel and perpendicular to the applied strain. Intrinsic and n-type silicon layers show a decrea-sing resistivity with increasing tensile strain whereas the p-type layer shows an opposite behavior.

This is attributed to the different ways strain affects valence and conduction bands. The effect is more pro-nounced the greater the order in a material; the smal-lest effect is thus found in amorphous silicon and the biggest in crystalline silicon. Those changes originate from the shift of valence and conduction bands with strain. The measured values have been used to esti-mate a possible influence of an applied external strain on the current-voltage characteristics of PV-devices (analytical and numerical).

These calculations have shown changes in the cell effi-ciency of at most 0.3% for applied strains of 0.75%. The strain-limiting layers in a p-i-n cell will most likely be the transparent conductive oxides because both ITO and ZnO:Al already show an irreversible resistivity increase (which is most likely linked to cracks) at a strain of about 0.5%. Silicon layers can withstand higher strains (up to roughly 1% in tension and compression). Tensile tests inside an electron microscope (see picture) with n-type µ-Si:H have proven the origin of irreversible resistivity changes to be cracks perpendicular to the applied strain.

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C. Bastien BRUNEAU

Defended October 6, 2015

Supervisors: Erik Johnson



Control of radio frequency capacitively coupled plasma asymmetries using Tailored Voltage Waveforms

The aim of this PhD thesis was to characterize and model the influence of strain on thin film solar cells in order to evaluate if strain might enhance solar cell efficiency.

This thesis deals with the study of capacitively coupled plasmas excited with Tailored Voltage Waveforms (TVWs) and their use in silicon thin film fabrication for photovoltaic applications. Tailoring the voltage wave-forms applied to an electrode of a plasma reactor is a recent technique. Compared to standard sinewaves, TVWs are composed of a fundamental frequency, in the radio frequency domain, and its first harmonics. Any waveform can then be approximated by selecting the first components of its Fourier series. It has been shown for example that the ion energy can be controlled by the amplitude asymmetry of the waveform, the ion flux being kept constant.

In the first part, we make use of the ability of TVWs to vary the ion energy to understand its effect on silicon thin film deposition. We find well-defined energy thresholds above which we observe, through several characteriza-tion techniques, a lower nucleation density in the case of microcrystalline silicon growth (Fig. 1-a). This indicates that ions with energy above these thresholds can break the crystalline bonds. We confirm this result by studying low-temperature epitaxial growth on a crystalline silicon wafer. Above the energy threshold, epitaxy breakdown occur, and distinct amorphous columns are observed in cross-section transmission electron microscopy images, which are initiated at locations where the crystal was broken by high energy ions.

Because of unexpected different nucleation densi-ties for sawtooth-up and down waveforms, despite the similar ion energies, we further explore the effect of these waveforms on plasma discharges. We find, using simulations in argon, that the asymmetry in the sheaths motion created by these waveforms induces a strong localization of the ionization events close to one electrode (Fig. 1-b), and consequently a strong ion flux asymmetry. This localization is confirmed experi-mentally with phase-resolved optical emission spec-troscopy (PROES). Surprisingly, we find that, for a given waveform, this asymmetry can be reversed for different chemistries (namely CF4 plasmas), because of different electron heating mechanisms. This new asymmetry can be fully controlled via the rise-to-fall ratio of the wave-form, i.e. its temporal asymmetry, and therefore opens the path for new applications of TVWs.

Fig. 1. (a) Schematic of microcrystalline growth as a function of maximum ion energy, (b) spatio-temporal excitation rate obtained from simulations (PIC) and experiments (PROES) for an argon discharge excited with a sawtooth waveform.

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D. Zheng FAN

Defended December 21, 2015

Supervisors: Pere Roca i Cabarrocas

Co-supervisor: Linwei Yu

Funding: CSC

In-plane silicon nanowires and their self-assembly for FET applications

My Ph.D. thesis is the first to study the growth of in-plane silicon nanowires (SiNWs) and to develop a method to generate self-organising SiNWs for nano-electronics applications. The growth of in-plane SiNWs is based on plasma-enhanced chemical vapour deposition (PECVD) thin film techniques, and results from the interaction between a liquid metal drop and a hydrogenated amor-phous silicon (a-Si:H) thin film. This in-plane solid-li-quid-solid (IPSLS) growth mode can be considered as a reactive-spreading and moving behaviour of a liquid on a-Si:H, activated by the establishment of a surface energy gradient due to the phase transition from a-Si:H at the advancing NP edge to c-Si at the NP receding edge.

In order to integrate IPSLS SiNWs in nano-electronic devices it is of prime importance to organise them in a controllable way. We propose a growth-in-place strategy which enables the SiNWs to grow along pre-patterned guiding steps. Moreover, this technique is applied to form In NPs on the sidewall of buried ITO layers, which facilitates the localization of the step-guided SiNWs.

This work sets the basis for the fabrication of SiNW field effect transistors on insulator, which also allows for gate engineering and the obtaining of Fully Depleted Sili-con-On-Insulator (FD-SOI).

Acknowledgements: This PhD project was done in col-laboration with Laboratoire de Photonique et de Nanos-tructures (LPN)-CNRS and partly supported by France National Nanofabrication Network (RENATECH).

Key words in-plane silicon nanowires, solid-liquid-solid growth mode, reactive spreading, step-guided growth, Indium nanoparticles formation, SiNW field effect tran-sistors.

Fig 1. A typical in-plane silicon naonwires via a solid-liquid-solid

growth mode.

Fig 2. Example of step-guided growth of in-plane silicon nanowire

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IV. Self introduction of PhD work PhD Candidates

E. Alienor TOGONAL

Started September 2011


Rusli (NTU)

Funding: NTU

Silicon Nanowires for Photovoltaics: from the material to the device

My thesis focuses on the fabrication of disordered SiNWs using a low-cost top-down approach named the Metal-Assisted-Chemical-Etching process (MACE). In 2015, disordered and ordered SiNW arrays have been integrated into two types of solar cells: heterojunction with intrinsic thin layer (HIT) and hybrid devices. SiNW based HIT devices were fabricated by RF-PECVD and the optimization of the process conditions has allowed us to reach efficiency as high as 12.9% with excellent fill factor above 80%.

Hybrid solar cells based on the combination of SiNWs with an organic layer have also been studied and cha-racterized.

Inclined-view SEM picture of a hybrid solar cell consisting of SiNWs coated with PEDOT:PSS

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F. Paul NARCHI

Started December 2013


Co-supervisor: Martin Foldyna

Funding: TOTAL CIFRE

Local Characterization of silicon solar cells using scan-ning probe microscopy techniques

In 2015, Kelvin Probe Force Microscopy (KPFM) mea-surements were carried out on the cross-section of crystalline silicon solar cell devices. The effect of exter-nal excitation (illumination and voltage bias) was obser-ved at the nanoscale on the surface potential images (figure). Using these measurements, a technique to investigate the electric field along the PIN junction was developed and presented at EUPVSEC conference. This work was rewarded with a Student Award.

In parallel, measurements with other nanoscale resolu-tion techniques were performed to compare with KPFM measurements, through many collaborations.

Influence of illumination on KPFM measurements on the PIN junc-

tion of an epitaxial silicon solar cell

G. Jean-Maxime ORLACH

Started October 2013


Vincent Giovangigli

Co-supervisor: Tatiana Novikova

Funding: DIM NanoK IdF

Modelling silicon nanocrystal dynamics in silane discharges: Relationship with nanostructured and epi-taxial growth for thin film photovoltaics

In this thesis we develop a self-consistent numerical axisymmetrical fluid plasma model required for the understanding and control of the formation and trans-port of small nanoparticles (1-10nm) generated in silane/hydrogen plasmas[1], which are typically used for thin film deposition in PECVD (Plasma Enhanced Che-mical Vapour Deposition) reactor for photovoltaic appli-cations[2].

We have so far derived a complete fluid model for non-thermal polyatomic partially ionized chemically reactive plasmas from kinetic theory[3]. This model was then implemented in a FORTRAN program and used for simulation of a capacitively-coupled radio-frequency discharge. In the final year of my thesis, this model will be further enriched with the incorporation of silicon nanoparticle dynamics.

[1] T. Novikova et al. “Numerical modeling of capacitively coupled

hydrogen plasmas: effects of frequency and pressure”, J.

Appl. Phys., 93(6), 3198 (2003).

[2] R. Cariou, M. Labrune, P. Roca i Cabarrocas. “Thin crystalline

silicon solar cells based on epitaxial films growth at 165 ■C by

RF-PECVD”, Solar Energy Materials and Solar Cells, 95, 2260-

2263 (2011).

[3] V. Giovangigli, Multicomponent Flow Modeling, MESST Series,

Birkhauser, Boston (1999).

21

H. Farah HADDAD


Supervisor: Jean-Luc Maurice

Co-supervisor: Tatiana Novikova



Transmission electron microscopy analysis of crystalli-zation mechanisms of silicon deposited by PECVD

My thesis aims to understand the crystallization mecha-nisms of silicon during thin film growth by PECVD, by means of Transmission Electron Microscopy (TEM). I am using mechanical polishing and ion milling for TEM sample preparation.

In 2015, I characterized epitaxial silicon deposited by Ronan Léal using a SiF4/H2/Ar plasma, in order to unders-tand how low-temperature epitaxy is established. Our preliminary results on the first stages of growth reveal nano-scale surface roughness1.

I also performed TEM observations on p-type silicon containing carbon and oxygen obtained by Prabal Goyal using SiH4/H2/HMDSO/B2H6/Ar. An interesting five-fold symmetry2 was observed, resulting from epitaxial growth and multiple twin formation.

1. High-resolution TEM image showing c-Si islands on top of sili-

con wafer.

2. Diffraction pattern showing an interesting fivefold symmetry.

I. Jian TANG


Supervisor: Erik Johnson

Co-supervisor: Pere Roca i Cabarrocas,

Martin Foldyna


DRE Scholarship

Optoelectrical modeling of PV devices from nanostruc-tured silicon with different degrees of crystallinity from nanocrystalline to monocrystalline

A comprehensive study of the silicon nanowire (NW) growth process has been carried out. Silicon NWs were grown by plasma-assisted-vapor-solid method using tin as a catalyst. We have focused on the evolution of the silicon NW density, morphology, and crystallinity. As shown in the figure below, we provide direct evidence of the merging of Sn catalyst droplets and the formation of Si nanowires. We found that during the first stages of NW growth, the density of Sn droplets decreases from ~9000 Sn to 2000 droplets/µm2 after just 10 seconds of growth. The long and straight NW density decreases from 170/µm2 after 2 minutes of growth to less than 10/µm2 after 90 minutes. This strong reduction in NW density is accompanied by an evolution of their mor-phology from cylindrical to conical, then to bent conical, and finally to a bent inverted conical shape.

22

J. Alice DEFRESNE


Supervisor: P. Roca i Cabarrocas,

Olivier Plantevin

Funding: Univ. Paris Sud

Robust c-Si wafer passivation through ion bombardment

The p-doped passivation layer used in HIT structures does not withstand the temperatures above 200°C necessary to anneal the TCO layers. In this work, we show that an irradiation by an argon ion beam allows us to anneal our solar cell precursor up to 400°C while maintaining a lifetime above 1 ms.

In order to study the impact of defects as a function of their location in the solar cells, we homogeneously implanted argon ions with an IRMA implanter. The samples are irradiated with Ar+ ions at room tempera-ture, with energies of 5, 10, 17 and 30 keV, in order to create defects at different depths in the samples. For pi/n/in samples irradiated at 10 keV, we can even get higher lifetimes after irradiation and annealing than the as-deposited lifetime.

K. Zuzana MRAZKOVA


Supervisor: P. Roca i Cabarrocas,

Kamil Postava

Co-supervisor: Martin Foldyna

Funding: French Government

Scholarship

Modeling and characterization of materials and nanos-tructures for photovoltaic applications

During my second year I have built on my previous work realized in collaboration between LPICM and Nanotech-nology Centre (CNT), Ostrava. I have focused on opti-cal characterization and modeling of crystalline silicon wafers with random pyramidal textures, produced by wet chemical etching with different chemical recipes, coated by thin films (such as passivation layers, trans-parent conductive oxide layers, antireflection coatings, etc.) deposited by plasma-enhanced chemical vapor deposition.

Different etching recipes lead to textures with different vertex angles of pyramids, which affect the solar cell performance. I have used scanning electron micros-copy and optical measurements for advanced characte-rization of pyramidal textures. Developed optical model for systems of thin films deposited on the pyramidal wafers enabled me to optimize chosen parameters (such as the thickness of the antireflection coating) for the maximum solar cell efficiency.

23

L. Fatme JARDALI

Started January 2014

Supervisor: Holger Vach

Funding: Hariri Foundation

for Sustainable Human Development

Ab initio Molecular Dynamics Simulations of Advanced Silicon Materials

Two-Dimensional films of silicon, ‘silicene’, have been raising considerable interest recently. Being compatible with existing silicon technologies and possessing inte-resting properties close to the ones of graphene, e.g. a Dirac cone band structure, silicene could become the material of the future. Therefore, the present research is in the forefront of novel materials with potential applica-tions in electronics, photonics, and other related areas.

My thesis aims at employing the methods of ab initio molecular dynamics (AIMD) simulations to explain and assist the ongoing experimental work in synthesizing silicene monolayers on supporting substrates. AIMD simulations allow us to investigate the growth dyna-mics of the silicene layer, at an atomistic level, pro-viding crucial information about the interaction of Si atoms with the underlying substrate atoms. We have studied the growth mechanism of the silicene layer on Ag (111) surface, a well-studied case, and showed that most reported silicene layers might have actually been alloyed with metal atoms destroying the properties of genuine silicene.

We are presently extending our studies toward the search for suitable substrates that suppress the inte-raction at the interface and permit the formation of a true honeycomb silicene layer.

M. Ronan LEAL

Started February 2014


Funding: TOTAL CIFRE

Low temperature silicon epitaxy by RF-PECVD using SiF4/H2/Ar gas mixtures for emitter formation in crys-talline solar cells

My PhD work in 2015 consists of performing low tempe-rature (around 200°C) doped epitaxy to make the emit-ter and the back surface field of a crystalline solar cell. In 2015, we obtained a thick (2.5µm) and high-quality epitaxy by RF-PECVD using SiF4/H2/Ar gas mixtures at temperature as low as 200°C. A high quality interface between these layers and the wafer has been shown by HR-TEM, which is crucial to have good electronic properties. Diffraction patterns have also been done at different area of the epitaxial layers and on the subs-trate. Their identical patterns reveal the good structu-ral quality of the sample over the 2.5µm.We have also identified two crucial (and easy to measure) factors to perform epitaxy: 1) a very high H2 depletion, and 2) low ion bombardment energy. We are currently transferring this process to a semi-industrial PECVD reactor.

24

N. Rasha KHOURY




Doctoral School Scholarship (EDX))

Innovative point-contacting technique for thin-film sili-con solar cells

In my PhD work, I explore an alternative, low-cost tech-nique to forming nano-scale point contacts on thin-film and crystalline silicon solar cells using nanoparticles (NP’s) of polystyrene as a sacrificial mask.

In 2015, NP’s with diameters of 50nm and 100nm were dispersed by spin coating onto a crystalline silicon test substrate. Room-temperature deposition of SiO2 was performed by a high density plasma technique. After removing the NP’s by dissolving them in toluene, the samples were characterized by using SEM, and by local resistance mapping using an AFM.

Just recently, the same process, using 100nm NP’s, were repeated on a substrate of ITO deposited on metallic layers. This substrate will act as a back reflector with NIP a-Si:H.

SEM image, showing SiO2 with nanoholes, on a substrate of ITO deposited on metallic layer.

O. Junkang WANG



Funding: CSC

Study of silicon thin film materials: application of MDE-CR-PECVD and TVW

During the past year, my work has been focused on the study of silicon thin film materials deposited by MDE-CR-PECVD and RF-PECVD systems.

Specifically, a SiF4/H2 chemistry, which has recently attracted interest as a precursor for silicon thin film depo-sition due to the resilient optoelectronic performance of the resulting material and devices, was combined with the MDECR system to study the growth of µc-Si:H with much higher deposition rates. Careful measurements have shown the presence of clear thresholds in IBE for the material crystallinity (shown in the figure).

As an application of TVW, another part of my work is about the study of a-Si:H and µc-Si:H deposited by RF-PECVD system. Resulting from the decoupling of IBE and ion flux by using TVW during deposition processes, different electrical properties of a-Si:H and µc-Si:H, like minority carrier diffusion length, have been observed. Further study of the resulting solar cell devices, including stability under illumination, is ongoing.

25

P. Fabien LEBRETON


Supervisor: François Silva

Funding: Total Cifre

Advanced passivation materials, processes and charac-terizations for crystalline silicon

During my first year of Ph.D., I worked on the surface passivation of p-type silicon. I developed a passivation stack consisting of a few nanometers of aluminum oxide deposited by ALD and a capping layer of silicon nitride deposited by PECVD.

Such passivation stacks present very good passivation properties (SRV < 4cm.s-1) but as the ALD method has a very low deposition rate, the effect of aluminum oxide thickness reduction has been studied. This layer reduc-tion required a better understanding of the interaction between the ALD and PECVD layers because strong electrostatic interactions between these layers degrade the passivation properties of the stack. A buffer layer of silicon carboxyde deposited by PECVD has also been inserted between the two in order to reduce electrosta-tic interactions.

To study layer interaction, an in-situ photoluminescence tool has been set-up. For the moment this tool allows one to follow passivation degradation during plasma and thermal treatment up to 250°C (monitoring PL intensity). An upgrade of the tool is in progress in order to obtain lifetime tracking during plasma and thermal treatment up to 500°C.

Q. Gwenaelle HAMON

Started February 2015


Funding: Total Cifre

Multi-junction solar cells: Combining III-V materials and Si grown by PECVD

My PhD is being performed within the framework of the IMPETUS project, whose goal is the monolithic integra-tion of III-V and Si for PV. In our inverted metamorphic approach, we first use MOVPE to grow the AlGaAs top cell on a lattice matched substrate, and then perform low temperature (LT-) PECVD heteroepitaxial growth of SiGe for the bottom cell, as shown in Figure 1.

In 2015, we have shown the first structural and electrical characterizations of Si(PECVD)/III-V(MOVPE) interfaces. Furthermore, the growth of highly doped crystalline Si by low-temperature PECVD enables us to fabricate hybrid tunnel junctions with low resistivity and a high current, suitable to interconnect the two subcells of the tandem III-V/Si solar cell.

26

R. Guillaume FISCHER



Funding: IPVF

Plasma assisted etching of silicon using Tailored Vol-tage Waveforms (TVWs) for PV applications

My project focuses on dry techniques (including clea-ning, etching and texturing) for the treatment of crys-talline silicon surfaces for PV applications. This study takes place in the framework of Project A on c-Si of the IPVF, which funds my PhD.This study finds its originality in the introduction of TVWs (which have been explored mostly for deposi-tion studies so far) for etching processes. In my earlier internship in the PVSiXT team, it has been shown that TVWs excitation (compared to standard RF) has a great potential for the comprehension and optimization of black silicon surfaces, i.e. nanotextured silicon surfaces (see SEM image) with very low reflectance obtained by RIE in an SF6/O2 gas mixture.

The first year of the PhD should give me the opportunity to explore the physics of the formation of black silicon and a better comprehension of its outstanding optical properties.

SEM tilted view of a black silicon surface obtained by RIE, composed of nanocones.

S. Mutaz AL-GHZAIWAT


Supervisor: Martin Foldyna

Co-Supervisors: Erik Johnson,

Pere Roca i Cabarrocas

Funding: ANR

Fabrication and study of solar cell modules based on silicon nanowire based radial junction solar cells

The main goal of my thesis is to develop, test and opti-mize new methods to integrate single and tandem radial junction solar cells on mini-modules, thus demonstra-ting their commercial potential and the compatibility of such modules with standard industrial processes.

Development of the mini-modules includes fabrication of single and tandem radial junction devices, integra-ting plasma enhanced vapor-liquid-solid growth into a module fabrication process flow in the most economi-cal way, development of new contacting strategies, and proper testing of the performance of the devices.

The thesis planning for my first year includes literature review, training on characterization tools and PECVD film deposition, fabrication of first radial junction SiNW solar cell, learning to use laser scribing and encapsula-tion with the Solems company, and fabrication of the first mini-module prototype.

The work will be carried out in close relation with the project Solarium funded by ANR in collaboration with other laboratories and the Solems company.

27

T. Laurent GUIN


Supervisor: Pere Roca i Cabarrocas,

Michel Jabbour

Funding: IDEX Paris Saclay

The influence of strain and processing on the electronic transport and evolution of microstructure in thin-film and nanowire semiconductors

Regardless of their technologies (crystalline silicon, thin-film, nanowire-array), solar cells may be subjected to deformation as a result of the processing, the condi-tion of use or applied strains. Yet, the electronic proper-ties of semiconductors such as the mobility of carriers and the band energy levels significantly depend on the strain state even for strains as low as 0.1%. Hence, the current voltage characteristic of solar cells as well as their power conversion efficiency are modified with the strain.

To compute the magnitude of this effect, we use 1D models of p-n and p-i-n solar cells based on the drift-diffusion formalism including the inhomogeneities in electronic properties that come from the strain. We thus first estimate if strains found in flexible applica-tions (thin-films) may affect the energy produced and second if applied deformations (crystalline silicon and thin films) may offer a way to improve the efficiency in rigid applications.

29

PVSiXT’s greatest asset is the researchers and staff that maintain a high-level research environment in all aspects. Below is a list of personnel that contributed to the daily operation of PVSiXT in 2015.

Ahmed Ben Slimane(Post-Doc Total)

PV material deposition

Nacib BenmmamarTotal R&D Technician)Vacuum and process

systems

Jean Francois Besnier (Total R&D Technician)

Rym Boukhicha(Post Doc CNRS)Material and device

characterization

Bastien Bruneau(CNRS Post-doc)Plasma processes

and modeling

Pavel Bulkin(CNRS IR)

Vacuum and plasmasystems, chemistry

Jerome Charliac(CNRS TCN)

Process equipmentmaintenance, Health

and Safety Officer

Wanghua Chen(Post Doc CNRS)

Atom probe tomography,nanowire growth

Dmitri Daineka(CNRS IR)

Microelectronics andgeneral vacuum processing

Fabrice Devaux(Total Manager)

III-V optoelectronics,R&D management

V. Update on research personnel

30

Amjad Deyine(Total R&D Scientist)Micro-structural analysis

Etienne Drahi (Total R&D Scientist)

c-Si solar cell processingand characterization

Bernard Drevillon(CNRS DR)

Optics, ellipsometry, andpolarimetry

Frederic Farci(CNRS TCN)

Mechanical designand fabrication

Jara Fernandez(Total R&D Scientist)PV material deposition

Sergej Filonovich(Total R&D Scientist)

PECVD growth of Si-based material

Ileana Florea(X IR)

High-resolution TEM

Martin Foldyna(CNRS CR)

Optical simulationsand nanoscalecharacterization

Enric Garcia-Caurel (X IR)

Optics and polarimetry

Nada Habka (Total R&D Scientist)Material and advanced

characterization

Ludovic Hudanski (Total R&D Scientist)

Reliability and PV panels

Cyril Jadaud(X IR)

Mechanical and thermaldesign, Health and Safety

Officer

Erik Johnson (CNRS CR)

Plasma processing, PVdevice characterization,

RF engineering

Jean Luc Maurice (CNRS DR)

High-resolution TEM

Jean Luc Moncel (CNRS IR)

Project Mgmt,developments

& Communication

31

Tatiana Novikova(X IR)

Numerical simulationof plasmas and optical

processes

Gilles Poulain(Total R&D Scientist)

PV cell fabricationtechnology

Patricia Prod’homme(Total Program Manager)Reliability and PV panels

Celine Richard(Post-doc Total)

PV panel technology

Pere Roca i Cabarrocas(CNRS DR)

From plasma processesto large area electronic

devices

Garry K. Rose (X TSEF)

Electronicsand automation

Christoph Sachs(Total R&D Scientist)

Silicon fabricationtechnology

Martin Sander(CNRS Post-doc)

Crystalline silicon cellsand module metrology

François SILVA(CNRS IR)

PECVD growth process,µwave plasmas, num.

simulation

Jacqueline Tran (X IE)

Optics, Measurementequipment maintenance

Sandrine Tussaut-Nenez

(CNRS IR)High-Resolution XRD

for Thin Films

Jean Charles Vanel (CNRS IR)

Signal acquisition,processing,

and automation

Federico Ventosinos (CNRS Post-doc)

Material and devicecharacterization

Holger Vach (CNRS DR)

Ab initio simulation ofadvanced silicon materials

32

A. Movement of Personnel and Alumni

PVSiXT has experienced less turnover in 2015 than in the previous year.

People leaving PVSiXT or changing position:– Joaquim Nassar– Gennaro Picardi– Igor Sobkowicz– Jean-Christophe Dornstetter– Sofia Gaiaschi– Romain Cariou– Rym Boukhicha

People joining PVSiXT:– Christoph Sachs (Total researcher)– Amjad Deyine (Total researcher)– Guillaume Fischer (PhD)– Gwenaelle Hamon (PhD)– Mutaz Al-Ghzaiwat (PhD)– Laurent Guin (PhD)

The former PhD students have communicated their present position:

– Igor SOBKOWICZ is a PV designer at Cupertino Elec-tric in the San Francisco Bay Area

– Jean-Christophe DORNSTETTER has joined a finan-cial software company, Misys

– Sofia GAIASCHI has been hired by HORIBA as a GD Application Engineer

– Romain CARIOU has joined the Fraunhofer ISE for a post-doc on III-V/Si tandem cells with a Marie-Curie Fellowship.

33

The main clean room of PVSiXT has been upgraded to allow more repeatable processing. For this purpose, the air management system has been improved significantly.

Furthermore the room has been reorganized to accom-modate the reference ARCAM reactor, a new high-tem-perature oven and, for 2016, a new high-temperature PECVD cluster tool, an ALD and an LPCVD reactor.

While waiting for its building in 2017, IPVF has installed some of its new tools in LPICM. The SemiLab µPCD is the first instance.

The main tools of the PVSiXT platform at the end of 2015 are the following.

A. Processing

A.1. PE-CVD– MVS Cluster tool– ARCAM (100 & 200)– ATOS – PHILIX – PLASFIL– PLASMAT– VENUS– CAMELEON– NEXTRAL

A.2. Other deposition– e-beam (Oxford)– Evaporators (BOC Edwards, Custom)– Sputtering (Alliance Concept)

A.3. Oven– Annealing Oven (Heraus)– RTA (ATV)– High-temperature

A.4. Others– HF chemical hood– RIE etching (NEXTRAL)

VI. Update of tool platform

34

B. MeasurementB.1. Material– KFM– Ellipsometer Uvisel 1 and 2, + in-situ– Raman spectrometer– SemiLab µPCD– Lifetime measurement (Sinton)– FTIR– XRD (Bruker D8)– SSPG (TFSC)– Hall Effect– UV/Vis TR (Perkin Elmer)

B.2. Device– Sun simulator– EQE (x3)– Sub-cell JV curve measurement system

C. Module– Laminator– Electroluminescence– Climatic chamber– Contact Angle Measurement

35

Erik Johnson Fabrice Devaux

It was another busy year for the PVSiXT joint PV research team in 2015. We continue to work both on public pro-jects (ANR and EU) as well as on independent projects. This year saw the successful completion of one public project (EU FP7 project PhotoNVoltaics), the start of three more (ANR projects Platofil, Solarium, INDEED), while other projects continued to progress and produce good scientific results (ANR Projects Nathisol and IMPETUS).

This year also marked the beginning of technical work for the Ile-de-France Photovoltaics Institute (IPVF), where Project A on crystalline silicon saw the first PERT cell fabricated by the consortium. We look forward to 2016 being a breakthrough year for this project, with the arrival of new equipment and PhD students paid by the IPVF beginning work in earnest.

We continue to focus on plasma processes for nanos-tructured materials for large-area optoelectronics, and the hottest research topics this year were:

– low-temperature silicon epitaxy by PECVD, both homoepitaxy for doped layers and heteroepitaxy for growth of Si on III-V’s,

– pursuing greater understanding of c-Si solar cells, both by using surface nano-probes and through ion implantation as a research tool,

– the use of Tailored Voltage Waveforms for a variety of plasma processes (thin film deposition, silicon etching, graphene cleaning and doping, as well as fundamental studies on a new type of “sawtooth waveform”),

– low temperature, passivation layers with existing and new materials for high-efficiency solar cells, understanding physical mechanisms controlling passivation and its stability,

– developing a quantifiable understanding of the role of ions in silicon film growth,

– nano-photonics for enhancing absorption in thin crystalline silicon solar cells,

– nanowire devices (solar cells and transistors) and growth,

– novel multijunctions and multiterminal devices, and the techniques needed to characterize them,

– molecular dynamics modeling of silicene and aro-matic silicon molecules, as well as their experimen-tal observation,

– multiscale plasma modelling to understand the role of plasma formed nanoparticles in low-temperature epitaxial processes,

As an integral part of all these topics, our exchanges with the LPICM microscopists (and their PhD students) have given us a chance to see and understand many growth processes for the first time, a strength that is

VII. Outlook

36

increasing with the arrival of the NanoMax HRTEM in 2015. This is a tool that will give us new insights in 2016.

Four successful PVSiXT PhD defences were held in 2015: Soumyadeep Misra on vertical nanowires with incredible light trapping for PV, Fan Zheng on in-plane nanowires for flexible electronics, Bastien Bruneau on new aspects of plasma excitation by Tailored Voltage Waveforms, and Dennis Lange on the effects of strain on transport properties of silicon thin films and related solar cell efficiency.

The end of 2015 saw a flurry of activity that will serve us well in 2016. Our main process room got an upgrade,

and in 2016 it will host three PECVD reactors, a new ALD machine, a new MOCVD reactor, and a new furnace for oxide growth. All these tools will come online in 2016, so after the expected learning curve, we look forward to applying these tools in creative ways to generate dis-ruptive and insightful results.

This year also saw another important milestone – the second bilateral collaborative project within PVSiXT. This project, judged to be of significant strategic inte-rest of industrial application to Total and its affiliate, marks a new maturity in the joint research team and a model for future interaction.

37

a. Publications in peer-reviewed journals

11. B. Bruneau, T. Lafleur, T. Gans, D. O’Connell, A. Greb, I. Korolov, A. Derzsi, Z. Donkó, S. Brandt, E. Schüngel, J. Schulze, P. Diomede, D.J. Economou, S. Longo, E. Johnson, J.-P. Booth, (2015). Effect of gas pro-perties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: compari-son of Ar, H2 and CF4, Plasma Sources Sci. Technol. 25, 01LT02.

12. S. Gaiaschi, M.E. Gueunier-Farret, C. Longeaud, E.V. Johnson, (2015). Electrical properties of hydroge-nated microcrystalline silicon carbon alloys: effect of deposition parameters and light soaking J. Phys. D: Appl. Phys. 48, 285101 doi:10.1088/0022-3727/48/28/285101

13. B. Bruneau, T. Gans, D. O’Connell, A. Greb, E.V. Johnson, J.-P. Booth, (2015). Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms, Phys. Rev. Lett. 114, 125002.

14. B. Bruneau, T. Novikova, T. Lafleur, J.-P. Booth, E.V. Johnson, (2015). Control and optimization of the slope asymmetry effect in Tailored Voltage Wave-forms for Capacitively Coupled Plasmas, Plasma Sources Sci. Technol. 24, 015021.

15. Sergio Manzetti, Tian Lu, Hadi Behzadi, Mehdi D. Estrafili, Ha-Linh Thi Le and Holger Vach, (2015). Intriguing properties of unusual silicon nanocrys-tals, RSC Adv., 5, 78192.

,

16. H. Vach, L.V. Ivanova, Q.K. Timerghazin, F. Jardali, H.L.T. Le, (2015). A deeper insight into strain for the sila‐bi [6] prismane (Si18H12) cluster with its endohedrally trapped silicon atom, Si19H12. Journal of computational chemistry, 36(28), 2089-2094.

17. N.C. Forero-Martinez, H.L.T. Le, N. Ning, H. Vach, H.C. Weissker, (2015). Temperature dependence of the radiative lifetimes in Ge and Si nanocrystals. Nanoscale, 7(11), 4942-4948.

18. H. Arwin, R. Magnusson, E. Garcia-Caurel, C. Fallet, K. Järrendahl, M. Foldyna, A. De Martino, R. Ossi-kovski, (2015). Optics Express 23, 1951.

19. A. Messanvi, H. Zhang, V. Neplokh, F.H. Julien, F. Bayle, M. Foldyna, C. Bougerol, E. Gautier, A. Babichev, C. Durand, J. Eymery, M. Tchernycheva, (2015). ACS applied materials & interfaces 7, 21898.

10. E. Dogmus, M. Zegaoui, L. Largeau, M. Tcherny-cheva, V. Neplokh, S. Weiszer, F. Schuster, M. Stutzmann, M. Foldyna, F. Medjdoub, (2015). Phy-sica Status Solidi (c) 12, 1412.

11. A.S. Togonal, M. Foldyna, W. Chen, J.X. Wang, V. Neplokh, M. Tchernycheva, J. Nassar, P. Roca i Cabarrocas, Rusli, (2015). Core-Shell Heterojunction Solar Cells Based on Disordered Silicon Nanowire Arrays. The Journal of Physical Chemistry C.

VIII. Publications

38

12. A. Fejfar, M. Hývl, A. Vetushka, P. Pikna, Z. Háj-ková, M. Ledinský, J. Kočka, P. Klapetek, A. Marek, A. Mašková, J. Vyskočil, (2015). Correlative micros-copy of radial junction nanowire solar cells using nanoindent position markers. Solar Energy Mate-rials and Solar Cells, 135, pp.106-112. http://dx.doi.org/10.1016/j.solmat.2014.10.027.

13. R. Cariou, J. Tang, N. Ramay, R. Ruggeri, P. Roca i Cabarrocas, (2015). “Low temperature epitaxial growth of SiGe absorber for thin film heterojunc-tion solar cells”. Solar Energy Materials and Solar Cells 134, 15-21. http://dx.doi.org/10.1016/j.sol-mat.2014.11.018.

14. S.N. Abolmasov and P. Roca i Cabarrocas, (2015). “In situ photoluminescence system for studying surface passivation in silicon heterojunction solar cells”. J. Vac. Sci. Technol. A 33, 021201. http://dx.doi.org/10.1116/1.4902014

15. Zhongwei Yu, Shengyi Qian, Linwei Yu, Sou-myadeep Misra, Pei Zhang, Junzhuan Wang, Yi Shi, Ling Xu, Jun Xu, Kunji Chen, Pere Roca i Cabarro-cas, (2015). “Boosting light emission from Si-based thin film over Si and SiO2 nanowires architecture”. Optics Express 23. DOI:10.1364/OE.23.005388.

16. Mingkun Xu, Zhaoguo Xue, Linwei Yu, Shengyi Qian, Zheng Fan, Junzhuan Wang, Jun Xu, Yi Shi, Kunji Chen and Pere Roca i Cabarrocas, (2015). “Operating principles of in-plane silicon nanowires at simple step-edges”. Nanoscale. DOI: 10.1039/C4NR06531J

17. I. Cosme, R. Cariou, W. Chen, M. Foldyna, R. Boukhi-cha, P. Roca i Cabarrocas, K.D. Lee, C. Trompoukis, V. Depauw, (2015). “Lifetime assessment in crys-talline silicon: From na nopatterned wafer to ultra-thin crystalline films for solar cells”. Solar Energy Materials and Solar Cells 135, 93. http://dx.doi.org/10.1016/j.solmat.2014.10.019.

18. A. Defresne, O. Plantevin, I.P. Sobkowicz, J. Bour-çois, P. Roca i Cabarrocas, (2015). “Interface defects in a-Si:H/c-Si heterojunction solar cells”. Nuclear Ins-truments and Methods in Physics Research B. DOI 10.1016/j.nimb.2015.04.009

19. S. Misra, L. Yu, M. Foldyna, P. Roca i Cabarrocas, (2015). ‘New approaches to improve theperfor-mance of radial junction thin film solar cells built over silicon nanowire arrays’, IEEE JPV, 5, 40. DOI 10.1109/JPHOTOV.2014.2366688

20. M. Müller, M. Hývl, M. Kratzer, C. Teichert, S. Misra, M. Foldyna, L. Yu, P. Roca i Cabarrocas, T. Itoh, Z. Hájková, A. Vetushka, (2015). Investigating inho-mogeneous electronic properties of radial junction solar cells using correlative microscopy. Japanese Journal of Applied Physics, 54(8S1), p.08KA08. http://dx.doi.org/10.7567/JJAP.54.08KA08.

21. Shengyi Qian, Soumyadeep Misra, Jiawen Lu, Zhongwei Yu, Linwei Yu, Jun Xu, Junzhuan Wang, Ling Xu, Yi Shi, Kunji Chen, Pere Roca i Cabarro-cas, (2015). “Full potential of radial junction Si thin film solar cells with advanced junction materials and design”. Appl. Phys. Lett. 107, 043902. http://dx.doi.org/10.1063/1.4926991.

22. Jiawen Lu, Shengyi Qian, Zhongwei Yu, Sou-myadeep Misra, Linwei Yu, Jun Xu, Yi Shi, Pere Roca i Cabarrocas, Kunji Chen, (2015). “How tilting and cavity-mode-resonant absorption contribute to light harvesting in 3D radial junction solar cells”. Optics Express. 23, AA1288. DOI:10.1364/OE.23.0A1288 | OPTICS EXPRESS A1288

23. Wanghua Chen, Philippe Pareige, Celia Castro, Tao Xu, Bruno Grandidier, Didier Stiévenard, Pere Roca i Cabarrocas, (2015). “Atomic characteriza-tion of Au clusters in vapor-liquid-solid grown sili-con nanowires”. J. Appl. Phys. 118, 104301. http://dx.doi.org/10.1063/1.4930143

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24. Zhongwei Yu, Jiawen Lu, Shengyi Qian, Soumyadeep Misra, Linwei Yu, Jun Xu, Ling Xu, Junzhuan Wang, Yi Shi, Kunji Chen, and Pere Roca i Cabarrocas, (2015). “Bi-Sn alloy catalyst for simultaneous mor-phology and doping control of silicon nanowires in radial junction solar cells”. Appl. Phys. Lett. 107, 163105, http://dx.doi.org/10.1063/1.4933274

25. Dennis Lange, Pere Roca i Cabarrocas, Nick Trian-tafyllidis, Dmitri Daineka, (2015). “Piezoresisti-vity of thin film semiconductors with application to thin film silicon solar cells”. Solar Energy Mate-rials and Solar Cells 145, p. 93-103. http://dx.doi.org/10.1016/j.solmat.2015.09.014.

26. D. Murias, M. Moreno, C. Reyes-Betanzo, A. Torres, R. Ambrosio, P. Rosales, J. Martinez, I. Vivaldo, P. Roca i Cabarrocas, (2015). “Plasma-Texturing Pro-cesses and a-Si:H Surface Passivation on c-Si Wafers for Photovoltaic Applications”. Journal of Solar Energy Engineering, American Society of Mechanical Engineers, 137, 051010. DOI: 10.1115/1.4031105.

27. Leon Hamui, B. Marel Monroy, Ka-Hyun Kim, Ale-jandra Lopez, Jaime Santoyo. Maximo Lopez, Pere Roca i Cabarrocas, Guillermo Santana Rodriguez, (2016). “Effect of Deposition Temperature on Poly-morphous Silicon Thin Films by PECVD: Role of Hydrogen”. Materials Science in Semiconductor Pro-cessing 41, 390–397. http://dx.doi.org/10.1016/j.mssp.2015.10.005.

b. Invited international conferences

11. J.L. Maurice, In situ treatments of nano-objects in the transmission electron microscope. WCAM (World Congress of Advanced Materials) Chongqing, China, 27-29 may 2015.

12. H. Vach, Aromatic Silicon Nanocrystals - outstan-ding properties and future applications, EMN Mee-

ting on Quantum Technology, Beijing, China, April 14-17 (2015). http://www.emnmeeting.org/quan-tum/low-dimensional-structure.

13. Erik V. Johnson, Gaining greater control and unders-tanding of processing plasmas through Tailored Vol-tage Waveforms, Gaseous Electronics Conference (GEC/ICRP 2015, October 2015, Hawaii, USA).

14. B. Bruneau, T. Lafleur, T. Novikova, T. Gans, D. O’Connell, A. Greb, J.-P. Booth, E.V. Johnson, Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms, 4th Workshop on Radio Frequency Discharges (WRF-2015, 22 – 23 June 2015, Dublin City Univer-sity, Ireland).

15. M. Foldyna, S. Misra, A.S. Togonal, W. Chen, J. Tang, Rusli, P. Roca i Cabarrocas, “Optical modeling of radial junction solar cells based on crystalline silicon nanowire arrays,” Workshop onTheory and Modeling for PV, Marseille, France, October 1-2, 2015.

16. R. Cariou, W. Chen, G. Hamon, M. Foldyna, R. Lachaume, J. Alvarez, J.-L. Maurice, J. Decobert, J.-P. Kleider, Pere Roca i Cabarrocas, “Low tem-perature plasma epitaxy of Silicon on III-V for tan-dem solar cells”. E-MRS Spring Meeting, Symp. C: ADVANCED INORGANIC MATERIALS AND STRUC-TURES FOR PHOTOVOLTAICS, 11-15 Mai 2015 Lille, France.

17. Pere Roca i Cabarrocas, “Low temperature plasma deposition processes: from amorphous silicon to epitaxial growth”. 20th International Colloquium on Plasma Processes. CIP2015 Saint Etienne. Saint Etienne France, 1-5 Juin 2015.

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c. Conference proceedings

11. Roelene Botha, Silvia Schwyn Thöny, Martin Grössl, Safer Mourad, Clau Maissen, Jacobus I. Venter, Tho-mas Südmeyer, Martin Hoffmann, Pavel V. Bulkin, Sabine Linz-Dittrich, David Bischof, Markus Michler, Stefan J. Rinner, Andreas Ettemeyer, Comparative study of the laser damage threshold and optical cha-racteristics of Ta2O5 - SiO2 multilayers deposited using various methods. Proc. SPIE 9632, Laser-In-duced Damage in Optical Materials: 2015, 963203 (November 23, 2015); doi:10.1117/12.2194084.

12. Soumyadeep Misra, Ileana Florea, Martin Foldyna, Pere Roca i Cabarrocas, “Radial Junction architec-ture: A new approach to stable and higly efficient silicon thin film solar cells”. MRS Online Proc. Library (2015) DOI: 10.1557/opl.2015.831

13. Wook Jun Nam, Jean-Christophe Dornstetter, M. Foldyna, Pere Roca i Cabarrocas, Zachary Gray, Shawn Waggoner, Atilla O. Cakmak, Douglas Nei-dich, and Stephen J. Fonash, “Attaining 46% Utiliza-tion of the AM1.5G Photons Impinging on a 400nm Thick nc-Si Cell Using Nanoelement-Array Light Trap-ping Structures”. Proc. 30th EUPVSEC, Hambourg, 14-18 septembre 2015.

14. G. Hamon, R. Cariou, R. Lachaume, J. Decobert, K. Louarn, W. Chen, J. Alvarez, J.P. Kleider and P. Roca i Cabarrocas: “Investigation of hybrid tunnel junction architectures for III-V/Si tandem solar cells”. Proc. 31th EUPVSEC, Hambourg, 14-18 septembre 2015.

15. Paul Narchi, Gennaro Picardi, Romain Cariou, Martin Foldyna, Patricia Prod’homme, Pere Roca i Cabarro-cas, “Kelvin probe force microscopy study of elec-tric field homogeneity in epitaxial silicon solar cells cross-section”. Proc. 31th EUPVSEC, Hambourg, 14-18 septembre 2015.

d. Oral conference presentations

11. Prabal Goyal, Elias Urrejola, Junegie Hong, Pere Roca i Cabarrocas, Erik Johnson, Liquid Si precur-sor-based p-type silicon oxide layers as B diffu-sion source for n-type c-Si substrates Internatio-nal Conference on Amorphous and Nanocrystalline Semiconductors (ICANS 2015, September 2015, Aachen, Germany).

12. Farah Haddad, Prabal Goyal, Junegie Hong, Erik Johnson, Jean-Luc Maurice, Pere Roca i Cabarrocas, Five-fold symmetries in the multiple twinning found in p-type microcrystalline silicon films grown from hexamethyldisiloxane International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS 2015, September 2015, Aachen, Germany).

13. B. Bruneau, E.V. Johnson, T. Gans, D. O’Connell, A. Greb, I. Korolov, A. Derszi, Z. Donko, E. Schun-gel, S. Brandt, J. Schulze, P. Diomede, D.J. Econo-mou, S. Longo, T. Lafleur, J.P. Booth, Comparison of the effect of sawtooth-like voltage waveforms on discharge dynamics of Ar, H2, and CF4 plasmas, Gaseous Electronics Conference (GEC/ICRP 2015, October 2015, Hawaii, USA). Won student award.)

14. Junkang Wang, Pavel Bulkin, Ileana Florea, Jean-Luc Maurice, Erik Johnson, Microcrystalline silicon thin films deposited by Matrix Distributed Electron Cyclo-tron Resonance PECVD using an SiF4/H2 chemistry, 62nd Meeting of the American Vacuum Society (AVS 62, October 2015, San Jose USA)

15. F. Ventosinos, B. Fakes, P.G. O’Brien, N.P. Kherani, E.V. Johnson, (2015). Four-wire thin-film silicon devices: Towards high efficiency, 42ND IEEE PHO-TOVOLTAIC SPECIALISTS CONFERENCE (IEEE-PVSC 42, June 14-19, 2015 New Orleans, USA).

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6. R. Cariou, W. Chen, G. Hamon, M. Foldyna, R. Lachaume, J. Alvarez, J. L Maurice, J. Decobert, J.P. Kleider and Pere Roca i Cabarrocas, “Low tempera-ture plasma epitaxy of Silicon on III-V for tandem solar cells”. E-MRS Spring Meeting, Symp. C: ADVANCED INORGANIC MATERIALS AND STRUCTURES FOR PHO-TOVOLTAICS, 11-15 Mai 2015 Lille, France

17. Pere Roca i Cabarrocas, “Low temperature plasma deposition processes: from amorphous silicon to epitaxial growth”. 20th International Colloquium on Plasma Processes. CIP2015 Saint Etienne. Saint Etienne France, 1-5 Juin 2015.

18. P. Roca i Cabarrocas*, W. Chen, R. Leal, J.M. Orlac’h, F. Haddad, R. Cariou, J.-C. Dornstetter, B. Bruneau, H.L.T Lee, T. Novikova, H. Vach, F. Silva, E. John-son, Low temperature plasma epitaxy. How does it work?. International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS 2015, Sep-tember 2015, Aachen, Germany)

e. Book Chapters

11. W. Chen and P. Pareige, “Using atom probe tomo-graphy in the study of semiconductor nanowires”, in “Semiconductor Nanowires: Materials, Synthesis, Characterization and Applications”, (éd. J. Arbiol and Q. Xiong), Elsevier, Ch. 11. (2015).

f. Patents

11. Patent Filing n° 15 58267Filing date: 07 septembre 2015by École polytechnique, Universite Paris-Sud 11 and Centre National de la Recherche Scientifique.Inventors: Alice Defresne, Pere Roca i Cabarrocas, Olivier Plantevin.

12. Patent Filing EP14307132.2Filed December 22, 2014.Filed by 1/TOTAL Marketing Services2/École polytechnique3/Centre National de la Recherche Scientifique. Inventors: Pere Roca i Cabarrocas, Wanghua Chen, Martin Foldyna, Gilles Poulain.

13. Patent Filing n° 153063383-1551Filed31/08/2015Filed by Total Marketing services et al.Inventors: Erik Johnson, Bastien Bruneau,Pere Roca i Cabarrocas et Pavel Bulkin.

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