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Final Report Summary - R2R-CIGS (Roll-to-roll manufacturing of high efficiency and low cost flexible CIGS solar modules)

Executive Summary:
The research and industry consortium of the FP7 EU project “R2R-CIGS” has been working on the development of cost-effective roll-to-roll (R2R) processes and technologies for the continuous, high-volume manufacturing of flexible CIGS modules.
Aim is to up-scale and implement innovative processes such that flexible CIGS modules can be produced competitively in large volume factories with comparable performance to CIGS technology on glass.
In order to meet this aim, the project pursues 20% cell efficiency and 16% mini-module efficiency on flexible substrates, and to develop on pilot level R2R processes for CIGS, Cd free buffer, TCO and barrier layer deposition as well as for laser scribing for making monolithic interconnected modules.
Promising lab-scale processes were up-scaled and pilot line equipment designed and built for demonstrating the processes in either a 300 mm wide all-in-one vacuum R2R CIGS pilot line or in conjunction with a 500 mm wide modular R2R CIGS pilot line. Substrate material was polyimide foil.
On lab-scale cells and mini-modules were made with 20% and 16.9% efficiency as targeted, using multi-stage gradient CIGS deposition and a post deposition treatments with alkali elements. Eventually this process was up-scaled and transferred to the modular R2R CIGS, where at the end of the project modules were made with 11.6% and cells with 16% efficiency.
In a parallel development, a multistage process was also incorporated in the all-in-one vacuum R2R CIGS pilot line. Demonstrated was for the first time the manufacturing of CIGS modules with back contact, absorber, buffer and TCO all deposited without vacuum break.

Three alternatives for chemical bath deposited (CBD) CdS buffer layers were developed and up-scaled. Two, atmospheric R2R spatial atomic layer deposition (ALD) and batch R2R-CBD of Zn(O,S), resulted in cells and modules with similar performance as reference material with a CdS buffer. The third, evaporation of In2S3 was incorporated in the all-in-one vacuum line.

Concerning the front electrode, atmospheric chemical vapour deposition as well as vacuum sputter techniques were investigated,. The first resulted in a new plasma enhanced injector with an innovative electrode concept and the latter in the integration of economic DC sputter processes with rotatable targets in the two pilot lines.

A key advantage of thin film solar cells is the monolithic interconnection, successfully implemented on glass but still a challenging task for flexible substrates in a R2R setting. To enable the latter a R2R laser scribing machine with on the fly scribe line detection and scribing technology has been built and installed in the modular pilot line. Functional monolithic interconnected flexible CIGS modules have been produced using polyimide foil of 500 mm width as substrate..

In order to ensure a sufficient product lifetime of over twenty years, the modules need protection by a weatherproof front sheet and a humidity barrier with a water vapour transmission rate lower than 5x10-4 g/m2/d. The latter can be achieved with ALD Al2O3 layers.
Two routes were explored for its application. The first route by laminating a barrier coated PEN or PET foil onto a weatherproof sheet material, for laminating the PV modules. Using a 500 mm wide R2R-ALD tool, a barrier coating was applied on a PEN foil, which proved sturdy enough for industrial scale and speed lamination on transparent UV resistant fluor-free polymer sheet also developed in this project. PV modules laminated with the resulting front sheet performed almost as good as (more expensive) commercial materials in damp heat tests. In the second route, the barrier is directly coated on top of CIGS modules. Excellent results in damp heat tests were shown for CIGS on glass modules and the feasibility of applying the barrier coating on a CIGS roll in a R2R-ALD tool was demonstrated.

Project Context and Objectives:
In a world with increasing global energy demand and predicted severe climate changes the development of sustainable, secure and competitive energy sources is an issue of topmost priority. The European Commission has defined the needs and binding targets in the European Strategic Energy Technology (SET) plan to reduce the greenhouse gas emissions by 20% and to ensure a 20% share of renewable energy generation in the EU energy mix by 2020. Photovoltaic (PV) technology has shown its potential to contribute significantly to the renewable energy sources. The PV market is dominated by crystalline silicon PV technology, but especially if flexibility and weight matters, as in the building or transport sector for example, thin film PV technologies are very valuable. Among the thin film technologies, CIGS absorber based PV offers highest efficiencies. Most mature manufacturing technologies for CIGS modules use rigid glass substrates.
In order to stimulate use and manufacturing of flexible thin film PV in Europe, the R2R-CIGS project aims to contribute to industrial large scale production of flexible high efficiency CIGS solar cell devices based on processes for roll-to-roll manufacturing. The aim is to develop efficient solar modules by implementing innovative cost-effective processes such that production costs below 0.5 €/Wp can be achieved in large volume factories with annual capacity of 500MWp in future. This ambitious target will be achievable when the efficiencies of flexible solar cells are comparable to those based on glass substrates and high throughput process with less energy and material consumption are implemented.

In order to meet this target the project pursues 20% cell efficiency and 16% mini-module efficiency on polyimide flexible substrates, and to develop on pilot line scale R2R processes for CIGS, Cd free buffer, TCO and barrier layer deposition as well as for laser scribing for making monolithic interconnected modules. Moisture barrier coatings for efficient and low-cost encapsulation will be developed to overcome the problems of efficiency degradation due to corrosion of electrical contacts.

The overall objective of R2R-CIGS to implement innovative and advanced concepts manufacturing and device processing in pilot-line scale, is pursued by the following set of sub-aims:
- Solar cells on polymer film with 20% efficiency and mini-module with 16% efficiency by control of composition gradient, surface, and interface properties on nano-scale.
- Scaling-up of static multi-stage CIGS deposition process towards inline R2R compatible processes.
- Implementation of the up-scaled multi-stage CIGS deposition process into R2R pilot lines yielding flexible CIGS modules with 14% efficiency.
- Cadmium free buffer layer deposition, by R2R spatial atomic layer deposition (ALD), chemical bath deposition (CBD) and evaporation.
- Front electrode deposition by R2R DC sputtering and scaled-up AP PE CVD processes.
- Automatic R2R laser scribing of thin-film layers on insulating polymer substrates enabling monolithic interconnection of solar cells to form flexible solar modules.
- Development of R2R-ALD moisture barriers with WVTR below 5x10-4 g/m2/d for integration in a polymer front sheet and application directly on CIGS modules.

The up-scaled and R2R processes are implemented into or tested in conjunction with a 500 mm wide modular R2R pilot line or a 300 mm wide all-in-one vacuum R2R CIGS pilot line.

Project Results:
The first 18 months of the R2R-CIGS project were mostly devoted to the design and development of processes and equipment for later testing, use and/or installment in pilot lines. High temperature stable polyimide films is the substrate material for the CIGS modules in this project. In the second 18 months the focus was on realization of the equipment and transferring the processes to pilot lines. At the end of the project, after 42 months, this resulted in the following main achievements:

1 CIGS layer deposition

- Modular R2R CIGS pilot line
Flexible CIGS solar cells on polyimide substrate with efficiencies above 19.5% and 20% within error margin were made as well as flexible CIGS mini-modules with efficiency of 16.9% by a static multi-stage low-temperature co-evaporation process, as aimed for. The mini-module’s absorber was deposited with a low-temperature multi-stage deposition process and a post deposition treatment with elements was given. A CdS buffer layer was applied with reduced thickness to decrease absorption in the near UV part of the spectrum.
Since the overall aim is up-scaling of lab processes, process windows were determined of the multi-stage absorber deposition and of the alkali element post-deposition treatments to prepare for R2R CIGS module production. The multi-stage CIGS process was transferred successfully to a 500 mm wide R2R CIGS coater in a modular pilot line for further testing and development. Module efficiencies of 11.7% and cell efficiencies of 16% were achieved at the project end.

- All in one vacuum R2R CIGS pilot line
In an alternative development, successful runs were made with back electrode, absorber, buffer layer and front electrode deposition on a 300 mm wide web without break in an all-in-one vacuum R2R CIGS pilot line.

2 Buffer layer

Three routes have been pursued for R2R application of a Cd-free buffer layer. Two focused on Zn(O,S) layers, for application in a modular R2R CIGS line, the third on In2S3 for use in an all-in-one vacuum R2R CIGS line. Both layers have as advantage over CdS by, besides not being toxic, having a reduced near UV light absorption.

- R2R spatial atomic layer deposition of Zn(O,S)
On lab-scale functional CIGS devices on glass and polyimide foil were made with a spatial ALD applied Zn(O,S) buffer layer with same or higher efficiency as reference devices with a batch CBD CdS buffer layer. In parallel an atmospheric R2R-ALD tool for a 500 mm wide web has been designed, built for Zn(O,S) and i-ZnO deposition.
The combination of the developed lab-scale process and the R2R-ALD tool resulted in successful truly R2R Zn(O,S) and i-ZnO deposition on 500 mm wide Mo & CIGS coated polyimide web material, out of which CIGS cells and mini-modules were made with efficiencies near that of reference material with a CdS buffer layer.

- R2R chemical bath deposition of Zn(O,S)
CBD Zn(O,S) recipes have been developed and transferred to a batch-mode R2R-CBD tool. This resulted in functional modules with similar efficiencies as for CdS references.

- R2R evaporation of In2S3
An evaporation process for In2S3 buffer layer application has been implemented and demonstrated successfully in an all-in-one vacuum R2R CIGS pilot line.
3 Front electrode

- Atmospheric deposition
A full size (50 cm deposition width) AP PE CVD injector reactor has been designed, assembled and integrated into an industrial scale CVD oven at TNO and includes an innovative plasma electrode, more robust and less prone to discharge damage. With this set-up ZnO:Al layers have been made at a 200oC substrate temperature with 50 Ohm sq resistivity.

- Vacuum deposition
DC sputtering process with a rotating target has been optimized for ZnO:Al window layers which resulted in resistivity values below 10 Ohm sq and relative transparency values above 85%, as targeted. The process was optimized further to manufacture CIGS devices with the same or better efficiency as with planar targets.
Accelerated stress tests (~1 sun illumination by white light from LEDs at 80°C in controlled atmosphere - synthetic air, 500 mbar - atmosphere) showed superior stability of CIGS modules with DC sputter-deposited ZnO:Al layers from a rotatable cathode compared to RF reference samples.
The processes have been implemented in the R2R CIGS pilot lines.

4 Monolithic Interconnection

A complete R2R laser scribing system (R2R-LSM) has been developed with on the fly line detection and scribing technology.
At first a laser scribing test rig was built, which is able to make P1 scribes (in the Mo back contact) on fixed, moving (flat) substrates, with a scribe spacing accuracy better than 15μm. Using the gained experience, and after solving the Illumination for the P1 tracking, this scribing subsystem was improved and a complete R2R-LSM tool was for a 500 mm wide web has been built and installed in the modular R2R CIGS pilot line. Complete functional monolithically interconnected CIGS modules have been made.

5 Encapsulation

Barrier coatings have been deposited using ALD with a water vapour transmission rate (WVTR) below 5x10-4 g/m2/d. Two approaches are followed to apply this barrier layer: (i) coating of a flexible polymer film, which will be laminated onto a weather proof polymer sheet layer to make a flexible front sheet and (ii) depositing directly on the TCO layer of CIGS modules.

- Front sheet with barrier
A transparent UV resistant weather-proof sheet has been developed with reduced costs using a Fluor-free formulation. R2R-ALD Al2O3 coated barrier foil samples with a WVTR of 5*10-4 g/m2/d are produced for lamination on this front sheet. The weather-proof layer and barrier foil can be laminated in production equipment at high speed without losing the barrier functionality.
Modules encapsulated with this front sheet prototype performed well in (85oC / 85 RH) damp heat tests, almost as good as commercially (and more expensive) front sheets.

- Barrier applied directly on TCO layer of CIGS modules
Barrier films were applied directly on the front electrode layer of CIGS modules, which did perform well in damp heat tests.

Potential Impact:
1 Potential impact
R2R-CIGS is expected to contribute to the development of the following new industrial scale processes for production of flexible CIGS modules:
- Roll-to-roll processing of flexible substrates
- R2R inline multi-stage deposition of nano-graded CIGS layers
- R2R deposition of Cd free buffer, i-ZnO and metal oxide barrier coatings
- Front electrode deposition at reduced temperatures using either APPE CVD or rotatable target sputtering
- Monolithic interconnection of cells on flexible substrates by automated R2R laser scribing
- Flexible, low cost encapsulation

The project accelerated the market introduction of flexible CIGS modules by providing industrial scale process and equipment solutions for the production of high efficiency, low cost and light weight CIGS modules. The industrial take-up of the results is stimulated and accelerated by the active participation of industrial CIGS producing partners and equipment suppliers.
The polyimide substrates enable integration of CIGS modules in low cost and light weight building components and transport applications.
R2R-CIGS technologies and processes are also relevant for manufacturing of large area flexible electronics, lighting and displays. Packaging with added sensing, monitoring or information functionalities are dependent on the development of high throughput processes and equipment such as those developed within R2R-CIGS

2 Main dissemination activities
The project and results have been disseminated through presentations at numerous workshops and conferences, such for example as the solar EU PVSEC and International PVSEC conferences, the materials MRS and E-MRS conferences, the precision engineering ASPE conference or the AIMCAL web coating & handling conference. Nine articles were published in peer reviewed scientific journals such as Progress in Photovoltaics: Research and Applications, Thin Solid Films, IEEE Journal of Photovoltaics or Journal of Vacuum Science and Technology A. An invited talk ““Atmospheric vapor-phase-deposition of TCOs for PV” was given at the 39th Int. Conf. on Advanced Ceramics and Composites. The R2R-CIGS project was present on the EuroNanoForum conferences in 2013 (with a booth) and in 2015. Other dissemination channels beside the website were a Youtube project movie, leaflets and posters.

3 Exploitation of results

The participation in R2R-CIGS helped the consortium partners to grow or position their specific activities in solar and other sectors through the created new knowledge and collaboration with other partners.
At the end of the project, thirteen key exploitable results were identified with a high TRL level (6 to 7) an/or a high potential or value to the consortium partner concerned. These include a cost effective Fluor free weatherproof polymer front sheet, an atmospheric R2R atomic layer (ALD) deposition tool and process for applying multi-component oxide layers (such as buffer layers on CIGS foil), a R2R ALD tool and process for applying a robust humidity barrier on a polymer foil or directly on CIGS foil and a R2R laser scribing tool and process for making monolithic interconnected modules.
The project enabled the companies Beneq, VDL Flow and Mondragon to develop new or to modify their roll to roll (R2R) equipment for CIGS module manufacturing. This concerns the web handling and/or the integration of new coating (e.g. spatial ALD) or material processing (e.g. laser scribing) in their equipment. This contributes also to their business development outside CIGS PV: in other thin film and hybrid (tandem) PV technologies as well as non-solar sectors such as large area electronics, OLED lighting, packaging. Mondragon is already using the knowledge acquired in roll to roll processing to develop new equipment for customers.
Manz, as CIGS line equipment developer and manufacturer and Flisom, engaged in industrialization of R2R CIGS technologies, gained through the exposure to the new technologies, indirect or directly through trials, insights in status and merits of alternatives for their present practice, for direct or future use. Such results contribute to the expansion of Flisom, which has grown their staff requirement from 15 to 60 over the period of the project indicating the high growth potential of flexible PV.

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