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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ARCIGS-M (Advanced aRchitectures for ultra-thin high-efficiency CIGS solar cells with high Manufacturability)

Teaser

Summary

ARCIGS-M aims to further develop the CIGS thin film solar cell technology towards higher efficiency (up to 23 % for solar cells and up to 18 % for solar cell modules), using novel approaches in passivation, patterning and optical design developed by the partners. These advances of the technology enable the use of ultrathin CIGS layers and the use of steel as substrate materials along with glass. Key competences regarding equipment for up-scaling of the patterning and deposition techniques are among the partners of the consortium. The final target of ARCIGS-M is to demonstrate a new CIGS solar cell architecture, with increased efficiency, at reduced cost and with high potential for new applications and markets, where the project will focus on building-integrated photovoltaics.

ARCIGS-M aims at supporting European industry (partners and non-partners) related to photovoltaics, directly by supporting companies producing photovoltaic modules, materials and equipment for production of photovoltaics and indirectly by informing power companies and building companies about the possibilities of using the ARCIGS-M results. Possible impact is also by enabling a higher rate of photovoltaic installations in buildings, thus increasing the power generation by photovoltaics and reducing green-house gas emissions.

A strategy for reducing cost has been identified by lowering the amount of scarce and expensive materials, most notably indium. This is obtained by reducing the thickness of the CIGS absorber layers to about 25 % of the thickness normally used, in combination with reflective and possibly also scattering back contacts. Passivation layer with nano-openings are added in order to reduce the back contact recombination that results from a higher generation of charge carriers close to the back contact.

ARCIGS-M consists of 13 partners from University (Uppsala University, University of Ljubljana, UniversitÃ© Catholique de Louvain), research institutes (imec, INL, CNRS and TNO), SMEÂ´s (Obducat AB, Midsummer AB), R&D companies (Solibro Research AB, AC&CS) and large companies (EDF and Arcelor Mittal). Out of 55 research-active persons, 18 (32%) are female (among which, the project coordinator and 2 WP leaders) and 37 (68%) are male.

Work performed

In WP1, coated steel substrates are developed as well as passivation layers made from aluminum oxide. Patterning of the samples has been made using electron beam lithography, which is less suitable for up-scaling. The work with nano-imprint lithography, the method of choice for upscaling is based on results from electron beam lithography, where both size of holes as well as distributions are investigated in real devices.

In WP2, high quality reflective stacks have been tested in experiments. About 90 % of the current density for a solar cell with 500 nm thick CIGS was obtained as compared to a solar cell with full CIGS thickness. Modeling revealed that the additional losses were due to escape of photons through the front contact after reflection at the back. The conclusion is that scattering needs to be employed for full current.

The partners active in WP3 have developed CIGS deposition processes suitable for thicknesses below 500 nm. Best results are 13.8 % efficiency for a 500 nm thick CIGS layer fabricated by co-evaporation. The substrate was Mo-coated glass, i.e. with neither reflective, nor passivated back contact. Up to now this is the best total efficiency for a solar cell.

In WP4 a first version of a full electro-optical model for a solar cell with 500 nm CIGS thickness has been obtained. Both optical simulations as well as charge carrier generation distribution are included and the model is based on material characterization. The model has been verified experimentally, see Figure 1.

A first module prototype is being developed in WP5, together with activities related to upscaling of the different process steps of the ARCIGS-M solar cell device. Some challenges have been identified, including patterning of the reflective/passivated structures for module manufacturing. The life cycle analysis has been waiting for the PEFCR rules for photovoltaics, due October 2018.

WP6 handles management, which includes arranging meetings and writing of reports. One key task is the organization of sample transfers. ARCIGS-M is a truly collaborative project with a high rate of sample exchanges and since the partners are spread out around Europe â€“ rigorous documentation and tracking of sample and sample exchange are needed. Hence, for that reason a database has been constructed, with data and status of the samples, in transit between labs and in progress.

In WP7, the dissemination of project objectives, progress and results is handled in addition to the immaterial rights and innovation. A homepage was launched at an early stage of the project, including filmed interviews of the partners. Daily twitter feeds report about news from the photovoltaics world and from the project. Four scientific papers have been published with results from ARCIGS-M. All papers from ARCIGS-M are gathered at the digital library DiVA at Uppsala University.

Final results

Until Midterm, ARCIGS-M has developed, from scattered groups to a focused team, where close to all partners are involved in many of the samples. This has paved the ground for break-throughs in the second and last period.

We can identify some break-throughs beyond state-of-the art at Midterm:
1. Fabrication of high quality passivation layers, suitable for upscaling, fabricated using either atomic layer deposition or sputtering
2. Reflective layer stacks with very high reflectance, enabling close to full reflection even after the exposure to the harsh conditions (selenium atmosphere and high temperature) during CIGS deposition
3. CIGS layers of high quality, leading to best solar cell devices with close to 14 % efficiency for a CIGS layer thickness of only 500 nm (without back contact passivation or reflection)
4. First version of a complete electro-optical model, to be used for feedback on progress of reflective stacks and passivation, also including all optical features of the ARCIGS-M technology

Until the end of ARCIGS-M, the main focus will be on prototyping, where all electrical and optical features in the ARCIGS-M technology will be implemented in solar cells, for basic characterization and further scientific understanding and in small to mid-sized modules. The mid-sized prototypes require several of the up-scaling efforts to be fully operational in pre-industrial scale. The implementation of all features in one single device, including steel substrate, isolation coating, reflective stacks, passivation layers, CIGS layers and front surface passivation layers and window layers - is challenging. The full electro-optical model is a key to understand and to mitigate possible bottlenecks and problems in obtaining the highest possible efficiency. If proven successful, the ARCIGS-M technology opens up for new building-integrated solutions with low cost and high efficiency. Thin film photovoltaic industry may benefit from several of the solutions, separately. Examples of these benefits are: Isolated low cost steel substrates, passivation layers, enhanced CIGS material quality and modeling, which are useful even without thinning down the CIGS layer.

The final goal for efficiency remains at 23 % efficiency for a solar cell with sub-micrometer CIGS thickness and 18 % efficiency for a prototype module.