By Dr. Ralf Hunger, Head of Process Integration, Solibro Hi-tech GmbH, Germany.
Solibro is an international group of research, engineering, and production companies, that in the course of its 15 years historical development have taken its proprietary CIGS solar cell technology from small-scale lab technology to world-wide large-scale thin-film PV module production with annual output capacity in the Gigawatt range. The Solibro BU is part of the China-based Hanergy Thin Film Power Group. In order for the “second generation” thin-film technology CIGS to compete with “first generation” silicon technology in the markets, continuous and substantial improvements in module efficiency and production cost were necessary:
At the German manufacturing site in Bitterfeld/Wolfen, the continuous improvement of the basic depositions steps by sputtering, CIGS coevaporation, and wet chemical deposition, and of the patterning steps, has realized an industrial-scale production of PV modules with aperture efficiency up to 16.6 % on 0.88 m2 aperture area.
R&D efforts have demonstrated the potential of productive technology potential with a champion prototype module of 18.7% aperture efficiency. This is the world record number for commercial size thin film and CIGS modules. Core components of this efficiency improvement have been the optimization of the current collecting (and transparent) top contact by the application of a metal grid on top of the TCO front contact. In university lab research, applying top contact grid for cm2 device measurement is standard technology, but Solibro developed equipment and processes to accomplish the controlled and uniform deposition of a collection grid on m2 module scale. Further development lines go along the improvement of the CIGS, TCO and CdS process and layer properties, which resulted in reduced recombination and increased front contact transmission.
For the further reduction of production cost, apart from efficiency increase, another lever is the increase of the throughput of fabrication tools. By the optimization of the CIGS deposition, a 20% increase of CIGS deposition speed at equal average module efficiencies could be demonstrated. Increasing the deposition speed further up to 150% of standard, an increased device metastability (light soak effects) and more non-uniformity in local cell performance have been observed.
In this contribution, the CIGS core technology, device properties, and some of the development aspects will be illustrated.
Dr. Ralf Hunger had studied Physics at the Berlin University of Technology, where he had also acquired his PhD. For 12 years, he had conducted research on chalcopyrite thin films at the Helmholtz-Zentrum-Berlin, the Advanced Institutes of Industrial Science and Technology Tsukuba (Japan), and the Darmstadt University of Technology. Thin film growth and characterization, deposition and vacuum technology, semiconductor surfaces and interfaces were his fields of academic interest. 10 years ago, Dr. Hunger had joined the Solibro GmbH and since then devoted himself to the industrialisation of the Cu(InGa)Se 2 deposition processes and CIGS module technology, first as senior process engineer CIGS, later as team leader CIGS development, now as head of process integration.