OpenCorporates doesn't have data for German companies (yet?).
Is there a source with open data for German companies?
Aswath Damodaran, Professor at Stern School of Business at New York University, has been compiling information on major corporations since 1998. His EU dataset contains data on 6,000 EU public corporations, including those in Germany.
Quandl has free datasets on current and historical stock prices for companies listed on the Frankfurt Stock Exchange: