One of the year's hottest stocks on the Hong Kong market, Hanergy Thin Film Power Group Ltd., plunged nearly 50% in morning trading—wiping US$18.6 billion from its market value.

The Chinese solar-power company's share price had nearly tripled this year—after nearly quadrupling in 2014—at one point making its chairman, Li Hejun, China's richest man on paper.

But on Wednesday the stock opened lower and then took a sharp downturn on heavy volume. Trading in the stock was halted about an hour after the market opened, with the price down 47% at 3.91 Hong Kong dollars.

There no apparent immediate cause for the heavy selloff but it followed the overnight implosion of another Chinese solar company, Yingli Green Energy, whose share price fell 37% in Tuesday trading in New York. Yingling had said in a filing to the U.S. Securities and Exchange Commission Friday that "there is substantial doubt as to our ability to continue as a going concern."

Access Investor Kit for Yingli Green Energy Holding Co. Ltd.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US98584B1035

Subscribe to WSJ: http://online.wsj.com?mod=djnwires