The controversial boss of Chinese solar energy firm Hanergy Thin Film Power Group -- once listed as China's richest man -- has stepped down as chairman and executive director, the company said, almost a year after its shares imploded.

Li Hejun stepped down from Friday "for the reason of strengthening corporate governance", Hanergy said in a statement to the Hong Kong stock exchange, where its shares have been suspended since May last year after plunging 47 percent in less than half an hour, wiping $19 billion off its market capitalisation.

The territory's securities regulators also announced a probe into the company.

Hanergy had captured the attention of investors after growing more than sixfold to became the world's largest solar power company by market value before the collapse, amid questions over the firm's share price and revenue sources.

The meteoric rise in share price once gave Li the title of China's richest man, according to one wealth survey, but he has since fallen well down the rankings and regulatory filings showed he later sold some of his shares at a 95 percent discount.

He remains the chairman and chief executive officer of parent company Hanergy Holding Group, according to the parent.

The statement said there were no matters relating to Li's resignation that needed to be brought to the attention of the stock exchange or shareholders.

Hong Kong's Securities and Futures Commission has yet to make a public announcement about its investigation, almost a year after it started.

Two other executive directors of Hanergy Thin Film also stepped down, according to the latest statement, with Yuan Yabin, a deputy general director of the parent taking over as chairman and an executive director of the listed unit.

The collapse of Hanergy and some other top-performing stocks on the Hong Kong stock market prompted critics to question the oversight of regulators.