First Solar Inc Chairman and former Chief Executive Michael Ahearn sold more than $68.5 million of the company's stock in three days this week, a move that reduced his holdings by nearly 90 percent.

Ahearn sold more than 689,000 First Solar shares in the open market between August 8 and August 10, according to a filing with the U.S. Securities and Exchange Commission late on Wednesday. As recently as June 30, he owned a total of 789,395 shares, according to Thomson Reuters data.

The shares were sold at prices ranging between $96.74 and $106.65.

Spokespeople for both First Solar and Ahearn's venture capital firm were not immediately available for comment on the sales.

Ahearn left First Solar's top post in late 2009, but has remained chairman.

He has reduced his holdings of First Solar stock significantly since stepping aside as CEO. In April of 2010, for instance, he had over 1.5 million shares.

At the same time, Ahearn has been raising funds for his newly-launched $300 million venture capital firm, True North Venture Partners.

First Solar is the world's most valuable solar company, but its stock has been hammered this year because pullbacks in government incentives for solar power in Europe have weakened demand and sent prices on panels down sharply. The stock has lost more than 20 percent of its value this year and is off 42 percent from a 52-week high reached in February.

First Solar shares were trading at $102.12 in midday trade on the Nasdaq.