The tribunal has received heavy criticism over political interference related to cases 003 and 004.
Donors are meeting in New York on Friday to review the budget proposal.
The court’s international side is expected to run out of funds in a few months, if more funding is not allocated.
In his confession during a hearing in March 2009, Duch said he reported to brother number two Nuon Chea, the regime’s chief ideologue.
The 27-page document released by the tribunal shows Duch expressing remorse.
Rare photos taken by American journalist Elizabeth Becker are now on display in Phnom Penh.
A meeting on funding is scheduled at the UN for Feb. 24, according to a donor country diplomat who spoke on condition of anonymity.
Nuon Chea has so far been the only one of three leaders on trial for atrocity crimes to repeatedly address the court in his defense.
Through an attorney, victims of the regime told the UN-backed court of crimes of torture and execution.
The three leaders—Noun Chea, Khieu Samphan and Ieng Sary—are charged with atrocity crimes that include genocide.
Cambodian deputy prosecutor Veng Huot said Khieu Samphan had attained the movement’s highest rank through two decades of resistance.
In an e-mail interview, Royce said he has sent a letter to Secretary of State noting his concern for an ongoing culture of impunity.