(1) According to the meaning of the question, one electron absorbs two photons to produce photoelectric effect, and the maximum initial kinetic energy of photoelectrons is obtained from the photoelectric effect equation:

Ekm=2hγ-W0，

② According to the kinetic energy theorem: -eU=0-Ekm, then:

U=2hv？ W0e。

A: ① The maximum initial kinetic energy of photoelectrons is 2h-w0.

(2) When the reverse voltage between the two poles of the photovoltaic cell increases to 2 kV? When W0e, the photocurrent is exactly zero.