  Polymer solar cells are considered as very promising candidates for
development of photovoltaics of the future. They are cheap and easy to
fabricate, however, up to now, they possess fundamental drawback, low
effectiveness. In the most popular BHJ (bulk heterojunction) architecture the
actual record of efficiency is about 13 percent. One ask the question how
fundamental this limitation is. In our paper we propose the simple model which
examines the limitations of efficiency by analysis of geometrical aspects of
the BHJ architecture. In this paper we considered two dimensional model. We
calculated the effective length of the donor-acceptor border in the random
mixture of donor and acceptor nanocrystals and further compared it with an
ideal comb architecture. It turns out that in the BHJ architecture, this
effective length is about 2 times smaller than in the comb architecture.
