  Solar cells based on organometal halide perovskites have recently become very
promising among other materials because of their cost-effective character and
improvements in efficiency. Such performance is primarily associated with
effective light absorption and large diffusion length of charge carriers. Our
paper is devoted to the explanation of large diffusion lengths in these
systems. The transport mean free path of charged carriers in a perovskite/${\it
TiO_2}$ heterojunction that is an important constituent of the solar cells have
been analyzed. Large transport length is explained by the planar diffusion of
indirect excitons. Diffusion length of the coupled system increases by several
orders compared to single carrier length due to the correlated character of the
effective field acting on the exciton.
