Large diffusion lengths of excitons in perovskite and ${\it TiO_2}$   heterojunction
  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. 
Zhyrair Gevorkian, Vladimir Gasparian and Yurii Lozovik
cond-mat.mtrl-sci