Four-dimensional (x, y, z, t) photoluminescence dynamics revealing efficient vertical charge transport in perovskite solar cells.

Perovskite has attracted significant attention in the field of solar cells due to their high-power conversion efficiency and low-cost manufacturing potential. Fast charge carrier diffusion is essential for efficient charge collection in perovskite solar cells. There has existed a discrepancy between low diffusion coefficients previously reported and near-unity charge collection efficiencies achieved in practical solar cells. We attribute this discrepancy to an insufficient understanding of nanoscale charge dynamics in perovskites.
  In this preparation, we review our recent work on investigating a new technique of four-dimensional (x, y, z, t) charge carrier tracking to quantitatively analyze and visualize vertical charge diffusion. The result reveals that the diffusion coefficients of intragrain and intergrain regions differ significantly, and both direct intragrain pathways and indirect detours play a crucial role in charge collection efficiency. The diffusivity obtained using conventional methods is 0.02cm2/s, whereas the diffusivity calculated from four-dimensional tracking is 0.25 cm2/s, which is an order of magnitude higher. This value rationalizes the high performance observed in practical solar cells. Further control of polycrystal growth is expected to enable solar cells with micrometer thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.