This is our new paper in Nano Letters
Victoria Gonzalez-Pedro, Emilio J. Juarez-Perez, Waode-Sukmawati Arsyad, Eva M. Barea, Francisco Fabregat-Santiago, Ivan Mora-Sero and Juan Bisquert
We think we have achieved some important new insights. Here are the conclusions:
We have observed the transport coupled with recombination process in perovskite solar cells by the standard signature of transmission line pattern in the impedance spectroscopy spectra. We studied different types of cells: thin film (TF) devices on the one hand, and nanostructured devices (NS) including a metal oxide framework, on the other. Despite the conspicuous differences of the samples in terms of perovskite material, growth method, sample configuration (nanostructured and thin film) or selective contact to electron, similar behavior has been observed indicating a common and general working mechanism in perovskite solar cells.
The observation of TL allowed us to determine important cell parameters as carrier conductivity, recombination resistance, and diffusion length. The transport rate is nearly the same in the different cells indicating that the dominant transport pathway is the perovskite absorber.
The diffusion length plays a key role in the photovoltaic performance of perovskite solar cells, limiting the active layer thickness to few hundreds of nm. We found that a large diffusion length is achieved in compact film of CH3NH3PbI3-xClx, while the metal oxide nanostructure increases the of CH3NH3PbI3 perovskite.
The model discussed in this work is highly consistent basically by three facts: i) there are many physical processes producing an arc in an impedance spectra but the observation of the straight line feature clearly indicates the presence of a transmission line or Gerischer impedance, that recognizes coupled transport and recombination and separates the correspondent resistance elements, as we discussed. ii) Diffusion lengths obtained applying our model are in good agreement with the measured ones for thin film CH3NH3PbI3-xClx measured by a completely different technique (time resolved photoluminescence). iii) Perovskite conductivities applying our model are in good agreement with the previously reported ones, also measured by completely different techniques.
The results here reported will contribute to the development of a complete model of the working principles in perovskite solar cells and could have important implications in the optimization and characterization of this technology.