Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response

The implementation of efficient hole blocking layers (BLs) is of vital importance to achieve high efficiency in solar cells using organolead trihalide materials as the solar light absorber. BLs permit electronic charge separation and avoid the recombination of charges by blocking the hole transfer to the anode. In this study, BLs have been prepared by aerosol spray pyrolysis and spin-coating using various solution compositions. The morphological, optical, microstructural and crystalline properties as well as the phase composition of these layers have been characterized by scanning electron microscopy, Raman spectroscopy, and optical measurements. Their blocking ability has been evaluated by cyclic voltammetry. A strong relationship has been found between these properties and the solar cell JV characteristics and performances. Overall, we figured out that the sprayed BLs were thin, highly compact, covering, and conformal. No cracks or pinholes were found because the precursor underwent degradation and condensation directly at high temperature. They were made of the pure anatase phase and were perfectly blocking. They produced high-efficiency perovskite solar cells. The BL has also an influence on the impedance spectroscopy response of the cells over the whole frequency range from several hundreds of Hz to some tens of mHz. We have found that the inductive loop if present for all the cells was more pronounced and observed over a larger potential range in the case of the most poorly working devices with inaccurate BLs. An equivalent electrical circuit is proposed to fit the full spectra. The various electrical parameters of the circuit have been compared for the various BLs and thoroughly analyzed. Influences of the BL on the internal resistance, recombinations, interfacial traps, and the inductive response are detailed.