Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells

The hole transport material (HTM) layer is a key component of the perovskite solar cells (PSCs) that must be optimized to reach high efficiency. The development of new HTMs alternative to Spiro-OMeTAD and the understanding of the role of doping agents on these layers are important research axes in the field. It requires the use of appropriate characterization tools enabling us to discriminate the bulk and interface effects. In the present paper, we fully analyze the effect of HTM doping and of the material on the impedance response of PSCs. The approach has been implemented on two different molecular HTMs, Spiro-OMeTAD and a new molecular carbazole HTM, called B186, and with various doping levels. We show that limitations by poor doping are characterized by an extra high frequency impedance loop for which capacitance and resistance analysis gives the dielectric constant and conductivity of the material, respectively. However, the low-frequency part of the spectra provides important information on the charge accumulation/outflow and on the recombination levels. More generally, the presented approach is of high practical interest for the development of new organic HTMs and for the optimization of the layer doping.