On the relation between mobile ion kinetics, device design, and doping in double-cation perovskite solar cells

Abstract

Contribution of mobile ions to current transport in double cation FA_1−xCs_xPbI₃ p–i–n solar cell structures with NiO_x hole transport layers and electron transport layers (ETLs) formed by Phenyl-C61-butyric acid methyl ester (PCBM) or C60 was studied by means of our previously developed suite of techniques comprising admittance spectroscopy, Deep Levels Transient Spectroscopy with standard (DLTS) and reversed (RDLTS) sequence of biasing/pulsing, and Photoinduced Open-Circuit Voltage V_oc Spectroscopy (PIVTS). These studies show that the amplitudes of mirror-like peaks in DLTS/RDLTS, the hallmark of mobile ions, strongly decrease when switching from PCBM to C60 and further decrease with the addition of Cl to the growth solution. A similar effect is observed in PIVTS spectra monitoring changes in long-time open circuit V_oc relaxations with temperature. The activation energies of peaks in DLTS, PIVTS, and low-frequency admittance spectra are similar to each other. The results indicate that switching the ETL type from PCBM to C60 and adding Cl in the perovskite ink for improved crystallization result in alleviating adverse effects of mobile ions, which correlates with observed changes in solar cell performance and reliability. Possible reasons for the described phenomena will be discussed.