Self-assembled monolayer-dependent ionic defect restructuring in perovskite solar cells: a deep-level transient spectroscopy study.

Self-assembled monolayers (SAMs) based on carbazole derivatives have become a dominant hole-selective contact choice for high-efficiency perovskite solar cells, yet their influence on the distribution of ionic defects in the adjacent perovskite absorber remains poorly understood. Here, we use deep-level transient spectroscopy (DLTS) to probe the ionic defects of perovskite solar cells employing two widely used SAM hole-transport layers (HTLs), the parent carbazole (2PACz) and its dimethoxy-substituted analogue (MeO-2PACz). Photovoltaic performance of bifacial CH3NH3PbI3 (methylammonium lead iodide, MAPbI3) solar cells reveals that the 2PACz-based devices outperform their MeO-2PACz counterparts, reaching a power conversion efficiency (PCE) of 16.4% compared with 14.1%. DLTS resolves two positively charged mobile-ion signatures in each device. On the basis of their diffusion coefficients and Meyer-Neldel rule, CP1 (2PACz) is tentatively assigned to an iodide vacancy (VI+), whereas CP2, CM1 and CM2 appear to belong to a single defect family, most plausibly the methylammonium interstitial (MAi+). The corresponding activation energies (EA) are 0.51 ± 0.04 eV (CP1) and 0.63 ± 0.05 eV (CP2) for the 2PACz-based device, and 0.13 ± 0.003 eV (CM1) and 1.07 ± 0.04 eV (CM2) for the MeO-2PACz-based device. Quantitative analysis of the DLTS amplitudes further places the mobile-ion concentrations of all four defects in the range of ∼1016-1017 cm-3. The perovskite films grown on the two SAMs are morphologically and structurally equivalent, and the device with the larger mobile-ion density delivers the higher VOC, which argues against ion-induced non-radiative recombination as the dominant VOC loss, although the mechanism remains open. These results show that subtle substitution of the carbazole core reshapes the ionic defect distribution in the perovskite absorber and identify DLTS as a sensitive diagnostic for rationally selecting SAM HTLs.