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

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, C_P1 (2PACz) is tentatively assigned to an iodide vacancy (V_I⁺), whereas C_P2, C_M1 and C_M2 appear to belong to a single defect family, most plausibly the methylammonium interstitial (MA_i⁺). The corresponding activation energies (E_A) are 0.51 ± 0.04 eV (C_P1) and 0.63 ± 0.05 eV (C_P2) for the 2PACz-based device, and 0.13 ± 0.003 eV (C_M1) and 1.07 ± 0.04 eV (C_M2) 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 V_OC, which argues against ion-induced non-radiative recombination as the dominant V_OC 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.