Silver coordination-induced n-doping of PCBM for stable and efficient inverted perovskite solar cells

The study addresses the operational instability of inverted (p-i-n) perovskite solar cells (PSCs) caused by ion diffusion and chemical reactions between metal electrodes (e.g., Ag) and perovskite layers. Degradation mechanisms include iodide migration to Ag forming insulating AgI, redox interactions with Pb2+, and metal diffusion into the perovskite forming insulating halides or defect states. Existing physical barrier strategies (graphene, chromium/bismuth interlayers, amorphous films) and chemical anticorrosion layers (TTTS, BTA, calix[4]pyrrole) delay but do not prevent iodine/metal migration under heat/light, limiting device lifetimes (<2000 h at room temperature, <1500 h above 60 °C under MPPT). The hypothesis is that introducing a bipyridine derivative (4,4'-dicyano-2,2'-bipyridine, DCBP) into the PCBM electron transport layer can coordinate with Ag to both block bidirectional Ag/I migration and induce n-doping in PCBM, thereby enhancing electron extraction, reducing interfacial recombination, and improving efficiency and stability.

Prior work has shown that metal migration (Ag, Al, Cu) and halide diffusion drive degradation in inverted PSCs under illumination or electric fields. Physical barriers such as graphene, chromium/bismuth interlayers, and amorphous films can impede diffusion but allow iodine permeation under stress. Chemical anticorrosion approaches using coordinating molecules (e.g., 1,3,5-triazine-2,4,6-trithiol trisodium salt (TTTS), benzotriazole (BTA), calix[4]pyrrole (C[4]P)) improve stability but often do not enhance, and can reduce, power conversion efficiency. Coordination-activated n-doping via metal–ligand complexes (e.g., bipyridines) has been effective in organic electronics to create stable low-work-function contacts and enhanced electron injection, suggesting potential applicability to PSC electron transport layers.

Strategy: Incorporate 4,4'-dicyano-2,2'-bipyridine (DCBP) into PCBM to form a coordination-induced n-doping (CIN) system upon interaction with Ag. The pyridine nitrogens chelate Ag, oxidizing Ag from 0 to +1 and releasing electrons captured by PCBM (n-doping). The cyano groups interact strongly with iodide and FA (formamidinium), suppressing FA/I vacancy formation and iodine migration. Computational: DFT (CP2K, PBE-D3, DZVP-MOLOPT, GTH pseudopotentials) on FAI-terminated FAPbI3 (001) with defects (VFA, VI) to assess charge redistribution and interactions with DCBP; Gaussian 09 for ESP, HOMO/LUMO, dipole; Multiwfn processing. Device fabrication: ITO/NiOx/PTAA/Al2O3/perovskite/PCBM(±DCBP)/BCP/Ag. Perovskite: FA0.95Cs0.05PbI3 from DMF/DMSO (4/1) solution, anti-solvent CB, anneal 100 °C 30 min. PCBM in CB (23 mg/mL) spin-coated; DCBP (optimized 5×10^-3 mmol/mL) added to PCBM for target devices; BCP interlayer spin-coated from saturated IPA solution; Ag (~100 nm) thermally evaporated. Active areas 0.09 cm^2 and 1 cm^2. Characterization: TOF-SIMS depth profiling and 3D mapping after 800 h AM1.5 illumination to track Ag and I distribution; XRD for phase (AgI, PbI2) and perovskite crystallinity; SEM (cross-section) and optical microscopy of Ag electrode morphology pre/post aging; XPS/AES for Ag 3d binding energy and Auger kinetic energy to determine Ag valence; time-of-flight mass spectrometry to detect [Ag(DCBP)+]; FTIR, XPS (N 1s, I 3d), and 1H NMR for coordination and I–N interactions; ESR for fullerene radical anions; UPS for band alignment; KPFM cross-sectional potential mapping and derived electric field under short-circuit; SCLC for trap density; PL/TRPL and PLQY (integrating sphere) for non-radiative recombination and QFLS; transient photocurrent, Mott–Schottky, and capacitance-frequency; JV (Newport-2612A, Keithley 2400), EQE (Newport-74125). Stability testing: Unencapsulated devices under MPPT at one sun (LED, ~45 °C) in N2; encapsulated devices under ISOS-L-2 at 85 °C; damp-heat at 85 °C/85% RH (ISOS-T-1).

- The DCBP additive forms strong coordination with Ag via pyridine nitrogens (Ag valence changes from 0 to +1: Ag 3d binding energy shifts to 368.7 eV; Auger kinetic energy 355.1 eV), releasing electrons that dope PCBM n-type; time-of-flight MS detects [Ag(DCBP)+] at m/z 313.5. - DCBP’s cyano groups interact strongly with I− and FA, suppressing iodine and FA vacancy formation (DFT charge redistribution) and hindering Ag/I bidirectional migration (TOF-SIMS shows minimal Ag and I interdiffusion after 800 h illumination in target vs substantial in control). - Structural/electrode stability: After aging, control Ag electrodes develop deep pits and perovskite shows AgI and PbI2 formation and degraded crystallinity; target devices show uniform Ag surface with negligible AgI, limited PbI2, and preserved perovskite crystallinity (XRD, SEM, optical microscopy). - N-doping evidence and transport enhancement: ESR shows strong paramagnetic signal indicating fullerene radical anions in PCBM@DCBP/Ag; UPS indicates more favorable band alignment for PCBM@DCBP; KPFM reveals increased potential drop and stronger local electric field at PVSK/ETL interface in target devices; electron mobility and conductivity increase with aging in PCBM@DCBP/Ag due to progressive CIN reaction, while they decrease in PCBM/Ag controls. - Trap density reduced in DCBP-modified perovskite/ETL stacks (SCLC): target 1.60×10^25 cm^-3 vs control 2.26×10^25 cm^-3; ideality factor decreases from 1.51 (control) to 1.27 (target), indicating suppressed non-radiative recombination; faster transient photocurrent decay (0.48 µs target vs 1.22 µs control); higher built-in potential Vbi. - Performance: Target devices achieve 26.03% PCE after 800 h aging, with stabilized 25.52%; certified PCE 25.51% with Jsc = 26.24 mA/cm^2, Voc = 1.170 V, FF = 83.1%; 1 cm^2 devices reach 24.17% after aging. Statistical averages: control drops from 23.25 ± 0.41% to 17.76 ± 1.50%; target increases from 23.73 ± 0.25% to 25.25 ± 0.34% (after 800 h illumination aging). After aging, EQE remains higher for target; integrated Jsc: 25.35 mA/cm^2 (target) vs 21.13 mA/cm^2 (control). - Voltage losses: Radiative Voc limit 1.310 eV; non-radiative voltage loss ~126 mV (Voc = 1.184 V). PLQY-derived QFLS indicates DCBP passivates perovskite bulk and PVSK/PCBM interface; transport losses are minimal (QFLS ~ Voc). - Stability: Under MPPT at one sun (~45 °C), target PCE peaks at 25.90% at 889 h and retains 22.77% at 2500 h (~95% of initial), whereas control falls to 11.51% at 2400 h. Encapsulated at 85 °C (ISOS-L-2), target retains >85% after 1000 h; control drops to 6.7%. Damp-heat (85 °C/85% RH): target retains 90% after 1500 h; control retains 54.4%.

The CIN strategy directly addresses the primary degradation pathways in inverted PSCs by simultaneously preventing Ag/I interdiffusion and enhancing electron extraction. Coordination of DCBP with Ag both immobilizes Ag at the electrode side and scavenges migrating iodide via strong I–N interactions, mitigating AgI formation and preserving perovskite stoichiometry and crystallinity. Concurrently, Ag-to-DCBP coordination releases electrons that are captured by PCBM, producing n-doped ETLs with improved conductivity, mobility, and band alignment. The stronger local electric field at the PVSK/ETL interface facilitates charge collection and reduces interfacial carrier accumulation, thereby suppressing non-radiative recombination. These mechanisms collectively yield higher Voc and FF, along with improved stability under light, heat, and humidity stress. The low non-radiative voltage loss (126 mV) and close QFLS–Voc agreement confirm that bulk and interfacial recombination losses are minimized without introducing significant transport penalties. Overall, the approach enhances both performance and durability, overcoming the typical trade-off seen with inert barrier layers.

Incorporating DCBP into the PCBM ETL establishes a silver coordination-induced n-doping system that: (i) chelates Ag to block bidirectional Ag/I migration and prevent AgI formation, protecting electrodes and perovskite; (ii) injects electrons into PCBM, yielding n-doped ETLs with favorable band alignment and enhanced electron transport; and (iii) passivates perovskite surface defects via strong interactions with iodide and FA. Devices achieve 26.03% champion PCE (certified 25.51%) with only 126 mV non-radiative voltage loss and exhibit outstanding operational and damp-heat stability (≈95% PCE retention after 2500 h MPPT at one sun; 90% after 1500 h at 85 °C/85% RH). This strategy opens avenues to improve long-term stability and reliability of high-efficiency inverted PSCs under realistic operating conditions.