Impact of Non-Uniform Interfacial Ionic Accumulation on Perovskite Solar Cells Performance Degradation Studied via In Situ Impedance Spectroscopy Coupled with ISOS Stability Tests

Adapted characterization methodologies to correlate key degradation mechanisms in full devices under operation with the variation in opto-electric properties are required for improving the long-term performance stability of perovskite solar cells (PSC). Particularly, complex dynamics of slow ionic redistribution at the perovskite (PK)/transport layers (TLs) interfaces, intrinsic to the PK layer physico-chemical stability and mediated by photo-thermal and voltage bias conditions, can impede the efficient charge extraction and trigger different types of hysteresis on current-voltage curves (I-V)¹ and metastability².<br/><br/>Impedance spectroscopy (IS) is a frequency-dependent technique increasingly used in PSC for the identification of multiple electro-chemical processes occurring at different timescales and operation modes. In this work, PSC accelerated degradation is studied by coupling in-situ IS with ISOS protocols³: ISOS-LC-1 light/dark (6h/18h) cycling and ISOS-L-1 light-soaking; both at 1 sun equivalent illumination and open circuit (Voc) load. During PSC degradation, the in-situ IS response is modelled using equivalent electric circuits (EC) composed of resistive (R), capacitive (C) and/or inductive (L) elements. A physical interpretation of the EC elements evolution is addressed in hand with drift-diffusion (DD) simulations⁴ and with the use of additional ex-situ opto-electrical characterization techniques: I-V curves, suns-Voc, external-quantum efficiency (EQE), electroluminescence (EL) and photoluminescence (PL). Single junction cells and series-interconnected double junction modules of small area (&lt; 0.64 cm²) are characterized, with NiOx/SAMs/CsFAMAPb(I_xBr_1-x)₃/C₆₀/SnO₂inverted structures (bottom emission) deposited over glass-FTO substrates and with opaque (gold) or semitransparent (ITO) top-contacts.<br/><br/>Combining I-V and IS measurements with DD simulations proposes that specific hallmarks on the IS are correlated to: (i) the formation of interfacial energetic barriers when increasing voltage bias on initially fresh devices with gold contacts and in all the architectures after ISOS-LC-1 degradation; and (ii) to a rise in mobile ions concentration for aged devices, with higher capacitive polarization at low frequencies due to charge accumulation at the PK/TLs interfaces. Simultaneously, the coupling of IS with EL points out that the redistribution of high ionic concentrations occurs via non-uniform aggregates formation at PK/TLs interfaces when approaching flat band condition, acting as “locally rectifying” Schottky barriers. As a result, marked inductive components appear on the IS response in dark and turn more prominent at high injection levels, whilst they are partially masked with the increase of photogenerated carriers upon illumination.<br/><br/>A reversibility in the aggregates formation was observed after prolonged light-soaking under ISOS-L-1, mainly through the homogenization of EL and hyperspectral-PL emission along with a reduction of the IS hallmarks linked to interfacial energetic barriers. Thus, for this case study it is identified a relation between ionic aggregates formation (photo-thermal and voltage bias dependent) and the evolution of local barriers for charge injection/extraction. This can be juxtaposed with I-Vcurves fill factor reduction and s-shape transitions observed after the dark relaxation periods of ISOS-LC-1, and with performance metastability during the posterior recovery under sustained ISOS-L-1 light soaking. Lastly, the dependence of the coupled analysis on the PK bulk vs interfaces quality of charge extraction (EQE), and the influence of shunts recombination and transport losses (IS vs suns-Voc) are evaluated.<br/><br/>REFERENCES<br/>[1] D. Jacobs, <i>et al</i>. <i>J. Appl. Phys</i>. 124, 225702 (2018)<br/>[2] L. Jiang, et al. <i>Nano Energy</i> 58, 687–694 (2019)<br/>[3] M. Khenkin, <i>et al</i>.<i> Nat Energy</i> 5, 35–49 (2020)<br/>[4] P. Calado, et al. <i>J Comput Electron </i>21, 960–991 (2022)