Enhancing Chemical Stability and Suppressing Ion Migration in CH₃NH₃PbI₃ Perovskite Solar Cells via Direct Backbone Attachment of Polyesters on Grain Boundaries

Abstract

Organic−inorganic halide perovskites feature excellent optoelectronic properties but poor chemical stability. While passivating perovskite grain boundary (GB) by polymers shows prospects on long-term performance of perovskite solar cells (PSCs), its detailed impact on the ion migration phenomenon, which largely deteriorates the PSC stability, remains less probed. Here, we introduce a new polar polymer, polycaprolactone (PCL), to passivate GBs of methylammonium lead triiodide (MAPbI 3 ) perovskite with only 1−2 polymer monolayers via direct backbone attachment. The PSCs with passivated MAPbI 3 , using a classic but less stable Spiro-OMeTAD (2,2′,7,7′tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene) hole transport layer (HTL), exhibit improved power conversion efficiencies up to 20.1%, with 90% of the initial PCE being preserved after 400 h ambient storage, and 80% even after 100 h, 85 °C aging. The improved PSC stability indicates critical roles of PCL GB passivation in retarding moisture-induced decomposition and suppressing ion migration within the perovskite. Time-of-flight secondary ion mass spectrometry reveals that I − ions can actively migrate into the electrode, HTL, and their interface in nonpassivated PSCs, even without an externally applied electric field, while such migration is significantly mitigated in PCL-passivated PSCs. This effective GB passivation by PCL suggests an important potential of polymer additives toward the development of stable high-performance PSCs.

Organic–inorganic hybrid perovskite for low-cost and high-performance xerographic photoreceptors

“…34 When temperature was elevated to 243 K, ionic conductivity began to dominate the total conductivity with an E a of 0.17 eV, which is comparable to the values reported previously. [35][36][37] For the PEG-containing perovskite lm, a similar activation energy of $43 meV was recorded for the electron conduction, indicating that the incorporation of PEG do not affect the exciton dissociation signicantly. But for the lm with PEG, a higher transition temperature of 300 K from the electronic to ionic migration-dominated conduction was found with a higher ionic conduction E a of 0.74 eV, which is about four times larger than that for the pristine perovskite lm.…”

Emerging Perovskite Solar Cell Technology: Remedial Actions for the Foremost Challenges

“…[ 233–238 ] For instance, Zhou et al has incorporated polycaprolactone polymers to passivate the MAPbI 3 grain boundaries, resulting in a fabricated device that showed stability up to 90% of its initial PCE even after 400 h of storage in ambient. [ 239 ] Basically, the grain boundaries (GBs) provide the pathways as an instantaneous entrance for moisture and oxygen from the surroundings. [ 235 ] Eventually, these spread throughout the entire grain, and degradation of the film starts.…”

Suppressing Ion Migration across Perovskite Grain Boundaries by Polymer Additives

“…As shown in Figure 2d, the peaks at 3173 and 3130 cm −1 belonging to the NH stretching from MA + of the perovskites exhibit no shift after incorporating the PCE10 and PS additives, indicating that there is also no interaction between the polymer additives and MA + ions. [9,19] We further performed steady-state photoluminescence (PL) and time-resolve PL (TRPL) spectra of perovskite films with and without polymers. As shown in Figure S1, Supporting Information, the steady-state PL spectra show no shift of the emission peak and the perovskite films exhibit similar carrier lifetime, indicating no passivation effect after adding polymers in perovskite films.…”

“…have introduced polycaprolactone polymers into perovskite films and found the polar CO groups can bond with the under‐coordinated Pb 2+ ions at perovskite grain boundaries as revealed by FTIR and X‐ray photoelectron spectroscopy (XPS) results. [ 9 ] Although the authors ascribed the improved device performance to the defects passivation effect of polymer additives via such “polymer−perovskite” interaction, it is still not clear on how such additional bonding can directly correlate to the changes in defect states in the perovskite film. In fact, the additives in some cases could have even larger impact on the perovskite crystal and film formation.…”