Passivation of Grain Boundary by Squaraine Zwitterions for Defect Passivation and Efficient Perovskite Solar Cells

Abstract

Unavoidable defects in grain boundaries (GBs) are detrimental and critically influence the organometal halide perovskite performance and stability. To address this issue, semiconducting molecules have been employed to passivate traps along perovskite GBs. Here, we designed and synthesized three squaraine molecules (SQ) with zwitterionic structure to interact with under-coordinated Pb2+ and passivate Pb–I antisite defects. Density functional theory calculation shows symmetric O atoms could coordinate with perovskite grains simultaneously, resulting in continuous charge distribution at the SQ–perovskite interface. The energetic traps distribution in CH3NH3PbI3 perovskite is influenced significantly by the interaction between SQ and perovskite as analyzed by thermally stimulated current, in which the deep-level defects are considerably reduced due to efficient SQ passivation. In addition, we explore how SQ molecules with different energy offset affect the charge extraction, which is suggested to facilitate exciton separation at the perovskite–SQ interface. These benefits lead to enhanced perovskite efficiency from 15.77 to 18.83% with the fill factor approaching 80%, which is among the highest efficiency reported for MAPbI3 solar cells fabricated in an ambient environment at 60% relative humidity (RH). Considerable retardation of perovskite device degradation was achieved, retaining 90% of initial efficiency when kept 600 h at 60 ± 5% RH.

Dopant‐Free Squaraine‐Based Polymeric Hole‐Transporting Materials with Comprehensive Passivation Effects for Efficient All‐Inorganic Perovskite Solar Cells

“…In general, the cyclobutadione core in the SQs can contribute two special signals; one is ascribed to C=O stretching vibration at around 1760 cm −1 , while another one is originated from C‐O stretching vibration at around 1630 cm −1 . In most cases, the C=O stretching vibration will not appear due to the formation of resonance‐stabilized zwitterionic core . As shown in Figure e and Figure S2, both DPASQ and PSQ1 do not show any C=O characteristic vibration in their FT‐IR spectra, which, however, can appear when doping Pb 2+ ions into them, attributed to the formation of an adduct between carbonyl group and Pb 2+ ion according to the previous reports .…”

“…In most cases, the C=O stretching vibration will not appear due to the formation of resonance‐stabilized zwitterionic core . As shown in Figure e and Figure S2, both DPASQ and PSQ1 do not show any C=O characteristic vibration in their FT‐IR spectra, which, however, can appear when doping Pb 2+ ions into them, attributed to the formation of an adduct between carbonyl group and Pb 2+ ion according to the previous reports . By contrast, the C=O characteristic vibration can be still found in the spectrum of PSQ2 , meaning the charge does not fully delocalize in the cyclobutadione core due to the change of polymerization positions of DPASQ.…”

“…By contrast, the C=O characteristic vibration can be still found in the spectrum of PSQ2 , meaning the charge does not fully delocalize in the cyclobutadione core due to the change of polymerization positions of DPASQ. Nonetheless, the C–O stretching vibrations in all three spectra show a shift to the low wavenumber, a clear evidence of the interaction between oxygen onion and Pb 2+ ions . Overall, based on these fluorescent and FT‐IR results, our designed polysquaraines are expected to passivate the defects of perovskites effectively to reduce the trap density at the perovskite/HTM interface.…”

“…Their zwitterionic, resonance stabilized quinoid structure not only can help to increase the conductivity without doping, but also is expected to enable a close contact with perovskite layer due to the ionic nature of perovskites . Moreover, the electron‐deficient cyclobutadione core could naturally provide the desired passivation effect by forming an adduct with uncoordinated Pb 2+ ions . Benefiting from these merits, we consider that SQs would be one class of ideal material candidates as dopant‐free HTMs for PVSCs, provided that the problem of their relatively low hole mobilities of only around 10 −5 cm 2 V −1 s −1 can be overcome .…”

A Dopant‐Free Zwitterionic Conjugated Polyelectrolyte as a Hole‐Transporting and Interfacial Material for Perovskite Solar Cells

“…Zwitterions have been widely studied in photovoltaic research for interfacial optimization in recent years. [ 17–19 ] They have excellent solubility in polar solvents and are easy to process in solution. They form dipoles at the interface and improve carrier transfer.…”