Do you know why there is a difference in wettability between the perovskite precursor solution ITO and glass

• Summary: Me-4PACz self-assembled monomolecular film serves as a hole transport layer for perovskite solar cells with high efficiency. However, there is a significant challenge in fabricating efficient perovskite devices due to poor interaction between the perovskite precursor solution and the underlying Me-4PACz, making it difficult for the perovskite to attach to Me-4PACz. In order to improve the interaction between the perovskite ink and the Me-4PACz coating substrate and obtain a uniform perovskite layer, a ternary co-solvent system consisting of DMF, DMSO, and NMP was used. Compared with inks based on DMF and DMSO, the addition of NMP showed higher binding energy of perovskite ink with Me-4PACz, which was revealed by density functional theory calculations. By optimizing the ternary co-solvent ratio, the perovskite device achieved power conversion efficiencies higher than 20%, 19.5%, and about 18.5% on the effective areas of 0.16 cm2, 0.72 cm2, and 1.08 cm2, respectively. Importantly, this perovskite ink-substrate interaction method is general and helps to obtain uniformity for other perovskite combinations such as MAPbI3, FA1−xMAxPbI3–yBry, and MA-free FA1−xCsxPbI3–yBry. layer and high photovoltaic device performance.
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• Research background: The interaction between perovskite ink and the bottom charge transfer layer (CTL) in perovskite solar cells is crucial for device performance. This interaction affects the uniformity, charge transfer properties, recombination process, repeatability, and stability of perovskite thin films. Uniform deposition of perovskite can be achieved on hydrophilic CTLs using highly polar solvents such as DMF and DMSO, but it is difficult to obtain a uniform layer on hydrophobic HTLs such as PTAA and poly TPD. The study focused on the advantages of specific self-assembled monolayers such as Me-4PACz as hole transport layers, but the poor interaction between perovskite inks and them makes it difficult to obtain a uniform perovskite layer. To address this issue, a study proposes the use of Lewis acid Lewis base additives to improve the interaction between perovskite ink and underlying HTL (such as Me-4PACz), in order to obtain high-quality perovskite solar cells. This is crucial for achieving the repeatability performance of small and large area devices.

  
  

  Research content:

  1. Hydrophobicity of Me-4PACz: The hydrophobicity of Me-4PACz is related to its methyl (- CH3) and/or long alkyl chains (C4H8). Compared to MeO-2PACz, which is less hydrophobic, its water contact angle is smaller because it contains less hydrophobic methoxy (- OCH3) groups and shorter alkyl chains (C2H4). When Me-4PACz is deposited on patterned (or etched) ITO substrates, the calcium titanium layer is only formed in the ITO portion, not in the glass portion of the substrate. This may be due to the horizontal self-assembly of Me-4PACz SAM on the vertical side of ITO, which leads to the formation of dense monolayers of long alkyl chains (C4H8). In addition, for glass, solvents tend to adsorb more on the surface, and polar solvents have a higher adsorption heat. Therefore, the use of highly polar solvents such as ethanol (EtOH) to dissolve Me-4PACz may lead to the formation of heterogeneous, random, and dense orientations of hydrophobic Me-4PACz in the glass portion. The ternary co solvent system strategy significantly improved the calcium titanium layer coverage of Me-4PACz SAM deposited on patterned ITO substrates.
  

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• 2. The important role of NMP in ternary solvent systems: The importance of NMP (N-methylpyrrolidone) is to improve the coverage of perovskite on Me-4PACz. NMP has a lower dipole moment compared to DMF and DMSO, making it a suitable solvent for improving the wettability of hydrophilic and hydrophobic surfaces. In addition, the hydrophilicity hydrophobicity balance (HLB) can also be considered, which consists of 20 × (1- (ML/M)) definition, where ML is the hydrophobic part and M is the total molecular weight. Due to the presence of multiple non polar - CH2 groups in NMP, NMP exhibits a lower HLB index compared to DMF and DMSO (see Table 1), indicating that NMP interacts with hydrophobic Me-4PACz (through perovskite ink) and forms a uniform perovskite film.
  

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• To further understand this, density functional theory calculations were conducted and the binding energy of the solvent perovskite Me-4PACz HTL adduct was calculated. Density functional theory has calculated the binding energies of solvent molecules with perovskite, Me-4PACz, and perovskite Me-4PACz systems. The calculation results indicate that  NMP has the highest binding energy with perovskite, indicating a strong interaction between NMP and the overall perovskite cluster. In addition, Me-4PACz SAM interacts with all solvent molecules through hydrogen bonding, but the binding energy of NMP-Me-4PACz is lower compared to DMF-Me-4PACz and DMSO-Me-4PACz, indicating that NMP solvents have a higher affinity for Me-4PACz SAM. Through contact angle measurement, it was further confirmed that NMP based perovskite ink interacts with Me-4PACz to improve the interaction, and forms a uniform layer of perovskite on Me-4PACz.
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• 3. Film quality of ternary solvent system: XRD results show that as the proportion of NMP solvent increases, the residual PbI2 peak gradually disappears and a new peak corresponding to the NMP PbI2 crystalline solvent intermediate complex appears. DMSO and NMP are both Lewis bases, forming crystal intermediate complexes with PbI2 (Lewis acid), further proving the existence of NMP in perovskite films through experiments. The SEM results showed that there was no significant difference in the surface morphology of perovskite films from DMF: DMSO and DMF: DMSO: NMP.
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• 4. Device performance: By optimizing the ternary co solvent ratio, perovskite devices achieved power conversion efficiencies of over 20%, 19.5%, and approximately 18.5% on effective areas of 0.16 cm2, 0.72 cm2, and 1.08 cm2, respectively.
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• Original link: https://onlinelibrary.wiley.com/journal/16163028