When mixed, solutions of positive and negative polyelectrolytes may spontaneously phase-separate into blended, hydrated complexes or coacervates (PECs). Charge-pairing interactions between ...oppositely-charged polyelectrolytes within PECs are weakened with the addition of salt MA. With a sufficiently high concentration of MA, the PEC may dissociate back into the individual polyelectrolytes, reversing the liquid–liquid phase separation induced by charge pairing and other interactions. This critical salt concentration (CSC), or “salt resistance,” has been extensively used to compare the stability and strength of association in PECs. However, the CSC is not always observed, and it shows a strong dependence on the type of ions comprising MA. In addition, the CSC is more likely to be observed with PECs assembled from polycarboxylates, a type of weak polyelectrolyte. Here, it is shown that a lack of experimental CSC is correlated with the preferred role of ions M+ and A– in the PEC, counterion versus co-ion, or the specificity of a particular ion for a particular polyelectrolyte repeat unit, revealed by calorimetric measurements. The importance of the enthalpy of ionization of weak polyelectrolytes in providing an experimentally measurable CSC is quantitatively demonstrated.
Surface passivation of metal halide perovskite thin films has proved to be critical for efficient and stable perovskite solar cells (PSCs),
i.e.
suppressing charge recombination at the interfaces ...between perovskite and charge transport layers for high device efficiency and preventing the penetration of degrading agents into the perovskite layer for high device stability. Here, we report the use of a new class of materials, phosphonium halide salts,
i.e.
triphenyl(9-ethyl-9
H
-carbazol-3-yl)-phosphonium bromide (TCPBr) and iodide (TCPI), as surface passivation agents for efficient and stable PSCs. Suppressed nonradiative recombination and enhanced interfacial hole extraction, enabled by this facile phosphonium halide passivation, resulted in PSCs with power conversion efficiencies (PCEs) of >20.1% (∼18.5% for pristine MAPbI
3
devices). Phosphonium halide passivation also reduced the hysteresis index from 11.2% (pristine device) to 5.4% (TCPBr passivated device). Moreover, the hydrophobic phosphonium halide passivation layers significantly suppressed moisture penetration and ion migration, greatly enhancing the device stability. It was found that unencapsulated solar cells based on TCPBr treated MAPbI
3
thin layers retained more than 90% of the initial PCEs after more than 1000 hours storage under ambient conditions. Our work established the potential of phosphonium-based materials for surface passivation of metal halide perovskites, providing a new route towards efficient and stable perovskite optoelectronic devices.
A new class of materials, phosphonium halide salts, are used as surface passivation agents for efficient and stable PSCs.
Various charged groups may be used as a repeat unit in polyelectrolytes to provide physical interactions between oppositely charged polymers leading to phase separation. The materials formed thus are ...termed polyelectrolyte complexes or coacervates (PECs). The strength of pairing between positive, Pol+, and negative, Pol–, repeat units depends on the specific identity of the monomer repeat unit. In this work, the pairing strength of the thiouronium group, a cation closely related to guanidinium, is evaluated using a polythiouronium polyelectrolyte. Polymers containing guanidinium, notably polyarginine, a peptide, are known for their unusual behavior, such as the formation of like-charge ion pairs and hydrogen bonding. It is shown here that some of this behavior is carried over to polythiouroniums, which results in exceptionally strong interactions with polyanions such as polysulfonates and polycarboxylates. The resilience of the polythiouronium/Pol– interaction was evaluated using the buildup of polyelectrolyte multilayers at various salt concentrations and by breaking up preformed PECs with high concentrations of added salt. The thiouronium group even interacts strongly enough with polymeric zwitterions to enable complexation with this nominally weakly interacting, net-neutral polymer.
Surface passivation of perovskite solar cells (PSCs) using a low‐cost industrial organic pigment quinacridone (QA) is presented. The procedure involves solution processing a soluble derivative of QA, ...N,N‐bis(tert‐butyloxycarbonyl)‐quinacridone (TBOC‐QA), followed by thermal annealing to convert TBOC‐QA into insoluble QA. With halide perovskite thin films coated by QA, PSCs based on methylammonium lead iodide (MAPbI3) showed significantly improved performance with remarkable stability. A PCE of 21.1 % was achieved, which is much higher than 18.9 % recorded for the unmodified devices. The QA coating with exceptional insolubility and hydrophobicity also led to greatly enhanced contact angle from 35.6° for the pristine MAPbI3 thin films to 77.2° for QA coated MAPbI3 thin films. The stability of QA passivated MAPbI3 perovskite thin films and PSCs were significantly enhanced, retaining about 90 % of the initial efficiencies after more than 1000 hours storage under ambient conditions.
A low‐cost industrial organic pigment, quinacridone (QA), was applied as surface passivation agent for perovskite solar cells (PSCs) by solution processing of a soluble QA derivative followed by thermal annealing to convert it into insoluble QA. Passivation with strong interactions between QA molecules and metal halides, together with the hydrophobicity of QA coating, enabled highly efficient PSCs with remarkable stability.
The interactions of charged polymers, polyelectrolytes, on surfaces and in solution have gained a lot of attention throughout the years. Depositing alternating layers of oppositely charged ...polyelectrolytes forms multilayers, PEMUs. Their association in solution leads to phase separations forming materials termed polyelectrolyte complexes, PECs. The repeat unit of the oppositely charged polyelectrolyte can be varied. The functional group linked to the repeat units dictates the strength of ion pairing. Their self-assembly in solution creates polymer nanoparticles and, in a network, produces polymer nanocomposites. These materials have been used for a variety of applications such as coatings, electronic devices, and drug delivery systems. Adding a certain salt concentration to the material can influence the polymer/polymer interaction within the complex. This work features three main interfaces at which polyelectrolyte interactions were studied: on a substrate through PEMUs, in solution through polymer nanoparticles and in nanocomposites. The pairing strength of a polythiouronium polyelectrolyte having a positively charged thiouronium group, like the guanidinium, was evaluated by the formation of PEMUs at various salt concentrations. Polyguanidiniums such as polyarginine are known to form like-charge ion pairings and hydrogen bonding. The same unusual characteristics cause the strong interactions of the polythiouronium with polyanions. High concentrations of salt dissociated the strong polythiouronium/polyanions complexes. The polythiouronium was selected to seek another example of a strong polycation and to form a PEMU with polyzwitterions, which are known for their weakly interacting strength and net-neutral charge.The second interface consists of the self-assembly of a polydimethylsiloxane (PDMS) with positively charged end groups in solution, and as a nanofiller, in a nanocomposite. Polydimethylsiloxane is biocompatible and highly hydrophobic; however, when self-assembled exhibits poor stability and tends to collapse. Introducing a glassy shell of polystyrene sulfonate enhances the stability of these polymer nanoparticles and improves the mechanical properties of the nanocomposite which exhibited thermal resistance. Using x-ray scattering and atomic force microscopy, the nanocomposite’s unique morphology was deduced. Its hydrophobic properties were probed using a solvatochromic dye and were used to encapsulate inorganic nanoparticles. The gold nanoparticles are hydrophobically capped and they appeared to be encapsulated within the PDMS core. Their assembly was studied by x-ray scattering in the presence of a PSS shell in solution and the loading capability of the nanoparticle was determined by transmission electron microscopy (TEM). The newly formed material is a novel polymer nanoparticle/polymer nanocomposite that takes advantage of the PDMS malleability and biocompatibility. The nanoparticle can thus be used for biological applications such as bio-imaging and sensing.The last section compares the interactions of different polycations with lipid layers (monolayer or bilayer) to mimic the mechanism of surface sterilization. The functional group varied between a primary or tertiary ammonium, a guanidinium and two different thiouronium groups. Ammoniums are known for their ability to disrupt lipid bilayers and for their use in disinfectants. Polyarginine, a strong positively charged polymer, is recognized for its cell-penetrating ability due to its unique salt-bridge like interactions. Radioactive unilamellar phosphocholine vesicles were prepared and used to deposit a monolayer on a hydrophobic scintillator and a bilayer on a hydrophilic one. The interactions of the layers and the deposition were monitored by radiocounting. Polyarginine and the polymer with a higher composition of a hydrophobic thiouronium exhibited the strongest interactions. These investigations of the interactions between polyelectrolytes at different interfaces serve as a guide to design a variety of functional materials. Understanding the structure-property polymer relationship allows the build-up of materials with tuned properties.
Nanocomposites with unusual and superior properties often contain well-dispersed nanoparticles. Polydimethylsiloxane, PDMS, offers a fluidlike or rubbery (when cross-linked) response, which ...complements the high-modulus nature of inorganic nanofillers. Systems using PDMS as the nanoparticulate, rather than the continuous, phase are rare because it is difficult to make PDMS nanoparticles. Aqueous dispersions of hydrophobic polymer nanoparticles must survive the considerable contrast in hydrophobicity between water and the polymer component. This challenge is often met with a shell of hydrophilic polymer or by adding surfactant. In the present work, two critical advances for making and using aqueous colloidal dispersions of PDMS are reported. First, PDMS nanoparticles with charged amino end groups were prepared by flash nanoprecipitation in aqueous solutions. Adding a negative polyelectrolyte, poly(styrene sulfonate), PSS, endowed the nanoparticles with a glassy shell, stabilizing them against aggregation. Second, when compressed into a nanocomposite, the small amount of PSS leads to a large increase in bulk modulus. X-ray scattering studies revealed the hierarchical nanostructuring within the composite, with a 4 nm PDMS micelle as the smallest unit. This class of nanoparticle and nanocomposite presents a new paradigm for stabilizing liquidlike building blocks for nanomaterials.
To stabilize and transport them through complex systems, nanoparticles are often encapsulated in polymeric nanocarriers, which are tailored to specific environments. For example, a hydrophilic ...polymer capsule maintains the circulation and stability of nanoparticles in aqueous environments. A more highly designed nanocarrier might have a hydrophobic core and a hydrophilic shell to allow the transport of hydrophobic nanoparticles and pharmaceuticals through physiological media. Polydimethylsiloxane, PDMS, is a hydrophobic material in a liquid-like state at room temperature. The preparation of stable, aqueous dispersions of PDMS droplets in water is problematic due to the intense mismatch in surface energies between PDMS and water. The present work describes the encapsulation of hydrophobic metal and metal oxide nanoparticles within PDMS nanodroplets using flash nanoprecipitation. The PDMS is terminated by amino groups, and the nanodroplet is capped with a layer of poly(styrenesulfonate), forming a glassy outer shell. The hydrophobic nanoparticles nucleate PDMS droplet formation, decreasing the droplet size. The resulting nanocomposite nanodroplets are stable in aqueous salt solutions without the use of surfactants. The hierarchical structuring, elucidated with small-angle X-ray scattering, offers a new platform for the isolation and transport of hydrophobic molecules and nanoparticles through aqueous systems.
Organometal halide perovskite materials, in particular colloidal perovskite nanocrystals (NCs), have been investigated extensively as next-generation light-emitting materials. However, producing ...highly efficient and stable perovskite thin films from colloidal NCs is not trivial, as dissociation of surfactants often occurs during the thin-film formation. Here, we demonstrate a facile solution-processing approach to prepare perovskite nanocomposite thin films by using phosphonium as the capping ligand for methylammonium lead bromide (MAPbBr3) NCs. The photoluminescence and stability of thin films containing in situ formed perovskite NCs were greatly enhanced after phosphonium passivation, with the photoluminescence quantum efficiency reaching 78% and only 5% decrease of the intensity after one month’s exposure in ambient conditions. Electrically driven light-emitting diodes (LEDs) based on pristine perovskite neat thin films and organic–perovskite nanocomposite thin films were fabricated, and we observed a 10-fold improvement in the external quantum efficiency of these LEDs (from 0.6% to 6.3%) resulting from the in situ formation of perovskite NCs with phosphonium passivation.
Abstract
Surface passivation of perovskite solar cells (PSCs) using a low‐cost industrial organic pigment quinacridone (QA) is presented. The procedure involves solution processing a soluble ...derivative of QA,
N
,
N
‐bis(tert‐butyloxycarbonyl)‐quinacridone (TBOC‐QA), followed by thermal annealing to convert TBOC‐QA into insoluble QA. With halide perovskite thin films coated by QA, PSCs based on methylammonium lead iodide (MAPbI
3
) showed significantly improved performance with remarkable stability. A PCE of 21.1 % was achieved, which is much higher than 18.9 % recorded for the unmodified devices. The QA coating with exceptional insolubility and hydrophobicity also led to greatly enhanced contact angle from 35.6° for the pristine MAPbI
3
thin films to 77.2° for QA coated MAPbI
3
thin films. The stability of QA passivated MAPbI
3
perovskite thin films and PSCs were significantly enhanced, retaining about 90 % of the initial efficiencies after more than 1000 hours storage under ambient conditions.
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