Realizing solution-processed heterostructures is a long-enduring challenge in halide perovskites because of solvent incompatibilities that disrupt the underlying layer. By leveraging the solvent dielectric constant and Gutmann donor number, we could grow phase-pure two-dimensional (2D) halide perovskite stacks of the desired composition, thickness, and bandgap onto 3D perovskites without dissolving the underlying substrate. Characterization reveals a 3D–2D transition region of 20 nanometers mainly determined by the roughness of the bottom 3D layer. Thickness dependence of the 2D perovskite layer reveals the anticipated trends for n-i-p and p-i-n architectures, which is consistent with band alignment and carrier transport limits for 2D perovskites. We measured a photovoltaic efficiency of 24.5%, with exceptional stability of T 99 (time required to preserve 99% of initial photovoltaic efficiency) of >2000 hours, implying that the 3D/2D bilayer inherits the intrinsic durability of 2D perovskite without compromising efficiency.

We study the effects of different electrolyte anions on the mixed ionic/electronic transport properties of organic electrochemical transistors (OECTs) based on poly(3-hexylthiophene-2,5diyl). We show that the transport properties depend on the anion present in the electrolyte, with greater source-drain currents resulting from the use of molecular anions such as hexafluorophosphate and trifluoromethanesulfonylimide than from the use of smaller atomic anions such as fluoride or chloride. Using spectroelectrochemistry, we show the maximum doping level that can be achieved in an aqueous environment is also anion-dependent. Furthermore, we find that the average electronic carrier mobility at a given doping level depends on the chemistry of the compensating counterion. We further investigate this dependence by electrochemical quartz crystal microbalance measurements, showing the solvation of the dopant anions within the polymer is drastically different depending on the choice of the anion. Surprisingly, we find that the kinetics of the doping process in these OECTs is faster for bulkier anions. Finally, we use electrochemical strain microscopy to resolve ion-dependent differences in doping and local swelling at the nanoscale, providing further insight into the coupling between local structure and ion uptake. These measurements demonstrate that the identity of the compensating ion and its interaction with the polymer and solvent are important considerations for benchmarking and designing polymer materials for mixed ionic/electronic conduction applications.

Ionic transport phenomena in organic semiconductor materials underpin emerging technologies ranging from bioelectronics to energy storage. The performance of these systems is affected by an interplay of film morphology, ionic transport and electronic transport that is unique to organic semiconductors yet poorly understood. Using in situ electrochemical strain microscopy (ESM), we demonstrate that we can directly probe local variations in ion transport in polymer devices by measuring subnanometre volumetric expansion due to ion uptake following electrochemical oxidation of the semiconductor. The ESM data show that poly(3-hexylthiophene) electrochemical devices exhibit voltage-dependent heterogeneous swelling consistent with device operation and electrochromism. Our data show that polymer semiconductors can simultaneously exhibit field-effect and electrochemical operation regimes, with the operation modality and its distribution varying locally as a function of nanoscale film morphology, ion concentration and potential. Importantly, we provide a direct test of structure-function relationships by correlating strain heterogeneity with local stiffness maps. These data indicate that nanoscale variations in ion uptake are associated with local changes in polymer packing that may impede ion transport to different extents within the same macroscopic film and can inform future materials optimization.

We study the interaction of mobile ions and electronic charges to form nonradiative defects during electric biasing of methylammonium lead triiodide (MAPbI 3 ) and formamidinium lead triiodide (FAPbI 3 ) thin films. Using multimodal microscopy that combines in situ photoluminescence and scanning Kelvin probe microscopy in a lateral electrode geometry, we correlate temporal changes in radiative recombination with the spatial movement of ionic and electronic charge carriers. Importantly, we compare trap formation with both charge injecting and blocking contacts. Even though ion migration takes place in both cases, we observe the formation of new nonradiative defects in MAPbI 3 only in the presence of injected electrons, suggesting that redox processes play a key role. On the basis of density functional theory (DFT) simulations, we propose that reduction of Pb 2+ to Pb 0 is responsible for the new defects formed in our films. These results underscore that defect properties in metal halide perovskites are not only determined by the migration of mobile ions but are also highly sensitive to their interaction with injected electronic charge.

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