Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current-voltage (I-V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. We show that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I-V hysteresis in perovskite-based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups were utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial-adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I-V hysteresis. Moreover, the ion adsorbing properties of this interface was exploited for perovskite-based memristors. This fundamental study of I-V hysteresis in perovskite-based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.

particular polyelectrolytes due to their wettability by the perovskite precursor solution. [ 43 ] For example, Choi et al. reported a water/methanol-processed polyelectrolyte as a hole extraction material in inverted perovskite solar cells, affording a maximum PCE of 12.5%, [ 43 ] while Li et al. developed a water soluble polyelectrolyte for inverted perovskite solar cells that required thermal annealing at 140 °C for 30 min to achieve a maximum PCE of 16.6%. [ 44 ] To effi ciently extract holes from perovskite active layer and generate large built-in potential ( V bi ) across the devices, we sought a material that possesses relatively high work function ( W ) and simplifi es the preparation of high-quality perovskite layers. Recently, we found that poly(arylene-vinylene) (PAVs) with polar side chains can be synthesized by the Horner-Wadsworth-Emmons coupling/polymerization in water without using toxic reagents/catalysts. [ 45 ] This method provides an avenue to produce PAVs with a broad backbone modifi cation, especially to facilitate the introduction of electron-defi cient monomers into the polymer backbone, which lowers the highest occupied molecular orbital (HOMO) energy level of the resulting conjugated polymer and thus increases the work function of the material. [ 45 ] Also, this new method provides a route to fabricate PAVs with reasonably high molecular weight and a high degree of trans-vinylene linkages that promote planarization of the polymer backbone by removing torsional interactions between aryl-rings, thus extending conjugation. Here, we show that a PAV-based conjugated polyelectrolyte (PVBT-SO 3 ; Figure 1 a) developed through this polymerization strategy can be used as a hole extraction material for effi cient inverted perovskite solar cells that can be cast from aqueous solutions and used without thermal annealing.Fullerene/perovskite planar heterojunction solar cells, shown in Figure 1 a, were fabricated by a one-step deposition process starting from indium tin oxide (ITO) substrates (hole extracting electrode). Aqueous solutions of PEDOT:PSS or PVBT-SO 3 were spin-coated onto ITO substrates to serve as the HEL. PVBT-SO 3 formed uniform fi lms on ITO substrates, even for fi lms 5 nm in thickness, with no aggregation observable by optical microscopy ( Figure S1, Supporting Information). The perovskite precursor solution (Pb(OAc) 2 and methylammonium iodide (MAI)) was spin-coated onto an ITO/HEL substrate, followed by mild thermal annealing (at 90 °C for 5 min) to form the photoactive layer. As shown in Figure 1 b,c, uniform and continuous perovskite fi lms formed on the ITO/HEL substrates. Notably, perovskite fi lms on ITO/PVBT-SO 3 substrate had larger crystallites (crystal size տ200 nm) and were free of pinholes when compared to fi lms on ITO/PEDOT:PSS substrates. Powder X-ray diffraction ( Figure S2, Supporting Information) showed

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals' photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries.