Solution Processing Techniques Research Articles

Crystallization and Stability of Flexible P-Type Semiconductors and Their Thin Film Transistor Application

In recent years, electronic devices that require superior mechanical flexibility have been extremely sought for potential applications in next-generation displays, sensors, bio-implantable and disposable medical devices. Polymeric materials, of which the flexibility far exceeds that of conventional brittle inorganic counterparts, are widely considered for various flexible devices due to their excellent stretchable, foldable and bendable characteristics.Oxidative chemical vapor deposition (oCVD) is an emerging technique that provides not only a capability to process mechanically flexible polymeric films but also strategies to address current challenges claimed in typical solution-based polymer processing1-2. The oCVD technique allows for greater uniformity, excellent conformality, enhanced performance (e.g., conductivity) and an ability of large area polymer processing that are limited in conventional solution-processed polymers3-4.Crystallization and air stability of organic materials have been issues that have to be mitigated to warrant high performance and sustainable device applications. The oCVD technique, in addition to the advantages described above, could also address the issues of crystallization and air stability to enhance the carrier transport and overall conductivity, and to secure sustainability. We recently found that an in-situ oCVD capability of substrate heating during depositions resulted in an substantial increase in ordered structures in conjugated polymers, leading to enhanced crystallization of the resulting oCVD polymers5. Consequently, the enhancement of the carrier transport and conductivity was obtained due to the increase in favorable conduction paths in the crystalline phase. Further, the vapor phase processing of oCVD may be of crucial relevance to achieve air stability since oCVD does not require any monomer solubility in specific solvents and, therefore, is able to polymerize insoluble materials6 that are not available for conventional solution processing techniques such as spin-coating or dip-casting. The reactivity of insoluble monomers is quite limited due to their rigid backbone structures; however, this limited reactivity is the key to achieve air stability since the reactions between insoluble materials and water (H2O)/oxygen, which are the main origins to cause instabilities of organic materials in air, are significantly prohibited.In this presentation, we report our recent findings of enhanced crystallization and air-stability of oCVD polymers. Particularly, the structure, chemical and electrical properties of two oCVD polymers, poly(3,4-ethylenedioxythiophene) (i.e., PEDOT) and unsubstituted polythiophene, will be compared with focuses on the crystalline state, dope-ability and air-stability. Further, thin film transistors (TFTs) that employ the two oCVD polymers as TFT channel are presented and the device performance of output and transfer characteristics will be compared with focuses on the carrier mobility and on/off switching properties. Acknowledgement: This work was supported by Purdue University and partly by an NSF grant, Award No. ECCS-193108. References Lock, J. P.; Im, S. G.; Gleason, K. K., Oxidative chemical vapor deposition of electrically conducting poly(3,4-ethylenedioxythiophene) films. Macromolecules 2006, 39 (16), 5326-5329.Im, S. G.; Gleason, K. K., Systematic control of the electrical conductivity of poly(3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition. Macromolecules 2007, 40 (18), 6552-6556.Coclite, A. M.; Howden, R. M.; Borrelli, D. C.; Petruczok, C. D.; Yang, R.; Yaguee, J. L.; Ugur, A.; Chen, N.; Lee, S.; Jo, W. J.; Liu, A.; Wang, X.; Gleason, K. K., 25th Anniversary Article: CVD Polymers: A New Paradigm for Surface Modification and Device Fabrication. Adv. Mater. 2013, 25 (38), 5392-5422.Kovacik, P.; Hierro, G. d.; Livernois, W.; Gleason, K. K., Scale-up of oCVD: large-area conductive polymer thin films for next-generation electronics. Materials Horizons 2015, 2 (2), 221-227.Wang, X.; Zhang, X.; Sun, L.; Lee, D.; Lee, S.; Wang, M.; Zhao, J.; Shao-Horn, Y.; Dincă, M.; Palacios, T.; Gleason, K. K., High electrical conductivity and carrier mobility in oCVD PEDOT thin films by engineered crystallization and acid treatment. Science Advances 2018, 4 (9), eaat5780.Borrelli, D. C.; Lee, S.; Gleason, K. K., Optoelectronic properties of polythiophene thin films and organic TFTs fabricated by oxidative chemical vapor deposition. Journal of Materials Chemistry C 2014, 2 (35), 7223-7231. Figure 1

On the Colloidal Stability of PbS Quantum Dots Capped with Methylammonium Lead Iodide Ligands.

Phase-transfer exchange of pristine organic ligands for inorganic ones is essential for the integration of colloidal quantum dots (CQDs) in optoelectronic devices. This method results in a colloidal dispersion (ink) which can be directly deposited by various solution-processable techniques to fabricate conductive films. For PbS CQDs capped with methylammonium lead iodide ligands (MAPbI3), the most commonly employed solvent is butylamine, which enables only a short-term (hours) colloidal stability and thus brings concerns on the possibility of manufacturing CQD devices on a large scale in a reproducible manner. In this work, we studied the stability of alternative inks in two highly polar solvents which impart long-term colloidal stability of CQDs: propylene carbonate (PC) and 2,6-difluoropyridine (DFP). The aging and the loss of the ink’s stability were monitored with optical, structural, and transport measurements. With these solvents, PbS CQDs capped with MAPbI3 ligands retain colloidal stability for more than 20 months, both in dilute and concentrated dispersions. After 17 months of ink storage, transistors with a maximum linear mobility for electrons of 8.5 × 10–3 cm2/V s are fabricated; this value is 17% of the one obtained with fresh solutions. Our results show that both PC- and DFP-based PbS CQD inks offer the needed shelf life to allow for the development of a CQD device technology.

Controllable deposition of organic metal halide perovskite films with wafer-scale uniformity by single source flash evaporation

Conventional solution-processing techniques such as the spin-coating method have been used successfully to reveal excellent properties of organic–inorganic halide perovskites (OHPs) for optoelectronic devices such as solar cell and light-emitting diode, but it is essential to explore other deposition techniques compatible with large-scale production. Single-source flash evaporation technique, in which a single source of materials of interest is rapidly heated to be deposited in a few seconds, is one of the candidate techniques for large-scale thin film deposition of OHPs. In this work, we investigated the reliability and controllability of the single-source flash evaporation technique for methylammonium lead iodide (MAPbI3) perovskite. In-depth statistical analysis was employed to demonstrate that the MAPbI3 films prepared via the flash evaporation have an ultrasmooth surface and uniform thickness throughout the 4-inch wafer scale. We also show that the thickness and grain size of the MAPbI3 film can be controlled by adjusting the amount of the source and number of deposition steps. Finally, the excellent large-area uniformity of the physical properties of the deposited thin films can be transferred to the uniformity in the device performance of MAPbI3 photodetectors prepared by flash evaporation which exhibited the responsivity of 51 mA/W and detectivity of 9.55 × 1010 Jones.

Microfluidic spray dried and spray freeze dried uniform microparticles potentially for intranasal drug delivery and controlled release

Microparticulate formulations of intranasal drug administration, i.e. nasal powder, have aroused widespread interest in academia and industry. However, low drug bioavailability due to poor hydrophilicity and rapid clearance of nasal mucosa remains an important challenge. Herein, with resveratrol as poorly water-soluble model drug, hydroxypropyl-β-cyclodextrin as complexation agent and excipient, chitosan as mucoadhesive enhancer, a series of uniform microparticles were fabricated using a self-designed microfluidic spray granulation platform. The effects of feed solution composition and process technique on particle size, morphology, density and flowability were investigated. The obtained powder presented the significantly enhanced resveratrol dissolubility over 1800 times pure resveratrol and the excellent antioxidant activity with Net AUC value exceeding 60. Chitosan played an important role in adjusting resveratrol release behavior and improving particle mucoadhesive property. This study shows promise for facile and scalable production of high-quality nasal powder using combined complexation method with spray drying and spray freeze drying technique.

Highly Stable Porous Polyimide Sponge as a Separator for Lithium-metal Secondary Batteries.

To inhibit Li-dendrite growth on lithium (Li)-metal electrodes, which causes capacity deterioration and safety issues in Li-ion batteries, we prepared a porous polyimide (PI) sponge using a solution-processable high internal-phase emulsion technique with a water-soluble PI precursor solution; the process is not only simple but also environmentally friendly. The prepared PI sponge was processed into porous PI separators and used for Li-metal electrodes. The physical properties (e.g., thermal stability, liquid electrolyte uptake, and ionic conductivity) of the porous PI separators and their effect on the Li-metal anodes (e.g., self-discharge and open-circuit voltage properties after storage, cycle performance, rate capability, and morphological changes) were investigated. Owing to the thermally stable properties of the PI polymer, the porous PI separators demonstrated no dimensional changes up to 180 °C. In comparison with commercialized polyethylene (PE) separators, the porous PI separators exhibited improved wetting ability for liquid electrolytes; thus, the latter improved not only the physical properties (e.g., improved the electrolyte uptake and ionic conductivity) but also the electrochemical properties of Li-metal electrodes (e.g., maintained stable self-discharge capacity and open-circuit voltage features after storage and improved the cycle performance and rate capability) in comparison with PE separators.

Probing the Dielectric Effects on the Colloidal 2D Perovskite Oxides by Eu3+ Luminescence.

Solution-processing technique of two-dimensional (2D) materials requires the knowledge of the dielectric effects on mutual interaction of each component and the layer structure variation. This research uses the magnetic dipole (MD) transition of intentionally doped Eu3+, which is dependent on the dielectric environments, as an optical probe to study the dielectric effects on the colloidal charge-bearing [Ca1.8Eu0.1Na0.1Nb3O10]- perovskite nanosheets (NSs) in various solvents. Results reveal that the solvent molecules with longer alkyl chain could more easily impact the ligands on the surface of the NSs, leading to a weaker interaction between the ligands and the NSs as well as less distortion of Eu3+ site (Ca2+ site) at the inner layer of the NSs. The large-sized ligands would impede the stacking of the NSs, while H+ would make the H+-modified NSs restack more easily. With the assistance of density functional theory (DFT) simulation, it is found that the ligands or the dielectric solvents could distort or relax the surface covalent polyhedra [NbO6]7- to a larger extent than the inner polyhedra. Small-sized ligands and a large thickness with more atomic layers of the NSs can resist structural variation caused by solvents. The acquired knowledge in this research benefits the understanding of the solution-processing technique for industrial application of 2D materials.

Role of electronically coupled in situ grown silver sulfides (Ag2S) nanoparticles with TiO2 for the efficient photoelectrochemical H2 evolution

A solution-processed in situ grown synthesis method has been developed to grow electronically coupled silver sulfide (Ag2S) nanoparticles (NPs) inside a titanium oxide (TiO2) thin film. Taking the advantage of better charge transport between Ag2S and TiO2, this composite thin film has been utilized for electro-photocatalytic H2 generation. This thin film growth requires three successive steps, including sol-gel derived ion-conducting thin film fabrication containing loosely bound light ion (Li+) followed by ion-exchange (with Li+↔Ag+) and subsequent sulfurization process. This entire solution-processed deposition technique is capable to fabricate cost-effective large area Ag2S–TiO2 thin film containing Ag2S NPs ranging ~10–70 nm. Since, Ag2S has a lower band gap and consider as a promising material for photoelectrochemical H2 generation, therefore Ag2S (NPs)-TiO2 thin film is grown on three different substrates, including fluorine-doped tin oxide (FTO), FTO/TiO2 (sol-gel), and FTO/TiO2 (NPs) to fabricate photoanode for this study. A comparative photo-electrocatalytic measurement of these three different Ag2S(NPs)-TiO2 thin film coated photoanodes showed that sample on FTO/TiO2 (NPs) substrate generate highest photocurrent of density ~50 mA cm−2 at 0.5 V vs NHE in 1 M KOH solution which is three orders higher than pure TiO2 and stable for more than 1.5 h, indicating it's excellent potential application for photoelectrochemical water splitting. The photocurrent generation of this Ag2S–TiO2 thin film is significantly higher than earlier reported Ag2S–TiO2 system, which is originated due to the reduced carrier recombination from electronically coupled Ag2S/TiO2 interface state of such in situ grown Ag2S NPs.

Controllable surface contact resistance in solution-processed thin-film transistors due to dimension modification

The solution-processed thin-film transistors (TFTs) have become the core requirement for the flexible and transparent electronics industry since they are fabricated using cost-effective techniques. However, the fabrication of low-dimensional TFTs by the solution-processing technique is still a challenge due to the surface contact resistance (ρ c). The current study introduces a controllable ρ c involving the effect of dimensions (channel width/length = W/L) on TFTs. The ρ c was measured to be 2.04 × 109 µm VA−1 when the channel length (L) of the TFT was 40 µm. A substantial drop in the surface contact resistance to 1.8 × 108 µm VA−1 was found with L = 5 µm. Thus, a 91% control on ρ c was obtained when L reduced to 40 µm from 5 µm. Such a controllable ρ c was observed with respect to different dimensions of the TFTs. Indeed, an extreme control was obtained in the drain saturation current from 8 µA to 0.8 µA with respect to variation in dimension (W/L) from 15 to 2.5.

Energy efficiency improvement of water pumping system using synchronous reluctance motor fed by perovskite solar cells

Perovskite solar cells (PSCs) are in the forefront of third-generation of photovoltaics and gained a lot of attention as a very promising green technology toward direct solar energy conversion to electricity. PSCs are fabricated following solution-processed techniques at low temperature and they present high power conversion efficiency exceeding 25%, enabling them to be attractive alternative to the silicon-based devices. This research work proposes an efficient and cost-effective photovoltaic (PV) pumping system based on PSCs. For this purpose, lab-scale PSCs were fabricated and their characteristics were determined. In parallel, the geometry of a synchronous reluctance motor (SynRM) driving a 350 m(3)/day water pump was optimized for maximizing the output power, while minimizing the torque ripple simultaneously. In addition, a perovskite solar array feeding the SynRM via an inverter was designed and implemented. The inverter was properly regulated by a control system which optimized the maximum available power of the PSCs solar array and the SynRM characteristics. Finally, laboratory measurements were performed, including a power generator simulating the behavior of the PSCs array feeding the SynRM. The obtained results confirmed the experimental validation of the proposed approach.

Alkyl-Substituted Selenium-Bridged V-Shaped Organic Semiconductors Exhibiting High Hole Mobility and Unusual Aggregation Behavior.

Toward the development of high-performance organic semiconductors (OSCs), carrier mobility is the most important requirement for next-generation OSC-based electronics. The strategy is that OSCs consisting of a highly extended π-electron core exhibit two-dimensional (2D) aggregated structures to offer effective charge transport. However, such OSCs, in general, show poor solubility in common organic solvents, resulting in limited solution processability. This is a critical trade-off between the development of OSCs with simultaneous high carrier mobility and suitable solubility. To address this issue, herein, five-membered ring-fused selenium-bridged V-shaped binaphthalene with decyl substituents (C10-DNS-VW) is developed and synthesized by an efficient method. C10-DNS-VW exhibits significantly high solubility for solution processes. Notably, C10-DNS-VW forms a one-dimensional π-stacked packing motif (1D motif) and a 2D herringbone (HB) packing motif (2D motif), depending on the crystal growth condition. On the other hand, the fabrication of thin films by means of both solution process and vacuum deposition techniques forms only the 2D HB motif. External stress tests such as heating and exposure to solvent vapor indicated that 1D and 2D motifs could be synergistically induced by the total balance of intermolecular interactions. Finally, the single-crystalline films of C10-DNS-VW by solution process exhibit carrier mobility up to 11 cm2 V-1 s-1 with suitable transistor stability under ambient conditions for more than two months, indicating that C10-DNS-VW is one of the most promising candidates for breaking the trade-off in the field of solution-processed technologies.

Spray-deposited PbS colloidal quantum dot solid for near-infrared photodetectors

Colloidal PbS quantum dots (QDs) are promising candidates for various optoelectronic applications based on solution-processed thin-film techniques. In this work, a versatile layer-by-layer (LBL) spray deposition of the QDs is introduced aiming for a future large-scale fabrication process of optoelectronic devices. As compared to spin-coated QD solids, a smaller inter-dot distance and a better-ordered superlattice stacking behavior of the QDs are found in the spray-deposited QD solids as confirmed by grazing-incidence small-angle X-ray scattering (GISAXS). The spectral mapping combined time-resolved photoluminescence analysis indicates a longer charge carrier lifetime and better order of the energy state distribution of the spray-deposited QD solid comparing with the spin-coated one. Thus, photodetectors based on spray deposition of QD solids demonstrate an excellent device performance, with the responsivity achieving 365.1 A/W and the detectivity reaching up to 1.4 × 1012 Jones under an illumination power of 63.5 μW/cm2 at a wavelength of 1250 nm. The spray-deposited device performances indicate a great potential of spray deposition of large sized QDs in large-scale fabrications for optoelectronics using longer wavelengths.

Clout of carbon in polyacrylonitrile/sulfur composite cathode via solution processing technique for lithium-sulfur batteries

A bamboo-like carbon scaffold poly (acrylonitrile) (PAN)–sulfur–composite electrode was prepared by a solution-processing technique. The Kombucha scoby derived carbon (KC) contains ample meso porous nature which is not only for anchoring active sulfur but also preventing the dissolution of polysulfides during the cycling process. A PAN–sulfur–carbon cathode made with 56 wt% sulfur loading delivered an initial discharge capacity of 1444 mAh g−1 at 0.1 C rate with 98% of Coulombic efficiency and high capacity retention of 1044 mAh g− 1 during the 60th cycle due to its peculiar morphology of the prepared composite cathode material obtained by solution processing technique.

Exploring the effect of Ga3+ doping on structural, electronic and optical properties of CH3NH3PbCl3 perovskites: an experimental study

Pure and gallium (Ga)-doped CH3NH3PbCl3 perovskites are systematically prepared via a simple solution-processed technique. The as-synthesized perovskites are then analysed for their structural, optical, morphological, elemental and electronic properties using X-ray powder diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (UV–Vis-DRS), field emission gun-scanning electron microscopy (FEG-SEM), energy-dispersive spectroscopy (EDS) and photoluminescence spectroscopy (PL), respectively. The detailed structural study reveals the formation of the cubic structure with space group pm3m for pristine and Ga-doped CH3NH3PbCl3 perovskites. Microstrain and lattice dislocation density of doped perovskites increase with increasing Ga composition, compared to the undoped CH3NH3PbCl3. Optical properties show the bandgap narrowing up to 10% Ga doping, and any further increase in doping concentrations again increases the bandgap. This blue shift in absorption edge, i.e. bandgap widening behaviour, of highly doped CH3NH3PbCl3 (20% and 30%) perovskites justifies the Burstein–Moss effect in doped semiconductors. The highest Urbach tail energy is observed for 10% Ga-doped compound, which in turn results in the highest electron–phonon interaction. Morphological imaging shows that optimum Ga doping (10%) can improve the characteristic in terms of homogeneity and defect-free surface, compared to pristine perovskite. However, doping beyond the optimal value can further deteriorate the surface morphology by enhancing the surface inhomogeneity and defects. XRD and EDS both confirm the purity of the synthesized materials. Here, doping increases non-radiative recombination which suppresses the radiative recombination of pure CH3NH3PbCl3. These observations unfold the prospect for a new insight of post-transition metals, to fabricate lead-free perovskite materials with advanced optoelectronic properties.

Crystalline Nature of Colloids in Methylammonium Lead Halide Perovskite Precursor Inks Revealed by Cryo-Electron Microscopy.

Metal halide perovskites have generated interest across many fields for the impressive optoelectronic properties achievable in films produced using facile solution-processing techniques. Previous research has revealed the colloidal nature of perovskite precursor inks and established a relationship between the colloid distribution and the film optoelectronic quality. Yet, the identity of colloids remains unknown, hindering our understanding of their role in perovskite crystallization. Here, we investigate precursor inks of the prototypical methylammonium lead triiodide perovskite using cryo-electron microscopy (cryo-EM) and show, for the first time, that the colloids are neither amorphous nor undissolved lead iodide, as previously speculated, but are a crystalline, non-perovskite material. We identify this as a perovskite precursor phase and discuss this as a potential means to understanding the role of chloride in processing. This work establishes cryo-EM as a viable technique to elucidate the nature of colloids in perovskite inks, a vital step toward a fundamental understanding of thin-film crystallization.

Flexible inkjet-printed dual-gate organic thin film transistors and PMOS inverters: Noise margin control by top gate

Low operating voltage dual-gate p-type organic thin film transistors (OTFTs) and PMOS inverters have been fabricated on 125 μm-thick flexible polyethylene naphthalate (PEN) substrates by solution-processed techniques. Inkjet-printed silver electrodes are used for gate, source, and drain contacts while the active organic semiconducting material is printed by a dispenser system over parylene dielectric. Bottom channel, top channel, and dual-gate OTFTs separately display average charge carrier mobilities of 0.03, 0.11, and 0.37 cm 2 V −1 s −1 , respectively. Top-gate is used to independently tune the threshold voltage for bottom-gate OTFT and vice versa. Two PMOS inverters are fabricated with high gain and low supply voltage operation. Ideal symmetric voltage transfer characteristics with high noise margin are achieved by controlling the threshold voltage of the driver OTFT by top-gate bias for low supply voltage, V DD = −2.5 V. • Flexible low voltage dual-gate OTFTs are fabricated by inkjet printing technique. • Dual-gate OTFTs are characterized in bottom-, top-, and dual-gate mode. • V Th of bottom channel OTFT is linearly control by top gate and vice versa. • DLL and ZVLL, PMOS inverters are compared in terms of gain and noise margin. • Ideal PMOS inverters are achieved by controlling V Th of bottom OTFT by top gate.

Carrier selective solution processed molybdenum oxide silicon heterojunctions solar cells with over 12% efficiency

We have fabricated and characterized the performance of carrier selective molybdenum oxide (MoOx)/n-Si heterojunction solar cells (HSCs), with the MoOx prepared by both E-beam evaporation (e-MoOx) and solution process (s-MoOx) techniques. The s-MoOx/Si HSC demonstrates the best power conversion efficiency of 12.5 %, with an open circuit voltage of 555 mV, short circuit current density of 33.3 mA cm−2, and fill factor of 67.4% and it is higher than the efficiencies reported to date for s-MoOx/Si HSC. The efficiency achieved is approaching the 13.3% obtained for the e-MoOx/Si HSC. The slightly lower efficiency of s-MoOx/Si HSC is attributed to its carrier extraction loss arising from a higher contact resistivity at the MoOx/n-Si interface. This is also reflected in the selectivity of the contacts, which measures 10.1 for the s-MoOx contact, relative to 11.5 for the e-MoOx contact. Trap-assisted tunneling at the interface is believed to be the main mechanism that influences the cell performance such as open circuit voltage and fill factor, and results in a high ideality factor of ∼3.

Impact of ultrasonication on carbon nanotube demixing and damage in polymer nanocomposites

AbstractWhile ultrasonication is universally employed for dispersion and distribution of carbon nanotubes (CNTs) in a solvent or polymer solution, the current work focuses on the underlying mechanisms of CNT demixing and CNT damage that can occur during processing. Here, multi‐walled CNTs were dispersed in a polycaprolactone polymer matrix using an established solution processing technique. Electrical, rheological, and mechanical characterization results suggest that once nanocomposite property enhancements reach an optimal level, further sonication leads to a decrease in the corresponding properties due to a combination of CNT damage and demixing mechanisms. Evidence of CNT damage from transmission electron microscopy, poor CNT distribution from optical image analysis and shear‐induced crystallization results, and reagglomeration observed from ultraviolet–visible results, taken together, suggest that mechanisms of demixing and damage of the CNTs coexist for excessive sonication times.

Bright infra-red quantum dot light-emitting diodes through efficient suppressing of electrons

Colloidal quantum dots are promising materials for near infrared light emitting diodes (NIR QLEDs) owing to the widely tunable wavelength of emitted light, high quantum efficiency, and full integration with solution processing techniques. However, the imbalance of charge carriers in NIR QLEDs greatly limits their performance, which in turn narrows the scope of their application. Here, we propose an approach for improving the brightness and stability of NIR PbS QLEDs through balancing the device current by an ultra-thin inorganic aluminum oxide (Al2O3) electron suppressing layer. This modification resulted in a 7.42 W/sr/m2 peak radiance at 1.3 μm wavelength. Moreover, the halide-capped PbS-based NIR QLEDs remained stable under constant current drive for over 144 h.

Towards Lead-Free Perovskite Solar Cells: Hurdles and Prospects

Consistent increase in the power conversion efficiency (PCE) of lead-based halide perovskite solar cells (PSCs) has been observed over the past one decade. PCE is currently slightly above 25 %. Various components within PSC devices are responsible for cell performance, however the most important component is the perovskite layer. Lead (Pb) has taken the lead in the achievement of impressive PCEs of PSCs; Pb has been a major part of the perovskite structure because of its excellent optoelectronic properties. Pb is however known to be toxic, with irreversible detrimental consequences upon human exposure. Plying less expensive routes such as solution process techniques in obtaining direct current electricity through photovoltaics is considered a huge advancement in photovoltaic technology; nevertheless, technological advancement should not be detrimental to health. Here, we provide up-to-date highlights on major progresses made towards achieving high efficiency in the fabrication of lead-free perovskites for photovoltaics application and hence make considerable suggestions on ways forward.

(Invited) Crystallization and Stability of Flexible P-Type Semiconductors and Their Thin Film Transistor Application

In recent years, electronic devices that require superior mechanical flexibility have been extremely sought for potential applications in next-generation displays, sensors, bio-implantable and disposable medical devices. Polymeric materials, of which the flexibility far exceeds that of conventional brittle inorganic counterparts, are widely considered for various flexible devices due to their excellent stretchable, foldable and bendable characteristics.Oxidative chemical vapor deposition (oCVD) is an emerging technique that provides not only a capability to process mechanically flexible polymeric films but also strategies to address current challenges claimed in typical solution-based polymer processing1-2. The oCVD technique allows for greater uniformity, excellent conformality, enhanced performance (e.g., conductivity) and an ability of large area polymer processing that are limited in conventional solution-processed polymers3-4.Crystallization and air stability of organic materials have been issues that have to be mitigated to warrant high performance and sustainable device applications. The oCVD technique, in addition to the advantages described above, could also address the issues of crystallization and air stability to enhance the carrier transport and overall conductivity, and to secure sustainability. We recently found that an in-situ oCVD capability of substrate heating during depositions resulted in an substantial increase in ordered structures in conjugated polymers, leading to enhanced crystallization of the resulting oCVD polymers5. Consequently, the enhancement of the carrier transport and conductivity was obtained due to the increase in favorable conduction paths in the crystalline phase. Further, the vapor phase processing of oCVD may be of crucial relevance to achieve air stability since oCVD does not require any monomer solubility in specific solvents and, therefore, is able to polymerize insoluble materials6 that are not available for conventional solution processing techniques such as spin-coating or dip-casting. The reactivity of insoluble monomers is quite limited due to their rigid backbone structures; however, this limited reactivity is the key to achieve air stability since the reactions between insoluble materials and water (H2O)/oxygen, which are the main origins to cause instabilities of organic materials in air, are significantly prohibited.In this presentation, we report our recent findings of enhanced crystallization and air-stability of oCVD polymers. Particularly, the structure, chemical and electrical properties of two oCVD polymers, poly(3,4-ethylenedioxythiophene) (i.e., PEDOT) and unsubstituted polythiophene, will be compared with focuses on the crystalline state, dope-ability and air-stability. Further, thin film transistors (TFTs) that employ the two oCVD polymers as TFT channel are presented and the device performance of output and transfer characteristics will be compared with focuses on the carrier mobility and on/off switching properties. Acknowledgement: This work was supported by Purdue University and partly by an NSF grant, Award No. ECCS-193108.