Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers.

A series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in organic electrochemical transistors (OECTs) affords state-of-the-art n-type performance with p(g7NC10N) recording an OECT electron mobility of 1.20 × 10-2 cm2 V-1 s-1 and a µC* figure of merit of 1.83 F cm-1 V-1 s-1. In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for p(g7NC4N), with a maximum electrical conductivity of 7.67 S cm-1 and a power factor of 10.4 µW m-1 K-2. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam organic mixed ionic-electronic conductors (OMIECs) highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.

Infrared Organic Photodetectors Employing Ultralow Bandgap Polymer and Non-Fullerene Acceptors for Biometric Monitoring.

Recent efforts in the field of organic photodetectors (OPD) have been focused on extending broadband detection into the near-infrared (NIR) region. Here, two blends of an ultralow bandgap push-pull polymer TQ-T combined with state-of-the-art non-fullerene acceptors, IEICO-4F and Y6, are compared to obtain OPDs for sensing in the NIR beyond 1100 nm, which is the cut off for benchmark Si photodiodes. It is observed that the TQ-T:IEICO-4F device has a superior IR responsivity (0.03 AW-1 at 1200 nm and -2 V bias) and can detect infrared light up to 1800 nm, while the TQ-T:Y6 blend shows a lower responsivity of 0.01 AW-1 . Device physics analyses are tied with spectroscopic and morphological studies to link the superior performance of TQ-T:IEICO-4F OPD to its faster charge separation as well as more favorable donor-acceptor domains mixing. In the polymer blend with Y6, the formation of large agglomerates that exceed the exciton diffusion length, which leads to high charge recombination, is observed. An application of these devices as biometric sensors for real-time heart rate monitoring via photoplethysmography, utilizing infrared light, is demonstrated.

Correlating Charge-Transfer State Lifetimes with Material Energetics in Polymer:Non-Fullerene Acceptor Organic Solar Cells.

Minimizing the energy offset between the lowest exciton and charge-transfer (CT) states is a widely employed strategy to suppress the energy loss (Eg/q - VOC) in polymer:non-fullerene acceptor (NFA) organic solar cells (OSCs). In this work, transient absorption spectroscopy is employed to determine CT state lifetimes in a series of low energy loss polymer:NFA blends. The CT state lifetime is observed to show an inverse energy gap law dependence and decreases as the energy loss is reduced. This behavior is assigned to increased mixing/hybridization between these CT states and shorter-lived singlet excitons of the lower gap component as the energy offset ΔECT-S1 is reduced. This study highlights how achieving longer exciton and CT state lifetimes has the potential for further enhancement of OSC efficiencies.

Acene Ring Size Optimization in Fused Lactam Polymers Enabling High n-Type Organic Thermoelectric Performance.

Three n-type fused lactam semiconducting polymers were synthesized for thermoelectric and transistor applications via a cheap, highly atom-efficient, and nontoxic transition-metal free aldol polycondensation. Energy level analysis of the three polymers demonstrated that reducing the central acene core size from two anthracenes (A-A), to mixed naphthalene-anthracene (A-N), and two naphthalene cores (N-N) resulted in progressively larger electron affinities, thereby suggesting an increasingly more favorable and efficient solution doping process when employing 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) as the dopant. Meanwhile, organic field effect transistor (OFET) mobility data showed the N-N and A-N polymers to feature the highest charge carrier mobilities, further highlighting the benefits of aryl core contraction to the electronic performance of the materials. Ultimately, the combination of these two factors resulted in N-N, A-N, and A-A to display power factors (PFs) of 3.2 µW m-1 K-2, 1.6 µW m-1 K-2, and 0.3 µW m-1 K-2, respectively, when doped with N-DMBI, whereby the PFs recorded for N-N and A-N are among the highest reported in the literature for n-type polymers. Importantly, the results reported in this study highlight that modulating the size of the central acene ring is a highly effective molecular design strategy to optimize the thermoelectric performance of conjugated polymers, thus also providing new insights into the molecular design guidelines for the next generation of high-performance n-type materials for thermoelectric applications.

Modification of Indacenodithiophene-Based Polymers and Its Impact on Charge Carrier Mobility in Organic Thin-Film Transistors.

The polymer indacenodithiophene-co-benzothiadiazole (IDT-BT) has been thoroughly studied for its use in p-type organic thin-film transistors over the course of the past decade. While a variety of modifications have been made to its structure, few analogues have been able to match or surpass the hole mobility that can be obtained by IDT-BT. Here, we discuss the rationale behind the chemical modifications that have been utilized and suggest design principles toward high-mobility indacenodithiophene-based polymers. It is clear that planarizing intramolecular interactions, which exist between the peripheral thiophene of the IDT unit and the benzothiadiazole, are imperative for achieving high hole mobilities in this relatively amorphous polymer. Moreover, despite the less ordered backbones of the extended fused-ring cores that have recently been utilized (TIF-BT and TBIDT-BT), high mobilities were still attained in these polymers owing to additional interchain charge transfer. Thus, maintaining the beneficial thiophene-benzothiadiazole intramolecular interactions, while further extending the IDT core to promote interchain charge transfer, is a logical strategy toward high-mobility p-type polymers.

Low-Temperature Cross-Linking Benzocyclobutene Based Polymer Dielectric for Organic Thin Film Transistors on Plastic Substrates.

The synthesis of a new benzocyclobutene based polymer, PSBBB, designed as a dielectric material for use in organic thin film transistors was reported. Compared to conventional benzocyclobutene-based materials, the introduction of a butoxide substituent at the 7-position of the benzocyclobutene pendant unit on the polymer allowed PSBBB to be cross-linked at temperatures of 120 °C, thus rendering it compatible with the processing requirements of flexible plastic substrates. The cross-linking behavior of PSBBB was investigated by Fourier transform infrared spectroscopy and differential scanning calorimetry, demonstrating cross-linking of the polymer after curing at 120 °C. Bottom-gate bottom-contact organic thin film transistors were fabricated using PSBBB as dielectric, affording a performance comparable to that of other dielectric polymeric materials.

Moisture-Resilient Perovskite Solar Cells for Enhanced Stability.

With the rapid rise in device performance of perovskite solar cells (PSCs), overcoming instabilities under outdoor operating conditions has become the most crucial obstacle toward their commercialization. Among stressors such as light, heat, voltage bias, and moisture, the latter is arguably the most critical, as it can decompose metal-halide perovskite (MHP) photoactive absorbers instantly through its hygroscopic components (organic cations and metal halides). In addition, most charge transport layers (CTLs) commonly employed in PSCs also degrade in the presence of water. Furthermore, photovoltaic module fabrication encompasses several steps, such as laser processing, subcell interconnection, and encapsulation, during which the device layers are exposed to the ambient atmosphere. Therefore, as a first step toward long-term stable perovskite photovoltaics, it is vital to engineer device materials toward maximizing moisture resilience, which can be accomplished by passivating the bulk of the MHP film, introducing passivation interlayers at the top contact, exploiting hydrophobic CTLs, and encapsulating finished devices with hydrophobic barrier layers, without jeopardizing device performance. Here, existing strategies for enhancing the performance stability of PSCs are reviewed and pathways toward moisture-resilient commercial perovskite devices are formulated.

Semitransparent Organic Photovoltaics Utilizing Intrinsic Charge Generation in Non-Fullerene Acceptors.

In organic semiconductors, a donor/acceptor heterojunction is typically required for efficient dissociation of excitons. Using transient absorption spectroscopy to study the dynamics of excited states in non-fullerene acceptors (NFAs), it is shown that NFAs can generate charges without a donor/acceptor interface. This is due to the fact that dielectric solvation provides a driving force sufficient to dissociate the excited state and form the charge-transfer (CT) state. The CT state is further dissociated into free charges at interfaces between polycrystalline regions in neat NFAs. For IEICO-4F, incorporating just 9 wt% donor polymer PTB7-Th in neat films greatly boosts charge generation, enhancing efficient exciton separation into free charges. This property is utilized to fabricate donor-dilute organic photovoltaics (OPV) delivering a power conversion efficiency of 8.3% in the case of opaque devices with a metal top-electrode and an active layer average visible transmittance (AVT) of 75%. It is shown that the intrinsic charge generation in low-bandgap NFAs contributes to the overall photocurrent generation. IEICO-4F-based OPVs with limited PTB7-Th content have high thermal resilience demonstrating little drop in performance over 700 h. PTB7-Th:IEICO-4F semitransparent OPVs are leveraged to fabricate an 8-series connected semitransparent module, demonstrating light-utilization efficiency of 2.2% alongside an AVT of 63%.

Sublimed C60 for efficient and repeatable perovskite-based solar cells.

Thermally evaporated C60 is a near-ubiquitous electron transport layer in state-of-the-art p-i-n perovskite-based solar cells. As perovskite photovoltaic technologies are moving toward industrialization, batch-to-batch reproducibility of device performances becomes crucial. Here, we show that commercial as-received (99.75% pure) C60 source materials may coalesce during repeated thermal evaporation processes, jeopardizing such reproducibility. We find that the coalescence is due to oxygen present in the initial source powder and leads to the formation of deep states within the perovskite bandgap, resulting in a systematic decrease in solar cell performance. However, further purification (through sublimation) of the C60 to 99.95% before evaporation is found to hinder coalescence, with the associated solar cell performances being fully reproducible after repeated processing. We verify the universality of this behavior on perovskite/silicon tandem solar cells by demonstrating their open-circuit voltages and fill factors to remain at 1950 mV and 81% respectively, over eight repeated processes using the same sublimed C60 source material. Notably, one of these cells achieved a certified power conversion efficiency of 30.9%. These findings provide insights crucial for the advancement of perovskite photovoltaic technologies towards scaled production with high process yield.

Vacuum-Deposited Donors for Low-Voltage-Loss Nonfullerene Organic Solar Cells.

The advent of nonfullerene acceptors (NFAs) enabled records of organic photovoltaics (OPVs) exceeding 19% power conversion efficiency in the laboratory. However, high-efficiency NFAs have so far only been realized in solution-processed blends. Due to its proven track record in upscaled industrial production, vacuum thermal evaporation (VTE) is of prime interest for real-world OPV commercialization. Here, we combine the benchmark solution-processed NFA Y6 with three different evaporated donors in a bilayer (planar heterojunction) architecture. We find that voltage losses decrease by hundreds of millivolts when VTE donors are paired with the NFA instead of the fullerene C60, the current standard acceptor in VTE OPVs. By showing that evaporated small-molecule donors behave much like solution-processed donor polymers in terms of voltage loss when combined with NFAs, we highlight the immense potential for evaporable NFAs and the urgent need to direct synthesis efforts toward making smaller, evaporable compounds.

Efficient and reliable encapsulation for perovskite/silicon tandem solar modules.

Perovskite/silicon tandem solar cells have a tremendous potential to boost renewable electricity production thanks to their very high performance combined with promising cost structure. However, for actual field deployment, any solar cell technology needs to be assembled into modules, where the associated processes involve several challenges that may affect both the performance and stability of the devices. For instance, due to its hygroscopic nature, ethylene vinyl acetate (EVA) is incompatible with perovskite-based photovoltaics. To circumvent this issue, we investigate here two alternative encapsulant polymers for the packaging of perovskite/silicon tandems into minimodules: a thermoplastic polyurethane (TPU) and a thermoplastic polyolefin (TPO) elastomer. To gauge their impact on tandem-module performance and stability, we performed two internationally established accelerated module stability tests (IEC 61215): damp heat exposure and thermal cycling. Finally, to better understand the thermomechanical properties of the two encapsulants and gain insight into their relation to the thermal cycling of encapsulated tandems, we performed a dynamic mechanical thermal analysis. Our understanding of the packaging process of the tandem module provides useful insights for the development of commercially viable perovskite photovoltaics.

n-Type organic semiconducting polymers: stability limitations, design considerations and applications.

This review outlines the design strategies which aim to develop high performing n-type materials in the fields of organic thin film transistors (OTFT), organic electrochemical transistors (OECT) and organic thermoelectrics (OTE). Figures of merit for each application and the limitations in obtaining these are set out, and the challenges with achieving consistent and comparable measurements are addressed. We present a thorough discussion of the limitations of n-type materials, particularly their ambient operational instability, and suggest synthetic methods to overcome these. This instability originates from the oxidation of the negative polaron of the organic semiconductor (OSC) by water and oxygen, the potentials of which commonly fall within the electrochemical window of n-type OSCs, and consequently require a LUMO level deeper than ∼-4 eV for a material with ambient stability. Recent high performing n-type materials are detailed for each application and their design principles are discussed to explain how synthetic modifications can enhance performance. This can be achieved through a number of strategies, including utilising an electron deficient acceptor-acceptor backbone repeat unit motif, introducing electron-withdrawing groups or heteroatoms, rigidification and planarisation of the polymer backbone and through increasing the conjugation length. By studying the fundamental synthetic design principles which have been employed to date, this review highlights a path to the development of promising polymers for n-type OSC applications in the future.

Incidence and Secondary Transmission of SARS-CoV-2 Infections in Schools.

BACKGROUND: In an effort to mitigate the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), North Carolina closed prekindergarten through grade 12 public schools to in-person instruction on March 14, 2020. On July 15, 2020, North Carolina's governor announced schools could open via remote learning or a hybrid model that combined in-person and remote instruction. In August 2020, 56 of 115 North Carolina school districts joined The ABC Science Collaborative (ABCs) to implement public health measures to prevent SARS-CoV-2 transmission and share lessons learned. We describe secondary transmission of SARS-CoV-2 within participating school districts during the first 9 weeks of in-person instruction in the 2020-2021 academic year. METHODS: From August 15, 2020 to October 23, 2020, 11 of 56 school districts participating in ABCs were open for in-person instruction for all 9 weeks of the first quarter and agreed to track incidence and secondary transmission of SARS-CoV-2. Local health department staff adjudicated secondary transmission. Superintendents met weekly with ABCs faculty to share lessons learned and develop prevention methods. RESULTS: Over 9 weeks, 11 participating school districts had >90 000 students and staff attend school in person. Among these students and staff, 773 community-acquired SARS-CoV-2 infections were documented by molecular testing. Through contact tracing, health department staff determined an additional 32 infections were acquired within schools. No instances of child-to-adult transmission of SARS-CoV-2 were reported within schools. CONCLUSIONS: In the first 9 weeks of in-person instruction in North Carolina schools, we found extremely limited within-school secondary transmission of SARS-CoV-2, as determined by contact tracing.

Nonfullerene-Based Organic Photodetectors for Ultrahigh Sensitivity Visible Light Detection.

It is well established that for organic photodetectors (OPDs) to compete with their inorganic counterparts, low dark currents at reverse bias must be achieved. Here, two rhodanine-terminated nonfullerene acceptors O-FBR and O-IDTBR are shown to deliver low dark currents at -2 V of 0.17 and 0.84 nA cm-2, respectively, when combined with the synthetically scalable polymer PTQ10 in OPD. These low dark currents contribute to the excellent sensitivity to low light of the detectors, reaching values of 0.57 µW cm-2 for PTQ10:O-FBR-based OPD and 2.12 µW cm-2 for PTQ10:O-IDTBR-based OPD. In both cases, this sensitivity exceeds that of a commercially available silicon photodiode. The responsivity of the PTQ10:O-FBR-based OPD of 0.34 AW-1 under a reverse bias of -2 V also exceeds that of a silicon photodiode. Meanwhile, the responsivity of the PTQ10:O-IDTBR of 0.03 AW-1 is limited by the energetic offset of the blend. The OPDs deliver high specific detectivities of 9.6 × 1012 Jones and 3.3 × 1011 Jones for O-FBR- and O-IDTBR-based blends, respectively. Both active layers are blade-coated in air, making them suitable for high-throughput methods. Finally, all three of the materials can be synthesized at low cost and on a large scale, making these blends good candidates for commercial OPD applications.

Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T-2DT/Non-Fullerene Organic Solar Cells.

The temperature dependent aggregation behavior of PffBT4T polymers used in organic solar cells plays a critical role in the formation of a favorable morphology in fullerene-based devices. However, there has been little investigation into the impact of donor/acceptor ratio on morphology tuning, especially for non-fullerene acceptors (NFAs). Herein, the influence of composition on morphology is reported for blends of PffBT4T-2DT with two NFAs, O-IDTBR and O-IDFBR. The monotectic phase behavior inferred from differential scanning calorimetry provides qualitative insight into the interplay between solid-liquid and liquid-liquid demixing. Transient absorption spectroscopy suggests that geminate recombination dominates charge decay and that the decay rate is insensitive to composition, corroborated by negligible changes in open-circuit voltage. Exciton lifetimes are also insensitive to composition, which is attributed to the signal being dominated by acceptor excitons which are formed and decay in domains of similar size and purity irrespective of composition. A hierarchical morphology is observed, where the composition dependence of size scales and scattering intensity from resonant soft X-ray scattering (R-SoXS) is dominated by variations in volume fractions of polymer/polymer rich domains. Results suggest an optimal morphology where polymer crystallite size and connectivity are balanced, ensuring a high probability of hole extraction via such domains.

Recycling Perovskite Solar Cells To Avoid Lead Waste.

Methylammonium lead iodide (MAPbI3) perovskite based solar cells have recently emerged as a serious competitor for large scale and low-cost photovoltaic technologies. However, since these solar cells contain toxic lead, a sustainable procedure for handling the cells after their operational lifetime is required to prevent exposure of the environment to lead and to comply with international electronic waste disposal regulations. Herein, we report a procedure to remove every layer of the solar cells separately, which gives the possibility to selectively isolate the different materials. Besides isolating the toxic lead iodide in high yield, we show that the PbI2 can be reused for the preparation of new solar cells with comparable performance and in this way avoid lead waste. Furthermore, we show that the most expensive part of the solar cell, the conductive glass (FTO), can be reused several times without any reduction in the performance of the devices. With our simple recycling procedure, we address both the risk of contamination and the waste disposal of perovskite based solar cells while further reducing the cost of the system. This brings perovskite solar cells one step closer to their introduction into commercial systems.

Who chooses vasectomy in Rwanda? Survey data from couples who chose vasectomy, 2010-2012.

BACKGROUND: Vasectomy is safe and highly effective; however, it remains an underused method of family planning (FP) in Africa. In view of this, three Rwandan physicians were trained in no-scalpel vasectomy with thermal cautery and fascial interposition on the prostatic end as vasectomy trainers in 2010, and this initiative has resulted in over 2900 vasectomy clients from February 2010 to December 2012. STUDY DESIGN: This cross-sectional descriptive study describes vasectomy clients (n=316) and their wives (n=300) from 15 randomly selected hospitals in Rwanda. RESULTS: The vasectomy clients were mainly over age 40, had young children (age <3) and were married and cohabiting. Limited financial resources, satisfaction with existing family size and avoiding side effects from hormonal methods (wives') were key motivators for vasectomy uptake. High rates of previous FP use and high degree of interspousal communication are known correlates of higher FP use. CONCLUSIONS: Future and current Rwandan FP programs and other interested parties will benefit from understanding which couples elect vasectomy, their motivations for doing so and their service utilization experiences. Better integration of vasectomy counseling and postvasectomy procedures will benefit the program. IMPLICATIONS: Until this project, vasectomy projects in sub-Saharan Africa were viewed as unrealistic. This study confirms factors influencing vasectomy uptake identified in earlier research, but does so within a robust sample of vasectomy users and their wives and provides a strong understanding of who likely vasectomy users are in this context. Promotion of vasectomy services should be considered as an essential element of a healthy contraceptive method mix.