Blending Self-Assembled Monolayers for Enhanced Band Alignment and Improved Morphology in p-i-n Perovskite Photodetectors.

Perovskite photodetectors, devices that convert light to electricity, require good extraction and low noise levels to maximize the signal-to-noise ratio. Self-assembling monolayers (SAMs) have been shown to be effective hole transport materials thanks to their atomic layer thickness, transparency, and energetic alignment with the valence band of the perovskite. While efforts are being made to reduce noise levels via the active layer, little has been done to reduce noise via SAM interfacial engineering. Herein, we report hybrid perovskite photodetectors with high detectivity by blending two different SAMs (2-PACz and Me-4PACz). We find that with a 1:1 2-PACz:Me-4PACz ratio (by weight), the devices achieved a low noise of 1 × 10-13 A Hz-1/2, a high responsivity of 0.41 A W-1 at 710 nm, and a specific detectivity of 6.4 × 1011 Jones at 710 nm at -0.5 V, outperforming its two counterparts. In addition to the improved noise levels in these devices, impedance spectroscopy revealed that higher recombination lifetimes of 0.85 µs were achieved for the 1:1 2-PACz:Me-4PACz-based photodetectors, confirming their low defect density.

Octupole moment driven free charge generation in partially chlorinated subphthalocyanine for planar heterojunction organic photodetectors.

In this study, high-performance organic photodetectors are presented which utilize a pristine chlorinated subphthalocyanine photoactive layer. Optical and optoelectronic analyses indicate that the device photocurrent is primarily generated through direct charge generation within the chlorinated subphthalocyanine layer, rather than exciton separation at layer interfaces. Molecular modelling suggests that this direct charge generation is facilitated by chlorinated subphthalocyanine high octupole moment (-80 DÅ2), which generates a 200 meV shift in molecular energetics. Increasing the thickness of chlorinated subphthalocyanine leads to faster response time, correlated with a decrease in trap density. Notably, photodetectors with a 50 nm thick chlorinated subphthalocyanine photoactive layer exhibit detectivities approaching 1013 Jones, with a dark current below 10-7 A cm-2 up to -5 V. Based on these findings, we conclude that high octupole moment molecular semiconductors are promising materials for high-performance organic photodetectors employing single-component photoactive layer.

Enhancing Circularly Polarized Electroluminescence through Energy Transfer within a Chiral Polymer Host.

Organic light-emitting diodes (OLEDs) that are able to emit high levels of circularly polarized (CP) light hold significant promise in numerous future technologies. Such devices require chiral emissive materials to enable CP electroluminescence. However, the vast majority of current OLED emitter classes, including the state-of-the-art triplet-harvesting Thermally Activated Delayed Fluorescence (TADF) materials, produce very low levels of CP electroluminescence. Here we showcase a host-guest strategy that allows for energy transfer between a chiral polymer host and a representative chiral TADF emitter. Such a mechanism results in large amplification of the circular polarization of the emitter. As such, this study presents a promising avenue to further boost the performance of CP-OLED devices, enabling their further development and eventual commercialization. This article is protected by copyright. All rights reserved.

Pathways to Upscaling Highly Efficient Organic Solar Cells Using Green Solvents: A Study on Device Photophysics in the Transition from Lab-to-Fab.

As the rise of nonfullerene acceptors (NFA) has allowed lab-scale organic solar cells (OSC) to reach 20% efficiency, translating these devices into roll-to-roll compatible fabrication still poses many challenges for researchers. Among these are the use of green solvent solubility for large-scale manufacture, roll-to-roll compatible fabrication, and, not least, information on charge carrier dynamics in each upscaling step, to further understand the gap in performance. In this work, the reproducibility of champion devices using slot-die coating with 14% power conversion efficiency (PCE) is demonstrated, under the condition that the optimal thickness is maintained. It is further shown that for the donor:acceptor (D:A) blend PM6:Y12, the processing solvent has a more significant impact on charge carrier dynamics compared to the deposition technique. It is found that the devices processed with o-xylene feature a 40% decrease in the bimolecular recombination coefficient compared to those processed with CB, as well as a 70% increase in effective mobility. Finally, it is highlighted that blade-coating yields devices with similar carrier dynamics to slot-die coating, making it the optimal choice for lab-scale optimization with no significant loss in translation toward up-scale.

Dark Current in Broadband Perovskite-Organic Heterojunction Photodetectors Controlled by Interfacial Energy Band Offset.

Lead halide perovskite and organic semiconductors are promising classes of materials for photodetector (PD) applications. State-of-the-art perovskite PDs have performance metrics exceeding silicon PDs in the visible. While organic semiconductors offer bandgap tunability due to their chemical design with detection extended into the near-infrared (NIR), perovskites are limited to the visible band and the first fraction of the NIR spectrum. In this work, perovskite-organic heterojunction (POH) PDs with absorption up to 950 nm are designed by the dual contribution of perovskite and the donor:acceptor bulk-heterojunction (BHJ), without any intermediate layer. The effect of the energetics of the donor materials is systematically studied on the dark current (Jd) of the device by using the PBDB-T polymer family. Combining the experimental results with drift-diffusion simulations, it is shown that Jd in POH devices is limited by thermal generation via deep trap states in the BHJ. Thus, the best performance is obtained for the PM7-based POH, which delivers an ultra-low noise current of 2 × 10-14 A Hz-1/2 and high specific detectivity of 4.7 × 1012 Jones in the NIR. Last, the application of the PM7-based POH devices as NIR pulse oximeter with high-accuracy heartbeat monitoring at long-distance of 2 meters is demonstrated.

A Conjugated Carboranyl Main Chain Polymer with Aggregation-Induced Emission in the Near-Infrared.

Materials exhibiting aggregation-induced emission (AIE) are both highly emissive in the solid state and prompt a strongly red-shifted emission and should therefore pose as good candidates toward emerging near-infrared (NIR) applications of organic semiconductors (OSCs). Despite this, very few AIE materials have been reported with significant emissivity past 700 nm. In this work, we elucidate the potential of ortho-carborane as an AIE-active component in the design of NIR-emitting OSCs. By incorporating ortho-carborane in the backbone of a conjugated polymer, a remarkable solid-state photoluminescence quantum yield of 13.4% is achieved, with a photoluminescence maximum of 734 nm. In contrast, the corresponding para and meta isomers exhibited aggregation-caused quenching. The materials are demonstrated for electronic applications through the fabrication of nondoped polymer light-emitting diodes. Devices employing the ortho isomer achieved nearly pure NIR emission, with 86% of emission at wavelengths longer than 700 nm and an electroluminescence maximum at 761 nm, producing a significant light output of 1.37 W sr-1 m-2.

Conjugated Polymer Heteroatom Engineering Enables High Detectivity Symmetric Ambipolar Phototransistors.

Solution-processed high-performing ambipolar organic phototransistors (OPTs) can enable low-cost integrated circuits. Here, a heteroatom engineering approach to modify the electron affinity of a low band gap diketopyrrolopyrole (DPP) co-polymer, resulting in well-balanced charge transport, a more preferential edge-on orientation and higher crystallinity, is demonstrated. Changing the comonomer heteroatom from sulfur (benzothiadiazole (BT)) to oxygen (benzooxadiazole (BO)) leads to an increased electron affinity and introduces higher ambipolarity. Organic thin film transistors fabricated from the novel PDPP-BO exhibit charge carrier mobility of 0.6 and 0.3 cm2 Vs⁻1 for electrons and holes, respectively. Due to the high sensitivity of the PDPP-based material and the balanced transport in PDPP-BO, its application as an NIR detector in an OPT architecture is presented. By maintaining a high on/off ratio (9 × 104), ambipolar OPTs are shown with photoresponsivity of 69 and 99 A W⁻1 and specific detectivity of 8 × 107 for the p-type operation and 4 × 109 Jones for the n-type regime. The high symmetric NIR-ambipolar OPTs are also evaluated as ambipolar photo-inverters, and show a 46% gain enhancement under illumination.

Rational Design of New Conjugated Polymers with Main Chain Chirality for Efficient Optoelectronic Devices: Carbo[6]Helicene and Indacenodithiophene Copolymers as Model Compounds.

The unique properties of conjugated polymers (CPs) in various optoelectronic applications are mainly attributed to their different self-assembly processes and superstructures. Various methods are utilized to tune and control CP structure and properties with less attention paid to the use of chirality. CPs with main chain chirality are rare and their microscopic and macroscopic properties are still unknown. In this work, the first experimental results are provided along these lines by synthesizing a series of racemic and enantiopure CPs containing statistical and alternating carbo[6]helicene and indacenodithiophene moieties and evaluating their microscopic (optical, energy levels) and macroscopic properties (hole mobilities, photovoltaic performance). It is demonstrated that a small statistical insertion of either the racemic or enantiopure helicene into the polymer backbone finely tunes the microscopic and macroscopic properties as a function of the statistical content. The microscopic properties of the enantiopure versus the racemic polymers with the same helicene loading remain similar. On the contrary, the macroscopic properties, and more interestingly those between the two enantiomeric forms, are altered as a function of the statistical content. Once incorporated into a solar cell device, these chiral CPs display better performance in their enantiopure versus racemic forms.

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%.

Fast Near-Infrared Photodetectors Based on Nontoxic and Solution-Processable AgBiS2.

Solution-processable near-infrared (NIR) photodetectors are urgently needed for a wide range of next-generation electronics, including sensors, optical communications and bioimaging. However, it is rare to find photodetectors with >300 kHz cut-off frequencies, especially in the NIR region, and many of the emerging inorganic materials explored are comprised of toxic elements, such as lead. Herein, solution-processed AgBiS2 photodetectors with high cut-off frequencies under both white light (>1 MHz) and NIR (approaching 500 kHz) illumination are developed. These high cut-off frequencies are due to the short transit distances of charge-carriers in the ultrathin photoactive layer of AgBiS2 photodetectors, which arise from the strong light absorption of this material, such that film thicknesses well below 120 nm are sufficient to absorb >65% of NIR to visible light. It is also revealed that ion migration plays a critical role in the photo-response speed of these devices, and its detrimental effects can be mitigated by finely tuning the thickness of the photoactive layer, which is important for achieving low dark current densities as well. These outstanding characteristics enable the realization of air-stable, real-time heartbeat sensors based on NIR AgBiS2 photodetectors, which strongly motivates their future integration in high-throughput systems.

Operando dynamics of trapped carriers in perovskite solar cells observed via infrared optical activation spectroscopy.

Conventional spectroscopies are not sufficiently selective to comprehensively understand the behaviour of trapped carriers in perovskite solar cells, particularly under their working conditions. Here we use infrared optical activation spectroscopy (i.e., pump-push-photocurrent), to observe the properties and real-time dynamics of trapped carriers within operando perovskite solar cells. We compare behaviour differences of trapped holes in pristine and surface-passivated FA0.99Cs0.01PbI3 devices using a combination of quasi-steady-state and nanosecond time-resolved pump-push-photocurrent, as well as kinetic and drift-diffusion models. We find a two-step trap-filling process: the rapid filling (~10 ns) of low-density traps in the bulk of perovskite, followed by the slower filling (~100 ns) of high-density traps at the perovskite/hole transport material interface. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (~50 times), improving the device performance substantially. Moreover, the activation energy (~280 meV) of the dominant hole traps remains similar with and without surface passivation.

Photo-Chemical Stimulation of Neurons with Organic Semiconductors.

Recent advances in light-responsive materials enabled the development of devices that can wirelessly activate tissue with light. Here it is shown that solution-processed organic heterojunctions can stimulate the activity of primary neurons at low intensities of light via photochemical reactions. The p-type semiconducting polymer PDCBT and the n-type semiconducting small molecule ITIC (a non-fullerene acceptor) are coated on glass supports, forming a p-n junction with high photosensitivity. Patch clamp measurements show that low-intensity white light is converted into a cue that triggers action potentials in primary cortical neurons. The study shows that neat organic semiconducting p-n bilayers can exchange photogenerated charges with oxygen and other chemical compounds in cell culture conditions. Through several controlled experimental conditions, photo-capacitive, photo-thermal, and direct hydrogen peroxide effects on neural function are excluded, with photochemical delivery being the possible mechanism. The profound advantages of low-intensity photo-chemical intervention with neuron electrophysiology pave the way for developing wireless light-based therapy based on emerging organic semiconductors.

A polymer library enables the rapid identification of a highly scalable and efficient donor material for organic solar cells.

The dramatic improvement of the PCE (power conversion efficiency) of organic photovoltaic devices in the past few years has been driven by the development of new polymer donor materials and non-fullerene acceptors (NFAs). In the design of such materials synthetic scalability is often not considered, and hence complicated synthetic protocols are typical for high-performing materials. Here we report an approach to readily introduce a variety of solubilizing groups into a benzo[c][1,2,5]thiadiazole acceptor comonomer. This allowed for the ready preparation of a library of eleven donor polymers of varying side chains and comonomers, which facilitated a rapid screening of properties and photovoltaic device performance. Donor FO6-T emerged as the optimal material, exhibiting good solubility in chlorinated and non-chlorinated solvents and achieving 15.4% PCE with L8BO as the acceptor (15.2% with Y6) and good device stability. FO6-T was readily prepared on the gram scale, and synthetic complexity (SC) analysis highlighted FO6-T as an attractive donor polymer for potential large scale applications.

Biorenewable Solvents for High-Performance Organic Solar Cells.

With the advent of nonfullerene acceptors (NFAs), organic photovoltaic (OPV) devices are now achieving high enough power conversion efficiencies (PCEs) for commercialization. However, these high performances rely on active layers processed from petroleum-based and toxic solvents, which are undesirable for mass manufacturing. Here, we demonstrate the use of biorenewable 2-methyltetrahydrofuran (2MeTHF) and cyclopentyl methyl ether (CPME) solvents to process donor: NFA-based OPVs with no additional additives in the active layer. Furthermore, to reduce the overall carbon footprint of the manufacturing cycle of the OPVs, we use polymeric donors that require a few synthetic steps for their synthesis, namely, PTQ10 and FO6-T, which are blended with the Y-series NFA Y12. High performance was achieved using 2MeTHF as the processing solvent, reaching PCEs of 14.5% and 11.4% for PTQ10:Y12 and FO6-T:Y12 blends, respectively. This work demonstrates the potential of using biorenewable solvents without additives for the processing of OPV active layers, opening the door to large-scale and green manufacturing of organic solar cells.

Enhanced sub-1 eV detection in organic photodetectors through tuning polymer energetics and microstructure.

One of the key challenges facing organic photodiodes (OPDs) is increasing the detection into the infrared region. Organic semiconductor polymers provide a platform for tuning the bandgap and optoelectronic response to go beyond the traditional 1000-nanometer benchmark. In this work, we present a near-infrared (NIR) polymer with absorption up to 1500 nanometers. The polymer-based OPD delivers a high specific detectivity D* of 1.03 × 1010 Jones (-2 volts) at 1200 nanometers and a dark current Jd of just 2.3 × 10-6 ampere per square centimeter at -2 volts. We demonstrate a strong improvement of all OPD metrics in the NIR region compared to previously reported NIR OPD due to the enhanced crystallinity and optimized energy alignment, which leads to reduced charge recombination. The high D* value in the 1100-to-1300-nanometer region is particularly promising for biosensing applications. We demonstrate the OPD as a pulse oximeter under NIR illumination, delivering heart rate and blood oxygen saturation readings in real time without signal amplification.

Enhanced Organic Electrochemical Transistor Performance of Donor-Acceptor Conjugated Polymers Modified with Hybrid Glycol/Ionic Side Chains by Postpolymerization Modification.

Emergent bioelectronic technologies are underpinned by the organic electrochemical transistor (OECT), which employs an electrolyte medium to modulate the conductivity of its organic semiconductor channel. Here we utilize postpolymerization modification (PPM) on a conjugated polymer backbone to directly introduce glycolated or anionic side chains via fluoride displacement. The resulting polymers demonstrated increased volumetric capacitances, with subdued swelling, compared to their parent polymer in p-type enhancement mode OECTs. This increase in capacitance was attributed to their modified side chain configurations enabling cationic charge compensation for thin film electrochemical oxidation, as deduced from electrochemical quartz crystal microbalance measurements. An overall improvement in OECT performance was recorded for the hybrid glycol/ionic polymer compared to the parent, owing to its low swelling and bimodal crystalline orientation as imaged by grazing-incidence wide-angle X-ray scattering, enabling its high charge mobility at 1.02 cm2·V-1·s-1. Compromised device performance was recorded for the fully glycolated derivative compared to the parent, which was linked to its limited face-on stacking, which hindered OECT charge mobility at 0.26 cm2·V-1·s-1, despite its high capacitance. These results highlight the effectiveness of anionic side chain attachment by PPM as a means of increasing the volumetric capacitance of p-type conjugated polymers for OECTs, while retaining solid-state macromolecular properties that facilitate hole transport.

Wide-Humidity Range Applicable, Anti-Freezing, and Healable Zwitterionic Hydrogels for Ion-Leakage-Free Iontronic Sensors.

Hydrogels have entered the spotlight for applications in soft electronics. It is essential and challenging to obtain hydrogels that can function properly under varying environmental circumstances, that is, 30-90% relative humidity (RH) and -20 to 40 °C due to their intrinsic nature to lose and absorb water upon variations in humidity and temperature. In this work, a green solvent, solketal, is introduced into poly 3-dimethyl-2-(2-methylprop-2-enoyloxy)ethyl azaniumyl propane-1-sulfonate (poly(DMAPS)) zwitterionic hydrogels. Compared to glycerol, solketal endows hydrogels with greater possibility for further modification as well as improved water content and mechanical performance consistency over 30-90% RH. Encouragingly, the optimized hydrogel demonstrates its unique merits as a dielectric layer in iontronic sensors, featuring non-leaky ions, high sensitivity (1100 kPa-1 ), wide humidity, and temperature range applicability. A wide-humidity range healable and stretchable electrode is attained by combining the hydrogel substrate with Ag paste. A full-device healable and highly-sensitive sensor is developed. This study is a pioneering work that tackles the broad humidity range applicability issue of hydrogels, and demonstrates the ion-leakage-free ionic skins with zwitterionic dielectrics. The outcomes of the study will considerably promote advancements in the fields of hydrogel electronics and iontronic sensors.

Microstructure-driven annihilation effects and dispersive excited state dynamics in solid-state films of a model sensitizer for photon energy up-conversion applications.

Bimolecular processes involving exciton spin-state interactions gain attention for their deployment as wavelength-shifting tools. Particularly triplet-triplet annihilation induced photon energy up-conversion (TTA-UC) holds promise to enhance the performance of solar cell and photodetection technologies. Despite the progress noted, a correlation between the solid-state microstructure of photoactuating TTA-UC organic composites and their photophysical properties is missing. This lack of knowledge impedes the effective integration of functional TTA-UC interlayers as ancillary components in operating devices. We here investigate a solution-processed model green-to-blue TTA-UC binary composite. Solid-state films of a 9,10 diphenyl anthracene (DPA) blue-emitting activator blended with a (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) green-absorbing sensitizer are prepared with a range of compositions and examined by a set of complementary characterization techniques. Grazing incidence X-ray diffractometry (GIXRD) measurements identify three PtOEP composition regions wherein the DPA:PtOEP composite microstructure varies due to changes in the packing motifs of the DPA and PtOEP phases. In Region 1 (≤2 wt%) DPA is semicrystalline and PtOEP is amorphous, in Region 2 (between 2 and 10 wt%) both DPA and PtOEP phases are amorphous, and in Region 3 (≥10 wt%) DPA remains amorphous and PtOEP is semicrystalline. GIXRD further reveals the metastable DPA-ß polymorph species as the dominant DPA phase in Region 1. Composition dependent UV-vis and FT-IR measurements identify physical PtOEP dimers, irrespective of the structural order in the PtOEP phase. Time-gated photoluminescence (PL) spectroscopy and scanning electron microscopy imaging confirm the presence of PtOEP aggregates, even after dispersing DPA:PtOEP in amorphous poly(styrene). When arrested in Regions 1 and 2, DPA:PtOEP exhibits delayed PtOEP fluorescence at 580 nm that follows a power-law decay on the ns time scale. The origin of PtOEP delayed fluorescence is unraveled by temperature- and fluence-dependent PL experiments. Triplet PtOEP excitations undergo dispersive diffusion and enable TTA reactions that activate the first singlet-excited (S1) PtOEP state. The effect is reproduced when PtOEP is mixed with a poly(fluorene-2-octyl) (PFO) derivative. Transient absorption measurements on PFO:PtOEP films find that selective PtOEP photoexcitation activates the S1 of PFO within ∼100 fs through an up-converted 3(d, d*) PtII-centered state.

Correction: Best practices in the measurement of circularly polarised photodetectors.

[This corrects the article DOI: 10.1039/D2TC01224C.].

Photonic nanostructures mimicking floral epidermis for perovskite solar cells.

Here, we report photonic nanostructures replicated from the adaxial epidermis of flower petals onto light-polymerized coatings using low-cost nanoimprint lithography at ambient temperature. These multifunctional nanocoatings are applied to confer enhanced light trapping, water repellence, and UV light and environmental moisture protection features in perovskite solar cells. The former feature helps attain a maximum power conversion efficiency of 24.61% (21.01% for the reference cell) without any additional device optimization. Added to these merits, the nanocoatings also enable stable operation under AM 1.5G and UV light continuous illumination or in real-world conditions. Our engineering approach provides a simple way to produce multifunctional nanocoatings optimized by nature's wisdom.