Charge Generation Process Research Articles

Controllable Exciton Diffusion Length and Ultrafast Charge Generation in Ternary Organic Solar Cells

Comprehensive SummaryCharge generation, a critical process in the operation of organic solar cell (OSC), requires thorough investigation in an ultrafast perspective. This work demonstrates that the utilization of alloy model for the non‐fullerene acceptor (NFA) component can regulate the crystallization properties of active layer films, which in turn affects exciton diffusion and hole transfer (HT), ultimately influencing the charge generation process. By incorporating BTP‐eC7 as a third component, without expanding absorption range or changing molecular energy levels but regulating the ultrafast exciton diffusion and HT processes, the power conversion efficiency (PCE) of the optimized PM6:BTP‐eC9:BTP‐eC7 based ternary OSC is improved from 17.30% to 17.83%, primarily due to the enhancement of short‐circuit current density (JSC). Additionally, the introduction of BTP‐eC7 also reduces the trap state density in the photoactive layer which helps to reduce the loss of JSC. This study introduces a novel approach for employing ternary alloy models by incorporating dual acceptors with similar structures, and elucidates the underlying mechanism of charge generation and JSC in ternary OSCs.

Comprehensive Insight into the Structure Contribution of A2‐A1‐D‐A1‐A2 Acceptor to Performance of P3HT Solar Cells

AbstractPoly(3‐hexylthiophene) (P3HT) acting as one of the most popular and low‐cost polymers is quite suitable for commercialization of organic solar cells but suffers from low power conversion efficiency (PCE) because of the limited matching non‐fullerene acceptors (NFAs). One important reason that restricts the enhancement but remains unresolved is the undisclosed contributions of the subtle structure modification to the obvious performance change. In combination with the previous reports and this work, herein A2‐A1‐D‐A1‐A2 type NFAs with single, dual, and triple modifications based on parent BTA3 is designed, including benzyl‐substitution on A2 group (namely Bn modification), fluorine‐substitution on A1 group (namely F modification), and thieno[3,2‐b]thiophene‐substitution on D group (namely TT modification). It is finally found that the binary devices of P3HT with these NFAs underwent unexpected variations in the aspect of molecular optoelectronic property, blend morphological feature and charge generation process. The triple modification (including Bn, F, and TT) gives full play to their unique advantages and consequently increases PCE by 60%. To the knowledge, the obtained optimal PCE is one of the highest values for A2‐A1‐D‐A1‐A2 type NFAs. This study provides clearer insights into the rational substitutions on the A2‐A1‐D‐A1‐A2 acceptors matched with P3HT for high‐performance organic photovoltaics.

Unfolding the facts of structural parameters of needle punched nonwoven in influencing its triboelectrification phenomenon: a detailed experimental analysis

Triboelectrification has been a well-known phenomenon for a long time. Although there are numerous debates regarding the actual mechanism involved in this, there are few interesting works uncovering its unique signatures. In this work, triboelectrification of needle punched nonwoven structures has been explored from a few important perspectives. The effect of linear density of constituent fibre on the triboelectric charge generation ability of needle punched nonwoven structures has been investigated. Further, the influence of antistatic agent on the triboelectric charge generation has been enlightened. An attempt has also been made to correlate the structural parameters of nonwoven with its triboelectrification performance. This research work not only reveals the triboelectrification phenomenon of this fibrous structure but also it is a food for novel thoughts on triboelectric charge generation process.

Ultrafast electron dynamics in excited states of conjugated thiophene-fluorene organic polymer (pF8T2) thin films.

The electronic states of poly(9,9-dioctylfluorenyl-alt-bithiophene) pF8T2 on H/Si(100) substrates, prototypical for organic photovoltaics, were investigated by ultrafast photoelectron spectroscopy and by time-resolved fluorescence studies. Occupied and unoccupied electronic states were analysed by ultraviolet photoelectron spectroscopy (UPS), static and dynamic femtosecond two-photon photoemission (2PPE), and time-correlated single photon counting (TCSPC). Time-resolved measurements allow assessment of population lifetimes of intermediate states. The combination of time-resolved photoelectron spectroscopy and fluorescence excitation allows following the electronic dynamics in excited states from the femtosecond to the nanosecond time scale. For this prototypical material the electron kinetic energy resolved lifetimes range from about a few tens of femtoseconds up to hundreds of picoseconds. After annealing these types of organic thin films the efficiency of organic solar cells usually increases. We show that annealing does not influence the initial ultrafast charge generation processes, but the long-lived states. However, the nanosecond scale fluorescence lifetimes measured by TCSPC are prolonged after annealing, which therefore is identified as the cause of a greater exciton diffusion range and thus is beneficial for charge carrier extraction.

High-Performance Wearable Organic Photodetectors by Molecular Design and Green Solvent Processing for Pulse Oximetry and Photoplethysmography.

White-light detection from the visible to the near-infrared region is central to many applications such as high-speed cameras, autonomous vehicles, and wearable electronics. While organic photodetectors (OPDs) are being developed for such applications, several challenges must be overcome to produce scalable high-detectivity OPDs. This includes issues associated with low responsivity, narrow absorption range, and environmentally friendly device fabrication. Here, an OPD system processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes is reported. The newly designed OPD is employed in wearable devices to monitor heart rate and blood oxygen saturation using a flexible OPD-based finger pulse oximeter. In achieving this, a framework for a detailed understanding of the structure-processing-property relationship in these OPDs is also developed. The bulk heterojunction (BHJ) thin films processed from 2-MeTHF are characterized at different length scales with advanced techniques. The BHJ morphology exhibits optimal intermixing and phase separation of donor and acceptor moieties, which facilitates the charge generation and collection process. Benefitting from high charge carrier mobilities and a low shunt leakage current, the newly developed OPD exhibits a specific detectivity of above 1012 Jones over 400-900nm, which is higher than those of reference devices processed from chlorobenzene andortho-xylene.

Gas-solid two-phase flow triboelectric generator for powder triboelectrification energy collection and road early warning

With the rapid development of modern industrial processes, electrostatic safety issues caused by powder in production and daily life have attracted people's attention. Since powder is small in size and easily agglomerates, it is difficult to observe or design devices to monitor the electrostatic process generated by powder. Herein, a continuous gas-solid two-phase flow triboelectric nanogenerator (GS-TENG) was developed to harvest triboelectricity from the collision process between the powder-air flow and the solid surface. Compared with the signals under non-continuous airflow, the short-circuit current and output voltage signals generated under continuous airflow impact exhibit an unidirectional direct current peak characteristic. Additionally, in order to analyze the relationship between motion of powder airflow and the triboelectric signals, the two-phase flow more comprehensively, a high-speed camera was used to record motion during the charge generation process. Results showed that 3 mm nylon sphere generated normal alternating current signals upon contact and separation from PTFE, while nylon power with the diameter of about 30 µm produced direct current triboelectric signals owing to the effect of point discharge. At a wind speed of 32 m/s, 5 cm distance, 90° impact angle, the short-circuit current and output voltage can reach the maximum values of 20 µA and 130 V, respectively, which can serve as a power source sufficient to power 63 commercial LEDs. Based on the gas-solid two-phase flow triboelectric nanogenerator, a self-powered road warning lights was designed, indicating application prospects in dusty weather.

Charge-Transfer State Dissociation Efficiency Can Limit Free Charge Generation in Low-Offset Organic Solar Cells.

We investigate the charge-generation processes limiting the performance of low-offset organic bulk-heterojunction solar cells by studying a series of newly synthesized PBDB-T-derivative donor polymers whose ionisation energy (IE) is tuned via functional group (difluorination or cyanation) and backbone (thiophene or selenophene bridge) modifications. When blended with the acceptor Y6, the series present heterojunction donor-acceptor IE offsets (ΔEIE) ranging from 0.22 to 0.59 eV. As expected, small ΔEIE decrease nonradiative voltage losses but severely suppresses photocurrent generation. We explore the origin of this reduced charge-generation efficiency at low ΔEIE through a combination of opto-electronic and spectroscopic measurements and molecular and device-level modeling. We find that, in addition to the expected decrease in local exciton dissociation efficiency, reducing ΔEIE also strongly reduces the charge transfer (CT) state dissociation efficiency, demonstrating that poor CT-state dissociation can limit the performance of low-offset heterojunction solar cells.

The Role of Device Mobility on the Charge Generation Process in Polymerized Small-Molecule Acceptor Based Organic Solar Cells

We investigate the role of backbone tuning of contemporary polymerized small-molecule acceptors (PSMAs), namely PJ2, PJTVT, and PJTET, on the charge generation process when blended with JD40 polymers to explain the difference between their device short circuit current, JSC. We utilize transient absorption (TA) spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements to demonstrate the face-on π–π and lamellar stackings impact on the diffusion of excitons in pristine films. Comparing the charge generation process to data obtained from space-charge-limited current (SCLC) measurements, we show that the generation of free charge carriers, although limited by geminate recombination of charge transfer (CT) pairs, is improved with a higher charge-carrier device mobility. A higher mobility is found to also drive charge generation limited by the same recombination more efficiently than blend systems where the charges recombine nongeminately. We show that an increment of ∼0.1 eV corresponds to an ∼5% increase in the device EQE for the same mobility. Based on the data compiled from the literature and the study herein, a mobility of at least 10–3 cm2 V–1 s–1 is postulated for an EQE of at least 90%. Finally, we observe an apparent logarithmic relationship between the charge-carrier mobility and the generation of photocurrent, in lieu of a linear relationship introduced by the Braun–Onsager model. The same trend remains for surveyed literature data of contemporary Y-series acceptor-based OPV devices. This work highlights that to improve the JSC of current state-of-art OPV devices, the mobility of a device can be improved through material designs and device engineering for a higher charge generation efficiency.

Toward More Efficient Organic Solar Cells: A Detailed Study of Loss Pathway and Its Impact on Overall Device Performance in Low‐Offset Organic Solar Cells

AbstractLow‐offset organic solar cell systems have attracted great interest since nonfullerene acceptors came into the picture. While numerous studies have focused on the charge generation process in these low‐offset systems, only a few studies have focused on the details of each loss channel in the charge generation process and their impact on the overall device performance. Here, several nonfullerene acceptors are blended with the same polymer donor to form a series of low‐offset organic solar cell systems where significant variation in device performance is observed. Through detailed analyses of loss pathways, it is found that: i) the donor:acceptor interfaces of PM6:Y6 and PM6:TPT10 are close to the optimum energetic condition, ii) energetics at the donor:acceptor interface are the most important factor to the overall device performance, iii) exciton dissociation yield can be field‐dependent owing to the sufficiently small energetic offset at the donor:acceptor interface, and iv) the change in substituents in the terminal group of Y‐series acceptors in this work mainly affects energetics at the donor:acceptor interface instead of the interface density in the active layer. In general, this work presents a path toward more efficient organic solar cells.

The Dynamics of Delocalized Excitations in Organic Solar Cells with Nonfullerene Acceptors

Recently, the performance of organic solar cells has been markedly improved benefiting from the development of nonfullerene acceptors (NFAs) with acceptor-donor-acceptor structures. Arising from the intermolecular electronic interactions between the electron donating and accepting units, intramoiety and interfacial delocalized excitations make a substantial contribution to the photocurrent generation. In this Perspective, we discuss recent studies on the excited-state dynamics responsible for the working mechanism in NFA-based organic solar cells and emphasize the dynamics of delocalized excitations in charge generation and recombination processes. The intramoiety delocalized excitations in NFAs enable charge separation without forming interfacial charge-transfer excitons first, allowing efficient photocharge generation in planar heterojunctions with reduced interfacial energy loss. We suggest a few research directions in elucidating the performance-limited processes toward the further optimization of NFA-based devices.

Origin of electric field-dependent charge generation in organic photovoltaics with planar and bulk heterojunctions

The electric field dependence in the charge generation process of organic solar cells in planar heterojunction and bulk heterojunction structures is related to the energetics and molecular orientation at the donor/acceptor interfaces.

19.10% Efficiency and 80.5% Fill Factor Layer-by-Layer Organic Solar Cells Realized by 4-Bis(2-Thienyl)Pyrrole-2,5-Dione Based Polymer Additives for Inducing Vertical Segregation Morphology.

The morphology plays a key role in determining the charge generation and collection process, thus impacting the performances of organic solar cells (OSCs). The limited selection pool of additives to optimize the morphology of OSCs, especially for the emerging layer-by-layer (LbL) OSCs, impeding the improvements of photovoltaic performances. Herein, a new method of using conjugated polymers as the additives to optimize the morphology for improving the photovoltaic performances of LbL-OSCs is reported. Four polymers of PH, PS, PF, and PCl are developed with different side chains. These polymers exhibit poor performances as donor materials and additives in the BHJ devices, due to the unsuitable energy level alignment and unfavorable molecular interactions. By contrast, they can be served as efficient additives to optimize the PM6 fibril matrix for facilitating the penetration of BTP-eC9 and forming an intertwined D/A bicontinuous network with a vertical segregation. Such morphology is optimized by side chain engineering, which enables the progressive improvement of the charge separation and collection. As a result, adding a small amount of PCl as the additive, the optimized morphology contributes to a champion PCE of 19.10% with a high FF of 80.5%.

Singlet-Triplet Energy Gap as a Critical Molecular Descriptor for Predicting Organic Photovoltaic Efficiency.

In contrast to the inorganic and perovskite solar cells, organic photovoltaics (OPV) depend on a series of charge generation and recombination processes, which complicates molecular design to improve the power conversion efficiencies (PCEs). Herein, we first propose the singlet-triplet energy gap (ΔEST ) as a critical molecular descriptor for predicting the PCE considering that minimizing ΔEST is beneficial to simultaneously reduce voltage loss and triplet recombination. Remarkably, the results from data-driven machine learning verify that the prediction accuracy of the ΔEST (Pearson's correlation coefficient r=0.72) is apparently superior to that of two commonly used molecular descriptors in OPV, i.e., the optical gap (r=0.65) and the driving force (r=0.53). Moreover, an impressive prediction accuracy of r=0.81 is achieved just by combining the three descriptors. This work paves the way toward rapid and precise screening of efficient OPV materials.

Singlet‐Triplet Energy Gap as a Critical Molecular Descriptor for Predicting Organic Photovoltaic Efficiency

AbstractIn contrast to the inorganic and perovskite solar cells, organic photovoltaics (OPV) depend on a series of charge generation and recombination processes, which complicates molecular design to improve the power conversion efficiencies (PCEs). Herein, we first propose the singlet‐triplet energy gap (ΔEST) as a critical molecular descriptor for predicting the PCE considering that minimizing ΔEST is beneficial to simultaneously reduce voltage loss and triplet recombination. Remarkably, the results from data‐driven machine learning verify that the prediction accuracy of the ΔEST (Pearson's correlation coefficient r=0.72) is apparently superior to that of two commonly used molecular descriptors in OPV, i.e., the optical gap (r=0.65) and the driving force (r=0.53). Moreover, an impressive prediction accuracy of r=0.81 is achieved just by combining the three descriptors. This work paves the way toward rapid and precise screening of efficient OPV materials.

Improving the performance of PM6 donor polymer by random ternary copolymerization of BDD and DTBT segments

Developing high-performance wide bandgap polymer donors to match the rapidly growing non-fullerene small molecule acceptors (NF-SMAs) is of great importance to further improve the photovoltaic performances of organic solar cells (OSCs). PM6 and D18 are two leading wide bandgap donor polymers, but their performances are highly sensitive to the molecular weights, which have distinct impact on the morphology and thereby the charge generation and recombination processes. In this work, we reported the random ternary copolymerization of PM6 and D18 segments to develop random terpolymers with finely tuned photoelectronic properties and optimized blend morphology for high-performance OSCs. The resulting random terpolymers (PMD-5, PMD-10, PMD-15 and PMD-20) exhibited finely tuned absorptions, energy levels and blend morphology. The optimal morphology of PMD-15:L8-BO contributed to the fast exciton diffusion and dissociation, efficient charge extraction with suppressed recombination. Thus, the power conversion efficiency was improved from 17.22% of the control PM6 devices and 17.47% of the control D18 devices to 18.12% of PMD-15 based devices, which is the top performance of random terpolymers. Moreover, the PMD-15 exhibited much higher batch-to-batch reproducibility at a broad molecular weight range than that of PM6 and D18, showing promising for practical applications.

Validation of a Textile Material’s Electrostatic Characterization Device for Different Parameters and Their Effect on the Electrostatic Charge Generation

This research aims to validate an electrostatics characterization device to better understand the process of static charge generation in textile materials and to see how different factors affect it. This electrostatic device offers a variety of settings for controlling sample electrostatic activation and has a sample size range of up to one square meter. It can move in both horizontal and vertical directions in a controlled manner, providing a variety of possibilities for testing the effect of various movement features on electrostatic charge formation. Not only the textile polymer but also the motion characterizations influence the generation of electrostatic charges in textiles. The influence of frequency, pressure, dwell time between moves, test duration, effect of different sample sizes, and amplitude of movement on electrostatic charge generation was studied in greater detail. Two different parameters of the electrostatic waveform (peak voltage and peak-to-peak voltage) were investigated. The generation of electrostatic charges is proportional to the peak voltage and peak-to-peak voltage of the electrostatic waveform. Overall electrostatic charge generation increases with increasing frequency, stepping height, applied pressure at the same frequency, and sample size, but decreases with increasing dwell time between moves at the same frequency. The charge also increases with test duration until a saturation point is reached.

A New Polymer Donor Enables Binary All-Polymer Organic Photovoltaic Cells with 18% Efficiency and Excellent Mechanical Robustness.

The development of polymerized small-molecule acceptors has boosted the power conversion efficiencies (PCEs) of all-polymer organic photovoltaic (OPV) cells to 17%. However, the polymer donors suitable for all-polymer OPV cells are still lacking, restricting the further improvement of their PCEs. Herein, a new polymer donor named PQM-Cl is designed and its photovoltaic performance is explored. The negative electrostatic potential and low average local ionization energy distribution of the PQM-Cl surface enable efficient charge generation and transfer process. When blending with a well-used polymer acceptor, PY-IT, the PQM-Cl-based devices deliver an impressive PCE of 18.0% with a superior fill factor of 80.7%, both of which are the highest values for all-polymer OPV cells. The relevant measurements demonstrate that PQM-Cl-based films possess excellent mechanical and flexible properties. As such, PQM-Cl-based flexible photovoltaic cells are fabricated and an excellent PCE of 16.5% with high mechanical stability is displayed. These results demonstrate that PQM-Cl is a potential candidate for all-polymer OPV cells and provide insights into the design of polymer donors for high-efficient all-polymer OPV cells.

Plasmonic Photocatalysis: Activating Chemical Bonds through Light and Plasmon

AbstractThe exceptionally large charge‐carrier density in photoexcited plasmonic nanoparticles (NPs) can be used for the making and breaking of high‐energy chemical bonds, which forms the basis of plasmonic photocatalysis. This review showcases the roadmap of important events and major bottlenecks in plasmonic photocatalysis, along with highlighting a few probable solutions for achieving the desired targets. The review starts with a discussion on various excitation and relaxation pathways, followed by the section on initial use of plasmons in enhancing the photocatalytic properties of semiconductor materials. Next, the sole use of plasmonic NPs in driving useful and industrially relevant chemical transformations is discussed. This is followed by a critical assessment of various challenges and opportunities in the area, along with a discussion on emerging experiments capable of overcoming these challenges. Decades of research have provided a clear understanding on charge generation and decay processes in plasmonic NPs. However, achieving an efficient separation and utilization of charge carriers is still a roadblock in realizing the full potential of plasmonic NPs in catalysis. In short, doing chemistry with plasmons is attractive; but it is high time to develop strategies that can quantitatively utilize the charge carriers for driving chemical transformations in a selective and efficient way.

Role of Exciton Diffusion and Lifetime in Organic Solar Cells with a Low Energy Offset.

Despite general agreement that the generation of free charges in organic solar cells is driven by an energetic offset, power conversion efficiencies have been improved using low-offset blends. In this work, we explore the interconnected roles that exciton diffusion and lifetime play in the charge generation process under various energetic offsets. A detailed balance approach is used to develop an analytic framework for exciton dissociation and free-charge generation accounting for exciton diffusion to and dissociation at the donor–acceptor interface. For low-offset systems, we find the exciton lifetime to be a pivotal component in the charge generation process, as it influences both the exciton and CT state dissociation. These findings suggest that any novel low-offset material combination must have long diffusion lengths with long exciton lifetimes to achieve optimum charge generation yields.

Application of A-DA'D-A non-fullerene acceptor with Benzotriazole Core in poly(3-hexylthiophene)-based organic solar cells

The advent of A-DA'D-A non-fullerene acceptors (NFAs) containing electron-deficient A′ unit such as benzothiadiazole (BT), benzotriazole (BTA) and quinoxaline (Qx) has driven the significant development of organic solar cells (OSCs) using donor-acceptor (D-A) type polymer as electron donor. However, how to use A-DA'D-A type NFAs to accelerate the development of poly(3-hexylthiophene) (P3HT) device is still not determined, because only limited BT-based A-DA'D-A molecules have been used in P3HT-based devices. Herein, a new A-DA'D-A molecule named TPBTA-RCN with BTA as the A′ unit and 2-(1,1-dicyanomethylene) rhodanine (RCN) as the A unit was designed. On the basis of our previous report, we concluded here that the terminal group plays a larger role than the central fused A′ unit in regulating charge generation and recombination processes of P3HT:A-DA'D-A-type NFA systems. In summary, we not only demonstrate the broad prospects of A-DA'D-A NFAs to pair with P3HT, but also illustrates further synthetic directions of this kinds of materials, which will greatly facilitate the progress of P3HT-based OSCs.