Yingzhi Jin

A direct-write printing process is controlled by two parameters to extrude aqueous carbon nanotube (CNT) solutions with liquid crystalline behavior, drawing highly conductive and flexible traces, sensors, and circuits with integrated circuit elements onto a variety of flexible substrates. The high CNT concentration and lack of fillers allow high final conductivity.

Abstract

Manufacturing of printed electronics relies on the deposition of conductive liquid inks, typically onto polymeric or paper substrates. Among available conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at low temperatures, and intrinsic mechanical flexibility. However, the electrical conductivity of printed CNT structures has been limited by CNT quality and concentration, and by the need for nonconductive modifiers to make the ink stable and extrudable. This study introduces a polymer-free, printable aqueous CNT ink, and, via an ambient direct-write printing process, presents the relationships between printing resolution, ink rheology, and ink-substrate interactions. A model is constructed to predict printed feature sizes on impermeable substrates based on Wenzel wetting. Printed lines have conductivity up to 10 000 S m⁻¹. The lines are flexible, with <5% change in DC resistance after 1000 bending cycles, and <3% change in DC resistance with a bending radius down to 1 mm. Demonstrations focus on i) conformality, via printing CNTs onto stickers that can be applied to curved surfaces, ii) interactivity using a CNT-based button printed onto folded paper structure, and iii) capacitive sensing of liquid wicking into the substrate itself. Facile integration of surface mount components on printed circuits is enabled by the intrinsic adhesion of the wet ink.

Nature Energy, Published online: 10 May 2021; doi:10.1038/s41560-021-00820-x

An efficient polythiophene-based organic solar cell (OSC) is demonstrated based on a fluorinated polythiophene donor with deep highest occupied molecular orbital (HOMO) level and appropriate miscibility with the acceptor. With further interfacial modification by a fullerene self-assembled monolayer, a record power conversion efficiency (PCE) of 13.65% for polythiophene-based OSCs is achieved with the device processed from nonhalogenated solvent.

Abstract

Benefiting from low cost and simple synthesis, polythiophene (PT) derivatives are one of the most popular donor materials for organic solar cells (OSCs). However, polythiophene-based OSCs still suffer from inferior power conversion efficiency (PCE) than those based on donor–acceptor (D–A)-type conjugated polymers. Herein, a fluorinated polythiophene derivative, namely P4T2F-HD, is introduced to modulate the miscibility and morphology of the bulk heterojunction (BHJ)-active layer, leading to a significant improvement of the OSC performance. The Flory–Huggins interaction parameters calculated from the surface energy and differential scanning calorimetry results suggest that P4T2F-HD shows moderate miscibility with the popular nonfullerene acceptor Y6-BO (2,2′-((2Z,2′Z)-((12,13-bis(2-butyloctyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2′,3′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile), while poly(3-hexylthiophene) (P3HT) is very miscible with Y6-BO. As a result, the P4T2F-HD case forms desired nanoscale phase separation in the BHJ film while the P3HT case forms a completely mixed BHJ film, as revealed by transmission electron microscopy (TEM) and grazing-incidence wide-angle X-ray scattering (GIWAXS). By optimizing the cathode interface and the morphology of the P4T2F-HD:Y6-BO films processed from nonhalogenated solvents, a new record PCE of 13.65% for polythiophene-based OSCs is demonstrated. This work highlights the importance of controlling D/A interactions for achieving desired morphology and also demonstrates a promising OSC system for potential cost-effective organic photovoltaics.

A novel multi-selenophene-containing polymer acceptor PFY-3Se with a narrow band gap, high electron mobility, and improved intermolecular interaction was developed. In all-polymer solar cells, batch-to-batch insensitive PFY-3Se obtained an impressive power conversion efficiency (PCE) of over 15 % with high reproducibility, which is much better than its analogue, selenophene-free PFY-0Se (13.0 %).

Abstract

All-polymer solar cells (all-PSCs) progressed tremendously due to recent advances in polymerized small molecule acceptors (PSMAs), and their power conversion efficiencies (PCEs) have exceeded 15 %. However, the practical applications of all-PSCs are still restricted by a lack of PSMAs with a broad absorption, high electron mobility, low energy loss, and good batch-to-batch reproducibility. A multi-selenophene-containing PSMA, PFY-3Se, was developed based on a selenophene-fused SMA framework and a selenophene π-spacer. Compared to its thiophene analogue PFY-0Se, PFY-3Se shows a ≈30 nm red-shifted absorption, increased electron mobility, and improved intermolecular interaction. In all-PSCs, PFY-3Se achieved an impressive PCE of 15.1 % with both high short-circuit current density of 23.6 mA cm⁻² and high fill factor of 0.737, and a low energy loss, which are among the best values in all-PSCs reported to date and much better than PFY-0Se (PCE=13.0 %). Notably, PFY-3Se maintains similarly good batch-to-batch properties for realizing reproducible device performance, which is the first reported and also very rare for the PSMAs. Moreover, the PFY-3Se-based all-PSCs show low dependence of PCE on device area (0.045–1.0 cm²) and active layer thickness (110–250 nm), indicating the great potential toward practical applications.

Nonvolatile antioxidant additive–based dibutylhydroxytoluene with polar cyanide and perfluorinated (PF) alkyl chains is introduced in organic solar cells (OSCs). Adding the antioxidant containing the PF alkyl chain leads to superior photo-oxidation stability and high power conversion efficiency, simultaneously. Our results provide a promising method for effective fabrication of OSC modules under severe photo-oxidation conditions.

Apart from power conversion efficiency (PCE) being the most important feature that requires improvement for organic solar cells (OSCs), their long-term stability is another key factor for their successful commercialization. In fact, the lifetime of OSCs is severely limited by photoinduced oxidation, which occurs because of light radiation and the ingress of moisture (H₂O) and oxygen (O₂) within an ambient atmosphere. Herein, dibutylhydroxytoluene (BHT)-based nonvolatile antioxidant additives with polar cyanide (CN) and perfluorinated alkyl chains (designated as BHT–CN and BHT–PF) are developed, demonstrating that the OSCs will have significantly improved long-term stability by using them when exposed to the combined action of all the aforementioned stresses. In particular, the use of BHT–PF in the various given test-bed OSC systems can remarkably enhance the long-term stability, as well as the high initial PCEs similar to the maximized values obtained from the highly optimized OSCs with each well-known suitable solvent additive. The promising results are attributed to the simultaneously enhanced dielectric and radical scavenging properties induced by the BHT–PF embedded in the active-layer matrices. Taking its easy applicability into consideration, the BHT–PF is very useful in fabricating OSC modules that should be stable under severe photo-oxidation conditions.

Indoor photovoltaic is a powerful power source for the future development of Internet of Things (IoT). It provides a constant supply of energy for a large number of sensors and actuators of IoT working in indoor environments. Herein, the indoor photovoltaic cells in recent years are reviewed and the current challenges and opportunities are also provided.

Due to the continuous development and advances in the Internet of Things, wireless sensors, actuators for human−interactive machines, and indoor low-power devices require a continuous supply of energy. Photovoltaic cells working under indoor light are suitable candidates for charging these devices because of their high voltages (up to 5 V), low costs, and environmental friendliness. Herein, the research on organic photovoltaic, dye-sensitized, and perovskite cells in indoor photovoltaic applications with respect to the active layer, modified layer, and preparation process is summarized. The performance enhancement of indoor photovoltaic cells is outlined, including photoactive material selection, bandgap optimization, modification layer function, and device structure design, followed by the prospects and challenges of future developments in indoor photovoltaic cells. With this review an important perspective for the advancement of indoor photovoltaics is offered.

The power conversion efficiencies (PCEs) of single‐junction organic solar cells have now reached over 18%. Recent progress that has been made in understanding the morphology and the device photophysics of high performing polymer:non‐fullerene acceptor blends and some of the major challenges that must be overcome to attain PCEs of over 20% are highlighted.

Abstract

The power conversion efficiencies (PCEs) of single‐junction organic solar cells (OSC) have now reached over 18%. This rapid recent progress can be attributed to the development of new nonfullerene electron acceptors (NFAs) that are paired with suitable high performing polymer electron donors. Substantial improvements in the PCEs and long‐term stability enabled by NFA OSCs have allowed the development and integration of these systems into many niche and novel applications. Here, the recent progress that has been made in understanding the device photophysics of high performing polymer:NFA blends is highlighted. As the bulk heterojunction morphology is intrinsically linked to the device photophysics, this review focuses on studies that have provided noteworthy morphological insights using advanced techniques such as solid‐state NMR and resonant soft X‐ray scattering. Through this, some of the major challenges that must be overcome to attain PCEs of over 20% in NFA OSCs are addressed.

In this review, the recent advances in blade-coated perovskite solar cells and organic solar cells are summarized. The technological details and issues are discussed with an outlook.

Abstract

High-efficiency perovskite solar cells (PSCs) and organic solar cells (OSCs) are promising alternatives for silicon-based solar cells. At present, the key point for commercialization of PSCs and OSCs is realizing large-scale production while maintaining the same high efficiency as small-area ones. In this review, the blade-coating method for preparing large-area films is introduced first and the recent advances of blade-coated OSCs and PSCs are summarized. Then, the effects of blading parameters on the crystal growth and film formation of the light-harvesting materials are discussed. Moreover, the limitations and advantages of making high-quality films via blade-coating are discussed. Finally, some strategies for the up-scaling of solar cells via blade-coating are proposed.

The effects of post treatments of thermal annealing (TA) and solvent annealing (SVA) on morphology evolution and efficiency are systematically investigated. The results show that CS₂ annealing induces better molecular interconnection and lower non-radiative recombination than that of TA treatment, which enables the best voltage and fill factor improvements and gives a record efficiency of 15.39%.

Abstract

Post-treatment is of great importance to form nanoscale phase-separated morphology for all-small-molecule organic solar cells (ASM-OSCs), while the reasons for the difference between thermal annealing (TA) and solvent annealing (SVA) remain unclear. In this work, the influences of TA and SVA (with three common solvents of THF, CS₂, and CF) are systematically investigated based on BT-2F:N3 through characterization of photovoltaic performance, molecular stacking, charge transfer, etc. The results indicate that: i) solvents with good solubility induce stronger molecular interaction than that of TA treatment, and thus endowing molecules with better mobility to migrate for crystallization and phase separation, which leads to better J-aggregation and molecular interconnection. ii) Donor-selectively dissolved CS₂ is better for optimizing the donor domain for its suitable domain size, improved molecular interaction and interconnection, and reduced trap states. iii) CS₂ imposes a small impact on N3 acceptors and thus alleviates the increment of non-radiative recombination. As a result, CS₂ SVA with unique multifunctions enables a PCE of 15.39% with simultaneously improved voltage (0.845 V) and fill factor (75.02%), which is much higher than 14.66% of TA treatment. Moreover, 15.39% efficiency is also the highest value in binary ASM-OSCs.

A flexible and durable self-powered integrated system composed of a silicon film solar cell, inkjet-printed micro-supercapacitor and a temperature sensor, is demonstrated, where aqueous MXene/MXene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) hybrid inks serve as microelectrodes for micro-supercapacitors, current collector for temperature sensor, and metal-free interconnection.

Abstract

Despite intense development of inkjet printing for scalable and customizable fabrication of power sources, one major shortcoming is the lack of eco-friendly aqueous inks free of additives (e.g., toxic solvents, surfactants). Here, an aqueous printable MXene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (MP) hybrid ink is demonstrated that has an adjustable viscosity to directly inkjet-print micro-supercapacitors (MP-MSCs) with excellent performance, seamless integration, and desirable customization, which is crucial for scalable industrialization of self-powered integrated systems. The MP-MSCs deliver an unprecedented volumetric capacitance of 754 F cm⁻³ and a remarkable energy density of 9.4 mWh cm⁻³, superior to previously reported inkjet-printed MSCs. Such outstanding performance is partly attributed to highly conductive PH1000 that prevents restacking of MXene nanosheets, enabling fast electron and ion diffusion throughout the microelectrodes. Moreover, MP-MSCs present exceptional miniaturization and superior modularization featuring high voltage output up to 36 V from 60 serially connected cells and impressive areal voltage of 5.4 V cm⁻² connected in tandem. Further, a printable temperature sensor integrated with the MP-MSC and a flexible solar cell exhibits an exceptional response of 2% and mechanical flexibility without any bias voltage input. Therefore, the MXene inks are expected to create various opportunities for miniaturization and innovative construction of flexible, self-sustaining, energy harvesting–storing–consuming microsystems for printable electronics.

In this work, tandem solar cells using wide bandgap hydrogenated amorphous silicon and narrow bandgap organic bulk heterojunction photovoltaics are explored. By chemically texturing transparent conductive oxide layers, the current matching between two subcells can be optimized to give a power-conversion efficiency of 15% while greatly improving operational stability compared to single junction organic photovoltaics.

Abstract

Monolithically stacked tandem solar cells present opportunities to absorb more of the sun's radiation while reducing the degree of energetic loss through thermalization. In these applications, the bandgap of the tandem's constituent subcells must be carefully adjusted so as to avoid competition for photons. Organic photovoltaics based on nonfullerene acceptors (NFAs) have recently exploded in popularity owing to the ease with which their electrical and optical properties can be tuned through chemistry. Here, highly complementary and efficient 2-terminal tandem solar cells are reported based on a wide bandgap amorphous silicon absorber, and a narrow bandgap NFA bulk-heterojunction with power conversion efficiencies (PCEs) exceeding 15%. Interface engineering of this tandem device allows for high PCEs across a wide range of light intensities both above and below “1 sun.” Furthermore, the addition of an inorganic silicon subcell enhances the operational stability of the tandem by reducing the light-stress experienced by the bulk heterojunction, resolving a long-standing stumbling block in organic photovoltaic research.

This review summarizes some important challenges in bringing organic photovoltaic technology from the laboratory to the market focusing on the: i) Upscaling of indium-tin-oxide (ITO)-based single cells and modules of interconnected cells (single cells versus modules); ii) upscaling using vacuum-based versus vacuum-free processing; iii) upscaling using ITO-based versus ITO-free substrates; and iv) encapsulation and lifetime improvement of upscaled devices.

Abstract

Organic photovoltaic (OPV) cells have recently undergone a rapid increase in power conversion efficiency (PCE) under AM1.5G conditions, as certified by the National Renewable Energy Laboratory (NREL), which have jumped from 11.5% in October 2017 to 18.2% in December 2020. However, the NREL certified PCE of large area OPV modules is still lagging far behind (11.7% in July 2020). Additionally, there has been a rapidly growing interest in the use of OPVs for dim light indoor applications, with reported PCE of some large area (≥1 cm²) devices, under 1000 lux, well above 20%. The transition of OPV from the lab to the market requires the development of effective manufacturing processes that can scale-up laboratory-scale devices into large area devices, without sacrificing performance and simultaneously minimizing associated manufacturing costs. This review article focuses on four important challenges that OPV technology has to face to achieve a reliable lab-to-fab transfer, namely: i) The upscaling of indium-tin-oxide (ITO)-based single cells and the interconnection of single cells into large area modules; ii) the development of alternatives to vacuum processing; iii) the development of alternatives to ITO-based substrates; and iv) strategies for improving the lifetime of large area OPV devices.

In this study, efficient inverted organic photovoltaics using hexagonal boron nitride as an electron blocking layer is fabricated and the device stability, as compared to the reference devices, is improved.

Abstract

In this study, monolayer hexagonal boron nitride (h-BN) grown via chemical vapor deposition (CVD) as an effective electron blocking layer (EBL) for the organic photovoltaics (OPVs) is proposed. Unexpectedly, it is found that h-BN can replace the commonly used hole transport layers (HTLs), i.e., molybdenum trioxide (MoO₃) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in an inverted device architecture. Here, a wet-transfer technique is employed and a single layer of h-BN on top of the PV2000:PC₆₀BM blend is successfully placed. Analysis of the bandgap diagram shows that the monolayer h-BN makes smaller barrier for holes but significantly larger barrier for electrons. This makes the h-BN effective in blocking electrons while creating a possible path for the holes through tunneling to the electrode, due to the low energy barrier at the PV2000/h-BN interface. Using h-BN as an EBL, efficient inverted OPVs are achieved with an average solar-to-power conversion efficiency of 6.13%, which is comparable with that of reference devices based on MoO₃ (7.3%) and PEDOT:PSS (7.6%) as HTLs. Interestingly, the devices with h-BN shows great light-soak stability. The study reveals that the monolayer h-BN grown by CVD could be an effective alternative EBL for the fabrication of efficient, lightweight, and stable OPVs.

Hydrogen-bonding plays a major role in the high value of V _oc for the symmetrical supercapacitor based on PEDOT electrodes. One electrode is soaked in organic acid and the device shows a V _oc value of 0.9 V; moreover, this V _oc value is stable in time. The symmetrical supercapacitor has much higher areal energy and power densities.

Abstract

Poly(3,4-ethylenedioxythiophene) is one of the semiconducting polymers that has attracted attention as electroactive materials for many different applications such as electrochromic devices, light-emitting diodes, biosensors, and supercapacitors. The fundamental understanding of the origin of its energy storage ability will lead to the proper design of such devices. Generally, the charge storage in supercapacitors is due to the formation of an electrical double layer and/or redox reactions. Recently, it is shown that the formation of cation radicals in PEDOT is induced by the hydrogen-bond formation between formic acid and polymer during electrochemical polymerization. The induced cation radicals play a major role in the charge storage ability of PEDOT, as studied in the current work. Furthermore, the presence of hydrogen bonds in PEDOT leads to the stable in time open circuit potential of 900 mV. This new knowledge leads to the designing of a symmetrical supercapacitor based on PEDOT as active material where hydrogen-bonds play a crucial role in the improved performance of the device.

A novel polymer acceptor PY2F-T with difluoro-monobromo end groups on monomer sub-units is synthesized, exhibiting extended absorption and stronger crystallinity compared to its non-fluorinated counterpart (PY-T). When employed in all-polymer solar cells, the PY2F-T based device yields an outstanding efficiency of 15.22% and retains a decent performance of 13.05% when processed under ambient conditions with an eco-friendly solvent (o-xylene, no additive).

Abstract

In this paper, a difluoro-monobromo end group is designed and synthesized, which is then used to construct a novel polymer acceptor (named PY2F-T) yielding high-performance all-polymer solar cells with 15.22% efficiency. The fluorination strategy can increase the intramolecular charge transfer and interchain packing of the previous PY-T based acceptor, and significantly improve photon harvesting and charge mobility of the resulting polymer acceptor. In addition, detailed morphology investigations reveal that the PY2F-T-based blend shows smaller domain spacing and higher domain purity, which significantly suppress charge recombination as supported by time-resolved techniques. These polymer properties enable simultaneously enhanced J _SC and FF of the PY2F-T-based devices, eventually delivering device efficiencies of over 15%, significantly outperforming that of the devices based on the non-fluorinated PY-T polymer (13%). More importantly, the PY2F-T-based active layers can be processed under ambient conditions and still achieve a 14.37% efficiency. They can also be processed using non-halogenated solvent o-xylene (no additive) and yield a decent performance of 13.05%. This work demonstrates the success of the fluorination strategy in the design of high-performance polymer acceptors, which provide guidelines for developing new all-PSCs with better efficiencies and stabilities for commercial applications.

External thermally activated delayed fluorescence (TADF) enhancement by exciplex matrixes is investigated with time-resolved photoluminescence and electroluminescence spectroscopies, which indicates reverse intersystem crossing and charge pre-separation provided by exciplex matrixes for TADF enhancement and quenching suppression. The unitary reverse intersystem crossing efficiency and dramatically reduced singlet and triplet nonradiative rate constants of CDBP:2DBSOSPO support its yellow TADF diodes with a record power efficiency of 114.9 lm W⁻¹.

Abstract

The understanding of the external enhancement effects from host matrixes on thermally activated delayed fluorescence (TADF) emitters is crucial but incomprehensive at present. Herein, a series of phosphine oxide (PO) acceptors mDBSOSPO (m = 2, 3, and 4, corresponding to PO substitution position) and 4,4'-bis(9-carbazolyl)-2,2'-dimethylbiphenyl (CDBP) as donor is used to construct CDBP:mDBSOSPO exciplex matrixes with typical TADF behaviors. After doped with a conventional yellow TADF emitter 4CzTPNBu, the exciplex matrixes dramatically elevate the reverse intersystem crossing (RISC) efficiencies up to 99%, effectively reduce triplet nonradiative rate constant, and tenfold increase singlet radiative/nonradiative ratio beyond 30 in the case of CDBP:2DBSOSPO:3% 4CzTPNBu. The time-resolved photoluminescence and electroluminescence (EL) spectra demonstrate that in contrast to single-molecular hosts, besides the additional RISC channel for TADF facilitation, the exciplexes support the charge preseparation for the step-by-step charge transfer-based energy transfer featuring effective quenching suppression. These external enhancement effects of the exciplex matrixes lead to the state-of-the-art EL performances of their yellow TADF diodes, including the recording power and quantum efficiencies of 114.9 lm W⁻¹ and 30.3% to date.

In an ordinal exciplex-based organic light-emitting diode (OLED), injected holes and electrons immediately recombine to emit light. Meanwhile, an organic long-persistent luminescent (OLPL) system can store holes and electrons for a long time and exhibit luminescence by recombination. By optimizing the light-emitting layer of the exciplex-based OLED, OLPL from an OLED is achieved.

Abstract

Organic long-persistent luminescent systems (OLPLs) exhibiting long-lasting emission after photoexcitation consist of organic electron donors and acceptors, that are widely used in organic light-emitting diodes (OLEDs). Although OLPLs and OLEDs include very similar excitonic processes, long-lasting emission has never been observed in OLEDs. This study confirms the presence of long-persistent luminescence (LPL) under electrical excitation.

In article number 2007177, Feng Liu and co‐workers report the fabrication of ternary organic solar cells, achieving a significant J _SC boost, by virtue of their optimized crystalline feature, with the formation of eutectic crystalline fibrils. The optimal morphology suppresses energetic disorder and nongeminate recombination, and increases charge transfer and transport, yielding a high efficiency of 17.84% with significant current amplification.

Direct photogeneration of free charge carriers enabled by remarkably low exciton binding energies is demonstrated in the state-of-the-art nonfullerene acceptor of Y6 by a joint experimental and theoretical study. This results in efficient charge generation under small interfacial energy offsets in the high-efficiency nonfullerene organic solar cells.

Abstract

Organic solar cells (OSCs) with nonfullerene acceptors (NFAs) exhibit efficient charge generation under small interfacial energy offsets, leading to over 18 % efficiency for the single-junction devices based on the state-of-the-art NFA of Y6. Herein, to reveal the underlying charge generation mechanisms, we have investigated the exciton binding energy (E _b) in Y6 by a joint theoretical and experimental study. The results show that owing to strong charge polarization effects, Y6 has remarkable small E _b of −0.11–0.15 eV, which is even lower than perovskites in many cases. Moreover, it is peculiar that the photoluminescence is enhanced with temperature, and the energy barrier for separating excitons into charges is evidently lower than the thermal energy according to the temperature dependence of photoluminescence, manifesting direct photogeneration of charge carriers enabled by weak E _b in Y6. Thus, charge generation in NFA-based OSCs shows little dependence on interfacial driving forces.