Yingzhi Jin

A noncovalently fused-ring electron acceptor (NFREA) NoCA-5 containing two terminal side-chains (T-SCs) is reported. Introduction of T-SCs can improve molecular rigidity, which is confirmed by the smaller Stokes shift value and lower reorganization free energy. Combining noncovalent conformational locks and T-SC engineering, the NoCA-5-based device exhibits a record power conversion efficiency (PCE) of 14.82 % and a certified PCE of 14.5 %.

Abstract

Side-chain engineering is an effective strategy to regulate the solubility and packing behavior of organic materials. Recently, a unique strategy, so-called terminal side-chain (T-SC) engineering, has attracted much attention in the field of organic solar cells (OSCs), but there is a lack of deep understanding of the mechanism. Herein, a new noncovalently fused-ring electron acceptor (NFREA) containing two T-SCs (NoCA-5) was designed and synthesized. Introduction of T-SCs can enhance molecular rigidity and intermolecular π–π stacking, which is confirmed by the smaller Stokes shift value, lower reorganization free energy, and shorter π–π stacking distance in comparison to NoCA-1. Hence, the NoCA-5-based device exhibits a record power conversion efficiency (PCE) of 14.82 % in labs and a certified PCE of 14.5 %, resulting from a high electron mobility, a short charge-extraction time, a small Urbach energy (E _u), and a favorable phase separation.

The involvement of guanidinium in perovskite bulk film and CH₃O-PEABr passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of perovskite films increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a record efficiency of 40.1%.

Abstract

Perovskite solar cells exhibit not only high efficiency under full AM1.5 sunlight, but also have great potential for applications in low-light environments, such as indoors, cloudy conditions, early morning, late evening, etc. Unfortunately, their performance still suffers from severe trap-induced nonradiative recombination, particularly under low-light conditions. Here, a holistic passivation strategy is developed to reduce traps both on the surface and in the bulk of micrometer-thick perovskite film, leading to a record efficiency of 40.1% under 301.6 µW cm⁻² warm light-emitting diode (LED) light for low-light solar-cell applications. The involvement of guanidinium into the perovskite bulk film and 2-(4-methoxyphenyl)ethylamine hydrobromide (CH₃O-PEABr) passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of the perovskite film increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a high open-circuit voltage (V _oc) of 1.00 V, a high short-circuit current (J _sc) of 152.10 µA cm⁻², and a fill factor (FF) of 79.52%. Note that this performance also stands as the highest among all photovoltaics measured under indoor light illumination. This work of trap passivation for micrometer-thick perovskite film paves a way for high-performance, self-powered IoT devices.

Narrowband near-infrared organic photodiodes are reported based on a dipolar squaraine dye. J-type coupling in the solid state allows SQ-H thin films to combine favorable NIR absorption at 1040 nm and charge carrier mobility. The bulk-heterojunction with PC₆₁BM yields an organic photodiode with external quantum efficiency of 12.3% at 1050 nm with a full-width half-maximum of 85 nm under short-circuit condition.

Abstract

A highly sensitive short-wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC₆₁BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm⁻¹) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.

Organic planar heterojunctions comprising a nonfullerene small molecular acceptor (NFA) and a polymer donor are successfully fabricated by transferring polymer films on top of water to NFA layers, demonstrating ultrafast charge transfer. They are rendered into bilayer organic solar cells, achieving a power conversion efficiency of up to 7.47% and high stability in ambient air.

Herein, planar heterojunctions comprising a nonfullerene small molecular acceptor (NFA) and a polymer donor are demonstrated by transferring polymer films on a water surface on top of NFA layers. So far, most solution-processed layer-by-layer architectures have been reported as sequentially deposited bulk heterojunctions or pseudo-bilayers because mixed regions at the donor/acceptor interface are inevitable in these methods. By virtue of the unique properties of conjugated polymers such as hydrophobicity and spontaneous film formation on a water surface, the fabrication of NFA/polymer bilayer nanostructures is clearly demonstrated by dramatically simplified methods. These bilayers are successfully rendered into bilayer organic solar cells achieving a power conversion efficiency of up to 7.47%. This reflects that these bilayers have appropriate morphological and optoelectrical properties to be operated as photoactive layers in photovoltaic devices. Further, ultrafast charge transfer from the polymer donor to the NFA and fast carrier mobility are investigated by transient-absorption spectroscopy and photoinduced charge-extraction measurements. Fast carrier dynamics are observed, which are essential for the efficient harvest of excitons in photovoltaic devices. It is believed that the formation of planar heterojunctions on water can offer technical diversity for the fabrication methods of the photovoltaic devices.

This article reports two low-cost A-DA′D-A type non-fullerene acceptors (Y25,Y26) with pentacyclic fused backbone as the DA′D electron-deficient core and explains the reason why the PCE of the Y26-based device is higher than that of Y25 through the analysis of morphology and physicochemical properties. It would be a reference for the further design of non-fullerene acceptors with high performance.

Recent studies have almost focused on finding active layer materials with extended π-conjugation structures for high-performance organic solar cells (OSCs). However, with the extension of conjugate length, the synthesis difficulty and cost of materials will increase. Achieving high efficiency while reducing material costs is a prerequisite for the commercialization of OSCs. Herein, two low-cost A-DA′D-A-type (where A and D represent an electron-withdrawing unit and an electron-donating unit, respectively) nonfullerene acceptors (Y25,Y26) are synthesized with pentacyclic fused backbone as the DA′D electron-deficient core and 5,6-difluoro-3-(dicyandiamethyl) indigo as the end groups. Compared with classical Y series acceptors with heptacyclic backbone, although Y25 and Y26 own the reduced conjugated length, they still show moderate performance (11.65% and 13.34%), and the cost of synthesis is significantly reduced. Therefore, we provide a new molecular design idea for commercially efficient nonfullerene OSCs acceptors. We also find that adding alkyl chains to the β site of thiophenes is beneficial to obtaining the reduced energetic disorder, dominant molecular stacking, and desirable morphology, which can facilitate charge carrier transport and prompt higher short-circuit current density (J _sc) as well as fill factor.

Based on the synergistic effect of the higher dielectric property of non-fullerene acceptors and corresponding photoactive films and the energy transfer from donor to acceptor on charge separation of selected non-fullerene-based photovoltaic devices, these results well interpret the high device performance with a tiny driving force, and the intrinsic physical working mechanism on non-fullerene-based photovoltaic devices is proposed.

Abstract

In non-fullerene-based photovoltaic devices, it is unclear how excitons efficiently dissociate into charge carriers under small driving force. Here, we developed a modified method to estimate dielectric constants of PM6 donor and non-fullerene acceptors. Surprisingly, most non-fullerene acceptors and blend films showed higher dielectric constants. Moreover, they exhibited larger dielectric constants differences at the optical frequency. These results are likely bound to reduced exciton binding energy and bimolecular recombination. Besides, the overlap between the emission spectrum of donor and absorption spectra of non-fullerene acceptors allowed the energy transfer from donor to acceptors. Hence, based on the synergistic effect of dielectric property and energy transfer resulting in efficient charge separation, our finding paves an alternative path to elucidate the physical working mechanism in non-fullerene-based photovoltaic devices.

Asymmetry and halogenation are employed to design a fused-ring non-fullerene electron acceptor, and demonstrate the synergistic effect of tuning optoelectronic properties and enhancing molecular stacking, leading to the highest device efficiency.

Abstract

Fused-ring non-fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC-M, this work systematically designs and synthesizes an asymmetric counterpart ITCC-M-2F, halogenated counterpart ITCC-Cl, and asymmetric and halogenated counterpart IDTT-Cl-2F. Among these NFAs, IDTT-Cl-2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB-T-2Cl, IDTT-Cl-2F-based OSCs yield the highest PCE of 13.3% with an open-circuit voltage of 0.96 V, short-circuit current of 19.20 mA cm^–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2-(2-chloro-6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.

A “two-in-one” strategy is applied to form an acceptor alloy for fine-tuning the donor/acceptor energy alignment and blend morphology. Enhanced hole transfer and suppressed charge recombination in the alloy acceptor consisting of AQx-3 and Y6 enable a power conversion efficiency of over 18%, which is the highest documented for ternary organic solar cells utilizing two nonfullerene acceptors.

Abstract

The trade-off between the open-circuit voltage (V _oc) and short-circuit current density (J _sc) has become the core of current organic photovoltaic research, and realizing the minimum energy offsets that can guarantee effective charge generation is strongly desired for high-performance systems. Herein, a high-performance ternary solar cell with a power conversion efficiency of over 18% using a large-bandgap polymer donor, PM6, and a small-bandgap alloy acceptor containing two structurally similar nonfullerene acceptors (Y6 and AQx-3) is reported. This system can take full advantage of solar irradiation and forms a favorable morphology. By varying the ratio of the two acceptors, delicate regulation of the energy levels of the alloy acceptor is achieved, thereby affecting the charge dynamics in the devices. The optimal ternary device exhibits more efficient hole transfer and exciton separation than the PM6:AQx-3-based system and reduced energy loss compared with the PM6:Y6-based system, contributing to better performance. Such a “two-in-one” alloy strategy, which synergizes two highly compatible acceptors, provides a promising path for boosting the photovoltaic performance of devices.

The conjugated polymer poly(3-hexylthiophene) (P3HT) is used as a conductivity-reducing additive for low-density polyethylene (LDPE). An ultralow amount of only 0.0005 wt% of P3HT decreases the electrical conductivity of LDPE threefold, which may facilitate the design of better insulation materials for high-voltage direct-current power cables.

Abstract

Poly(3-hexylthiophene) (P3HT) is found to be a highly effective conductivity-reducing additive for low-density polyethylene (LDPE), which introduces a new application area to the field of conjugated polymers. Additives that reduce the direct-current (DC) electrical conductivity of an insulation material at high electric fields have gained a lot of research interest because they may facilitate the design of more efficient high-voltage direct-current power cables. An ultralow concentration of regio-regular P3HT of 0.0005 wt% is found to reduce the DC conductivity of LDPE threefold, which translates into the highest efficiency reported for any conductivity-reducing additive to date. The here-established approach, i.e., the use of a conjugated polymer as a mere additive, may boost demand in absolute terms beyond the quantities needed for thin-film electronics, which would turn organic semiconductors from a niche product into commodity chemicals.

The quadrupole moment of non-fullerene acceptors (NFAs) is an important parameter, which controls exciton quenching, charge generation, and recombination in NFA-based blends. Using two structurally similar NFAs, namely, O-IDTBR and O-IDTBCN, with very different quadrupole moments (Q₂₀), the precise impact on efficiency-limiting processes is revealed by a combination of computational and transient optical and electro-optical spectroscopy studies.

Abstract

Advancing non-fullerene acceptor (NFA) organic photovoltaics requires the mitigation of the efficiency-limiting processes. Acceptor end-group and side-chain engineering are two handles to tune properties, and a better understanding of their specific impact on the photophysics could facilitate a more guided acceptor design. Here, the device performance, energetic landscape, and photophysics of rhodanine and dicyanovinyl end-capped IDT-based NFAs, namely, O-IDTBR and O-IDTBCN, in PCE10-based solar cells are compared by transient optical and electro-optical spectroscopy techniques and density functional theory calculations. It is revealed how the acceptors’ quadrupole moments affect the interfacial energetic landscape, in turn causing differences in exciton quenching, charge dissociation efficiencies, and geminate versus non-geminate recombination losses. More precisely, it is found that the open circuit voltage (V _OC) is controlled by the acceptors’ electron affinity (EA), while geminate and non-geminate recombination, and the field dependence of charge generation, rely on the acceptors’ quadrupole moments. The kinetic parameters and yields of all processes are determined, and it is demonstrated that they can reproduce the performance differences of the devices’ current–voltage characteristics in carrier drift-diffusion simulations. The results provide insight into the impact of the energetic landscape, specifically the role of the quadrupole moment of the acceptor, beyond trivial considerations of the donor–acceptor energy offsets.

A new non-fullerene acceptor named BTP1O-4Cl-C12 which contains chlorinated end groups, extended inner side chains and asymmetric alkyl and alkoxy outer side chains is reported. These modifications help BTP1O-4Cl-C12-based devices achieve high efficiency of 17.1% and show its potential application in ternary organic solar cells.

Abstract

Chemical modifications of non-fullerene acceptors (NFAs) play vital roles in the development of high efficiency organic solar cells (OSCs). In this work, on the basis of the previously reported molecule named Y6-1O, chlorination and inner side-chain engineering are adopted to endow the corresponding devices with higher open-circuit voltage (V _OC) and short-circuit current density (J _SC) as well as good morphology for high fill factor (FF). As a result, the molecule named BTP1O-4Cl-C12 can help achieve a higher power conversion efficiency (PCE) of 17.1% than that of Y6-1O (16.1%). Furthermore, the following comparisons between BTP1O-4Cl-C12 and the two symmetric acceptors named BTP2O-4Cl-C12 and BTP-4Cl-C12 demonstrate the effect of asymmetric alkoxy substitution on the outer side chains, which not only achieves a balance between V _OC and J _SC, but also help obtain appropriate morphology for efficient charge dissociation and suppressed charge recombination. Therefore, the asymmetric BTP1O-4Cl-C12 can achieve a higher PCE compared to the symmetric BTP2O-4Cl-C12 and BTP-4Cl-C12. The work not only reports an excellent NFA for high-performance OSCs, but also puts forward a series of methods for consecutive chemical modifications on Y-series acceptors, which can be further applied to boost the PCE of OSCs to a higher level.

Ultrathin and ultra-lightweight organic solar cells (total thickness of less than 3 μm) with a stabilized power conversion efficiency of 15.5% and unprecedented power-per-weight of 32.07 W g⁻¹ at a weight of 4.83 g m⁻² are realized, which could be applied to almost any surface of wearable electronic devices, and can withstand the associated mechanical deformation.

Abstract

Ultraflexible and ultra-lightweight organic solar cells (OSCs) have attracted great attention in terms of power supply in wearable electronic systems. Here, ultrathin and ultra-lightweight OSCs, with a total thickness of less than 3 µm, with excellent mechanical properties in terms of their flexibility and ability to be stretched are demonstrated. A stabilized power conversion efficiency (PCE) of 15.5% and unprecedented power-per-weight of 32.07 W g⁻¹ at a weight of 4.83 g m⁻² is achieved, which represents one of the best-performing OSCs based on ultrathin foils substrate reported to date. The ternary strategy introduces the third component of amorphous conformation of the PC₇₁BM molecule, which can slightly reduce crystallization and aggregates without decreasing the electron mobility, thereby reducing rigidity and brittleness of the active layer. The increase in the ductility of the active layer significantly improves the mechanical flexibility of the device, resulting in over 90% retention in the PCE after 200 stretching–compression cycles. In addition, the ternary device exhibits excellent stability when stored in a N₂-filled glove box, resulting in the PCE retaining over 95% of its initial efficiency even after 1000 h. This ultraflexible and ultra-lightweight photovoltaic foils constitute a major step toward the integration of power supply into malleable electronic textiles.

A blend of starch and a biocompatible intrinsically conducting polymer PEDOT:PSS can be used as a stretchable strain sensor to monitor food processing in real time, because its resistance changes with food volume expansion and shrinkage during food processing including fermentation, steaming, refreshing, and storage. This can be used to optimize the processing conditions and control the food quality.

Abstract

Because stretchable strain sensors that have a resistance or capacitance sensitive to strain can sense skin deformation during physical movement, they have been extensively studied as wearable devices for healthcare monitoring. In principle, they can be used to monitor starch-based food processing in real time, since starch-based food can have remarkable volume change during processing. Monitoring starch-based food processing in real time can help achieve high quality and high productivity while reducing energy consumption. Nevertheless, there is no such report in the literature. Here, a blend of starch and a biocompatible intrinsically conducting polymer, poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS), is reported as a stretchable strain sensor to monitor starch-based food processing including fermentation, steaming, storage, and refreshing in a real-time manner. The resistance of the blends increases during the food volume expansion mainly caused by the fermentation, steaming, and refreshing, and it decreases as a result of the food volume shrinkage during cooling or storage. The signals can be thus used to optimize the processing conditions and control the food quality. This technology can be easily combined with the Internet of Things.

A photomultiplication-type organic photodiode with dramatically increased response speed is demonstrated. Introduction of a quantum dot interlayer effectively blocks dark current of the device while it rapidly and effectively multiplies photocurrent when illuminated, resulting in high detectivity, external quantum efficiency, and bandwidth of the device. Dependence of device response speed on charge trapping dynamics at the interlayer is also discussed.

Abstract

A novel photomultiplication (PM)-type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM-type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM-inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM-type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD-interlayer PM-type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast-responding PM-type OPDs but also provides comprehensive understanding of the underlying device physics.

The design principle for lead-free perovskites and the progress of typical lead-free perovskite photodetectors are reviewed and discussed. The outlook for future research and applications is then explored.

Abstract

State-of-the-art photodetectors which apply hybrid perovskite materials have emerged as powerful candidates for next-generation light sensing. Among them, lead-based ones are the most popular beyond doubt on account of their unique and superior optoelectronic properties. Nevertheless, trade-off toward commercialization exists between nontoxicity and high performance, with the poor stability of lead-based perovskites, indicating that it is indispensable to substitute lead with nontoxic element meanwhile bringing about a comparable figure of merit of photodetectors and relatively long-term stability. Herein, recent advances in lead-free perovskite photodetectors are reviewed, analyzing the principle while designing new materials and highlighting some remarkable progress, which are comparable, even superior, to lead-based photodetectors. Furthermore, their potential strategy in optical communication, image sensing, narrowband photodetection, etc., is examined and a perspective on developing new materials and photodetectors with superior properties for more practical applications is provided.