以昇陳

Publication date: April 2020

Source: Nano Energy, Volume 70

Author(s): Xiaoling Ma, Jian Wang, Qiaoshi An, Jinhua Gao, Zhenghao Hu, Chunyu Xu, Xiaoli Zhang, Zhitian Liu, Fujun Zhang

Graphical abstract

Based on highly efficient binary OPVs, Br-ITIC and PC₇₁BM were successively incorporated into the active layer as the third and the fourth component, leading to the step-by-step PCE improvement from 15.7% to 16.4% and then to 16.8%.

Publication date: Available online 18 January 2020

Source: Nano Energy

Author(s): Congcong Wu, Kai Wang, Munkhbayar Batmunkh, Abdulaziz S.R. Bati, Dong Yang, Yuanyuan Jiang, Yuchen Hou, Joseph G. Shapter, Shashank Priya

Graphical abstract

Third-generation PV technologies along with multiple nanomaterials and nanostructures.

Organic solar cells based on ternary systems have promise as high efficiency and high stability systems. Finding the performance sweet‐spot is, however, a monumental task. In article number https://doi.org/10.1002/aenm.2019024171902417, Mariano Campoy‐Quiles and co‐workers present a simple high‐throughput method to map the ternary parameter space and efficiently discover optimum conditions.

The efficiency of colloidal quantum dot solar cells (CQDSCs) is improved by employing molecularly engineered π‐conjugated polymer‐based organic hole transport materials (HTMs). Their optical and charge generation/collection properties are optimized, and the CQDSC with an efficiency of 11.53%, which is the best among CQDSCs using organic HTMs, is achieved.

Abstract

Organic p‐type materials are potential candidates as solution processable hole transport materials (HTMs) for colloidal quantum dot solar cells (CQDSCs) because of their good hole accepting/electron blocking characteristics and synthetic versatility. However, organic HTMs have still demonstrated inferior performance compared to conventional p‐type CQD HTMs. In this work, organic π‐conjugated polymer (π‐CP) based HTMs, which can achieve performance superior to that of state‐of‐the‐art HTM, p‐type CQDs, are developed. The molecular engineering of the π‐CPs alters their optoelectronic properties, and the charge generation and collection in CQDSCs using them are substantially improved. A device using PBDTTPD‐HT achieves power conversion efficiency (PCE) of 11.53% with decent air‐storage stability. This is the highest reported PCE among CQDSCs using organic HTMs, and even higher than the reported best solid‐state ligand exchange‐free CQDSC using pCQD‐HTM. From the viewpoint of device processing, device fabrication does not require any solid‐state ligand exchange step or layer‐by‐layer deposition process, which is favorable for exploiting commercial processing techniques.

The role of reverse intersystem crossing in the acceptor molecule on the device stability of exciplex‐based organic light‐emitting diodes is revealed. The addition of an electron‐donating unit onto the acceptor core can prevent molecular degradation, and the thermally activated delayed fluorescence (TADF) ability can also recycle triplet energy as exciplex emission, resulting in significant improvement of the device lifetime.

Abstract

Exciplex system exhibiting thermally activated delayed fluorescence (TADF) holds a considerable potential to improve organic light‐emitting diode (OLED) performances. However, the operational lifetime of current exciplex‐based devices, unfortunately, falls far behind the requirement for commercialization. Herein, rationally choosing a TADF‐type electron acceptor molecule is reported as a new strategy to enhance OLEDs' operating lifetime. A comprehensive study of the exciplex system containing 9,9′,9′′‐triphenyl‐9H,9′H,9′′H‐3,3′:6′,3′′‐tercarbazole (Tris‐PCz) and triazine (TRZ) derivatives clarifies the relationship between unwanted carrier recombination on acceptor molecules, TADF property of acceptors, and the device degradation event. By employing a proposed “exciton recycling” strategy, a threefold increased operational lifetime can be achieved while still maintaining high‐performance OLED properties. In particular, a stable blue OLED that employs this strategy is successfully demonstrated. This research provides an important step for exciplex‐based devices toward the significant improvement of operational stability.

The efficiency of photocurrent generation is studied in the high‐efficiency nonfullerene PM6:Y6 blend, using a combination of field‐ and temperature‐dependent optoelectronic measurements. These experiments reveal barrierless free charge generation, despite a small driving force. Theoretical modeling suggests the existence of a large electrostatic interfacial field, which pushes charges away from the donor–acceptor interface.

Abstract

Organic solar cells are currently experiencing a second golden age thanks to the development of novel non‐fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high‐performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near‐unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.

By designing N‐functionalized asymmetrical acceptors N7IT and N8IT, the effects of nitrogen (N) atom on reducing nonradiative recombination loss (ΔE ₃) and dipole moment on morphology are revealed.

Abstract

Energy loss (E _loss) consisting of radiative recombination loss (ΔE ₁ and ΔE ₂) and nonradiative recombination loss (ΔE ₃) is considered as an important factor for organic solar cells (OSCs). Herein, two N‐functionalized asymmetrical small molecule acceptors (SMAs), namely N7IT and N8IT are designed and synthesized, to explore the effect of N on reducing E _loss with sulfur (S) as a comparison. N7IT‐based OSCs achieve not only a higher PCE (13.8%), but also a much lower E _loss (0.57 eV) than those of the analogue (a‐IT)‐based OSCs (PCE of 11.5% and E _loss of 0.72 eV), which are mainly attributed to N7IT's significantly enhanced charge carrier density (promoting J _SC) and largely suppressed nonradiative E _loss by over 0.1 eV (enhancing V _OC). In comparison, N8IT, with an extended π‐conjugated length, shows relatively lower photovoltaic performance than N7IT (but higher than a‐IT) due to the less favorable morphology caused by the excessively large dipole moment of the asymmetrical molecule. Finally, this work sheds light on the structure–property relationship of the N‐functionalization, particularly on its effects on reducing the E _loss, which could inspire the community to design and synthesize more N‐functionalized SMAs.

Nature Photonics, Published online: 20 January 2020; doi:10.1038/s41566-019-0573-5

Publication date: June 2020

Source: Materials Today Energy, Volume 16

Author(s): Dongxue Wang, Wen Cui, Feng Li, Mingqun Yang, Xin Jing, Liangmin Yu, Mingliang Sun

Publication date: April 2020

Source: Materials Science and Engineering: R: Reports, Volume 140

Author(s): Hang Yin, Cenqi Yan, Hanlin Hu, Johnny Ka Wai Ho, Xiaowei Zhan, Gang Li, Shu Kong So

Semiconducting polymers based on naphthobisthiadiazole (NTz) are designed and synthesized by introducing fluorine atoms on the NTz and bithiophene moieties. The impact of induced noncovalent sulfur–fluorine interaction position on the electronic structures, ordering structures, and photovoltaic performance is systematically studied. The newly developed polymer exhibits the power conversion efficiency of 10.8% that is one of the highest values for polymer/fullerene organic solar cells.

Abstract

Controlling the energetics and backbone order of semiconducting polymers is essential for the performance improvement of polymer‐based solar cells. The use of fluorine as the substituent for the backbone is known to effectively deepen the molecular orbital energy levels and coplanarize the backbone by noncovalent interactions with sulfur of the thiophene ring. In this work, novel semiconducting polymers are designed and synthesized based on difluoronaphthobisthiadiazole (FNTz) as a new family of naphthobisthiadiazole (NTz)–quaterthiophene copolymer systems, which are one of the highest performing polymers in solar cells. The effect of the fluorination position on the energetics and backbone order is systematically studied. It is found that the dependence of the solar cell fill factor on the active layer thickness is very sensitive to the fluorination position. It is thus further investigated and discussed how the structural features of the polymers influence the photovoltaic parameters as well as the diode characteristics and bimolecular recombination. Further, the polymer with fluorine on both the naphthobisthiadiazole and quaterthiophene moieties exhibits a quite high power conversion efficiency of 10.8% in solar cells in combination with a fullerene. It is believed that the results would offer new insights into the development of semiconducting polymers.

Vibration‐induced emission (VIE) is coined for rationalization of the large Stokes shift and dual emission of V‐shaped N,N′‐disubstituded‐dihydribenzo[a,c]phenazines (DHPs), which involve a bent‐to‐planar vibration and the reverse in the excited state. The interconnection of conformational and electronic properties of DHPs is systematically examined in the excited and ground states. VIE‐active luminogens serve as excellent multicolor emitters and ratiometric fluorescent probes.

Abstract

Organic fluorophores with dual‐emission and large Stokes shifts are attracting great attention due to their importance in fundamental research and technique applications. This Progress Report gives an account on how a novel luminescence mechanism termed vibration‐induced emission (VIE) is established. The VIE mechanism is coined for the rationalization of an alterable dual emission of V‐shaped N,N′‐disubstituded‐dihydribenzo[a,c]phenazines (DHPs), which are originated from a bent‐to‐planar vibration and the reverse in the excited state. The validation of the VIE mechanism is highlighted, such as the work reporting the utilization of chemically‐locked strategy to snapshot the excited‐state planarization of DHPs, and the application of the approach of steric hindrance‐induced planarization to tune the ground‐state geometry of DHPs. Moreover, the emerging applications of this VIE concept in photoelectric and biomedical disciplines are summarized. Additionally, further development of the VIE systems as well as the remaining challenges are prospected. This report could arouse wide interest from various fields to the specific area of VIE, which would not only broaden the VIE territory but also enlarge the scope of advanced functional materials.

The efficiency of colloidal quantum dot solar cells (CQDSCs) is improved by employing molecularly engineered π‐conjugated polymer‐based organic hole transport materials (HTMs). Their optical and charge generation/collection properties are optimized, and the CQDSC with an efficiency of 11.53%, which is the best among CQDSCs using organic HTMs, is achieved.

Abstract

Organic p‐type materials are potential candidates as solution processable hole transport materials (HTMs) for colloidal quantum dot solar cells (CQDSCs) because of their good hole accepting/electron blocking characteristics and synthetic versatility. However, organic HTMs have still demonstrated inferior performance compared to conventional p‐type CQD HTMs. In this work, organic π‐conjugated polymer (π‐CP) based HTMs, which can achieve performance superior to that of state‐of‐the‐art HTM, p‐type CQDs, are developed. The molecular engineering of the π‐CPs alters their optoelectronic properties, and the charge generation and collection in CQDSCs using them are substantially improved. A device using PBDTTPD‐HT achieves power conversion efficiency (PCE) of 11.53% with decent air‐storage stability. This is the highest reported PCE among CQDSCs using organic HTMs, and even higher than the reported best solid‐state ligand exchange‐free CQDSC using pCQD‐HTM. From the viewpoint of device processing, device fabrication does not require any solid‐state ligand exchange step or layer‐by‐layer deposition process, which is favorable for exploiting commercial processing techniques.

Power of twist : A twisted heptagonal diimide acceptor with well‐balanced structural rigidity and rotatability has been developed for the synthesis of a highly efficient aggregation‐induced delayed fluorescence material, which was used to make state‐of‐the‐art performance non‐doped OLEDs.

Abstract

The development of efficient non‐doped organic light‐emitting diodes (OLEDs) is highly desired but very challenging because of a severe aggregation‐caused quenching effect. Herein, we present a heptagonal diimide acceptor (BPI), which can restrict excessive intramolecular rotation and inhibit close intermolecular π–π stacking due to well‐balanced rigidity and rotatability of heptagonal structure. The BPI‐based luminogen (DMAC‐BPI ) shows significant aggregation‐induced delayed florescence with an extremely high photoluminescence quantum yield (95.8 %) of the neat film, and the corresponding non‐doped OLEDs exhibit outstanding electroluminescence performance with maximum external quantum efficiency as high as 24.7 % and remarkably low efficiency roll‐off as low as 1.0 % at 1000 cd m⁻², which represents the state‐of‐the‐art performance for non‐doped OLEDs. In addition, the synthetic route to DMAC‐BPI is greatly streamlined and simplified through oxidative Ar−H/Ar−H homo‐coupling reaction.

The luminescence mechanism of aggregation‐induced emission‐based deep blue organic light‐emitting diodes is investigated by magneto‐electroluminescence. A conversion process from the higher energy triplet sates (T₂) to the lowest singlet state (S₁) is found in the devices. By regulating the exciton utilization according to this process, the external quantum efficiency is improved from 6.41% to 7.93%.

Abstract

Aggregation‐induced emission (AIE) materials are highly attractive because of their excellent properties of high efficiency emission in nondoped organic light‐emitting diodes (OLEDs). Therefore, a deep understanding of the working mechanisms, further improving the electroluminescence (EL) efficiency of the resulting AIE‐based OLEDs, is necessary. Herein, the conversion process from higher energy triplet state (T₂) to the lowest singlet state (SS₁) is found in OLEDs based on a blue AIE material, 4′‐(4‐(diphenylamino)phenyl)‐5′‐phenyl‐[1,1′:2′,1′′‐terphenyl]‐4‐carbonitrile (TPB‐AC), obviously relating to the device efficiency, by magneto‐EL (MEL) measurements. A special line shape with rise at low field and reduction at high field is observed. The phenomenon is further clarified by theoretical calculations, temperature‐dependent MELs, and transient photoluminescence emission properties. On the basis of the T₂‐S₁ conversion process, the EL performances of the blue OLEDs based on TPB‐AC are further enhanced by introducing a phosphorescence doping emitter in the emitting layer, which effectively regulates the excitons on TPB‐AC molecules. The maximum external quantum efficiency (EQE) reaches 7.93% and the EQE keeps 7.57% at the luminance of 1000 cd m⁻². This work establishes a physical insight for designing high‐performance AIE materials and devices in the future.

A novel wide bandgap polymer donor PNDT‐ST and a near infrared nonfullerene acceptor Y6‐T are developed for highly efficient organic solar cells. The high lowest unoccupied molecular orbital energy of Y6‐T and the high crystallinity of PNDT‐ST as well as the compatible PNDT‐ST:PNDT‐ST:Y6‐T ternary blend enable the significantly improved power conversion efficiency of 16.57% with minimal energy loss of 0.521 eV.

Abstract

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V _oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E _loss) of 0.521 eV. Such small E _loss is the best record for polymer solar cells with PCEs over 16% to date.