以昇陳

Efficient electron injection into organic semiconductors is achieved by hydrogen bond formation between the host materials and bases that are commonly used in organic synthesis as catalysts. The electron‐injection efficiency in inverted organic light‐emitting diodes, the electron‐injection layer of which consists of the host and bases, is found to be almost proportional to the basicity of the bases.

Abstract

Molecular n‐dopants that can lower the electron injection barrier between organic semiconductors and electrodes are essential in present‐day organic electronics. However, the development of stable molecular n‐dopants remains difficult owing to their low ionization potential, which generally renders them unstable. It is shown that the stable bases widely used in organic synthesis as catalysts can lower the electron injection barrier similar to that in conventional n‐doping in organic optoelectronic devices. In contrast to conventional n‐doping, which is based on the electron transfer from dopants with low ionization potential, the reduction of the injection barrier caused by adding bases is determined by the formation of hydrogen bonds between the hosts and the bases, providing energy‐level‐independent electron injection. The observation of the efficient electron injection induced by hydrogen bonding affords new perspectives on the method for controlling the behavior of electrons unique to organic semiconductors.

Organic photovoltaic (OPV) cells promise to have a good photovoltaic performance under the indoor light environment. Via optimizing the active layers, 1 cm² OPV cells are fabricated and a top power conversion efficiency of 22% under 1000 lux illumination is demonstrated.

Abstract

Organic photovoltaic (OPV) technologies have the advantages of fabricating larger‐area and light‐weight solar panels on flexible substrates by low‐cost roll‐to‐toll production. Recently, OPV cells have achieved many significant advances with power conversion efficiency (PCE) increasing rapidly. However, large‐scale solar farms using OPV modules still face great challenges, such as device stability. Herein, the applications of OPV cells in indoor light environments are studied. Via optimizing the active layers to have a good match with the indoor light source, 1 cm² OPV cells are fabricated and a top PCE of 22% under 1000 lux light‐emitting diode (2700 K) illumination is demonstrated. In this work, the light intensities are measured carefully. Incorporated with the external quantum efficiency and photon flux spectrum, the integral current densities of the cells are calculated to confirm the reliability of the photovoltaic measurement. In addition, the devices show much better stability under continuous indoor light illumination. The results suggest that designing wide‐bandgap active materials to meet the requirements for the indoor OPV cells has a great potential in achieving higher photovoltaic performance.

A novel red thermally activated delayed fluorescence (TADF) emitter, TPA–PZCN, is designed and synthesized. It simultaneously possesses a high Φ _PL of 97% and a small ΔE _ST of 0.13 eV. Red, deep‐red, and near‐infrared organic light‐emitting diodes (LEDs) based on it achieve record external quantum efficiencies of 27.4%, 28.1%, and 5.3%, respectively, which are the best performances in comparison with LEDs having a similar device structure.

Abstract

Researchers have spared no effort to design new thermally activated delayed fluorescence (TADF) emitters for high‐efficiency organic light‐emitting diodes (OLEDs). However, efficient long‐wavelength TADF emitters are rarely reported. Herein, a red TADF emitter, TPA–PZCN, is reported, which possesses a high photoluminescence quantum yield (Φ _PL) of 97% and a small singlet–triplet splitting (ΔE _ST) of 0.13 eV. Based on the superior properties of TPA–PZCN, red, deep‐red, and near‐infrared (NIR) OLEDs are fabricated by utilizing different device structure strategies. The red devices obtain a remarkable maximum external quantum efficiency (EQE) of 27.4% and an electroluminescence (EL) peak at 628 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.65, 0.35), which represents the best result with a peak wavelength longer than 600 nm among those of the reported red TADF devices. Furthermore, an exciplex‐forming cohost strategy is adopted. The devices achieve a record EQE of 28.1% and a deep‐red EL peak at 648 nm with the CIE coordinates of (0.66, 0.34). Last, nondoped devices exhibit 5.3% EQE and an NIR EL peak at 680 nm with the CIE coordinates of (0.69, 0.30).

By rationally tuning the molecular interaction and energy level alignments of the donors and acceptors, when both donor and acceptor are fluorinated or both are not fluorinated, high‐performance organic photovoltaics can be realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, devices based on the PBDB‐T‐F/Y1‐4F blend and PBDB‐T‐F/Y6 exhibit power conversion efficiencies as high as 14.8% and 15.9%, respectively.

Abstract

The performance of organic photovoltaics (OPVs) has rapidly improved over the past years. Recent work in material design has primarily focused on developing near‐infrared nonfullerene acceptors with broadening absorption that pair with commercialized donor polymers; in the meanwhile, the influence of the morphology of the blend film and the energy level alignment on the efficiency of charge separation needs to be synthetically considered. Herein, the selection rule of the donor/acceptor blend is demonstrated by rationally considering the molecular interaction and energy level alignment, and highly efficient OPV devices using both‐fluorinated or both‐nonfluorinated donor/acceptor blends are realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, the devices based on the PBDB‐T‐F/Y1‐4F blend and PBDB‐T‐F/Y6 exhibit champion power conversion efficiencies as high as 14.8% and 15.9%, respectively.

The use of liquid exfoliated 2D WS₂ and MoS₂ as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS₂ yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS₂ and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS₂ is introduced as HTL in PBDB‐T‐2F:Y6:PC₇₁BM organic solar cells, a maximum PCE value of 17% is achieved.

Abstract

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J _SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J _SC of 26 mA cm⁻². Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.

A series of wide bandgap donor polymers are designed and synthesized by incorporating a monothiophene functionalized with both a fluorine atom and an ester group. Fabricated from nonhalogenated solvent, power conversion efficiencies of 11.39% and 12.11% are achieved for binary and ternary nonfullerene solar cells, respectively.

Abstract

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V _oc of 1.01 V and a J _sc of 17.89 mA cm⁻². Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs.

Mechanochromic response is presented as a method to tune optical properties of photonic metasurfaces. A subwavelength array of silicon nitride/molybdenum oxide and molybdenum disulfide nanowires due to heat‐activated mechanical strain alters its optical properties drastically in the visible part of the spectrum with transmission change up to 197% and mechanically bistable response.

Abstract

The change of optical properties that some usually natural compounds or polymeric materials show upon the application of external stress is named mechanochromism. Herein, an artificial nanomechanical metasurface formed by a subwavelength nanowire array made of molybdenum disulfide, molybdenum oxide, and silicon nitride changes color upon mechanical deformation. The aforementioned deformation induces reversible changes in the optical transmission (relative transmission change of 197% at 654 nm), thus demonstrating a giant mechanochromic effect. Moreover, these types of metasurfaces can exist in two nonvolatile states presenting a difference in optical transmission of 45% at 678 nm, when they are forced to bend rapidly. The wide optical tunability that photonic nanomechanical metasurfaces, such as the one presented here, possess by design, can provide a valuable platform for mechanochromic and bistable responses across the visible and near infrared regime and form a new family of smart materials with applications in reconfigurable, multifunctional photonic filters, switches, and stress sensors.

Nature Energy, Published online: 23 September 2019; doi:10.1038/s41560-019-0466-3

Aggregation‐induced emission dots (AIE dots) with ultrabright NIR‐I emission (peak at ≈700 nm and a quantum yield of 19%) are synthesized for NIR‐II (1200 nm) excited in vivo two‐photon fluorescence (2PF) imaging of deep tumor vasculatures. The tumor vasculatures labeled with AIE dots exhibit enhanced 2PF, which leads to the tumor blood vessels being able to be distinguished from normal ones in deep tissues.

Abstract

Intravital fluorescence imaging of vasculature morphology and dynamics in the brain and in tumors with large penetration depth and high signal‐to‐background ratio (SBR) is highly desirable for the study and theranostics of vascular‐related diseases and cancers. Herein, a highly bright fluorophore (BTPETQ) with long‐wavelength absorption and aggregation‐induced near‐infrared (NIR) emission (maximum at ≈700 nm) is designed for intravital two‐photon fluorescence (2PF) imaging of a mouse brain and tumor vasculatures under NIR‐II light (1200 nm) excitation. BTPETQ dots fabricated via nanoprecipitation show uniform size of around 42 nm and a high quantum yield of 19 ± 1% in aqueous media. The 2PF imaging of the mouse brain vasculatures labeled by BTPETQ dots reveals a 3D blood vessel network with an ultradeep depth of 924 µm. In addition, BTPETQ dots show enhanced 2PF in tumor vasculatures due to their unique leaky structures, which facilitates the differentiation of normal blood vessels from tumor vessels with high SBR in deep tumor tissues. Moreover, the extravasation and accumulation of BTPETQ dots in deep tumor (more than 900 µm) is visualized under NIR‐II excitation. This study highlights the importance of developing NIR‐II light excitable efficient NIR fluorophores for in vivo deep tissue and high contrast tumor imaging.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

Recent progress in bandgap engineering strategies including the two main, widely used impurity and pressure as well as intermediate band, external electric field, and steric methods are reviewed comprehensively. Their underlying mechanism, achievements, and challenges are outlined. Additionally, future research directions are provided to realize direct and gap size continually tunable perovskites for further enhancing solar cell performance.

Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single‐junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

An electron‐deficient unit containing B←N bonds, namely BNIDT, is developed to construct polymer acceptors for photovoltaic applications. Desirable optoelectronic properties such as broad absorption profiles, low‐lying energy levels, ambipolar charge transport properties, and strong electron‐affinity are found for these polymers. All‐polymer solar cells using these B←N embedded polymers as acceptor materials exhibit an enhanced efficiency of 8.78%.

Abstract

In the field of all‐polymer solar cells (all‐PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron‐deficient units to build high‐performance polymer acceptors is critical. A novel electron‐deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single‐crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4‐difluorothiophene, two novel conjugated polymers named BN‐T and BN‐2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350–800 nm, low‐lying energy levels, and ambipolar film‐transistor characteristics. Using PBDB‐T as the donor and BN‐2fT as the acceptor, all‐PSCs afford an encouraging efficiency of 8.78%, which is the highest for all‐PSCs excluding the devices based on imide and dicyanoethylene‐type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron‐deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

In bulk heterojunctions with small energetic offsets between donor and acceptor materials, the donor polymer can assist the electron transport by providing “bridges” or a “shortcut” for electron transport across the small‐molecular domains and facilitates the overall electron transport. This finding can be also applied to other fields to tune the charge transport property of organic materials or slush blends.

Abstract

Conventional organic solar cell (OSC) systems have significant energy offsets between the donor and acceptor both at the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Because of this, in a bulk heterojunction (BHJ) system, electrons typically transport in acceptors, whereas holes typically transport in donors. It is not favorable for electrons to hop back and forth between the donor and acceptor because the hopping is energetically disfavored. In such conventional OSC systems, the addition of donor polymer to acceptor films should typically reduce the electron mobility. In this study, a surprisingly large increase (up to 30×) in electron mobility is observed in an OSC blend when introducing a polymer donor into small molecular acceptor. By ruling out morphology reasons, it is shown that the donor polymer can assist the electron transport by providing “bridges” or a “shortcut” for electron transport across the domains of small molecular acceptors. This can happen because, for these systems, the LUMO offset is small. The study shows the benefits of donor‐assisted electron transport in BHJ systems with small energetic offsets. This finding could be also applied to other fields to tune the optimized charge transport property of organic materials or slush blends.

Nature Communications, Published online: 10 September 2019; doi:10.1038/s41467-019-12132-6

Publication date: December 2019

Source: Materials Today Energy, Volume 14

Author(s): Tao Lei, Ruixiang Peng, Like Huang, Wei Song, Tingting Yan, Liqiang Zhu, Ziyi Ge

A high power conversion efficiency of 11.76%, the best efficiency for all‐polymer solar cells, is achieved by printing fabrication based on PTzBI‐Si:N2200 processing with 2‐methyltetrahydrofuran. A Multi‐length‐scaled morphology is found in the bulk heterojunctions, which ensures fast transfer of carriers and facilitates exciton separation, and boosts carrier mobility and current density, thus improving the device performance.

Abstract

All‐polymer solar cells (all‐PSCs) exhibit excellent stability and readily tunable ink viscosity, and are therefore especially suitable for printing preparation of large‐scale devices. At present, the efficiency of state‐of‐the‐art all‐PSCs fabricated by the spin‐coating method has exceeded 11%, laying the foundation for the preparation and practical utilization of printed devices. A high power conversion efficiency (PCE) of 11.76% is achieved based on PTzBI‐Si:N2200 all‐PSCs processing with 2‐methyltetrahydrofuran (MTHF, an environmentally friendly solvent) and preparation of active layers by slot die printing, which is the top efficient for all‐PSCs. Conversely, the PCE of devices processed by high‐boiling point chlorobenzene is less than 2%. Through the study of film formation kinetics, volatile solvents can freeze the morphology in a short time, and a more rigid conformation with strong intermolecular interaction combined with the solubility limit of PTzBI‐Si and N2200 in MTHF results in the formation of a fibril network in the bulk heterojunction. The multilength scaled morphology ensures fast transfer of carriers and facilitates exciton separation, which boosts carrier mobility and current density, thus improving the device performance. These results are of great significance for large‐scale printing fabrication of high‐efficiency all‐PSCs in the future.