Yingzhi Jin

The rapid rise in efficiencies of lead halide perovskite solar cells has hastened the case for commercialization. One of the key barriers, however, is stability of devices. In this issue of Joule, Zhou et al. studied the degradation mechanism of a FA-Cs mixed cation perovskite-based solar cell with experimental and modeling techniques to show that phase separation into FA-rich and Cs-rich compositions account for the observed degradation.

A non-fullerene acceptor (DTY6) with long-branched alkyl chains was synthesized and showed a power conversion efficiency (PCE) of 16.1% when using a non-halogen solvent. Encouragingly, the large-area modules (18 cm2) exhibited an outstanding PCE of 14.4% (certified to be 13.98% by a third-party accredited institute), which is by far the highest module efficiency reported to date.

High‐efficiency solution‐processed hybrid tandem photovoltaic devices, employing inorganic perovskite and organic bulk‐heterojunction as the photoactive layers, are demonstrated. A PCE of 18.04% in the hybrid tandem device is achieved, which is significantly higher than the comparable single‐junction devices, owing to a near‐optimal absorption spectral match.

Abstract

Although the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI₂Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI₂Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

A novel BDT‐based random polymeric hole‐transporting layer (asy‐ranPBTBDT) is developed with irregularity from asymmetric substitution and random copolymerization. The resulting low crystallinity from the irregularity leads to superior solubility capacity and suppressed charge recombination and morphological changes. Therefore, the colloidal quantum dot solar cells using asy‐ranPBTBDT‐based device show highly efficient power conversion efficiency of 13.2% with superior operational stability.

Abstract

Next‐generation solution‐processed solar cells will hopefully be processed using green solvents, and will unite high performance with operating stability. Colloidal quantum dot/polymer hybrid solar cells are of interest for their harvest of the visible as well as the near infrared; however, today's best polymer hole‐transporting layers (HTLs) rely on processing using hazardous solvents such as chlorobenzene. This stems from the strong polymer–polymer attraction in polymeric p‐type materials, which accounts for their limited solubility. Here, a new random polymeric HTL (asy‐ranPBTBDT) is reported that is soluble in green solvents such as 2‐methylanisole without compromising ultimate device power conversion efficiency. The new polymer structure induces a strong π–π stacking face‐on orientation and less lateral grain growth compared to control asy‐PBTBDT, leading to reduced charge recombination and improved device stability. The resulting device exhibits a power conversion efficiency (PCE) of 13.2% and retains 89% of its initial efficiency after 120 h of continuous device operation at the maximum power point, compared to a PCE of 11.4% and 71% degradation for control devices.

To generate cost‐efficient and high‐performanced polymers, a simple chemical steric effect (SE) is introduced to benzothiophene (BDT)‐based side chains. The polymeric crystallinity and miscibility are rebalanced and a power conversion efficiency (PCE) of 14.53% is achieved. Thus, the SE applied in crystalline polymer pave an easier and cheaper route to realize the coordination of low‐cost fabrication and high‐performance.

Abstract

Low synthetic cost and high performance are becoming a new challenge in designing polymer donors for large‐scaled polymer solar cells (PSCs) fabrication; however, complicated synthetic routes and high material costs hamper the widespread commercial application of OPVs. Here, a simple and low‐cost chemical steric effect (SE) is introduced to BDT‐based side chains. Through adjusting alkyl side chains, the polymeric crystallinity and miscibility are rebalanced and subsequently the photovoltaic device based on the meta‐positioned alkyl polymer outperforms its para‐positioned counterpart. The champion device based on the polymer with the meta‐positioned side chains affords a PCE of 14.53% without sacrificing its high fill factor of 0.77, which could be attributed to a more balanced charge‐carrier transport ability and optimized morphology. This is the highest PCE value reported in BTZ based polymer donors to date. Thus, it shows that the SE applied in high crystalline polymer could pave an easier and cheaper chemical route to realize the coordination of low‐cost fabrication and high‐performance.

This study reports the deposition of a TiO₂ electron transporting layer for perovskite solar cells by spray coating using a stable TiO₂ colloidal aqueous solution, which is synthesized via the self‐condensation of a titanium peroxide complex under hydrothermal conditions. Although the whole fabrication process for the cells is performed at 100 °C, 22.7% efficiency is achieved.

Abstract

TiO₂ is one of the most efficient and widely used materials for electron‐transporting layer (ETLs) in perovskite solar cells (PSCs). The formation of efficient TiO₂ layers is generally carried out at high temperature by baking at a temperature >400 °C or by vacuum deposition (e.g., atomic layer deposition and E‐beam). In this study, the preparation of a TiO₂ ETL for PSCs is reported with excellent properties at low temperatures based on the synthesis of a stable TiO₂ colloidal aqueous solution and spray coating. The prepared TiO₂ colloids are able to produce a dense and uniform ETL even if it is simply dried at 100 °C after spray coating. It is believed that this is owing to the peroxo functional group remaining on the surface of the TiO₂ colloids. The TiO₂ ETLs, combined with the TiO₂ underlayer formed by chemical bath deposition, and the sprayed TiO₂ colloids allowed the fabrication of PSCs with performance similar to those of PSCs produced by annealing at 450 °C with a TiO₂ paste. The PSCs fabricated entirely at 100 °C demonstrated power conversion efficiency of 22.7% in small cells, and 19.0% in mini‐modules.

In article number https://doi.org/10.1002/aenm.2020004532000453, Weili Yu, Chunlei Guo and co‐workers demonstrate that by combining the space‐limiting technique and the anti‐temperature crystallization method, millimeter sized MAPbI₃ perovskite monocrystals with thickness from tens of nanometers to micrometers can be fabricated. Solar cells based on the 300 nm thick perovskite monocrystal are prepared, which show 3% enhancement in power conversion efficiency (PCE) compared to their polycrystalline counterparts. This work provides a scalable way to synthesize high quality perovskite crystals with less grains and grain boundaries, is believed a key step to develop perovskite single crystal solar cells with high performance.

In article number https://doi.org/10.1002/aenm.2020016102001610, Zonghao Liu, Liyuan Han, Wei Chen and co‐workers, review the stability improvement strategy of perovskite solar cells from the view point of barrier designs. The barriers can address adverse issues like product volatilization, ion diffusion, electrode corrosion, and fight off the harmful influence of external stresses including sunlight, heat, H₂O/O₂, electric bias, etc.

The recent progress of CsPbI₃ perovskite for highly efficient and stable photovoltaics is summarized. Furthermore, those important phase stabilization strategies for black‐phase CsPbI₃ are also discussed. With the advancing of fundamental studies on CsPbI₃ perovskite material properties, the CsPbI₃ perovskite and other inorganic perovskites will become more and more promising for high‐efficiency and stable perovskite solar cells.

Abstract

Research on chemically stable inorganic perovskites has achieved rapid progress in terms of high efficiency exceeding 19% and high thermal stabilities, making it one of the most promising candidates for thermodynamically stable and high‐efficiency perovskite solar cells. Among those inorganic perovskites, CsPbI₃ with good chemical components stability possesses the suitable bandgap (≈1.7 eV) for single‐junction and tandem solar cells. Comparing to the anisotropic organic cations, the isotropic cesium cation without hydrogen bond and cation orientation renders CsPbI₃ exhibit unique optoelectronic properties. However, the unideal tolerance factor of CsPbI₃ induces the challenges of different crystal phase competition and room temperature phase stability. Herein, the latest important developments regarding understanding of the crystal structure and phase of CsPbI₃ perovskite are presented. The development of various solution chemistry approaches for depositing high‐quality phase‐pure CsPbI₃ perovskite is summarized. Furthermore, some important phase stabilization strategies for black phase CsPbI₃ are discussed. The latest experimental and theoretical studies on the fundamental physical properties of photoactive phase CsPbI₃ have deepened the understanding of inorganic perovskites. The future development and research directions toward achieving highly stable CsPbI₃ materials will further advance inorganic perovskite for highly stable and efficient photovoltaics.

An organic mixed‐ion–electron n‐type conductor based on highly crystalline and reduced perylene bisimide is developed, achieving a record ZT value of 0.23 that enables a quasi‐constant thermoelectric power output. This work paves the way for the design and development of efficient organic thermoelectrics by rational control of the mobility of the electronic and ionic carriers.

Abstract

Low‐cost, non‐toxic, abundant organic thermoelectric materials are currently under investigation for use as potential alternatives for the production of electricity from waste heat. While organic conductors reach electrical conductivities as high as their inorganic counterparts, they suffer from an overall low thermoelectric figure of merit (ZT) due to their small Seebeck coefficient. Moreover, the lack of efficient n‐type organic materials still represents a major challenge when trying to fabricate efficient organic thermoelectric modules. Here, a novel strategy is proposed both to increase the Seebeck coefficient and achieve the highest thermoelectric efficiency for n‐type organic thermoelectrics to date. An organic mixed ion–electron n‐type conductor based on highly crystalline and reduced perylene bisimide is developed. Quasi‐frozen ionic carriers yield a large ionic Seebeck coefficient of −3021 μV K⁻¹, while the electronic carriers dominate the electrical conductivity which is as high as 0.18 S cm⁻¹ at 60% relative humidity. The overall power factor is remarkably high (165 μW m⁻¹ K⁻²), with a ZT = 0.23 at room temperature. The resulting single leg thermoelectric generators display a high quasi‐constant power output. This work paves the way for the design and development of efficient organic thermoelectrics by the rational control of the mobility of the electronic and ionic carriers.

A paradigm shift in the doping strategy of p‐type conjugated polymers is proposed by using another p‐type material having partly filled HOMO level instead of an acceptor molecule with low‐lying LUMO value. This strategy results in extraordinary thermally and environmentally stable doped semiconductor systems with high electrical conductivity suitable for thermoelectric applications.

Abstract

Unlike the conventional p‐doping of organic semiconductors (OSCs) using acceptors, here, an efficient doping concept for diketopyrrolopyrrole‐based polymer PDPP[T]₂‐EDOT (OSC‐1) is presented using an oxidized p‐type semiconductor, Spiro‐OMeTAD(TFSI)₂ (OSC‐2), exploiting electron transfer from HOMO_OSC‐1 to HOMO_OSC‐2. A shift of work function toward the HOMO_OSC‐1 upon doping is confirmed by ultraviolet photoelectron spectroscopy (UPS). Detailed X‐ray photoelectron spectroscopy (XPS) and UV–vis–NIR absorption studies confirm HOMO_OSC‐1 to HOMO_OSC‐2 electron transfer. The reduction products of Spiro‐OMeTAD(TFSI)₂ to Spiro‐OMeTAD(TFSI) and Spiro‐OMeTAD is also confirmed and their relative amounts in doped samples is determined. Mott–Schottky analysis shows two orders of magnitude increase in free charge carrier density and one order of magnitude increase in the charge carrier mobility. The conductivity increases considerably by four orders of magnitude to a maximum of 10 S m⁻¹ for a very low doping ratio of 8 mol%. The doped polymer films exhibit high thermal and ambient stability resulting in a maximum power factor of 0.07 µW m⁻¹ K⁻² at a Seebeck coefficient of 140 µV K⁻¹ for a very low doping ratio of 4 mol%. Also, the concept of HOMO_OSC‐1 to HOMO_OSC‐2 electron transfer is a highly efficient, stable and generic way to p‐dope other conjugated polymers.

The first example of flexible transparent zinc‐mesh electrodes is demonstrated for assembly of large‐scale Zn‐anode‐based electrochromic windows, and solar‐charging smart windows with sunlight‐intermittency issues perfectly addressed are presented. These findings facilitate new opportunities for the development of next‐generation transparent electrochemical devices.

Abstract

Newly born zinc‐anode‐based electrochromic devices (ZECDs), incorporating electrochromic and energy storage functions in a single transparent platform, represent the most promising technology for next‐generation transparent electronics. As the existing ZECDs are limited by opaque zinc anodes, the key focus should be on the development of transparent zinc anodes. Here, the first demonstration of a flexible transparent zinc‐mesh electrode is reported for a ZECD window that yields a remarkable electrochromic performance in an 80 cm² device, including rapid switching times (3.6 and 2.5 s for the coloration and bleaching processes, respectively), a high optical contrast (67.2%), and an excellent coloration efficiency (131.5 cm² C⁻¹). It is also demonstrated that such ZECDs are perfectly suited for solar‐charging smart windows as they inherently address the solar intermittency issue. These windows can be colored via solar charging during the day, and they can be bleached during the night by supplying electrical energy to electronic devices. The ZECD smart window platform can be scaled to a large area while retaining its excellent electrochromic characteristics. These findings represent a new technology for solar‐charging windows and open new opportunities for the development of next‐generation transparent batteries.

Comprehensive insights into the material, mechanical, and tribological properties of MXene nanolayers are provided. Potential applications and the recent advancements attained in MXene‐reinforced nanocomposites are discussed, starting with the synthesis, fabrication techniques, intricacies of the underlying physics and mechanisms, and finally focusing on the progress in experimental and computational studies.

Abstract

MXenes are recently discovered 2D nanomaterial with superior mechanical, thermal, and tribological properties, being commonly employed in a wide variety of critical research areas, ranging from cancer therapy to energy and environmental applications. Due to their special properties, such as mechanoceramic nature with excellent mechanical performance, thermal stability and rich surface properties, MXenes have tremendous potential as advanced composite structures, especially those based on polymers due to a great affinity between macromolecules and the terminating groups of 2D MXenes. MXenes have been extensively explored in metal matrix nanocomposites as well as in solid‐ or liquid‐based lubrication systems owing to the 2D structure and antifriction characteristics. The purpose of the this paper is to provide a comprehensive insight into the material, mechanical, and tribological properties of the MXene nanolayers with discussions on the recent advancements attained from MXene‐reinforced nanocomposites starting with the synthesis, fabrication techniques, intricacies of the underlying physics and mechanisms, and finally focusing on the progress in computational studies. This analysis of MXene‐based composites will stimulate an emerging field with innumerable opportunities and ample potentials to produce newfangled materials and structures with targeted properties.

The development of the bulk heterojunction, in terms of materials design, device engineering, and the underpinning physical understanding, has led to significant improvements in organic photovoltaics. Looking forward, the bulk heterojunction concept is likely to allow even greater solar cell efficiencies and interestingly, can be applied to other organic electronic applications, such as organic photodetectors and photocatalysts.

Abstract

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction‐based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar‐cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

The performance and industrial viability of organic photovoltaics are strongly influenced by the functionality and stability of the interface layers. In article number https://doi.org/10.1002/adma.2020029732002973, Na Li, Ning Li, Xinghua Wang, Li Nian, and co‐workers present an advanced aqueous solution‐processed cathode interface layer based on cost‐effective organosilica nanodots (OSiNDs). Devices based on the OSiNDs interlayer show improved efficiency and highly enhanced operational stability compared to devices based on the commonly used ZnO interlayer.

Solid‐state ¹⁹F magic angle spinning nuclear magnetic microscopy and elemental mapping are introduced to probe the structures of ternary and quaternary blends. The presence of the individual guest paths minimizes the influence on charge generation and transport of the host system, allowing cooperation of the parallel‐like subcells, producing impressive 17.2% efficiency via a quaternary strategy.

Abstract

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.

To generate cost‐efficient and high‐performanced polymers, a simple chemical steric effect (SE) is introduced to benzothiophene (BDT)‐based side chains. The polymeric crystallinity and miscibility are rebalanced and a power conversion efficiency (PCE) of 14.53% is achieved. Thus, the SE applied in crystalline polymer pave an easier and cheaper route to realize the coordination of low‐cost fabrication and high‐performance.

Abstract

Low synthetic cost and high performance are becoming a new challenge in designing polymer donors for large‐scaled polymer solar cells (PSCs) fabrication; however, complicated synthetic routes and high material costs hamper the widespread commercial application of OPVs. Here, a simple and low‐cost chemical steric effect (SE) is introduced to BDT‐based side chains. Through adjusting alkyl side chains, the polymeric crystallinity and miscibility are rebalanced and subsequently the photovoltaic device based on the meta‐positioned alkyl polymer outperforms its para‐positioned counterpart. The champion device based on the polymer with the meta‐positioned side chains affords a PCE of 14.53% without sacrificing its high fill factor of 0.77, which could be attributed to a more balanced charge‐carrier transport ability and optimized morphology. This is the highest PCE value reported in BTZ based polymer donors to date. Thus, it shows that the SE applied in high crystalline polymer could pave an easier and cheaper chemical route to realize the coordination of low‐cost fabrication and high‐performance.

Nature Energy, Published online: 20 July 2020; doi:10.1038/s41560-020-0653-2

The upscaling of layer treatments and processing that afford high efficiency and stability in small-area perovskite solar cells remains challenging. Liu et al. show how the efficiency and stability of perovskite modules can be improved using an integrated approach to interface and layer engineering.

Nature Energy, Published online: 31 August 2020; doi:10.1038/s41560-020-00684-7

Donor–acceptor systems with low energy-level offset enable high power efficiency in organic solar cells yet it is unclear what drives charge generation. Classen et al. show that long exciton lifetimes enable efficient exciton splitting and thus generation of free charges while also suppressing voltage losses.

A sky‐blue‐exciplex‐forming host constructed by π–donor and π–acceptor with bipolar π‐spacers is used to fabricate single‐emissive‐layer all‐fluorescent white organic light‐emitting diodes, leading to stable warm white emission with a record‐high power efficiency of 69.6 lm W⁻¹ and a long T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m⁻².

Abstract

Exciplex‐forming hosts with thermally activated delayed fluorescence (TADF) provide a viable opportunity to unlock the full potential of the yet‐to‐be improved power efficiencies (PEs) and stabilities of all‐fluorescent white organic light‐emitting diodes (WOLEDs), but this, however, is hindered by the lack of stable blue exciplexes. Here, an advanced exciplex system is proposed by incorporating bipolar charge‐transport π‐spacers into both the electron‐donor (D) and the electron‐accepter (A) to increase their distance for hypsochromic‐shifted emission while maintaining the superior transporting ability. By using spirofluorene as the π‐spacer, 3,3′‐bicarbazole as the D‐unit, and 2,4,6‐triphenyl‐1,3,5‐triazine as the A‐unit, a π–D and π–A exciplex with sky‐blue emission and fast reverse intersystem crossing process is thereof constructed. Combining this exciplex‐forming host, a blue TADF‐sensitizer, and a yellow conventional fluorescent dopant in a single‐emissive‐layer, the fabricated warm‐white‐emissive device simultaneously exhibits a low driving voltage of 3.08 V, an external quantum efficiency of 21.4%, and a remarkable T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m⁻², accompanied by a new benchmark PE of 69.6 lm W⁻¹ among all‐fluorescent WOLEDs.

The relatively large non‐radiative recombination voltage loss (ΔV _non‐rad) is the main challenge for the development of organic solar cells (OSCs). ΔV _non‐rad of OSCs can be effectively reduced to 0.16 V by adopting material combinations that deliver high E _CT (the energy of charge‐transfer state) and low ΔE _CT (energetic difference between singlet excited state and CT state), together with chlorination in donors.

Abstract

Compared with inorganic or perovskite solar cells, the relatively large non‐radiative recombination voltage losses (ΔV _non‐rad) in organic solar cells (OSCs) limit the improvement of the open‐circuit voltage (V _oc). Herein, OSCs are fabricated by adopting two pairs of D–π–A polymers (PBT1‐C/PBT1‐C‐2Cl and PBDB‐T/PBDB‐T‐2Cl) as electron donors and a wide‐bandgap molecule BTA3 as the electron acceptor. In these blends, a charge‐transfer state energy (E _CT) as high as 1.70–1.76 eV is achieved, leading to small energetic differences between the singlet excited states and charge‐transfer states (ΔE _CT ≈ 0.1 eV). In addition, after introducing chlorine atoms into the π‐bridge or the side chain of benzodithiophene (BDT) unit, electroluminescence external quantum efficiencies as high as 1.9 × 10⁻³ and 1.0 × 10⁻³ are realized in OSCs based on PBTI‐C‐2Cl and PBDB‐T‐2Cl, respectively. Their corresponding ΔV _non‐rad are 0.16 and 0.17 V, which are lower than those of OSCs based on the analog polymers without a chlorine atom (0.21 and 0.24 V for PBT1‐C and PBDB‐T, respectively), resulting in high V _oc of 1.3 V. The ΔV _non‐rad of 0.16 V and V _oc of 1.3 V achieved in PBT1‐C‐2Cl:BTA3 OSCs are thought to represent the best values for solution‐processed OSCs reported in the literature so far.

End‐group π−π stacking is proved to be able to effectively reduce the singlet−triplet energy difference in narrow‐bandgap A−D−A acceptors, which is beneficial in simultaneously decreasing the voltage loss in exciton dissociation and suppressing triplet recombination. Furthermore, the absorption spectra can be broadened or redshifted, thus improving the light‐harvesting efficiencies. These results pave the way toward high‐efficiency organic photovoltaics.

Abstract

To improve the power conversion efficiencies for organic solar cells, it is necessary to enhance light absorption and reduce energy loss simultaneously. Both the lowest singlet (S1) and triplet (T1) excited states need to energertically approach the charge‐transfer state to reduce the energy loss in exciton dissociation and by triplet recombination. Meanwhile, the S1 energy needs to be decreased to broaden light absorption. Therefore, it is imperative to reduce the singlet−triplet energy gap (ΔE _ST), particularly for the narrow‐bandgap materials that determine the device T1 energy. Although maximizing intramolecular push−pull effect can drastically decrease ΔE _ST, it inevitably results in weak oscillator strength and light absorption. Herein, large oscillator strength (≈3) and a moderate ΔE _ST (0.4−0.5 eV) are found for state‐of‐the‐art A−D−A small‐molecule acceptors (ITIC, IT‐4F, and Y6) owing to modest push−pull effect. Importantly, end‐group π−π stacking commonly in the films can substantially decrease the S1 energy by nearly 0.1 eV, but the T1 energy is hardly changed. The obtained reduction of ΔE _ST is crucial to effectively suppress triplet recombination and acquire small exciton dissociation driving force. Thus, end‐group π−π stacking is an effective way to achieve both small energy loss and efficient light absorption for high‐efficiency organic photovoltaics.

A tunable dual‐color imaging photodetector with a fast response speed (≈10² ns) is developed by constructing a type‐I p–n heterojunction of CH₃NH₃PbBr₃/CO _i 8DFIC. Dual‐color imaging can be switched to single‐color imaging by applying a small bias voltage.

Abstract

An electrically modulated single‐/dual‐color imaging photodetector with fast response speed is developed based on a small molecule (CO _i 8DFIC)/perovskite (CH₃NH₃PbBr₃) hybrid film. Owing to the type‐I heterojunction, the device can facilely transform dual‐color images to single‐color images by applying a small bias voltage. The photodetector exhibits two distinct cut‐off wavelengths at ≈544 nm (visible region) and ≈920 nm (near‐infrared region), respectively, without any power supply. Its two peak responsivities are 0.16 A W⁻¹ at ≈525 nm and 0.041 A W⁻¹ at ≈860 nm with a fast response speed (≈10² ns). Under 0.6 V bias, the photodetector can operate in a single‐color mode with a peak responsivity of 0.09 A W⁻¹ at ≈475 nm, showing a fast response speed (≈10² ns). A physical model based on band energy theory is developed to illustrate the origin of the tunable single‐/dual‐color photodetection. This work will stimulate new approaches for developing solution‐processed multifunctional photodetectors for imaging photodetection in complex circumstances.

A fused‐ring electron acceptor, FINIC, with fluorination of both end groups and side chains is designed and synthesized, and compared with its nonfluorinated analogue, INIC. FINIC exhibits 3D molecular stacking, exciton transport and charge transport. FINIC‐based organic solar cells yield an efficiency of 14.0%, far exceeding that of the INIC‐based devices (5.1%).

Abstract

A new fluorinated electron acceptor (FINIC) based on 6,6,12,12‐tetrakis(3‐fluoro‐4‐hexylphenyl)‐indacenobis(dithieno[3,2‐b ;2′ ,3′ ‐d ]thiophene) as the electron‐donating central core and 5,6‐difluoro‐3‐(1,1‐dicyanomethylene)‐1‐indanone as the electron‐deficient end groups is rationally designed and synthesized. FINIC shows similar absorption profile in dilute solution to the nonfluorinated analogue INIC. However, compared with INIC, FINIC film shows red‐shifted absorption, down‐shifted frontier molecular orbital energy levels, enhanced crystallinity, and more ordered molecular packing. Single‐crystal structure data show that FINIC molecules pack into closer 3D “network” motif through H‐bonding and π–π interaction, while INIC molecules pack into incompact “honeycomb” motif through only π–π stacking. Theoretical calculations reveal that FINIC has stronger electronic coupling and more molecular interactions than INIC. FINIC has higher electron mobilities in both horizontal and vertical directions than INIC. Moreover, FINIC and INIC support efficient 3D exciton transport. PBD‐SF/FINIC blend has a larger driving force for exciton splitting, more efficient charge transfer and photoinduced charge generation. Finally, the organic solar cells based on PBD‐SF/FINIC blend yield power conversion efficiency of 14.0%, far exceeding that of the PBD‐SF/INIC‐based devices (5.1%).

A novel robot‐based high‐throughput film‐fabrication technique, introduced by Loïc M. Roch, Thomas Heumueller, and co‐workers in article number https://doi.org/10.1002/adma.2019078011907801, enables quaternary semiconductor blends to be fully screened for organic photovoltaic applications. This is realized by drop‐casting 96 individual solutions onto a pre‐structured glass plate, resulting in smooth and homogeneous layers. Moreover, the use of machine‐learning algorithms enables an autonomous operation toward highly efficient optimization routines, leading to a 30‐fold faster rate than conventional high‐throughput approaches.

The active layer morphology of all‐small‐molecule organic solar cells (SM‐OSCs) is tuned by side chain engineering of the donor molecules and thermal annealing (TA) of the devices. An SM‐OSC based on A–D–A‐structured SM1‐F with fluorine and alkyl substituents as the donor and Y6 as the acceptor, and with TA, demonstrates a high power conversion efficiency of 14.07%.

Abstract

It is very important to fine‐tune the nanoscale morphology of donor:acceptor blend active layers for improving the photovoltaic performance of all‐small‐molecule organic solar cells (SM‐OSCs). In this work, two new small molecule donor materials are synthesized with different substituents on their thiophene conjugated side chains, including SM1‐S with alkylthio and SM1‐F with fluorine and alkyl substituents, and the previously reported donor molecule SM1 with an alkyl substituent, for investigating the effect of different conjugated side chains on the molecular aggregation and the photophysical, and photovoltaic properties of the donor molecules. As a result, an SM1‐F‐based SM‐OSC with Y6 as the acceptor, and with thermal annealing (TA) at 120 °C for 10 min, demonstrates the highest power conversion efficiency value of 14.07%, which is one of the best values for SM‐OSCs reported so far. Besides, these results also reveal that different side chains of the small molecules can distinctly influence the crystallinity characteristics and aggregation features, and TA treatment can effectively fine‐tune the phase separation to form suitable donor–acceptor interpenetrating networks, which is beneficial for exciton dissociation and charge transportation, leading to highly efficient photovoltaic performance.