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Three fused-ring electron acceptors (FXIC-1, FXIC-2, and FXIC-3) were designed and synthesized. This FXIC series has similar electron-rich central units and the same electron-poor termini. Due to the different steric structures of fluorene, bifluorenylidene, and spirobifluorene, FXIC-1 is a one-dimensional (1D) crystal, while FXIC-2 and FXIC-3 are multidimensional (MD) amorphous materials. The conformations of the FXIC series have a slight impact on their absorption and energy levels. FXIC-1 has higher electron mobility than FXIC-2 and FXIC-3. When blending with different polymer donors (PTB7-Th, J71, and PM7), the FXIC-1-based organic solar cells have efficiencies higher than those of the FXIC-2/FXIC-3-based cells. Meanwhile, the ternary-blend cells based on PTB7-Th:F8IC with FXIC-1, FXIC-2, and FXIC-3 show similar efficiencies, which are all better than those of the binary-blend devices.

The performance of tin-based perovskite solar cells has been substantially hampered by voltage loss caused by energy level mismatch, charge recombination, energetic disorder, and other issues. Here, a fused-ring electron acceptor based on indacenodithiophene (IDIC) was for the first time introduced as a transition layer between a tin-based perovskite layer and a C60 electron transport layer, leading to better matched energy levels in the device. In addition, coordination interactions between IDIC and perovskite improved the latter's crystallinity. The introduction of IDIC raised the power conversion efficiency from 8.98% to 11.5% and improved the device's stability. The decomposition mechanism of tin-based perovskite was also revealed by detecting the optical properties of perovskite microdomains through innovative integration of confocal laser scanning microscopy and photoluminescence spectroscopy.

Comprehensive Summary: We propose a strategy to improve performance of unidirectionally extended fused‐ring electron acceptors by using pyrrolo[3,2‐b]pyrrole to replace pyrrole ring, and design two asymmetric nonfullerene acceptors 1PIC and 2PIC. Replacing pyrrole in 1PIC with pyrrolo[3,2‐b]pyrrole remarkably red‐shifts absorption peak by 109 nm, elevates the HOMO and LUMO energy levels, and improves electron mobility. The photovoltaic devices based on blend of PM6 donor and 2PIC acceptor exhibit power conversion efficiency as high as 12.6%, which is much higher than that of PM6:1PIC (3.53%), due to more efficient exciton generation and dissociation, faster and more balanced carrier transport and less charge recombination. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: We propose a strategy to improve performance of unidirectionally extended fused‐ring electron acceptors by using pyrrolo[3,2‐b]pyrrole to replace pyrrole ring, and design two asymmetric nonfullerene acceptors 1PIC and 2PIC. Replacing pyrrole in 1PIC with pyrrolo[3,2‐b]pyrrole remarkably red‐shifts absorption peak by 109 nm, elevates the HOMO and LUMO energy levels, and improves electron mobility. The photovoltaic devices based on blend of PM6 donor and 2PIC acceptor exhibit power conversion efficiency as high as 12.6%, which is much higher than that of PM6:1PIC (3.53%), due to more efficient exciton generation and dissociation, faster and more balanced carrier transport and less charge recombination. [ABSTRACT FROM AUTHOR]

We synthesize and compare four near-infrared absorbing fused-ring electron acceptors named as nTTIC (n=2, 3, 4, and 5), based on different number of thieno[3,2-b]thiophene (TT) unit as the electron-donating core. With increasing the TT unit, absorption spectrum of the TTIC series red shifts, and the highest occupied molecular orbital (HOMO) upshifts notably. It is worth noting that 4TTIC and 5TTIC exhibit absorption edges approaching 1100 nm, which is the photoresponse limit of solar cells based on crystal silicon. When the TTIC series acceptors are blended with polymer donor PM6, the binary-blend organic solar cells based on 3TTIC show the best power conversion efficiency (PCE) of 13.1%. In contrast, 2TTIC-based devices exhibit relatively lower PCE of 8.32%, mainly caused by the larger energy loss and blue-shifted absorption. Due to insufficient driving force of charge separation caused by very high HOMO, 4TTIC and 5TTIC show poor PCEs lower than 3%. [ABSTRACT FROM AUTHOR]

Two kinds of nitrogen-bridged star-shaped fused-ring electron acceptors based on benzo[1,2-b:3,4-b’-5,6-b’’]tri(4-H-dithieno[3,2-b:2’,3’-d]pyrrole) (BTDTP) core with linear and branched alkyl side chains on bridging nitrogen atoms are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption bands at 500–800 nm (molar extinction coefficient of 3.0 × 105 M −1 cm−1) with narrow optical bandgaps around 1.58 eV, but reveal different molecular packing and morphology. Compared to acceptor with linear side chains, the acceptor bearing branched side chains exhibits ordered face-on orientation with tight π-π stacking, as well as more favorable phase separation morphology and balanced charge transport when blended with polymer donor PM6. Organic solar cell device based on acceptors bearing branched side chains achieves a power conversion efficiency (PCE) of 9.95% with a fill factor of 74.98% and a short-circuit current density of 14.90 mA cm−2, which is much superior to the device using acceptors bearing linear side chains (a PCE of 6.74%).

Abstract A new fused‐ring electron acceptor FNIC3 with dynamics controlled aggregation behavior was synthesized. FNIC3 shows strong absorption in 600–900 nm, HOMO/LUMO energy levels of −5.59/−4.04 eV, and electron mobility of 1.2 × 10−3 cm2 V−1 s−1. The aggregation of FNIC3 shows strong dependency on film formation time. Prolongation of film formation time promotes the crystallization of FNIC3, leading to improved crystallinity and enlarged aggregate sizes. Aggregation of FNIC3 significantly influences the photovoltaic device parameters. Appropriate aggregation red‐shifts the absorption and improves the mobilities of the blend, which contributes to high photocurrent and fill factor thus high power conversion efficiency (PCE). Overaggregation leads to increased nonradiative energy loss and insufficient charge generation, resulting in decreased open‐circuit voltage and short‐circuit current density. The blends based on PM6:FNIC3 fabricated under proper film formation time exhibit a PCE of 12.3%, higher than those fabricated under short and long film formation time (10.0–10.5%).

To achieve high performance organic photovoltaic hydrogen-evolution (OPH), it is crucial to extend the exciton lifetime of organic photovoltaic catalysis. This is primarily because it increases the exciton diffusion length. In this work, a new asymmetric organic photovoltaic catalysis (BTP-eC9-B4F) has been developed, which achieves a longer exciton lifetime of 1.25 ns and a higher fluorescence quantum yield of 9.4 % compared to symmetric structures. This has been made possible by using asymmetric end groups. Experimental results demonstrate that the photocatalyst with a longer exciton lifetime exhibits a more efficient average hydrogen evolution rate of 121.57 mmol h−1 g−1 under AM 1.5G, 100 mW cm−2 for 5 h. Therefore, extending the lifetime of excitons has been shown to be an effective approach to achieve high efficiency OPH. High performance organic photocatalytic hydrogen evolution is realized by prolonging exciton lifetime and enhancing exciton transport. [Display omitted] • The asymmetric non-fullerene electron acceptor BTP-eC9-B4F was synthesized. • A n effect of 121.57 mmol h−1 g−1 in photocatalytic hydrogen evolution was achieved. • BTP-eC9-B4F showed long exciton lifetime of 1.25 ns? [ABSTRACT FROM AUTHOR]

Asymmetric molecular design strategy has received increasing attention in the development of fused-ring electron acceptors (FREAs). Benzo [2,1-b: 3,4-b': 5,6-b"] trithiophene (asym-BTT) with intrinsically asymmetric and electron-rich π-plane is an ideal unit to construct asymmetric FREAs, whereas asym-BTT-based FREAs are still very limited. Herein, two asymmetric FREAs based on the π-expended asym-BTT fused-core with para -hexylphenyl and meta -hexylphenyl side chains (p-FABTT and m-FABTT) have been developed. Owing to strong electron-donating ability and planar configuration of π-expended asym-BTT fused-core, two acceptors exhibit effective intramolecular charge transfer and intermolecular interaction, leading to near infrared absorption above 900 nm with narrow bandgaps of around 1.34 eV. Although the same π-conjugated backbones of two acceptors afford the nearly identical energy levels and solution absorptions, side chain isomerization leads to significant differences in film absorption, molecular stacking orientation and crystallinity. In neat film, the m-FABTT with meta -hexylphenyl side chains exhibits slightly redshifted absorption with stronger 0-0 absorption peak from J-aggregation compared to p-FABTT with para -hexylphenyl side chains. More orderly molecular stacking and stronger π-π interactions with shorter π-π stacking distance in both the neat film and active layer can be observed for m-FABTT. When blended with donor polymer D18, m-FABTT exhibits broadened spectral region and proper phase separation morphology for effective exciton dissociation and charge transport. Therefore, the OSCs device based on m-FABTT achieves an optimal PCE of 7.14 % with a higher J SC of 15.39 mA cm−2 and an improved FF of 57.96 % as comparing with p-FABTT -based device (a PCE of 4.81 % with a J SC of 11.67 mA cm−2 and a FF of 48.51 %). [Display omitted] • Two FREAs based on π-expended asymmetric benzotrithiophene core (p-FABTT and m-FABTT) are synthesized. • The acceptors achieve NIR absorption above 900 nm with low bandgaps of around 1.34 eV. • The aggregation and photovoltaic performance of two acceptors are regulated by side chain isomerization. • m-FABTT with meta- hexylphenyl side chains display stronger J-aggregation, higher crystallinity and improved PCE. [ABSTRACT FROM AUTHOR]

A series of fused-ring electron acceptors with indacenodithiophene as core, 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile as end groups, thiophene with different side chains as π bridge are designed, synthesized and applied in non-fullerene organic solar cells (OSCs). The effects of alkyl side chains on absorption, energy level, molecular packing, domain size, domain purity, charge transport and photovoltaic performance are systematically investigated. Via the steric hindrance effect, increase in carbon number of side chains slightly downshifts HOMO energy levels, upshifts LUMO energy levels and thereby increases open circuit voltage from 0.84 to 0.95 V gradually. Short circuit current, fill factor and efficiency vary non-monotonically with aliphatic carbon number of alkyl chains and reach maximum value simultaneously when carbon number is six. [ABSTRACT FROM AUTHOR]

A fused-ring electron acceptor IDT-2BR1 based on indacenodithiophene core with hexyl side-chains flanked by benzothiadiazole rhodanine was designed and synthesized. In comparison with its counterpart with hexylphenyl side-chains (IDT-2BR), IDT-2BR1 exhibits higher highest occupied molecular orbital (HOMO) energy but similar lowest unoccupied molecular orbital (LUMO) energy (IDT-2BR1: HOMO=−5.37 eV, LUMO=−3.67 eV; IDT-2BR: HOMO=−5.52 eV, LUMO=−3.69 eV), red-shifted absorption and narrower bandgap. IDT-2BR1 has higher electron mobility (2.2×10 cm V s) than IDT-2BR (3.4×10 cm V s) due to the reduced steric hindrance and ordered molecular packing. Fullerene-free organic solar cells based on PTB7-Th:IDT-2BR1 yield power conversion efficiencies up to 8.7%, higher than that of PTB7-Th:IDT-2BR (7.7%), with a high open circuit voltage of 0.95 V and good device stability. [ABSTRACT FROM AUTHOR]

Surface defect modulation utilizing Fused-ring electron acceptor molecules achieved a dramatically enhanced open-circuit voltage from 1.079 V to 1.278 V and a record efficiency up to 3.31% for double Cs 2 AgBiBr 6 perovskite solar cells. [Display omitted] • Fused-ring acceptors were used to passivate Cs 2 AgBiBr 6 perovskite. • Nonradiative recombination was suppressed by passivating Ag-exposed surface defects. • The modified double PSCs achieved a record efficiency of 3.31%. • Excellent long-term light-heat stabilities were achieved, with > 97% PCE sustained. Lead-free double perovskites have drawn increasing attention for addressing the stability and toxicity challenges from lead-based halide perovskites. However, their power conversion efficiencies (PCE) are still far behind that of Pb-based perovskite solar cells (PSCs) mainly because of the severe energy loss. Herein, we successfully employed the fused-ring electron acceptor (FREA) molecules to passivate defects in Cs 2 AgBiBr 6 -based double PSCs to realize a dramatically enhanced open-circuit voltage (V OC) from 1.079 V to 1.278 V and a champion PCE up to 3.31%, which is the highest efficiency for double PSCs to date. The strong binding of C≡N and N=C–S groups on FREA with Ag-exposed surface of Cs 2 AgBiBr 6 effectively decreased surface trap densities and considerably suppressed non-radiative recombination. Moreover, the passivated devices showed superior long-term stability, which maintained 98.5% and 97.2% of the initial efficiency under continuous AM 1.5 G illumination and 85 ℃ heating for 300 h, respectively. This work manifests the importance of the rational design of functional passivation molecules to improve the performance and stability of double PSCs. [ABSTRACT FROM AUTHOR]

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport., (© 2022 Wiley-VCH GmbH.)

The terminal electron-withdrawing group is thought to strongly influence π-π stacking interactions and thereby the charge transport properties of fused-ring electron acceptors (FREAs). In this work, we designed and synthesized three asymmetric/symmetric small molecule FREAs ( LC301 , LC302 , and LC303 ), in which three electron-withdrawing functional groups with different polarities (phenyl-fused indanone < thienyl-fused indanone < F-modified phenyl-fused indanone) were selected as the terminal groups. Photophysical properties, electrochemistry, charge transport, and crystalline properties of the materials were studied to investigate the effect of electron-withdrawing abilities of the terminal groups on the properties of FREAs. Starting from the symmetric LC302 (two thienyl-fused indanone terminal groups), we have found that by simply replacing only one thienyl-fused indanone terminal group in symmetric LC302 with one difluorinated phenyl-fused indanone terminal group, the asymmetric LC301 -based organic solar cells (OSCs) can yield a high power conversion efficiency (PCE) of 17.21% and a promising fill factor (FF) of 78.1%, which are higher than those of symmetric LC302 -based OSCs (PCE = 15.18%, FF = 73.3%) and the asymmetric LC303 -based OSCs bearing one thienyl-fused indanone terminal group and one nonfluorinated phenyl-fused indanone terminal group (PCE = 14.28%, FF = 70.3%). The results indicate that the rational selection of terminal groups with different electron-withdrawing capabilities is highly desirable for designing high-performance asymmetric FREAs.

We present the first report incorporating strongly electron-donating moiety pyrrolo[3,2- b ]pyrrole into fused-ring electron acceptors, and design and synthesize two narrow-bandgap nonfullerene acceptor isomers PPIC1 and PPIC2. These two molecules bear the same electron-withdrawing terminal groups and isomeric eight-fused-ring cores. The two compounds exhibit strong light-harvesting ability in the near-infrared region beyond 1000 nm. Compared with the symmetric PPIC2, the asymmetrical PPIC1 shows remarkably blue-shifted absorption, wider bandgap, down-shifted highest occupied molecular orbital, up-shifted lowest unoccupied molecular orbital, and lower electron mobility. Paired with the classical polymer donor PTB7-Th, PPIC1-based photovoltaic devices exhibit higher open-circuit voltage, short-circuit current and efficiency than the PPIC2-based devices. [Display omitted] • Pyrrolo[3,2- b ]pyrrole-based fused-ring electron acceptor isomers PPIC1 and PPIC2 are designed. • Two isomers exhibit strong near-infrared absorption beyond 1000 nm. • The asymmetrical PPIC1 shows better photovoltaic performance than the symmetric PPIC2. [ABSTRACT FROM AUTHOR]

Perovskite solar cell (PSC) has attracted great attention due to its high power conversion efficiency (PCE), low cost and solution processability. The well-designed interface and the modification of electron transport layer (ETL) are critical to the PCE and long-term stability of PSCs. In this article, a fused-ring electron acceptor is employed as the interfacial material between TiO 2 and the perovskite in rigid and flexible PSCs. The modification improves the surface of TiO 2 , which decreases the defects of ETL surface. Moreover, the modified surface has lower hydrophilicity, and thus is beneficial to the growth of perovskite with large grain size and high quality. As a result, the interfacial charge transfer is promoted and the interfacial charge recombination can be suppressed. The highest PCE of 19.61% is achieved for the rigid PSCs after the introduction of ITIC, and the hysteresis effect is significantly reduced. Flexible PSC with ITIC obtains a PCE of 14.87%, and the device stability is greatly improved. This study provides an efficient candidate as the interfacial modifier for PSCs, which is compatible with low-temperature solution process and has a great practical potential for the commercialization of PSCs. [Display omitted] • A fused-ring electron acceptor ITIC is employed as the interfacial material for rigid and flexible perovskite solar cells. • ITIC not only suppresses the surface defects of TiO 2 film but also promotes the growth of perovskite crystals. • ITIC improves the electron extraction and reduces the charge recombination which contributes to the performance of PSCs. • The highest power conversion efficiency of 19.61% is achieved for the rigid PSCs after the introduction of ITIC. • Flexible PSC with ITIC obtains a power conversion efficiency of 14.87%, and the device stability is greatly improved. [ABSTRACT FROM AUTHOR]

[Display omitted] • Efficient PBDB-T ternary solar cells using two alloy-forming acceptors with distinct structural orders. • Förster resonance energy transfer between two acceptors; • Optimal phase-separation and structural order achieved with less crystalline third component; • A champion PCE of 14.55% for PBDB-T single-junction solar cells. Ternary organic solar cells (OSCs) have demonstrated great potential in boosting the power conversion efficiencies (PCEs) of single-junction devices. Screening suitable third component is crucial but challenging for different host binary OSCs. We report herein an efficient ternary strategy in achieving highly efficient PBDB-T based devices by incorporating two alloy-forming fused-ring electron acceptors (FREAs) with distinct structural orders. INPIC-Si as the third component showcases a relatively narrower bandgap and higher energy levels than the host acceptor IDTO-T-4F. Featuring good compatibility and alloy-forming blends, two FREAs generate Förster resonance energy transfer from IDTO-T-4F to INPIC-Si, facilitating efficient exciton dissociation and charge carrier transport. Importantly, the low crystalline INPIC-Si decreases the strong aggregation of PBDB‐T:IDTO-T-4F blends for optimal phase-separation and structural order. Consequently, the best PBDB‐T:IDTO-T-4F:INPIC-Si ternary OSCs deliver a champion PCE of 14.55% with simultaneously improved device parameters over binary ones. To the best of our knowledge, this performance is among the highest for PBDB-T-based ternary OSCs in literature. Our work demonstrates the incorporation of a narrow bandgap FREA with low structural order into heavily aggregated binary blends can optimize the blend morphology for reduced energy loss and significantly improved photovoltaic performance in ternary OSCs. [ABSTRACT FROM AUTHOR]

Asymmetric fused-ring electron acceptors (a-FREAs) have proved to be a promising type of electron acceptor for high-performance organic solar cells (OSCs). However, the relationship among molecular structures of a-FREAs and their nanoscale morphology, charge-carrier dynamics, and device performance remains unclear. In this contribution, two FREAs differing in conjugated backbone geometry with an asymmetric conformation (IPT-2F) or symmetric one (INPIC-2F) are selected to systematically explore the superiorities of the asymmetric conformation. Despite the frailer extinction coefficient and weaker crystallinity, IPT-2F shows stronger dipole interactions in the asymmetrical backbone, which would induce a closer lamellar packing than that of the symmetrical counterpart. Using PBDB-T as the electron donor, the IPT-2F-based OSCs achieve the best power conversion efficiency of 14.0%, which is ca. 67% improvement compared to the INPIC-2F-based ones (8.37%), resulting from a simultaneously increased short-circuited current density ( J sc ) and fill factor. Systematical investigations on optoelectronic and morphological properties show that the asymmetric conformation-structured IPT-2F exhibits better miscibility with the polymer donor to induce a favorable blend ordering with small domain sizes and suitable phase separation compared to the INPIC-2F symmetric molecule. This facilitates an efficient charge generation and transport, inhibits charge-carrier recombination, and promotes valid charge extraction in IPT-2F-based devices.

The ternary strategy has been widely used in high-efficiency organic solar cells (OSCs). Herein, we successfully incorporated a mid-band-gap star-shaped acceptor, FBTIC, as the third component into the PM6/Y6 binary blend film, which not only achieved a panchromatic absorption but also significantly improved the open-circuit voltage ( V OC ) of the devices due to the high-lying lowest unoccupied molecular orbital (LUMO) of the FBTIC. Morphology characterizations show that star-shaped FBTIC molecules are amorphously distributed in the ternary system, and the finely tuned ternary film morphology facilitates the exciton dissociation and charge collection in ternary devices. As a result, the best PM6/Y6/FBTIC-based ternary OSCs achieved a power conversion efficiency (PCE) of 16.7% at a weight ratio of 1.0:1.0:0.2.

Selecting suitable outermost aromatic rings of the central cores is of particular importance for the design of fused-ring electron acceptors (FREAs) because the direct electronic communication between the outermost aromatic rings and termini exerts a strong impact on the optoelectronic properties of FREAs. In most cases, the outermost rings of the FREA cores are thiophene. This work reported the first example of using pyrrole as the outermost rings of the core. Fused hexacyclic electron acceptor, P6IC, using pyrrole in place of the often-used thiophene as the outermost rings of the central core was synthesized. Compared with its structural analogue F6IC with thiophene as the outermost rings, P6IC exhibits a remarkably upshifted highest occupied molecular orbital energy level (P6IC: -5.43 eV, F6IC: -5.71 eV), a slightly upshifted lowest unoccupied molecular orbital energy level (P6IC: -3.94 eV, F6IC: -4.00 eV), 54 nm red-shifted absorption, a narrower band gap (P6IC: 1.30 eV, F6IC: 1.37 eV), and an enhanced mobility (P6IC: 8.8 × 10 -4 cm 2 V -1 s -1 , F6IC: 7.4 × 10 -4 cm 2 V -1 s -1 ). Organic photovoltaic cells using PTB7-Th/P6IC as a photoactive layer exhibit an efficiency of 12.2%, far surpassing that based on PTB7-Th/F6IC active layer (5.57%). The semitransparent devices using PTB7-Th/P6IC as the active layer yield efficiency of 10.2% with an average visible transmittance (AVT) of 17.0%, far surpassing that based on PTB7-Th/F6IC (5.26% with an AVT of 18.4%).

A new fused-chrysene electron-donating core is synthesized, where chrysene is condensed with two thiophenes via two dihydrobenzene rings. Based on this building block coupled with two electron-accepting end groups of 1,1-dicyanomethylene-3-indanone, a new Z-shaped fused-ring electron acceptor, FCIC, is designed and synthesized. FCIC shows intense absorption in the 500-850 nm region, with a maximum molar absorptivity of 1.5 × 10 5 M -1 cm -1 , a bandgap of 1.50 eV, and a charge mobility of 2.5 × 10 -4 cm 2 V -1 s -1 . The ternary organic photovoltaic cells based on PTB7-Th/F8IC/FCIC yield an efficiency of 12.6%, higher than that of the binary cells of PTB7-Th/F8IC (10.7%) and PTB7-Th/FCIC (7.21%). Relative to the PTB7-Th/F8IC binary blend, the addition of FCIC leads to improvement in the open-circuit voltage, short-circuit current, and fill factor.

In order to utilize the near-infrared (NIR) solar photons like silicon-based solar cells, extensive research efforts have been devoted to the development of organic donor and acceptor materials with strong NIR absorption. However, single-junction organic solar cells (OSCs) with photoresponse extending into >1000 nm and power conversion efficiency (PCE) >11% have rarely been reported. Herein, three fused-ring electron acceptors with varying core size are reported. These three molecules exhibit strong absorption from 600 to 1000 nm and high electron mobility (>1 × 10 -3 cm 2 V -1 s -1 ). It is proposed that core engineering is a promising approach to elevate energy levels, enhance absorption and electron mobility, and finally achieve high device performance. This approach can maximize both short-circuit current density (  J SC ) and open-circuit voltage (V OC ) at the same time, differing from the commonly used end group engineering that is generally unable to realize simultaneous enhancement in both V OC and J SC . Finally, the single-junction OSCs based on these acceptors in combination with the widely polymer donor PTB7-Th yield J SC as high as 26.00 mA cm -2 and PCE as high as 12.3%., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Much effort has been devoted to the development of new donor materials for small-molecule organic solar cells due to their inherent advantages of well-defined molecular weight, easy purification, and good reproducibility in photovoltaic performance. Herein, we report two small-molecule donors that are compatible with both fullerene and nonfullerene acceptors. Both molecules consist of an (E)-1,2-di(thiophen-2-yl)ethane-substituted (TVT-substituted) benzo[1,2-b:4,5-b']dithiophene (BDT) as the central unit, and two rhodanine units as the terminal electron-withdrawing groups. The central units are modified with either alkyl side chains (DRBDT-TVT) or alkylthio side chains (DRBDT-STVT). Both molecules exhibit a medium bandgap with complementary absorption and proper energy level offset with typical acceptors like PC 71 BM and IDIC. The optimized devices show a decent power conversion efficiency (PCE) of 6.87% for small-molecule organic solar cells and 6.63% for nonfullerene all small-molecule organic solar cells. Our results reveal that rationally designed medium-bandgap small-molecule donors can be applied in high-performance small-molecule organic solar cells with different types of acceptors.