Indacenodithiophene Research Articles

Controlling the Cyano‐Containing A2 Segments in A2‐A1‐D‐A1‐A2 Type Non‐Fullerene Acceptors to Combine with a Benzotriazole‐Based p‐Type Polymer: “Same‐Acceptor‐Strategy” for High VOC Organic Solar Cells

To achieve efficient organic solar cells (OSCs), the design of promising non‐fullerene small molecular acceptors (SMAs) is crucially important and the relationship between the chemical structure and optoelectronic properties needs to be further investigated. Herein, an A2‐A1‐D‐A1‐A2 molecular skeleton is adopted to study the effect of end‐capped A2 groups containing different numbers of cyano units, where D and A1 are fixed as indacenodithiophene (IDT) and benzotriazole (BTA) units, respectively. Utilizing the “same‐acceptor‐strategy,” three BTA‐based SMAs, named as BTA701, BTA3, and BTA703, are paired with a BTA‐based p‐type polymer J71. The open‐circuit voltage (VOC) gradually decreases with the enhancement of electron‐accepting ability of terminal A2 units, from 1.32 V (BTA701) to 1.20 V (BTA3) and to 0.85 V (BTA703). The device J71:BTA3 eventually shows the best power conversion efficiency (PCE) of 8.60% with a VOC up to 1.2 V because of the complementary light absorption, high and balanced hole and electron mobility, suitable phase separation, and crystallinity. This study indicates that appropriate cyano‐containing units in BTA‐based SMAs can effectively modulate the absorption, energy levels, charge mobility and surface free energy, which can provide valuable insights to the further design of SMAs. In addition, these results prove that the “same‐acceptor‐strategy” is simple and effective to realize a VOC as high as 1.2V.

Monolithic nonlinear optical chromophores with extended conjugate bridge: Large refractive index, high thermal and electro-optic stability

Monolithic electro-optic (EO) molecular glasses are promising materials for optical waveguide device owing to their high chromophore loading density, high refractive indices and large EO coefficients (r33 values) for the optimization of in-device figure of merit (n3r33). However, the stability of EO molecular glasses including the poling induced r33 stability, chemical stability under both high temperature and high voltage in poling is still an obstruction for their real application. Herein, we used the chemical structural optimization, by means of introducing a large conjugated electron-bridge indacenodithiophene (IDT), to synthesize a monolithic EO molecular glass IDTC. Comprehensive properties of excellent glass-forming ability, high thermal stability (Td = 315 °C), and high glass transition temperature (Tg = 137 °C) was achieved in monolithic IDTC thin films. More significantly, refractive index of monolithic film was exceptionally high (n = 1.8294 @ 1310 nm and n = 1.7645 @1550 nm). The poled IDTC film exhibited the r33 value of 87.6 pm/V with large figure of merit (n3r33: 536 pm/V @1310 nm), and the EO activity can be retained over 85% of the initial value after 300 h annealing at 85 °C. Meanwhile, it was found that there is no decomposition of IDTC in poled films after high temperature and high voltage poling as well as long term annealing. The poled chromophores are recyclable EO materials for the sustainable application in low-cost EO devices.

Controlling Molecular Packing and Orientation via Constructing a Ladder-Type Electron Acceptor with Asymmetric Substituents for Thick-Film Nonfullerene Solar Cells.

A nonfullerene acceptor, IDTT-OB, employing indacenodithieno[3,2- b]thiophene (IDTT) decorated with asymmetric substituents as the core, is designedly prepared. In comparison with the analogue IDT-OB, extending the five-heterocyclic indacenodithiophene (IDT) core to seven-heterocyclic fused ring endows IDTT-OB with more broad absorption and elevated highest occupied molecular orbital energy level. In addition, IDTT-OB shows a more intense molecular packing and a higher crystalline behavior with a strong face-on orientation in the neat film and the PBDB-T:IDTT-OB blend film. Furthermore, an ideal nanomorphology with a domain size of 19 nm can be obtained, which is in favor of exciton diffusion and charge separation. Accordingly, PBDB-T:IDTT-OB-based polymer solar cells demonstrate a maximum power conversion efficiency (PCEmax) of 11.19% with an impressive fill factor of 0.74, comparable to the state-of-the-art acceptors with similar molecular backbones. More importantly, IDTT-OB-based devices show good tolerance to the film thickness, which maintain a high PCE of 10.20% with a 250 nm thick active layer, demonstrating that the asymmetric acceptor is profound for fabricating high-efficiency thick-film nonfullerene solar cells.

Angular/linear-shaped indacenodithiophene (IDT) for donor-acceptor copolymers: Geometric shape effects on physical properties and photovoltaic performance

Angular-indacenodithiophene (a-IDT) as the analogue of linear-IDT (l-IDT), in which the central phenyl ring was linked on the β-position of the thiophene ring but fused on its α-position, was designed and synthesized through intramolecular annulation. This designed dibrominated a-IDT monomer was copolymerized with 4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) as the acceptor unit through Stille coupling reaction to furnish an alternating D-A conjugated polymer Pa-IDTDTBT. Similarly, l-IDT unit and its D-A conjugated polymer Pl-DTDTBT were also synthesized for comparison. The geometric shape of l-IDT and a-IDT subunits have tremendous influence on the conjugated backbone curvature for Pl-IDTDTBT and Pa-IDTDTBT, mainly reflects in changes observed in their photoelectric properties, hole mobilities, film morphology and photovoltaic performance. Theoretical calculation concludes that Pl-IDTDTBT possesses a more linear conjugated backbone and hence stronger intermolecular π–π interactions, leading to more red-shifted absorption spectra of Pl-IDTDTBT in the solution as well as the thin film relative to those of Pa-IDTDTBT. XRD measurements demonstrated that Pl-IDTDTBT has more ordered π-stacking than Pa-IDTDTBT counterpart as a result of the less curvature of the Pl-IDTDTBT conjugated backbone. BHJ PSCs device based on Pl-IDTDTBT/PC71BM achieved a PCE of 5.34%, is higher than Pa-IDTDTBT (3.64%). As expected, Pa-IDTDBT with the more conjugated backbone curvature showed a high Voc of 1.02 V owing to its deeper HOMO energy level (−5.47 eV) relative to Pl-IDTDTBT, which is one of the highest Voc values of IDT-based polymers. Our work demonstrates that the geometric shape of subunits should be considered in designing the novel photovoltaic polymers and other organic optoelectronic materials.

High‐Performance Eight‐Membered Indacenodithiophene‐Based Asymmetric A‐D‐A Type Non‐Fullerene Acceptors

Indacenodithiophene (IDT) has been widely used as the central core to design high‐performance acceptor‐donor‐acceptor (A‐D‐A)‐type non‐fullerene acceptors (NFAs). NFAs based on five‐, six‐, seven‐, and nine‐membered IDT have been successfully prepared. However, less research attention has been paid to the eight‐membered IDT derivative. In this study, a novel asymmetric TTPTTT building block as an eight‐membered IDT unit are designed and synthesized. The effect of nonfluorinated, monofluorinated, and difluorinated end‐capping groups on the photovoltaic performance of TTPTTT‐based NFAs are specifically investigated. By blending with the polymer donor PBT1‐C, organic solar cells (OSCs) based on TTPTTT‐IC, TTPTTT‐2F, and TTPTTT‐4F exhibited power conversion efficiencies (PCEs) of 7.91, 11.52, and 12.05%, respectively. Our results indicate that the asymmetric TTPTTT building block as an eight‐membered IDT derivative is a promising central core unit for designing high‐performance A‐D‐A type NFAs.

Effect on photovoltaic performance of D–A–π–A motif polymer dye sensitizers by adopting the complex of metal with diamine as auxiliary electron acceptor

Donor–acceptor–π bridge–acceptor (D–A–π–A) dyes are promising sensitizers in dye-sensitized solar cells (DSSCs). With the aim to obtain high conversion efficiencies solar cells, three novel D–A–π–A efficient sensitizers coded PIDT–NCo, PIDT–NZn and PIDT–NCd had been synthesized and applied to DSSCs. These sensitizers adopting Co(II), Zn(II) or Cd(II) complexes as auxiliary electron acceptors (A), indacenodithiophene (IDT) derivatives as electron donor (D) and 8-quinolinol derivative as π bridge and acceptor (A). The results of photovoltaic test indicated that the three polymeric metal complex dye sensitizers have relatively higher performance among polymer dye sensitizers, and the device sensitized by PIDT–NCd shows the highest power conversion efficiency of 6.43% under simulate AM 1.5 G solar irradiation. Besides all dyes exhibited excellent thermal stability (Td > 320 °C). The research data have potential values for the development of coordination chemistry and provided a brand new idea to design efficient polymer dyes for DSSCs.

End-chain effects of non-fullerene acceptors on polymer solar cells

Abstract Four acceptor1-acceptor2-donor-acceptor2-acceptor1 (A1-A2-D-A2-A1) structural electron acceptors with different end-chains were designed and synthesized which all possessed indacenodithiophene (IDT) core, benzothiadiazole (BT) bridge as acceptor2, and rhodanine (R) end groups as acceptor1. The non-fullerene acceptor attached with ethyl group is called IDT-BT-R2 and used as control compound. And the other three of them are attached with methoxymethyl, trifluoroethyl and 1-piperidino groups generating IDT-BT-RO, IDT-BT-RF3 and IDT-BT-RN, respectively. The influence of end-chains on their optoelectronic properties were compared between four non-fullerene acceptors. Compared with IDT-BT-R2, the molecule IDT-BT-RF3 show red-shifted light absorption and lower LUMO level because of the electron withdrawing property of fluorine atoms. OSCs based on IDT-BT-RF3 display more efficient charge separation and lower degree of monomolecular recombination, allowing OSCs to show higher short-circuit current (Jsc) than the system of IDT-BT-R2. OSCs based on IDT-BT-RO also show more efficient charge separation and less monomolecular recombination. Due to the elevated LUMO level of the acceptor IDT-BT-RN, organic solar cells (OSCs) utilizing this material as acceptor display high open-circuit voltage (Voc) of 1.10 eV and low energy loss of 0.49 eV when maintaining a relatively high power conversion efficiency (PCE) of 7.09%. We demonstrated that the end-chain engineering could finely tune the light absorption properties and energy levels of novel non-fullerene acceptors and eventually improved OSCs performance can be harvested.

The regulation of π-bridge of indacenodithiophene-based donor-π-acceptor conjugated polymers toward efficient polymer solar cells

The instinct feature of π-bridges of donor-π-acceptor (D-π-A) conjugated polymers plays a critical role in tailoring the polymer basic properties as well as the photovoltaic performance. In this work, we designed two new D-π-A polymers PSeBDDIDT and PTzBDDIDT based on donor unit indacenodithiophene (IDT), acceptor unit benzo[1,2-c:4,5-c’]dithiophene-4,8-dione (BDD) with varying π-conjugated bridges of selenophene and thiazole units. Compared to the counterpart polymer PThBDDIDT with thiophene π-bridge, polymers PSeBDDIDT and PTzBDDIDT exhibited enhanced intermolecular interactions, expanded absorption range and better molecular planarity arising from the suppressed steric hindrance between the respective bridge and the adjacent BDD moiety. Conventional polymer solar cells were fabricated with PC71BM as the acceptor. The results witness a greatly improved efficiency for PSeBDDIDT based solar cell compared to PThBDDIDT based counterpart device (7.04%). As a result, the champion PCE of 8.65% was recorded with VOC of 0.899 V, JSC of 16.04 mA cm−2 and FF of 0.60. It is worth highlighting that this is the highest efficiency among all IDT-based polymeric donors reported till now. As a contrast, the devices fabricated from PTzBDDIDT demonstrated a low efficiency of 2.75%, with increased VOC of 0.968 V, JSC of 8.12 mA cm−2 and FF of 0.35. The inferior photocurrent and fill factor of PTzBDDIDT based device was in accord with the poor exciton dissociation probability (38%) and low hole and electron charge transport inside the polymer solar cell. In conclusion, this work demonstrates a state of the art IDT-based polymeric donor with selenophene π-bridge and the importance of rational regulation of π-bridges on polymer properties and photovoltaic efficiency of polymer solar cells.

Electron-Affinity-Triggered Variations on the Optical and Electrical Properties of Dye Molecules Enabling Highly Efficient Dye-Sensitized Solar Cells.

The synthesis, characterization, and photovoltaic performance of a series of indacenodithiophene (IDT)-based D-π-A organic dyes with varying electron-accepting units is presented. By control of the electron affinity, perfectly matching energy levels were achieved with a copper(I/II)-based redox electrolyte, reaching a high open-circuit voltage (>1.1 V) while harvesting a large fraction of solar photons at the same time. Besides achieving high power conversion efficiencies (PCEs) for dye-sensitized solar cells (DSCs), that is, 11.2 % under standard AM 1.5 G sunlight, and 28.4 % under a 1000 lux fluorescent light tube, this work provides a possible method for the design and fabrication of low-cost highly efficient DSCs.

Electron‐Affinity‐Triggered Variations on the Optical and Electrical Properties of Dye Molecules Enabling Highly Efficient Dye‐Sensitized Solar Cells

AbstractThe synthesis, characterization, and photovoltaic performance of a series of indacenodithiophene (IDT)‐based D‐π‐A organic dyes with varying electron‐accepting units is presented. By control of the electron affinity, perfectly matching energy levels were achieved with a copper(I/II)‐based redox electrolyte, reaching a high open‐circuit voltage (>1.1 V) while harvesting a large fraction of solar photons at the same time. Besides achieving high power conversion efficiencies (PCEs) for dye‐sensitized solar cells (DSCs), that is, 11.2 % under standard AM 1.5 G sunlight, and 28.4 % under a 1000 lux fluorescent light tube, this work provides a possible method for the design and fabrication of low‐cost highly efficient DSCs.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells.

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.

Synthesis and photovoltaic properties of a small molecule acceptor with thienylenevinylene thiophene as π bridge

A new n-type organic semiconductor (n-OS) acceptor with (E)-5-(2-(5-(2-ethylhexyl)thiophen-2-yl)vinyl)-thiophene-2-yl (TVT) functional group as π-bridge to link indacenodithiophene (IDT) core and 3-(dicyanomethylidene)indan-1-one (IC) terminals, named ITVT, was synthesized and applied in polymer solar cells (PSCs). It is the first time employing TVT unit in n-OS small molecule acceptor. Benefited from the conjugated TVT side groups, the small molecule ITVT shows a strong absorption at the short wavelength range (300–500 nm). In addition, ITVT exhibits a low optical band gap (Egopt) of 1.48 eV and a medium electron mobility of 1.85 × 10−4 cm2 V-1 s-1. A wide band gap polymer PBZ was chosen as donor to fabricate an inverted BHJ PSCs because of the complementary absorption and matched molecular energy levels. The as-cast PSCs yield a power conversion efficiency (PCE) of 5.84% with an open-circuit voltage (Voc) of 0.96 V and a short-circuit current density (Jsc) of 14.22 mA cm−2 under the illumination of AM 1.5 G at 100 mW cm−2.

Effect of Ring‐Fusion on Miscibility and Domain Purity: Key Factors Determining the Performance of PDI‐Based Nonfullerene Organic Solar Cells

AbstractCompared to the rapid development of nonfullerene organic solar cells (OSCs) based on the state‐of‐the‐art indacenodithiophene (IDT)‐based small molecule acceptors (SMAs), the progress for perylene diimide (PDI)‐based electron acceptors has lagged behind owing to the lack of understanding on the structure–morphology–performance relationship of PDI SMAs. Given the ease of synthesis for PDIs and their high intrinsic electron mobility, it is crucial to identify key material parameters that influence the polymer:PDI blend morphology and to develop rational approaches for molecular design toward high‐performance PDI‐based SMAs. In this study, three pairs of PDI‐based SMAs with and without ring‐fusion are investigated and it is found that ring‐fusion and domain purity are the key structural and morphological factors determining the fill factors (FFs) and efficiencies of PDI‐based nonfullerene OSCs. This data shows that nonfullerene OSCs based on the ring‐fused PDI‐based SMAs exhibit much higher average domain purity and thus increased charge mobilities, which lead to enhanced FFs compared to those solar cells based on nonfused PDIs. This is explained by higher Florry Huggins interaction parameters as observed by melting point depression measurements. This study suggests that increasing repulsive molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is the key to improve the FF and performance of PDI‐based devices.

Synthesis of B←N embedded indacenodithiophene chromophores and effects of bromine atoms on photophysical properties and energy levels

Developing novel fused π-conjugated chromophores has been an energetic research realm and knowing how to adjust their photophysical properties and energy levels through structure tailoring is of pivotal importance. Herein, based on the ladder-type π-conjugated indacenodithiophene (IDT) moiety, four B←N embedded IDT structures, namely, BNIDT, BNIDT-2Br, BNIDT-4Br, and BNIDT-6Br are synthesized and fully characterized. The influences of B←N unit embedded in the IDT backbone and Br atoms anchoring at the periphery on the photophysical properties and energy levels are discussed systematically. From IDT to BNIDT, a new intra-molecular charge transfer (ICT) transition band appears at lower energy (400–600 nm) in the absorption spectra with reduced optical bandgaps (Eg) from 3.25 eV to 2.11 eV and the fluorescence emission peaks red-shift from 390 nm to 565 nm along with remarkably extended fluorescence lifetimes from 1.2 ns to 12.4 ns due to the introduction of electron-deficient B←N into the backbone. Further anchoring Br atoms at the periphery of the backbone gives rise to depressed optical bandgaps, decreased fluorescence quantum yields (Φ), and shortened fluorescence lifetimes (τ) from BNIDT (Eg = 2.11 eV, Φ = 0.46, τ = 12.4 ns), BNIDT-2Br (Eg = 2.08 eV, Φ = 0.18, τ = 4.9 ns), BNIDT-4Br (Eg = 1.67 eV, non-emission) to BNIDT-6Br (Eg = 1.61 eV, non-emission). The HOMO and LUMO levels estimated from ultraviolet photoelectron spectroscopy (UPS) and optical bandgaps also experience synergetic lowering from IDT to BNIDT-6Br. This work indicates that backbone modification with electron-deficient B←N unit and side groups tailoring with halogen atoms are powerful to manipulate the optical properties and energy levels of fused π-conjugated chromophores.

Beyond Fullerenes: Indacenodithiophene-Based Organic Charge-Transport Layer toward Upscaling of Low-Cost Perovskite Solar Cells.

Phenyl-C61-butyric acid methyl ester (PCBM) is universally used as the electron-transport layer (ETL) in the low-cost inverted planar structure of perovskite solar cells (PeSCs). PCBM brings tremendous challenges in upscaling of PeSCs using industry-relevant methods due to its aggregation behavior, which undermines the power conversion efficiency and stability. Herein, we highlight these, seldom reported, challenges with PCBM. Furthermore, we investigate the potential of nonfullerene indacenodithiophene (IDT)-based molecules by employing a commercially available variant, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b'] dithiophene (ITIC), as a PCBM replacement in ambient-processed PeSCs. Films fabrication by laboratory-based spin-coating and industry-relevant slot-die coating methods are compared. Although similar power-conversion efficiencies are achieved with both types of ETL in a simple device structure fabricated by spin-coating, the nanofibriller morphology of ITIC compared to the aggregated morphology of PCBM films enables improved mechanical integrity and stability of ITIC devices. Upon slot-die coating, the aggregation of PCBM is exacerbated, leading to significantly lower power-conversion efficiency of devices than spin-coated PCBM as well as slot-die-coated ITIC devices. Our results clearly indicate that IDT-based molecules have great potential as an ETL in PeSCs, offering superior properties and upscaling compatibility than PCBM. Thus, we present a short summary of recently emerged nonfullerene IDT-based molecules from the field of organic solar cells and discuss their scope in PeSCs as electron or hole-transport layer.

A small molecular electron acceptor based on asymmetric hexacyclic core of thieno[1,2-b]indaceno[5,6-b′]thienothiophene for efficient fullerene-free polymer solar cells

A novel A-D-A (acceptor-donor-acceptor) type non-fullerene small molecule, A201, consisting of an asymmetric thieno[1,2-b]indaceno[5,6-b′]thienothiophene (TITT) unit as middle D part and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) groups as end-capped A parts was designed and synthesized. The asymmetric TITT building block showed a higher dipole moment of 0.85 Debye (1 Debye = 3.33564 × 10−30 cm) compared with the symmetric analogues of indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) of 0.098 and 0.13 Debye, respectively. The solution-processed bulk heterojunction solar cells using a benzotriazole (BTA)-based polymer of J71 as donor and A201 as acceptor, showed a power conversion efficiency (PCE) of 9.36% with an open-circuit voltage (Voc) of 0.88 V, a short-circuit current (Jsc) of 13.15 mA cm−2, and a fill factor (FF) of 0.67, under the illumination of AM 1.5G at 100 mW cm−2. The high PCE of this material combination could be attributed to its broad absorption spectrum and the high hole mobility (μh) and electron mobility (μe) of 9.56 × 10−4 and 5.17 × 10−4 cm2 V−1 s−1, respectively. These results indicate that the asymmetric electron-donating segments are promising to construct A-D-A type small molecular acceptors, which could largely enhance the diversity of building blocks to design photovoltaic materials.

Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells

We report two novel small-molecule non-fullerene acceptors, IF-TN and IDT-TN, with indenofluorene (IF) and indacenodithiophene (IDT) as their respective central electron-rich cores and naphthyl-fused indanone (N) as the electron-withdrawing end-groups. By pairing these non-fullerene acceptors with a widely used polymer PBDB-T, the fabricated polymer solar cells based on PBDB-T:IDT-TN obtain a power conversion efficiency up to 5.89%, associated with a relatively high open-circuit voltage of 0.97 V and a remarkably low non-radiative open-circuit voltage loss of 0.23 V. These findings provide an effective approach for the rational molecular design of small-molecule acceptors with low non-radiative open-circuit voltage loss.

Synthesis of Conjugated Wide-Bandgap Copolymers Bearing Ladder-Type Donating Units and Their Application to Non-Fullerene Polymer Solar Cells

Two new wide-bandgap conjugated copolymers, T-3MT and 2T-3MT, containing methyl-3-thiophenecarboxylate (3MT) as a weak accepting unit and indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) as ladder-type donating units, were synthesized through Stille coupling polymerization. The effects of the IDT and IDTT units on the physical, optical, and electrochemical properties of these copolymers were investigated. Polymer solar cells (PSCs) were constructed using these new copolymers and different acceptor molecules for inter-comparative analysis of their performance. The PSCs bearing the as-cast active layers with 2,2′-((2Z,2′Z)-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′] dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IM-IDT) displayed the highest PCEs of 3.78−3.96%, mainly owing to the predominant face-on orientation of blend film, and the relatively high open circuit voltage exceeding 1.0 V.

Nonfullerene Acceptor with "Donor-Acceptor Combined π-Bridge" for Organic Photovoltaics with Large Open-Circuit Voltage.

In this work, a kind of "donor-acceptor (D-A) combined π-bridge" based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) to link a thiophene ring has been designed to construct a new A-π-D-π-A-type small molecular acceptor (IDT-FBTR) with indacenodithiophene (IDT) as a central core (D) and 3-octyl-2-(1,1-dicyanomethylene)rhodanine as an electron-withdrawing terminal group (A). Because of the strong intramolecular push-pull electron effect, the IDT-FBTR shows a strong and broad intramolecular charge-transfer absorption band in the range of 500-750 nm. Especially, as an electron-deficient FBT unit (A') and an electron-rich thiophene ring (D') in "D-A combined π-bridge" exert an "offset effect" to regulate the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels of the molecule, a relatively high LUMO energy level can be maintained for IDT-FBTR that is helpful to enhance the open-circuit voltage ( Voc) for highly efficient organic solar cells (OSCs). Therefore, the optimized OSC device based on IDT-FBTR as the acceptor and PTB7-Th as the donor shows a much high Voc of 1.02 V with a relatively low Eloss of 0.56 eV and a best power conversion efficiency of 9.14%.

Utilizing Benzotriazole and Indacenodithiophene Units to Construct Both Polymeric Donor and Small Molecular Acceptors to Realize Organic Solar Cells With High Open-Circuit Voltages Beyond 1.2 V.

Devolopment of organic solar cells with high open-circuit voltage (VOC) and power conversion efficiency (PCE) simutaniously plays a significant role, but there is no guideline how to choose the suitable photovoltaic material combinations. In our previous work, we developed “the Same-Acceptor-Strategy” (SAS), by utilizing the same electron-accepting segment to construct both polymeric donor and small molecular acceptor. In this study, we further expend SAS to use both the same electron-accepting and electron-donating units to design the material combination. The p-type polymer of PIDT-DTffBTA is designed by inserting conjugated bridge between indacenodithiophene (IDT) and fluorinated benzotriazole (BTA), while the n-type small molecules of BTAx (x = 1, 2, 3) are obtained by introducing different end-capped groups to BTA-IDT-BTA backbone. PIDT-DTffBTA: BTAx (x = 1–3) based photovolatic devices can realize high VOC of 1.21–1.37 V with the very small voltage loss (0.55–0.60 V), while only the PIDT-DTffBTA: BTA3 based device possesses the enough driving force for efficient hole and electron transfer and yields the optimal PCE of 5.67%, which is among the highest value for organic solar cells (OSCs) with a VOC beyond 1.20 V reported so far. Our results provide a simple and effective method to obtain fullerene-free OSCs with a high VOC and PCE.