Dithienopicenocarbazole-Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss

High-Performance All-Polymer Solar Cells Enabled by n-Type Polymers with an Ultranarrow Bandgap Down to 1.28 eV

Compared to organic solar cells based on narrow-bandgap nonfullerene small-molecule acceptors, the performance of all-polymer solar cells (all-PSCs) lags much behind due to the lack of high-performance n-type polymers, which should have low-aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near-infrared region. Herein, two novel polymer acceptors, DCNBT-TPC and DCNBT-TPIC, are synthesized with ultranarrow bandgaps (ultra-NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm² V^-1 s^-1 obtained for DCNBT-TPC. Blended with a polymer donor, PBDTTT-E-T, the resultant all-PSCs based on DCNBT-TPC and DCNBT-TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short-circuit currents up to 19.44 and 22.52 mA cm^-2 , respectively. This is the first example that a PCE of over 10% can be achieved using ultra-NBG polymer acceptors with a photoresponse reaching 950 nm in all-PSCs. These results demonstrate that ultra-NBG polymer acceptors, in line with nonfullerene small-molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all-PSCs.

Comparison of Fused-Ring Electron Acceptors with One- and Multidimensional Conformations

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.

High-Performance Fluorinated Fused-Ring Electron Acceptor with 3D Stacking and Exciton/Charge Transport

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%).

Near-Infrared Electron Acceptors with Unfused Architecture for Efficient Organic Solar Cells

The absorption of nonfullerene acceptors (NFAs) at near-infrared (NIR) regions is crucial for obtaining high current densities in organic solar cells (OSCs). Herein, two narrow-band gap NFAs with unfused backbones possessing broad (600-900 nm) and strong absorption are developed by the conjugation of a benzothiadiazole core to halogenated end groups through a cyclopentadithiophene bridge. Compared with the fluorinated counterpart BCDT-4F, the chlorinated NFA BCDT-4Cl shows stronger J-aggregation and closer molecular packing, leading to an optimized blend morphology when paired with the polymer donor, PBDB-T. Thus, an obvious improvement in external quantum efficiency response was obtained for BCDT-4Cl-based OSCs, presenting a higher efficiency of 12.10% than those (9.65%) based on BCDT-4F. This work provides a design strategy for NIR acceptors in the combination of electron-deficient core and halogenated terminal in unfused backbones, which results in not only fine-tuning the optoelectronic properties but also simplifying the synthetic complexities of molecules.

Enhancing Performance of Fused-Ring Electron Acceptor Using Pyrrole Instead of Thiophene

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 cm² V^-1 s^-1, F6IC: 7.4 × 10^-4 cm² 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%).

With PBDB-T as the Donor, the PCE of Non-Fullerene Organic Solar Cells Based on Small Molecule INTIC Increased by 52.4

At present, most high-performance non-fullerene materials are centered on fused rings. With the increase in the number of fused rings, production costs and production difficulties increase. Compared with other non-fullerenes, small molecule INTIC has the advantages of easy synthesis and strong and wide infrared absorption. According to our previous report, the maximum power conversion efficiency (PCE) of an organic solar cell using PTB7-Th:INTIC as the active layer was 7.27%. In this work, other polymers, PTB7, PBDB-T and PBDB-T-2F, as the donor materials, with INTIC as the acceptor, are selected to fabricate cells with the same structure to optimize their photovoltaic performance. The experimental results show that the optimal PCE of PBDB-T:INTIC based organic solar cells is 11.08%, which, thanks to the open voltage (VOC) increases from 0.80 V to 0.84 V, the short circuit current (JSC) increases from 15.32 mA/cm2 to 19.42 mA/cm2 and the fill factor (FF) increases from 60.08% to 67.89%, then a 52.4% improvement in PCE is the result, compared with the devices based on PTB7-Th:INTIC. This is because the PBDB-T:INTIC system has better carrier dissociation and extraction, carrier transportation and higher carrier mobility.

BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis, Aromaticity, and Reactivity

BN-embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X-ray crystallography, optical spectroscopy, and cyclic voltammetry. The B-N bonds show delocalized double-bond characteristics and the conjugation can be extended through the trans-orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN-embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN-embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge-transfer interactions.

A Near-Infrared Absorption Small Molecule Acceptor for High-Performance Semitransparent and Colorful Binary and Ternary Organic Photovoltaics

An acceptor-donor-acceptor (A-D-A)-type non-fullerene acceptor (NFA), PTTtID-Cl, featuring thieno[3,2-b]thieno[2''',3''':4'',5'']-pyrrolo[2'',3'':4',5']thieno[2',3':4,5]thieno-[2,3-d]pyrrole (DTPTt) as the electron-rich core and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (ID-Cl) as the peripheral electron-deficient terminal group was synthesized and characterized. PTTtID-Cl exhibited strong absorption in the range of 700-850 nm in CHCl₃ and redshifted absorption centered at 881 nm in a thin film. The near infrared (NIR)-absorption of PTTtID-Cl was combined with a low-bandgap polymer donor (PTB7-Th) to achieve binary and semitransparent organic photovoltaics (OPVs) with a power conversion efficiency (PCE) of 8.9 % and 7.7 % (with an average visible transmittance (AVT) of 16.7 %), respectively. A ternary device with a ratio of PM7/PTTtID-Cl/IT-4F=1:0.15:0.85 (w/w) achieved a short-circuit current density of 19.46 mA cm^-2 , an open-circuit voltage of 0.87 V, and a fill factor of 71.2 %, giving a PCE of 12.0 %. In addition, by employing the Ag/ITO/Ag microcavity structure, semitransparent colorful binary organic photovoltaics (OPVs) with superior transparency of 27.9 % at 427 nm and 22.7 % at 536 nm for blue and green devices, respectively, were prepared. The semitransparent colorful devices based on the optimized ternary blend gave PCEs of 8.7 %, 8.4 %, and 9.1 % for blue, green, and red devices, respectively. These results indicate the promising potential of PTTtID-Cl as a NIR-absorption NFA for applications in semitransparent colorful binary and ternary OPVs.

Dithieno[3,2-b:2',3'-d]pyrrol-Fused Asymmetrical Electron Acceptors: A Study into the Effects of Nitrogen-Functionalization on Reducing Nonradiative Recombination Loss and Dipole Moment on Morphology

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

BODIPY bearing alkylthienyl side chains: a new building block to design conjugated polymers with near infrared absorption for organic photovoltaics

A new benzene-fused BODIPY unit for designing polymer donors with near-infrared absorption for organic photovoltaics.

Acceptor–donor–acceptor type molecules for high performance organic photovoltaics – chemistry and mechanism

The chemical structure–property relationships and mechanism for high performance organic photovoltaics of acceptor–donor–acceptor type molecules are discussed.

Simple-Structured NIR-Absorbing Small-Molecule Acceptors with a Thiazolothiazole Core: Multiple Noncovalent Conformational Locks and D-A Effect for Efficient OSCs

Developing simple-structured and efficient near-infrared (NIR) absorbing small-molecule acceptors (SMAs) remains of great importance for commercial applications in organic solar cells (OSCs). Herein, we construct two novel thiazolothiazole-centered NIR-absorbing SMAs by alkoxy/alkylthio side chains (TTz3/TTz4) and multiple noncovalent conformational locks of S···N, S···O, and S···S. These conformational locks make both simple-structured SMAs exhibit an extended planar configuration and a red-shifted NIR absorption in comparison with the SMA with an alkyl side chain (TTz1). As expected, both SMAs show high-efficiency photovoltaic performance in the solution-processing OSCs using J71 as a donor. A higher power conversion efficiency of 8.76% with a low energy loss (0.57 eV) is obtained in the TTz3-based OSCs. It is the first report on the simple-structured and efficient NIR-absorbing SMAs, which have great potential applied in the semitransparent OSCs.

Design of New n-Type Porphyrin Acceptors with Subtle Side-Chain Engineering for Efficient Nonfullerene Solar Cells with Low Energy Loss and Optoelectronic Response Covering the Near-Infrared Region

A series of tailor-made highly efficient and near-infrared (NIR) porphyrin-based acceptors is designed and synthesized for fullerene-free bulk-heterojunction (BHJ) organic solar cells. Constructing BHJ active layers using a PTB7-Th donor and porphyrin acceptors (P-x), which have complementary absorption, accomplishes panchromatic photon-to-current conversion from 300 to 950 nm. Our study shows that side chains of the porphyrin acceptors fairly influence the molecular ordering and nanomorphology of the BHJ active layers. Significantly, the porphyrin acceptor with four dodecoxyl side chains (P-2) achieves an open-circuit voltage (V_OC) of 0.80 V, short-circuit current density (J_SC) of 13.94 mA cm^-2, fill factor of 64.8%, and overall power conversion efficiency of 7.23%. This great performance is attributable to the ascendant light-harvesting capability in the visible and near-infrared region, a high-lying LUMO energy level, a relatively high and more balanced carrier mobilities, and more ordered face-on molecular packing, which is beneficial for obtaining high V_OC and J_SC.

Regulation of Molecular Packing and Blend Morphology by Finely Tuning Molecular Conformation for High-Performance Nonfullerene Polymer Solar Cells

The asymmetric thienobenzodithiophene (TBD) structure is first systematically compared with the benzo[1,2-b:4,5-b']dithiophene (BDT) and dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) units in donor-acceptor (D-A) copolymers and applied as the central core in small molecule acceptors (SMAs). Specific polymers including PBDT-BZ, PTBD-BZ, and PDTBDT-BZ with different macromolecular conformations are synthesized and then matched with four elaborately designed acceptor-donor-acceptor (A-D-A) SMAs with structures comparable to their donor counterparts. The resulting polymer solar cell performance trends are dramatically different from each other and highly material-dependent, and the active layer morphology is largely governed by polymer conformation. Because of its more linear backbone, the PTBD-BZ film has higher crystallinity and more ordered and denser π-π stacking than those of the PBDT-BZ and PDTBDT-BZ films. Thus, PTBD-BZ shows excellent compatibility with and strong independence on the SMAs with varied structures, and PTBD-BZ-based cells deliver high power conversion efficiency (PCE) of 10-12.5%, whereas low PCE is obtained by cells based on PDTBDT-BZ because of its zigzag conformation. Overall, this study reveals control of molecular conformation as a useful approach to modulate the photovoltaic properties of conjugated polymers.

One-step synthesis of ball-shaped metal complexes with a main absorption band in the near-IR region

The design of near-IR materials is highly relevant to energy and pharmaceutical sciences due to the high proportion of near-IR irradiation in the solar spectrum and the high penetration of near-IR light in biological samples. Here, we show the one-step synthesis of hexacoordinated ruthenium and iron complexes that exhibit a main absorption band in the near-IR region. For that purpose, novel tridentate ligands were prepared by condensation of two diimines and four cyanoaryl derivatives in the presence of ruthenium and iron template ions. This method was applied to a wide variety of cyanoaryl, diimine, and metal ion combinations. The relationship between the structure and the optical and electrochemical properties in the resulting complexes was examined, and the results demonstrated that these compounds represent novel near-IR materials whose physical properties can be controlled based on rational design guidelines. The intense absorption bands in the 700–900 nm region were assigned to metal-to-ligand charge transfer (MLCT) transitions, which should allow applications in materials with triplet excited states under irradiation with near-IR light.

Efficient Non-Fullerene Organic Solar Cells Based on a Wide-Bandgap Polymer Donor Containing an Alkylthiophenyl-Substituted Benzodithiophene Moiety

Two wide-bandgap polymer donors containing an alkylthiophenyl substituted benzo[1,2-b : 4,5-b']dithiophene moiety, namely PTZPO and PTZPS, were designed and synthesized. Both polymers exhibit relatively wide optical bandgap of 1.95 V with similar absorption profiles. The polymer PTZPS with alkylthiophenyl substituted benzo[1,2-b : 4,5-b']dithiophene units showed enhanced light-harvesting capabilities, leading to improved short-circuit current densities. The PTZPS : ITIC film shows more appreciable film morphology and phase separation than the film composed of a blend of ITIC with alkoxyl substitutions containing copolymer PTZPO, which facilitates exciton dissociation and charge transport. The PTZPS : ITIC-based non-fullerene organic solar cells show clearly improved short-circuit current density and an impressively high power conversion efficiency of more than 11 %. These observations demonstrate the great promise of using PTZPS as electron-donating materials for high-performance non-fullerene organic solar cells.

Modulating Structure Ordering via Side-Chain Engineering of Thieno[3,4-b]thiophene-Based Electron Acceptors for Efficient Organic Solar Cells with Reduced Energy Losses

Nonfullerene-based organic solar cells (OSCs) have made a huge breakthrough in the recent years. Introducing a proper side chain on the π-conjugated backbone plays a vital role for further improving the power conversion efficiency (PCE) of OSCs due to easy tuning of the physical properties of the molecule such as absorption, energetic level, solid-state stacking, and charge transportation. More importantly, the side chain significantly affected the blend film's morphology and thus determined the PCEs of the devices. In this work, two low-band-gap nonfullerene acceptors, ATT-4 and ATT-5, with an alkyl or branched alkyl substitute on indacenodithiophene (IDT) and thieno[3,4-b]thiophene (TbT) backbone were synthesized for investigating the effect of the substituent on the performance of the nonfullerene acceptors (NFAs). In comparison to ATT-1 with p-hexylphenyl-substituted IDT and n-octyl-substituted TbT moieties, ATT-4 and ATT-5 exhibit better crystallinity with shorter interchain distance and ordered molecular structure in neat and the corresponding blend films. The tailored ATT-5 exhibits a high PCE of 12.36% with a V_oc of 0.93 V, J_sc of 18.86 mA cm^-2, and fill factor (FF) of 0.71, blending with a wide-band-gap polymer donor PBDB-T. Remarkably, although ATT-4 and ATT-5 exhibit broader light absorption, the devices obtained higher V_oc than that of ATT-1 mainly due to the reduced nonradiative recombination in the blend films. These results implied that side-chain engineering is an efficient approach to regulate the electronic structure and molecular packing of NFAs, which can well match with polymer donor, and obtain high PCEs of the OSCs with improved V_oc, J_sc, and FF, simultaneously.

Boosting Efficiency and Stability of Organic Solar Cells Using Ultralow-Cost BiOCl Nanoplates as Hole Transporting Layers

A novel nanomaterial, bismuth oxychloride nanoplates (BiOCl NPs), was first applied in organic solar cells (OSCs) as hole transporting layers (HTLs). It is worth noting that the BiOCl NPs can be facilely synthesized at ∼1/200 of the cost of the commercial PEDOT:PSS and well dissolved in green solvents. Different from the PEDOT:PSS interlayer, the deposition of BiOCl HTL is free of post-treatment at elevated temperature, which reduces device fabrication complexity. To demonstrate the universality of BiOCl in improving photovoltaic performance, OSCs containing various representative active layers were investigated. The power conversion efficiencies (PCEs) of the P3HT:PC₆₁BM, PTB7-Th:PC₇₁BM, and PM6:Y6-based OSCs with the BiOCl HTL boosted from 3.62, 8.78, and 15.63 to 4.24, 9.92, and 16.11%, respectively, compared to the PEDOT:PSS-based ones. It was found that the superior performances of the BiOCl-based OSCs are mainly attributed to the sufficient oxygen vacancies and improved interfacial contact. Moreover, the BiOCl-based OSCs show a much better stability than the cells with the PEDOT:PSS interfacial layer.

Z-Shaped Fused-Chrysene Electron Acceptors for Organic Photovoltaics

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 × 10⁵ M^-1 cm^-1, a bandgap of 1.50 eV, and a charge mobility of 2.5 × 10^-4 cm² 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.

Revealing the Position Effect of an Alkylthio Side Chain in Phenyl-Substituted Benzodithiophene-Based Donor Polymers on the Photovoltaic Performance of Non-Fullerene Organic Solar Cells

In this work, position effects of an alkylthio side chain were investigated by designing and synthesizing two copolymers based on a phenyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDTP) and difluorobenzotriazole (FTAZ). The polymer based on the meta-position-alkylthiolated BDTP, named m-PBDTPS-FTAZ, showed a relatively broader bandgap (2.00 vs 1.96 eV) and lower highest occupied molecular orbital (HOMO) energy level (-5.40 vs -5.32 eV) than its para-positioned structural isomeric analogue polymer (named p-PBDTPS-FTAZ), that is, m- and p-PBDTPS-FTAZ with the side chain structured as ethylhexyl- in the phenyl unit and hexyldecyl- in the FTAZ moiety. When blended with ITIC, m-PBDTPS-FTAZ showed a comparable crystallinity but more uniform morphology compared to that of p-PBDTPS-FTAZ. A high power conversion efficiency of 13.16% was achieved for m-PBDTPS-FTAZ:ITIC devices with a high open circuit voltage (V_OC) of 0.95 V, which is higher than that of p-PBDTPS-FTAZ:ITIC devices (10.86%) with a V_OC of 0.89 V. Therefore, m-BDTPS could be an effective donor unit to construct high-efficiency polymers due to its effectively decreased HOMO energy level of polymers while still maintaining good molecular stacking.

Star-Shaped Fused-Ring Electron Acceptors with a C3h-Symmetric and Electron-Rich Benzotri(cyclopentadithiophene) Core for Efficient Nonfullerene Organic Solar Cells

Classical fused-ring electron acceptors (FREAs) with a linear acceptor-donor-acceptor (A-D-A) architecture continuously break records of power conversion efficiency (PCE) in nonfullerene organic solar cells. In contrast, the development of star-shaped FREAs still lags behind. Herein, a new C_3h-symmetric and electron-rich core, benzotri(cyclopentadithiophene) (BTCDT) in which the central benzo[1,2-b:3,4-b':5,6-b″]trithiophene fused with three outer thiophenes via three cyclopentadienyl rings, is synthesized and used for the construction of star-shaped FREAs (BTCDT-IC and BTCDT-ICF). Owing to the strong electron-donating ability of the BTCDT unit, both acceptors exhibit the effective intramolecular charge transfer, leading to the strong absorption in the region of 500-800 nm with narrow band gaps below 1.70 eV as well as suitable highest occupied molecular orbital and lowest unoccupied molecular orbital levels. Compared with nonfluorinated BTCDT-IC, fluorinated BTCDT-ICF red-shifts the absorption peak to 688 nm and reduces the band gap to 1.62 eV, which induces a broader external quantum efficiency (EQE) response ranging from 300 to 800 nm and a higher maximum EQE of 70% while blending with a wide band gap polymer donor J61. The J61:the BTCDT-ICF blend film exhibits more suitable phase morphology compared with the J61:BTCDT-IC blend film, which is responsible for the enhanced EQE value, increased short-circuit current density (J_SC), and fill factor (FF) in organic solar cell devices. As a result, the J61:BTCDT-ICF-based device yields a best PCE of 8.11% with a high J_SC of 16.93 mA cm^-2 and a high FF of 65.6%, demonstrating that the BTCDT-based star-shaped FREAs hold great potential for nonfullerene organic solar cells.

Isomer-free: Precise Positioning of Chlorine-Induced Interpenetrating Charge Transfer for Elevated Solar Conversion

The influence caused by the position of the chlorine atom on end groups of two non-fullerene acceptors (ITIC-2Cl-δ and ITIC-2Cl-γ) was intensely investigated. The single-crystal structures show that ITIC-2Cl-γ has a better molecular planarity and closer π-π interaction distance. More importantly, a 3D rectangle-like interpenetrating network is formed in ITIC-2Cl-γ and is beneficial to rapid charge transfer along multiple directions, whereas only a linear stacked structure could be observed in ITIC-2Cl-δ. The two acceptor-based solar cells show power conversion efficiencies (PCEs) over 11%, higher than that of the ITIC-2Cl-m-based device (10.85%). An excellent PCE of 13.03% is obtained by the ITIC-2Cl-γ-based device. In addition, the ITIC-2Cl-γ-based device also shows the best device stability. This study indicates that chlorine positioning has a great impact on the acceptors; more importantly, the 3D network structure may be a promising strategy for non-fullerene acceptors to improve the PCE and stability of organic solar cells.

Noncovalently fused-ring electron acceptors with near-infrared absorption for high-performance organic solar cells

Non-fullerene fused-ring electron acceptors boost the power conversion efficiency of organic solar cells, but they suffer from high synthetic cost and low yield. Here, we show a series of low-cost noncovalently fused-ring electron acceptors, which consist of a ladder-like core locked by noncovalent sulfur–oxygen interactions and flanked by two dicyanoindanone electron-withdrawing groups. Compared with that of similar but unfused acceptor, the presence of ladder-like structure markedly broadens the absorption to the near-infrared region. In addition, the use of intramolecular noncovalent interactions avoids the tedious synthesis of covalently fused-ring structures and markedly lowers the synthetic cost. The optimized solar cells displayed an outstanding efficiency of 13.24%. More importantly, solar cells based on these acceptors demonstrate very low non-radiative energy losses. This research demonstrates that low-cost noncovalently fused-ring electron acceptors are promising to achieve high-efficiency organic solar cells.

Recently, the non-fullerene acceptors with fused rings enable high-efficiency organic solar cells but they are not ideal in terms of synthetic cost and yield. Here, Huang et al. report ‘less fused’ acceptors with non-covalent S⋅⋅⋅O interactions and solar cell efficiency of up to 13%.

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.

Simple non-fused electron acceptors for efficient and stable organic solar cells

The flexibility in structural design of organic semiconductors endows organic solar cells (OSCs) not only great function-tunabilities, but also high potential toward practical application. In this work, simple non-fused-ring electron acceptors are developed through two-step synthesis from single aromatic units for constructing efficient OSCs. With the assistance of non-covalent interactions, these rotatable non-fused acceptors (in solution) allow transiting into planar and stackable conformation in condensed solid, promoting acceptors not only feasible solution-processability, but also excellent film characteristics. As results, decent power conversion efficiencies of 10.27% and 13.97% can be achieved in single and tandem OSCs consisting of simple solution-cast blends, in which the fully unfused acceptors exhibit exceptionally low synthetic complexity index. In addition, the unfused acceptor and its based OSCs exhibit promising stabilities under continuous illumination. Overall, this work reveals valuable insights on the structural design of simple and effective electron acceptors with great practical perspectives.

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.

Double Acceptor Block-Containing Copolymers with Deep HOMO Levels for Organic Solar Cells: Adjusting Carboxylate Substituent Position for Planarity

A deep highest occupied molecular orbital (HOMO) level is a prerequisite for polymer donor material to boost the organic solar cells (OSCs) performance by achieving high open circuit voltage ( V_oc). Abandoning the traditional concept of donor-acceptor (D-A) structure, two copolymers PBTZ-4TC and PBTZ-C4T based on acceptor₁-π-acceptor₂ (A₁-π-A₂) architecture, where thiophene as the bridge, the difluorinated benzotriazole (BTZ) as A₁ unit alternating copolymerized with 4,4'-dicarboxylate-substituted difluorotetrathiophene (4TC) and 3,3'-dicarboxylate-substituted difluorotetrathiophene (C4T) as A₂, respectively, are developed. Because of the double acceptor blocks with high electron affinity, both A₁-π-A₂ type copolymers possess the lower HOMO levels of 5.52-5.56 eV, which are lower than most D-A type donors. Polymer PBTZ-4TC and PBTZ-C4T have the same backbone but only differ with the position of carboxylate substituent on the A₂ unit. Intriguingly, subtle optimizing the position of the carboxylate-substitute causes a significantly difference on the properties of the A₁-π-A₂ type copolymers. PBTZ-C4T with more planar geometry is demonstrated with better light absorption, higher crystallinity, more pronounced temperature-dependent aggregation effect, and favorable bulk heterojunction morphology but with slightly higher HOMO level and more emission energy loss relative to the PBTZ-4TC. The PBTZ-C4T device exhibits the higher power conversion efficiency (PCE) of 9.34% than the PBTZ-4TC-based one (8.75%). These results reveal that concept of A₁-π-A₂ type copolymers not only can afford more flexibility in tuning the energy levels to achieve the deep HOMO levels but also can provide a facial strategy to greatly enrich the types of polymer donors for high-performance OSCs.

Water/Alcohol Soluble Thickness-Insensitive Hyperbranched Perylene Diimide Electron Transport Layer Improving the Efficiency of Organic Solar Cells

The electron transport layer (ETL) is very crucial for enhancing the device performance of polymer solar cells (PSCs). Meanwhile, thickness-insensitive and environment-friendly water/alcohol soluble processing are two essential requirements for large-scale roll-to-roll commercial application. Based on this, we designed and synthesized two new n-type ETLs with tetraethylene pentamine or butyl sulfonate sodium substituted tetraethylene pentamine as the branched side chains and high electron affinities perylene diimide (PDI) as the central core, named as PDIPN and PDIPNSO₃Na. Encouragingly, both PDIPN and PDIPNSO₃Na can effectively reduce the interfacial barrier and improve the interfacial contact. In addition, both of them can exhibit strong n-type self-doping effects, especially the PDIPN with higher density of negative charge. Consequently, compared to bare ITO, the PCE of the devices with ITO/PDIPN and ITO/PDIPNSO₃Na ETLs has increased to 3–4 times. Our research results indicate that n-type self-doping PDI-based ETL PDIPN and PDIPNSO₃Na could be promising candidates for ETL in environment-friendly water/alcohol soluble processing large-scale PSCs.

A Simple Phenyl Group Introduced at the Tail of Alkyl Side Chains of Small Molecular Acceptors: New Strategy to Balance the Crystallinity of Acceptors and Miscibility of Bulk Heterojunction Enabling Highly Efficient Organic Solar Cells

Research on fused-ring small-molecular-acceptors (SMAs) has deeply advanced the development of organic solar cells (OSCs). Compared to fruitful studies of ladder-type cores and end-caps of SMAs, the exploration of side chains is monotonous. The widely utilized alkyl and aryl side chains usually produce a conflicting association between SMAs' crystallinity and miscibility. Herein, a fresh idea about the modification of side chains is reported to explore the subtle balance between the crystallinity and miscibility. Specifically, a phenyl is introduced to the tail of the alkyl side chain whereby a new acceptor IDIC-C4Ph is reported. Moderately weakened crystallinity is observed, while maintaining preferred absorption profiles and face-on orientations. Concurrently, remarkably improved heterojunction morphologies and stacking orientations are detected. PBDB-T:IDIC-C4Ph devices exhibit greater efficiency of 11.50% than devices from alky and aryl modified acceptors. Notably, the as-cast OSCs of PBDB-TF:IDIC-C4Ph reveal outstanding FF over 76% with the best efficiency up to 13.23%. The annealed devices reveal further increased efficiency exceeding 14% with the state of the art FF of 78.32%. Overall, an effective but easily navigable approach is demonstrated to modulate the crystallinity of SMAs toward synergistically improved morphologies and molecular orientations of bulk heterojunction enabling highly efficient OSCs.

Simplified synthetic routes for low cost and high photovoltaic performance n-type organic semiconductor acceptors

The application of polymer solar cells (PSCs) with n-type organic semiconductor as acceptor requires further improving powder conversion efficiency, increasing stability and decreasing cost of the related materials and devices. Here we report a simplified synthetic route for 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b’] dithiophene by using the catalyst of amberlyst15. Based on this synthetic route and methoxy substitution, two low cost acceptors with less synthetic steps, simple post-treatment and high yield were synthesized. In addition, the methoxy substitution improves both yield and efficiency. The high efficiency of 13.46% was obtained for the devices with MO-IDIC-2F (3,9-bis(2-methylene-5 or 6-fluoro-(3-(1,1-dicyanomethylene)-indanone)-4,4,9,9-tetrahexyl-5,10-dimethoxyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b’] dithiophene) as acceptor. Based on the cost analysis, the PSCs based on MO-IDIC-2F possess the great advantages of low cost and high photovoltaic performance in comparison with those PSCs reported in literatures. Therefore, MO-IDIC-2F will be a promising low cost acceptor for commercial application of PSCs.

Non-fullerene organic molecule acceptors have been the hot research subject to drive the efficiency of the organic solar cells higher, but their synthetic costs are high. Here Li et al. design a simplified synthetic route for the state-of-the-art acceptors and bring down their cost by more than 15%.

Designing Alternative Non-Fullerene Molecular Electron Acceptors for Solution-Processable Organic Photovoltaics

Until recently, solution-processable organic photovoltaics (OPVs) mainly relied on fullerene derivatives as the n-type material, paired with a p-type conjugated polymer. However, fullerene derivatives have disadvantages that limit OPV performance, thus fueling research of non-fullerene acceptors (NFAs). Initially, NFAs showed poor performance due to difficulties in obtaining favorable blend morphologies. One example is our work with 2,6-dialkylamino core-substituted naphthalene diimides. Researchers then learned to control blend morphology by NFA molecular design. To limit miscibility with polymer while preventing excessive self-aggregation, non-planar, twisted or 3D structures were reported. An example of a 3D structure is our work with homoleptic zinc(II) complexes of azadipyrromethene. The most recent design is a planar A-D-A conjugated system where the D unit is rigid and has orthogonal side chains to control aggregation. These have propelled power conversion efficiencies (PCEs) to ∼14 %, surpassing fullerene-based OPVs. These exciting new developments prompt further investigations of NFAs and provide a bright future for OPVs.