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A carbon-oxygen-bridged hexacyclic ladder-type building block for low-bandgap nonfullerene acceptors

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A carbon-oxygen-bridged hexacyclic ladder-type building block for low-bandgap nonfullerene acceptors

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Abstract

A hexacyclic carbon-oxygen-bridged ladder-type unit, COi6, was developed. Three nonfullerene acceptors (COi6IC, COi6FIC and COi6DFIC) based on COi6 were prepared. They present low optical bandgaps of 1.31-1.37 eV and strong absorbance in the near-infrared region. A 9.12% power conversion efficiency was achieved from the solar cells based on COi6FIC and a wide-bandgap copolymer donor (FTAZ).

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... [14][15][16][17][18][19][20][21]27,[31][32][33][34] However, a more in-depth understanding of these requirements is lacking. For example, there have been many studies on the synthesis and performance of new FREAs, 1,12,13,23,[35][36][37][38][39][40][41][42][43] but little work has focused on the molecular packing of these materials (i.e., requirement (d) above). To obtain high efficiency, OPV electron acceptors need a high electron mobility in order to extract electrons from the active layer and transport them to the cathode before they recombine. ...
... The hole mobility of the FTAZ: IDTCF blend was measured to be 7.9 Â 10 À6 cm 2 V À1 s À1 , which is over two orders of magnitude lower than the hole mobility generally observed for FTAZ-based blends. [39][40][41][42]69,70,[79][80][81] Recall that the GIWAXS results found that IDTCF disrupts the packing of the FTAZ chains as seen by the larger (010) peak in the blend film. This effect will directly hinder the hole transport and would contribute to the low J sc value observed for the FTAZ:IDTCF device. ...
Article
Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ~ 1 Å, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.
... PTB7-Th:CO i 10DFIC cells gave higher V oc than the reported PTB7-Th:CO i 8DFIC cells due to the higher LUMO of CO i 10DFIC [5]. The best PTB7-Th:CO i 10DFIC:CO i 8DFIC (D:A 1 :A 2 ) ternary cells gave a PCE of 13.48%, with a V oc of 0.71 3 (Tables S4, S5 and S6 online). Compared with the binary cells, the ternary cells afforded much higher J sc due to the stronger and broader EQE response. ...
... Their A-D-A acceptors show lower bandgaps and stronger light-harvesting capability due to enhanced intramolecular charge transfer (ICT). Ding et al. have designed pentacyclic [2], hexacyclic [3], heptacyclic [4] and octacyclic [5,6] CO-bridged units, and corresponding A-D-A acceptors. Among CO-bridged units, the octacyclic unit CO i 8 shows the strongest electron-donating capability and the largest molecular plane, rendering A-D-A acceptor CO i 8DFIC a very low bandgap of 1.26 eV and good electron mobility. ...
... These nonfullerene acceptors have strong visible-NIR light-harvesting capability and good electron mobility, delivering higher short-circuit current density (J sc ) and PCE in solar cells than fullerene acceptors. To further enhance light absorption, Ding et al. developed stronger light-harvesting A-D-A acceptors by using more electron-rich carbon-oxygen-bridged (CO-bridged) core units [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. The acceptor CO i 8DFIC, based on an octacyclic CO-bridged unit, CO i 8, presents a narrow bandgap of 1.26 eV and strong absorption at 600-1000 nm [15]. ...
Article
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High-performance donors matching low-bandgap nonfullerene acceptors are still limited. Only a few D-A copolymer donors, such as PM6, PTQ10, P2F-EHp, D16 and W1, have delivered >15% PCEs. Developing more high-performance donors would keep lifting the efficiency, which is a great mission of the field! Previously, our group reported highly efficient wide-bandgap copolymers based on fused-ring acceptor units. These units have strong electron-withdrawing capability and extended molecular planes, gifting copolymers deep HOMO levels, enhanced - stacking and high hole mobility. These lead to simultaneously improved Voc, Jsc and FF. For example, the copolymer D16 based on a fused-ring thiolactone unit, 5H-dithieno[3,2-b:2',3'-d]thiopyran-5-one (DTTP), gave a PCE of 16.72% when blending with Y6 (Sci. Bull., 2019, 64, 1573). In continuing our effort, here we report a more efficient copolymer donor D18 by using a fused-ring acceptor unit, dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT). Compared with DTTP, DTBT has a larger molecular plane and gifts D18 a higher hole mobility. D18: Y6 solar cells demonstrated a PCE of 18.22% (certified 17.6%), which is the highest efficiency for organic solar cells to date.
... Lin et al. [13] synthesized fused ring small molecular acceptors with acceptor-donor-acceptor backbone (ITIC and IDIC) having high electron mobility and high light harvesting capability. Xiao et al. [9,[13][14][15] improved the NFAs by using carbonoxygen bridge. These NFAs exhibit strong absorption in visible and infrared range (600-1000 nm) with band gap value of 1.26 eV [16]. ...
Article
Recently, end-capped acceptors tailoring approach has attracted many researchers because of unceasing higher power conversion efficiencies (PCEs) of resulted compounds. By keeping in view, the crucial role of NFAs in bulk-heterojunction OSCs, herein, we molecularly engineered five new non-fullerene acceptor materials (Y6A1-Y6A5) by modifying a recently synthesized Y6 molecule (R), having 18% power conversion efficiency when combined with D18 donor polymer. The structural-elemental connection, physical-chemical, optoelectronic, and photovoltaic characteristics of novel deigned and reference material (R) are studied with advanced quantum-chemical modulations. Density functional theory and time dependent-density functional theory has been employed through various basis sets to investigate the designed molecules theoretically. Interestingly, all of the newly modeled materials displayed lower excitation energies with lower HOMO-LUMO energy-gaps in-contrast with R molecule. Moreover, a red-shifted absorption and lower reorganizational energies of electron and hole are also a novel feature of these designed materials. The lower binding energy values of modeled materials offers better charge separation and high photo-current density (J sc) as compared to R. Transition density analysis, open circuit voltage, and molecular electrostatic potential analysis suggested that end-capped acceptors alteration of R molecule is an efficient approach for tuning the optoelectronic properties of non-fullerene-based acceptor molecules (Y6A1-Y6A5). In last, composite study of donor: acceptor (D18:Y6A2) complex has also been carried-out to realize the charge transfer process at the donor-acceptor interface. After all investigations, we hope that our theoretical modeled materials are superior than Y6 molecule, therefore, we endorse these materials for the synthesis to prepare highly-efficient BHJ-OSCs devices.
... 13,23 Among the reported A-D-A type NFAs, the majority of them exhibit weak absorption in the near-infrared (NIR) region, which reduces the utilization of solar energy. [24][25] Very recently, Zou et al. pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene)) bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile), with the absorption onset at 931 nm. Y6 is based on A-DA'D-A structure, with the 2-(5,6difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (2FIC) as A unit and electron-deficient fused ring (di-thienothiophen[3.2-b]-pyrrolobenzothiadiazole) as A' unit. ...
Article
Non-fullerene acceptors have been utilized to construct efficient organic solar cells (OSCs). In this work, two new molecular acceptors, TPQx-4F and TPQx-6F, with quinoxaline (Qx)-fused ring, were designed and synthesized. The single-junction inverted OSCs device of two NFAs, blended with PM6 as the donor material, were systemically investigated. The introduction of fluorine atoms into Qx-containing fused central core not only downshifted energy levels, but also greatly optimized the morphology of PM6:TPQx-6F blend films for the first time. The power conversion efficiency (PCE) of the PM6:TPQx-4F based devices improved from 6.02% to 7.75% through thermal annealing (TA) treatment. Meanwhile, PM6:TPQx-6F based devices showed a PCE of 12.28%, in which a remarkable PCE of 14.62% was received after TA treatment optimization. The differences performance of devices based PM6:TPQx-4F and PM6:TPQx-6F were mainly attributed to the variation of short-circuit current density (Jsc) and fill factor (FF). Further investigations revealed that PM6:TPQx-6F blend films presented higher electron mobilities, more efficient charge dissociation and uniform nanophase separation than those of TPQx-4F-based devices, giving rise to higher Jsc and FF. In addition, the enhanced Jsc and FF were simultaneously realized by simple TA treatment for TPQx-4F- and TPQx-6F-based devices. This work provides more insight into the underlying reason of fine-tuning energy levels and optimizing active layer morphology on high-performance NFAs based OSCs.
... Compared with other FTAZ:NFA containing systems, FTAZ:ITBC showed a higher roughness value, which could also contribute to the relatively low device performance. 24,49,53 ■ CONCLUSIONS In summary, the use of benzothiophene dioxide as the endcapping group for a nonfullerene acceptor has been investigated for the first time with the synthesis of the novel small-molecule acceptor ITBC. The electron-withdrawing benzothiophene dioxide moiety gives ITBC a near IR absorption and low frontier energy levels, providing a new and simple approach to achieve red-shifted absorption and reduced energy levels compared to other end-cap engineering methods such as the introduction of fluorine by multiple step synthesis. ...
... Recently, non-fullerene acceptors (NFAs) have become increasingly common as a replacement for traditional fullerene derivatives, and there have been a few reports of fluorinated acceptors which outperform their non-fluorinated counterparts in OPV devices, similar to the donor polymers discussed previously [21][22][23][24][25][26][27]. For example, Zhao et al. synthesized the fluorinated acceptor ITIC-Th1, which has an indacenodithieno[3,2-b]thiophene (IDTT) core and fluorinated 1,1-dicyanomethylene-3-indanone (INCN) end groups [28]. ...
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Fluorination of the donor polymer or non-fullerene acceptor (NFA) in an organic photovoltaic device is an effective method to improve device efficiency. Although there have been many studies on donor polymer fluorination, blends containing both a fluorinated donor and fluorinated NFA have rarely been reported. In this study, we use two donor polymers (4′-FT-HTAZ and 4′-FT-FTAZ) and two NFAs (ITIC-Th and ITIC-Th1) with different amounts of fluorine (from 2F to 6F) to investigate how the degree of fluorination in a blend impacts device performance. We find that fluorinating the NFA leads to a higher short-circuit current density (Jsc) and fill factor (FF), however, the open-circuit voltage (Voc) is decreased due to a depressed lowest unoccupied molecular orbital (LUMO) level. Adding additional fluorine to the donor polymer does not have a large effect on the Jsc or FF, but it does lead to an improved Voc. By fluorinating the NFA and having more fluorine on the donor polymer, we obtain both a high Jsc and Voc simultaneously, leading to a power conversion efficiency over 10% in the case of 4′-FT-FTAZ:ITIC-Th1, which has the most amount of fluorine (6F).
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Indacenodithiophene (IDT) is a promising building block for designing organic semiconductors. In this work, a new decacyclic building block (m-IDTIDT) with a doubled IDT unit and meta-alkyl-phenyl substitutions was synthesized and used for designing a new non-fullerene electron acceptor (m-IDTIDT-FIC). Compared to p-IDTIDT-IC, m-IDTIDT-FIC has smaller π−π distance, larger crystalline coherence length, and a higher electron mobility. Thus, polymer solar cells (PSCs) based on PCE10 : m-IDTIDT-FIC can achieve a higher PCE (8.27%) than that of PCE10 : p-IDTIDT-IC -based PSCs (6.48%). In addition, PSCs based on J71 : m-IDTIDT-FIC show better photovoltaic performances than PSCs based on PCE10 : m-IDTIDT-FIC. This is because J71 : m-IDTIDT-FIC blend film has more balanced charge transport ability, smooth surface, and higher relatively domain purity than PCE10 : m-IDTIDT-FIC blend film. PSCs based on J71 : m-IDTIDT-FIC obtained the best photovoltaic performances with a PCE of 11.32%, an open-circuit voltage (Voc) of 0.92 V, a short-circuit current (Jsc) of 18.01 mA cm-2, and an fill factor (FF) of 68.30%. These results indicate that m-IDTIDT with meta-alkyl-phenyl substitutions is a promising IDT-based building block for high-performance non-fullerene electron acceptors.
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It is promising, yet challenging to employ molecules made from little synthesis complexibility for constructing efficient and low-cost organic solar cells (OSCs). Herein, two unfused acceptors, DF-TCIC and HF-TCIC, are developed for OSC application, in which 3,4-difluorothiophene core connects, through cyclopentadithiophene (CPDT) bridge, to 1,1-dicyanomethylene-3- indanone derivatives (IC for DF-TCIC and DFIC for HF-TCIC, respectively). As mediated by the intramolecular non-covalent interaction, these unfused acceptors allow exhibiting nearly planar structure, and strong charge transfer effect. As results, the HF-TCIC based OSCs show high short-circurt current density (Jsc ) of 20.04 mA cm-2 and power conversion efficiency (PCE) of 9.86%, outperforming DF-TCIC based devices (Jsc of 16.39 mA cm-2 and PCE of 8.23 %). This work reveals unfused acceptor with reduced synthesis complexity is also promising for OSC applications.
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Three nonfullerene acceptors CO5DFIC, CO5DFIC-OT and CO5DFIC-ST were developed. CO5DFIC-OT and CO5DFIC-ST have alkoxythiophene and alkylthiothiophene -bridges, respectively, and they show higher LUMO and enhanced light-harvesting capability than CO5DFIC without -bridges. CO5DFIC-OT and CO5DFIC-ST solar cells gave higher open-circuit voltage, short-circuit current density and power conversion efficiency than CO5DFIC cells.
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Two non-fullerene electron acceptors (BT-SFIC and BT-FIC) bearing a dithienyl[1,2-b:4,5-b']benzodithiophene core were designed and synthesized. Due to the strong electron-withdrawing ability of fluorine atoms, these two non-fullerene acceptors both show near-infrared absorption around 900 nm, with a narrow bandgap of 1.44 and 1.39 eV for BT-SFIC and BT-FIC, respectively. BT-SFIC and BT-FIC have complementary absorption and matched energy level with PTB7-Th. PSCs based on PTB7-Th donor and these two acceptors both show good charge dissociation and collection, small charge recombination and good active layer morphology. However, BT-FIC bearing much more fluorine have much higher electron mobility. And the PSCs based on PTB7-Th: BT-FIC gives a maximum PCE of 10.10% with a VOC of 0.73 eV, a JSC of 21.28 mA cm⁻² and an FF of 65%, which is among in the high-performance polymer organic solar cells based on PTB7-Th.
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In this work, three near‐infrared (NIR) absorption nonfullerene small‐molecule acceptors (NF‐SMAs) (BDSeIC, BDSeIC2Br, and BDSeIC4Br) based on a fused benzo[1,2‐b:4,5‐b′]diselenophene unit as the electron‐rich central core and 2‐(2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)propanedinitrile (INCN) without or with one or two bromine substituents as the electron‐deficient group have been synthesized for polymer solar cells. Compared to BDSeIC without bromine substitution, these multibrominated materials BDSeIC2Br and BDSeIC4Br exhibit lower energy levels, stronger absorption in the range of 500–900 nm, better crystalline quality, and enhanced electron mobility. The optimal BDSeIC2Br‐based devices with PM6 as the donor, achieved a high power conversion efficiency (PCE) of up to 12.5% with a relatively low energy loss (Eloss) of 0.52 eV. The PCE of 12.5% for the BDSeIC2Br‐based devices are much higher than those devices based on PM6:BDSeIC (7.1%) or PM6:BDSeIC4Br (9.6%) blend films, and it is the highest reported PCE in binary PSCs with the brominated INCN end‐capped NF‐SMAs. Such outstanding PCE of BDSeIC2Br‐based device is attributed to more balanced electron/hole mobility, higher charge dissociation and charge collection efficiency, and more proper phase separation features. These results indicate that introducing a benzo[1,2‐b:4,5‐b′]diselenophene core unit and bromine substiution on the end groups is an effective way to achieve high‐performance NIR absorption NF‐SMAs. Three nonfullerene acceptors based on a fused benzo[1,2‐b:4,5‐b′]diselenophene as the electron‐rich central core with zero to four bromine atoms on the electron‐deficient group are synthesized for polymer solar cells (PSCs). The PM6:BDSeIC2Br based device achieves a PCE of 12.5% with a relatively low Eloss of 0.52 eV, which is the highest PCE in brominated INCN end‐capped NF‐SMAs based binary PSCs.
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In this work, two wide bandgap polymers of PDTT-TZNT and PDTF-TZNT were developed by Stille-coupling of naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) acceptor unit with bithiophene (DTH) and fluorinated bithiophene (DTF), respectively. These polymers exhibited a wide bandgap over 1.84 eV. The fluorinated PDTF-TZNT had lower highest occupied molecular orbital HOMO level (− 5.24 eV), higher molar absorption coefficient (1.28 × 10⁵ M⁻¹ cm⁻¹), and higher molecular packing order. Using a low bandgap 3,9-bis(2-methylene-(5&6-methyl-(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 (IT-M) as the electron acceptor, the PDTF-TZNT:IT-M devices generated a higher power conversion efficiency (PCE) of 10.05%. To make up the weak absorption of above binary blend of PDTF-TZNT:IT-M in the short wavelength region and increase the device performance further, a large bandgap small molecular acceptor of 5,5,10,10,15,15-hexabutyl-2,7,12-tri(4-(3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile-benzothiadiazole)-truxene (meta-TrBRCN) was added as the second acceptor material to fabricate ternary blend PSCs. The meta-TrBRCN could not only expand the absorption range but also fine-tune the blend morphology by stepwise changing its content. When 0.2 of meta-TrBRCN was added, the PCE of PDTF-TZNT:IT-M devices was improved to 11.48%.
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Ladder-type non-fullerene acceptors develop very rapidly since 2015 as they have tunable structure, broad absorption region and good morphology control. The power conversion efficiency (PCE) of non-fullerene organic solar cell (NF-OSC) increased from below 6% up to over 13%, and. In this review, we analyze the fundamental concepts of organic bulk heterojunction (BHJ) solar cells and different building blocks of non-fullerene acceptors to have an overview of high-performance photovoltaic materials. Meanwhile, we also discuss several design guidelines for the chemical construction and modification, which majorly focus on the backbone variation, side-chain engineering and end-cap group substitution. The PCE of several backbones, such as IDT, IDTT, BDCPDT etc. have successfully exceeded 10% PCE, which demonstrate great potential for future optimization. The side-chains can affect the intermolecular packing by forming a 3-D conformation along with the rigid backbones. The end-cap groups can regulate the electronic property of the acceptors directly, and provide the main electron-transport channel which influence frontier orbital properties. Although design principles and mechanistic understanding of ladder-type acceptors have been explored, there are also many opportunities and challenges of device mechanism and molecular optimization which need to have deep study and investigation. We hope this review could assist readers to better understand the design principles of high-performance photovoltaic materials, molecular design and structure-property relationship.
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“The Same‐Acceptor‐Strategy” (SAS) adopts benzotriazole (BTA)‐based p‐type polymers paired with a new BTA based non‐fullerene acceptor BTA13 to minimize the trade‐off between the open‐circuit voltage (VOC) and short circuit current (JSC). The fluorination and sulfuration are introduced to lower the highest occupied molecular orbitals (HOMO) of the polymers. The fluorinated polymer of J52‐F shows the higher power conversion efficiency (PCE) of 8.36% than the analog polymer of J52, benefited from a good balance between an improved VOC of 1.18 V and a JSC of 11.55 mA cm⁻². Further adding alkylthio groups on J52‐F, the resulted polymer, J52‐FS, exhibits the highest VOC of 1.24 V with a decreased energy loss of 0.48 eV, compared with 0.67 eV for J52 and 0.54 eV for J52‐F. However, J52‐FS shows an inferior PCE (3.84%) with a lower JSC of 6.74 mA cm⁻², because the small ΔEHOMO between J52‐FS and BTA13 (0.02 eV) gives rise to the inefficient hole transfer and high charge recombination, as well as low carrier mobilities. The results of this study clearly demonstrate that the introduction of different atoms in p‐type polymers is effective to improve the SAS and realize the high (VOC) and PCE.
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A heptacyclic carbon-oxygen-bridged ladder-type unit, COi7, was designed. Two corresponding nonfullerene acceptors, COi7IC and COi7DFIC, were developed. The single crystal structure of COi7IC indicates an S-shaped backbone and a packing...
Article
Molecular engineering of non-fullerene acceptors (NFAs) is a promising strategy to uncover new stucture-property relationships and design principles for developing next-generation high perofrmance n-type materials. Two dithienocyclopentacarbazole (DTC) -based NFAs, DTC(4R)-IC and DTC(4R)-4FIC, are synthesized to elucidate the effects of incorporating aliphatic side chains and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC) end groups on the thermal and optoelectronic properties of these NFAs. Inverted organic solar cell architecture of ITO/ZnO/J71:NFA/MoO3/Ag is empoloyed. By replacing phenyl-based side chains with aliphatic side chrains on the carbon bridges of carbazole-based core, the device containing DTC(4R)-IC exhibits an enhanced PCE of 9.81%, a Voc of 0.96 V, a Jsc of 15.61 mA cm-2, and a FF of 65.49 %, whereas the device containing DTC(4Ph)-IC exhibits a PCE of 7.76%. The incorporation of 2FIC end groups affords DTC(4R)-4FIC featuring better device performance relative to DTC(4R)-IC, more red-shifted absorption edge, and broader absorption range upon fluroination. Notbably, the device fabricated with DTC(4R)-4FIC affords a Voc of 0.82 V, a Jsc of 18.92 mA cm-2, a FF of 70.22 % and a highest PCE of 10.89%, which is by far the highest PCE for NFA containing heptacyclic carbzole core.
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Small-molecule donors have critical effect on properties of the small molecule-based organic solar cells (SM-OSCs). In order to develop novel small-molecule donors, two (A'-D)2A type small molecules (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT are design -ed and synthesized, in which 5,6-difluoro-2,1,3-benzothiadiazole (DFBT) and novel asymmetry electron-rich thieno[3,2-c] isochromene (TiC) are introduced as the central electron-accepting (A) and the armed electron-donating (D) units, respectively. And their photoelectronic properties are studied. A significant effect of the TiC and electron- accepting (A') end units on these properties is observed. A deep HOMO energy level of -5.45 eV with a medium bandgap of 1.77 eV is exhibited. An significantly improved power conversion efficiency (PCE) of 7.55% is received in the (IDO-TiC-T)2-DFBT/ PC71BM based OSCs with a high open circuit voltage (VOC) of 0.9 V, which is 1.38 times higher than that in the (Rh-TiC-T)2-DFBT/PC71BM based cells. Our results illuminate that asymmetry TiC unit has a great potential to build up small-molecule donors for OSCs with high PCE and VOC. Keywords: thieno[3,2-c]isochromene, small-molecule donor, asymmetric electron-rich unit, fullerene, organic solar cells.
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A new synergistic strategy using electron-rich core units and altering the aromatic structure-based 1,1-dicyanomethylene-3-indanone (IC) as end-groups for nonfullerene PSCs is reported and investigated how to obtain excellent PCE with extreme low Eloss simultaneously. Specifically, two Benzo[1,2-b:4,5-b']diselenophene-based, A-D-A-type chlorinated NF-SMAs (BDSeThCl and BDSePhCl) are synthesized, which are linked with a new 2-chlorothienyl-based IC and a conventional monochlorinated phenyl-based IC as end-group, respectively. BDSePhCl exhibited a wider and red-shifted absorption and downshifted energy levels than that of BDSeThCl. The blend films of BDSePhCl:PM7 perform better charge generation properties, more suitable phase separation, and more balanced charge mobilities in comparison with that of BDSeThCl:PM7. Therefore, compared to the best PCE of 11.91% with Eloss of 0.58 eV for BDSeThCl:PM7 blends, the optimal BDSePhCl:PM7 blends show an enhanced PCE of 13.68% with reducing Eloss of 0.49 eV. Notably, the excellent PCE of 13.68% is the highest record for A-D-A-type NF-SMAs with monochlorinated IC group in binary PSCs. The Eloss of 0.49 eV is the lowest value reported for A-D-A typed NF-SMAs in binary PSCs with PCE >13%. These results demonstrate that tailoring the monochlorinated aromatic ring-based IC is an effective strategy to improve the PCE and reduce the Eloss simultaneously in binary PSCs.
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The performance of a wide-bandgap copolymer donor PDTPO-BDTT in nonfullerene solar cells was investigated. These solar cells presented broad photoresponse and high short-circuit current density. PDTPO-BDTT:IT-4F and PDTPO-BDTT:NNFA-4F solar cells with more efficient photoluminescence quenching and better film morphology gave decent power conversion efficiencies of 10.96% and 10.04%, respectively, which are much higher than that of the previously reported PDTPO-BDTT:fullerene solar cells.
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Three new chlorine substituted non-fullerene acceptors (COi6-2Cl-γ, COi6-2Cl-δ and COi6-2Cl-m) based on a highly electron-rich core, COi6, are designed and synthesized, showing ultra-narrow bandgaps as low as 1.31 eV. The oxygen-containing conjugated COi6 possesses a strong electron-donating capacity, resulting in strong intramolecular charge transfer and intermolecular π–π stacking at the derived acceptors. Three kinds of chlorine-substituted end groups have been chosen to react with the COi6 core in order to study the effect of the chlorine atom positioning on the performance of the final acceptor materials. The solar cells based on these three acceptor materials and a spectrally complementary polymer donor, PTB7-Th, have been systemically studied. It is noted that the mixed acceptor, COi6-2Cl-m, shows a relatively high power conversion efficiency (PCE) of 9.22%, which is better than that of the other two isomer-free acceptors. With detailed investigations on five different mixing ratios of ternary cells using two isomer-free small molecules as a co-acceptor (COi6-2Cl-γ : COi6-2Cl-δ), the highest PCE of 9.30% was obtained in ternary cells when the ratio of COi6-2Cl-γ and COi6-2Cl-δ was 1 : 1. From this observation, it is demonstrated that mixed materials in appropriate proportions are better than certain structure materials, COi6-2Cl-γ and COi6-2Cl-δ, in our system.
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A wide-bandgap copolymer donor PPN4T-2F based on phenanthridin-6(5H)-one unit was developed. PPN4T-2F has a large optical bandgap of 2.15 eV and a deep the highest occupied molecular orbital level of -5.43 eV. Solar cells based on PPN4T-2F and a nonfullerene acceptor, IT-4F, afforded a power conversion efficiency (PCE) of 8.54%. PPN4T-2F is comparable to those efficient ultra-wide-bandgap copolymer donors.
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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 non-covalent conformational locks of S···N, S···O, 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 alkyl side chain (TTz1). As expected, both SMAs show high-efficiency photovoltaic performance in the solution-processing OSCs using J71 as 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.
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Two phenazine copolymer donors P1 and P2 were developed. P1 and P2 have deep the highest occupied molecular orbital levels, enhanced light-harvesting capability and good hole mobility. Solar cells based on P2 and a nonfullerene acceptor, Y6, gave a power conversion efficiency (PCE) of 15.14%, which is the highest value achieved by phenazine-donor-based solar cells to date.
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A medium-bandgap acceptor IBCT based on an 2-(1-oxo-1,2-dihydro-3H-benzo[b]cyclopenta[d]thiophen-3-ylidene)malononitrile end unit was developed. IBCT has an optical bandgap of 1.65 eV and afforded a power conversion efficiency of 11.26% and an open-circuit voltage of 1.02 V in single-junction organic solar cells when blending with a wide-bandgap copolymer donor L1. The L1:IBCT solar cell was further used as the front cell in tandem solar cells, yielding a 15.25% efficiency. IBCT is among the best medium-bandgap acceptors.
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As a common feature in a majority of malignant tumors, hypoxia has become the Achilles’ heel of photodynamic therapy (PDT). The development of type‐I photosensitizers that show hypoxia‐tolerant PDT efficiency provides a straightforward way to address this issue. However, type‐I PDT materials have rarely been discovered. Herein, a π‐conjugated molecule with A–D–A configuration, COi6‐4Cl, is reported. The H2O‐dispersible nanoparticle of COi6‐4Cl can be activated by an 880 nm laser, and displays hypoxia‐tolerant type I/II combined PDT capability, and more notably, a high NIR‐II fluorescence with a quantum yield over 5%. Moreover, COi6‐4Cl shows a negligible photothermal conversion effect. The non‐radiative decay of COi6‐4Cl is suppressed in the dispersed and aggregated state due to the restricted molecular vibrations and distinct intermolecular steric hindrance induced by its four bulky side chains. These features make COi6‐4Cl a distinguished single‐NIR‐wavelength‐activated phototheranostic material, which performs well in NIR‐II fluorescence‐guided PDT treatment and shows an enhanced in vivo anti‐tumor efficiency over the clinically approved Chlorin e6, by the equal stresses on hypoxia‐tolerant anti‐tumor therapy and deep‐penetration imaging. Therefore, the great potential of COi6‐4Cl in precise PDT cancer therapy against hypoxia challenges is demonstrated. An A–D–A‐type π‐conjugated molecule, COi6‐4Cl, is synthesized and exploited in tumor theranostics. COi6‐4Cl absorbs 880 nm light to emit bright NIR‐II emission and exert type‐I/II combined photodynamic reactions, which can realize deep‐penetration fluorescence imaging and hypoxia‐tolerant photodynamic therapy simultaneously by the same NIR‐light irradiation. These features make COi6‐4Cl a distinguished single‐NIR‐wavelength‐activated phototheranostic material.
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Solar energy conversion has nowadays attracted research interests of the community, wherein conjugated polymers (CPs) become a class of workhorses on photon-to-electron and photon-to-fuel conversion studies. In recent years, exciting breakthroughs have been made in these multi-interdisciplinary fields, with the assistance of the intrinsic flexibilities on tuning optoelectronic, mechanical and structural properties of CPs. In this review, we summarize the recent notable development of CPs in polymer solar cells, perovskite solar cells, and photocatalysts, wherein CPs function well as light-capture and conversion components for polymer solar cells and photocatalysts as well as charge extraction materials for perovskite solar cells. By analyzing the principles, status, and structural-properties of these areas, we outline the design strategies and perspectives of CPs for further advancing solar energy conversions.
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The development of A-D-A type heteroacenes for bulk-heterojunction solar cells has gained immense interest in the last 5-6 years due to synthetic accessibility via new molecular design, facile functionalization, tunable optoelectronic properties, cost-effective device fabrications and high efficiencies. Specifically, they have reached power conversion efficiencies (PCE) up to 18% as electron acceptors, demonstrating their effectiveness to replace fullerene derivatives in OSCs. This review explores the important aspects of the structural modifications of A-D-A-type heteroacenes and their application in high performance binary, ternary and tandem OSCs. The modulation of functional groups and their influence on the frontier orbital tuning, blend morphology, charge transport properties have been comprehensively discussed as a tool to correlate molecular structure-properties with device performance beneficial for new material design. Finally, their applications and prospects in semitransparent OSCs are discussed as a potential for future technology.
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Organic solar cells are composed of electron donating and accepting organic semiconductors. Whilst a signifcant palette of donors has been developed over three decades, until recently only a small number of acceptors have proven capable of delivering high power conversion efciencies. In particular the fullerenes have dominated the landscape. In this perspective, the emergence of a family of materials–the nonfullerene acceptors (NFAs) is described. These have delivered a discontinuous advance in cell efciencies, with the signifcant milestone of 20% now in sight. Intensive international efforts in synthetic chemistry have established clear design rules for molecular engineering enabling an ever-expanding number of high efciency candidates. However, these materials challenge the accepted wisdom of how organic solar cells work and force new thinking in areas such as morphology, charge generation and recombination. This perspective provides a historical context for the development of NFAs, and also addresses current thinking in these areas plus considers important manufacturability criteria. There is no doubt that the NFAs have propelled organic solar cell technology to the efciencies necessary for a viable commercial technology–but how far can they be pushed, and will they also deliver on equally important metrics such as stability?
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A series of acceptor-donor-acceptor type of fused-ring non-fullerene acceptors, FOR-IN, FOR-1F and FOR-2F, were designed and synthesized, featuring with the same pyran-composed backbone and different fluorine substituted 1,1-dicyanomethylene-3-indanone terminals. The...
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Organic photovoltaics (OPVs) represent one of the potential candidates of next-generation solar cells for converting the green and sustainable solar energy into electrical power. An OPV cell utilizes a blend of electron donor (D) and electron acceptor (A) materials as the photo-active layer, where the photogenerated excitons are separated into mobile electrons and holes. Relative to the fullerene acceptors, nonfullerene small-molecule acceptors (NF-SMAs) have several advantages such as the synthesis-facile chemical modifications and straightforward tunability in the absorptivity, spectral coverage, optical band gap, and frontier molecular orbitals. In recent 3 years, the progress in design and synthesis of the fused-ring NF-SMAs with perpendicular side-chains on the electron-rich core, and again, on the design and synthesis of the wide/medium/low band gap polymer donors have led to realizations of over 13% power conversion efficiencies (PCEs). The rapid advances requires timely review articles. In this review article, we will focus on this type of fused-ring NF-SMAs reported in the past 3 years, with sepcial attention on their molecular structure design and structure-property relationship.
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We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0 to 2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550-850 nm) and strong absorption with high extinction coefficients of (2.1-2.5) ×10(5) M(-1) cm(-1). Fluorine substitution down shifts LUMO energy level, red shift absorption spectrum, and enhance electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.
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A D-A conjugated polymer (PThTPTI) was developed by using a brand new pentacyclic aromatic lactam acceptor unit (TPTI). PThTPTI possesses good light absorption, thermal stability, and a deep HOMO level. PThTPTI/PC71BM cells afford an outstanding PCE up to 7.80%, with high Voc (0.87 V), Jsc (13.69 mA cm−2) and FF (65.6%), and over 70% EQE in the range of 435-640 nm.
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Power conversion efficiency (PCE) has surpassed 10% for single junction organic solar cells (OSCs) mainly through the design and synthesis of novel donor materials, the optimization of film morphology and the evolution of the devices. However, the development of novel acceptor materials is relatively sluggish compared with the donor compounds. Nowadays, fullerene derivatives, such as PC61BM and PC71BM, are still the dominant acceptors due to their superior charge transporting properties. Unfortunately, these two acceptors suffer from some intrinsic shortcomings such as limited absorption, difficult functionalization, and high production cost. Therefore, developing novel non-fullerene acceptors that can overcome the above-mentioned disadvantages is highly desirable. As a matter of fact, research on non-fullerene acceptors has made considerable progress in the last two years and a highest PCE of around 12% has been achieved. In this review, we will summarize recent research progress in non-fullerene small molecule acceptors and compare these molecules' performances in OSCs employing the same donor materials. Moreover, the acceptors with excellent photovoltaic performance are highlighted and the reasons are elaborated. Finally, the implications and the challenges are proposed.
Article
Nowadays, organic solar cells (OSCs) with efficiencies over 10% have been achieved through the elaborate design of electron donors and fullerene acceptors. However, the drawbacks of fullerene acceptors, like poor absorption, limited chemical and energetic tunabilities, high-cost purification and morphological instability, have become the bottlenecks for the further improvement of OSCs. To overcome the mentioned shortages from fullerene, research studies on non-fullerene electron acceptors have boomed. To date, the highest efficiency of fullerene-free OSCs has been pushed to be 12%, which surpasses that of fullerene-based OSCs. In this perspective, we focus on summarizing the development of small molecule electron acceptors designed to replace the fullerene derivatives. Since it has been revealed that the search for matched donor:acceptor pairs is important for accomplishing high efficiencies, we therefore divide electron acceptors into several categories according to the donors used in fullerene-free OSCs. After the introduction of these acceptors, we outline the designing rules as well as perspectives for the development of non-fullerene acceptors. We believe that the development of non-fullerene electron acceptors will make organic photovoltaics closer to practical applications.
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Isomer-free fullerene bisadducts were predicted to be high-performance acceptors for bulk-heterojunction (BHJ) solar cells. They can afford higher open-circuit voltage compared with monoadducts because of the higher LUMO levels, and can also provide higher short-circuit current and fill factor compared with the isomer mixture because of reduced energetic disorder. However, the application of isomer-free fullerene bisadducts in BHJ solar cells met synthetic challenges. In this work, a new regioselective synthesis method, so-called prebisaddition-confined bisfunctionalization (PCB), was developed for preparing isomer-free C60 bisadducts. This synthesis is highly selective for obtaining homo- and hetero-bisadducts with “e configuration”. Isomer-free C60 bisadducts, e-C60[C(CO2tBu)2]2, e-NC60BA and e-PPMF, were successfully synthesized via PCB. Gram-scale synthesis of e-C60[C(CO2tBu)2]2 was realized. Single crystals of e-PPMF and two intermediates were obtained. Electrochemical studies proved that the single isomer has reduced energetic disorder. e-PPMF featuring a compact methano group and a solubilizing side chain showed good performance in P3HT and low-bandgap polymer solar cells. A decent power conversion efficiency (PCE) of 8.11% was achieved from solar cells consisting of e-PPMF and the lowbandgap polymer, PPDT2FBT. The outstanding photovoltaic performance of e-PPMF among the fullerene bisadducts results from its better packing and reduced energetic disorder.
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We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = -5.66 eV, LUMO = -3.93 eV; ITIC: HOMO = -5.48 eV, LUMO = -3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-bandgap polymer donors and wide-bandgap polymer donors. ITIC-Th has higher electron mobility (6.1 X 10-4 cm2 V-1 s-1) than ITIC (2.6 X 10-4 cm2 V-1 s-1) due to enhanced intermolecular interaction induced by sulfur-sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-bandgap and wide-bandgap polymer donors. In one case a power conversion efficiency of 9.57% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.
Article
Although fullerenes and their derivatives, such as PCBM, have been the dominant electron-acceptor materials in organic photovoltaic cells (OPVs), they suffer from some disadvantages, such as weak absorption in the visible spectral region, limited spectral breadth and difficulty in variably tuning the band gap. It is necessary to explore non-fullerene electron acceptors that will not only retain the favorable electron-accepting and transporting properties of fullerenes but also overcome their insufficiencies. After a decade of mediocrity, non-fullerene acceptors are undergoing rapid development and are emerging as a hot area of focus in the field of organic semiconductors. Solution-processed bulk heterojunction (BHJ) OPVs based on non-fullerene acceptors have shown encouraging power conversion efficiencies of over 4%. This article reviews recent developments in several classes of solution-processable non-fullerene acceptors for BHJ OPVs. The remaining problems and challenges along with the key research directions in the near future are discussed.
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