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End-cap Group Engineering of a Small Molecule Non-Fullerene Acceptor: The Influence of Benzothiophene Dioxide
... Cao et al. and our group have reported two NFAs with sulfonyl-based ending groups. 38,39 However, the device efficiency is lower than that of the ketone-based counterpart. In this work, we combine the sulfonyl-based ending groups with a 2,1,3-benzothiadiazolebased core to obtain a new NFA, BTP-IS. ...
... We also mentioned that the reported sulfonyl-based NFA (ITBC), which adopts an indacenodithieno[3,2-b]-thiophene (IDTT) core, exhibited very low electron mobility. 38 Compared to IDTT, the 2,1,3benzothiadiazole-based core has larger conjugated system and better planarity. Thus, the 2,1,3-benzothiadiazole-based core is beneficial to attenuate the steric hindrance of the sulfonyl groups. ...
... The balance charge transport should be vital in the OSCs with sulfonyl-based NFAs since the low-efficiency ITBC device exhibited a very poor balance in charge transport. 38 We then tested the J−V curves under different light intensities to assess the charge recombination. Generally, the relationship between J SC and light intensity (P) obeys a power law, J SC ∝ P α , where the exponent α value is related to bimolecular recombination. ...
Ending groups play a vital role in regulating the band gap and energy level of low-band gap nonfullerene acceptors (NFAs). In this work, a novel NFA, BTP-IS, is synthesized by adopting sulfonyl-based ending groups. Compared to the ketone counterpart BTP-IC, BTP-IS exhibits a red-shift in absorption spectra with lower lowest unoccupied molecular orbital level. More importantly, the BTP-IS-based organic solar cells with PM6 as donor present a high power conversion efficiency (PCE) of 12.79%, which is much higher than that of the BTP-IC device (PCE of 7.54%). The efficient charge transfer between the polymer donor and NFA acceptor, the balance charge transport, and the fine photoactive morphology bear on the effective exciton dissociation and charge collection in the BTP-IS device, which induces high short circuit current (JSC) and fill factor (FF) values. This research has shed light on designing novel NFAs from the perspective of ending groups.
... 131 The substitution of the outer thiophene ring in 7-34 with a phenyl ring leads to structure 7-35 (ITBC) 134 and affects the energy levels by lowering them again. Compared to 7-34, the PV parameters of 7-35 with PBDB-T are improved to a J SC of 19.9 mA cm −2 , a FF of 65%, a PCE of 12.1%, and a decreased V OC of 0.94 V. 131,134 In acceptor 7-36 (ITBC), 138 the oxo group of INCN was replaced with a SO 2 functionality. This downshifted the energy levels but maintained a similar band gap as ITIC. ...
... Combined with the polymer FTAZ, it leads to a quite low V OC , J SC , and FF with a PCE of 4.17%. 138 In molecule 7-37 (IDTT-R), 140 a 2-(1,1-dicyanomethylene)rhodanine (RCN) acceptor unit is introduced, which heightens the HOMO/ LUMO energy levels compared to ITIC. Due to a blue-shift in the absorption of 7-37, the optical band gap is widened to 1.84 eV. ...
Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.
... In this investigation, femtosecond ND2PA spectroscopy (ND2PAS) is used to simultaneously obtain changes in ND2PA spectra and their respective magnitudes for all compounds (Figure 2) 34 and ITBC. 39 Steady-State Absorption Spectroscopy. UV−vis-NIR measurements were performed on a Shimadzu UV3101-PC dual arm spectrometer using 1 cm quartz cuvettes. ...
... 1−10 μM) for seven-ring, indacene-based compounds ITIC, ITIC3, F7IC, and ITBC are plotted in Figure 6. The reported reduction potentials for films of these compounds shift to increasingly reducing (more negative) potential in the order ITIC 31 < ITIC3 34 < F7IC 33 < ITBC, 39 suggesting that the acceptor strength of the end groups increases in the same order. Previous studies of quadrupolar A-D-A chromophores 10,48 show bathochromic shifts of the 1PA band with increasing acceptor strength; the successively bathochromically shifted 1PA and concomitantly increased M ge values in the present series, as summarized in Table 3, is consistent with this behavior and with the acceptor strength implied by the reduction potentials. ...
The two-photon absorption (2PA) properties are investigated for two series of organic, π-conjugated, fused-ring, quadrupolar A-π-D-π-A chromophores of the type originally developed as non-fullerene acceptors for organic photovoltaics. These molecules are found to exhibit large nondegenerate two-photon absorption (ND2PA) cross-sections (ca. 6-27 × 10³ GM) in the near infrared (NIR). In the first series, involving molecules of varying core size, ND2PA spectra and cross-sections characterized by femtosecond ND2PA spectroscopy in chloroform solutions reveal that increases in core size, and thus conjugation length, leads to substantially red-shifted and enhanced 2PA. In a second series, variation of the strength of the terminal acceptor (A) with constant core size (7 rings, indacene-based) led to less dramatic variation in the 2PA properties. Among the two core types studied, compounds in which the donor has a thieno[3,2-b]thiophene center demonstrate larger 2PA cross-sections than their indacene-centered counterparts, due to the greater electron-richness of their cores amplifying intramolecular charge transfer. Excited-state absorption (ESA) contributions to nonlinear absorption measured by open-aperture Z-scans are deduced for some of the compounds by analyzing the spectral overlap between 2PA bands and NIR ESA transitions obtained by ND2PA and transient absorption measurements, respectively. ESA cross-sections extracted from transient absorption and irradiance-dependent open-aperture Z-scans are in reasonable agreement and their moderate magnitudes (ca. 10⁻²¹ m²) suggest that, although ESA contributions are non-negligible, the effective response is predominantly instantaneous 2PA.
... Synthesis of 2,5-Di(2-triisopropylsilylselenopheno[3,2-b]thiophen-5-yl)terephthalic Acid Diethyl Ester (7). To a two-neck flask were added 6 (15.8 g, 25 mmol), 2,5-dibromoterephthalic acid diethyl ester (4.56 g, 12 mmol), PdCl 2 (PPh 3 ) 2 (505 mg, 6 mol %), and anhydrous toluene (50 mL) under an argon atmosphere. ...
A set of isomeric ladder-type non-fullerene acceptors (NFAs), SRID-4F and TRID-4F, was designed and synthesized to systematically investigate the structure-property relationship of selenium substitution on A-D-A type NFAs. It was found that regio-specific selenium substitution not only affects optical and electrochemical properties, but also changes the morphology of thin films. Photovoltaic devices based on SRID-4F showed high power conversion efficiency (PCE) over 13 % when paired with polymeric donor PBDB-T-2F.
The assembly of conjugated organic molecules from solution to solid-state plays a critical role in determining the thin film morphology and optoelectronic properties of solution-processed organic electronics and photovoltaics. During evaporative solution processing, π-conjugated systems can assemble via various forms of intermolecular interactions, forming distinct aggregate structures that can drastically tune the charge transport landscape in the solid-state. In blend systems composed of donor polymer and acceptor molecules, assembly of neat materials couples with phase separation and crystallization processes, leading to complex phase transition pathways which govern the blend film morphology. In this review, we provide an in-depth review of molecular assembly processes in neat conjugated polymers and nonfullerene small molecule acceptors and discuss their impact on the thin film morphology and optoelectronic properties. We then shift our focus to blend systems relevant to organic solar cells and discuss the fundamentals of phase transition and highlight how the assembly of neat materials and processing conditions can affect blend morphology and device performance.
The electron acceptor materials in organic solar cells (OSCs) play an essential role in enhancing power conversion efficiency (PCE). Although Y6‐based nonfullerene acceptors (NFAs) have shown fascinating experimental progress, molecular engineering is an effective strategy for further improvement of PCE. Herein, a series of Y6‐based symmetric and asymmetric NFAs are designed using the Y6‐based core with various end‐group units. The impact of end group units on geometric, electronic, and excited state properties of NFAs is studied using state‐of‐the‐art density functional theory methods. Various selection criteria are used to screen potential NFAs among 18 newly designed NFAs. The interfacial electronic properties between conjugated polymer and NFAs are thoroughly studied in the excited state to analyze the potentiality of screened NFAs. More importantly, the screened asymmetric NFAs have shown improved charge mobilities with a lower charge recombination rate than prototype FRY6. It is noticed that screened NFAs have multiple charge transfer pathways through direct excitation, hot excitons, and intermolecular electric field mechanisms. Overall, the results obtained from this computational study give useful guidance for developing NFAs for high‐performance OSCs.
We investigated the optoelectronic and photovoltaic properties of three types of acceptor-donor-acceptor-based non-fullerene acceptor (NFA) molecules for organic solar cell (OSC) applications. Density functional theory and its time-dependent variant were employed to compute the quadrupole moment perpendicular to the π-system (Q20), open circuit voltage (VOC), and other relevant solar cell parameters. The role of functionalization in the acceptor unit on the overall device performance was explored by incorporating halogen and methoxy-based electron-withdrawing groups. The electronegativity differences between the halogen atoms and the methoxy group demonstrated contrasting effects on the energy levels, molecular orbitals, and absorption maximum. We observed a trade-off between short-circuit current (JSC) and VOC, which was further substantiated by an inverse correlation between Q20 and VOC. We found an optimum value of Q20 in the range of 80 to 130 ea02 to achieve an optimized solar cell performance. Among the designed systems, Se-derived NFAs with a small band gap, red-shifted absorption maximum, high-oscillator strength, small exciton binding energy, and optimum Q20 turned out to be potential candidates for future applications. These criteria can be generalized to design and screen next-generation non-fullerene acceptors to achieve improved OSC performance.
For effective supplementary acceptor molecules (A 2 ) in ternary organic solar cell (TOSC) devices, a series of ITIC derivatives was designed and synthesized by incorporating symmetrically or asymmetrically functional termini of...
Here is reported an expedient synthesis implementing enabling technologies of a family of thiophene-based heptamers alternating electron donor (D) and acceptor (A) units in a D–A′–D–A–D–A′–D sequence. The nature of the peripheral A groups (benzothiadiazole vs. thienopyrrole-dione vs. thiophene-S,S-dioxide) and the strength of the donor units (alkyl vs. thioalkyl substituted thiophene ring) have been varied to finely tune the chemical-physical properties of the D–A oligomers, to affect the packing arrangement in the solid-state as well as to enhance the photovoltaic performances. The optoelectronic properties of all compounds have been studied by means of optical spectroscopy, electrochemistry, and density functional theory calculations. Electrochemical measurements and Kelvin probe force microscopy (KPFM) predicted a bifunctional behaviour for these oligomers, suggesting the possibility of using them as donor materials when blended with PCBM, and as acceptor materials when coupled with P3HT. Investigation of their photovoltaic properties confirmed this unusual characteristic, and it is shown that the performance can be tuned by the different substitution pattern. Furthermore, thanks to their ambivalent character, binary non-fullerene small-molecule organic solar cells with negligible values of HOMO and LUMO offsets were also fabricated, resulting in PCEs ranging between 2.54–3.96%.
We introduce two novel solution-processable electron acceptors based on an isomeric core of the much explored diketopyrrolopyrrole (DPP) moiety, namely pyrrolo[3,2-b]pyrrole-1,4-dione (IsoDPP). The newly designed and synthesized compounds, 6,6′-[(1,4-bis{4-decylphenyl}-2,5-dioxo-1,2,4,5-tetrahydropyrrolo[3,2-b]pyrrole-3,6-diyl)bis(thiophene-5,2-diyl)]bis[2-(2-butyloctyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione] (NAI-IsoDPP-NAI) and 5,5′-[(1,4-bis{4-decylphenyl}-2,5-dioxo-1,2,4,5-tetrahydropyrrolo[3,2-b]pyrrole-3,6-diyl)bis(thiophene-5,2-diyl)]bis[2-(2-butyloctyl)isoindoline-1,3-dione] (PI-IsoDPP-PI) have been synthesized via Suzuki couplings using IsoDPP as a central building block and napthalimide or phthalimide as end-capping groups. The materials both exhibit good solubility in a wide range of organic solvents including chloroform (CF), dichloromethane (DCM), and tetrahydrofuran (THF), and have a high thermal stability. The new materials absorb in the wavelength range of 300–600 nm and both compounds have similar electron affinities, with the electron affinities that are compatible with their use as acceptors in donor-acceptor bulk heterojunction (BHJ) organic solar cells. BHJ devices comprising the NAI-IsoDPP-NAI acceptor with poly(3-n-hexylthiophene) (P3HT) as the donor were found to have a better performance than the PI-IsoDPP-PI containing cells, with the best device having a VOC of 0.92 V, a JSC of 1.7 mAcm−2, a FF of 63%, and a PCE of 0.97%.
Oxidized thiophene incorporated molecules show great potential as luminescent materials. However, most works focused on thiophene-S, S-dioxides (TDO) moiety, rarely on thiophene-S-oxide (TO) incorporated molecules. Herein, TO and TDO incorporated ladder-type molecules, namely IDT2O–H and IDT4O–H, have been designed and synthesized to study their luminescent properties. Compared to IDT4O–H, IDT2O–H not only shows stronger and bathochromic fluorescence emission in solution and film states but also exhibits unusual anti-Kasha phosphorescence in film state. The experimental spectra and theoretical calculations demonstrate that the distinct energy levels and orbital features originated from their structures are crucial for the different luminescent properties of IDT2O–H and IDT4O–H.
A series of non-fullerene small molecules (BT-T-IDC and BT-T-tB-IDC) with the architecture of A1-π-A2-π-A1 (acceptor 1-π-acceptor 2-π-acceptor 1), electron-deficient benzothiadiazole (BT) used as the core (A2), electron-rich thiophene used as the π-bridge (π), and electron-deficient dicyanovinylindandione (IDC) or t-butyl substituted IDC used as end groups (A1), have been synthesized. IDC groups are widely used as the acceptor unit due to the strong electron-withdrawing property that can lead to deep LUMO energy levels. Materials showed good thermal stability with the onset decomposition temperature (Td) upon 310 °C at 5 wt% loss. BT-T-tB-IDC showed better solubility in chloroform and chlorobenzene than that of BT-T-IDC due to t-butyl groups on the IDC. The HOMO/LUMO energy levels of BT-T-IDC and BT-T-tB-IDC were −5.60/−3.90 and −5.71/−3.87 eV, respectively. Organic solar cell based on PBDBT-T (poly[(2,6-(4,8-bis (5-(2-ethylhexyl) thiophen-2-yl)-benzo [1,2-b:4,5-b′] dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl) benzo[1′,2′-c:4′,5′-c′] dithiophene-4,8-dione))])) as the donor and BT-T-IDC as the acceptor showed low power conversion efficiency (PCE) of 0.06% due to poor solubility. However, the PCE of the device based on BT-T-tB-IDC showed a better PCE of 1.55%.
We report 4 fused-ring electron acceptors (FREAs) with the same end-groups and side-chains but different cores, whose sizes range from 5 to 11 fused rings. The core size has considerable effects on the electronic, optical, charge transport, mor-phological and photovoltaic properties of the FREAs. Extending the core size leads to red-shift of absorption spectra, up-shift of the energy levels, enhancement of molecular packing and electron mobility. From 5 to 9 fused rings, the core size extension can simultaneously enhance open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) of organic solar cells (OSCs). The best efficiency of the binary-blend devices increases from 5.6% to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3% to 12.6% as the acceptor core size extends.
To simultaneously achieve the low photon energy loss (Eloss) and the broad spectral response, the molecular design of the efficient wide band-gap (WBG) donor polymer with a deep HOMO level is of critical importance in fullerene-free polymer solar cells (PSCs). Herein, we developed a new benzodithiophene unit, i.e., DTBDT-EF and conducted systematic investigations on a WBG DTBDT-EF-based donor polymer, namely, PDTB-EF-T. Due to the synergistic electron-withdrawing effect of the fluorine atom and ester group, PDTB-EF-T exhibits higher oxidation potential, i.e. deeper HOMO level (ca. -5.5 eV) than most well-known donor polymers. Hence, a high open-circuit voltage of 0.90 V was obtained when paired with a fluorinated small molecule acceptor (IT-4F), corresponding to a low Eloss of 0.62 eV. Furthermore, side-chain engineering demonstrated that fine side-chain modulation of the ester greatly influences the aggregation effects and molecular packing of polymer PDTB-EF-T. Benefiting from the stronger interchain π-π interaction, the improved ordering structure, and thus the highest hole mobility, the most symmetric charge transport and reduced recombination are achieved for the linear decyl-substituted PDTB-EF-T (P2)-based PSCs, leading to the highest short-circuit current density and fill factor (FF). Due to the high Flory-Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. By taking advantage of the vertical phase distribution of the P2:IT-4F blend, a remarkably high power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 was recorded for inverted devices. These results demonstrate the great potential of DTBDT-EF unit for future organic photovoltaic applications.
Fluorine‐contained polymers, which have been widely used in highly efficient polymer solar cells (PSCs), are rather costly due to their complicated synthesis and low yields in the preparation of components. Here, the feasibility of replacing the critical fluorine substituents in high‐performance photovoltaic polymer donors with chlorine is demonstrated, and two polymeric donors, PBDB‐T‐2F and PBDB‐T‐2Cl, are synthesized and compared in parallel. The synthesis of PBDB‐T‐2Cl is much simpler than that of PBDB‐T‐2F. The two polymers have very similar optoelectronic and morphological properties, except the chlorinated polymer possess lower molecular energy levels than the fluorinated one. As a result, the PBDB‐T‐2Cl‐based PSCs exhibit higher open circuit voltage (Voc) than the PBDB‐T‐2F‐based devices, leading to an outstanding power conversion efficiency of over 14%. This work establishes a more economical design paradigm of replacing fluorine with chlorine for preparing highly efficient polymer donors.
Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near-infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high-performance fullerene-free OSCs based on a combination of a medium-bandgap polymer donor (FTAZ) and a narrow-bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single-junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.
Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction organic solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochemical stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused aromatic cores with strong electron-accepting end groups. The key structure–property relationships, donor–acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field.
We choose the high-performance nonfullerene acceptor ITIC-Th as an example, and incorporate electron-donating methoxy and electron-withdrawing F groups onto the terminal group 1,1-dicyanomethylene-3-indanone (IC) to construct a small library of four fused-ring electron acceptors. With this series, we systematically investigate the effects of the substituents on end-groups on electronic properties, charge transport, film morphology, and photovoltaic properties in ITIC-Th series. Electron-withdrawing ability increases from methoxylated to unsubstituted, fluorinated, and difluorinated IC, leading to downshift of energy levels and redshift of absorption spectra. The optimized organic solar cells based on ITIC-Th series show power conversion efficiencies ranging from 8.88% to 12.1%.
A series of new non-fullerene small molecule acceptors (NTIC, NTIC-Me, NTIC-OMe and NTIC-F) based on the acceptor-donor-acceptor (A-D-A) architecture, using hexacyclic naphthalene-(cyclopentadithiophene) as the central unit were designed and synthesized. The non-fullerene OSC device based on PBDB-T:NTIC showed a highest PCE of 8.63%. With the relative high-lying LUMO level of NTIC-OMe, the PBDB-T:NTIC-OMe based device obtained a comparatively high Voc of 0.965 V and a PCE of 8.61% simitanousely. The results demonstrate that naphthalene core is a promising building block for constructing high efficicent non-fullerene accepotors and furthur boost the photovaltaic perfomance of the devices.
A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 10(5) m(-1) cm(-1) , higher than that of ITIC1 (1.5 × 10(5) m(-1) cm(-1) ). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (-5.43 eV) and lowest unoccupied molecular orbital (LUMO) (-3.80 eV) energy levels relative to ITIC1 (HOMO: -5.48 eV; LUMO: -3.84 eV), and higher electron mobility (1.3 × 10(-3) cm(2) V(-1) s(-1) ) than that of ITIC1 (9.6 × 10(-4) cm(2) V(-1) s(-1) ). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.
A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
Harvesting energy directly from sunlight using photovoltaic technology has become an essential component of future global energy production. Although silicon-based inorganic materials have still played a dominant role in the market, problems for inorganic materials-based solar cells mainly lie in (1) high cost of production and technical difficulties in the fabrication of large-area cells and (2) the highly purified silicon supply problem. In contrast to inorganic photovoltaic materials, organic/polymer photovoltaic materials have currently become of broader interest owing to their great potential to bring about a major breakthrough in reducing the cost of solar cells. As a promising building block to construct narrow bandgap semiconductors for high efficient photovoltaics, structurally well-defined indacenodithiophene (IDT) and its derivatives have inspired great interest in the industrial and academic communities over recent years due to their rigidified and tunable coplanar fused ring aromatic structures, and 3D conformation as well. The aromatic fused-ring blocks efficiently restrain rotational disorder and consequently lower reorganization energy. An encouraging power conversion efficiency of more than 12% has been achieved in the IDT-based polymer solar cells. This review surveys recent research advances in the area of IDT-based conjugated materials for photovoltaic applications. The factors affecting the bandgaps, molecular energy levels, film morphologies, as well as the photovoltaic performance of these materials have also been discussed.
To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.
Development of high-performance donor–acceptor (D–A) copolymers
is vital in the research of polymer solar cells (PSCs). In this work, a lowbandgap
D–A copolymer based on dithieno[3,2-b:2′,3′-d]pyridin-5(4H)-one
unit (DTP), PDTP4TFBT, is developed and used as the donor material
for PSCs with PC71BM or ITIC as the acceptor. PDTP4TFBT:PC71BM and
PDTP4TFBT:ITIC solar cells give power conversion efficiencies (PCEs) up to
8.75% and 7.58%, respectively. 1,8-Diiodooctane affects film morphology
and device performance for fullerene and nonfullerene solar cells. It inhibits
the active materials from forming large domains and improves PCE for
PDTP4TFBT:PC71BM cells, while it promotes the aggregation and deteriorates
performance for PDTP4TFBT:ITIC cells. The ternary-blend cells based on
PDTP4TFBT:PC71BM:ITIC (1:1.2:0.3) give a decent PCE of 9.20%.
A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).
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.
Fullerene-free organic solar cells show over 11% power conversion efficiency, processed by low toxic solvents. The applied donor and acceptor in the bulk heterojunction exhibit almost the same highest occupied molecular orbital level, yet exhibit very efficient charge creation.
Fine energy-level modulations of small-molecule acceptors (SMAs) are realized via subtle chemical modifications on strong electron-withdrawing end-groups. The two new SMAs (IT-M and IT-DM) end-capped by methyl-modified dicycanovinylindan-1-one exhibit upshifted lowest unoccupied molecular orbital levels, and hence higher open-circuit voltages can be observed in the corresponding devices. Finally, a top power conversion efficiency of 12.05% is achieved.
Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.
In this work, we designed, calculated and synthesized a planar electron acceptor IDT-2BR with an A-D-A structure, using indacenodithiophene (IDT) unit as a core and 5-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)-3-ethyl-2-thioxothiazo -lidin-4-one (BR) unit as end-capping electron-withdrawing groups. Theoretical calculations revealed that IDT-2BR adopted a nearly flat backbone configuration while the hexylphenyl groups on the IDT moiety exhibited a dihedral angle of ca. 115[degree] to the backbone plane. IDT-2BR exhibited excellent thermal stability, broad and strong absorption from 300 to 750 nm with a maximum extinction coefficient of 1.3 [times] 105 M-1 cm-1 at 636 nm, appropriate LUMO (-3.69 eV) and HOMO (-5.52 eV) energy levels matched with those of P3HT, and relatively high electron mobility of 3.4 [times] 10-4 cm2 V-1 s-1. Fullerene-free PSCs based on P3HT: IDT-2BR blended films gave PCEs of up to 5.12%, which is much higher than that of PC61BM-based control devices (3.71%) and is the highest value
Low lying excited states of the fullerene anion promote a faster charge separation in organic solar cells containing fullerene derivatives as electron acceptors. Alternative electron acceptors, not based on fullerenes but that share the same property, can be easily designed. On the other hand, it is unlikely for a generic electron acceptor to replicate this fullerene characteristic by chance.
Significance
For producing electricity, polymer solar cells (PSCs) offer properties tunability, light weight, scalability, and earth-abundant materials. PSC active layers typically consist of donor polymer and fullerene acceptor blends having discrete conduits for photogenerated hole and electron conduction. The spherical fullerene shape, which enables close packing, orbital degeneracies, and low charge-transfer reorganization energies, is thought to be essential for efficient photocurrent generation and high power conversion efficiencies (PCEs). However, the recent advent of irregularly shaped indacenodithienothiophene (IDTT) acceptors yielding higher PCEs challenges the fullerene paradigm. In a combined experimental and theoretical study with two new isomeric IDTT derivatives, we shed light on the basis of this performance in terms of surprisingly close molecular packing, strong electronic coupling, and low reorganization energies.
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).
Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications.
Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC → donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.
We compared indacenodithiophene(IDT)-based fused-ring electron acceptor IDIC1 with its counterpart IHIC1 in which the central benzene unit is replaced by naphthalene unit, and investigated effects of benzene/naphthalene core on optical and electronic properties as well as performance of organic solar cells (OSCs). Compared with benzene-cored IDIC1, naphthalene-cored IHIC1 has a larger π-conjugation with stronger intermolecular π-π stacking. Relative to benzene-cored IDIC1, naphthalene-cored IHIC1 shows a higher lowest unoccupied molecular orbital energy level (IHIC1: −3.75 eV, IDIC1: –3.81 eV) and a higher electron mobility (IHIC1: 3.0 × 10−4 cm2 V−1 s−1, IDIC1: 1.5 × 10−4 cm2 V−1 s−1). When paired with the polymer donor FTAZ that has matched energy levels and complementary absorption spectrum, IHIC1-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency relative to the IDIC1-based control devices. These results demonstrate that extending benzene in IDT to naphthalene is a promising approach to upshift energy levels, enhance electron mobility, and finally achieve higher efficiency in nonfullerene acceptor-based OSCs.
The post-fullerene indacenodithiophene acceptor ITIC is a highly effective n-type component of high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs) for reasons that are not well-understood. Here, the impact of the ITIC alkyl sub-stituent architecture on PSC active layer film morphology, charge transport, and photovoltaic (PV) performance is investigated with the donor polymers PBDB-T and PBDB-TF. On progressing from n-propyl to n-hexyl to n-nonyl ITIC substituents, PSC power conversion efficiency (PCE) increases from <0.1 % to 9.31% to 10.24%, respectively. BHJ blend morphology, carrier recombina-tion dynamics, and PV performance with both donor polymers as probed by AFM, XRD, GIWAXS, and light intensity dependence correlate with marked differences in ITIC acceptor crystallinity. The DSC cold crystallization temperatures of the n-hexyl and n-nonyl-functionalized acceptors are found to closely track the anneal-ing temperatures for optimum PSC performance. These results identify a promising strategy for optimizing the performance of post-fullerene acceptor PSCs.
Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10−3 cm2 V−1 s−1 vs IHIC2: 5.0 × 10−4 cm2 V−1 s−1). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.
Donor polymer fluorination has proven to be an effective method to improve the power conversion efficiency of fullerene-based polymer solar cells (PSCs). However, this fluorine effect has not been well-studied in systems containing new, non-fullerene acceptors (NFAs). Here, we investigate the impact of donor polymer fluorination in NFA-based solar cells by fabricating devices with either a fluorinated conjugated polymer (FTAZ) or its non-fluorinated counterpart (HTAZ) as the donor polymer and a small molecule NFA (ITIC) as the acceptor. We found that, similar to fullerene-based devices, fluorination leads to an increased open circuit voltage (Voc) from the lowered HOMO level and improved fill factor (FF) from the higher charge carrier mobility. More importantly, donor polymer fluorination in this NFA-based system also led to a large increase in short circuit current (Jsc), which stems from the improved charge transport and extraction in the fluorinated device. This study demonstrates that fluorination is also advantageous in NFA-based PSCs and may improve performance to a higher extent than in fullerene-based PSCs. In the context of other recent reports on demonstrating higher photovoltaic device efficiencies with fluorinated materials, fluorination appears to be a valuable strategy in the design and synthesis of future donors and acceptors for PSCs.
A fused-ring thiophene-thieno[3,2-b]thiophene-thiophene (4T)-based low-band gap electron acceptor, 4TIC, has been designed and synthesized for non-fullerene solar cells. The utilization of the 4T center core enhances the charge mobility of 4TIC and extends its absorption band edge to 900 nm, which facilitates its function as a very efficient low-band gap electron acceptor. The rigid, conjugated framework of 4T also offers a lower reorganization energy to facilitate lower VOC energy loss. Femtosecond transient spectroscopy showed a level of polaron generation in 4TIC results in the more efficient transfer of energetic carriers higher than that seen with the benchmarked molecule, ITIC. Film morphology analysis has also shown that 4TIC has structural order that is more prominent than that of ITIC with a multiscale phase separation in the blend with donor polymer PTB7-Th. As a result, solar cells based on PTB7-Th and 4TIC exhibit a high power conversion efficiency of 10.43% and a relatively low non-ideal photon energy loss of 0.33 V. The low band gap and small energy loss make 4TIC suitable for tandem solar cells as a back-cell to reduce the transmission loss. As a demonstration, we fabricated series connection tandem solar cells incorporating 4TIC, which exhibts a high device performance of 12.62%.
Fullerene-free OSCs employing n-type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non-fullerene acceptors exhibit great potential for realizing high-performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non-fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.
Side group of ITIC-like small molecular acceptor (SMA) plays a critical role in crystallization property. In this article, two new SMAs with n-hexylthienyl and n-hexylselenophenyl as side chain, namely ITCPTC-Th and ITCPTC-Se, are designed and synthesized by employing newly developed thiophene-fused ending group (CPTCN). And thiophene and selenophene side group substituted effects of SMA-based fullerene-free polymer solar cells (PSCs) are investigated. A stronger σ-inductive effect between selenophene side group and electron-donating backbone endows ITCPTC-Se with better optical absorption and higher LUMO level, ITCPTC-Th-based PSCs deliver a higher power conversion efficiency of 10.61%. Charge transport and collection, recombination loss mechanism, and morphology of blend films are intensively studied. These results confirm that side group substituted effects of SMAs are multiple and thiophene is a superior option to selenophene as aromatic side group of ITIC-like SMAs.
A A new small molecule, ITTC, bearing indacenodithieno[3,2-b]thiophene core and 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile end group, was designed, synthesized and characterized as non-fullerene electron acceptor. The ITTC possesses strong and broad light absorption, high and balanced charge mobility and nanoscale interpenetrating morphology when blending with a recently synthesized hexafluoroquinoxaline based polymer donor-HFQx-T. HFQx-T was obtained from a Stille coupling copolymerization of 2, 6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b’]dithiophene electron donating unit and hexafluoroquinoxaline based electron accepting unit. The device employing HFQx-T as donor and ITTC as acceptor delivered a power conversion efficiency (PCE) of 8.19% without any post-treatment. After thermal annealing, an impressive PCE of 10.4% was obtained. This performance is among the highest PCEs reported for fullerene-free polymer solar cells up to date. This study demonstrates the great potential of ITTC as n-type material for organic electronics.
The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.
A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.
The device efficiency of polymer:fullerene bulk heterojunction solar cells has recently surpassed 11%, as a result of synergistic efforts among chemists, physicists, and engineers. Since polymers are unequivocally the "heart" of this emerging technology, their design and synthesis have consistently played the key role in the device efficiency enhancement. In this article, the first focus is a discussion on molecular engineering (e.g., backbone, side chains, and substituents), then the discussion moves on to polymer engineering (e.g., molecular weight). Examples are primarily selected from the authors contributions; yet other significant discoveries/developments are also included to put the discussion in a broader context. Given that the synthesis, morphology, and device physics are inherently related in explaining the measured device output parameters (Jsc , Voc and FF), we will attempt to apply an integrated and comprehensive approach (synthesis, morphology, and device physics) to elucidate the fundamental, underlying principles that govern the device characteristics, in particular, in the context of disclosing structure-property correlations. Such correlations are crucial to the design and synthesis of next generation materials to further improve the device efficiency.
Low bandgap n-type organic semiconductor (n-OS) ITIC has attracted great attention for the application as acceptor with medium bandgap p-type conjugated polymer as donor in non-fullerene polymer solar cells (PSCs) because of its attractive photovoltaic performance. Here we report a modification on the molecular structure of ITIC by side chain isomerization with meta-alkyl-phenyl substitution, m-ITIC, to further improve its photovoltaic performance. Compared with its isomeric counterpart ITIC with para-alkyl-phenyl substitution, m-ITIC shows a higher film absorption coefficient, a larger crystalline coherence and higher electron mobility. These inherent advantages of m-ITIC resulted in a higher power conversion efficiency (PCE) of 11.77% for the non-fullerene PSCs with m-ITIC as acceptor and a medium bandgap polymer J61 as donor, which is significantly improved over that (10.57%) of the corresponding devices with ITIC as acceptor. To the best of our knowledge, the PCE of 11.77% is one of the highest values reported in literatures to date for non-fullerene PSCs. More importantly, m-ITIC-based device shows less thickness-dependent photovoltaic behavior than ITIC-based devices in the active-layer thickness range of 80~360 nm, which is beneficial for large area device fabrication. These results indicate that m-ITIC is a promising low bandgap n-OS for the application as acceptor in PSCs and the side chain isomerization could be an easy and convenient way to further improve the photovoltaic performance of the donor and acceptor materials for high efficiency PSCs.
Herein, we design and synthesize a perylene diimide derivative with a fully fused backbone, FITP, which possesses an elevated lowest unoccupied molecular orbital level and high electron mobility. Consequently, polymer solar cells with FITP as the acceptor can provide the best efficiency of 7.33% with a high voltage of 0.99 V.
From hybrid perovskites to semiconducting polymer/fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation.
A nonfullerene-based polymer solar cell (PSC) which significantly outperforms fullerene-based PSCs with respect to the power-conversion efficiency is demonstrated for the first time. An efficiency >11%, which is among the top values in the PSC field, and excellent thermal stability is obtained using PBDB-T and ITIC as donor and acceptor, respectively.
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.
Non-fullerene organic solar cells (NF-OSCs), in which an n-type organic molecule instead of a fullerene derivative is utilized as the electron-acceptor material, have recently emerged as a new topic in the field of organic solar cells. Replacement of the traditional fullerene acceptor in the photoactive layer of a normal organic solar cell with the organic acceptor gives rise to several advantages, like light absorption and energy level tunability, diversity of donor-to-acceptor combination, and large-scale production of acceptor materials. Studies on NF-OSCs can be traced back to 1986, when the first bilayered organic solar cell was proposed. Unfortunately, they has been advancing very slowly and the power-conversion-efficiency (PCE) was only approaching or exceeding 2% up to 2012. Fast advances have been driven forward since 2013, when the PCE value first broke through 4%, and the reported PCE value has now reached about 8% after a short period of 3 years. If we turn to natural systems such as the photosynthesis systems I and II, in which Nature utilizes organic molecules to accomplish high-efficiency solar-to-chemical energy conversion through the cascade unidirectional electron-hole transfer paths, we can rationally expect an even higher PCE and a convincing future for NF-OSCs. In this review, we will address recent new progress in this sub-branch of organic solar cells.
In the past two years, non-fullerene acceptors including polymers and small molecules have become the focus of many research efforts. Fullerene-free organic solar cells (OSCs) have shown efficiencies of up to 6.8% for solution-processed devices, and even up to 8.4% for vacuum-deposited devices, which have been significantly improved relative to those disclosed 2 years ago (generally <4%). Non-fullerene acceptor materials are a new focus in the OSC field. Tailoring extended fused-rings with electron-deficient groups is an effective strategy for design of acceptors. Here, very recent developments in several systems of fused ring-based electron acceptors, such as halogenated (sub or subna)phthalocyanine, imide-functionalized rylene, and linear fused-rings end capped with electron deficient blocks, are reviewed.
The study emphasizes the correlation between chemical structures, physical properties, and resulting device performance of the solution-processed polymers/small molecules, and tries to provide a comprehensive understanding from materials design to a variety of device applications. To achieve near-IR absorbing/emitting abilities, strong donor-acceptor interaction and stabilization of the quinoid resonant structure are required. However, due to the smaller one-dimensional quantum well length, bandgap lowering is more difficult and normally requires very strong donor or acceptor units in the case of small molecules. Anthopoulos and Patil showed that the triethylene glycol side chain can enhance molecular self-assembly and increase the FET mobility for DPP-based polymers. Gong applied PS-TPD-PFCB, a commonly used electron-blocking material in light-emitting diodes, in a novel IR photodetector and realized very low dark current and high detection sensitivity. Yang and Li's recent work shows a clear link between the polymers' molecular compatibility (preferred orientations) and the resulting multidonor system morphology, which has a profound impact on the carrier transport and device efficiency.
A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings, for example, weak absorption in the visible spectral region and limited energy level variability, of fullerene acceptors is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion efficiencies of up to 6.8%, a record for fullerene-free PSCs.
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A new push–pull molecule, containing an (alkylamino)thiophene as electron donor and 1,3-bis(dicyanomethylidene)-2-indenylindene as electron acceptor, has been synthesized and characterized in the solid state by X-ray crystallography, in solution by UV/Vis and NMR spectroscopy and by theoretical calculations. The quinoidal zwitterionic form of this compound was underscored both in the solid state and in moderately polar solvents. Experimental and theoretical analyses were also performed on two similar push–pull molecules, selectively chosen to vary basic chemical features such as the resonance energy of the aromatic group and the steric hindrance of the attractor. Comparative analysis of the three compounds has shown that steric hindrance, resonance energy and the polarity of the environment play a significant role in stabilizing one limiting resonance form over the other. By varying these chemical variables, fine tuning of the electronic ground state of a push–pull molecule, from the neutral aromatic form to the charge-separated zwitterion, passing through a form in which the two limiting forms contribute almost equally (the cyanine limit), can be achieved.
Different strategies for the synthesis of 2′-hydroxy-4′-methylsulfonylacetophenone are reported in the present paper. This compound is considered as a key synthon for the synthesis of new flavonoid derivatives designed as potential cyclooxygenase-2 inhibitors. The retrosynthetic approach via 3′-methylsulfonylacetophenone, which included three synthetic pathways, did not allow us to obtain the expected compound. However, a synthesis from 3-mercaptophenol led to the desired acetophenone in three steps: thiophenol methylation, Friedel–Crafts acetylation and oxidation of the sulphide to the corresponding sulfone. The desired compound, 2′-hydroxy-4′-methylsulfonylacetophenone, will be used as a synthon for the preparation of novel flavonoid derivatives, such as 2′-hydroxychalcones, flavanones, flavones, and flavonols.
Careful selection of processing solvents to tune optimal active layer morphology in a donor-acceptor copolymer blend [PTB7:P(NDI2OD-T2)] yields bulk heterojunction microstructures with intradomain percolative pathways and enhanced charge transport. Such microstructures afford all-polymer solar cells with power conversion efficiencies as high as 2.7%.
The synthesis of a series of bright deeply colored styryl dyes derived from the title compounds and various aromatic and heteroaromatic aldehydes is described. Some of the dyes were subjected to oxidative cyanation to obtain the cyanated analogues. The absorption-emission spectra of the dyes were recorded and the dyeing properties on polyester of some of the dyes was studied.
Second-order non-linear optical (NLO) poly(phenylquinoxalines) with high glass transition temperatures were prepared by reaction of bis(1,2-diketone)chromophore monomer and a tetramine at room temperature. Glass transition temperatures in the range of 187–260 °C were obtained. Thin spincoated films of the polymers were corona-poled and analysed by second-harmonic generation (SHG). Second-order susceptibility values d33(ω) up to 114 pm/V were obtained. Poled order stability measurements over a period of 750 h resulted in up to 91% of remaining NLO-response at 125 °C.