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Abstract

Two polymer donors, FTAZ and J71, and two fused-ring electron acceptors, ITIC1 and ITIC2, are used to investigate the effects of conjugation dimension on the performance of organic solar cells (OSCs). FTAZ and J71, ITIC1 and ITIC2 share the same molecular backbone, respectively, while J71 and ITIC2 possess conjugated thienyl side chains. The addition of conjugated side chains slightly red-shift the absorption spectra and lower the bandgap due to the extended 2D conjugation. Conjugated side chains on acceptor induce the self-aggregation of acceptors, while conjugated side chains on donor increase the miscibility of donors and acceptors, thus optimize the morphology of active layers. The blends based on mixed combinations, namely 1D donor/2D acceptor and 2D donor/1D acceptor, show better performance relative to 1D donor/1D acceptor and 2D donor/2D acceptor.

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... This characteristic face-on molecular orientation is of importance for OSCs with a vertical orientate charge transport behavior. [26][27][28][29] However, we can still find some minor differences as for the crystallinity intensities. With a comparable film thickness, the π-π diffraction intensity of iso-IDIC is a little lower than that of IDIC. ...
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The bulk-heterojunction (BHJ) blend plays an important role to determine the charge transport behaviors and efficiencies of organic solar cells. The side chain engineering has been recognized as one feasible way to tune the BHJ networks. As one promising side chain species, the well-aligned alkyl chains could promote the ordered molecular assembly, but often induce inferior self-aggregated BHJ morphology ascribed to the overly strong crystallinity. Hereon, we provide an isomerized strategy to modulate the crystallinity of alkyl chain attached acceptor. The linear alkyl chain is isomerized to branched structure with a bulky isopropyl terminal. As a consequence, the acceptor crystallinity is mildly reduced but maintaining the preferred molecular orientations, absorption and energy level properties. As for the as-cast devices, the isomerized iso-IDIC generate better-developed BHJ morphology and greater efficiency of 13.10% relative to linear chain attached IDIC (12.37%). Impressively, the slightly decreased crystallinity of iso-IDIC reveals good tolerance to pre/post-treatments to the BHJ, with the thermal annealing delivering over 13.50% efficiency. However, the IDIC based devices produce negative response to the pre/post-treatments, ascribed to the seriously aggregated BHJ networks. Therefore, the BHJ should be carefully modulated and the isomerized side chain proposed here provides one promising approach to finely tune the acceptor crystallinity and the resulting morphology.
... Nonfullerene acceptors have attracted much attention owing to their strong visible-near-infrared light-harvesting capability and tunable energy levels. In addition, they exhibit higher short-circuit current density (J sc ) and PCE in PSCs than fullerene acceptors, leading to a PCE of 18% that was reported recently [6][7][8]. ...
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Three two-dimensional donor–acceptor conjugated copolymers consisting of a benzo[1,2-b:4,5-b′]dithiophene derivative and thieno[3,2-b]thiophene with a conjugated side chain were designed and synthesized for use in bulk heterojunction (BHJ) or nonfullerene polymer solar cells (PSCs). Through attaching various acceptor end groups to the conjugated side chain on the thieno[3,2-b]thiophene moiety, the electronic, photophysical, and morphological properties of these copolymers were significantly affected. It was found that the intermolecular charge transfer interactions were enhanced with the increase in the acceptor strength on the thieno[3,2-b]thiophene moiety. Moreover, a better microphase separation was obtained in the copolymer: PC71BM or ITIC blend films when a strong acceptor end group on the thieno[3,2-b]thiophene moiety was used. As a result, BHJ PSCs based on copolymer:PC71BM blend films as active layers exhibited power conversion efficiencies from 2.82% to 4.41%, while those of nonfullerene copolymer:ITIC-based inverted PSCs ranged from 6.09% to 7.25%. These results indicate the side-chain engineering on the end groups of thieno[3,2-b]thiophene unit through a vinyl bridge linkage is an effective way to adjust the photophysical properties of polymers and morphology of blend films, and also have a significant influence on devices performance.
... To date, significant progresses have been achieved to improve the optoelectronic properties and photovoltaic performance by the optimization of absorption and energy level alignment. [6][7][8][9][10][11][12][13][14] Nevertheless, one critical challenge, the manipulation of morphology of active layer, is not yet solved. The morphology of blend film is determined by the intermolecular interaction of donor and acceptor. ...
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To investigate the effect of bridge atom on the properties of non-fullerene acceptors, we designed two analogues by changing only one atom and report the first germanium based non-fullerene acceptor GDIC-C8. Compared to its analogous IDIC-C8, alternating the bridge atom from carbon to germanium dramatically leads to a more planar molecular geometry. GDIC-C8 tends to form 1D lamellar/slip-stack packing while IDIC-C8 represents reticular motif packing in single crystal while. This unique structure greatly enhance the crystallinity of GDIC-C8 in single crystal and neat film, evidenced by X-ray single crystal diffraction and atomic force microscope meausurments. However, GDIC-C8 prefers to form edge‐on π‐π stacking when blended with donor polymer PM6. In addition, the extremely high crystallinity of GDIC-C8 restrains competitively the crystallization of PM6 in blend film. These features lead to the inferior photovoltaic performance of GDIC-C8 compared to IDIC-C8. This work systematically studies the effect of bridge atoms on molecular geometry and packing, and reveals the importance of the bridge atom in the fused‐ring semiconductors for the modulation of optoelectronic properties and device performance.
... The aim of this research was to explore the efficient organic molecule by investigating the optoelectronic properties, energy level, charge transport and morphology among the newly designed acceptor moieties. The photovoltaic properties of these newly designed molecules are investigated and compared with the reference molecule ITIC2 by using density functional theory (DFT) [19]. ...
Article
In present study, four A-D-A type of 2D conjugated molecules (M1-M4) are designed to find their opto-electronic properties. These molecules have interlinked with donor and acceptor moieties through thiophene unit, 2-(5,6-difluoro-2-methlyene-3-oxo-2,3-dihydro-1H-iden-1-ylidene)malononitrile (M1), 2-((2-methyl-2H-benzo[d][1], [2], [3] triazol-4-yl) methylene)malononitrile (M2), 2-((5,6-difluoro-2-methyl-2H-benzo[d][1], [2], [3] triazol-4-yl) methylene)malononitrile (M3), and 3-methyl-5-methylene-2-thioxothiazolidin-4-one (M4) by density functional theory (DFT). The photovoltaic properties of these newly designed molecules are evaluated and compared with reference molecule R (ITIC2) contains electron rich benzo[1,2-b:4,5-b′]di(cyclopenta[2,1-b:3,4-b′]dithiophene) core, electron-deficient end groups, conjugated 5-(2-ethylhexyl)thiophene side chains, and nonconjugated 4-hexylbenzene side chains. The effect of end acceptor groups on absorption, energy level, charge transport, morphology, and photovoltaic properties of the designed molecules (M1-M4) were investigated by TD-DFT B3LYP/6-31G basic level of theory and compared with reference molecule R. Among all molecules, M1 and M3 showed lower band gap and exhibited more absorption with B3LYP/6-31G (d, p) level of theory due to highly extended conjugation between electron withdrawing end-capped acceptor moieties. The reorganization energy calculation showed that λe of M2 and M3 was lowered because of their good charge transfer ability as compared to other molecules. M3 also showed least coefficient interaction between acceptor and donor groups in TDM analysis which showed the easier and highest dissociation at the excited state. Overall, designed structure M3 was found to be more effective and efficient acceptor molecule for solar cell application. The findings provide novel information for the development of ITIC based acceptors for OPVs.
... 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
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Organic solar cells (OSCs) employing non-fullerene acceptors (NFAs) have drawn significant research attention in recent years. Molecular stacking and aggregation of electron donors and acceptors within the photoactive layer is vitally important to light absorption and the photon-to-electricity conversion process. Herein, we present the versatile molecular stacking of typical A-D-A type NFAs, as well as the affecting factors including the chemical structures of NFAs and physical processing conditions. We highlight in particular experimental approaches to regulate the molecular stacking and aggregation,and summarize the influences of these features on optoelectronic and photovoltaic properties of NFA-based OSCs, which provides crucial guidance for further development of high performance OSCs.
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With an indenoindene core, a new thieno[3,4-b]thiophene-based small-molecule electron acceptor, 2,2′-((2Z,2′Z)-((6,6′-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-π-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated π-conjugation with 14 π-electrons and one more sp3 carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 × 105m−1 cm−1 in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.
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Suppression of carrier recombination is critically important in realizing high-efficiency polymer solar cells. Herein, it is demonstrated difluoro-substitution of thiophene conjugated side chain on donor polymer can suppress triplet formation for reducing carrier recombination. A new medium bandgap 2D-conjugated D–A copolymer J91 is designed and synthesized with bi(alkyl-difluorothienyl)-benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit, for taking the advantages of the synergistic fluorination on the backbone and thiophene side chain. J91 demonstrates enhanced absorption, low-lying highest occupied molecular orbital energy level, and higher hole mobility, in comparison with its control polymer J52 without fluorination on the thiophene side chains. The transient absorption spectra indicate that J91 can suppress the triplet formation in its blend film with n-type organic semiconductor acceptor m-ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene). With these favorable properties, a higher power conversion efficiency of 11.63% with high VOC of 0.984 V and high JSC of 18.03 mA cm−2 is obtained for the polymer solar cells based on J91/m-ITIC with thermal annealing. The improved photovoltaic performance by thermal annealing is explained from the morphology change upon thermal annealing as revealed by photoinduced force microscopy. The results indicate that side chain engineering can provide a new solution to suppress carrier recombination toward high efficiency, thus deserves further attention.
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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.
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A new acceptor-donor-acceptor (A-D-A) type small molecule, namely NFBDT, with a heptacyclic benzodi(cyclopentadithiophene) (FBDT) unit based on BDT as the central building block, was designed and synthesized. Its optical, electrical, thermal properties and photovoltaic performances were systematically investigated. NFBDT exhibits a low optical bandgap of 1.56 eV resulting in wide and efficient absorption in the range from 600 to 800 nm, and suitable energy levels as an electron acceptor. With the widely used and successful wide bandgap polymer PBDB-T selected as the donor material, an optimized PCE of 10.42% was obtained for the PBDB-T:NFBDT-based device with an outstanding short-circuit current density of 17.85 mA cm-2 under AM 1.5G irradiation (100 mW cm-2), which is so far among the highest performance of NF-OSC devices. These results demonstrate that BDT unit could also be applied for designing NF-acceptors, and the fused-ring benzodi(cyclopentadithiophene) unit is a promising block for designing new and high performance NF-acceptors.
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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%).
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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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Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si–C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm−2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.
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A highly efficient fullerene-free polymer solar cell (PSC) based on PDCBT, a polythiophene derivative substituted with alkoxycarbonyl, has achieved an impressive power conversion efficiency of 10.16%, which is the best result in PSCs based on polythiophene derivatives to date. In comparison with a poly(3-hexylthiophene):ITIC-based device, the photovoltaic and morphological properties of PDCBT:ITIC-based device are carefully investigated and interpreted.
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In this work, the authors design and synthesize two novel wide bandgap copolymers based on selenophene substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTSe) as the donor unit and fluorinated benzotriazole as the acceptor unit for high performance non‐fullerene polymer solar cells (NF‐PSCs). A larger maximum molar extinction coefficient (ϵ) of 8.54 × 104 M−1 cm−1 is achieved when introducing sulfur atom onto the two‐dimensional (2D) BDTSe units, which should realize the better complementary absorption with ITIC as the acceptor, leading to a higher Jsc of 19.51 mA cm−2. Furthermore, a lower highest occupied molecular orbital (HOMO) energy level with almost no change in bandgap can be also achieved after inserting the sulfur atoms, thus resulting in an enhanced open‐circuit voltage (Voc) of 0.84 V without sacrificing the short‐current density (Jsc). In addition, the higher crystallinity and optimized morphology are found to be beneficial to more efficient exciton dissociation and charge transport, giving rise to a higher fill factor (FF) of 75.1% and an elevated power conversion efficiency (PCE) of 12.31%. The results indicate that the strategy of alkylthioselenyl side chains on the BDT unit for constructing the donor‐acceptor (D‐A) copolymer donor materials is an excellent approach for realizing highly efficient NF‐PSCs. Two novel wide bandgap copolymers with conjugated selenyl side chains, PBDT‐Se‐TAZ, and PBDTS‐Se‐TAZ, are synthesized successfully for efficient non‐fullerene polymer solar cells (NF‐PSCs). The side chain engineering of alkylthioselenyl group promotes the resulting copolymers to exhibit high PCE of 12.31%.
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We develop a fused-ring electron acceptor (IOIC3) based on naphtho[1,2-b:5,6-b′]dithiophene core with alkoxy side-chains and compare it with its counterpart (IOIC2) with alkyl side-chains. Change in the side-chains affects electronic, optical, charge transport, and morphological properties of the analogues. Because of π-conjugative effect and σ-inductive effect of the oxygen atoms, IOIC3 exhibits a slightly upshifted HOMO level (−5.38 eV) and a downshifted LUMO level (−3.84 eV) relative to IOIC2 (HOMO = −5.41 eV, LUMO = −3.78 eV), leading to red-shifted absorption and smaller optical bandgap of 1.45 eV than that of IOIC2 (1.54 eV). IOIC3 exhibits a higher electron mobility of 1.5 × 10–3 cm² V–1 s–1 than IOIC2 (1.0 × 10–3 cm² V–1 s–1). Organic solar cells (OSCs) based on PTB7-Th:IOIC3 exhibit power conversion efficiency (PCE) as high as 13.1%, significantly higher than that of PTB7-Th:IOIC2 (9.33%). The semitransparent OSCs based on PTB7-Th:IOIC3 afford PCEs of up to 10.8% with an average visible transmittance (AVT) of 16.4%, higher than those of PTB7-Th:IOIC2 (PCE = 7.32%, AVT = 13.1%).
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We demonstrate that voltage losses due to both radiative and non-radiative recombination of charge carriers are strongly dependent on D/A phase separation. By processing the active layer with various solvent additives, we create distinct morphologies that lead to significantly different device open-circuit voltages (VOC), even though the charge transfer state energy (ECT) of the D/A blend remains rather constant. We find that radiative recombination losses are significantly increased for a finely intermixed morphology, due to the large D/A interface area. This leads to a total recombination loss of ECT - qVOC ≈ 0.7 eV. However, considerably smaller losses (0.5 eV), due to suppressed non-radiative recombination, are possible in solar cells where the D/A materials are organized to only allow for selective charge carrier extraction. Using a drift diffusion model, we show that the origin of the reduced non-radiative recombination losses is related to an effect which has not been considered for ‘optimized’ solar cells - the suppression of minority carrier diffusion to the ‘wrong’ contact. Our results suggest that the built-in field is not sufficiently strong even in ‘optimized’ organic solar cells and that selective carrier extraction is critical for further improvements in VOC.
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A novel wide‐bandgap copolymer of PBDT‐ODZ based on benzo[1,2‐b:4,5‐b′ ]dithiophene (BDT) and 1,3,4‐oxadiazole (ODZ) blocks is developed for efficient nonfullerene polymer solar cells (NF‐PSCs). PBDT‐ODZ exhibits a wide bandgap of 2.12 eV and a low‐lying highest occupied molecular orbital (HOMO) level of −5.68 eV, which could match well with the low‐bandgap acceptor of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylthienyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐dithiophene (ITIC‐Th), inducing a good complementary absorption from 300 to 800 nm and a minimal HOMO level offset (0.1 eV). The PBDT‐ODZ:ITIC‐Th devices exhibit a large open‐circuit voltage (Voc) of 1.08 eV and a low energy loss (Eloss) of 0.50 eV, delivering a high power conversion efficiency (PCE) of 10.12%. By adding a small amount of copper(I) iodide (CuI) as an additive to form coordination complexes in the active blends, much higher device performances are achieved due to the improved absorption and crystallinity. After incorporating 4% of CuI, the PCE is elevated to 12.34%, with a Voc of 1.06 V, a Jsc of 17.1 mA cm⁻² and a fill factor of 68.1%. This work not only provides a novel oxadiazole‐containing wide‐bandgap polymeric donor candidate for high‐performance NF‐PSCs but also presents an efficient morphology‐optimization approach to elevate the PCE of NF‐PSCs for future practical applications.
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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.
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Two new non-fullerene (NF) acceptors, namely BDTIT-M and BDTThIT-M, were rationally designed to optimize the energy levels and optical bandgap. BDTIT-M is derived by changing end-group of NFBDT into slightly weak DCI-M, and BDTThIT-M is obtained by adding two conjugated thiophene side-chains into ladder-type core of BDTIT-M. By incorporating with polymer donor PBDB-T, BDTIT-M based organic solar cells (OSCs) deliver a higher PCE of 11.31% than that of NFBDT based cells, which is mainly attributed to the increased VOC and FF. A higher PCE of 12.12% with small energy loss of ~ 0.588 eV is achieved compared with BDTThIT-M based OSCs, benefiting from the elevated LUMO level, narrowed bandgap, enhanced absorption coefficient and electron mobility of BDTThIT-M compared with BDTIT-M. The combination of methyl-modified end-group and conjugated side-chain should be an efficient strategy to elevate the LUMO and HOMO levels with different amplitude for realizing simultaneous improvement in VOC and JSC.
Article
A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600-950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 105 m-1 cm-1 , low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10-3 cm2 V-1 s-1 . Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.
Article
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.
Article
Despite rapid advances in the field of nonfullerene polymer solar cells (NF-PSCs), successful examples of random polymer-based NF-PSCs are limited. In this study, it is demonstrated that random donor polymers based on thieno[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) containing two simple thiophene (T) and bithiophene (2T) electron-rich moieties (PTTI-Tx) can be promising materials for the fabrication of highly efficient NF-PSCs. With negligible influence on optical bandgaps and energy levels, the crystalline behavior of PTTI-Tx polymers was modulated by varying the T:2T ratio in the polymer backbone; this resulted in the formation of different microstructures upon blending with a nonfullerene m-ITIC acceptor in NF-PSCs. In particular, a PTPTI-T70:m-ITIC system enabled favorable small-scale phase separation with an increased population of face-on oriented crystallites, thereby boosting the processes of effective exciton dissociation and charge transport in the device. Consequently, the highest power conversion efficiency of 11.02% with an enhanced short-circuit current density of 17.12 mA cm−2 is achieved for the random polymer-based NF-PSCs thus far. These results indicate that random terpolymerization is a simple and practical approach for the optimization of a donor polymer toward highly efficient NF-PSCs.
Article
We have developed a kind of novel fused-ring small molecular acceptor, whose planar conformation can be locked by intramolecular noncovalent interaction. The formation of planar supramolecular fused-ring structure by conformation locking can effectively broaden its absorption spectrum, enhance the electron mobility, and reduce the nonradiative energy loss. Polymer solar cells (PSCs) based on this acceptor afforded a power conversion efficiency (PCE) of 9.6%. In contrast, PSCs based on similar acceptor, which can't form a flat conformation, only gave a PCE of 2.3%. Such design strategy, which can make the synthesis of small molecular acceptor much easier, will be promising in developing new acceptor for high efficiency polymer solar cells.
Article
A synergetic effect of molecular weight (Mn) and fluorine (F) on the performance of all-polymer solar cells (all-PSCs) is comprehensively investigated by tuning the Mn of the acceptor polymer poly((N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) and the F content of donor polymer poly(2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-dyl-alt-thiophene-2,5-diyl). Both Mn and F variations strongly influence the charge transport properties and morphology of the blend films, which have a significant impact on the photovoltaic performance of all-PSCs. In particular, the effectiveness of high Mn in increasing power conversion efficiency (PCE) can be greatly improved by the devices based on optimum F content, reaching a PCE of 7.31% from the best all-PSC combination. These findings enable us to further understand the working principles of all-PSCs with a view on achieving even higher power conversion efficiency in the future.
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Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.
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Ternary organic solar cells are emerging as a promising strategy to enhance device power conversion efficiency by broadening the range of light absorption via the incorporation of additional light-absorbing components. However, how to find compatible materials that allow comparable loadings of each component remains a challenge. In this article, we focus on studying the donor polymer compatibilities in ternary systems from a morphological point of view. Four typical donor polymers with different chemical structures and absorption ranges were mutually combined to form six distinct ternary systems with fullerene derivative acceptors. Two compatible ternary systems were identified as showing significant improvements of efficiency from both binary control devices. Ternary morphologies were characterized by grazing incident X-ray scattering and correlated with device performance. We find that polymers that have strong lamellar interactions and relatively similar phase separation behaviors with the fullerene derivative are more likely to be compatible in ternary systems. This result provides guidance for polymer selection for future ternary organic solar cell research while relaxing the limitation of chemical structure similarity and greatly extends the donor candidate pool.
Article
The effect of contact barriers on the light-intensity dependence of the open-circuit voltage of organic solar cells is investigated in experiments and simulations. Reduced light-intensity dependence is found when the open-circuit voltage surpasses the built-in voltage, leading to a slope of kT/2q for a device with one non-ohmic contact and a slope of zero for a device with two non-ohmic contacts. The reduced light-intensity dependence of the open-circuit voltage is not caused by entering a contact-recombination-limited regime but by the absence of band bending in the vicinity of a non-ohmic contact.
Article
Advances in the design and application of highly efficient conjugated polymers and small molecules over the past years have enabled the rapid progress in the development of organic photovoltaic (OPV) technology as a promising alternative to conventional solar cells. Among the numerous OPV materials, benzodithiophene (BDT)-based polymers and small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking 10% efficiency barrier in the single junction OPV devices. Remarkably, the OPV device featured by BDT-based polymer has recently demonstrated an impressive PCE of 11.21%, indicating the great potential of this class of materials in commercial photovoltaic applications. In this review, we offered an overview of the organic photovoltaic materials based on BDT from the aspects of backbones, functional groups, alkyl chains, and device performance, trying to provide a guideline about the structure-performance relationship. We believe more exciting BDT-based photovoltaic materials and devices will be developed in the near future.
Article
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
A planar fused-ring electron acceptor (IC-C6IDT-IC) based on indacenodithiophene is designed and synthesized. IC-C6IDT-IC shows strong absorption in 500-800 nm with extinction coefficient of up to 2.4 × 105 M-1 cm-1 and high electron mobility of 1.1 × 10-3 cm2 V-1 s-1. The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibit power conversion efficiencies of up to 8.71%.