Developing a high-performance donor polymer is critical for achieving efficient non-fullerene organic solar cells (OSCs). Currently, most high-efficiency OSCs are based on a donor polymer named PM6, ...unfortunately, whose performance is highly sensitive to its molecular weight and thus has significant batch-to-batch variations. Here we report a donor polymer (named PM1) based on a random ternary polymerization strategy that enables highly efficient non-fullerene OSCs with efficiencies reaching 17.6%. Importantly, the PM1 polymer exhibits excellent batch-to-batch reproducibility. By including 20% of a weak electron-withdrawing thiophene-thiazolothiazole (TTz) into the PM6 polymer backbone, the resulting polymer (PM1) can maintain the positive effects (such as downshifted energy level and reduced miscibility) while minimize the negative ones (including reduced temperature-dependent aggregation property). With higher performance and greater synthesis reproducibility, the PM1 polymer has the promise to become the work-horse material for the non-fullerene OSC community.
The synergistic effect of fluorination on molecular energy level modulation is realized by introducing fluorine atoms onto both the donor and the acceptor moieties in a D–A polymer, and as a result, ...the polymer solar cell device based on the trifluorinated polymer, PBT‐3F, shows a high efficiency of 8.6%, under illumination of AM 1.5G, 100 mW cm−2.
Conspectus As researchers continue to develop new organic materials for solar cells, benzo1,2-b:4,5-b'dithiophene (BDT)-based polymers have come to the fore. To improve the photovoltaic properties ...of BDT-based polymers, researchers have developed and applied various strategies leading to the successful molecular design of highly efficient photovoltaic polymers. Novel polymer materials composed of two-dimensional conjugated BDT (2D-conjugated BDT) have boosted the power conversion efficiency of polymer solar cells (PSCs) to levels that exceed 9%. In this Account, we summarize recent progress related to the design and synthesis of 2D-conjugated BDT-based polymers and discuss their applications in highly efficient photovoltaic devices. We introduce the basic considerations for the construction of 2D-conjugated BDT-based polymers and systematic molecular design guidelines. For example, simply modifying an alkoxyl-substituted BDT to form an alkylthienyl-substituted BDT can improve the polymer hole mobilities substantially with little effect on their molecular energy level. Secondly, the addition of a variety of chemical moieties to the polymer can produce a 2D-conjugated BDT unit with more functions. For example, the introduction of a conjugated side chain with electron deficient groups (such as para-alkyl-phenyl, meta-alkoxyl-phenyl, and 2-alkyl-3-fluoro-thienyl) allowed us to modulate the molecular energy levels of 2D-conjugated BDT-based polymers. Through the rational design of BDT analogues such as dithienobenzodithiophene (DTBDT) or the insertion of larger π bridges, we can tune the backbone conformations of these polymers and modulate their photovoltaic properties. We also discuss the influence of 2D-conjugated BDT on polymer morphology and the blends of these polymers with phenyl-C61 (or C71)-butyric acid methyl ester (PCBM). Finally, we summarize the various applications of the 2D-conjugated BDT-based polymers in highly efficient PSC devices. Overall, this Account correlates the molecular structures of the 2D-conjugated BDT-based polymers with their photovoltaic properties. As a result, this Account can guide the molecular design of organic photovoltaic materials and the development of organic materials for other types of optoelectronic devices.
In this work, polymer solar cells are fabricated based on the blend of PTB7‐Th: PC71BM by using a mixed solvent additive of 1,8‐diiodooctane and N‐methyl pyrrolidone to optimize the morphology of the ...blend. A high power conversion efficiency (PCE) of 10.8% has been achieved with a simple conventional device. In order to deeply investigate the influence of the mixed solvent additives on the morphology and device performance, the variations of the molecular packing and bulk morphology of the blend film cast from ortho‐dichlorobenzene with single or binary solvent additives are measured. Although all the blend films exhibit similar domain size and nanoscale phase separation, the blend film processed with mixed solvent additive shows the highest domain purity, resulting in the least bimolecular recombination, relatively high Jsc and FF, and hence enhanced PCE. Therefore, the best photovoltaic performance with the Voc of 0.82 V, Jsc of 19.1 mA cm−2, FF of 69.1%, and PCE of 10.8% are obtained for the device based on the blend with binary solvent additive treatment.
For the PTB7‐Th:PC71BM blend system, the binary solvent additives of 1,8‐diiodooctane and N‐methyl pyrrolidone are used to enhance the domain purity, so that a power conversion efficiency of 10.8% is achieved with the conventional device structure, which is much higher than that of the devices without or with single solvent additive treatment.
High‐performance ternary organic solar cells are fabricated by using a wide‐bandgap polymer donor (bithienyl‐benzodithiophene‐alt‐fluorobenzotriazole copolymer, J52) and two well‐miscible ...nonfullerene acceptors, methyl‐modified nonfullerene acceptor (IT‐M) and 2,2′‐((2Z,2′Z)‐((5,5′‐(4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydros‐indaceno1,2‐b:5,6‐b′dithiophene‐2,7‐diyl)bis(4‐((2‐ethylhexyl)oxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short‐circuit current density of 19.7 mA cm−2 and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.
Ternary organic solar cells show over 11% power conversion efficiency by using two compatible nonfullerene acceptors with complementary absorption spectra, similar chemical structures, and similar lowest unoccupied molecular orbital levels. Broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to improve the performance of OSCs.
In addition to the innovation of nonfullerene acceptors, the development of highly efficient nonfullerene organic solar cells requires the design of new polymer donors and fundamental understanding ...of their structural and morphological properties. Utilizing meta-alkoxy-phenyl-substituted benzodithiophene and benzodithiophene-4,8-dione building blocks, we designed and prepared a new class of structurally similar photovoltaic polymers named PBDx (x = 1–4), which are capable of being processed from nonchlorinated solvents. From PBD1 to PBD4, the total carbon number of the alkyl side chains in each repeat unit increased by four in turn. The effect of side chain structure variation on the molecular aggregation, molecular arrangement, mesoscale phase separation, charge transport, and nonfullerene solar cell performance was systematically studied. Our hard and soft X-ray scattering results indicate that small side chain variation yields vastly different molecular packing and mesoscale morphology for these analogues. It was found that PBD1 with the shortest alkyl side chain exhibited the strongest molecular aggregation, most attractive interaction with solvent additive, highest composition variation at a small length scale of 30 nm, and best photovoltaic performance of over 12% efficiency among the four polymers. Moreover, the structure–performance connections were discussed in the context of polymer thermodynamics, and the composition of the mixed phase was most likely quenched closer to the percolation threshold for the PBD1:IDIC system according to solubility limit measurements. This work thus elaborates the origin of such disparity in morphology and performance of nonfullerene solar cells.
Four D–A copolymers of tetradodecyl-substituted indacenodithiophene (IDT) donor unit with different acceptor units including bis(thiophen-2-yl)-bithiazole (BTz), bis(thiophen-2-yl)thiazolothiazole ...(TTz), bis(thiophen-2-yl)-tetrazine (TZ), and bis(thiophen-2-yl)-benzothiadiazole (DTBT), PIDT-BTz, PIDT-TTz, PIDT-TZ, and PIDT-DTBT, were synthesized for the application as donor materials in polymer solar cells (PSCs). The copolymers possess good solubility benefitted from the four alkyl side chains on IDT unit, deeper HOMO levels at ca. −5.2 eV thanks to the IDT unit and tunable bandgap depending on the acceptor units. Among the copolymers, PIDT-TTz has the highest hole mobility (μh) of 4.99 × 10–3 cm2/V s. The power conversion efficiency (PCE) of the PSC based on PIDT-TTz/PC70BM (1:2 w/w) reached 5.79%, under the illumination of AM1.5G, 100 mW/cm2. PIDT-DTBT film has the smallest bandgap of 1.68 eV and a higher μh of 2.24 × 10–3 cm2/V s. The PSC based on PIDT-DTBT/PC70BM (1:3 w/w) exhibited an even higher PCE of 6.17% with a J sc of 13.27 mA/cm2, a V oc of 0.82 V, and a FF of 56.9%.
A new conjugated polymer based on 5,7-bis(2-ethylhexyl)benzo1,2-c:4,5-c′dithiophene-4,8-dione, named as PBDTBDD, was designed, synthesized, and applied in polymer solar cells (PSCs). A power ...conversion efficiency (PCE) of 6.67% was obtained from the PBDTBDD/PC61BM-based PSC, which is a remarkable result for the PSCs using PC61BM as electron acceptor. The PBDTBDD/PC61BM-based device exhibits a narrow absorption band and excellent quantum efficiency in the range from 500 to 700 nm. Furthermore, PBDTBDD shows a strong aggregation effect in solution state, and the study indicates that although the temperature used in solution preparation has little influence on molecular orientation as well as crystallinity of the D/A blend, it plays an important role in forming proper domain size in the blend. This work provides a good example to reveal the correlation between the morphology of the blend films and the processing temperature of the solution preparation. Furthermore, the study in this work suggests an interesting and feasible approach to modulate domain size without changing crystallinity of the blend films in PSCs.
Attaching meta‐alkoxy‐phenyl groups as conjugated side chains is an easy and effective way to modulate the molecular energy level of D‐A polymer for photovoltaic application, and the polymer solar ...cells based on the polymer consisting meta‐alkoxy‐phenyl groups as conjugated side chain, PBT‐OP, shows an enhanced open circuit voltage and thus higher efficiency of 7.50%, under the illumination of AM 1.5G, 100 mW/cm2.
We designed and synthesized a series of fused-ring electron acceptors (FREAs) based on naphthalene-fused octacyclic cores end-capped by 3-(1,1-dicyanomethylene)-5,6-difluoro-1- indanone (NOICs) using ...a bottom-up approach. The NOIC series shares the same end groups and side chains, as well as similar fused-ring cores. The butterfly effects, arising from different methoxy positions in the starting materials, impact the design of the final FREAs, as well as their molecular packing, optical and electronic properties, charge transport, film morphology, and performance of organic solar cells. The binary-blend devices based on this NOIC series show power conversion efficiencies varying from 7.15% to 14.1%, due to the different intrinsic properties of the NOIC series, morphologies of blend films, and voltage losses of devices.
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