Effects of alkyl chain length and substituent pattern of fullerene bis-adducts on film structures and photovoltaic properties of bulk heterojunction solar cells

Structure-Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C61 Butyric Acid Methyl Ester

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10-2 cm2/(V s), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.

Highly Regioselective Synthesis of Bisadduct[C70] Additive toward the Enhanced Performance of Perovskite Solar Cells

The high-regioselective synthesis of bisadducts based on low-symmetry C₇₀ has been a challenging work due to the large amount of formed regioisomers, which require tedious separation procedures for isomeric purity and block their application in different fields. Herein, we successfully obtained a novel 1, 2, 3, 4-bis(triazolino)fullerene[C₇₀] 2 with high regioselectivity by the rigid tether-directed regioselective synthesis strategy and the corresponding molecular structure was unambiguously confirmed by single-crystal X-ray crystallography characterization. The crystal data clearly show that the addition occurs at the domain of corannulene moiety at the end of ellipse C₇₀ as well as the 1, 2, 3, 4-addition sites located at one hexagonal ring with a [6,6]-closed addition pattern. Furthermore, 2 was applied as an additive of perovskite layer to construct MAPbI3-based regular (n-i-p) perovskite solar cells, affording the power conversion efficiency (PCE) of 18.59%, which is a 7% enhancement relative to that of control devices without additive.

Thiophene- and Carbazole-Substituted N-Methyl-Fulleropyrrolidine Acceptors in PffBT4T-2OD Based Solar Cells

The impact of fullerene side chain functionalization with thiophene and carbazole groups on the device properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2';5',2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) as donor and C60 or C70 fulleropyrrolidines as acceptors. The photovoltaic performance clearly depended on the molecular structure of the fulleropyrrolidine substituents although no direct correlation with the surface morphology of the photoactive layer, as determined by atomic force microscopy, could be established. Although some fulleropyrrolidines possess favorable lowest unoccupied molecular orbital levels, when compared to the standard PC71BM, they originated OPV cells with inferior efficiencies than PC71BM-based reference cells. Fulleropyrrolidines based on C60 produced, in general, better devices than those based on C70, and we attribute this observation to the detrimental effect of the structural and energetic disorder that is present in the regioisomer mixtures of C70-based fullerenes, but absent in the C60-based fullerenes. These results provide new additional knowledge on the effect of the fullerene functionalization on the efficiency of organic solar cells.

Regioselective electrosynthesis of tetra- and hexa-functionalized [60]fullerene derivatives with unprecedented addition patterns

The efficient and regioselective electrosynthesis of tetra- and hexa-functionalized [60]fullerene derivatives with unprecedented addition patterns has been achieved. The tetra-functionalized [60]fullerene derivative with an intriguing 1,2,4,17-addition pattern is regioselectively obtained by cyclization reaction of the dianionic species generated electrochemically from a [60]fulleroindoline with 1,2-bis(bromomethyl)benzene at 0 °C, and can be converted to the more stable 1,2,3,4-adduct at 25 °C. Furthermore, the hexa-functionalized [60]fullerene derivative with the 1,2,3,4,9,10-addition pattern displaying a unique "S"-shaped configuration can be synthesized by protonation of the electrochemically generated dianion of the obtained tetra-functionalized 1,2,4,17-adduct. The structures of the tetra- and hexa-functionalized products have been determined by spectroscopic data and single-crystal X-ray analysis.

PffBT4T-2OD Based Solar Cells with Aryl-Substituted N-Methyl-Fulleropyrrolidine Acceptors

Novel C60 and C70 N-methyl-fulleropyrrolidine derivatives, containing both electron withdrawing and electron donating substituent groups, were synthesized by the well-known Prato reaction. The corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels were determined by cyclic voltammetry, from the onset oxidation and reduction potentials, respectively. Some of the novel fullerenes have higher LUMO levels than the standards PC61BM and PC71BM. When tested in PffBT4T-2OD based polymer solar cells, with the standard architecture ITO/PEDOT:PSS/Active-Layer/Ca/Al, these fullerenes do not bring about any efficiency improvements compared to the standard PC71BM system, however they show how the electronic nature of the different substituents strongly affects the efficiency of the corresponding organic photovoltaic (OPV) devices. The functionalization of C70 yields a mixture of regioisomers and density functional theory (DFT) calculations show that these have systematically different electronic properties. This electronic inhomogeneity is likely responsible for the lower performance observed in devices containing C70 derivatives. These results help to understand how new fullerene acceptors can affect the performance of OPV devices.

Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells

Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives often-but not always-exhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.

Investigation of Substituent Effect in Modified Nature-Sourced Polymers: Rational Side Chain Engineering to Control Yield, Design, and Properties

"Side chain engineering" research has yielded many promising and beneficial results, with applications in various fields. However, this research did not receive sufficient focus when nature-sourced polymers are concerned. In this study, we have performed side chain engineering on chitosan, a nature-sourced polysaccharide, by coupling it with a number of aliphatic aldehydes of varying chain lengths. The side chains' length and the pursuing effect on the modified products' properties were studied in great detail. In terms of coupling yields, it was found that some substituents have displayed more favorable results than others by a factor of over 35 times. When studying the modified polymers' physical and mechanical properties, some of them were found to exhibit more rigid mechanical properties by a factor of 3.5 times than others. The effect was also expressed through self-assembly concentrations and encapsulation capabilities of the modified polymers. Remarkably, the combined experimental and calculated kinetic studies showed the results do not necessarily follow a linear progression relating to substituent chain length, but rather a parabolic pattern with a specific extremum point. This study has assisted in shedding light on the inspected phenomenon, explaining that not only steric and electronic factors but also interfacial solubility related factors govern the coupling reaction and the resulting modified polymers' properties. As chemical protocols in various academic, clinical, and industrial studies around the world slowly shift their norms toward finding safer ways for the production of novel materials and technologies, nature-sourced polymers hold great promise as virtually inexhaustible raw materials. The perfection of their chemical modification is therefore relevant now more than ever, with far-reaching and diverse applicative prospects.

Anchoring Fullerene onto Perovskite Film via Grafting Pyridine toward Enhanced Electron Transport in High-Efficiency Solar Cells

Fullerene derivatives have been popularly applied as electron transport layers (ETLs) of inverted (p-i-n) planar heterojunction perovskite solar cells (iPSCs) due to their strong electron-accepting abilities, and so far, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) has been the most commonly used ETL, which suffers, however, from high cost due to the complicated synthetic route. Herein, novel pyridine-functionalized fullerene derivatives (abbreviated as C₆₀-Py) were synthesized facilely via a one-step 1,3-dipolar cycloaddition reaction and applied as ETLs superior to PCBM in iPSC devices. Three pyridine-functionalized fullerene derivatives with different alkyl groups, including methyl, n-butyl, and n-hexyl, grafted onto the pyrrolidine moiety (abbreviated as C₆₀-MPy, C₆₀-BPy, and C₆₀-HPy, respectively) were synthesized. According to cyclic voltammogram study, the chain length of the N-alkyl group has negligible influence on the molecular energy level of C₆₀-Py. However, the ETL performance of C₆₀-Py is sensitively dependent on the chain length of the N-alkyl group, with C₆₀-BPy exhibiting the highest power conversion efficiency (PCE) of 16.83%, which surpasses that based on PCBM ETL (15.87%). The PCE enhancement of C₆₀-BPy device is attributed to the coordination interactions between the pyridine moiety with the Pb²⁺ ion of CH₃NH₃PbI₃ perovskite, which anchor C₆₀-BPy onto perovskite film and reinforce the passivation of the trap state within the CH₃NH₃PbI₃ perovskite film and suppress the nonradiative electron-hole recombinations, leading to enhanced electron transport reflected by the increase of short-circuit current density ( J_sc). The ambient stability of C₆₀-HPy-based device is much better than that based on PCBM ETL since its long N-alkyl group can function as a superior encapsulating layer protecting the CH₃NH₃PbI₃ layer from contact with the ambient moisture.

Effect of dihydronaphthyl-based C60 bisadduct as third component materials on the photovoltaic performance and charge carrier recombination of binary PBDB-T : ITIC polymer solar cells

A dihydronaphthyl-based C60 bisadduct (NCBA) acceptor was introduced as a third component to typical poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b0]dithiophene))-alt-(5,5-(10,30-di-2-thienyl-50,70-bis(2-ethylhexyl)benzo[10,20-c:40,50-c0]dithiophene-4,8-dione))] (PBDB-T): 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene (ITIC) binary polymer solar cells (PSCs). NCBA plays a bridging role between the lowest unoccupied molecular orbital (LUMO) of PBDB-T and ITIC and provides more routes for charge carrier transfer at the interface between PBDB-T and ITIC, whereupon a higher open-circuit voltage (VOC) could be realized upon the addition of NCBA relative to the neat ITIC as an electron acceptor. With the strong visible light absorption in the range from 300 to 520 nm of the NCBA molecule, it had the effect of apparently complementary visible light absorption compared with the binary PBDB-T : ITIC layer. The crystallinity and surface morphology of the PBDB-T : NCBA : ITIC (1 : 0.1 : 0.9) thin films was similar to that of the binary PBDB-T : ITIC layer, which guaranteed suitable efficient exciton dissociation and charge carrier transport. The photocurrent density versus effective voltage (Jph-Veff) curves, short-circuit current density (JSC), and VOC as a function of incident light intensity as well as the transient photovoltage (TPV) and transient photocurrent (TPC) were measured, and the results illustrated the effects of NCBA as third component materials in terms of efficient exciton dissociation and reduced charge carrier recombination and loss. The PBDB-T : NCBA : ITIC (1 : 0.1 : 0.9)-based PSCs showed an optimized PCE value of 9.56% and better thermal stability after 10 h thermal annealing treatment (the normalized PCE value was 92.5% of the initial PCE value).

Photochemical site-selective synthesis of [70]methanofullerenes

Methanofullerenes such as the well-known [70]PCBM are commonly synthesized under harsh conditions to obtain the product as a mixture of site-isomers (namely α, β and minor γ) due to the D_5h symmetry of the C₇₀ cage. We report the first site-selective synthesis of [70]methanofullerenes under light irradiation and low temperatures, thus avoiding time-consuming and highly expensive HPLC separations. Pure major site-isomers α-[70]PCBM and α-[70]DPM have been thus efficiently prepared including the crystal structure of 5b. Photovoltaic preliminary results revealed a slightly beneficial performance for α-pure [70]PCBM site-isomer devices.

Structural variation determined by length-matching effects: towards the formation of flexible porous molecular crystals

The length ratio of the alkyl chain to the spacer will significantly influence the packing motif of gemini molecules.

cis-1 Isomers of tethered bismethano[70]fullerene as electron acceptors in organic photovoltaics

Isomer-controlled [70]fullerene bis-adducts can achieve high performance as electron-acceptors in organic photovoltaics (OPVs) because of their stronger absorption intensities than [60]fullerene derivatives, higher LUMO energy levels than mono-adducts, and less structural and energetic disorder than random isomer mixtures. Especially, attractive are cis-1 isomers that have the closest proximity of addends owing to their plausible more regular close packed structure. In this study, propylene-tethered cis-1 bismethano[70]fullerene with two methyl, ethyl, phenyl, or thienyl groups were rationally designed and prepared for the first time to investigate the OPV performances with an amorphous conjugated polymer donor (PCDTBT). The cis-1 products were found to be a mixture of two regioisomers, α-1-α and α-1-β as major and minor components, respectively. Among them, the cis-1 product with two ethyl groups (Et₂-cis-1-[70]PBC) showed the highest OPV performance, encouraging us to isolate its α-1-α isomer (Et₂-α-1-α-[70]PBC) by high-performance liquid chromatography. OPV devices based on Et₂-cis-1-[70]PBC and Et₂-α-1-α-[70]PBC with PCDTBT showed open-circuit voltages of 0.844 V and 0.864 V, respectively, which were higher than that of a device with typical [70]fullerene mono-adduct, [70]PCBM (0.831 V) with a lower LUMO level. However, the short-circuit current densities and resultant power conversion efficiencies of the devices with Et₂-cis-1-[70]PBC (9.24 mA cm⁻², 4.60%) and Et₂-α-1-α-[70]PBC (6.35 mA cm⁻², 3.25%) were lower than those of the device with [70]PCBM (10.8 mA cm⁻², 5.8%) due to their inferior charge collection efficiencies. The results obtained here reveal that cis-1 [70]fullerene bis-adducts do not guarantee better OPV performance and that further optimization of the substituent structures is necessary.

cis-1 Isomers of [70]fullerene bis-adducts were utilized as electron-acceptors in organic photovoltaic devices for the first time.

Impact of fullerene derivative isomeric purity on the performance of inverted planar perovskite solar cells

The effect of utilizing a pure cis-α-dimethoxy carbonyl fulleropyrrolidine C70 (DMEC70) isomer as the electron transporting material (ETM) in inverted perovskite solar cells (PSCs) was evaluated. The as-prepared C70 mono-adduct products are mixtures of regioisomers and the interest was to evaluate them independently as ETMs. Three different cis-DMEC70 isomers (α, β-endo and β-exo) (mix-DMEC70) were synthesized and purified by HPLC. It was found that PSCs based on the pure α-DMEC70 exhibit a substantially enhanced maximum power conversion efficiency (PCE) of 18.6% as compared to devices based on the mixed-DMEC70 isomers that yielded a PCE of 16.4%. A maximum PCE of 15.7% was observed for devices based on [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). This work points out the importance of using pure fullerene derivative isomers as ETMs to reduce the intrinsic energy disorder, which enhances the overall device performance.

Tightly Bound Double-Caged [60]Fullerene Derivatives with Enhanced Solubility: Structural Features and Application in Solar Cells

A series of novel highly soluble double-caged [60]fullerene derivatives were prepared by means of lithium-salt-assisted [2+3] cycloaddition. The bispheric molecules feature rigid linking of the fullerene spheres through a four-membered cycle and a pyrrolizidine bridge with an ester function CO₂ R (R=n-decyl, n-octadecyl, benzyl, and n-butyl; compounds 1 a-d, respectively), as demonstrated by NMR spectroscopy and X-ray diffraction. Cyclic voltammetry studies revealed three closely overlapping pairs of reversible peaks owing to consecutive one-electron reductions of fullerene cages, as well as an irreversible oxidation peak attributed to abstraction of an electron from the nitrogen lone-electron pair. Owing to charge delocalization over both carbon cages, compounds 1 a-d are characterized by upshifted energies of frontier molecular orbitals, a narrowed bandgap, and reduced electron-transfer reorganization energy relative to pristine C₆₀ . Neat thin films of the n-decyl compound 1 a demonstrated electron mobility of (1.3±0.4)×10^-3  cm²  V^-1  s^-1 , which was comparable to phenyl-C₆₁ -butyric acid methyl ester (PCBM) and thus potentially advantageous for organic solar cells (OSC). Application of 1 in OSC allowed a twofold increase in the power conversion efficiencies of as-cast poly(3-hexylthiophene-2,5-diyl) (P3HT)/1 devices relative to the as-cast P3HT/PCBM ones. This is attributed to the good solubility of 1 and their enhanced charge-transport properties - both intramolecular, owing to tightly linked fullerene cages, and intermolecular, owing to the large number of close contacts between the neighboring double-caged molecules. Test P3HT/1 OSCs demonstrated power-conversion efficiencies up to 2.6 % (1 a). Surprisingly low optimal content of double-caged fullerene acceptor 1 in the photoactive layer (≈30 wt %) favored better light harvesting and carrier transport owing to the greater content of P3HT and its higher degree of crystallinity.

Surfactant-Triggered Nanoarchitectonics of Fullerene C60 Crystals at a Liquid-Liquid Interface

Here, we report the structural and morphological modulation of fullerene C₆₀ crystals induced by nonionic surfactants diglycerol monolaurate (C₁₂G₂) and monomyristate (C₁₄G₂). C₆₀ crystals synthesized at a liquid-liquid interface comprising isopropyl alcohol (IPA) and a saturated solution of C₆₀ in ethylbenzene (EB) exhibited a one-dimensional (1D) morphology with well-defined faceted structure. Average length and diameter of the faceted rods were ca. 4.8 μm and 747 nm, respectively. Powder X-ray diffraction pattern (pXRD) confirmed a hexagonal-close packed (hcp) structure with cell dimensions ca. a = 2.394 nm and c = 1.388 nm. The 1D rod morphology of C₆₀ crystals was transformed into "Konpeito candy-like" crystals (average diameter ca. 1.2 μm) when the C₆₀ crystals were grown in the presence of C₁₂G₂ or C₁₄G₂ surfactant (1%) in EB. The pXRD spectra of "Konpeito-like" crystals could be assigned to the face-centered cubic (fcc) phase with cell dimensions ca. a = 1.4309 nm (for C₁₂G₂) and a = 1.4318 nm (for C₁₄G₂). However, clusters or aggregates of C₆₀ lacking a uniform morphology were observed at lower surfactant concentrations (0.1%), although these crystals exhibited an fcc crystal structure. The self-assembled 1D faceted C₆₀ crystals and "Konpeito-like" C₆₀ crystals exhibited intense photoluminescence (PL) (∼35 times greater than pC₆₀) and a blue-shifted PL intensity maximum (∼15 nm) compared to those of pC₆₀, demonstrating the potential use of this method for the control of the optoelectronic properties of fullerene nanostructures. The "Konpeito-like" crystals were transformed into high surface area nanoporous carbon with a graphitic microstructure upon heat-treatment at 2000 °C. The heat-treated samples showed enhanced electrochemical supercapacitance performance (specific capacitance is ca. 175 F g^-1, which is about 20 times greater than pC₆₀) with long cyclic stability demonstrating the potential of the materials in supercapacitor device fabrication.

Regioisomer effects of [70]fullerene mono-adduct acceptors in bulk heterojunction polymer solar cells

Regioisomer separations of [70]fullerene mono-adducts for polymer solar cell (PSC) applications were conducted for the first time.

Despite numerous organic semiconductors being developed during the past decade, C₇₀ derivatives are predominantly used as electron acceptors in efficient polymer solar cells (PSCs). However, as-prepared C₇₀ mono-adducts intrinsically comprise regioisomers that would mask individual device performances depending on the substituent position on C₇₀. Herein, we separate the regioisomers of C₇₀ mono-adducts for PSC applications for the first time. Systematic investigations of the substituent position effect using a novel symmetric C₇₀ mono-adduct ([70]NCMA) and a prevalent, high-performance one ([70]PCBM) reveals that we can control the structures of the blend films with conjugated polymers and thereby improve the PSC performances by regioisomer separation. Our approach demonstrates the significance of exploring the best-matching regioisomer of C₇₀ mono-adducts with high-performance conjugated polymers, which would achieve a remarkable progress in PSC devices.

Comment on "Fullerene-based materials for solar cell applications: design of novel acceptors for efficient polymer solar cells--a DFT study" by A. Mohajeri and A. Omidvar, Phys. Chem. Chem. Phys., 2015, 17, 22367

Though a linear correlation has been recently reported between mean polarizabilities of the fullerene derivatives and open-circuit voltages of organic solar cells based on them, we demonstrate that there is no general dependence between these two values and some related quantities (anisotropy of polarizability and LUMO levels).

Counting the Isomers and Estimation of Anisotropy of Polarizability of the Selected C60 and C70 Bisadducts Promising for Organic Solar Cells

Currently, bisadducts of C60 and C70 fullerenes are widely studied as electron-acceptor materials for organic solar cells. These compounds are usually used as mixtures of the positional isomers. However, as recently shown, the separate use of the purified isomers with lowest anisotropies of polarizability may enhance solar cell output parameters. To predict the structures of the compounds appropriate for this purpose, we calculated anisotropies of polarizability of four classes of fullerene bisadducts, namely, bis-[60]PCBM, [60]OQMF, bis-[70]PCBM, and [70]OQMF (18, 16, 41, and 42 positional isomers, respectively). As found, the anisotropies quadratically correlate with the interaddend distances in fullerene bisadducts, whereas there are no obvious correlations between the structures and lowest unoccupied molecular orbital levels, traditionally used for assessing the efficiency of candidates for organic solar cell electron acceptors. According to our calculations, bisadducts bis-[60]PCBM-ee-1, [60]OQMF-cis-3.2, [60]OQMF-trans-4.2, cc(1.1)cc(2'.1)-bis-[70]PCBM, and cc1cc(2'.1)-[70]OQMF have the lowest anisotropies of polarizability. These compounds have a primary interest for synthesis, purification, and further separate testing in solar cells. The structures of these adducts have a common feature, which we describe with the "not so close and not so far" rule: the distances between the addends in the most isotropic fullerene bisaddicts should be medium among the possible values. These are ee, ef, cis-3, and trans-4 positions in the case of the C60 bisadducts and cc bonds placed on the different poles and the same hemisphere of the C70 skeleton.

Synthesis and Isolation of cis-2 Regiospecific Ethylene-Tethered Indene Dimer-[70]Fullerene Adduct for Polymer Solar Cell Applications

Although the utilization of [70]fullerene bis-adducts can enhance the power conversion efficiencies of polymer solar cells (PSCs) owing to their strong absorption intensities and high-lying lowest unoccupied molecular orbital energy levels, this synthetic strategy typically yields a mixture of regioisomers that would mask the intrinsic device performances depending on the substituent pattern on the [70]fullerene derivatives. In this study, a single cis-2 regioisomer of C70 bis-adduct (cis-2-[70]BIEC) has been prepared for the first time by the same strategy that had been applied to [60]fullerene to obtain a regioisomerically pure C60 bis-adduct (cis-2-[60]BIEC). Diels-Alder reaction was conducted between a rationally designed ethylene-tethered indene dimer and [70]fullerene, followed by isolation using high-performance liquid chromatography suitable for the separation of fullerene derivatives. A series of structural analysis techniques including NMR spectroscopies and X-ray crystallography were used to identify the absolute configuration of the bis-adduct. A systematic study on the optical, electrochemical, and photovoltaic properties of cis-2-[70]BIEC as well as the corresponding regioisomer mixture (bis-[70]BIEC) and the monoadduct (α-mono-[70]BIEC) has been performed to examine the effect of the pure cis-2 regioisomer. More importantly, their properties are compared with those of cis-2-[60]BIEC to address the effect of fullerene cage structures, that is, C60 versus C70. The PSC based on cis-2-[70]BIEC and poly(3-hexylthiophene) showed a remarkable power conversion efficiency of 4.2%, which is higher than those with bis-[70]BIEC (2.2%), α-mono-[70]BIEC (2.2%), cis-2-[60]BIEC (2.8%), and even a prevalent high-performance C70 monoadduct ([70]PCBM, 3.8%). Our synthetic strategy will pave the way for further development on the rational design and isolation of single fullerene bis-adduct regioisomers exhibiting high device performances.

Hetero Bis-Addition of Spiro-Acetalized or Cyclohexanone Ring to 58π Fullerene Impacts Solubility and Mobility Balance in Polymer Solar Cells

Fullerene bis-adducts are increasingly being studied to gain a high open circuit voltage (Voc) in bulk heterojunction organic photovoltaics (OPVs). We designed and synthesized homo and hetero bis-adduct [60]fullerenes by combining fused cyclohexanone or a five-membered spiro-acetalized unit (SAF5) with 1,2-dihydromethano (CH2), indene, or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). These new eight 56π fullerenes showed a rational rise of the lowest unoccupied molecular orbital (LUMO). We perform a systematic study on the electrochemical property, solubility, morphology, and space-charge-limited current (SCLC) mobility. The best power conversion efficiency (PCE) of 4.43% (average, 4.36%) with the Voc of 0.80 V was obtained for poly(3-hexylthiophene) (P3HT) blended with SAF5/indene hetero bis-adduct, which is a marked advancement in PCE compared to the 0.9% of SAF5 monoadduct. More importantly, we elucidate an important role of mobility balance between hole and electron that correlates with the device PCEs. Besides, an empirical equation to extrapolate the solubilities of hetero bis-adducts is proposed on the basis of those of counter monoadducts. Our work offers a guide to mitigate barriers for exploring a large number of hetero bis-adduct fullerenes for efficient OPVs.

A single cis-2 regioisomer of ethylene-tethered indene dimer–fullerene adduct as an electron-acceptor in polymer solar cells

A single cis-2 isomer of fullerene bis-adduct has been synthesized, isolated and applied as an electron-acceptor in polymer solar cells.