Han-Hee Cho

Controlling Number of Indene Solubilizing Groups in Multiadduct Fullerenes for Tuning Optoelectronic Properties and Open-Circuit Voltage in Organic Solar Cells

The ability to tune the lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels of fullerene derivatives used as electron acceptors is crucial in controlling the optical/electrochemical properties of these materials and the open circuit voltage (V(oc)) of solar cells. Here, we report a series of indene fullerene multiadducts (ICMA, ICBA, and ICTA) in which different numbers of indene solubilizing groups are attached to the fullerene molecule. The addition of indene units to fullerene raised its LUMO and HOMO levels, resulting in higher V(oc) values in the photovoltaic device. Bulk-heterojunction (BHJ) solar cells fabricated from poly(3-hexylthiophene) (P3HT) and a series of fullerene multiadducts-ICMA, ICBA, and ICTA showed V(oc) values of 0.65, 0.83, and 0.92 V, respectively. Despite demonstrating the highest V(oc) value, the P3HT:ICTA device exhibited lower efficiency (1.56%) than the P3HT:ICBA device (5.26%) because of its lower fill factor and current. This result could be explained by the lower light absorption and electron mobility of the P3HT:ICTA device, suggesting that there is an optimal number of the solubilizing group that can be added to the fullerene molecule. The effects of the addition of solubilizing groups on the optoelectrical properties of fullerene derivatives were carefully investigated to elucidate the molecular structure-device function relationship.

Influence of intermolecular interactions of electron donating small molecules on their molecular packing and performance in organic electronic devices

Intermolecular interactions have a critical role in determining the molecular packing and orientation of conjugated polymers and organic molecules, leading to significant changes in their electrical and optical properties. Herein, we investigated the effects of intermolecular interactions of electron-donating small molecules on their structural, optical, and electrical properties, as well as on their performance in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). A series of dithienosilole-based small molecule donors were synthesized by introducing different terminal groups of ester and amide groups combined with three different versions of alkyl side chains. In comparison to dithienosilole-based small molecules with ester terminal groups, those with amide terminal groups exhibit strong intermolecular interaction by hydrogen bonding in a non-destructive manner. In addition, in order to control the intermolecular distance during assembly and thus fine-tune the interaction between the small molecule donors, three different alkyl side chains (i.e., n-octyl, n-decyl, and 2-ethylhexyl chains) were introduced into both small molecules with amide and ester terminal groups. The molecular packing and orientation of the small molecule donors were dramatically changed upon modifying the terminal groups and the alkyl side chains, as evidenced by grazing incidence X-ray scattering (GIXS) measurements. This feature significantly affected the electrical properties of the small molecules in OFETs. The trends in the activation energies for charge transport and the hole mobilities in OFETs were consistent with the molecular ordering and orientation propensity. In addition, the nano-scale morphology of small molecules blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was also influenced by the intermolecular interaction of small molecule donors. Power conversion efficiencies of more than 4.3% in OPVs were obtained from dithienosilole-based small molecules with ester terminal groups and linear side chains due to the optimized intermolecular interaction and morphology of the active layer.

Controlling side-chain density of electron donating polymers for improving their packing structure and photovoltaic performance

A simple and efficient approach of controlling the side-chain density in the electron donating polymers has been demonstrated to tune their 3-D packing structure and HOMO level, which increases the hole mobility and V(oc) values, thus improving the solar cell performance.

Surface Engineering of Graphene Quantum Dots and Their Applications as Efficient Surfactants

The surface properties of graphene quantum dots (GQDs) control their dispersion and location within the matrices of organic molecules and polymers, thereby determining various properties of the hybrid materials. Herein, we developed a facile, one-step method for achieving systematic control of the surface properties of highly fluorescent GQDs. The surfaces of the as-synthesized hydrophilic GQDs were modified precisely depending on the number of grafted hydrophobic hexylamine. The geometry of the modified GQDs was envisioned by conducting simulations using density functional theory. In stark contrast to the pristine GQDs, the surface-modified GQDs can effectively stabilize oil-in-water Pickering emulsions and submicron-sized colloidal particles in mini-emulsion polymerization. These versatile GQD surfactants were also employed in liquid-solid systems; we demonstrated their use for tailoring the dispersion of graphite in methanol. Finally, the particles produced by the GQD surfactants were fluorescent due to luminescence of the GQDs, which offers great potential for various applications, including fluorescent sensors and imaging.

Polarity and Air-Stability Transitions in Field-Effect Transistors Based on Fullerenes with Different Solubilizing Groups

A series of o-xylene and indene fullerene derivatives with varying frontier molecular orbital energy levels were utilized for assessing the impact of the number of solubilizing groups on the electrical performance of fullerene-based organic-field-effect transistors (OFETs). The charge-carrier polarity was found to be strongly dependent upon the energy levels of fullerene derivatives. The o-xylene C60 monoadduct (OXCMA) and indene C60 monoadduct (ICMA) exhibited unipolar n-channel behaviors with high electron mobilities, whereas the bis- and trisadducts of indene and o-xylene C60 derivatives showed ambipolar charge transport. The OXCMA OFETs fabricated by solution shearing and molecular n-type doping showed an electron mobility of up to 2.28 cm(2) V(-1) s(-1), which is one of the highest electron mobilities obtained from solution-processed fullerene thin-film devices. Our findings systematically demonstrate the relationship between the energy level and charge-carrier polarity and provide insight into molecular design and processing strategies toward high-performance fullerene-based OFETs.

Molecular structure-device performance relationship in polymer solar cells based on indene-C60 bis-adduct derivatives

Interfacial tension between two materials is a key parameter in determining their miscibility and, thus, their morphological behavior in blend films. In bulk heterojunction (BHJ)-type polymer solar cells (PSCs), control of the interfacial tension between the electron donor and the electron acceptor is critically important in order to increase miscibility and achieve optimized BHJ morphology for producing efficient exciton dissociation and charge transport. Herein, we report the synthesis of a series of indene-C60 bis-adducts (ICBA) derivatives by modifying their end-groups with fluorine (FICBA), methoxy (MICBA) and bromine (BICBA) functional units. We systematically studied the effects of their structural changes on the blend morphology with poly(3-hexylthiophene) (P3HT) and their performance in the PSCs. The end-group modification of ICBA derivatives induced a dramatic change in their interfacial tensions with P3HT (i.e., from 4.9 to 8.3mN m−1), resulting in large variations in the power conversion efficiency (PCE) of the PSCs, ranging from 2.9 to 5.2%.

Synthesis and side-chain engineering of phenylnaphthalenediimide (PNDI)-based n-type polymers for efficient all-polymer solar cells

We designed and synthesized a series of n-type conjugated polymers by introducing phenylnaphthalenediimide (PNDI) as a novel n-type building block, and investigated the effect of side-chain engineering of the polymer acceptors on the performance of all-polymer solar cells (all-PSCs). The optical, electrochemical, and structural properties of the polymers with three different side chains of 2-ethylhexyl (PPNDI-EH), 2-butyloctyl (PPNDI-BO), and 2-hexyldecyl (PPNDI-HD) groups were examined. Interestingly, the PNDI-based polymer having the longest side chain showed a higher degree of edge-on oriented intermolecular assembly in thin films, thereby resulting in the highest field-effect electron mobility among the three polymers. Also, we examined the performance of PNDI-based polymers as polymer acceptors in all-PSCs. Unlike the trend in the field-effect transistor, the PPNDI-BO-based all-PSCs exhibited the highest power conversion efficiency (PCE) of 4.25% among the three polymer blends. This was attributed to the well-balanced hole/electron transport and higher exciton dissociation probability in the PPNDI-BO-based all-PSCs, benefitted from the well-intermixed blend morphology between the polymer donor and PPNDI-BO.

Enhancing Mechanical Properties of Highly Efficient Polymer Solar Cells Using Size-Tuned Polymer Nanoparticles

The low mechanical durability of polymer solar cells (PSCs) has been considered as one of the critical hurdles for their commercialization. We described a facile and powerful strategy for enhancing the mechanical properties of PSCs while maintaining their high power conversion efficiency (PCE) by using monodispersed polystyrene nanoparticles (PS NPs). We prepared highly monodispersed, size-controlled PS NPs (60, 80, and 100 nm), and used them to modify the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) anode buffer layer (ABL). The PS NPs played two important roles; i.e., they served as (1) binders in the PEDOT:PSS films, and (2) interfacial modifiers between ABL and the active layer, resulting in remarkable improvement of the mechanical integrity of the PSCs. The addition of PS NPs enhanced the inherent mechanical toughness of the PEDOT:PSS ABL due to their elastic properties, allowing the modified ABL to tolerate higher mechanical deformations. In addition, the adhesion energy (Gc) between the active layer and the modified PEDOT:PSS layer was enhanced significantly, i.e., by a factor of more than 1.5. The Gc value has a strong relationship with the sizes of the PS NP, showing the greatest enhancement when the largest size PS NPs (100 nm) were used. In addition, PS NPs significantly improve the air-stability of the PSCs by suppressing moisture adsorption and corrosion of the electrodes. Thus, the modification of ABL with PS NPs effectively enhances both the mechanical and the long-term stabilities of the PSCs without sacrificing their PCE values, demonstrating their great potential as applications in flexible organic electronics.

Impact of highly crystalline, isoindigo-based small-molecular additives for enhancing the performance of all-polymer solar cells

We have developed a simple yet versatile approach for enhancing the performance of all-polymer solar cells (all-PSCs) using a highly crystalline small-molecular additive, 6,6′-dithiopheneisoindigo (DTI). The DTI additive in a blend of PTB7-Th donor and P(NDI2HD-T) acceptor enhances the power conversion efficiency of all-PSCs from 5.9 to 6.8%. Based on the analyses of the electrical, optical, and structural properties of all-PSCs, it is suggested that DTI additives promote tighter π–π packing and larger sizes of crystalline domains. Such morphological changes of the PTB7-Th:P(NDI2HD-T) blend films upon the addition of DTI enhance the exciton lifetime and diffusion length, which are proved by static and femtosecond-transient absorption spectroscopies and time-resolved photoluminescence. As a result of improved exciton dissociation probability and charge transport, the short-circuit current density of all-PSCs is greatly increased. Importantly, we also show that the DTI additive can be applied to other all-PSC systems with different polymer donors and naphthalene diimide (NDI)-based polymer acceptors, where the efficiencies of all-PSCs are enhanced by 10–20%.