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

Molecular structure-device performance relationship in polymer solar cells based on indene-C-60 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-C-60 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%