Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Abstract

The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C61‐butyric acid methyl ester) or the indene C60 bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.

Origin of fullerene-induced vitrification of fullerene:donor polymer photovoltaic blends and its impact on solar cell performance

“…For instance, it is still unclear why specic acceptors work well only in combination with certain donors. 1 Moreover, thorough knowledge has not been established why small variations in the chemical structure of the active materials in certain cases can lead to signicant differences in device performance while in other scenarios essentially identical device characteristics are obtained.…”

“…One reason for these differences that are observed in devices even for structurally very similar materials is that the manipulation of the chemical structure oen results in alteration of the energy levels, 1,3 which, in the case of the LUMO of the acceptor, should either promote charge dissociation or increase the open circuit voltage (V oc ) with a given donor. Chemical changes lead, however, also to a different miscibility of the two components, which can affect OPV device characteristics considerably, 4 as well as a different thermal behaviour.…”

Facile synthesis and photovoltaic applications of a new alkylated bismethano fullerene as electron acceptor for high open circuit voltage solar cells

“…Although many C 60 derivatives and a new C 84 derivative have been synthesized and utilized as acceptors in OPVs, only handful C 60 derivatives achieved good solubility in organic solvents, desired electron mobility to have good charge transport, broad absorption to contribute to the photocurrent and suitable electronic levels to have efficient charge dissociation. 19,20 The research effort devoted to new fullerene acceptor materials are increasing but the new fullerene derivative acceptors showing better photovoltaic performances than PCBM are very few. PCBM is still the most important acceptor material used in the organic solar cell studies.…”

An overview of molecular acceptors for organic solar cells

“…Nevertheless, a limiting factor to this polymer: C 60 bisadducts strategy was evidenced for some blends where a simultaneous dramatic loss in the J sc and lower electron mobilities resulted in a reduced PCE for most polymers other than P3HT [79]. It was identified that the principal 40401-p7 reason was the correlation between J sc and the free energy of the photoinduced charge generation ΔG CT .…”