Beyond PCBM: Understanding the Photovoltaic Performance of Blends of Indene-C60 Multiadducts with Poly(3-Hexylthiophene)

The effect of functionalization of the C ₆₀ cage with multiple indene groups in relation to the dynamics of photogenerated species in blends with poly(3-hexylthiophene) (P3HT) and the performance of P3HT:indene-C ₆₀ photovoltaic devices is reported. Despite the systematic decrease of the electron affinity of the acceptor with the number of additions, exciton dissociation is efficient in blends of P3HT with all three indene-C ₆₀ derivatives. By replacing the prototypical acceptor [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PCBM) with mono-indene-C ₆₀ (ICMA) or a sample of a mixture of bis-indene-C ₆₀ regioisomers (ICBA) the power conversion efficiency is enhanced, predominantly due to an increase in the open-circuit voltage that originates from the lower electron affinity of the indene-C ₆₀ acceptor. The use of an acceptor sample that represents a mixture of tris-indene-C ₆₀ (ICTA) regioisomers results in a reduction of the short-circuit current density, fill factor, and open-circuit voltage of the photovoltaic device. The electron mobility in ICTA domains is ca. a factor 10 lower than in ICMA and ICBA. Density functional theory calculations of the LUMO energies in ICTA isomers demonstrate that energetic disorder caused by the presence of regioisomers is unlikely to be responsible for the low electron mobility in ICTA. The observed deterioration in device performance is attributed to the formation of small ICTA clusters "coated" in insulating indene units that reduce electronic coupling between the molecules and cause the low electron mobility in ICTA domains. These findings indicate that while multiple additions to a fullerene cage provide a facile methodology for controlling the energy levels, they may have limited success in improving OPV device performance. Photovoltaic devices based on a bulk heterojunction of bis-indene-C ₆₀ with poly(3-hexylthiophene) show enhanced power conversion efficiency due to an improved open circuit voltage. The addition of a third indene unit to form tris-indene-C ₆₀ dramatically hinders both the local and bulk electron transport in the tris-indene-C ₆₀ phase due to the formation of small, isolated tris-indene-C ₆₀ clusters, resulting in poor device performance.