Figure 1.

Jablonski diagram illustrating the competition between charge generation (with time constant τ_d) and intersystem crossing (with time constant τ_ISC) of PCBM singlet excitons (S₁). The energies are relative to the ground state (S₀) and all arrows describe population transfer. The formation of PCBM triplet excitons (T₁) is detrimental to the device performance. Driven by the large energy difference, they can transfer into low-energy triplet excitons on the donor polymer, which are then quickly quenched by the charges. This loss pathway competes with charge generation, particularly in systems with high charge-transfer (CT) state energies. Note that the CT states with either spin-singlet or spin-triplet character have very similar energies due to the weak wavefunction overlap, and are therefore neglected here for simplicity. The triplet excitons can also limit the device lifetime through reacting with oxygen to form highly reactive superoxide radical anion, O₂⁻. The molecular structures of the systems studied are shown as insets.