Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics

We present time-resolved photo luminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ∼4-fold and ∼15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO ₂ solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO ₂ nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO ₂ solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO ₂.