Quantification of quantum efficiency and energy losses in low bandgap polymer:fullerene solar cells with high open-circuit voltage.

In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (E _D*) and the energy of the charge transfer state (E _CT) will optimize the open-circuit voltage (V _oc). In this work, this energy loss E _D*- E _CT is measured directly via Fourier-transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI-1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI-1 are similar (1.4 eV-1.5 eV), the optimized photovoltaic devices show large differences in V _oc and internal quantum efficiency (IQE). For P3TI:PC ₇₁BM blends a E _D*-E _CT of ∼ 0.1 eV, a V _oc of 0.7 V and an IQE of 87% are found. For PTI-1:PC ₆₁BM blends an absence of sub-gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher V _oc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI-1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between E _CT and qV _oc due to radiative and non-radiative recombination are quantified for both material systems, indicating that for the PTI-1:PC ₆₁BM material system, V _oc can only be increased by decreasing the non-radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (<0.1 eV) and a high V _oc. Polymer:fullerene photovoltaic devices comprising two isoindigo containing polymers show large differences in open-circuit voltage (V _oc) and external and internal quantum efficiencies. Instead of morphology, an almost vanishing energetic difference between the optical gap of the polymer and energy of the charge transfer state is responsible for the lower photocurrent produced by the high V _oc device.