Sub-bandgap absorption in polymer-fullerene solar cells studied by temperature-dependent external quantum efficiency and absorption spectroscopy

► Detection of very weak sub-bandgap (SBG) absorptions in organic solar cells. ► Temperature-dependent detection of very weak SBG EQE features. ► Critical review of literature-models for the SBG EQE origin. ► Strong decrease of the SBG EQE peak above RT contradicts the charge transfer model. ► Polaron transitions offer a consistent interpretation of absorption and EQE(T) data.