Sub-bandgap absorption in organic solar cells: experiment and theory

Most high-performance organic solar cells involve bulk-heterojunctions in order to increase the active donor-acceptor interface area. The power conversion efficiency depends critically on the nano-morphology of the blend and the interface. Spectroscopy of the sub-bandgap region, i.e., below the bulk absorption of the individual components, provides unique opportunities to study interface-related properties. We present absorption measurements in the sub-bandgap region of bulk heterojunctions made of poly(3-hexylthiophene-2,5-diyl) as an electron donor and [6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) as an electron acceptor and compare them with quantum-chemical calculations and recently published data on the external quantum efficiency (EQE). The very weak absorption of the deep sub-bandgap region measured by the ultra-sensitive Photothermal Deflection Spectroscopy (PDS) features Urbach tails, polaronic transitions, conventional excitons, and possibly charge-transfer states. The quantum-chemical calculations allow characterizing some of the unsettled spectral features.