Figure 2: Excitons and polaron pairs spectra of low-bandgap co-polymers.
From: Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics
Chemical structure of the repeat unit for the co-polymers: (a) PCPDT-BDT, (b) PCPDT-BT, (c) PCPDT-2TTP and (d) PCPDT-2TBT. The donor and acceptor moieties are indicated with grey and yellow/red shaded areas, respectively. Red shaded areas indicate acceptor moieties with larger electron affinities. Absorption spectra of: (e) PCPDT-BDT, (f) PCPDT-BT, (g) PCPDT-2TTP and (h) PCPDT-2TBT. The vertical dashed lines indicate the pump photon energies for the transient absorption experiments. (i) PCPDT-BDT, (j) PCPDT-BT, (k) PCPDT-2TTP, (l) PCPDT-2TBT doping-induced ΔOD spectra as solid lines obtained by adding SbCl5 to the respective polymer solutions. GB indicates ground-state bleaching, P1 first polaron band, Ex exciton absorption band, P2 second polaron band. The blue squares show the transient absorption spectra recorded at zero time delay between the pump and probe beam. The respective pump beam wavelengths are indicated on the corresponding linear absorption graphs. The vertical dashed red lines indicate the choice of probe photon energies for determining the polaron pair yields in the experiments of Fig. 5. The inset in (k) sketches the P1 and P2 polaron transitions.
