Subbandgap absorption in polymer-fullerene solar cells

We present external quantum efficiency (EQE) studies of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenylC(61)-butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction polymer solar cells with improved intensity resolution in the subbandgap (SBG) region, i.e., the energy range below the optical bandgaps of the pristine materials. Varying the P3HT: PCBM blending ratio, we find that in addition to a Gaussian profile an exponential tail is needed for a quantitative description of the SBG EQE spectra. While the exponential contribution can be reliably assigned to disorder effects, the SBG EQE Gaussian profile can be due to charge-transfer absorption between P3HT and PCBM or due to absorption of PCBM at the interface or in the polymer-rich phase. (C) 2010 American Institute of Physics. [doi:10.1063/1.3527077]