Optical properties of conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites

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Abstract
In this work, we present the evolution of optical constants as a function of [ 6 , 6 ] - phenyl C 61 -butyric acid methyl ester (PCBM) concentration for conjugated poly(3-hexylthiophene)/ [ 6 , 6 ] - phenyl C 61 -butyric acid methyl ester composites. The PCBM concentration of the utilized samples varies from 1 to 50 wt % . The dielectric functions for all these composites reveal electronic structural changes as a result of the addition of PCBM. We have deconvoluted the contribution of the substrate using a two-layer Fabry-Pérot structural model. The extracted optical properties contain crucial absorption peaks of singlet exciton states and vibronic sidebands for poly(3-hexylthiophene) (P3HT) conjugated polymer as well as two PCBM-related states at higher energies. With the addition of PCBM, we have observed a limit of 20 wt % PCBM beyond which two discrete energy levels (3.64 and 4.67 eV ) appear in the spectrum. For the highest concentration composite, the results suggest that the interchain interactions provide a small excitonic contribution in the absorptionspectrum at energies where the conjugated polymer absorbs ( 1.85 – 2.7 eV ) and a strong rise of PCBM states (3.64 and 4.67 eV ) which are responsible for the subsequent exciton dissociation. In addition, the energy gap between the higher occupied molecular orbitals and the lower unoccupied molecular orbitals of the highest concentration composite ( 50 wt % ) is 1.85 eV . The tuning of the optical properties of P3HT with the addition of PCBM shows that ellipsometry can be used to monitor layer concentration toward optimization of plastic solar cells.