Photoconductivity and photoexcitation spectra of acceptor substituted poly(3-butyl)thiophene

We have studied acceptor substituted poly(3-butyl)thiophene (PBT) in order to examine the usefulness of the acceptor as an intrinsic charge trap under laser excitations. The acceptor used is the chemically prepared high electron affinity monomer 3-(4-nitrobenzene-1,2-ethen) thiophene (NBT). The studies reported here include transient photoconductivity in the nanoseconds to μ-seconds time domain, and nearly steady state photoinduced absorption in thin films of PBT and co-polymers made of PBT and acceptor substituted PBT. The PA spectra revealed two main absorption bands; HE at 1.4-1.7 eV, which is due to contributions of both triplet excitons and polarons (or bipolarons) and LE at ≈0.5 eV which is due to polarons (or bipolarons) only. The photoconductivity studies show increased slow component (ten to hundreds of nanosecond) as the amount of acceptor substitution is increased, indicating charge separation under laser excitation.