Transient Magnetophotoinduced Absorption Studies of Photoexcitations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>π</mml:mi></mml:math>-Conjugated Donor-Acceptor Copolymers

Abstract

We have utilized a plethora of transient and steady state optical and magneto-optical spectroscopies in a broad spectral range (0.25-2.5 eV) for elucidating the primary and long-lived photoexcitations in a low band-gap π-conjugated donor-acceptor (DA) copolymer used for efficient photovoltaic solar cells. We show that both singlet excitons (SE) and intrachain triplet-triplet (TT) pairs are photogenerated in the DA-copolymer chains. From the picosecond transient magnetic field response of these species we conclude that the SE and TT spin states are coupled. The TT decomposition into two intrachain geminate triplet excitons maintains spin coherence and thus their spin entanglement lasts into the microsecond time domain.