  The saga of the primary photoexcitations in $\pi$-conjugated polymers has
been a source of extraordinary scientific curiosity that has lasted for more
than three decades. From soliton excitations in trans-polyacetylene, to singlet
and triplet excitons and polarons in other polymers, to charge transfer
excitons in blends of polymers and fullerenes, the field has been rich with a
variety of different photoexcitation species. Here we show the photogeneration
of a novel primary intrachain photoexcitation species, namely the composite
multi-exciton (CME) in $\pi$-conjugated donor-acceptor (DA)-copolymers used in
organic photovoltaic (OPV) solar cells. We utilized the magnetic field response
of the transient photoinduced absorption from sub-picosecond to millisecond to
show in pristine DA-copolymer early photogeneration of the CME species that is
composed of four coupled spin 1/2 particles, having unique optical and magnetic
signatures. This species decomposes into two independent triplets in the
microsecond time domain. Importantly in copolymer/fullerene blends the CME
ionization generates photocarriers by a unique process that may enhance the
photocurrent in OPV solar cells.
