Novel primary photoexcitations in $\pi$-conjugated donor-acceptor copolymers probed by transient magneto-photoinduced-absorption

TL;DR

This paper uncovers a new primary photoexcitation species, the composite multi-exciton (CME), in $\pi$-conjugated donor-acceptor copolymers, revealing its unique magnetic signatures and potential role in enhancing organic photovoltaic efficiency.

Contribution

It reports the discovery of the CME species in DA-copolymers and its photogeneration mechanism, a novel insight into primary excitations in these materials.

Findings

CME is generated early in DA-copolymers with unique magnetic signatures.

CME decomposes into two triplets within microseconds.

CME ionization may improve photocurrent in OPV devices.

Abstract

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.