Photovoltaic Processes of Singlet and Triplet Excited States in Organic Solar Cells

Abstract

This article reports the respective photovoltaic processes of singlet and triplet photoexcited states in dissociation and charge reactions based on the studies of magnetic‐field effects of photocurrents. The magnetic‐field effects of photocurrents reveal that weak donor‐acceptor interactions lead to a two‐step photovoltaic process: dissociation in polaron‐pair states evolved from singlet excitonic states and exciton‐charge reactions occurred in triplet excitonic states in the generation of the photocurrent. However, strong donor‐acceptor interactions yield a one‐step photovoltaic process: direct dissociation of both singlet and triplet excitons in bulk‐heterojunction organic solar cells. In addition, the magnetic‐field effects of photocurrents indicate that the dissociated electrons and holes form charge‐transfer complexes with singlet and triplet spin configurations at donor‐acceptor intermolecular interfaces. As a result, the magnetic‐field effects of photocurrents can deliver a critical understanding of singlet and triplet photovoltaic processes to design advanced solar‐energy materials and devices.

Microscopic modeling of magnetic-field effects on charge transport in organic semiconductors

“…However, Xu and Hu conjecture that the magnetoresistance is not caused by a difference in recombination rate of singlet and triplet e-h pairs, but by a difference in dissociation rate due to the more ionic nature of the singlet e-h pairs. 26 As the resulting influence on the MC for both mechanisms is almost identical, we will focus on the model of Prigodin et al, where we assume that there is a smaller recombination rate of triplet than singlet e-h pairs. Note that this is in sharp contrast to what is concluded by Kersten et al, 27 where the triplet e-h pairs have the largest recombination rate.…”

Bipolar polaron pair recombination in polymer/fullerene solar cells

“…The active area of the actual cell is about 4 mm 2 ; the rest of the substrate is needed to electrically contact the cell without disturbing the microwave resonator. Without further encapsulation the samples were bonded onto a sample holder and transferred into the spectrometer, exposing the samples to air in darkness (1)], the excitons generated diffuse to the interface, where the charges are separated, eventually forming polarons P + and P − [denoted by (2)]. Spin-dependent processes involving these polarons can be either unipolar, where polarons of the same charge form a bipolaron [denoted by (3)], or bipolar, where two polarons of different charge annihilate [denoted by (4)].…”

“…The low spin-orbit coupling of organic semiconductors and the resulting long spin lifetimes lead to a pronounced spin dependence of charge-transport processes, providing an opportunity for further improvements of the efficiency of light-emitting diodes (LEDs) [1] and solar cells [2,3] as well as for the development of spintronic devices [4,5]. Magnetic resonance methods are uniquely suited to study the influence of spins on electronic transport on time scales as fast as 100 ns, often allowing to microscopically identify relevant or rate-limiting processes.…”

Communication: Manipulating the singlet-triplet equilibrium in organic biradical materials

“…[4][5][6][7][8][9] When a ground or thermally accessible, paramagnetic state is generated, they also provide close insight into the behavior that is very promising for light harvesting, controllable reactivity, and spin valve applications. [10][11][12] 1,4-Diazidobenzene attracted our attention in this regard. Like several other aromatic azides, it undergoes a photochemical reaction to yield 1,4-phenylenedinitrene (Fig.…”