Magnetic Field Effects on Photocurrent and Photovoltaic Processes in Organic Solar Cells

Two mechanisms were investigated to understand the magnetic field effects on photocurrent (MFP), a phenomenon whereby an external magnetic field (MF) can increase or decrease photocurrent in organic solar cells (OSCs). The first one relates to MF-modulated intersystem crossing (ISC) between singlet and triplet polaron pairs and charge transfer (CT) complexes, accounting for the positive MFP in low and high magnetic field, respectively. The second mechanism originates from the interaction between triplet excited states and charge carriers, namely triplet-charge reaction (TCR). External magnetic field was found to reduce the rate constant of TCR, leading to negative MFP.

MFP measurements were used to study the dissociation processes of singlet and triplet excitons in single-layer solar cells. It was found that the triplet excitons mainly dissociate directly into free charge carriers at the metal-electrode interface due to their long diffusion length, while the singlet excitons experience significant bulk dissociation into polaron pairs due to their short diffusion length. The interfacial dissociation of triplet excitons at the metal electrode can lead to enhanced photovoltaic (PV) response from phosphorescent materials as compared to the bulk dissociation of singlet excitons in fluorescent materials.

measurements were also applied to study the charge carrier generation and recombination processes in bulk-heterojunction solar cells. It was found that dissociation of polaron pairs, triplet-charge reaction, dissociation of hot excitons, and photoinduced charge transfer are responsible for charge carrier generation in OSCs. High-field MFP reflects the recombination of dissociated electrons and holes towards the formation of singlet and triplet CT complexes in bulk-heterojunction OSCs. The annealing effect on the formation of CT complexes was also investigated.

The PV efficiency of bulk-heterojunction solar cells was improved by uniformly dispersing phosphorescent heavy-metal complexes. The MFP results suggest that the spin-orbit coupling and the density of triplet excited states in conjugated polymer MEH-PPV were enhanced by external heavy-atom effect. The increased triplet density facilitated exciton dissociation and reduced charge recombination, and consequently improved the PV response in bulk-heterojunction cells.