The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets

Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecul...

Full description

Saved in:
Bibliographic Details
Published in:Nature energy 2020-09, Vol.5 (9), p.711-719
Main Authors: Classen, Andrej, Chochos, Christos L., Lüer, Larry, Gregoriou, Vasilis G., Wortmann, Jonas, Osvet, Andres, Forberich, Karen, McCulloch, Iain, Heumüller, Thomas, Brabec, Christoph J.
Format: Article
Language:English
Subjects:
639/301/1005/1007
639/4077/909/4101/4096
639/766/1130/2799
Charge transfer
Current carriers
Economics and Management
Emitters
Energy
Energy Policy
Energy Storage
Energy Systems
Excitons
Lifetime
Molecular orbitals
Offsets
Open circuit voltage
Photovoltaic cells
Power efficiency
Quantum efficiency
Renewable and Green Energy
Solar cells
Solar energy
Voltage
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆ E HOMO ) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆ E HOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆ E HOMO . Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆ E HOMO . Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells. Donor–acceptor systems with low energy-level offset enable high power efficiency in organic solar cells yet it is unclear what drives charge generation. Classen et al. show that long exciton lifetimes enable efficient exciton splitting and thus generation of free charges while also suppressing voltage losses.
ISSN:2058-7546
2058-7546
DOI:10.1038/s41560-020-00684-7