Excitons to Charge Carriers in Molecular and Nanoscale Systems

NREL researchers strive to understand the complex manifold of excited states generated from photoexcitation of excitonic semiconductors.

The energies, oscillator strengths, lifetimes, spin multiplicity, Coulomb binding energy, and mobility (degree of localization) of these states all play crucial roles in determining the specific pathways required to convert these bound states into free charges.

Research Highlights

Struggle To Dissociate Triplet Excitons From Singlet Fission in Pentacene

We have successfully identified electron acceptors capable of dissociating triplet excitons from singlet fission in pentacene films. However, even at the optimal driving force, the rate constant for electron transfer is surprisingly small.

Research Details

  • Successfully dissociated triplets from singlet fission in pentacene into free carriers
  • Identified and found optimum driving force to follow Marcus formulation
  • Discovered rate constant for photoinduced electron transfer is 5–6 orders of magnitude slower than for singlet states

Significance and Impact

Slow dissociation process contrasts with singlet excitons, opening up other competing pathways that may prove to be an obstacle to the design of efficient singlet fission solar cells that are based on a direct dissociation process.

Partners

University of Colorado Boulder

Colorado State University

Imperial College London

University of Kentucky

King Abdullah University of Science and Technology

Endothermic Singlet Fission Through Oligomer Design

We discovered two triplet excitons born from one photon absorption event in chains of molecular absorbers are formed with no loss of energy and with lifetimes into the microsecond regime through the involvement of spatial separation and dynamic geometric isolation.

Research Details

  • Designed and synthesized perylene oligomers with varying numbers of chromophores that are strongly coupled through bridges
  • Found significant triplet yield only in trimer and longer structures
  • Spectroscopy and calculations showed that oligomers undergo planarization in the singlet excited state before singlet fission but that torsional motions subsequently isolate triplets

Significance and Impact

The design of strongly coupled yet flexible chromophores demonstrates a new paradigm for producing useful high-energy, triplet excitons in molecular architectures. They have potential as components in photovoltaics or photocatalysis schemes that previously were plagued by significant heat loss, poor efficiency of triplet separation, or short-lived triplet excitons.

Microsecond Charge Separation at Heterojunctions

We have demonstrated heterojunctions between transition metal dichalcogenides  and single-walled carbon nanotube with exceptionally long, microsecond timescale, charge separation following sub-picosecond interfacial charge transfer. These carrier lifetimes are orders of magnitude longer-lived than in other monolayer transition metal dichalcogenide heterojunctions.

Research Details

  • Monolayer molybdenum disulfide (MoS2) grown by chemical vapor deposition
  • Highly enriched (6,5) semiconducting single-walled carbon nanotubes
  • Ultra-fast time-resolved spectroscopy

Significance and Impact

Long-lived separated charge carriers are a prerequisite for efficiently converting photon energy to electricity or fuels in solar energy harvesting devices. With this research, we have counteracted ultrafast excited state decay in transition metal dichalcogenide monolayers by demonstrating that heterojunctions between molybdenum disulfide and single-walled carbon nanotubes enable remarkably long carrier lifetimes in the microsecond time range.

Contact

Jeffrey Blackburn

Group Research Manager

Jeffrey.Blackburn@nrel.gov
303-384-6649