Organic and Molecular Semiconductors
NREL aims to acquire a fundamental understanding of interfacial photoinduced electron transfer processes for semiconductors at molecular, nanoscale, and organic interfaces.
Specific organic systems that are examined include conjugated polymers, single-walled carbon nanotubes, molecular films, and organic colloidal nanoparticles.
Ultrafast Charge Transfer Cascade in a Mixed-Dimensionality Nanoscale Trilayer
We demonstrate a mixed-dimensionality (2D/1D/2D) trilayer of quantum-confined semiconductors, that enables ultrafast photoinduced exciton dissociation, followed by charge diffusion and slow recombination. The trilayer doubles charge carrier yield, relative to a 2D/1D bilayer, and enables the separated charges to overcome inter-layer exciton binding energies that limit the creation of unbound separated charges.
Research Details
- Highly enriched semiconducting single-walled carbon nanotubes (SWCNTs) and chemical vapor deposition-grown monolayer transition metal dichalcogenides (TMDCs)
- Detailed steady-state and time-resolved spectroscopy study
- Charge transfer times in the femtosecond to picosecond range with charge recombination lifetime exceeding 1 microsecond
Significance and Impact
The tunable electronic and optical properties of 2D TMDCs and 1D semiconducting SWCNTs make them good quantum confined model systems for fundamental studies on charge and exciton transfer across heterointerfaces. In analogy to the multistep charge transfer cascade found in the photosynthesis reaction center, these multi-component low-dimensional heterostructures open up new opportunities for directing charge flow and lengthening charge separation lifetimes.
Past Research Highlights
Reconciling the Driving Force and the Barrier to Charge Separation in Donor-Nonfullerene Acceptor Films, ACS Energy Letters (2021)
We discovered microwave conductivity measurements facilitate a candid assessment of how driving force calculations are performed. They also highlight how underestimated driving force values lead to precarious interpretations of charge separation dynamics in donor-nonfullerene acceptor films relevant to record-setting organic photovoltaics (OPVs).
Research Details
- Solution phase electrochemistry enabled quantitative driving force calculations for all-small-molecule films.
- Time-resolved microwave conductivity measurements observed dramatic dependence of charge yields on driving for sensitized and blended films.
- Photoluminescence quenching and lifetime measurements corroborated microwave data.
Significance and Impact
The research addresses the pitfalls for popular driving force calculation methods that obscure the subtle dependence of charge separation efficiency on driving force. In our call for standardized methods, we contributed to a unifying movement for OPVs at a time where coherent progress in devices and fundamental understanding is crucial.
Partner
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
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.
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.
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