Electronic Structure and Spectroscopy of Organic Semiconductors

Organic semiconductors consist of π‐conjugated molecular systems in which electrons delocalise across extended molecular backbones. Their electronic structure is defined by frontier molecular orbitals—the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)—the energy separation of which establishes the bandgap. Photoexcitation generates bound electron–hole pairs (excitons) whose binding energy often exceeds thermal energy, influencing charge‐carrier generation and recombination. Charge transport occurs via thermally activated hopping of polarons or through transient band‐like states, while polarisation and dielectric screening modulate energy‐level alignment at interfaces. Spectroscopic techniques such as photoelectron and inverse photoemission spectroscopy, optical absorption and photoluminescence, infrared vibrational probes and impedance measurements yield insights into density of states distributions, exciton dynamics and energy‐level offsets. Recent advances in device‐integrated methods permit in situ probing of operating films and interfaces, bridging fundamental understanding with practical applications in light‐emitting diodes, solar cells and transistors.

Research from Nature Portfolio

Recent studies have demonstrated a novel three‐terminal hot-electron transistor for direct measurement of molecular semiconductor energy gaps under operational bias, extracting both electron and hole injection barriers from current–voltage characteristics across materials such as PBDB-T-2Cl, C₆₀, PTCDA and Alq₃. Foundational work has shown that tuning the injection energy in a hot-electron molecular device enables access to different electron‐transfer regimes, revealing negative differential resistance arising from the Marcus inverted region. Earlier developments introduced a three‐terminal architecture to deconvolute interface and bulk contributions to energy‐level alignment at metal–molecule junctions, thereby reconciling spectroscopic data with real‐device performance.

Electronic Structure and Spectroscopy of Organic Semiconductors publication trend

The graph below shows the total number of articles in electronic structure and spectroscopy of organic semiconductors across all publications each year (not limited to Nature Index journals).

Technical terms

HOMO (Highest Occupied Molecular Orbital): The uppermost filled electronic state in a molecule.

LUMO (Lowest Unoccupied Molecular Orbital): The lowest energy vacant electronic state in a molecule.

Bandgap: The energy difference between HOMO and LUMO, determining optical absorption onset.

Exciton Binding Energy: The energy required to separate a bound electron–hole pair.

Density of States (DOS): The distribution of accessible electronic states as a function of energy.

Hot-electron transistor: A three-terminal device injecting high-energy electrons to probe electronic levels under operating conditions.

Marcus Inverted Region: A regime in electron-transfer theory where increasing driving force reduces transfer rate.

Energy-Resolved Electrochemical Impedance Spectroscopy: A technique to infer DOS by measuring impedance response as a function of applied potential.

References

1. Determination of transport levels of organic semiconductors by UPS and IPS. New Journal of Physics (2008).
2. Determination of energy level alignment at metal/molecule interfaces by in-device electrical spectroscopy. Nature Communications (2014).
3. Tuning the charge flow between Marcus regimes in an organic thin-film device. Nature Communications (2019).
4. Intrinsic Photoconductivity Spectral Dependence as a Tool for Prediction of Open-Circuit Voltage in Organic Solar Cells. Energies (2023).
5. Exciton Binding Energy of Non‐Fullerene Electron Acceptors. Advanced Energy and Sustainability Research (2022).
6. Mapping the Density of States Distribution of Organic Semiconductors by Employing Energy Resolved–Electrochemical Impedance Spectroscopy. Advanced Functional Materials (2020).