Charge Separation Mechanisms in Organic Photovoltaic Systems

The conversion of light into electric current in organic photovoltaics relies on a sequence of photoexcitation, exciton diffusion, interfacial charge transfer and separation, and charge collection. In bulk heterojunction architectures donor and acceptor materials form an interpenetrating network that enhances exciton dissociation at heterojunctions. Upon photon absorption, tightly bound excitons migrate to interfaces where an energetic offset drives the transfer of an electron or hole to generate charge-transfer states. The efficiency of this process depends on the interplay of energetic driving forces, electronic coupling, molecular ordering and dynamic processes such as coherence and delocalisation. Recent advances have revealed that sub-100 femtosecond formation of charge-separated species can be mediated by vibronic quantum coherence, that thermalised charge-transfer states can dissociate thermally without large driving energies, and that spatial delocalisation of carriers can minimise Coulombic binding. Control of energetic offsets at the donor–acceptor interface and the reduction of energetic disorder have been shown to diminish open-circuit voltage losses while preserving high internal quantum efficiency. Both polymer–fullerene and nonfullerene acceptor systems now exhibit power conversion efficiencies exceeding 18 per cent, underlining the practical impact of optimised charge separation pathways. Detailed spectroscopic and theoretical studies continue to uncover the balance between coherent and incoherent processes that governs free-charge generation and inform design rules for next-generation materials.

Research from Nature Portfolio

Studies have demonstrated that ultrafast vibronic coherence between excitons and polaron-pair states can drive charge separation in conjugated polymers, with quantum beats persisting for hundreds of femtoseconds and accelerating free-charge formation. Investigations of low-driving-energy heterojunctions have revealed that electronic order and low energetic disorder enable free-charge separation to outcompete trap-assisted recombination despite minimal energetic offsets. Further work on bulk heterojunction blends has measured the activation energy for photocurrent generation and shown that thermalised charge-transfer states at room temperature possess sufficient thermal energy to dissociate into free carriers without reliance on hot-exciton pathways.

Charge Separation Mechanisms in Organic Photovoltaic Systems publication trend

The graph below shows the total number of articles in charge separation mechanisms in organic photovoltaic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Exciton: A bound electron–hole pair created upon photon absorption in an organic semiconductor.

Charge-transfer (CT) state: An interfacial excited state in which an electron is partially transferred from donor to acceptor.

Bulk heterojunction: A nanoscale blend of donor and acceptor materials forming interpenetrating domains to facilitate exciton dissociation.

Polaron pair: A weakly bound electron–hole pair in which each charge carrier is stabilised by local molecular deformation.

Vibronic coherence: The quantum coupling of electronic and vibrational motions that can facilitate ultrafast charge separation.

Energetic offset: The difference in energy levels between donor and acceptor that drives charge transfer.

References

1. Understanding the correlation between energy‐state mismatching and open‐circuit voltage loss in bulk heterojunction solar cells. Carbon Energy (2024).
2. Activationless Charge Transfer Drives Photocurrent Generation in Organic Photovoltaic Blends Independent of Energetic Offset. Journal of the American Chemical Society (2024).
3. Ultrafast Coherent Hole Injection at the Interface between CuSCN and Polymer PM6 Using Femtosecond Mid-Infrared Spectroscopy. ACS Applied Materials & Interfaces (2024).
4. Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss. Nature Communications (2018).
5. Tracking the coherent generation of polaron pairs in conjugated polymers. Nature Communications (2016).
6. Role of coherence and delocalization in photo-induced electron transfer at organic interfaces. Scientific Reports (2016).
7. Probing the pathways of free charge generation in organic bulk heterojunction solar cells. Nature Communications (2018).