Trends in Chemistry

Trends in Chemistry

Volume 1, Issue 9, December 2019, Pages 815-829
Journal home page for Trends in Chemistry

Review
Engineering Charge-Transfer States for Efficient, Low-Energy-Loss Organic Photovoltaics

https://doi.org/10.1016/j.trechm.2019.08.001Get rights and content

Highlights

  • Mechanistic concepts related to charge transfer and dissociation at organic heterojunctions are provided.

  • The energetic landscape and dynamics of the charge-transfer state can be adjusted by the molecular structure and film morphology of bulk heterojunctions, enabling efficient photocurrent generation.

  • Theoretical studies based on Marcus theory, the energy-gap law, etc. quantify the relationship between charge-transfer state properties and open-circuit voltage losses.

Charge transfer (CT) between donors and acceptors following photoexcitation of organic photovoltaics (OPVs) gives rise to bound electron–hole pairs across the donor–acceptor interface, known as CT states. While these states are essential to charge separation, they are also a source of energy loss. As a result of reduced overlap between electron and hole wavefunctions, CT states are influenced by details of the film morphology and molecular structure. Here, we describe several important strategies for tuning the energy level and dynamics of the CT state and approaches that can enhance their dissociation efficiency into free charges. Furthermore, we provide an overview of recent physical insights into the key parameters that significantly reduce the Frenkel-to-CT energy offset and recombination energy losses while preserving a high charge-generation yield. Our analysis leads to critical morphological and molecular design strategies for achieving efficient, low-energy-loss OPVs.

Section snippets

The Role of CT States at Organic Heterojunctions (HJs)

Owing to the weak intermolecular coupling and low dielectric constants of organic semiconductors, photon absorption generates tightly bound electron–hole pairs (i.e., excitons) with binding energies of EB= 0.2 – 1.5 eV 1, 2, 3, 4. In OPVs and photodetectors, type II (staggered) HJs comprising electron donor and acceptor materials are usually employed to promote efficient exciton dissociation. As illustrated in Figure 1A, excitons generated in the donor diffuse to the donor–acceptor (D-A) HJ

Engineering CT States at D-A HJs

Table 1 summarizes the principal approaches to engineer the energetics and dynamics of CT states. The CT exciton energy is given by:

ECT=IED−EAA+ECTB,where IED is the ionization energy [or highest occupied molecular orbital (HOMO) energy] of the donor, EAA is the electron affinity [or lowest unoccupied molecular orbital (LUMO) energy] of the acceptor, and ECTB < 0 is the Coulomb binding energy of the CT state. All CT-state engineering strategies therefore focus on tuning either the molecular

Dependence of OPV Performance on the CT State

In this section, we describe physical insights underpinning the interdependence of OPV performance on CT-state properties with the goal of providing morphological and molecular design guidelines that can promote efficient and low-energy-loss photocurrent generation.

Concluding Remarks

We have outlined relationships between OPV performance, CT-state properties, and various morphological and molecular configurations. An overview of physical insights on CT-state dissociation mechanisms, CT energy loss, and CT exciton recombination losses leads to design strategies for efficient and low energy loss OPVs (Table 1).

  • •

    Introducing molecular interface dipoles, steric control of donor–acceptor molecular distance, and CT-state delocalization in local molecular aggregation can enhance the

Acknowledgments

We gratefully thank Professor Noel C. Giebink and YunHui L. Lin for helpful discussions. B.P.R. acknowledges support by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0012458. S.R.F. acknowledges support by the Office of Naval Research, Contract No. N00014-17-1-2211 and the US Department of Energy Solar Energy Technologies Office Award No. DE-EE0006708.

Glossary

Born–Oppenheimer approximation
the motions of electrons and nuclei are treated separately in the system, since the electrons move on a short timescale whereas the nuclei do not respond.
Charge-transfer (CT) recombination
the recombination process of charge carriers that occurs via the CT state across the donor–acceptor HJ.
Dark saturation current
a residual current due to thermally produced carriers under reverse bias.
Fermi’s golden rule
the expression that relates the transition rate between two

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