Chapter 7
First-Principles Investigations
of Electronically Excited States
in Organic Semiconductors
Takatoshi Fujita
7.1 Introduction
Understanding the charge photogeneration process is essential for the rational design
of novel materials for organic solar cells. The charge photogeneration process
includes several elementary steps [16]: (i) light absorption by an organic molecule
or polymer and the formation of an electronically excited state, which is frequently
refereed as an exciton state, (ii) the exciton diffusion to a donor/acceptor (D/A) interface, (iii) a charge-transfer reaction at the interface and the subsequent formation of
an interfacial charge-transfer state, comprising an electron in the acceptor material
and a hole in the donor material, and (iv) a charge-separation reaction, which gives
rise to free charge carriers. Although the organic solar cell with the internal quantum
efficiency near 100% has been reported [85], the underlying mechanism enabling efficient charge separation remains unclear [32, 46]. In addition, charge recombination
to the ground state has recently become a focus of research because the non-radiative
charge recombination results in a considerable energy loss in an open-circuit voltage
[9, 122]. A thorough understanding of electronically excited states is necessary for
elucidating the charge photogeneration process. The representative excited states
in the charge photogeneration process are an initial excited-state absorbing photon,
an interfacial CT states, and the charge-separated (CS) states; it is thus essential to
predict those energy levels and characterize the excited states.
An ab initio method solving the electronic Schrödinger equation has become a
powerful tool to explore the electronic states of organic semiconductors [4, 64, 94].
In addition to the energy, the wave function properties of excited states, such as
the electron-hole (e–h) separation and their spatial extent, can be directly obtained
within a given approximation. Because of increases in computational capabilities and
development of program packages [34, 65], an electronic structure calculation for an
T. Fujita (B)
Institute for Molecular Science, 38 Nishigo-Naka, Aichi 444-8585, Japan
e-mail: tfujita@ims.ac.jp
© Springer Nature Singapore Pte Ltd. 2021
M. Hiramoto and S. Izawa (eds.), Organic Solar Cells,
https://doi.org/10.1007/978-981-15-9113-6_7
155
First-Principles Investigations
of Electronically Excited States
in Organic Semiconductors
Takatoshi Fujita
7.1 Introduction
Understanding the charge photogeneration process is essential for the rational design
of novel materials for organic solar cells. The charge photogeneration process
includes several elementary steps [16]: (i) light absorption by an organic molecule
or polymer and the formation of an electronically excited state, which is frequently
refereed as an exciton state, (ii) the exciton diffusion to a donor/acceptor (D/A) interface, (iii) a charge-transfer reaction at the interface and the subsequent formation of
an interfacial charge-transfer state, comprising an electron in the acceptor material
and a hole in the donor material, and (iv) a charge-separation reaction, which gives
rise to free charge carriers. Although the organic solar cell with the internal quantum
efficiency near 100% has been reported [85], the underlying mechanism enabling efficient charge separation remains unclear [32, 46]. In addition, charge recombination
to the ground state has recently become a focus of research because the non-radiative
charge recombination results in a considerable energy loss in an open-circuit voltage
[9, 122]. A thorough understanding of electronically excited states is necessary for
elucidating the charge photogeneration process. The representative excited states
in the charge photogeneration process are an initial excited-state absorbing photon,
an interfacial CT states, and the charge-separated (CS) states; it is thus essential to
predict those energy levels and characterize the excited states.
An ab initio method solving the electronic Schrödinger equation has become a
powerful tool to explore the electronic states of organic semiconductors [4, 64, 94].
In addition to the energy, the wave function properties of excited states, such as
the electron-hole (e–h) separation and their spatial extent, can be directly obtained
within a given approximation. Because of increases in computational capabilities and
development of program packages [34, 65], an electronic structure calculation for an
T. Fujita (B)
Institute for Molecular Science, 38 Nishigo-Naka, Aichi 444-8585, Japan
e-mail: tfujita@ims.ac.jp
© Springer Nature Singapore Pte Ltd. 2021
M. Hiramoto and S. Izawa (eds.), Organic Solar Cells,
https://doi.org/10.1007/978-981-15-9113-6_7
155