8 Open-Circuit Voltage in Organic Solar Cells
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required to be dissociated into free charges within its lifetime, and in organic photovoltaics, the heterojunction of the D/A is used to promote this dissociation. In organic
semiconductors, the exciton moves around by the F ¨
orster energy transfer process,
with a typical exciton diffusion length of approximately 5–10 nm [12]. Thus, the size
of the donor and acceptor domains should be controlled at the ~10 nm scale, and
otherwise, the exciton will relax to the ground state without being separated from
the free charges before it reaches the D/A interface.
8.2.1.2 Charge Separation
The exciton may be formed in either the donor or the acceptor material. In the donor,
the exciton is then quenched by electron transfer from donor to acceptor, and in the
acceptor, it is quenched by hole transfer from acceptor to donor. The charge transfer
does not necessarily generate free charges, because the coulombic attraction force
between the charge pairs is large and is effective over a long range, owing to the
low dielectric constants of organic semiconductors. It has been reported that the estimated Coulomb capture length is approximately 4 nm [10]. Thus, the charge transfer
(CT) state, which is defined as an electron-hole pair stabilized by the Coulomb
attraction force at the D/A interface, is potentially formed before complete charge
separation [13]. This CT state could decay to the ground state (geminate recombination), resulting in the loss of the photocurrent. The charge separation process requires
dissociation of the CT state into free charges.
8.2.1.3 Charge Recombination
After charge dissociation, free electrons and holes are transported in the donor and
acceptor domains, respectively. Finally, free charges are collected at the electrodes.
When the free charge pairs meet at the D/A interface, a CT state is formed and can
decay to the ground state (non-geminate, or bimolecular, recombination: Fig. 8.2).
Charge recombination is further classified according to the mechanism of the process.
Bimolecular recombination with light emission is called radiative recombination,
whereas if it occurs with thermal relaxation, it is called non-radiative recombination
[14]. The recombination processes are sometimes explained using the theory of the pn
junction in inorganic SCs. Recombination between free electrons in the conduction
band and holes in the valence band is called band-to-band recombination. On the
other hand, recombination with a trapped charge in the trap state inside the bandgap is
called trap-assisted recombination [15]. The kinetics of these processes are strongly
related to the V OC in OSCs.
197
required to be dissociated into free charges within its lifetime, and in organic photovoltaics, the heterojunction of the D/A is used to promote this dissociation. In organic
semiconductors, the exciton moves around by the F ¨
orster energy transfer process,
with a typical exciton diffusion length of approximately 5–10 nm [12]. Thus, the size
of the donor and acceptor domains should be controlled at the ~10 nm scale, and
otherwise, the exciton will relax to the ground state without being separated from
the free charges before it reaches the D/A interface.
8.2.1.2 Charge Separation
The exciton may be formed in either the donor or the acceptor material. In the donor,
the exciton is then quenched by electron transfer from donor to acceptor, and in the
acceptor, it is quenched by hole transfer from acceptor to donor. The charge transfer
does not necessarily generate free charges, because the coulombic attraction force
between the charge pairs is large and is effective over a long range, owing to the
low dielectric constants of organic semiconductors. It has been reported that the estimated Coulomb capture length is approximately 4 nm [10]. Thus, the charge transfer
(CT) state, which is defined as an electron-hole pair stabilized by the Coulomb
attraction force at the D/A interface, is potentially formed before complete charge
separation [13]. This CT state could decay to the ground state (geminate recombination), resulting in the loss of the photocurrent. The charge separation process requires
dissociation of the CT state into free charges.
8.2.1.3 Charge Recombination
After charge dissociation, free electrons and holes are transported in the donor and
acceptor domains, respectively. Finally, free charges are collected at the electrodes.
When the free charge pairs meet at the D/A interface, a CT state is formed and can
decay to the ground state (non-geminate, or bimolecular, recombination: Fig. 8.2).
Charge recombination is further classified according to the mechanism of the process.
Bimolecular recombination with light emission is called radiative recombination,
whereas if it occurs with thermal relaxation, it is called non-radiative recombination
[14]. The recombination processes are sometimes explained using the theory of the pn
junction in inorganic SCs. Recombination between free electrons in the conduction
band and holes in the valence band is called band-to-band recombination. On the
other hand, recombination with a trapped charge in the trap state inside the bandgap is
called trap-assisted recombination [15]. The kinetics of these processes are strongly
related to the V OC in OSCs.