262
M. Hiramoto
10.3 Recombination Suppression
Recombination suppression has been a priority issue for inorganic solar cells since the
recombination is the main leakage of photocurrent. On the other hand, for organic
solar cells, the recombination has not attracted considerable attention due to the
small magnitude of photocurrent. However, considering that in the past decade, the
photocurrent density of organic solar cells exceeded 20 and reached 25 mAcm
−2
[12], researchers have started to focus on the recombination process in relation to
the V oc -loss (Chapter 8). Radiative recombination determines the thermodynamic
theoretical limitation of V oc (SQ-limit) (Fig. 10.7). V oc -loss of inorganic solar cells
such as GaAs is negligible, and V oc value (triangular-shaped point) has already
almost reached the SQ-limit. However, V oc -losses of organic solar cells reported so
far are more than 0.5 V, while V oc is far below SQ-limit (crosses). Therefore, nonradiative recombination causing V oc -loss for organic solar cells should be suppressed.
Presently, non-radiative recombination suppression is a cutting-edge issue for organic
solar cells.
10.3.1 Non-radiative Recombination Dissipated to Molecular
Vibration
Non-radiative recombination, which dissipates excited energy to the thermal molecular vibrational energy, is an essential leakage process in the organic solar cells.
Conventionally, the carrier generation process of organic semiconductors showing the
hopping conduction is represented by the localized model (Fig. 10.8a). In this case,
since there is a large Stokes shift (Fig. 10.8c) between the ground state and charge
transfer (CT) state due to the considerable reorganization energy, strong vibrational
coupling induces the non-radiative recombination (geminate recombination) [13].
Fig. 10.7 Dependence of
open-circuit photovoltage
(Voc) on the CT state energy
(E CT ). The black solid line is
the theoretical (SQ) limit of
Voc. The dashed black line
shows Voc = E CT /e − 0.5 V.
The crosses are Voc for all
types of organic solar cells
reported so far. A closed
triangle is Voc for GaAs
CT state energy (E CT ) / eV
V
oc / V
GaAs
V oc of organic solar cells
reported so far
M. Hiramoto
10.3 Recombination Suppression
Recombination suppression has been a priority issue for inorganic solar cells since the
recombination is the main leakage of photocurrent. On the other hand, for organic
solar cells, the recombination has not attracted considerable attention due to the
small magnitude of photocurrent. However, considering that in the past decade, the
photocurrent density of organic solar cells exceeded 20 and reached 25 mAcm
−2
[12], researchers have started to focus on the recombination process in relation to
the V oc -loss (Chapter 8). Radiative recombination determines the thermodynamic
theoretical limitation of V oc (SQ-limit) (Fig. 10.7). V oc -loss of inorganic solar cells
such as GaAs is negligible, and V oc value (triangular-shaped point) has already
almost reached the SQ-limit. However, V oc -losses of organic solar cells reported so
far are more than 0.5 V, while V oc is far below SQ-limit (crosses). Therefore, nonradiative recombination causing V oc -loss for organic solar cells should be suppressed.
Presently, non-radiative recombination suppression is a cutting-edge issue for organic
solar cells.
10.3.1 Non-radiative Recombination Dissipated to Molecular
Vibration
Non-radiative recombination, which dissipates excited energy to the thermal molecular vibrational energy, is an essential leakage process in the organic solar cells.
Conventionally, the carrier generation process of organic semiconductors showing the
hopping conduction is represented by the localized model (Fig. 10.8a). In this case,
since there is a large Stokes shift (Fig. 10.8c) between the ground state and charge
transfer (CT) state due to the considerable reorganization energy, strong vibrational
coupling induces the non-radiative recombination (geminate recombination) [13].
Fig. 10.7 Dependence of
open-circuit photovoltage
(Voc) on the CT state energy
(E CT ). The black solid line is
the theoretical (SQ) limit of
Voc. The dashed black line
shows Voc = E CT /e − 0.5 V.
The crosses are Voc for all
types of organic solar cells
reported so far. A closed
triangle is Voc for GaAs
CT state energy (E CT ) / eV
V
oc / V
GaAs
V oc of organic solar cells
reported so far