6 Charge Carrier Dynamics in Polymer Solar Cells
127
exciton, (2) exciton diffusion into a donor/acceptor interface, (3) charge transfer
at the interface, (4) charge dissociation into free charge carriers, and (5) charge
collection into each electrode. The charge dissociation is in competition with (4)’
geminate (monomolecular) recombination to the ground state. The charge collection
is in competition with (5)’ non-geminate (bimolecular) recombination to the ground
state, as shown in Fig. 6.1. Thus, the external quantum efficiency (EQE) of polymer
solar cells is equal to the product of the efficiency of each elementary process: EQE
= η A × η ED × η CT × η CD × η CC . The internal quantum efficiency (IQE) is given by
IQE = η ED × η CT × η CD × η CC .
These elementary processes in photovoltaic conversion range over a wide temporal
scale as described below. The photon absorption is an electronic transition and hence
occurs on a time scale of femtoseconds (~10
−15 s). The exciton diffusion typically
occurs on a timescale of picoseconds to sub-nanoseconds (10
−12 –10
−10 s) though it
depends on phase-separated structures. The charge transfer has been reported to be
promptly completed in the order of tens of femtoseconds (~10
−14 s). The geminate
recombination typically ranges from picoseconds to nanoseconds (10
−12 –10
−9 s).
The charge collection time typically ranges from sub-microseconds to microseconds
(10
−7 –10
−6 s), though it depends on the charge mobility, the thickness of the active
layer, and the electric field applied to the active layer. In other words, the elementary
processes in photovoltaic conversion range from femtoseconds to microseconds (over
nine orders of magnitude on a temporal scale). This chapter focuses on these rapid
photovoltaic conversion events studied by time-resolved optoelectronic measurements such as transient absorption spectroscopy and discusses the recent findings
obtained from the kinetics analyses.
6.3 Optoelectronic Measurements
6.3.1 Transient Absorption Spectroscopy
Transient absorption spectroscopy is one of the most powerful tools for studying
the dynamics of short-lived transient species such as excitons and charge carriers.
Historically, Norrish and Porter developed the flash photolysis technique around 1950
before the invention of lasers [17, 18]. Their pioneering work enabled us to directly
observe such short-lived transient products and therefore opened a new research field.
As a result, they were awarded the Nobel Prize in Chemistry 1967 with Eigen for their
studies of extremely fast chemical reactions, effected by disturbing the equilibrium by
means of very short pulses of energy [19, 20]. Owing to the emergence of short-pulsed
lasers, this technique has been further improved in temporal resolution. Currently, we
can directly observe ultrafast phenomena with a resolution of femtoseconds. In 1999,
Zeweil was awarded the Nobel Prize in Chemistry for his studies of the transition
states of chemical reactions using femtosecond spectroscopy [21]. These two typical
measurement methods are described below.
127
exciton, (2) exciton diffusion into a donor/acceptor interface, (3) charge transfer
at the interface, (4) charge dissociation into free charge carriers, and (5) charge
collection into each electrode. The charge dissociation is in competition with (4)’
geminate (monomolecular) recombination to the ground state. The charge collection
is in competition with (5)’ non-geminate (bimolecular) recombination to the ground
state, as shown in Fig. 6.1. Thus, the external quantum efficiency (EQE) of polymer
solar cells is equal to the product of the efficiency of each elementary process: EQE
= η A × η ED × η CT × η CD × η CC . The internal quantum efficiency (IQE) is given by
IQE = η ED × η CT × η CD × η CC .
These elementary processes in photovoltaic conversion range over a wide temporal
scale as described below. The photon absorption is an electronic transition and hence
occurs on a time scale of femtoseconds (~10
−15 s). The exciton diffusion typically
occurs on a timescale of picoseconds to sub-nanoseconds (10
−12 –10
−10 s) though it
depends on phase-separated structures. The charge transfer has been reported to be
promptly completed in the order of tens of femtoseconds (~10
−14 s). The geminate
recombination typically ranges from picoseconds to nanoseconds (10
−12 –10
−9 s).
The charge collection time typically ranges from sub-microseconds to microseconds
(10
−7 –10
−6 s), though it depends on the charge mobility, the thickness of the active
layer, and the electric field applied to the active layer. In other words, the elementary
processes in photovoltaic conversion range from femtoseconds to microseconds (over
nine orders of magnitude on a temporal scale). This chapter focuses on these rapid
photovoltaic conversion events studied by time-resolved optoelectronic measurements such as transient absorption spectroscopy and discusses the recent findings
obtained from the kinetics analyses.
6.3 Optoelectronic Measurements
6.3.1 Transient Absorption Spectroscopy
Transient absorption spectroscopy is one of the most powerful tools for studying
the dynamics of short-lived transient species such as excitons and charge carriers.
Historically, Norrish and Porter developed the flash photolysis technique around 1950
before the invention of lasers [17, 18]. Their pioneering work enabled us to directly
observe such short-lived transient products and therefore opened a new research field.
As a result, they were awarded the Nobel Prize in Chemistry 1967 with Eigen for their
studies of extremely fast chemical reactions, effected by disturbing the equilibrium by
means of very short pulses of energy [19, 20]. Owing to the emergence of short-pulsed
lasers, this technique has been further improved in temporal resolution. Currently, we
can directly observe ultrafast phenomena with a resolution of femtoseconds. In 1999,
Zeweil was awarded the Nobel Prize in Chemistry for his studies of the transition
states of chemical reactions using femtosecond spectroscopy [21]. These two typical
measurement methods are described below.