8 Open-Circuit Voltage in Organic Solar Cells
213
small LUMO-LUMO offset between the donor and the acceptor. V r and V nr were
also quantified using Eq. 8.9 and are shown in the bar graph (Fig. 8.10d). The device
based on 4SubPDI showed a V nr value of 0.28 V, significantly smaller than that of
RefPDI (0.38 V), while both devices exhibited similar V r values. The reason for the
suppression of V nr was the enhancement of radiative efficiency from the CT state.
This was realized by an intensity borrowing mechanism between CT state emission
and S 1 emission by hybridizing these states [42] by getting E CT close to E
opt
g as a
result of the bay-functionalization, which increases the LUMO of PDI.
8.4 Conclusion and Future Prospects
In this chapter, we began by describing the theoretical background of V OC in OSCs,
including the equivalent circuit model and a detailed balance of thermodynamics in
the devices. The important outcome of these theories is that they reveal the factors
determining V OC in OSCs, namely the energetics, types, and rates of charge recombination, both radiative and non-radiative. These processes occur at the D/A interface
and are thus closely related to each other. Our recent study on the relationship between
the D/A interfacial structure and V OC , and how V OC in OSCs can be increased by
modifying the D/A interface, was then presented. A brief summary of our results is as
follows. First, energy-level alignment is very important to determine the energetics at
the D/A interface and the value of V OC . V OC could change even though the same D/A
materials are used, because of the vacuum-level shift near the D/A interface induced
by E F alignment. E F can be controlled by doping, resulting in the control of V OC and
successful realization of pn homojunction OSCs. In particular, doping near the D/A
interface (within less than 10 nm) has a dominant effect on the device performance.
In the Sect. 8.3.4, the energy structure difference of the monolayer at the D/A interface was investigated. The research revealed that a monolayer cascade can weaken
the Coulomb binding energy at the interface; in this way, both reduction in E CT and
thermal-activation-free charge generation are realized. Thus, a monolayer cascade is
an ideal structure for achieving high V OC and FF. The suppression of charge recombination was presented in the Sects. 8.3.5 and 8.3.6. A model OSC device with high
mobility and highly crystalline donor and acceptor materials was demonstrated and
was able to reduce the V OC loss to less than 0.3 V and attain an FF of almost 0.8.
The results revealed that high crystallinity in several molecular layers near the D/A
interface is important for reducing the energy loss in the output voltage and realizing
ideal band-to-band recombination. In the Sect. 8.3.6, the use of bay-functionalization
of PDI for lifting the LUMO of the acceptor molecule was described. The energylevel control available leads to an E CT close to E
opt
g , leading to a reduction in energy
loss for charge separation, and to the suppression of non-radiative recombination as
a result of hybridization of the CT and S 1 states.
These results reveal that how we can maximize V OC in OSCs. First, the CT state
should be destabilized by weakening the Coulomb binding energy at the D/A interface. Second, E CT should be close to the E
opt
g of the smaller D/A bandgap material,
213
small LUMO-LUMO offset between the donor and the acceptor. V r and V nr were
also quantified using Eq. 8.9 and are shown in the bar graph (Fig. 8.10d). The device
based on 4SubPDI showed a V nr value of 0.28 V, significantly smaller than that of
RefPDI (0.38 V), while both devices exhibited similar V r values. The reason for the
suppression of V nr was the enhancement of radiative efficiency from the CT state.
This was realized by an intensity borrowing mechanism between CT state emission
and S 1 emission by hybridizing these states [42] by getting E CT close to E
opt
g as a
result of the bay-functionalization, which increases the LUMO of PDI.
8.4 Conclusion and Future Prospects
In this chapter, we began by describing the theoretical background of V OC in OSCs,
including the equivalent circuit model and a detailed balance of thermodynamics in
the devices. The important outcome of these theories is that they reveal the factors
determining V OC in OSCs, namely the energetics, types, and rates of charge recombination, both radiative and non-radiative. These processes occur at the D/A interface
and are thus closely related to each other. Our recent study on the relationship between
the D/A interfacial structure and V OC , and how V OC in OSCs can be increased by
modifying the D/A interface, was then presented. A brief summary of our results is as
follows. First, energy-level alignment is very important to determine the energetics at
the D/A interface and the value of V OC . V OC could change even though the same D/A
materials are used, because of the vacuum-level shift near the D/A interface induced
by E F alignment. E F can be controlled by doping, resulting in the control of V OC and
successful realization of pn homojunction OSCs. In particular, doping near the D/A
interface (within less than 10 nm) has a dominant effect on the device performance.
In the Sect. 8.3.4, the energy structure difference of the monolayer at the D/A interface was investigated. The research revealed that a monolayer cascade can weaken
the Coulomb binding energy at the interface; in this way, both reduction in E CT and
thermal-activation-free charge generation are realized. Thus, a monolayer cascade is
an ideal structure for achieving high V OC and FF. The suppression of charge recombination was presented in the Sects. 8.3.5 and 8.3.6. A model OSC device with high
mobility and highly crystalline donor and acceptor materials was demonstrated and
was able to reduce the V OC loss to less than 0.3 V and attain an FF of almost 0.8.
The results revealed that high crystallinity in several molecular layers near the D/A
interface is important for reducing the energy loss in the output voltage and realizing
ideal band-to-band recombination. In the Sect. 8.3.6, the use of bay-functionalization
of PDI for lifting the LUMO of the acceptor molecule was described. The energylevel control available leads to an E CT close to E
opt
g , leading to a reduction in energy
loss for charge separation, and to the suppression of non-radiative recombination as
a result of hybridization of the CT and S 1 states.
These results reveal that how we can maximize V OC in OSCs. First, the CT state
should be destabilized by weakening the Coulomb binding energy at the D/A interface. Second, E CT should be close to the E
opt
g of the smaller D/A bandgap material,