Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 34

are shown in Scheme 2.13 .PC 60 BM was crystalline dark-brown powder, and
possesses good solubility in common organic solvents such as chloroform, toluene,
and o-dichlorobenzene [ 65 ].
Absorption spectra of PC60BM and PC70BM are shown in Fig. 2.3 . It can be
seen that both the two materials show strong absorption at ultraviolet region, from
200 to 400 nm, but PC70BM shows stronger absorption in visible region com-
pared to PC60BM. Since, OPV devices using PC70BM as acceptor will harvest
more sunlight, many OPVs using PC70BM as acceptor show bigger J sc and hence
better PCEs than that of PC60BM-based devices.
The electrochemical properties and energy level of the fullerene derivatives is
very important for PSCs. The open-circuit voltage (V oc ) of PSCs is determined by
the difference between the LUMO energy level of the fullerene acceptors and the
HOMO energy level of the polymer donors [ 66 , 67 ]. Therefore, the LUMO energy
level of the fullerene derivatives is a key parameter for the application of an
acceptor to match with a polymer donor. LUMO level of electron donors or
acceptors can be measured by cyclic voltammogram (CV) method. LUMO level of
unsubstituted C60 and PCBM were *-4.2 and -4.0 eV, respectively [ 68 , 69 ].
Based on the difference of the LUMO levels of those two compounds, it is easy to
conclude that LUMO level of C60 can be elevated by adding the substituent. As
discussed in the above section, higher LUMO level of electron acceptor materials
would be helpful to get higher V oc , and for the purpose to get higher LUMO level,
the bisadducts and multiadducts of fullerene were used in OPVs. For example, the
LUMO level of bis-PCBM was 0.1-0.15 eV higher than that of PCBM, and when
bis-PCBM was used as electron acceptor in P3HT-based OPV device, a V oc of
0.72 V was recorded, which was 0.12 V higher than the PCBM/P3HT-based
devices [ 70 ]. Furthermore, multiadducts can also be used in OPVs, and higher V oc
of the device can be realized. However, since the substituent of PCBM is inert for
electron transport and the symmetric property of fullerene can be weakened,
electron transport properties of the bis- or multiadducts were not as good as that of
PCBM. Therefore, PC60BM and PC70BM are still among the best electron
acceptors in OPVs. The photovoltaic properties of the fullerene derivatives were
studied by fabricating the PSCs based on P3HT as donor and the fullerene
derivatives

as acceptor, and the results are listed in Table 2.5 [ 70 - 72 ]

Scheme 2.14 .

Scheme 2.13 Molecular
structures of PC 60 BM and
PC 70 BM