Fullerene and Non-Fullerene Acceptors
In the history of OSCs, fullerene materials played a prevailing role. After 2015, since the appearance of high-performance NFAs such as ITIC and Y6 [53,56], the PCE of OSCs has been significantly improved. Therefore, it is worth discussing both fullerene and NFAs in this chapter.
Fullerene and its derivatives have been widely used as acceptor materials in OSCs because of their LUMO energy levels, excellency as electron acceptors, and high isotropic electron mobility. However, fullerene and its derivatives show significant disadvantages such as high price, weak absorption in the visible region, poor stability (easy dimerization), and difficulty in adjusting energy levels through structural changes. Therefore, in recent years, high-performance non-fullerene materials have become the focus of OSCs. Specifically, the single-junction OSCs based on small molecular NFA Y6 have achieved a breakthrough in PCE of more than 17% [2,57,58].
Fullerene Materials
Ever since the first fullerene (C60) was synthesized in 1985 through laser evaporation of graphite by Richard Smalley, Harry Kroto, and Robert Curl, it has attracted the broad interest of global researchers [59]. Because the molecular structure of C60 is similar to that of football, it is also called “footballene.” In addition, it was inspired by the spherical dome shell structure of the Montreal World Expo in Canada designed by the architect Buckminster Fuller, so C60 is also called “Buckminster fullerene” or “buckyball.”
FIGURE 12.9 Molecular structures of small organic molecules mentioned in Table 12.1. (a) HI 1, (b) IDIC, (c) NCBDT. (d) DRTB-T. (e) IC-C6IDT-IC, (f) ITIC, (g) INIC3, (h) BDT- 2t-ID, (i) Y6. (j) PC71BM. (k) DF-PCIC, (1) DTSi(FBTTh2Cy)2/DTGe(FBTTh2Cy)2.
FIGURE 12.9 (Continued) Molecular structures of small organic molecules mentioned in
Table 12.1. (a) HI 1, (b) IDIC,(c) NCBDT. (d) DRTB-T. (e)IC-C6IDT-IC, (f) ITIC, (g) INIC3,(h) BDT-2t-ID, (i) Y6, (j) PC71BM, (k) DF-PCIC. (1) DTSi(FBTTh2Cy)2/DTGe(FBTTh2Cy)2.
C60 has been widely applied in biomedicine, catalysts, superconducting materials, organic photovoltaic, and other fields. The methods of making C60 include chemical synthesis, arc discharge, and laser heating [60].
Qo is composed of 20 hexagons and 12 pentagons. With a large cavity, Q„ may have some metal particles or small molecules inside to change its properties. Ever since the appearance of C60, more fullerene molecules have been synthesized. Besides C60, C70 (consisting of 25 hexagons), PC6,BM ([6,6]-phenyl-C6rbutyric acid methyl ester), and PC71BM ([6,6]-phenyl-C7rbutyric acid methyl ester) are also widely used in OSCs.
The chemical structures of two representative fullerenes, namely, C60 and its derivative PC61BM, are shown in Figure 12.11.
C60 and C70 are regarded as excellent electron acceptor materials in OSCs due to their conjugated spherical and ellipsoidal structures, respectively. In order to improve their solubility, functionalized C60 and C70 have been synthesized. Side chains like alkoxy group have been added to C60 and C70 balls to obtain the fullerene derivatives PC6,BM and PC71BM, and their solubility is improved. In 1992, Sariciftci et al. discovered that electrons in excited states of the donor could be quickly injected into C60 because the surface of spherical C60 is a large conjugated structure, which allows the electrons to be delocalized very well and stabilized [61,62]. This pioneering work inspired many researchers. Through the continuous efforts of global researchers, the efficiency of OSCs with fullerenes as electron acceptors has exceeded 12% [63,64].
However, fullerenes have the following inherent disadvantages:
First, it is difficult to modify and adjust the band gap and LUMO energy of fullerene. Functional groups (side chains) can be added to C60 and C70 to improve their solubility; however, the energy level of frontier molecular
FIGURE 12.10 Molecular structures of polymers mentioned in Table 12.1. (a) PBDB-T, (b) FTAZ. (c) PBDT-S-2TC, (d) PTB7-Th. (e) PTTbPDI, (f) PTbPDI. (g) N2200. (h) PNDI-2T- TR(x), (i) PM6. and (j) P2F-EHp.
FIGURE 12.10 (Continued) Molecular structures of polymers mentioned in Table 12.1. (a) PBDB-T, (b) FTAZ, (c) PBDT-S-2TC. (d) PTB7-Th, (e) PTTbPDI, (f) PTbPDI, (g) N2200, (h) PNDI-2T-TR(x). (i) PM6. and (j) P2F-EHp.
FIGURE 12.11 The chemical structures of СЫ) and PC61BM.
orbitals and light absorption ability cannot be adjusted through this method. Therefore, the selected donor materials have to match the energy level of fullerene acceptor in order to obtain the high open-circuit voltage. This greatly limits the choice of donor materials. Experiments have shown that the PCE of single-junction photovoltaic devices based on fullerene acceptor is limited [63]. Second, as mentioned above, the absorption of OSCs based on fullerene acceptor is limited in the visible and NIRs. The reason for the weak absorption of fullerene is attributed to the fact that the C60 and C70 are spherical and ellipsoidal structures with high symmetry, respectively. This directly leads to the failure of improving 7SC and PCE. Furthermore, fullerenes are easy to aggregate and dimerize due to their molecular structures, affecting the effective separation of charges and stability of OSCs. Finally, the voltage loss of fullerene-based OSC is generally much higher than that of silicon-based and perovskite solar cells, resulting in low open- circuit voltages.
The shortcomings of fullerenes as electron acceptors listed above have prevented the further improvement of the PCE of OSCs in the last two decades. Under this situation, it is very necessary for researchers to find new NFAs to overcome the problems of fullerene acceptors.
Compared to fullerene acceptors, NFAs have the characteristics such as tunable energy levels and absorption range, better solubility, and lower energy loss [17,29]. To date, a large number of NFA materials have been successfully synthesized and applied, achieving the highest PCE over 17% [3,30,37-39]. Among them, two NFA materials, namely, ITIC and Y6, are particularly noteworthy.
