Nanocarbons as Electron Donors and Acceptors in Photoinduced Electron-Transfer Reactions

Nanocarbons composed of hybridized sp 2 carbons such as fullerenes, carbon nanotubes (CNTs), carbon nanohorns (CNHs) and graphene have been successfully utilized as electron acceptors in electron donor-acceptor hybrids for photoinduced electron-transfer reactions to mimic well the function of the photosynthetic reaction center. However, sp 2 hybridized nanocarbons can also be used as electron donors when higher fullerenes such as C 76 and C 78 or endohedral metallofullerenes are employed or when nanocarbons are combined with relatively strong electron acceptors. This paper focuses on photoinduced electron-transfer reactions of nanocarbons including fullerenes, CNTs, CNHs and graphene, which act as not only as electron acceptors but also as electron donors with strong electron acceptors such as tetracyanoethylene (TCNE) and tetracyano- p -quinodimethan (TCNQ) to produce the radical cations efﬁciently because of small reorganization energies of electron transfer. Alternatively the oxidizing ability of electron acceptors was enhanced by binding Lewis acids to the radical anions, which enabled to oxidize nanocarbons. The important roles of nanocarbons such as fullerenes, carbon nanotubes and graphenes on the stability of rapidly emerging perovskite based solar cells with high power conversion efﬁciency have also been discussed in this paper.

The extensive research on sp 2 hybridized nanocarbons began with the discovery of the first fullerene molecule (C 60 ), made up of alternating 20 hexagons and 12 pentagons of sp 2 carbon atoms, in 1985 by Kroto and Smalley et al, 1 followed by the discovery of carbon nanotubes (CNTs) by Iijima in 1991. 2 In addition to 1D CNTs, 2D graphene has now become the most studied carbon allotropes since the first experimental discovery of graphene in 2004. 3 These sp 2 hybridized nanocarbons have highly delocalized π-electron systems, resulting in the low-lying end of the π * orbitals, which readily accept electrons with minimal structural change upon the electron-transfer reduction. Fullerenes have held great promise as three-dimensional electron acceptors because of the small reorganization energy resulting from the π-electron systems being delocalized over the threedimensional curved surface together with the rigid and confined structure of the aromatic π sphere. Thus, a number of electron donoracceptor systems using fullerenes as good electron acceptors have been developed extensively, mimicking the function of the photosynthetic reaction centers to undergo efficient charge separation and slow charge recombination upon photoexcitation. [4][5][6][7][8][9][10][11][12][13][14][15] Carbon nanotubes, in particular single-walled carbon nanotubes (SWNTs), have also been used as electron acceptors in electron donor-acceptor hybrids. [16][17][18][19][20][21][22][23][24][25][26] Nanocarbons can also act as electron donors provided that relatively strong electron acceptors are employed in nanocarbon-electron acceptor hybrids. This paper focuses on such electron donor-acceptor nanohybrids in which nanocarbons are used as not only electron acceptors but also as electron donors, expanding the scope of the applications of nanocarbons. The roles of nanocarbons as electron or charge transport materials in organic-inorganic hybrid perovskite based solar cells will also be discussed to achieve high power conversion efficiency and stability.

C 60 as Electron Donor and Acceptor Components
The LUMO of C 60 is triply degenerate to be able to accept up to 6 electrons to produce C 60 6-. [27][28][29] The 6-6 bond lengths in C 60 •are averaged at 1.397(2) A • , 30 which is slightly longer than the averaged 6-6 bond length of C 60 (1.391(8) A • ), 31 whereas the 5-6 bond length * Electrochemical Society Member. z E-mail: fukuzumi@chem.eng.osaka-u.ac.jp in C 60 •are averaged at 1.449(2)A • , 30 which is slightly shorter than the averaged 5-6 bonds of C 60 (1.455(8)A • ). 31 Such a change in the direction of the bond length of C 60 •is in accordance with the theory which predicts that the t 1u LUMO orbital is antibonding for the 6-6 bonds and bonding for the 6-5 bonds. 31 The magnitude of the Jahn-Teller distortion of C 60 •is very small, because the C 60 framework is very stiff and the extra electron is spread over many carbon atoms. 32 Nevertheless, the I h symmetry of C 60 is changed to D 3d in C 60 •-. 32 In contrast to the LUMO, the HOMO of C 60 (h u ) is quintetly degenerate to be able to remove 10 electrons . 27 However, the oxidation of C 60 is more difficult than the reduction to obtain stable C 60 •+ , 33 and only three subsequent one-electron oxidations of C 60 are observed in the presence of Bu 4 N + AsF 6 in dichloromethane at −55 • C to produce C 60 •+ , C 60 2+ and C 60 •3+ at E 1/2 (vs ferrocenium ion/ferrocene = 1.27, 1.71, and 2.14 V, respectively. 34 A C 60 •+ salt was successfully synthesized by using hexachlorocarborane anion (CB 11 H 6 Cl 6 -), which is an exceptionally inert anion with extremely low nucleophilicity. 35 [C 60 •+ ][CB 11 H 6 Cl 6 -] has been characterized by IR, NIR (λ max = 980 nm), and EPR spectroscopies. 35,36 The C 60 •+ salt is also obtained when combined with an appropriately chosen superhalogen radical (acting as an oxidizing agent). 37 C 60 •+ was produced by photoinduced electron transfer from the triplet excited state of C 60 to strong electron acceptors such as tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), exhibiting the absorption maximum at 960 nm. 38

Carbon Nanotubes, Nanohorns and Graphenes as Electron Donor and Acceptor Components
Single-walled carbon nanotubes (SWNTs) have the unusual features, which arise from the distinct nanostructure. SWNTs consist of hexagon networks of carbon atoms, which are rolled up from graphite and graphene to create seamless cylinders with various chiralities (Figure 3). 74 SWNTs can act as both electron donors and acceptors in the electrochemical oxidation and reduction, respectively. 75,76 Diameter-selective dispersion of HiPco SWNTs has been accomplished through noncovalent complexation of the non-destructive nanotubes with a flexible porphyrinic polypeptide bearing 16 porphyrin units (P(H 2 P) 16 ) in DMF at 298 K. 77 Supramolecular formation occurs through wrapping of peptidic backbone in P(H 2 P) 16 and π-π interaction between porphyrins and nanotubes to extract the large-diameter nanotubes (ca. 1.3 nm) as shown in Figure 4. 77 Photoinduced electron transfer from the excited states of P(H 2 P) 16 to SWNTs occurs in the supramolecular complex, in which SWNTs act as an electron acceptor, to produce the charge-separated state with a lifetime of 0.37 ± 0.03 ms. 77 Covalent functionalization of diameter sorted SWCNTs with porphyrins (MP) has also been achieved and the photoexcitation of the MP-SWCNT hybrid results in formation of the charge-separated radical cation and the radical anion as given by [MP •+ -SWCNT •-] with their characteristic absorption bands in the visible and near-IR regions. 78 The photoelectrochemical solar cells were built by using these MP-SWCNT on the modified FTO/SnO 2 electrodes gave best performance for ZnP-SWCNT, which has the longest lifetime of the charge-separated state. 78 SWNTs have also been successfully de-bundled and solubilized in N,N-dimethylformamide by wrapping with coenzyme Q 10 (CoQ 10 ) through the hydrophobic interaction between SWNT surface and isoprenoid side chain ( Figure 5). 79 The resulting donor-acceptor nanohybrid was characterized by UV-Vis-NIR spectroscopy and highresolution transmission electron microscopy. The photoexcitation of SWNT-CoQ 10 solution results in the efficient photoinduced electron transfer from SWNT to CoQ 10 in the supramolecular complex, in which SWNTs act as an electron donor, to produce the chargeseparated state (SWNT •+ and CoQ 10 •-). 79 SWNT also acts as an electron donor in photoinduced electron transfer from SWNT to the triplet excited state of azaxanthylium units, which are covalently bound to SWNT through 2-(ethylthio)ethylamido linkers. 80 Water-soluble electron-donor SWNT hybrids was synthesized based on the noncovalent immobilization of quaternized pyridyloxy zinc phthalocyanines (ZnPc) with a varying number of pyridyl substituents. 81 The synthesized SWCNT/ZnPc   hybrids have been used for energy conversion in photovoltaic devices, exhibiting the incident photon-to-current efficiency (IPCE) spectrum, which resembles the absorption features of ZnPc to show the maxima of 0.6% and 0.25% at the Soret-and Q-band wavelengths, respectively. 81 Double walled carbon nanotubes (DWCNTs) also act as an electron donor in the hybrids with different substituted perylenediimides (PDIs), which are attached selectively to the outer walls of DWCNTs to leave the inner walls intact and the fluorescence of PDIs in the hybrids was quenched by electron transfer from DWCNTs to the singlet excited state of PDIs. 82 Carbon nanohorns (CNHs) consist of pseudo-cylindrical singlewall tubules with a conical tip are closely related to CNTs. 83 A nanohybrid composed of two allotropic forms of carbon, i.e., carbon nanohorns (CNH) and C 60 , has been synthesized from a C 60 derivative bearing a benzocrown ether subunit (crown-C 60 ) and a CNH functionalized with NH 3 + groups (CNH-sp-NH 3 + F -) through ammonium-crown ether interactions ( Figure 6). 84 Photoinduced electron transfer from the singlet excited state of the C 60 moiety to the CNH occurs with a rate constant of 6.5 × 10 10 s -1 to produce the chargeseparated state, which decays with a lifetime of 1.0 ns via back elec- tron transfer to produce the triplet excited state of the CNH-sp-NH 3 + moiety. 84 CNHs were covalently coupled with oligo(thienylenevinylenes) (nTVs) to obtain CNH-nTV conjugates, in which CNHs also act as an electron acceptor in photoinduced electron transfer from nTVs to the CNHs to produce the charge-separated state with a long lifetime of 10 μs due to the efficient electron migration in CNHs. 85 Graphene, a recent addition to carbon allotropes, has also attracted increasing attention due to its many intrinsic useful properties. 86 Graphene oxide (GO), in which oxygen atoms are bound with the carbon scaffold has been used as an electron donor in an all-carbon donor-acceptor hybrid by combining GO and C 60 . 87 Laser flash photolysis measurements revealed the occurrence of photoinduced electron transfer from the GO electron donor to the C 60 electron acceptor in the conjugate. 87 Fast photoinduced charge separation was also reported in well-ordered self-assemblies of perylenediimide-graphene oxide (TAIPDI-GO) hybrid layers in aqueous environments. 88 The hole (i.e. radical cation) of GO formed after the photoinduced charge separation migrates rapidly on the smooth π-surface of a graphene sheet, resulting in slower charge recombination. 88 GO also acts as an electron donor in an electron donor-acceptor composite with an ionic endohedral metallofullerene (Li + @C 60 ) formed in neat water. 89 Photoexcitation of the GO-Li + @C 60 composite results in electron transfer from GO to the triplet excited state of Li + @C 60 , leading to photocurrent generation at an optically transparent electrode (OTE) of nanostructured SnO 2 (OTE/SnO 2 ) electrode. 89 Fullerene molecules were grafted to GO using hydrolyzed PCBM as a fullerene precursor. 90 In this case as well, GO acts as an electron donor and covalently attached fullerene molecules act as an electron acceptor for ultrafast electron transfer from GO to the singlet excited state of the fullerene to produce the charge-separated state. 90 Supramolecular donor-acceptor hybrids composed of few-layer GO as an electron acceptor rather than an electron acceptor and phthalocyanine or porphyrin bearing four pyrene entities as photosensitizer donors have been synthesized, undergoing ultrafast charge separation with the rate constant of the order of 10 12 s -1 . 91 Exfoliated GO-phthalocyanine (Pc) nanohybrids were also reported, undergoing photoinduced electron transfer from GO to the electron accepting Pcs with the lifetimes of <1 ps for the charge separation and 330 ± 50 ps for the charge recombination. 92

Carbon Nanomaterials as Hole and Electron Transport Materials in Perovskite Solar Cells
The rapidly increasing power conversion efficiency (PCE) of organic-inorganic metal halide perovskite solar cells (PKSCs) has attracted increasing attention of both the academic and industrial research communities, [93][94][95][96] because the PKSC cell has now achieved an efficiency of 22.1%, exceeding the PCEs of multicrystalline and thin-film silicon. 97 The remaining issue is the stability because the material is soluble in aprotic polar solvents, degrading quickly in the presence of moisture and under light illumination. [98][99][100][101][102] The perovskite layer in PKSCs not only acts as a light harvester but also a hole-transporting material (HTM) or an electron transporting material (ETM) at the same time. 103 However, up to now, the PCE of HTM or ETM free PKSCs is lower than that of their parent ones M3058 ECS Journal of Solid State Science and Technology, 6 (6) M3055-M3061 (2017) Figure 6. Structure of carbon nanohorn-(CNH)-C 60 hybrid made from CNH-sp-NH 3 + Fand crown C 60 .
with HTM or ETM, because the non-selective contact leads to high charge recombination rate, low fill factor (FF) and open-circuit voltage (V OC ). 104 Thus, the exploration of effective HTM and ETM for PKSCs is a particularly important. Because carbon nanomaterials such as fullerenes, carbon nanotubes and graphene act as both electron donors and acceptors, they are good candidates for HTM and ETM for PKSCs. Fullerene electron-transport layers like C 60 and PC 61 BM in PKSCs have been shown to improve the efficiency and reduce hysteresis, recombination losses and surface pin holes. [105][106][107][108][109][110][111][112] On the other had, a high-conductivity random network-type SWCNT film acts as the effective HTM-counter electrode (CE) of a PKSC with and without additional HTM. [113][114][115][116][117] The introduction of SWCNTs into a carbon CE for mesoscopic structured PKSCs enhanced the hole collection efficiency due to its' 1D excellent conduction with a high hole mobility. 118 Transient absorbance measurements indicate that holes are rapidly and efficiently transferred from the MAPbI 3 valence band to the adjacent SWCNT layer, providing mechanistic information regarding the role of SWCNT layers as efficient hole extraction layers in PKSCs. 119 Incorporation of multi-walled carbon nanotubes (MWCNTs) in the bulk of the active layer of PKSCs has been shown to reduce charge recombination and increase the open circuit voltage. 120 Such reduction in recombination losses due to selective charge transport in the bulk provides good opportunities for further material engineering toward higher PCE.
The incorporation of moderately reduced graphene oxide (rGO) into a hole transport layer resulted in improvement of the power conversion efficiency than that of PKSc without rGO. 121 Graphene and graphene-derived nanomaterials have also been utilized as lightharvesting layers, in new designs of PKSCs. 122

Conclusions
Nanocarbons such as fullerenes, carbon nanotubes (CNTs), carbon nanohorns (CNHs) and graphene have been utilized not only as electron acceptors but also as electron donors in a variety of electron donor-acceptor conjugates, undergoing efficient photoinduced charge separation with slow charge recombination. Such excellent electrontransfer properties have well been applied as electron and hole transport materials in rapidly emerging organic-inorganic metal halide perovskite solar cells (PKSCs). Nanocarbon materials will certainly keep providing scientific and technological excitement for scientists working in various disciplines.