Enhanced photoelectrochemical performance of composite photovoltaic cells of Li + @ C 60 – sulphonated porphyrin supramolecular nanoclusters †

Photoelectrochemical cells (PECs) have been widely investigated as a next-generation solar cell because of their simple structure. The photoinduced charge separation between the excited state of dye and the electrode plays an important role in improvement of PEC performance. In the natural photosynthetic reaction centre, the efficient photoinduced electron transfer occurs to give a long-lived charge separated (CS) state with high quantum yield. Extensive efforts have so far been devoted to design and synthesize electron donor–acceptor linked molecules to achieve efficient photoinduced charge separation for applications to PECs. However, the synthetic difficulty of the covalently linked donor–acceptor molecule has precluded the development of simple photovoltaic devices using such model compounds of the photosynthetic reaction centre. Among many candidates, porphyrins and fullerenes are a suitable combination for the construction of PECs, because porphyrins have strong visible absorption bands and fullerene exhibits efficient electron-transfer properties such as small reorganization energy due to the delocalized three-dimensional p-system. The supramolecular approaches for PECs are also investigated, however, there was no report of supramolecules with a strong binding between neutral porphyrins and fullerenes. We have recently designed and synthesized simple electron donor–acceptor supramolecular complexes composed of lithium ion encapsulated fullerene (Li@C60) and sulphonated mesotetraphenylporphyrin (MTPPS : M = Zn, H2), which have strong 1 : 1 supramolecular binding due to the cation–anion and p–p interactions (K = B10 M ). Photoexcitation of the supramolecule exhibited extremely slow charge-recombination of the CS state (t = 0.3 ms) in benzonitrile (PhCN). Li@C60 has been reported to act as a more effective electron acceptor than pristine C60. 9 The driving force of photoinduced electron transfer from MTPPS to the triplet excited state of Li@C60 is highly positive ( DGET = 0.98 eV for ZnTPPS and 0.67 eV for H2TPPS in polar PhCN), which is large enough to afford the CS states even under the non-polar environment in nanoclusters. We report herein photovoltaic cells using Li@C60–MTPPS 4 nanoclusters, which are assembled on the optically transparent electrode (OTE) of nanostructured SnO2 (OTE/SnO2). The photoelectrochemical behaviour of the nanostructured SnO2 film of supramolecular nanoclusters between Li@C60 and MTPPS 4 denoted OTE/SnO2–(MTPPS 4 –Li@C60)n is significantly higher than the single component films of MTPPS or Li@C60 clusters, denoted OTE/SnO2–(MTPPS 4 )n or OTE/SnO2–(Li @C60)n (Scheme 1). A solution of Li@C60–MTPPS 4 supramolecule was prepared by mixing Li@C60PF6 (2.5 10 4 M) and (Bu4N)4 MTPPS (2.5 10 4 M) in PhCN. The mother PhCN solution of 1 mL was injected to an acetonitrile (MeCN) solution (3 mL) to produce the suspension containing the supramolecular

Photoelectrochemical cells (PECs) have been widely investigated as a next-generation solar cell because of their simple structure. 1-3 The photoinduced charge separation between the excited state of dye and the electrode plays an important role in improvement of PEC performance. In the natural photosynthetic reaction centre, the efficient photoinduced electron transfer occurs to give a long-lived charge separated (CS) state with high quantum yield. 4 Extensive efforts have so far been devoted to design and synthesize electron donor-acceptor linked molecules to achieve efficient photoinduced charge separation for applications to PECs. 5,6 However, the synthetic difficulty of the covalently linked donor-acceptor molecule has precluded the development of simple photovoltaic devices using such model compounds of the photosynthetic reaction centre. Among many candidates, porphyrins and fullerenes are a suitable combination for the construction of PECs, because porphyrins have strong visible absorption bands and fullerene exhibits efficient electron-transfer properties such as small reorganization energy due to the delocalized three-dimensional p-system. 7 The supramolecular approaches for PECs are also investigated, however, there was no report of supramolecules with a strong binding between neutral porphyrins and fullerenes.
We have recently designed and synthesized simple electron donor-acceptor supramolecular complexes composed of lithium ion encapsulated fullerene (Li + @C 60 ) and sulphonated mesotetraphenylporphyrin (MTPPS 4À : M = Zn, H 2 ), which have strong 1 : 1 supramolecular binding due to the cation-anion and p-p interactions (K = B10 5 M À1 ). 8 Photoexcitation of the supramolecule exhibited extremely slow charge-recombination of the CS state (t = 0.3 ms) in benzonitrile (PhCN). 8 Li + @C 60 has been reported to act as a more effective electron acceptor than pristine C 60 . 9 The driving force of photoinduced electron transfer from MTPPS 4À to the triplet excited state of Li + @C 60 is highly positive (ÀDG ET = 0.98 eV for ZnTPPS 4À and 0.67 eV for H 2 TPPS 4À in polar PhCN), 8 which is large enough to afford the CS states even under the non-polar environment in nanoclusters.
A solution of Li + @C 60 -MTPPS 4À supramolecule was prepared by mixing Li + @C 60 PF 6 À (2.5 Â 10 À4 M) and (Bu 4 N + ) 4 MTPPS 4À (2.5 Â 10 À4 M) in PhCN. The mother PhCN solution of 1 mL was injected to an acetonitrile (MeCN) solution (3 mL) to produce the suspension containing the supramolecular nanoclusters [(MTPPS 4À -Li + @C 60 ) n ]. The suspension of (MTPPS 4À -Li + @C 60 ) n was transferred into a cuvette, in which the two electrodes OTE and OTE/SnO 2 were placed and kept at a distance of 5 mm using a Teflon spacer. Then, application of the DC electric field (B100 V cm À1 ) resulted in the deposition of (MTPPS 4À -Li + @C 60 ) n from the suspension to the electrode surface and the formation of a robust thin film of OTE/SnO 2 -(MTPPS 4À -Li + @C 60 ) n , as documented by discoloration of the suspension and the simultaneous coloration of the OTE/SnO 2 electrode. For reference purposes, a thin film of only Li + @C 60 or MTPPS was analogously deposited onto the electrode surface to form OTE/SnO 2 -(Li + @C 60 ) n or OTE/SnO 2 -(MTPPS) n . Steady-state UV-vis absorption spectroscopy was used to follow the deposition of the MTPPS 4À -Li + @C 60 supramolecular material onto the electrode surface. The UV-vis absorption spectra of OTE/ SnO 2 -(MTPPS 4À -Li + @C 60 ) n are shown in Fig. 1, exhibiting significant broadening as compared with those in PhCN solutions of MTPPS 4À . Such broadening behaviour indicates that the molecular environment on the OTE/SnO 2 surface is significantly perturbed because of the aggregation of the porphyrin molecules or the supramolecules by p-stacking. Thus, MTPPS 4À -Li + @C 60 is successfully deposited on OTE/SnO 2 . The broad absorption band at 725 nm shown in Fig. 1a may be assigned to the charge-transfer band between the porphyrin plane and the fullerene sphere in the 1 : 1 supramolecular complex as reported previously. 10 TEM was used to evaluate the topography of an OTE/SnO 2 -(MTPPS 4À -Li + @C 60 ) n film as shown in Fig. 2. The (MTPPS 4À -Li + @C 60 ) n films are composed of closely packed MTPPS 4À and Li + @C 60 composite clusters of about 80 nm in size, which renders a nanoporous morphology to the film. The cluster sizes were also evaluated by the dynamic light scattering (DLS) measurements (see Fig. S1 in the ESI †). The grape bunch morphology of the cluster assembly thus provides a high surface area to the electrophoretically deposited film of Li + @C 60 clusters. As indicated earlier, 11 charging of porphyrin and fullerene moieties in the DC electric field plays an important role in the growth and deposition process. These films are quite robust and can be washed with organic solvents to remove any loosely bound MTPPS 4À and Li + @C 60 nano-assemblies.
Photoelectrochemical measurements were performed using a standard two-electrode system consisting of a working electrode and a Pt wire gauze electrode in air-saturated MeCN containing 0.5 M LiI and 0.01 M I 2 (Scheme 1). In order to evaluate the response towards the photocurrent generation, a series of photocurrent action spectra were recorded. The IPCE (incident photon-to-photocurrent efficiency) values were calculated by normalizing the photocurrent values for incident light energy and intensity and using eqn (1), 12 where i sc is the short circuit photocurrent (A cm À2 ), I inc is the incident light intensity (W cm À2 ) and l is the wavelength (nm). The maximum IPCE values of OTE/SnO 2 -(Li + @C 60 ) n (black spectrum in Fig. 3a) and OTE/SnO 2 -(ZnTPPS 4À ) n (blue spectrum) are only 5% (425 nm) and 22% (445 nm), respectively. In contrast to the reference experiments, the IPCE value of OTE/ SnO 2 -(ZnTPPS 4À -Li + @C 60 ) n is much higher than the sum of the two individual IPCE values of the individual systems OTE/ SnO 2 -(ZnTPPS 4À ) n and OTE/SnO 2 -(Li + @C 60 ) n in the visible region. The maximum IPCE value attained in these experiments was 77% at 450 nm. The high ICPE value at the Q-band region was also observed to be 50% at 570 nm. Such a high IPCE value indicates that photocurrent generation is initiated via photoinduced electron transfer in supramolecules between ZnTPPS 4À and Li + @C 60 , followed by the charge transport to the collective surface of the OTE/SnO 2 electrode (Scheme 1). When ZnTPPS 4À was replaced by H 2 TPPS 4À , a significantly low IPCE value of 7% was observed at 440 nm (Fig. 3b) probably because of the self-aggregation of H 2 TPPS 4À without binding with Li + @C 60 . 13   We have also evaluated the power characteristics of the OTE/ SnO 2 -(ZnTPPS 4À -Li + @C 60 ) n electrode (Fig. S2 in the ESI †). The power conversion efficiency, Z, is calculated using eqn (2): 12 in which the fill factor (FF) is defined as FF = [IV] max /I sc V oc , and V oc is the open-circuit photovoltage and I sc is the short-circuit photocurrent. OTE/SnO 2 -(ZnTPPS 4À -Li + @C 60 ) n has a an overall Z value of 2.1% at an input power (W in ) of 28 mW cm À2 , whereas FF = 0.37, V oc = 460 mV and I sc = 3.4 mA cm À2 in the OTE/SnO 2 -(ZnTPPS 4À -Li + @C 60 ) n . Note that the Z value is two orders of magnitude greater than that of the previously reported simple porphyrin and C 60 composite system (B0.03%). 12 Such a significant enhancement of the Z value demonstrates that the strong ordering in the clusters and the efficient charge separation in (ZnTPPS 4À -Li + @C 60 ) n improved the light energy conversion properties.
In order to clarify the photocurrent generation mechanism, we examined formation of the CS state [(ZnTPPS 4À ) + -Li + @C 60 À ] by nanosecond laser flash photolysis measurements. Time-resolved transient absorption spectra of (ZnTPPS 4À -Li + @C 60 ) n dispersed in a deaerated MeCN-PhCN solution (3 : 1 v/v) are shown in Fig. 4a, which clearly exhibit a broad absorption band at around 1035 nm. 8,9 This is diagnostic of formation of Li + @C 60 À upon laser irradiation. , affording a rate constant of back electron transfer k BET = 4.6 Â 10 3 s À1 . The lifetime of the CS state is 220 ms, which is long enough to inject an electron from Li + @C 60 À of the CS state to the SnO 2 electrode before the charge recombination. Such a long-lived CS state was further detected by EPR under photoirradiation of an MeCN-PhCN solution (1 : 3 v/v) containing (ZnTPPS 4À -Li + @C 60 ) n at 77 K. The EPR signal was observed at g = 2.0020, which is attributable to the mixture of the porphyrin radical cation (g = 2.002) 14 and Li + @C 60 À (g = 2.0014) (see Fig. S3a in the ESI †). 15 When ZnTPPS 4À was replaced by H 2 TPPS 4À , the transient absorption bands due to the CS state was significantly smaller than the case of ZnTPPS 4À (Fig. S4, ESI †). This is the reason why the IPCE value of OTE/SnO 2 -(H 2 TPPS 4À -Li + @C 60 ) n was low as shown in Fig. 3b. Based on the above-mentioned results, the photocurrent generation is initiated by photoinduced electron transfer from ZnTPPS 4À to Li + @C 60 in the cluster to produce the CS state, (ZnTPPS 4À ) + -Li + @C 60 À . The reduced Li + @C 60 (Li + @C 60 À ) (E(Li + @C 60 /Li + @C 60 À ) = 0.14 V vs. SCE) 8,9 injects electrons into the conduction band of SnO 2 (0.2 V vs. SCE), 11 whereas the oxidized ZnTPPS 4À (E(ZnTPPS 4À /(ZnTPPS 4À ) + ) = 0.74 V vs. SCE) 8 undergoes the electron-transfer reduction with the iodide (I 3 À /I À = 0.7 V vs. SCE) 12 in the electrolyte solution.
In conclusion, the photoinduced electron transfer from ZnTPPS 4À to Li + @C 60 in the supramolecular cluster makes it possible to enhance the performance of the photoelectro-chemical cell. Thus, the use of Li + @C 60 as an electron acceptor in the supramolecular clusters with ZnTPPS 4À paves a new way for the design of high performance solar cells.