Natural Photosynthetic Carotenoids for Solution-Processed Organic

Article pubs.acs.org/JPCC

Natural Photosynthetic Carotenoids for Solution-Processed Organic Bulk-Heterojunction Solar Cells Xiao-Feng Wang,*,† Li Wang,‡ Zhongqiang Wang,† Yuwei Wang,† Naoto Tamai,‡ Ziruo Hong,† and Junji Kido† †

Research Center for Organic Electronics (ROEL), Graduate School of Science and Engineering, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan ‡ Faculty of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan S Supporting Information *

ABSTRACT: In this work, we demonstrate utilization of natural carotenoids (Cars), namely, fucoxanthin, β-carotene, and lycopene, as electron-donor molecules together with the electron-acceptor fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in organic solar cells (OSCs). Unlike fucoxanthin and β-carotene, which form amorphous films, lycopene readily forms aggregates through a simple spin coating process. A high carrier mobility of up to 2.1 × 10−2 cm2/(V s) was observed for lycopene, which is three orders of magnitude greater than those of fucoxanthin and β-carotene, with values of (8.1 and 1.8) × 10−5 cm2/(V s), respectively. OSCs with different Car:PCBM blend ratios were optimized for these Cars. The highest photovoltaic performance was obtained for lycopene with a blend ratio of 1:1, at which the film morphology and charge transport were optimized. Replacement of the acceptor molecule PCBM with a high-lowest-unoccupiedmolecular-orbital fullerene derivative indene-C60 bisadduct improved the overall conversion efficiency of lycopene-based OSCs by enhancing the open-circuit current (Voc). Interestingly, further investigation on charge-separation dynamics revealed that photocurrent is generated only from the S2 (1Bu+) state, and the others underwent ultrafast excitation relaxation through S2 → S1 (2Ag−) → S0 (ground state), leaving much room for further improvement.

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lying singlet states, including the optically allowed S2 (1Bu+) and the optically forbidden S1 (2Ag−), 1Bu−, and 3Ag− states, concerning transitions from to the ground S0 (1Ag−) state.7 These important roles and photochemical and photophysical properties of Cars strongly suggest applications of molecules of this type in opto-electric devices. In fact, previous studies on Car derivatives have proven that Cars have the potential of electron ejection to electron-acceptor molecules when they are covalently bound.8 Recently, one Car, β-carotene, was used in the fabrication of an organic field-effect transistor (OFET) and exhibited a hole mobility of up to 4 × 10−4 cm2/(V s).9 Such a moderate hole mobility is not sufficient for OFETs but should be enough for OSCs because typical OSC donor materials usually have moderate hole mobilities.10 In the present study, three typical Cars, that is, fucoxanthin, β-carotene, and lycopene, were used as p-type donor molecules in solution-processed bulk-heterojunction (BHJ) OSCs. The morphologies of the Car thin films were predicted from their absorption spectra and measured using atomic force microscopy (AFM). The correlations of photovoltaic performance

INTRODUCTION Organic solar cells (OSCs) have attracted much attention in recent years because of continued improvements in their photovoltaic performances and clarification of their working mechanisms.1 In OSC devices, the most important part is the photoactive layer, consisting of p-type donor and n-type acceptor molecules because the major performance-determining processes, including light harvesting, charge separation, and charge transport, all occur within this layer.2 To satisfy the needs of these critical processes, donor materials are usually designed and synthesized with a symmetric structure to reduce their polarity.3 In view of this, naturally occurring donor molecules with symmetric chemical structures are promising for use in OSCs because they have many merits such as abundant resources, cost-effectiveness, and environmental friendliness, whereas there have been very few reports on OSCs based on natural products due to lacking of information on their optical/ electronic properties.4 Carotenoids (Cars), along with chlorophylls, are the most abundant pigments found in nature.5 Cars play important roles in natural photosynthesis as they have light-harvesting, singletenergy-transferring, and photoprotective functions.6 These functions of Cars are determined by their extremely complicated low-lying singlet-excited-states. In particular, the all-trans conjugated chain with C2h symmetry gives rise to low© XXXX American Chemical Society

Received: October 2, 2012 Revised: December 27, 2012

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dx.doi.org/10.1021/jp309773b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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with film morphology and hole mobility are discussed. Subpicosecond time-resolved absorption spectroscopy (TAS) was used on Car/fullerene blend films from the best OSCs to reveal the charge-separation process at the donor/acceptor interface.

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RESULTS AND DISCUSSION Chemical and Physical Properties of Car Thin Films. Figure 1 shows the chemical structures of fucoxanthin, β-

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EXPERIMENTAL SECTION Fabrication of Bulk-Heterojunction OSCs. Carotenoids were purchased from Wako Chemicals, and fullerene derivatives were purchased from Lumtec. BHJ OSCs were fabricated with the following structure: ITO/MoO3 (5 nm)/ carotenoid:PCBM (80 nm)/Ca (20 nm)/Al (100 nm). Patterned ITO glass with a sheet resistance of 15 Ω per square were precleaned with a detergent, ultrasonicated in deionized water, acetone, and isopropyl alcohol, and then treated in an ultraviolet-ozone chamber for 30 min. A 5 nm thick layer of MoO3 is thermally evaporated onto the ITO substrates as anode contact in high vacuum ( β-carotene > lycopene, although the order was slightly affected by the Car/PCBM blend ratios. (3) Lycopene gave the largest fill factors (FFs) among these Cars. The FF value for lycopene-based OSCs increased with increasing Car/PCBM blend ratio up to 1:1 and then decreased with further increases in the Car/PCBM blend ratio. The FF values were less affected by the Car/PCBM blend ratio in the cases of fucoxanthin and β-carotene.

Figure 5. I−V curves and EQE profiles of OSCs based three carotenoids under optimal conditions.

I−V curves: the Jsc values integrated from the corresponding EQE profiles are ranked in the order lycopene > β-carotene > fucoxanthin, whereas the Voc values are ranked in the reverse order. The maximum EQE for the best OSC was β-carotene > lycopene, which is consistent with the observed Voc values. Furthermore, the aggregate nature of the lycopene film will create more opportunity of nongeminate recombination due to the delocalization of excitons between each lycopene molecules. Thus, the smallest Voc value for the OSC based on lycopene among these Cars must originated from the above dual effects The large FF value of lycopene, based on an OSC with a 1:1 weight ratio of the Car/PCBM blend film, seems to originate from its good film morphology because only lycopene forms aggregates on a solid substrate. Figure 6 shows the AFM height, phase, and 3-D images for lycopene/PCBM blend films with

the LUMOs, is largest for lycopene, followed by β-carotene and fucoxanthin. Moreover, the Voc values of OSCs using small-dyemolecules-based donor materials generally obey the following equation:14 VOC =

nkT ⎛ JSC ⎞ ΔE DA ln⎜⎜ ⎟⎟ + q 2q ⎝ JSO ⎠

(1)

where Jso is determined by a number of materials properties that determine the carrier generation/recombination rate, independent of the energy difference of the donor’s HOMO and acceptor’s LUMO, ΔEDA. Here the ΔEDA value for these E



Article

Table 2. Carrier Mobility Data Derived from Single Carrier Devices Using Space-Charge-Limited Current Method μh (cm2/(V s)) μe (cm2/(V s))

lycopene

2:1

1:1

1:2

1:4

PCBM

2.1 × 10−2

1.3 × 10−3 5.1 × 10−5

4.2 × 10−4 1.7 × 10−4

1.2 × 10−4 7.6 × 10−4

1.0 × 10−5 1.2 × 10−3

5.0 × 10−3

2:1, 1:1, and 1:2 mixing ratios. Compared with the bare lycopene film, the film became less rough in the presence of PCBM. At a blend ratio of 1:1, the RMS factor is the smallest, at 0.243 nm, which allows excellent phase separation of the donor and acceptor molecules. However, increasing either the lycopene or the PCBM content increased the film roughness, and the phase separation may become worse. The largest FF at a blend ratio of lycopene/PCBM 1:1 is therefore ascribable to the excellent phase separation at this blend ratio, which guarantees balanced electron and hole transport toward both electrodes. Table 2 shows the carrier mobilities of thin films containing lycopene and PCBM in different blend ratios. With increasing PCBM content, the hole mobility of the film decreases and the electron mobility of the film increases. At a blend ratio of 1:1, the hole mobility and electron mobility are closest to each other, indicating that this condition gives the most balanced charge transport. The mobility results therefore also support the observed photovoltaic performances. Indeed, the high hole mobility of the lycopene thin film suggests to fabricate OSCs with much thicker active layer because typical P3HT-based polymer solar cells have the thickness of active layer up to 200−300 nm. The thickness of 80 nm for the active layers in the present investigation may not be the optimal condition. In Figure S1 and Table S1 of the Supporting Information, the thickness dependence of active layer containing lycopene:PCBM on the photovoltaic performance was investigated. Interestingly, the Jsc and Voc values increase with reduced thickness of the active layer, whereas the FF values were more or less similar, except for the thinnest film that tends to easily affected by the electric field. The smaller Jsc values for OSCs with thicker active layers, that is, larger lightharvesting efficiency, should be originated from the nongeminate recombination process because the charge separation and the geminate recombination processes are equal for these OSCs with different thickness. This is also supported by the decreased Voc values with increased film thickness. Actually, the excellent carrier mobility of lycopene aggregates helps the holes moving around and provides a good opportunity for the nongeminate recombination to occur. Although lycopene gives the best photovoltaic performance among these Cars, it should still be possible to further enhance the photovoltaic performance by improving the Voc value. The replacement of PCBM with the high-LUMO acceptor indeneC60 bisadduct (ICBA) is one method.15 Also, ICBA offers shallower LUMO level for investigating the charge-transfer dynamics. The LUMO level of ICBA is −3.7 eV, which is 0.2 eV higher than that of PCBM. A solar cell based on a lycopene/ ICBA active layer was fabricated and compared with the cell based on a lycopene/PCBM active layer. Figure 7 shows the I− V curves and EQE profiles of lycopene-based OSCs using PCBM and ICBA as the electron acceptor, and Table 3 lists the relevant parameters obtained from the I−V curves. The blend ratio of lycopene/fullerene for both acceptors was fixed at a ratio of 1:1. The ICBA-based OSC gives a Voc value of 0.64 V, which is approximately 0.2 V higher than that of the OSC based on PCBM; this is in good agreement with the difference

Figure 7. I−V curves and EQE profiles of lycopene-based OSCs using different fullerene derivatives.

Table 3. Photovoltaic Performance of Lycopene-Based BHJ OSCs Using Different Fullerene Derivatives as Electron Acceptor acceptor

Jsc/mA cm−2

Voc/V

FF

η/%

PCBM ICBA

1.39 1.1

0.45 0.64

0.52 0.55

0.33 0.38

between the LUMO levels of these two acceptor molecules. However, the Jsc value of the OSC based on ICBA is much lower than that of the OSC based on PCBM. This reduced photocurrent can be attributed to the reduced energy gap between the LUMO levels of the donor and acceptor molecules. As a result, the photovoltaic performance is slightly improved upon replacement of PCBM with ICBA. Charge Separation and Recombination at Donor/ Acceptor Interface of Lycopene-Based OSCs Determined Using Subpicosecond TAS. As discussed in the previous section, lycopene-based BHJ OSCs have excellent phase separation in the active layer and excellent chargetransport properties but low photovoltaic performances as a result of the low EQE and resultant photocurrent. It is probable that such low photocurrent generation by lycopene originates from early charge-separation events because the excitation relaxation of lycopene is an ultrafast event and may compete with charge separation.7 We therefore performed subpicosecond TAS on the lycopene/fullerene BHJ films to study the dynamics of charge separation and recombination. Figure 8 F



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S1 state can be clearly observed for both lycopene/PCBM and lycopene/ICBA systems. Importantly, there is no sign of electron transfer from the S1 state to PCBM/ICBA because no increasing component of the PCBM•−/ICBA•− signal is observed. To confirm the above statement, Figure 10 shows the exponential fitting results of the decay profiles for lycopene/

Figure 8. Sub-picosecond transient absorption spectra of lycopene thin film.

shows the TAS of the bare lycopene thin film. After excitation at the red-most absorption peak [S2(0)] of the lycopene thin film, a broad bleaching signal at ∼500 nm, corresponding to the S2 state, immediately appeared, which underwent ultrafast relaxation down to the S1 state, within 0.4 ps, through the dark states. A clear transient absorption from the S1 state was observed between 600 and 700 nm, which then returned to the ground state before the delay time of 10 ps. We then measured the TAS for the Car/fullerene BHJ systems. Figure 9 shows the TAS of thin films containing

Figure 10. Kinetic fitting results of transition absorption spectra at 500 and 1080 nm for lycopene/PCBM blend film (a) and lycopene/ICBA blend film (b).

PCBM (a) and lycopene/ICBA (b) systems at both 500 and 1080 nm. The signal at 500 nm for the bare lycopene film has a lifetime τ = 1.2 ps. When the acceptor molecules are present, this signal can be fitted with two components, τ1 = 1.6 ps and τ2 = 88 ps for PCBM and τ1 = 2.0 ps and τ2 = 210 ps for ICBA. The first component corresponds to the excitation relaxation process of lycopene, which is in agreement with the excitedstate lifetime for the bare lycopene film. The second component originates from the charge-recombination process, which is on the same order as the decay kinetics of PCBM•−, with a lifetime of 110 ps, and of ICBA•−, with a lifetime of 545 ps. The slower charge recombination between ICBA•− and lycopene•+ compared with that between PCBM•− and lycopene•+ is one of the reasons why the Voc values of OSCs based on the former are larger than the values of those based on the latter. Only a fraction of the lycopene in the excited state therefore contributes to photocurrent generation, and the remainder undergoes an excitation relaxation process. The above TAS results nicely explain the observed photocurrent and EQE results in lycopene-based OSCs. As pointed out by one of the reviewers, the dissipation of energy through triplet state of these Cars may also be a factor to determine the low photovoltaic performance of OSCs. In Figure 4, the first triplet state (T1) of these Cars was also shown.17 The T1 energy levels for these Cars are below the LUMO level of acceptor molecules, indicating that charge separation is hardly occuring between the triplet state of Car

Figure 9. Sub-picosecond transient absorption spectra of lycopene/ fullerene films with 1:1 blend ratio.

Car:fullerene at a blend ratio of 1:1 in both the visible and nearinfrared regions for PCBM (upper panel) and ICBA (lower panel). After excitation of the blend film at the S2(0) of lycopene, the TAS in the visible region is similar to that observed for the bare lycopene film. In the near-infrared region, a broad signal at ∼1080 nm, which corresponds to PCBM•−/ ICBA•−, appears immediately after the excitation.16 The generation of PCBM•−/ICBA•− is caused by extremely fast electron transfer (much less than the laser pulse of 100 fs in the present study) between lycopene and PCBM/ICBA. However, the excitation relaxation of the S2 to the S1 state must occur as a parallel electron transfer mode because the TAS signal from the G



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and the LUMO of fullerene derivatives. Nevertheless, the yield of T1 state through intersystem crossing is very low with a value of ∼10−6,18 suggesting that the loss of excitation energy through triplet state is probably not the major reason responsible for the low photovoltaic performance of present system using Cars.

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CONCLUSIONS In summary, three typical natural Cars, namely, fucoxanthin, βcarotene, and lycopene, were used as donor materials in solution-processed OSCs. Among these Cars, only lycopene formed aggregates on a solid substrate during spin coating processes; this was supported by the absorption spectra and AFM images. The hole mobility of lycopene measured by the SCLC method was 2.1 × 10−2 cm2/(V s), which was three orders of magnitude greater than those of fucoxanthin and βcarotene, respectively. In OSCs, the optimum blend ratio of Car:PCBM was 1:1 for lycopene, which was much larger than those for fucoxanthin and β-carotene. The OSCs based on lycopene gave much better photovoltaic performances than those based on fucoxanthin and β-carotene, mainly because of the superior charge-carrier transport of the lycopene-based OSCs. The replacement of PCBM by ICBA in lycopene-based OSCs slightly improved the overall conversion efficiency by increasing the Voc value by 0.2 V. The low photocurrent generated by lycopene-based OSCs can be attributed to inefficient electron transfer from only the S2 state of lycopene, which competes with excitation relaxation from the S2 to the S1 state. This work not only opens a vast material library for OSCs and related optoelectronic devices but substantially deepens our knowledge about the charge transfer at the donor−acceptor interface of organic photovoltaics.

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ASSOCIATED CONTENT

* Supporting Information S

Dependence of photovoltaic performance on the lycopene:PCBM layer thickness. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], charles1976110@ hotmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Xiao-Feng Wang would like to thank Prof. Yasushi Koyama for his guidance to the research fields of photosynthesis and photovoltaics so that the idea behind this investigation can be generated. This work was partially supported by Grants-in-Aid for Young Scientists (A) (23686138) (to X.-F.W) from the Japan Society for the Promotion of Science (JSPS) and by Japan Regional Innovation Strategy program by the Excellence (J-RISE) from Japan Science and Technology Agency (JST).

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Natural Photosynthetic Carotenoids for Solution-Processed Organic

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