Near-infrared absorption bacteriochlorophyll derivatives as biomaterial electron donor for organic solar cells

Journal of Photochemistry and Photobiology A: Chemistry 347 (2017) 49–54

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Near-infrared absorption bacteriochlorophyll derivatives as biomaterial electron donor for organic solar cells Shengnan Duana , Guo Chenb , Mengzhen Lia , Gang Chena , Xiao-Feng Wanga,* , Hitoshi Tamiakic , Shin-ichi Sasakic,d a

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, PR China Key Laboratory of Advanced Display and System Applications (Ministry of Education), Shanghai University, Shanghai 200072, PR China Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan d Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan b c

A R T I C L E I N F O

Article history: Received 2 June 2017 Received in revised form 5 July 2017 Accepted 15 July 2017 Available online 17 July 2017 Keywords: Organic solar cells Biomaterials Bacteriochlorophyll aggregate Photosynthesis Planar-heterojunction

A B S T R A C T

In this paper, we investigate two bacteriochlorophyll (BChl) derivatives, namely methyl (pyro) bacteriopheophorbide a [BChl-(2)1], with excellent absorption in near-infrared regions as donor materials for organic solar cells (OSCs). In the meantime, we prepare two methyl (pyro)pheophorbide a [Chl-(2)1] as reference to make a comparison. Compared to the small red-shifts of absorption bands of Chls-1/2 after forming the thin film through spin coating of the homogeneous solution, the absorption spectra of BChls-1/2 exhibited larger red-shifts from the absorption peak at 750 nm in the solution to the absorption peak at 860 nm after forming the solid film. This significant absorption shift is attributed to the formation of J-type aggregates by intermolecular p-p stacking among the BChl-1/2 molecules. What’s more the absorbed sunlight of the BChls-1/2 aggregates at the near-infrared (NIR) region can be transferred into photocurrent which can be proved from the EQE indicating the energy and electron transferring between BChl aggregates are efficient. The optimized power conversion efficiency (PCE) value of 1.43% under standard AM1.5 mW/cm2 sunlight was achieved with the BChl-1: C70 planarheterojunction (PHJ) cells. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Organic solar cells (OSCs) have shown the potential to be the next generation of photovoltaic devices since they have many advantages including flexible, light weight, low cost and easy to manufacture [1–4]. In general, broadening the absorption range to near-infrared (NIR) wavelength in solar energy can enhance the generation of photocurrent [5,6]. Organic small molecule such as porphyrin are widely used in dye-sensitized solar cells [7–10]. Bacteriochlorophylls (BChls) which are analogues of porphyrin are capable of harvesting sunlight and transferring the excited energy in natural photosynthetic processes [11–13]. Another advantage of

using BChl molecules as the donor materials of OSCs is that the molecular structure of the BChls is tunable through replacing the molecular peripheral substituents thus we can design and synthesize the suitable molecules with a proper energy level, an excellent charge transport capacity, a high solubility and so on [14– 16]. The BChl and chlorophyll (Chl) molecules used in the present investigation are cyclic tetrapyrroles [17,18]. Such (B)Chls can readily self-assemble into J-type of aggregation and in general the formation of J-type aggregates can enlarge the carrier mobility and charge transportation ability [19–22]. In this work, we employed two BChls, methyl bacteriopheo-

Abbreviations: AFM, atomic force microscopy; (B)Chl, (bacterio)chlorophyll; BChl-1, methyl pyrobacteriopheophorbide a; BChl-2, methyl bacteriopheophorbide a; BCP, bathocuproine; Chl-1, methyl pyropheophorbide a; Chl-2, methyl pheophorbide a; CV, cyclic voltammogram; D/A, donor and acceptor; DELUMO, energy gap of LUMO between donor and acceptor material; ΔEDA, the energy gap between donor’s HOMO and acceptor’s LUMO; FF, fill factor; HOMO, highest occupied molecular orbital; ITO, indium tin oxide; IPCE, monochromatic incident photon-to-electron conversion efficiency; Jsc, short-circuit current; LUMO, lowest unoccupied molecular orbital; OSC, organic solar cell; PCE, power conversion efficiency; PHJ, planar-heterojunction; PL, photoluminescence; RMS, root-mean-square; UPS, ultraviolet photoelectron spectroscopy; Voc, open-circuit voltage. * Corresponding author. E-mail address: [email protected] (X.-F. Wang). http://dx.doi.org/10.1016/j.jphotochem.2017.07.014 1010-6030/© 2017 Elsevier B.V. All rights reserved.

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phorbide a (BChl-1), and methyl pyrobacteriopheophorbide a (BChl-2) as an electron donor and two Chls, methyl pheophorbide a (Chl-1), methyl pyropheophorbide a (Chl-2) as references. These semi-synthesized (B)Chls were employed in conjuncture with fullerene C70 as an electron acceptor in planar-heterojunction (PHJ) cells. The PHJ solar cells based on BChl-1 exhibit the power conversion efficiency (PCE) of 1.43% under AM1.5 solar illumination (100 mW/cm2) at room temperature. Since the devices were fabricated with BChl-1/2 aggregates absorbing the NIR light energy and can eventually transform the NIR light into photocurrent, they are inexpensive and less pollutive to the environment compared with other systems using fully synthesized chemicals.

2.2. Device fabrication PHJ solar cells were fabricated with the following structures: indium tin oxide (ITO)/MoO3 (5 nm)/(B)Chls (10 nm)/C70 (40 nm)/ BCP (8 nm)/Al (100 nm). The ITO substrates were precleaned with detergent, deionized water, acetone and isopropyl alcohol in an ultrasonic bath for 20 min in sequence and then exposed in ultraviolet-ozone for 15 min. Then the substrates were transferred into high-vacuum chamber where a 5 nm thick MoO3 layer were thermally deposited on the ITO substrates. After that, the (B)Chls in chloroform (10 mg/mL) were spin-coated on the MoO3 modified ITO substrate to reach a 10-nm thickness film. Then C70, BCP, and Al were sequentially thermally deposited in high vacuum.

2. Experimental section 2.3. Material characterization 2.1. Materials BChls-1/2 and Chls-1/2 are semi-synthesized as reported before [23–25]. Both the indium-tin oxide (ITO), MoO3, C70, bathocuproine (BCP), and Al were commercially available.

UV/Vis/NIR absorption spectra of (B)Chls on the solid film and in a tetrahydrofuran (THF) solution were recorded by a Shimadzu UV3100 spectrophotometer. The cyclic voltammograms (CVs) of (B) Chls dispersed in dichloromethane were recorded by a VSP

Fig. 1. The molecular structures of (B)Chls-1/2 as donor materials.

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multichannel galvanostatic-potentiostatic system (Bio-Logic SAS). The energy band values were measured with an integrated ultrahigh vacuum system equipped with multi-technique surface analysis system (VG Scienta R 3000) ultraviolet photoelectron spectroscopy (UPS). 2.4. Device characterization Thickness of (B)Chls film was measured by a Dektak 8 stylus profiler (Veeco). The morphologies of the (B)Chls film were analyzed through atomic force microscopy (AFM) in tapping mode under ambient conditions using a Bruker instrument. Both PCE and monochromatic incident photon-to-electron conversion efficiency (IPCE) characterization of solar cells were carried out on a CEP2000 integrated system (Bunkou Keiki) under standard measurement conditions. The device area of 0.04 cm2 was controlled by a metal mask. The performance data were averaged from four independently fabricated solar cells. 3. Results and discussion Fig. 1 shows the chemical structures of BChls-1/2 and Chls-1/2 which were used in this investigation. Both the (B)Chls share a cyclic tetrapyrrole as their backbone without the central metal and the 31-hydroxy group which were requisite for the formation of well-ordered J-aggregates in the previous reports [26,27]. As the substituents on the C3 position are an acetyl group for BChls-1/2 and a vinyl group for Chls-1/2. Additionally, at the C132 positions are a methoxycarbonyl group for (B)Chls-2, while no 132substituents are for (B)Chls-1. When they were spin-coated to form the solid films, molecular structure-dependent self-assemblies were observed in UV/Vis absorption spectra and AFM images. Fig. 2a shows the normalized UV/Vis/NIR absorption spectra of (B)Chls both in a THF solution and on the film which were made by spin-coating a chloroform solution of these (B)Chls (10 mg/mL) on a cleaned glass substrate. Table 1 lists the two intense absorption maxima. Compared with the Soret bands at the purple region and Qy bands at the red region of the BChls in a solution, their spincoated films moved them to a longer wavelength and broadened. The changes of the spectra originated from J-aggregates by intermolecular p-p stacking. There are much less red-shifts of Chls compared with the BChls indicating the states of Chls in the film are monomer or less aggregate while the states of BChls are aggregates in the film. These narrower Qy band of Chls compared with the BChls might show that the aggregates of Chls are more homogenous. Fig. 2b shows the first one-electron oxidation potentials (Eox) of the (B)Chls in dichloromethane, from which we can estimate the highest occupied molecular orbital (HOMO) level of these (B)Chls as follows [28]: HOMOðeVÞ ¼ eðEox þ 4:8Þ:

ð1Þ

Their lowest unoccupied molecular orbital (LUMO) levels in solid state were also calculated as the following equation: LUMOðeVÞ ¼ HOMOðeVÞ þ Eopt g ðfilmÞ:

ð2Þ

The HOMO and LUMO values of these (B)Chls are listed in Table 1. We also measured the HOMO and LUMO energy levels of BChls-1/2 by UPS (in Fig. S1 and Table S1), confirming the values estimated from the CV and absorption results. Fig. 3 shows the height and 3D images of (B)Chls-1/2 in the solid films spin-coated on the MoO3 modified substrate prepared from evaporation of the chloroform solution (10 mg/mL). The surfaces of BChl-1 and 2 films were rougher with the root-mean-square (RMS) values of 0.68 and 2.14 nm, respectively than those of Chl-1 (0.31 nm) and Chl-2 films (0.59 nm), which can be attributable to

Fig. 2. (a) Absorption spectra of (B)Chls-1/2 on the film and in THF and (b) their CVs in dichloromethane with the scanning rate 100 mV/s.

larger aggregates of the BChls-1/2. The RMS data show that Chls-1/ 2 and BChl-1 may be better for interface contacts than BChl-2. From AFM images, the aggregates morphology of Chls-1/2 is more ordered than those of the others, which is accordant with the absorption analysis. Fig. 4a depicts the device structure of the present OSCs and Fig. 4b shows their energy alignments. In these OSCs, a 5 nm thickness of MoO3 was prepared as hole transport layer as well as blocking the electrons. Then the donor layer was prepared by spincoating (B)Chls in a chloroform solution (10 mg/mL). Since the diffusion length of the (B)Chls is quite short, the optimized thickness of (B)Chls is only 10 nm in this research. Then 40 nm thickness of C70 was prepared as acceptor layer. When the sunlight illuminates the device, the (B)Chl molecules are electronically excited. Then the excitons generate and dissociate at donor and acceptor (D/A) interface, which means the electrons transfer from the LUMO of donor (B)Chls to the LUMO of acceptor C70 [29,30]. The energy gap of LUMO between donor and acceptor material (DELUMO) acts as driving force for excitons when the excitons dissociate on the D/A interface. Larger DELUMO of D/A materials can get much more efficient exciton dissociations which may produce

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Table 1 Specific UV/Vis/NIR absorption spectral and CV data of (B)Chls-1/2. Solution

l

Soret

Film

[nm]

l

Qy

[nm]

a Eopt g

b

lSoret [nm]

lQy [nm]

Eopt ½eV g

Eox

381 387 415 419

846 861 681 682

1.37 1.35 1.72 1.71

0.22 0.12 0.39 0.31

[V]

HOMO [eV]

LUMO [eV]

5.02 4.92 5.19 5.11

3.65 3.57 3.47 3.40

½eV BChl-1 BChl-2 Chl-1 Chl-2 a b

359 359 405 412

752 750 668 669

1.57 1.58 1.78 1.79

The optical bandgaps (Eopt g ) were calculated from the absorption edges. The first one-electron oxidation potentials Eox were calculated in dichloromethane vs. Ag/Ag+(Eox = 0.22 V vs. ferrocene/ferrocenium).

Fig. 3. AFM phase of (B)Chls-1/2 in the films on the MoO3 modified ITO substrate.

Fig. 4. (a) Device structure and (b) energy alignments of OSCs based on the (B)Chls-1/2 as their donor materials.

larger photocurrent. The energy gap between the HOMO of donor material and the LUMO of acceptor materials is proportional to the open-circuit voltage (Voc) of the device. Lower HOMO of donor energy level can result higher Voc in this PHJ solar cell as shown later. After that 8 nm thickness of BCP was thermal evaporated as blocking the holes. Then aluminum electrode of 100 nm thickness was thermal deposited as cathode. Fig. 5a shows the optimized current voltage (J-V) curves of OSCs based on (B)Chls as donor materials under standard AM 1.5 mW/ cm2 sunlight. Their photovoltaic parameters which are averaged by four devices are listed in Table 2 and their discrepancy of the

photovoltaic parameters of the four devices are listed in Fig. S2. The formula of calculating Voc is that [31,32]:   nkT J DEDA ln sc þ : ð3Þ Voc ¼ q J so 2q In this formula, Jso represents the magnitude of saturation current density for the charge recombination under thermal equilibrium and the DEDA represents the energy gap between donor’s HOMO and acceptor’s LUMO. Since the devices based on Chls-1/2 give the largest DEDA compared with the BChls-1/2, which is consistent with the Voc that we measured in J-V curves. The photovoltaic

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Fig. 5. (a) J-V curves and (b) IPCE profiles of OSCs based on (B)Chls as donor materials.

Table 2 Photovoltaic performance of OSCs based on (B)Chls-1/2 as donor materials under standard AM1.5 mW/cm2 sunlight. (B)Chl

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

BChl-1 BChl-2 Chl-1 Chl-2

4.63  0.09 4.73  0.09 5.03  0.10 5.41  0.10

0.50  0.01 0.36  0.01 0.66  0.01 0.64  0.01

0.62  0.01 0.56  0.01 0.49  0.01 0.47  0.01

1.43  0.03 0.96  0.02 1.63  0.03 1.62  0.03

performance of OSCs based on Chls-1/2 as donor materials are better compared with BChls-1/2 which is attributed to the larger Voc and short-circuit current (Jsc). And this is owing to the deeper HOMO energy levels of Chls-1/2 than that of BChls-1/2. A deeper HOMO energy level can shrink the energy barrier for the electron injection from (B)Chls-1/2 to the C70 and the electron collection at anode can be more efficient thus a higher Jsc can be obtained for Chls-1/2. The devices based on BChl-1 give the higher PCE of 1.43% compared with the BChl-2, which was calculated from Jsc of 4.63 mA/cm2, Voc of 0.50 V, and fill factor (FF) of 0.62. The BChl-1 gives a larger Voc which is originating from a larger DEDA as well as a larger FF, although BChl-2 gives a larger photocurrent compared with the BChl-1. Fig. 5b shows corresponding EQE profiles of the (B)Chls-1/2, and the integrated photocurrents calculated from EQE are within an error of 5% of the Jsc data measured in J-V curves. From the IPCE, (B)Chls-1/2 at both Soret band and Q band regions have a good response with the absorption spectra. This indicates that photoexcited photons in BChl-1/2 aggregates at Qy bands have already transferred into the electrons which can contribute to the photocurrent demonstrating the energy and electron transferring between BChl aggregates are efficient. 4. Conclusions In summary, two self-aggregates of BChl derivatives with NIR absorption maxima, named BChl-1 and BChl-2 have been semisynthesized as donor materials for PHJ cells and two Chl derivatives also have been employed as reference. The photovoltaic performances of Chls are better than that of BChls since the energy levels of Chls are much more matching. On the other hand, the better PCE of OSCs based on BChl-1 is due to a larger Voc and FF since the HOMO energy level of which is deeper compared with that of BChl-2. The BChls can readily self-assemble into aggregates through a spin-coating process which can be seen from absorption spectrum and AFM images while Chls-1/2 are monomers or less aggregates. And this self-assembled aggregate of BChls-1/2 with NIR absorption can eventually generate excitons which is contributed to the photocurrent as can be seen from EQE. The decent PCE value of BChl devices up to 1.43% was achieved based on BChl-1:C70 PHJ cells. This investigation opens abundant donor materials with NIR absorption and provides a low-cost as well as

nonpolluting materials library for the development of organic solar cells. Acknowledgements The authors thank Wenjie Zhao for the experimental assistance. We thank Dr. Daobin Yang of Yamagata University for measuring AFM. This work was partially supported by the Natural Science Foundation of China (No.11574111 to X-F.W.), JSPS KAKENHI Grant Number JP16K05826 in Scientific Research (C) (to S.S.), and JSPS KAKENHI Grant Number JP24107002 in Scientific Research on Innovative Areas “Artificial Photosynthesis (AnApple)” (to H.T.). This work was also supported by Natural Science Foundation of Jilin Province (No. 20160101303JC). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jphotochem.2017. 07.014. References [1] W.-L. Ma, C.-Y. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622. [2] C.W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183– 185. [3] B. Walker, X. Han, C. Kim, A. Sellinger, T.-Q. Nguyen, Solution-processed organic solar cells from dye molecules: an investigation of diketopyrrolopyrrole: vinazene heterojunctions, ACS Appl. Mater. Int. 4 (2012) 244–250. [4] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S.-J. Jia, S.P. Williams, Polymerfullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839–3856. [5] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photon. 6 (2012) 153–161. [6] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Polymer solar cells with enhanced open-circuit voltage and efficiency, Nat. Photon. 3 (2009) 649–653. [7] M. Urbani, M. Grätzel, M.K. Nazeeruddin, T. Torres, Meso-substituted porphyrins for dye-sensitized solar cells, Chem. Rev. 114 (2014) 12330–12396. [8] T. Higashino, H. Imahori, Porphyrins as excellent dyes for dye-sensitized solar cells: recent developments and insights, Dalton Trans. 44 (2015) 448–463. [9] L.L. Li, E.W.G. Diau, Porphyrin-sensitized solar cells, Chem. Soc. Rev. 42 (2013) 291–304. [10] W.M. Campbell, A.K. Burrell, D.L. Officer, K.W. Jolley, Porphyrins as light harvesters in the dye-sensitised TiO2 solar cell, Coord. Chem. Rev. 248 (2004) 1363–1379. [11] H. Scheer, The pigments, in: B.R. Green, W.W. Parson (Eds.), Light-Harvesting Antennas in Photosynthesis, Kluwer Academic Publishers, Dordrecht, the Netherlands, 2006, pp. 29–81. [12] T. Tomi, Y. Shibata, Y. Ikeda, S. Taniguchi, C. Haik, N. Mataga, K. Shimada, Energy and electron transfer in the photosynthetic reaction center complex of Acidiphilium rubrum containing Zn-bacteriochlorophyll a studied by femtosecond up-conversion spectroscopy, Biochim. Biophys. Acta Bioenergy 1767 (2007) 22–30. [13] H. Tamiaki, R. Shibata, T. Mizoguchi, The 17-propionate function of (bacterio) chlorophylls: biological implication of their long esterifying chains in photosynthetic systems, Photochem. Photobiol. 83 (2007) 152–162. [14] R.J. Cogdell, T.D. Howard, N.W. Isaacs, K. McLuskey, A.T. Gardiner, Structural factors which control the position of the Qy absorption band of

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