Novel Solid-State Solar Cell Based on Hole-Conducting MOF

Apr 4, 2017 - (15-27) It is noted that almost all the previous reports on MOF-sensitized solar cell have demonstrated solar to electrical energy conve...
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Novel Solid-State Solar Cell Based on Hole-Conducting MOFSensitizer Demonstrating Power Conversion Efficiency of 2.1% Do Young Ahn,†,‡ Deok Yeon Lee,†,‡ Chan Yong Shin,‡ Hoa Thi Bui,‡ Nabeen K. Shrestha,*,§ Lars Giebeler,⊥ Yong-Young Noh,§ and Sung-Hwan Han‡ ‡

Institute of Materials Design, Department of Chemistry, Hanyang University, Seongdong-gu, Seoul 133-791, Republic of Korea Department of Energy and Materials Engineering, Dongguk University, Seoul 100-715, Republic of Korea ⊥ Leibniz-Institute for Solid State and Materials Research (IFW) Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany §

S Supporting Information *

ABSTRACT: This work reports on designing of first successful MOFsensitizer based solid-state photovoltaic device, perticularly with a meaningful output power conversion efficiency. In this study, an intrinsically conductive cobalt-based MOFs (Co-DAPV) formed by the coordination between Co (II) ions and a redox active di(3diaminopropyl)-viologen (i.e., DAPV) ligand is investigated as sensitizer. Hall-effect measurement shows p-type conductivity of the Co-DAPV film with hole mobility of 0.017 cm2 V−1 s−1, suggesting its potential application as hole transporting sensitizer. Further, the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Co-DAPV are well-matched to be suitably employed for sensitizing TiO2. Thus, by layer-by-layer deposition of hole conducting MOF-sensitizer onto mesoporous TiO2 film, a power conversion efficiency of as high as 2.1% is achieved, which exceeds the highest efficiency values of MOF-sensitized liquid-junction solar cells reported so far. KEYWORDS: metal−organic-frameworks, sensitizer, hole conductor, solid-state, solar cell

M

through carboxylic linkage. However, unlike the dyes, MOFs can be hardly solubilized to make MOF solution. In addition, carboxylic groups are coordinated to the metal centers in carboxylic ligand based MOFs, and the remaining unsaturated carboxylic groups are generally enclosed inside the pores of the frameworks. Therefore, they are not available for coordination to the oxide surface. When deposited onto the oxide semiconductor film via layer-by-layer (LbL) approach, some of carboxylic groups of the ligand can be linked to the oxide surface in the first half cycle of the first LbL deposition cycle. Being not enough to cover all the oxide surface by a single LbL deposition cycle, several LbL cycles are essential. Thus, MOFs are not only deposited to the bared oxide surface, but they are also piled up in the subsequent LbL cycles. However, it should be noted that majority of MOFs are intrinsically electrical insulators. Consequently, the photoinduced charge carriers on top MOF layer can be hardly transferred to the beneath MOF layers, which ultimately have to be transported to the oxide surface. Recently, we have demonstrated that the hole doping of MOFs by incorporating iodine molecules into the frameworks improves the electrical conductivity, thereby improving

etal−Organic Frameworks (MOFs) are organic−inorganic hybrids with highly porous network structures possessing high surface area.1 They are traditionally employed as porous materials for gas storage, separations, and catalysis.2−4 However, the porous network structures and their functionality are tunable, and therefore MOFs can be targeted for particular applications by tailoring the chemistry of organic and inorganic components in their precursor solutions.5 This fascinated the extensive study of MOFs on diverse fields of applications including in electrochemical, electronic, and optoelectronic devices.6−14 In addition to the advantage of their solution-based processing for thin film formation, and their electrical conductivity and photoactivity depending on the nature of their metal centers or organic ligands, MOFs have also recently been characterized and explored as interfacial modifier or light-harvesting active materials in thin-film solar cells.15−27 It is noted that almost all the previous reports on MOF-sensitized solar cell have demonstrated solar to electrical energy conversion based on liquid-junction devices with a moderate photocurrent response showing power conversion efficiency less than 1.5%. In typical dye-sensitized solar cells (DSSCs), a TiO2 film is sensitized by adsorption of dye via immersion of the oxide film into the dye solution, and subsequently washing away the nonadsorbed dye, leaving the monomolecular layer of the dye anchoring to the oxide surface © XXXX American Chemical Society

Received: March 10, 2017 Accepted: April 4, 2017 Published: April 4, 2017 A

DOI: 10.1021/acsami.7b03487 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. SEM (a) top and (b) cross-sectional views of Co-DAPV film deposited by LbL technique on a glass substrate. Inset image in a shows TEM view of the Co-DAPV film, revealing large thin sheet form. (c) High-resolution TEM image of the Co-DAPV film, and the inset is the SAED image, revealing the single crystalline nature of the film. (d) XRD patterns of Co-DAPV bulk powder and thin film, showing their well-matched crystal structures.

higher electrical resistivity of 5.37 × 106 Ω cm.24 The resistivity of Co-DAPV film was further decreased to 0.24 Ω cm when the measurement was performed under 1 sun light irradiation, demonstrating the photoconductive characteristic of the CoDAPV film. The photoconductivity of the film is associated with the metal to ligand charge transfer, which involves the d−d transitions at the Co−metal centers under illumination followed by the transfer of excited electrons to the ligand due to electron accepting nature of the DAPV ligand.28 In line with the above conductivity results, the two-point probe conductivity measurement also exhibits a significantly higher conductivity of Co-DAPV film as compared to that of the iodine-induced hole-doped Co-NDC film, which in fact is intrinsically an insulator at the undoped state, as shown in Figure S1. The conductivity of Co-DAPV film was further characterized with respect to temperature, as shown in Figure S2, which demonstrates the decreasing trend of electrical resistance with increasing the temperature, revealing the semiconductor characteristic of Co-DAPV. To gain inside further the electrical properties, the Co-DAPV film was characterized using Hall-effect measurement, which showed the p-type characteristic with hole concentration and mobility of 3.35 × 10+8 cm3 and 0.017 cm2 V −1 s−1, respectively. It should be noted that this value of hole mobility is comparable to the highly doped P3HT with F4TCNQ.29 Optical absorption spectrum of the Co-DAPV film shows the maximum absorption of visible light at about 527 nm (Figure S3a). This absorption peak position is resembled to the absorption peak due to d-d transition of cobalt in CoCl2, which was used as metal ion precursor in synthesis of Co-DAPV, while di(3-diaminopropyl)-viologen (i.e., DAPV) used as organic linker does not have absorption peak at this region (Figure S3b). Further, the absorption edge of the Co-DAPV film is at about 755 nm, suggesting an energy gap of approximately 1.64 eV (Figure S3a). On the basis of the oxidation onset potential in cyclic

the photovoltaic performance of the MOF-based devices significantly.24,25 However, while designing the solid-state photovoltaic device, we found that the iodine induced conductivity was not enough to transport charge carriers in a solid state device. Herein, we employ an intrinsically p-type conductive Co-based MOFs, referred here as Co-DAPV, formed by the coordination between Co (II) ions and di(3diaminopropyl)-viologen dibromide (i.e., DAPV)28 as ligand. By constructing a p-n type heterojunction with thin mesoporous TiO2 (TiO2-mp) film, the resulting solid-state device achieved a maximum power conversion efficiency of 2.10%. MOF Film was grown by aqueous solution based layer-bylayer (LBL) technique (details are given in the Supporting Information). Figure 1a shows SEM top-view of the Co-DAPV film, which reveals that the film consists of large MOF sheets. This finding is also strongly supported by the TEM image of the film shown in the inset. The cross-sectional-view of the film in Figure 1b shows the uniform thickness of approximately 140 nm in 8 LbL deposition cycles. Figure 1c is the high-resolution TEM image of the Co-DAPV film and the inset is the SAED patterns, both of which indicate the single-crystalline nature of the MOF film. Further, the crystallinity of the MOF film was confirmed using X-ray diffraction (XRD). As evident in Figure 1d, both the bulk powder MOFs and the MOF film has dominant XRD peak at 2θ of 10.86°, suggesting the singlecrystalline nature of the Co-DAPV MOFs. In addition, XRD spectra of the bulk powder and thin film of MOFs are wellmatched to each other, suggesting that thin MOF film of CoDAPV were successfully deposited using LbL deposition approach. Four- point probe conductivity measurement of Co-DAPV film showed the electrical resistivity of 28.62 Ω cm. In contrast, it is worth noting that the iodine-induced hole-doped Co-NDC film measured under similar conditions showed a significantly B

DOI: 10.1021/acsami.7b03487 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

by deposition of about 4 μm-thick doctor bladed TiO2-mp film, which was subsequently sensitized with Co-DAPV by LbL deposition technique for 20 cycles. Figure S4a, b show the cross-sectional SEM view of the FTO/TiO2-bl/TiO2-mp electrode before and after sensitization of TiO2-mp film with Co-DAPV. The cross-section shown in Figure S4b reveals the deposition of Co-DAPV sensitizer throughout the TiO2-mp film. After deposition of approximately 100 nm-thick Au film, and illuminating under 1 sun radiation, the device demonstrated the characteristic photovoltaic J−V curve, as shown in Figure S4c. From this J−V curve, the flowing values of photovoltaic parameters were obtained: Voc = 0.62 V, Jsc = 3.51 mA cm−2, FF = 0.41, Eff = 0.78%. The poor device performance observed in this case could be due to the relatively thick TiO2mp film (i.e., ∼4 μm) that critically influences the interfacial recombination in solid-state devices, limiting the maximum power conversion efficiency. Hence, optimization on thickness of TiO2-mp film was performed by depositing the film using spin coating method at various revolution per minute (rpm) of the spin speed. Figure S5 shows the characteristic J−V curves of the devices with TiO2-mp film deposited at various rpm and sensitized with Co-DAPV for 20 LbL cycles. The values of photovoltaic parameters obtained from the J−V curves are tabulated in Table S1. Thus, the device with TiO2-mp film deposited at 2000 rpm demonstrated the highest power conversion efficiency of 1.12%. Further, keeping the 2000 rpm spin speed for deposition of TiO2-mp film, optimization on number of LbL cycle for deposition of Co-DAPV sensitizer was performed. With increasing the number of LbL cycles, the degree of visible light absorption increased, as shown by UV/ Visible absorption spectra in Figure S6, suggesting that amount of sensitizer increased with increasing the number of LbL cycles. Figure 3a shows the characteristic J−V curves of the devices with TiO2-mp film deposited at 2000 rpm and sensitized with Co-DAPV using various number of LbL cycles. The values of photovoltaic parameters obtained from the J−V curves are tabulated in Table 1. Hence, after optimizing the TiO2-mp film thickness and LbL cycles for sensitization, the cell demonstrated the best photovoltaic performance with Voc = 0.67 V, Jsc = 4.92 mA cm−2, FF = 0.57, and Eff = 2.10%. This is the maximum power conversion efficiency achieved among five device replicas, while the power conversion efficiency of as low as 1.80% was observed in this preliminary study. Further device

voltammogram (Figure S3c), the highest occupied molecular orbital (HOMO) energy state of the Co-DAPV is estimated to be at −5.40 eV, and thus revealing the lowest unoccupied molecular orbital (LUMO) energy state of −3.76 eV. The value of LUMO level is also closer to the value (i.e., −3.69 eV) indicated by the reduction onset potential in the cyclic voltammogram, revealing the reliability of energy profile of Co-DAP shown in Figure 2. On the basis of this energy profile,

Figure 2. Energy level diagram showing the band edge alignment at TiO2-DAPV heterojunction, showing favorable energy cascade for unidirectional charge transfer.

Co-DAPV can be a promising candidate for employment as a sensitizer. Further, considering the band edge positions and electron carrier property of anatase TiO2, sensitization of TiO2 film with Co-DAPV establishes an energy cascading system, whereby the photoinduced electrons can be driven to the FTO electrode, whereas holes to the Au-electrode, as shown in Figure 2. This implies that TiO2/Co-DAPV heterojunction can effectively absorb light and separate the photoinduced charge carriers, thereby working as a light-harvesting system. To test the light-harvesting property of the TiO2/Co-DAPV heterojunction, we constructed a solid-state photovoltaic device having the configuration of FTO/TiO2-bl/TiO2-mp/CoDAPV/Au by depositing approximately 50 nm-thick TiO2 blocking (TiO2-bl) layer25 on FTO glass substrate, followed

Figure 3. (a) J−V curves demonstrated under 1 sun illumination condition by mesoporous TiO2 film based solid-state solar cell sensitized with CoDAPV deposited at various LbL cycles, showing 15 LbL cycles as the optimum condition for sensitization. (b) SEM cross-sectional view of the photoanode prepared at optimum conditions, showing about 50 nm thick TiO2 blocking layer, about 500 nm thick mesoporous TiO2 film, and about 300 nm thick overlay of Co-DAPV layer. C

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ACS Applied Materials & Interfaces Table 1. Photovoltaic Parameters Obtained from J−V Curves of the Solid-State TiO2-Based Solar Cells Sensitized with Co-DAPV at Various LbL Cycles, Showing Highest Power Conversion Efficiency at 15 LbL Cycles of Sensitization no. of LbL cycles

Voc (V)

Jsc (mA cm−2)

FF

eff (%)

5 10 15 20 30

0.58 0.61 0.67 0.67 0.66

0.98 4.07 4.92 3.20 3.24

0.57 0.50 0.57 0.51 0.52

0.42 1.55 2.10 1.12 1.14

increases with the number of LbL cycles, and thereby increasing the degree of light absorption, as observed in Figure S6, Figure 3a, and Table 1 reveal that Voc, Jsc, and FF increase with increasing only up to certain LbL cycles. This implies that unnecessarily increasing the film thickness of Co-DAPV sensitizer increases the film resistance, thereby critically influencing the resistance for charge transportation between the TiO2-mp film and Co-DAPV sensitizer. The charge transport resistance at various interfaces of the devices were investigated using electrochemical impedance spectroscopy (EIS) particularly under very low incident light intensity of 0.1 sun. Under this low illumination intensity condition, conductivity of TiO2 film virtually does not change from dark condition, and is very low due to a low photogenerated electron density at the conduction band. The main feature of the EIS spectrum under this circumstance corresponds to the transport of electrons in TiO2 film.30 The impedance spectra of the devices with TiO2-mp film deposited at 2000 rpm and sensitized with Co-DAPV under various number of LbL cycles are shown in Figure 4a. The small and the big semicircles registered at lower and higher values of Z1, respectively are attributed correspondly to the charge transport resistance across FTO-TiO2 and TiO2-Co-DAPV interfaces. The size of these semicircles directly represents the charge transport resistance. Among the tested samples, the device with 15 LbL sensitization cycles shows the smallest charge transport resistance between TiO2 film and Co-DAPV sensitizer, revealing the swift charge transportation across the interface. Under this condition, highest charge collection efficiency of the photoanode is achieved, leading to the highest power conversion efficiency (Eff) of the device. This finding is also supported by the IPCE measurement of the devices. Figure 4b shows the IPCE spectra of the devices with various LbL sensitization cycles, resembling the peak and edge positions to those of the absorption spectrum of the Co-DAPV film shown in Figure S3a. Most importantly, the highest external quantum efficiency of the device is obtained at 15 LbL sensitization cycles, and the results are in line with J−V curves and Nyquist plots shown in Figures 3a and 4a, respectively. In summary, the present work demonstrates successfully for the first time on fabrication of a solid-state photovoltaic device sensitized and charge transported by a novel hole conducting MOF-sensitizer with output power conversion efficiency of 2.1%. The study shows that the optimization on film thickness

optimization and the detail study of deviations on device performance will be followed up in further work. It is worth being underlined that this is the first report demonstrating successfully on the MOF-sensitizer based solid-state solar cell with a meaningful output power conversion efficiency. Moreover, the power conversion efficiency of this solid-state device is higher than the previously reported liquid junction configuration with Ru-based and Co-based MOF sensitizers.23−26 Details on the performance of various MOFsensitizer-based solar cells reported previously have been provided in Table S2. SEM cross-sectional view of the optimized photoanode is shown in Figure 3b, which shows about 500 nm-thick TiO2-mp film and a 300 nm thick overlay of Co-DAPV on the top of the TiO2-mp film. Fine tuning of the overlay thickness, and employment of an additional appropriate hole transporting material, which can drop the energy gap between HOMO level of Co-DAPV and the valence band of TiO2, and thereby facilitating more efficient hole collection at Au-electrode, may further enhance the photovoltaic performance of the device. This aspect will be followed up in further work. SEM-top views of the optimized photoanode before and after sensitization with Co-DAPV are shown in Figure S7, which shows the existence of Co-DAPV sensitizer in the form of large thin sheets with the similar morphology as confirmed by TEM shown in Figure 1a. XRD study of the optimized photoanode before and after sensitization with Co-DAPV showed the similar XRD signals of single -crystalline Co-DAPV (Figure S8), as shown in Figure 1d. This finding further implies that the LbL sensitization of TiO2-mp film is incorporated with CoDAPV. Although the concentration of Co-DAPV sensitizer

Figure 4. (a) Nyquist plots measured under very low incident intensity of 0.1 sun illumination of the solid-state solar cells described in Figure 3, which were sensitized with Co-DAPV at various LbL cycles. Inset is the equivalent circuit employed for fitting the spectra. (b) Incident photon-tocurrent conversion efficiency (IPCE) spectra of the same solar cells. D

DOI: 10.1021/acsami.7b03487 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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of layer-by-layer deposited MOF-sensitizer can significantly improve the charge transportation across TiO2-MOF heterojunction, and thereby enhancing the power conversion efficiency. Hence, fine-tuning the relative film thickness of mesoporous TiO2 and MOFs, HOMO−LUMO levels of MOFs, and addition of an external appropriate hole-transporting material, the output device performance can plausibly be enhanced further. Further, unlike the typical MOFs, the CoDAPV used in this work was synthesized using aqueous solution of metal ion and ligand precursors. Therefore, the CoDAPV is completely stable against water, and this could be helpful for maintaining the stability of the Co-DAPV-based devices. The present work demonstrates an example to open a number of avenues for MOF-sensitized solid-state photovoltaic devices. As various metal ions and organic linkers can be coordinated through solution-based chemistry to form unlimited varieties of MOFs particularly in thin film form with tunable electrical conductivity and light harvesting properties, performance of the MOF-based solid-state devices can be enhanced unanimously in future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03487. Experimental details, additional electrical conductivity characterization, optical absorption spectra, cyclic voltammogram, SEM images, J−V curves, and tables containing photovoltaic parameters obtained from J−V curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nabeen K. Shrestha: 0000-0002-4849-4121 Lars Giebeler: 0000-0002-6703-8447 Author Contributions †

D.Y.A. and D.Y.L contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013009768). One of the authors (N. K. Shrestha) acknowledges the Alexander von Humboldt Foundation for supporting the research stay at Leibniz-Institute for Solid State and Materials Research (IFW) Dresden, Germany.

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REFERENCES

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DOI: 10.1021/acsami.7b03487 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX