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Efficient Vacuum-Deposited Ternary Organic Solar Cells with Broad Absorption, Energy Transfer, and Enhanced Hole Mobility Hyun-Sub Shim,† Chang-Ki Moon,† Jihun Kim,‡ Chun-Kai Wang,§ Bomi Sim,† Francis Lin,§ Ken-Tsung Wong,§ Yongsok Seo,‡ and Jang-Joo Kim*,† †
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea Intellectual Textile System Research Center (ITRC) and RIAM School of Materials Science and Engineering, College of Engineering, Seoul National University, Daehakro 1, Kwanakgu, Seoul, 151-744, South Korea § Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan ‡
S Supporting Information *
ABSTRACT: The use of multiple donors in an active layer is an effective way to boost the efficiency of organic solar cells by broadening their absorption window. Here, we report an efficient vacuum-deposited ternary organic photovoltaic (OPV) using two donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (DTTz) for visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) for near-infrared absorption, codeposited with C70 in the ternary layer. The ternary device achieved a power conversion efficiency of 8.02%, which is 23% higher than that of binary OPVs. This enhancement is the result of incorporating two donors with complementary absorption covering wavelengths of 350 to 900 nm with higher hole mobility in the ternary layer than that of binary layers consisting of one donor and C70, combined with energy transfer from the donor with lower hole mobility (DTTz) to that with higher mobility (DTDCTB). This structure fulfills all the requirements for efficient ternary OPVs. KEYWORDS: ternary layer, vacuum deposition, energy transfer, broad absorption, cascade energy levels
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INTRODUCTION The performance of organic photovoltaics (OPVs) has improved impressively in the past several years. The power conversion efficiency (PCE) has reached around 10% in singlejunction solar cells.1−4 Despite this progress, their efficiency is still lower than that of inorganic counterparts, mainly because of the narrow absorption band of organic materials, resulting in a lower short-circuit current density (JSC).5−7 The concept of ternary OPVs with two donors or acceptors in the active layer has been widely studied to boost efficiency by broadening the absorption spectra using a much simpler fabrication method than that used for tandem structures.8−14 To date, ternary OPVs obtained by the solution process using polymers have been successfully demonstrated to reach a PCE of 9.2%, with more than 20% efficiency enhancement compared to binary devices.12−15 In the vacuum process with small molecules, however, ternary OPVs have not shown much progress, although they have several advantages such as ease of © XXXX American Chemical Society
material purification via sublimation and no solvent use. In particular, in the solution process, different solvents are preferred in different binary devices because the selection of the solvent critically affects the film morphology.16−19 Therefore, it can be difficult to select a common solvent for the formation of the ternary layer where one more material needs to be dissolved in one solvent than in a binary layer. Vacuum-deposited ternary OPVs have only recently been reported using two donors, chloroindium phthalocyanine and boron subphthalocyanine chloride, codeposited with C60.20 The absorption range is successfully extended to the near-infrared (NIR) region because of the complementary absorption of the two donors. However, the devices showed poor performance and a PCE of 2.08%, which is lower than that of binary systems. Received: October 10, 2015 Accepted: December 30, 2015
A
DOI: 10.1021/acsami.5b09620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Chemical structures of DTTz and DTDCTB. (b) Energy levels of DTTz, DTDCTB, and C70. (c) Normalized absorbance spectra of DTTz and DTDCTB. (d) Absorbance spectra of ternary films with different compositions.
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RESULTS AND DISCUSSION The chemical structures of DTTz and DTDCTB used in the ternary OPVs are shown in Figure 1a, and their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are summarized in Figure 1b.21,22 Figure 1c displays the normalized absorption spectra of pristine DTTz and DTDCTB films. The visible region is absorbed mainly by DTTz, with an absorption peak at 530 nm, and the NIR is absorbed by DTDCTB at 670 nm. The complementary absorption of the two donors enables efficient use of the solar photon flux at wavelengths of 300 to 900 nm. In previous results, we used the two donors as subcells in tandem OPVs to achieve a PCE of 9.2%.23 The absorption profiles for ternary blend films with different compositions are shown in Figure 1d. The absorption at 450 to 650 nm is reduced and that from 650 to 900 nm is enhanced with increasing DTDCTB composition in the blend films, as expected. Figure 2a displays the current density−voltage (J−V) characteristics of binary and ternary OPVs with different compositions. The device structures are indium tin oxide (ITO)/1,1-bis(4-bis(4-methyl-phenyl)-amino-phenyl)cyclohexane (TAPC):MoO 3 (20 nm)/TAPC (3 nm)/ DTDCTB (5 nm)/active layer (50 nm)/C70 (5 nm)/ bathocuproine (BCP):C60 (5 nm)/BCP (5 nm)/Ag. The TAPC:MoO3 layer and intrinsic TAPC layer are used to enhance hole extraction from the active layer to ITO.23 To enhance the electrical properties of the devices, the BCP:C60 mixed layer is also inserted between the C70 and BCP layers.24 The composition of C70 in the active layer of the ternary OPVs is maintained at 50% because the binary devices based on DTTz and DTDCTB are both optimized at a composition of
A few requirements need to be met to improve the device performance of ternary OPVs compared to binary devices. First, the two donors or acceptors should have complementary absorption to efficiently use the solar photon flux. If the absorption overlap is large, the enhancement of JSC would be limited by the optical losses. Second, cascade energy levels are needed to ensure efficient charge transport. Without them, charges can be trapped and recombined in the active layer by the energetic barriers, reducing JSC and the fill factor (FF). In addition, energy transfer to molecules having higher mobility between two donors or acceptors in the ternary layer is favored to enhance the electrical properties of devices because charges can be transported to the electrode via molecules with higher mobility. In this paper, efficient vacuum-deposited ternary OPVs are demonstrated using two donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (DTTz) for visible absorption and 2-((7-(5-(diptolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) for NIR absorption, codeposited with C70 in the ternary layer. The ternary device achieves a PCE of 8.02%, which is 23% higher than that of binary OPVs. To the best of our knowledge, this is the highest efficiency and the first demonstration of improved device performance in vacuum-deposited ternary OPVs that is comparable to that of solution-processed devices. This enhancement is the result of incorporating two donors with complementary absorption and of increased hole mobility due to the cascade energy levels and energy transfer from the lowmobility donor to the high-mobility donor; these features meet all the requirements for efficient ternary OPVs. B
DOI: 10.1021/acsami.5b09620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) J−V characteristics and (b) IPCE of ternary OPVs with different compositions. (c) J−V characteristics and (d) IPCE of the devices as a function of the ternary layer thickness.
Table 1. Performance of Devices with Different Ternary Layer Compositionsa DTTz:DTDCTB:C70 1:0:1 0.7:0.3:1 0.5:0.5:1 0.3:0.7:1 0:1:1 a
PCE (%) 6.52 5.78 7.04 7.45 6.52
± ± ± ± ±
0.09 0.17 0.16 0.12 0.25
JSC (mA cm−2)
JSCb (mA cm−2)
± ± ± ± ±
11.86 13.21 13.86 14.56 12.85
12.61 13.22 14.00 14.64 13.17
0.14 0.29 0.19 0.18 0.17
VOC (V) 0.89 0.86 0.85 0.83 0.81
± ± ± ± ±
0.01 0.01 0.02 0.02 0.01
FF 0.58 0.51 0.60 0.61 0.61
± ± ± ± ±
0.01 0.01 0.01 0.01 0.02
RS (Ωcm2)
RP (× 105 Ωcm2)
3.05 2.25 2.62 2.03 2.28
13.95 0.68 2.54 16.56 5.45
Thickness of the ternary layer is 50 nm. bJSC is calculated from IPCE data.
complementary absorption by DTTz and DTDCTB molecules, which covers the entire visible and NIR spectra of sunlight, as shown in the incident-photon-to-electron conversion efficiency (IPCE) spectra (Figure 2b). The change in the IPCE is in good agreement with the absorption profiles shown in Figure 1d. Surprisingly, however, the IPCE at wavelengths of 400 to 650 nm remains almost constant even though the absorption by DTTz is reduced with increasing DTDCTB composition in the blend layer. This is related to the electrical properties of an active layer with energy transfer, which are discussed later. The device with the composition of 0.7:0.3:1 (DTTz:DTDCTB:C70) shows a higher field-dependent photocurrent with a low FF. This is likely related to the charge transport paths. It is difficult to form these paths at lower DTDCTB compositions because of the larger distances between molecules. In addition, charge transfer from DTDCTB to DTTz is also difficult because of the energetic barrier, especially at low electric field near the open-circuit potential. In other words, the hole mobility in the ternary layer with a low DTDCTB concentration in the solar cell must be much lower than the space-charge-limited current (SCLC) mobility, which
1:1 (donor:C70). The binary OPVs show similar efficiencies of 6.5%, as summarized in Table 1. The open-circuit voltage (VOC) of the DTTz:C70-based OPV is higher than that of the DTDCTB:C70-based OPV because of the higher gap between the HOMO level of the donor and the LUMO level of the acceptor. In the ternary devices, JSC increases significantly up to 14.64 mA cm−2 with the gradual reduction in VOC as the DTDCTB composition in the blend layer increases (Figure S2). The change in VOC is a common phenomenon when two donors or acceptors having different HOMO or LUMO levels are used in ternary OPVs.12,25−27 This can be explained by the formation of electronic alloy charge-transfer states.13,27 When the composition of the donor with the higher HOMO level is gradually increased in the blend layer, the interfacial bandgap becomes continuously smaller, which reduces VOC because VOC is proportional to the interfacial bandgap between the HOMO level of the donor and the LUMO level of the acceptor. In this system, DTDCTB has a higher HOMO level than DTTz. Therefore, VOC decreases gradually with increasing DTDCTB composition in the ternary layer. The enhancement of JSC originates in the additional photocurrent generated by the C
DOI: 10.1021/acsami.5b09620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 2. Performance of Devices with Different Ternary Layer Thicknesses nm 40 50 60 70 a
PCE (%) 6.77 7.45 8.02 7.77
± ± ± ±
0.10 0.12 0.09 0.09
JSC (mA cm−2)
JSCa (mA cm−2)
± ± ± ±
12.49 14.56 15.66 15.03
13.01 14.64 15.47 15.11
0.18 0.18 0.16 0.16
VOC (V) 0.84 0.83 0.83 0.82
± ± ± ±
0.01 0.02 0.01 0.02
FF 0.62 0.61 0.62 0.63
± ± ± ±
0.01 0.01 0.01 0.01
RS (Ωcm2)
RP (× 105 Ωcm2)
2.53 2.03 2.50 2.42
0.27 16.56 2.77 0.29
JSC is calculated from IPCE data.
was measured at a much higher electric field. As a result, the probability of charge recombination is increased in the blend layer under a given electric field, and a higher electric field is required to extract charges to the electrodes. The device structure was further optimized by varying the thickness of the ternary layer from 40 to 70 nm at the composition of 0.3:0.7:1 (DTTz:DTDCTB:C70). The device parameters are summarized in Table 2, and the results are shown in Figure 2c. JSC increases to 15.47 mA cm−2 as the thickness of the ternary layer is changed from 40 to 60 nm and decreases with further increases in the thickness, whereas VOC and the FF are not changed significantly; their values are 0.83 V and 0.62, respectively, at a thickness of 60 nm. In vacuumdeposited OPVs, the FF is reduced critically with increasing thickness of the active layer owing to the increase in the resistance of the film.28−30 In this system, however, the FF remains almost constant at a relatively high value of 0.63, although the thickness of the active layer is increased to 70 nm; thus, the electrical properties of the devices are not degraded with increasing thickness. The variation in the spectral shape and intensity of the IPCE with the thickness of the active layer originates from the optical effect, as revealed by a comparison of the experimental results with those calculated using the transfer matrix method (Figure S8). The optimized device shows a PCE of 8.02%, which represents a 23% enhancement compared to binary OPVs. To the best of our knowledge, this efficiency is the highest reported for vacuum-processed OPVs, and this is also the first demonstration of a ternary OPV having higher efficiency than binary OPVs fabricated by vacuum deposition. The transfer matrix method was used to understand the variation of the JSC value of the ternary devices with variations in the composition and thickness of the active layer.31,32 The refractive indices of ternary films with different compositions were obtained using a linear combination of binary films, as shown in Figure 3a. The absorbance spectra of films with different compositions calculated using the transfer matrix method and the refractive indices match the experimental ones in Figure S7 well, verifying that the linear combination works well in this system. This good match also indicates that there is no additional transition state or aggregation originating from interaction between two donors. The calculated IPCEs of the OPVs with different compositions without considering the electrical loss are shown in Figure 3b. The calculated IPCE is reduced at wavelengths from 450 to 650 nm and increased at 650 to 900 nm with decreasing DTTz composition in the ternary layer, which is consistent with the absorption spectra shown in Figure 1d. Interestingly, however, the calculated IPCEs differ from the experimental ones at 450 to 650 nm, the range in which absorption by DTTz molecules occurs. The experimental IPCE is not significantly reduced, although the absorption of the active layer is decreased. The difference between the experimental and calculated IPCEs can be understood in terms of energy transfer from
Figure 3. (a) Optical constants of ternary films with different compositions. (b) Calculated IPCE spectra of ternary OPVs.
DTTz to DTDCTB and increased hole mobility with increasing DTDCTB composition in the blend layers. Photoluminescence (PL) spectra of DTTz (30 nm), DTDCTB (70 nm), and DTTz:DTDCTB (3:7, 100 nm) films are shown in Figure 4. The thicknesses of the films were controlled to
Figure 4. PL spectra of intrinsic and blend films excited at a wavelength of 325 nm. The PL intensities of DTDCTB and DTTz:DTDCTB were enlarged five times for clarity. D
DOI: 10.1021/acsami.5b09620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
constant with increasing DTDCTB composition in the blend layer even though the absorption by DTTz is reduced can be understood in terms of the increase in the charge collection efficiency due to the increased hole mobility, which compensates for the reduction in the absorption due to the reduced number of DTTz molecules. All the facts reveal that the enhanced JSC of the ternary OPVs compared to binary devices can be ascribed to the broad absorption obtained by using two complementary donors covering the visible region to the NIR region and the higher hole mobility in the ternary layer than in binary layers consisting of one donor and C70, combined with energy transfer from the donor with lower hole mobility to that with higher mobility.
maintain the same number of molecules in the intrinsic and blend layers. The intrinsic DTTz film shows one PL peak at 780 nm, whereas the DTDCTB films have three peaks at 520, 745, and 880 nm. DTDCTB has absorption peaks at 305, 415, and 670 nm originating from electronic transitions from S0 to S4 and S5, S0 to S2 and S3, and S0 to S1, respectively.22 The PL peaks might be related to these transitions in the reverse direction. When DTTz is codeposited with DTDCTB, the peak of DTTz disappears, and three peaks appear at 490, 745, and 880 nm. Compared to the PL spectra of the intrinsic DTDCTB film, the first peak is blue-shifted, and the PL intensity at 880 nm is greater. The origin of the peak shift in the blend film is not clear yet. One plausible reason might be related to the increase in the distance between DTDCTB molecules when DTTz is mixed with DTDCTB, which reduces the intermolecular interaction. The increase in the PL intensity at 880 nm and lack of emission from DTTz indicates that energy transfer from DTTz to DTDCTB is efficient owing to the large overlap between the emission spectrum of DTTz and the S0 to S1 absorption of DTDCTB. The difference between the experimental and calculated IPCEs can also be understood in terms of the charge mobilities of the mixed films measured by single-carrier devices. The device structures are ITO/MoO3 (5 nm)/donor or blend layer (100 nm)/MoO3 (10 nm)/Al and Al/blend layer (100 nm)/ LiF (1 nm)/Al for hole- and electron-only devices, respectively. The mobilities extracted from the SCLC in the pristine and blend films (see Figure S9) are shown in Figure 5. The
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CONCLUSION In summary, we demonstrated remarkable enhancement of the device performance in vacuum-deposited ternary OPVs as compared to binary devices. A PCE of 8.02% was achieved, which is 23% higher than that of binary devices. The combination of DTTz and DTDCTB shows complementary absorption covering a wavelength range of 350 to 900 nm and higher hole mobility in the ternary layer than in binary layers consisting of one donor and C70, combined with energy transfer from the donor with lower hole mobility (DTTz) to that with higher mobility (DTDCTB). Thus, it satisfies all the requirements for efficient ternary OPVs. This system demonstrates the possibility of enhancing device performance by using vacuum deposition.
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Device Fabrication. A 150 nm-thick ITO-coated glass substrate with an insulator to define the active area was cleaned with acetone and isopropyl alcohol and exposed to UV−O3 before film deposition. All the materials were thermally evaporated with a base pressure of 6.0% Efficiency. Adv. Mater. 2012, 24, 2768−2773. (31) Pettersson, L. A. A.; Roman, L. S.; Inganäs, O. Modeling Photocurrent Action Spectra of Photovoltaic Devices Based on Organic Thin Films. J. Appl. Phys. 1999, 86, 487−496. (32) Shim, H.-S.; Kim, S.-Y.; Kim, J. W.; Kim, T.-M.; Lee, C.-H.; Kim, J.-J. An Efficient Interconnection Unit Composed of ElectronTransporting Layer/Metal/p-doped Hole-Transporting Layer for Tandem Organic Photovoltaics. Appl. Phys. Lett. 2013, 102, 203903.
characteristics of hole-only and electron-only devices to examine the mobility. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Brain Korea (BK) 21 Plus and the Mid-Career Researcher Program (2014R1A2A1A01002030) through a National Research Foundation (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP).
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DOI: 10.1021/acsami.5b09620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
