Low-Band-Gap Small Molecule for Efficient Organic Solar Cells with a

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Low-Band-Gap Small Molecule for Efficient Organic Solar Cells with a Low Energy Loss below 0.6 eV and a High Open-Circuit Voltage of over 0.9 V Daobin Yang,†,‡ Hisahiro Sasabe,*,†,‡,§ Takeshi Sano,*,†,‡,§ and Junji Kido*,†,‡,§ †

Department of Organic Device Engineering, Yamagata University, Yonezawa 992-8510, Japan Research Center for Organic Electronics, Yamagata University, Yonezawa 992-8510, Japan § Frontier Center for Organic Materials, Yamagata University, Yonezawa 992-8510, Japan ‡

S Supporting Information *

ABSTRACT: Small molecule organic solar cells (SMOSCs) have received considerable attention in recent years. However, one of the key factors limiting the performance of SMOSCs is their large energy loss (Eloss), which is typically between 0.6 and 1.0 eV, and therefore significantly higher than those of perovskite solar cells and inorganic solar cells (Eloss < 0.5 eV). Herein, we successfully report a new acceptor−donor−acceptor (A−D−A) type dimeric squaraine electron donor (D-IDTT-SQ) with a low optical band gap of 1.49 eV and deep HOMO energy level of −5.20 eV. Consequently, a high open-circuit voltage (Voc) of 0.93 V with an impressive power conversion efficiency (PCE) of 7.05% is achieved for solution-processed bulk heterojunction SMOSCs, showing an Eloss of only 0.56 eV. This is the first report wherein SMOSCs result in such a low Eloss, while simultaneously exhibiting a considerably high Voc over 0.9 V and an excellent PCE above 7.0%.

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rganic solar cells (OSCs) are considered as very promising next-generation green technology owing to their low cost, light weight, and flexibility.1,2 In comparison with their polymeric counterparts, small molecular (SM) photovoltaic materials have attracted considerable attention owing to their advantages including well-defined molecular structure, accurate molecular weight, and high purity without batch-to-batch variation.3,4 Although the record power conversion efficiency (PCE) of SM-based single-junction bulkheterojunction (BHJ) OSCs is as high as 10.1%,5 the PCEs of SMOSCs remain significantly lower than those of perovskite solar cells and inorganic solar cells.6 One of the key reasons behind the relatively low PCEs of SMOSCs is their significant loss in the energy of the opencircuit voltage (Voc), which is related to the optical band gap (Eg). The minimum energy loss in OSCs is defined as Eloss = Eg − eVoc.7 We provide a summary of all small-molecules-based OSCs exhibiting high PCEs of over 7% reported to date (a total of 50 OSCs) in Figure 1; the detailed data are shown in Table S1 (Supporting Information, SI). Nearly all Eloss of SMOSCs with high PCEs of over 7% are between 0.6 and 1.0 eV, and only two low-band-gap small-molecules-based OSCs are © XXXX American Chemical Society

Figure 1. Plot of eVoc vs Eg for the reported SMOSCs.

Received: July 10, 2017 Accepted: August 8, 2017

2021

DOI: 10.1021/acsenergylett.7b00608 ACS Energy Lett. 2017, 2, 2021−2025

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters Scheme 1. Chemical Structure of D-IDTT-SQ

simultaneously exhibits high Voc and excellent PCE values for SMOSCs. The desired compound D-IDTT-SQ was synthesized via a condensation reaction with a satisfactory yield of over 70%. The steps for synthesis and characterization are comprehensively described in the SI. D-IDTT-SQ is readily soluble in common organic solvents, such as chloroform, chlorobenzene, and dichlorobenzene, at room temperature (>20 mg mL−1). Additionally, high-quality films of D-IDTT-SQ can be obtained through spin-coating, which indicates that it is very suitable for solution-processing. As shown in Figure S1 (SI), the decomposition temperature of D-IDTT-SQ at 5% weight loss is as high as 340 °C, demonstrating that this compound exhibits high thermal stability. The normalized UV−vis−NIR absorption spectra of the DIDTT-SQ in a dilute chloroform solution (2.0 × 10−6 mol L−1) and thin film state are shown in Figure 2. The absorption band

located in the Eloss region below 0.6 eV. However, both of the Voc of their small-molecules-based OSCs are less than 0.8 V.8,9 In contrast, perovskite solar cells exhibit a very low Eloss of approximately 0.5 eV and a high Voc of over 1.0 V, thereby resulting in high PCEs of approximately 20%.10,11 Likewise, the Eloss for inorganic crystalline solar cells is normally between 0.3 and 0.6 eV.12 Additionally, some very recent reports show that polymer solar cells can obtain a very high Voc of approximately 1.0 V and PCEs of approximately10% with a low Eloss of around 0.5 eV.13−15 However, to the best of our knowledge, there is currently no report of an Eloss below 0.6 eV in small-moleculebased OSCs with a Voc above 0.9 V and PCEs of over 7% simultaneously. The main reason is the trade-off between Eg and HOMO/LUMO energy levels of the small molecular donor material.16 Therefore, it is extremely important to design and explore SMOSCs with a low band gap and a high Voc, in order to achieve a low Eloss and high efficiency. Our design strategy involves the A−D−A structural combination of large planar weak electron-donor (D) and strong electron-acceptor (A) units to afford the target lowband-gap small molecule.17,18 For this design to be successful, relatively deep HOMO levels are required for these units in order to obtain high Voc and low Eloss values.19 With regard to this, squaraines (SQ) represent an ideal building unit that meets these requirements: (1) An SQ unit possesses intense and broad absorption in vis−NIR regions, which results in a low band gap. (2) An SQ unit exhibits relatively deep HOMO and LUMO energy levels as well as low voltage loss ((AcceptorLUMO − DonorHOMO) − eVoc, approximately 0.3 eV), resulting in a high Voc. (3) The good structural tenability of SQ is convenient for molecular design and synthesis.20 The rigid and planar structure of indacenodithieno[3,2-b]thiophene (IDTT) is employed as the D unit, which can enhance πelectron delocalization and facilitate intramolecular charge transport.21,22 Hence, a novel A−D−A type dimeric squaraine, referred to as D-IDTT-SQ (Scheme 1), is designed and synthesized. In this design, each SQ unit comprises one cyano group with strong electron-withdrawing properties that can downshift the HOMO energy level, thereby promoting high Voc.23 Consequently, D-IDTT-SQ exhibits a low band gap of 1.49 eV, and a solution-processed BHJ OSC using D-IDTTSQ:PC71BM (1:5 in wt %) as the active layer simultaneously affords a very high Voc of 0.93 V, an excellent PCE of 7.05%, and a high maximum external quantum efficiency (EQEmax) value of 68%, which is the first report for low-band-gap smallmolecules-based OSCs. Interestingly, the Eloss of the prepared SMOSCs is as low as 0.56 eV, which approaches the Eloss reported for perovskite solar cells and inorganic solar cells. The experimental results state that it is possible to develop a lowband-gap small molecule with a very low E loss that

Figure 2. UV−vis−NIR absorption spectra of D-IDTT-SQ.

in the wavelength range 350−500 nm is attributed to a π−π* transition in the IDTT segment, while the broader and more red-shifted band in the 550−850 nm wavelength range arises from the intramolecular charge transfer (ICT).17,18 The absorption of the D-IDTT-SQ solution exhibits strong absorption in the 600−750 nm region, with a maximum molar extinction coefficient of 2.84 × 105 M−1 cm−1 at 705 nm. Relative to its solution, the absorption of the D-IDTT-SQ film exhibits a significantly broader and more red-shifted absorption band with a maximum optical density of 1.11 × 105 cm−1 at 722 nm. Such strong absorption in the vis−NIR region indicates that the delocalization of the π-electron between their constituent units is very effective. In this case, the efficient delocalization can be attributed to the extended conjugation that arises from the introduction of the IDTT core into the SQ molecular skeleton. The optical band gap estimates from the onset position of film absorption is as low as 1.49 eV, a value associated with low-band-gap photovoltaic materials.1 2022

DOI: 10.1021/acsenergylett.7b00608 ACS Energy Lett. 2017, 2, 2021−2025

Letter

ACS Energy Letters To estimate the energy levels of D-IDTT-SQ, its electrochemical properties are investigated by cyclic voltammetry. As shown in Figure S2 (SI), Eonset and Eonset ox red are determined as 0.40 and −1.20 V relative to Fc/Fc+, respectively. Hence, the HOMO and LUMO energy levels of D-IDTT-SQ are calculated to be −5.20 and −3.60 eV, respectively. Such a relatively deep HOMO energy level is a prerequisite for achieving high Voc.24 Moreover, the offset between the LUMO energy level of D-IDTT-SQ and PC71BM (LUMO = −4.00 eV) is 0.40 eV, which is close to the empirically determined and commonly accepted minimum value (approximately 0.30 eV), thereby suggesting that an efficient exciton dissociation and charge separation can be realized.24 The hole mobility of D-IDTT-SQ is evaluated using spacecharge-limited-current (SCLC) measurements.25 The D-IDTTSQ film exhibits a hole mobility of up to 2.51 × 10−4 cm2 V−1 s−1, which is one of the highest values among SQ-based photovoltaic materials, thereby ensuring effective charge carrier transport to electrodes.26 To evaluate the photovoltaic performance, devices with the ITO/MoO3 (8 nm)/D-IDTT-SQ:PC71BM (80 nm)/BCP (3 nm)/Al (100 nm) structure are fabricated. As shown in Figure S3 and Table S2 (SI), the optimized blend ratio of D-IDTT-SQ and PC71BM is established as 1:5 (w/w). The photovoltaic parameters of the as-cast and thermally annealed D-IDTTSQ:PC71BM-based devices are summarized in Table 1, and the

14.03 mA cm−2 is observed, leading to an improvement in PCE to 7.05%. The investigation of the charge carrier mobility of the blend films reveal that the strongly enhanced Jsc arises as a result of its increased hole mobility (2.14 × 10−4 vs 1.82 × 10−4 cm2 V−1 s−1) and electron mobility (5.24 × 10−4 vs 4.07 × 10−4 cm2 V−1 s−1) (Figure S4, SI). To the best of our knowledge, this is the first report for squaraines-based single-junction OSCs with a PCE of over 7%.29 As shown in Figure 4, the EQE curves show a broad spectrum response from 300 to 860 nm. The as-cast processed

Table 1. Photovoltaic Performances of SMOSCs

device exhibits an EQEmax value of 65% over a broad region. Upon thermal annealing, an enhancement in the EQE value is observed in the 350−770 nm region, and the EQEmax value increases to 68%. Although an Eloss below 0.6 eV can sometimes be achieved with SMOSCs, the EQE value often drops dramatically (typically below 60%), thereby resulting in inefficient charge generation that limits both Jsc and PCE.13,30 However, in this study, the photovoltaic parameters of EQE, Jsc, and PCE are kept at a considerably high level simultaneously. In summary, a new A−D−A-type dimeric squaraine DIDTT-SQ employing IDTT as a bridge unit and SQ as endcapping groups is developed for SMOSCs. This compound features a low band gap, deep HOMO and LUMO energy levels, and high hole mobility. As a consequence, D-IDTTSQ:PC71BM-based OSCs afford considerably high Voc, Jsc, PCE, and EQEmax values of up to 0.93 V, 14.03 mA cm−2, 7.05%, and 68% respectively, with a low Eloss of 0.56 eV. The Eloss of D-IDTT-SQ:PC71BM-based OSCs is close to those of perovskite solar cells and inorganic solar cells. To the best of our knowledge, D-IDTT-SQ is the first small molecule-based OSCs with an Eloss below 0.6 eV that exhibits a PCE above 7% and Voc of over 0.9 V simultaneously. This remarkable result indicates that the A−D−A-structured dimeric molecular strategy can be used as an effective way to obtain low Eloss and high Voc, thereby achieving high performance.

Devices As-cast Annealed a

Voca

(V)

0.94 (0.94) 0.93 (0.93)

a

−2

a

Jsc (mA cm )

FF

13.34 (13.20) 14.03 (13.61)

0.53 (0.52) 0.54 (0.54)

Figure 4. EQE curves of D-IDTT-SQ:PC71BM-based OSCs.

a

PCE (%) 6.65 (6.45) 7.05 (6.83)

Average values of 20 individual cells are given in parentheses.

Figure 3. J−V curves of D-IDTT-SQ:PC71BM-based OSCs.

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corresponding J−V curves are shown in Figure 3. An impressive Voc of up to 0.93 V is obtained for the BHJ SMOSCs, which is attributed to the relatively deep HOMO energy level of −5.20 eV and the low voltage loss of 0.27 eV ((PC71BMLUMO − DIDTT-SQHOMO) − eVoc).24,27,28 It is worth noting that the Eloss is as low as 0.56 eV, and the Voc of 0.93 V is extremely high for small molecules with a low band gap of less than 1.50 eV. The as-cast fabricated BHJ OSCs exhibit the best PCE of 6.65%, with a Voc of 0.94 V, a Jsc of 13.34 mA cm−2, and an FF of 0.53. When the D-IDTT-SQ:PC71BM BHJ layer is thermally annealed at 90 °C for 10 min, an enhancement in Jsc of up to

EXPERIMENTAL METHODS Electron acceptor PC71BM is purchased from Solarmer Energy, Inc. Small molecular organic solar cells are fabricated using indium−tin oxide (ITO) coated glass as substrate. The sheet resistance of ITO is 10 Ω sq−1. Patterned ITO-coated glass substrates are sequentially cleaned using detergent, deionized water, acetone, and isopropanol in an ultrasonic bath for 30 min each. The cleaned substrates are dried in an oven at 65 °C for 12 h before use. The substrates are treated by UV-ozone for 20 min and then immediately transferred into a high vacuum 2023

DOI: 10.1021/acsenergylett.7b00608 ACS Energy Lett. 2017, 2, 2021−2025

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ACKNOWLEDGMENTS We acknowledge financial support for this work by the Japan Science and Technology Agency (JST) and Ministry of Education, Culture, Sports, Science and Technology (MEXT) for financial support through the Center of Innovation (COI) Program.

chamber for deposition of 8 nm MoO3 at a pressure of less than 2 × 10−4 Pa with a rate of 0.20 Å s−1. Subsequently, photoactive layers (thickness: 80 ± 5 nm) are fabricated by spin-coating a blend of D-IDTT-SQ and PC71BM in chloroform with a total concentration of 20 mg mL−1 in a N2-filling glovebox at 35 °C. Afterward, the substrates are transferred back to the highvacuum chamber, where BCP (3 nm) and Al (100 nm) are deposited as the top electrode at pressures of less than 6 × 10−5 Pa with a rate of 0.20 Å s−1 and 2 × 10−4 Pa with a rate of 1.5− 5.0 Å s−1, respectively, resulting in a final OSC with the structure of ITO/MoO3 (8 nm)/D-IDTT-SQ:PC71BM (80 nm)/BCP (3 nm)/Al (100 nm). The active area of organic solar cells is 9 mm2. Current density−voltage (J−V) and external quantum efficiency (EQE) characterizations of organic solar cells are performed on a CEP-2000 integrated system manufactured by Bunkoukeiki Co. The integration of EQE data over an AM 1.5G solar spectrum yielded calculated Jsc values with an experimental variation of less than 5% relative to the Jsc measured under 100 mW cm−2 simulated AM 1.5G light illumination. Hole-only and electron-only devices are fabricated with the structure ITO/MoO3 (8 nm)/D-IDTT-SQ (80 nm) or DIDTT-SQ:PC71BM (80 nm)/MoO3 (8 nm)/Al (100 nm) and ITO/ZnO (20 nm)/D-IDTT-SQ:PC71BM (80 nm)/BCP (3 nm)/Al (100 nm), respectively. Mobility is extracted by fitting the current density−voltage curves using space charge limited current (SCLC); the J−V curves of the devices are plotted as J 0.5 versus V using the following equation: J=

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2 ⎛ 9 εr × ε0 × μ × V V⎞ exp⎜0.89β ⎟ 3 8 L⎠ ⎝ L

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00608. Synthetic procedures, TGA curve, summary of reported small molecules-based organic solar cells with PCE of over 7%, CV curve, photovoltaic performances, charge carrier mobility calculation (PDF)

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where J is the current density, L is the film thickness of the active layer, μh is the hole mobility, μe is the electron mobility, εr is the relative dielectric constant of the transport medium, ε0 is the permittivity of free space (8.85 × 10−12 F m−1), and V (= Vappl − Vbi) is the internal voltage in the device, where Vappl is the applied voltage to the device and Vbi is the built-in voltage due to the relative work function difference of the two electrodes.

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (T.S.). *E-mail: [email protected] (J.K.). ORCID

Daobin Yang: 0000-0001-5611-8209 Hisahiro Sasabe: 0000-0002-1312-0111 Notes

The authors declare no competing financial interest. 2024

DOI: 10.1021/acsenergylett.7b00608 ACS Energy Lett. 2017, 2, 2021−2025

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DOI: 10.1021/acsenergylett.7b00608 ACS Energy Lett. 2017, 2, 2021−2025