New D-A-A-configured Small Molecule Donors for High-Efficiency

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Organic Electronic Devices

New D-A-A-configured Small Molecule Donors for High-Efficiency Vacuum-Processed Organic Photovoltaics under Ambient Light Chia-Hsun Chen, Hao-Chun Ting, Ya-Ze Li, Yuan-Chih Lo, Pin-Hao Sher, Juen-Kai Wang, TienLung Chiu, Chi-Feng Lin, I-Sheng Hsu, Jiun-Haw Lee, Shun-Wei Liu, and Ken-Tsung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20415 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

New D-A-A-configured Small Molecule Donors for High-Efficiency Vacuum-Processed Organic Photovoltaics under Ambient Light Chia-Hsun Chen1, Hao-Chun Ting2, Ya-Ze Li3, Yuan-Chih Lo2, Pin-Hao Sher4, Juen-Kai Wang4,5, TienLung Chiu6, Chi-Feng Lin7, I-Sheng Hsu3,8, Jiun-Haw Lee1*, Shun-Wei Liu3,8*, Ken-Tsung Wong2,4* 1Graduate

Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taiwan 2Department 3Organic

of Chemistry, National Taiwan University, Taipei 10617, Taiwan

Electronics Research Center, Ming Chi University of Technology, New Taipei City 24301,

Taiwan 4Institute 5Center

of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan

for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan

6Department

of Electrical Engineering, Yuan Ze University, Taoyuan 32003, Taiwan

7Department

of Electro-Optical Engineering, National United University, Miaoli 36003, Taiwan

8Department

of Electrical Engineering, Ming Chi University of Technology, New Taipei City 24301,

Taiwan

*CORRESPONDING AUTHOR Jiun-Haw Lee, [email protected] Shun-Wei Liu, [email protected] Ken-Tsung Wong, [email protected] ACS Paragon Plus Environment

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ABSTRACT Four new donor-acceptor-acceptor (D-A-A) type molecules (DTCPB, DTCTB, DTCPBO, DTCTBO), where benzothiadiazole (BT) or benzoxadiazole (BO) serves as the central A bridging triarylamine (D) and cyano group (terminal A), have been synthesized and characterized. The intramolecular charge transfer character renders these molecules with strong visible light absorption and forms anti-parallel dimeric crystal packing with evident π-π intermolecular interactions. The characteristics of the vacuumprocessed photovoltaic device with BHJ active layer employing these molecules as electronic donors combining C70 as electronic acceptor were examined and concluded a clear structure-propertyperformance relationship. Among them, the DTCPB-based device delivers the best PCE up to 6.55% under AM 1.5 G irradiation. The study of PCE dependence on the light intensity indicates the DTCPBbased device exhibits superior exciton dissociation and less propensity of geminated recombination, which was further verified by steady photoluminescence study. The DTCPB-based device was further optimized to give an improved PCE up to 6.96% with relatively high stability under AM 1.5 G continuous light-soaking for 150 hours. This device can also perform a PCE close to 16% under a TLD840 fluorescent lamp (800 lux), indicating its promising prospect for indoor photovoltaics application.

KEYWORDS Organic photovoltaics, Vacuum process, Small molecule donor, D-A-A configuration, Ambient light, Device stability INTRODUCTION The emergences of new polymeric1-3 and small-molecule4-7 donors and non-fullerene acceptors (NFAs)8-13 have significantly facilitated the advance of organic photovoltaics (OPVs). Now, single-14-18 and multiple19,20-junction OPV cell with a peak power conversion efficiency (PCE) over 14% have been successfully achieved. It is obvious that the majority of reported high-efficiency OPV materials were designed based on the solution-processed approach. In spite of the high efficiency, the long-term stability of solution-processed OPV devices is still an unresolved and challenging issue. The use of ACS Paragon Plus Environment

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vacuum deposition can produce morphologically stable organic films for optoelectronics application that has been successfully witnessed by the great success of OLED technology.21 Therefore, vacuum deposition of active layers could be the appropriate future option for the fabrication of OPV with higher stability.22,23 However, as compared to the materials designed for popular solution-processing methods, the progress of small molecule materials suitable for vacuum-processed OPVs largely lags behind. Only limited examples that can achieve PCE over 9%,24,25 in which our molecular design strategy of highly polar electronic donors with donor-acceptor-acceptor (D-A-A) configuration made a significant contribution in this regard.26 As we are considering OPV to harvest the sunlight energy, the demands of new energy-harvesting technology that can convert environmental light energy (e.g. reflection of sunlight and artificial lighting etc.) to electric current for driving low-power consumption devices such as end nodes (sensor or actuator) for the internet of things (IoT) technology are emerging. Along with this line, high-efficiency indoor photovoltaics (IPV), which can efficiently harvest the indoorilluminated light energy (intensity range 200-1000 lux) will be an excellent solution for providing sustainable power to drive these devices. Importantly, IPVs only need to convert ambient light coming from day light effectively, fluorescent lamps, LED bulbs and halogen lamps, which are in different emission spectra, typically covering only visible wavelength region, and 100-1000 times lower of intensity as compared to sunlight. In addition, in contrast to materials with absorption extending to nearinfrared (NIR) for outdoor PVs, materials covering the absorption range between 350-650 nm with high molar extinction coefficient should be sufficiently good for IPVs. Recently, high efficiency dyesensitized solar cell (DSSC)-based IPV with PCE up to 28% has been successfully realized with an anthracene-cored dye under dim light (300 lux).27 The PCEs of DSSC-based IPVs can be improved by cocktail-type dye combination, allowing the best device achieves up to PCE of 31.8%28 under a warm white fluorescent tube light. A recent OPV-based IPV with an active layer composed of dithienobenzene-based donor and fullerene acceptor was reported by Tsoi and co-workers, giving PCE of 28% under fluorescent lamps (1000 lux).29 In addition to these organic material-based PVs, the low exciton binding energy of perovskite is particularly interesting, which should be advantageous for ACS Paragon Plus Environment

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indoor applications. In this regard, a perovskite-based IPV with a PCE of 27% under a fluorescent lamp (1000 lux) was reported by Lin and his co-workers.30 More recently, the PCE of perovskite-based IPV has been largely improved to 35.2% under a fluorescent lamp (1000 lux) by employing ionic liquid to modify the interface layer,31 indicating a bright and promising future of perovskite-based IPVs. It is optimistic to anticipate that the future large demand of sustainable energy source for low-power consumption applications will strongly stimulate the growth of IPVs technology. Since the indoor light intensity is much weaker as compared to outdoor irradiation, the typically inferior photo-stability of organic materials may not be as crucial as we ponder over conventional PVs. In addition, the demand for active materials only with strong visible light absorption greatly reduces the difficulty of molecular design of small molecules suitable for vacuum-processed OPVs. In this work, we report the syntheses and characterizations of four new small molecule donors DTCPB, DTCTB, DTCPBO, and DTCTBO stemmed from the modification of previously reported D-A-A donors,32 where benzothiadiazole (BT) or benzoxadiazole (BO) serves as the central A bridging triarylamine (D) and cyano group (terminal A). As compared to dicyanovinylene-substituted BT or BO, the weaker electron-withdrawing character of CN-substituted BT or BO -A-A block led these new donors to exhibit reduced molecular π-conjugation and dipolar characters.33,34 We adopted standard protocols established for the conventional OPVs research to examine the characteristics of vacuum-fabricated devices systematically. Based on the observed data, a clear chemical structure-physical property-device performance relationship has been established. Among them, the donor DTCPB as combined with C70 as acceptor gave the best device with a PCE up to 6.55%, open-circuit voltage (VOC) of 0.9 V, short-circuit current density (JSC) of 11.15 mA cm-2, and a fill factor (FF) of 65.4% under air mass 1.5 global (AM 1.5 G), 100 mW cm-2 solar radiation. We also examined the PCE of all devices as a function of the light intensity of a standard sunlight simulator. Over a wide range of light intensity, the DTCPB-based device exhibit higher performance and stability owing to the superior exciton dissociation and less propensity of geminated recombination, which was further verified by steady photoluminescence study. The DTCPB-based device was further optimized to improve the PCE up to 6.96%. The device was continuously lightACS Paragon Plus Environment

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soaked under AM 1.5 G for up to 150 hours, showing the superior stability. The resulting device was further investigated for its feasibility of potential IPV application. This device delivered a PCE of 15.78% (800 lux) and 14.98% (1200 lux) under a TLD-840 fluorescent lamp. To the best of knowledge, this result presents the first example of vacuum-processed high efficiency (PCE over 15%) OPV under ambient light, indicating the prospect of the small-molecule donor in IPV application.

RESULTS AND DISCUSSIONS The syntheses of small molecule donors DTCPB, DTCTB, DTCPBO and DTCTBO are depicted in Scheme 1. Pd-catalyzed Stille coupling reaction of 7-bromobenzo-1,2,5-thiadiazole-4-carbonitrile (1)35 with

4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)

phenylene

and

5-(N,N-ditolylamino)-2-(tri-n-

butylstannyl) thiophene gave DTCPB and DTCTB, respectively. The synthesis of BO-embedded donors DTCPBO and DTCTBO was easily achieved with the condensation of the aldehyde intermediates 2 and 331 with hydroxylamine hydrochloride.

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Scheme 1. The synthesis and structures of CN-terminated bipolar molecules DTCPB, DTCTB, DTCPBO and DPCTBO, and structures of homologues (DTDCPB, DTDCTB, DTDCPBO and DPDCTBO) end-capped with dicyanovinylene group.

Photophysical Properties Figure 1a depicts the absorption spectra of new D-A-A donors DTCPB, DTCTB, DTCPBO and DPCTBO in solution (CH2Cl2). The relevant data are summarized in Table 1. Obviously, the absorption maxima (λmax) of these new donors are significantly blue-shifted as compared to those of homologues end-capped with dicyanovinylene26,32 due to the reduced electron-withdrawing feature of CN-substituted BT or BO block, which also sequentially induces weaker intramolecular charge transfer (ICT) character, therefore, resulting in lower extinction coefficient. The absorption λmax of BO-based donors (DTCPBO, DTCTBO) is evidently red-shifted together with higher extinction coefficient as compared to those of BT-based donors (DTCPB, DTCTB), which can be mainly ascribed to the less electron deficient property of BT, resulting in wider optical energy gaps of BT-based cases. In addition, thiophene-bridged donors (DTCTB, DTCTBO), exhibit bathochromic shift absorption and higher extinction coefficient as compared to those of phenyl-linked donors (DTCPBO, DTCPB) due to the better quinoidal propensity of thiophene ring and smaller dihedral angle (which were discussed later in this article), leading to more efficient ICT character. The absorption spectra of these four donor molecules in the thin film are shown in Figure 1b. Basically, the absorption follows the same trends observed in solution but with red-shifted and broadening profiles because of intermolecular aggregations and/or π-π interactions typically ACS Paragon Plus Environment

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observed in the solid state. The bathochromic shift and wider spectral coverage will be beneficial for better light absorption, leading OSC device to have higher JSC. As compared to the homologues endcapped with dicyanovinylene group, such as molecules DTDCPB, DTDCTB, DTDCPBO and DPDCTBO (Scheme 1),

the λmax of these new dipolar molecules locates at shorter wavelength

-1

)

-1

(a)

3x 10

4

2x 10

4

1x 10

4

DTC PB DTC TB DTC PB O DTC TB O

0

300

400

500

600

W a velen g th (n m )

700

0.6

(b) DTC PB DTC TB DTC PB O DTC TB O

5

4

-1

4x 10

A b s o rp tio n C o e ffic ie n t (1 0 c m )

obviously due to the much weaker ICT character (Table S1, in Supporting Information).

E x tin c tio n c o e ffic ie n t ( M c m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

0.4

0.2

0.0

400

500 600 700 800 W avelen g th (n m )

900

Figure 1. The absorption coefficient spectra of dipolar molecules DTCPB, DTCPBO, DTCTB and DTCTBO (a) in dichloromethane solutions (10-5 M), (b) in thin film with a layer thickness of 50 nm deposited on the glass substrate by thermal evaporation with encapsulation under nitrogen condition.

Table 1. Summary of physical properties of dipolar molecules DTCPB, DTCPBO, DTCTB, and DTCTBO. λabs εa λabs Eox/Ered HOMOb LUMOc Egd Tme Tdf a 4 -1 -1 10 M cm eV eV eV °C °C nm nm(film) (V) DTCPB 491 1.82 515 0.46/-1.58 -5.26 -3.22 2.04 204 287 DTCTB 563 2.32 584 0.36/-1.50 -5.16 -3.30 1.86 250 292 DTCPBO 518 2.34 538 0.50/-1.42 -5.30 -3.38 1.92 188 287 DTCTBO 583 3.31 599 0.44/-1.39 -5.24 -3.41 1.83 227 276 a) Measured in dichloromethane solutions (10-5 M). b) Measured in dichloromethane solutions with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte, HOMO = –4.8 eV– e(Eoxi1/2 – EFc1/2). c) Measured in tetrahydrofuran solutions with 0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte. LUMO= –4.8 eV– e(Ered1/2 – EFc1/2). d) Calculated from the difference between HOMO and LUMO. e) Temperature obtained from DSC analysis under N2 at a heating rate of 10 °C/min. f) Temperature corresponding to 5% weight loss obtained from TGA analysis under N2 at a heating rate of 10 °C/min. ACS Paragon Plus Environment

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Theoretical calculations with density functional theory (DFT) and time-dependent DFT (TD-DFT) were conducted at a B3LYP/6-31g* level using dichloromethane as the matrix. All structural parameters for theoretical calculations were extracted from the crystal data. As summarized in Table 2, it is clearly that the HOMO is localized on the triarylamino group with limited contribution from the CN-substituted BT or –BO unit, whereas the LUMO is localized on the CN-substituted BT or –BO unit with limited contribution from the central aryl bridges (phenylene or thiophene) and the nitrogen center of the terminal triarylamino group. In addition, the TD-DFT calculations conclude that the S0 to S1 transition is a charge-transfer type transition mainly contributed from HOMO to LUMO (Table 3). In addition, the oscillator strength (ƒ) of the main transition increases as BT is replaced to BO, implying better light capturing capability in BO-based molecule. This result agrees with the observed higher extinction coefficient of BO-based cases comparing to that of BT-based counterparts. However, the calculated optical energy gap is inconsistent with the observed λmax as shown in Figure 1. The calculations reveal the stronger corresponding ground state (GS) dipole moment of BO-based molecules as compared to those of BT-based counterparts. This result is consistent with the reported stronger electronwithdrawing character of BO unit as compared to that of BT counterpart.32 The calculations also reveal the effect of bridge arene (phenylene vs. thiophene) on the dipolar features. For BT-based cases, the thiophene-bridged DTCTB shows higher GS dipole moment as compared to that of phenylene-bridged DTCPB, which is consistent to the reported data of molecules DTDCPT and DTDCTB.24 However, in CN-substituted BO system, the phenylene-bridged DTCPBO and thiophene-bridged DTCTBO exhibit comparable GS dipole moment, where DTCPBO shows slightly higher dipole that is different from the calculated trends of dicyanovinylene-capped homologues (DTDCPBO and DTDCTBO). In spite of various GS dipole moments depending on the structural features, the strong dipolar characters are beneficial for close intermolecular interactions, thus reduce the overall bulk dipole and enhance hole transport behavior.36


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Table 2. Calculated frontier orbital (HOMO/LUMO) density distributions of dipolar molecules DTCPB, DTCPBO, DTCTB and DTCTBO. Molecule

HOMO

LUMO

DTCPB

DTCTB

DTCPBO

DTCTBO

Table 3. Theoretical analyses of dipolar molecules DTCPB, DTCPBO, DTCTB and DTCTBO, and their relevant homologues. GS dipole Electronic E Oscillator Molecule Composition (Debye) transition (eV, nm) strength DTCPB 9.6 S0 -> S1 1.90 (653.9) HOMO -> LUMO (99.7 %) 0.2481 DTCTB 10.8 S0 -> S1 1.92 (645.2) HOMO -> LUMO (99.8 %) 0.4312 DTCPBO 13.7 S0 -> S1 1.98 (625.0) HOMO -> LUMO (99.9 %) 0.4887 DTCTBO 13.3 S0 -> S1 1.93 (641.0) HOMO -> LUMO (100 %) 0.5058 DTDCPB 14.5 S0 -> S1 1.88 (660.6) HOMO -> LUMO (99.7 %) 0.7587 DTDCTB 18.0 S0 -> S1 1.94 (638.6) HOMO -> LUMO (99.2 %) 0.9248 DTDCPBO 15.9 S0 -> S1 1.82 (680.4) HOMO -> LUMO (100 %) 0.9237 DTDCTBO 19.2 S0 -> S1 1.95 (635.4) HOMO -> LUMO (100 %) 1.0163 Gaussian 09, Crystal coordination, B3LYP/6-31g*, in dichloromethane


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Electrochemical Properties The electrochemical property of DTCPB, DTCTB, DTCPBO and DTCTBO was probed with cyclic voltammetry (CV). Figure 2 depicts the cyclic voltammograms of these four cyano-capped donors and the electrochemical data are summarized in Table 1. As shown, these molecules exhibit quasi-reversible oxidation and reduction behaviors, indicating their high electrochemical stability. As compared with previously reported dicyanovinylene-substituted homologous molecules (DTDCPB, DTDCPBO, DTDCTB and DTDCTBO, Scheme 1 and Table S1), cyano-containing BT or BO moiety possess the weaker electro-withdrawing ability, therefore, lead DTCPB, DTCTB, DTCPBO and DTCTBO to have higher reduction potentials, and thus higher LUMO energy levels. The oxidation potentials of these new D-A-A donors are only slightly changed, revealing the less evident effect on shifting the HOMO energy levels upon altering the terminal A unit from dicyanovinylene to cyano group. In these new donors, the thiophene-bridged cases (DTCTB, DTCTBO) exhibits lower oxidation potentials as compared to those of phenylene-linked cases (DTCPB, DTCPBO), and BO-centered cases performed lower reduction potentials than those of BT-embedded cases. These results manifest the changes of optical energy gaps (absorption profile) are dominated by the CN-substituted BO or BT -A-A block.

DTCPB DTCPBO DTCTB DTCTBO

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

+

Potential(V vs. Fc/Fc )

Figure 2. Cyclic voltammograms of dipolar molecules DTCPB, DTCPBO, DTCTB and DTCTBO. The oxidation potentials were measured in DCM solution with nBu4PF6 (0.1 M) as electrolyte and the


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reduction potentials were measured in N2-bubbled tetrahydrofuran (THF) solution with nBu4ClO4 (0.1 M) as electrolyte. Thermal Properties The thermal stability and morphological properties of these molecules (DTCPB, DTCPBO, DTCTB and DTCTBO) were investigated with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The data are summarized in Table 1. The decomposition temperatures (Td) referred to 5% weight loss of DTCPB, DTCPBO, DTCTB and DTCTBO are in the range of 276292 °C. Sharp endothermic peak correlated to the melting transition temperature (Tm) in DSC analysis was observed for individual donor molecule, indicating the crystalline characteristic of these polar molecules.

Crystallographic Analysis Crystal structures of these four new donor molecules were obtained by slow diffusion of dichloromethane/hexane (DTCPB, DTCTBO) or dichloromethane/methanol (DTCTB, DTCPBO). The relevant data for X-ray structures are summarized in Table S2. The phenylene-bridged molecules showed ortho-ortho hydrogen-hydrogen steric interactions that twist the conformation between phenylene and BT/BO ring with the dihedral angle of 27.3° for DTCPB and 25.6° for DTCPBO. In contrast, the lack of steric interactions between thiophene bridge and BT/BO ring renders rather coplanar conformations with the reduced dihedral angle between thiophene ring and BT/BO ring of 7.7° for DTCTB and 7.4° for DTCTBO. These D-A-A-configured molecules exhibit high propensity to pack into anti-parallel dimer due to the inherent dipole-dipole interactions (Fig. 3a).24,32,35,37 There are two types of anti-parallel dimeric arrangements between two neighboring DTCPB molecules. One dimeric pair packing (Fig. 3c) exhibits a face-to-face interaction between two neighboring BT units (S to S’ distance of 6.24 Å), which are also aligned into anti-parallel way with the shortest face-to-face distance calculated to be 3.47 Å, indicating evident π–π interactions. The second dimeric pair (Figure S1, Supporting Information) exhibits less co-facial stacking between two neighboring BT units (S to S’ ACS Paragon Plus Environment

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distance of 8.30 Å), in which only the phenylene rings of BT unit overlap with a face-to-face distance of (3.81 Å). In addition to the dimeric arrangement of two neighboring molecules, the packing behavior between neighboring dimeric pairs is even more crucial for governing the charge carrier transporting features.22 The two type dimeric pairs of DTCPB are arranged in two directions (Figure 3c) with herringbone-like manner, leading the crystal to have a density of 1.318 g/cm3. As the S atom of DTCPB changes to O atom in DTCPBO, the two DTCPBO molecules in close proximity were found to have less co-facial stacking even though these two phenylene-bridged donor molecules (DTCPB and DTCPBO) exhibits similarly twisted conformations. In one of the two dimeric pairs, the two BO rings of neighboring DTCPBO molecules are slightly slipped away from each other with a closet distance of 3.47 Å between two N atoms (S to S’ distance of 5.19 Å) of two neighboring BO rings (Figure 3e). The second dimeric pair arranged in different directions exhibits very similar configuration between two DTCPO molecules with slightly longer N to N’ (3.49 Å) and S to S’ (5.30 Å) distances. As compared to that of DTCPB, the packing behaviors of DTCPBO molecules are more complicate, rendering a slightly looser packing crystal with a density of 1.278 g/cm3. As view from the b-axis plane (Figure 3f), DTCPBO molecules are arranged into a lamellar structure, where a layer of DTCPBO molecules with the closest S to S’ distance of 5.30 Å is sandwiched by the layers packing along the b,c-plane with different packing manner in which the closest S to S’ distance is 5.19 Å. Closely inspect the crystal structure, one can find that the dimeric pairs of DTCPBO are arranged in four different ways within a unit cell, in which four dimeric pairs with two different packing directions are positioning on the b,c plane. In addition, there are also four dimeric pairs packing with two different directions within the middle layer (Figure S2, Supporting Information). It is evident and interesting that only one atom change (S of DTCPB to O of DTCPBO) makes significant influences on the crystal packing manners. As indicated in Figure 4, the thiophene-linked donors (DTCTB and DTCTBO) exhibit more closely packed co-facial dimeric pair because of the more coplanar molecular conformations and stronger dipolar characters. The terminal CN-group of one DTCTB molecule is positioning on top of thiophene ring of the neighboring DTCTB molecule, where a shortest face-to-face distance between two ACS Paragon Plus Environment

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neighboring molecules is 3.40 Å for DTCTB and 3.45 Å for DTCTBO, revealing close π–π interactions. Both DTCTB and DTCTBO are found to have only one type of dimeric pair, which is arranged with neighboring dimeric pair into much ordered brickwork stacking as compared to those of phenylene-bridged counterparts (DTCPB and DTCPBO), leading to denser crystal packing with a density of 1.387 g/cm3 for DTCTB and 1.327 g/cm3 for DTCTBO. The strong inherent molecular dipolar characters together with the resulting anti-parallel dimeric arrangement of two neighboring D-AA type donors will be beneficial for the efficiency of OPVs.24 (a)

(d)

(b)

(e)

(c)

(f)

Figure 3. The X-ray structures of DTCPB (top) and DTCPBO (bottom): (a)(d) dimeric pair, (b) (e) view from the long molecular axis, (c) (f) molecular packing. For clarity, the terminal p-tolyl groups are omitted for (b) (c) and (e) (f).


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(c)

(f)

Figure 4. The X-ray structures of DTCTB (top) and DTCTBO (bottom): (a)(d) dimeric pair, (b) (e) view from the long molecular axis, (c) (f) molecular packing. For clarity, the terminal p-tolyl groups are omitted for (b) (c) and (e) (f).

Photovoltaic characteristics The standard protocols well developed for conventional OPVs were employed to systematically examine the characteristics of devices with vacuum-processed bulk heterojunction active layers composing of these new D-A-A molecules as electron donors and C70 as the electron acceptor. The device structure was configured as: ITO/MoO3 (20 nm)/ donor: C70 (1:2.6, v/v) (70 nm) /BCP (7 nm)/Al (100 nm), where ITO (indium tin oxide) serves as the transparent anode, MoO3 serves as the holeextraction layer, BCP (bathocuproine) acts as the electron extraction layer and Al is the cathode. The thickness of DTCTBO-based device was 60 nm, which is 10-nm thinner than the other three cases due to strong recombination issue discussed later. Figure 5a depicts the J-V characteristics under 1-sun illumination from a simulated air mass 1.5 Global (AM 1.5G). The data of device performance are summarized in Table 2. Typically, the VOC of OPV device is linearly correlated to the energy level difference between HOMO of the electron donor and LUMO of the electron acceptor.36 From the CV ACS Paragon Plus Environment

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results, it is obvious that the phenylene-bridged D-A-A molecules (DTCPB, DTCPBO) exhibit lower HOMO, thus lead device to give higher VOC as compared to those of thiophene-linked counterparts (DTCTB, DTCTBO). In addition, the structural feature of central A unit (BT vs. BO) also modulates the HOMO energy level, in which the donor with BO moiety results in a deeper HOMO level compared to that of BT-centered ones. Therefore, the devices with BO-based donors typically performed higher VOC comparing to those of BT-based ones.38,39 The JSC values of devices are governed by the absorption spectra of electron donor and acceptor. To analyze the JSC, the external quantum efficiency (EQE) spectra were measured as shown in Figure 5b. The phenylene-bridged donors (DTCPB, DTCPBO) exhibit higher EQE as compared to those of thiophene-linked analogues (DTCTB, DTCTBO). The thin-film absorption spectra of organic electron donors strongly overlapped with the absorption spectrum of C70. Hence, the JSC values of devices contribute from both electron and acceptor materials. It is worthy to note that the absorption coefficient of C70 is rather high as compared to those of D-A-A donors, indicating its superior role-play of harvesting light energy in the active layer. The absorption spectra of four devices were measured to probe the individual contribution of donor and acceptor on JSC. As shown in Figure 5c, the absorption at short wavelength range (~500 nm) mainly contributes from C70, leading to similar profiles for these four devices. In contrast, there are evident differences on the longer absorption wavelength range (550~700 nm), which are nicely correlated to the respective absorption spectrum of four electron donors. Combine the EQE and device absorption profile; the internal quantum efficiency (IQE) of these four devices can be obtained as shown in Figure 5d. Typically, there is about 4% loss due to reflection at the interfaces (air/glass substrate and glass substrate/ITO),40 giving the theoretical maximum IQE of ~96% in the device. Among these four devices, the DTCPB-based device performs the best response with a plateau IQE value of ~90% covering from 450 to 600 nm. This result reveals that DTCPB-based device closely reaches to the theoretical IQE limit and can convert the photons (450-600 nm) to current efficiently, which will be highly beneficial for harvesting the ambient energy of interior lighting.


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The PCE value is the product of VOC, JSC, and FF. As shown in Table 4, FF values of four devices are quite different. Devices employing with donors (DTCPBO and DTCPB) containing phenylene bridge exhibit significant higher FF values as compared to those of thiophene-based donors (DTCTBO and DTCTB). It is also clear that D-A-A donors with BT as central A unit outperform the BO-embedded counterparts in term of FF value. In general, there is evident trade-off effect between VOC and JSC as the introduced electron donors possessing analogous molecular structure25. However, in this series of new D-A-A donors, the devices employing donors with longer absorption wavelength indeed exhibit lower VOC, but also accompany with slightly lower JSC mainly due to the inferior FF values as compared to those of phenylene-based donors. The best OPV device based on DTCPB as an electron donor and C70 as electron acceptor gives the VOC of 0.90 V and JSC of 11.15 mA cm-2 together with FF of 65.4% to afford PCE up to 6.55% under AM 1.5 illumination. The variations on FF values may be attributed to nano scale morphology of electron donor and acceptor materials, resulting in different exciton dissociation, geminated and/or non-geminated exciton recombination, and carrier extraction characteristics.41,42


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(a) 2

-2 -4

DTCTB DTCPB DTCPBO DTCTBO

-6 -8 -10

0.0 0.5 Voltage (V)

1.5


80 70 60 50 40 30 450

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-0.5

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-1

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Absorption Coefficient (10 cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 90 80 70 60 50 40 30 20 10 0 450


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550

Wavelengh (nm)

600

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800

Wavelength (nm)

Figure 5. (a) J-V characteristics under 1-sun illumination, (b) EQE and material absorption spectra, (c) device absorption spectra, and (d) IQE spectra of the organic solar cells with four different electron donors.

Table 4. Device performances of organic solar cells with four different electron donors. Device donor

VOC (V)

JSC (mA cm-2)

FF (%)

PCE (%)

RSH (kΩ cm2)

RS (Ω cm2)

DTCTB

0.81±0.01

10.11±0.05

53.37±0.87

4.40±0.06

0.50±0.01

14.22±1.36

DTCPB

0.90±0.01

11.15±0.12

65.42±1.05

6.55±0.10

1.38±0.18

7.74± 1.27

DTCTBO

0.87±0.01

10.33±0.15

50.32±0.71

4.54±0.11

0.44±0.01

10.96±1.43

DTCPBO

0.95±0.01

10.52±0.15

59.55±0.59

5.96±0.10

0.87±0.04

12.10±0.68

As the active layer exposures to light excitation, excitons are formed on the electron donors and/or acceptors. The excitons are possibly proceeding geminated recombination after reaching to the donor/acceptor interface for charge separation. The geminated recombination typically reduces the RSH ACS Paragon Plus Environment

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and FF value of the device. In addition, even though the excitons can be successfully dissociated at donor/acceptor interface to give polarons, which may possibly recombine during the course of transporting toward the counter electrodes, leading to non-geminated recombination. The study of device characteristics as a function of the incident light intensity is generally used to probe these recombination processes.43,44 The geminated recombination solely originates from the characteristics of organic materials, which should be independent on the light intensity. As shown in Figure 6a, at the low light intensity, the devices adopting donors with phenelyne moiety (DTCPB and DTCPBO) exhibit similar FF values (~68%), which are higher than those of thiophene-bridged ones (~60% for DTCTB and DTCTBO). This result clearly indicates that the donors with thiophene bridge are suffering from the intrinsic geminated recombination. As the light intensity increases, the FF of device based on donors (DTCPB, DTCTB and DTCPBO) subsequently increases and then starts to decrease as the light intensity reaches to around 0.1 sun. Obviously, the low FF of DTCTBO-based device is ascribed to the significant geminated and non-geminated recombinations. This FF-light intensity correlations also reveal the superior character of the donor with phenylene-bridge and/or BT system for suppressing exciton recombinations as compared to that of donors with thiophene-bridge and/or BO system, agreeing with the observed results of other D-A-A donors reported previously.24,32 As indicated by the X-ray structures, the coplanar conformation observed in thiophene-bridged donors (DTCTB and DTCTBO) renders close and order intermolecular interactions, which may possibly facilitate the formation of larger donor grains during the vacuum deposition for the better exciton recombinations.24 It is intriguing to see that the twisted conformation between the phenylene-bridge and BT (or BO) ring does not interrupt the formation of anti-parallel dimeric pair, but severely influences on the pair-to-pair arrangements. The balance between the close intermolecular interactions within the dimeric structure and slightly disordered inter-pair packing manners may render DTCPB to form suitable nanomorphology that can reduce the exciton recombinations. Therefore, the DTCPB-based device exhibits superior exciton dissociation capability (lowest geminated recombination) and reduced propensity for non-geminated recombination, giving the highest FF, JSC and the highest PCE. Figure 6b depicts the ACS Paragon Plus Environment

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PCE of four devices as a function of light intensity, in which the PCE of the DTCPB-based device can be improved to ~7% under low light intensity (0.3-0.8 sun).

75

(a)

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70 65

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PCE (%)

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FF (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


55 DTCPB DTCTB DTCPBO DTCTBO

50 45 40 1E-3

0.01

5 4

DTCPB DTCTB DTCPBO DTCTBO

3 0.1

1

2

0.0

Intensity (sun)

0.5

1.0

Intensity (sun)

Figure 6. (a) FF and (b) PCE of organic solar cells under different light intensity illumination.

The PCE of the OPV devices was determined by VOC, JSC, and FF, which are related to RS and RSH. Among the optimized OPV devices with these four donors, DTCPB was the best one. The VOC reached the second-high level (0.90 V), as predicted from the HOMO levels. Although DTCPB exhibits the shortest absorption wavelength and lowest absorption coefficient, it can effectively dissociate the excitons as it contacts to C70 (as shown in IQE spectra). Such a trade-off resulted in the highest JSC (11.15 mA/cm2), but not a big difference with the others (~10 mA/cm2). One should note that the major difference came from the FF ranging from 50.32% to 65.43%, resulting in 30.0% difference in PCE, which was the key for the highest PCE of DTCPB-based OPV device. From light-intensity experiment, it pointed out that carrier transport and collection in this device were superior than others with less geminated and non-geminated recombination, which also echoed the results of highest RSH and lowest RS value in Table 2. From the IQE measurement, it implied that the efficiency of the whole process from exciton dissociation, carrier transport, and collection reached almost 100% at a certain wavelength. In the following, we focus on the exciton dissociation of DTCPB-based thin films.


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In addition to the weak tendency of exciton recombinations observed in DTCPB, the exciton dissociation between DTCPB and C70 was also probed with the photophysical study. Figure 7a shows the steady-state photoluminescence (PL) spectra of the pristine DTCPB film as well as the DTCPB:C70 (1:1) mixed film. It is worthy to note that the D/A ratio is different from the device because of the emission intensity of DTCPB:C70 (1:2.6) blend was too weak to be detected, indicating the superior dissociation capability as the ratio of C70 increases in DTCPB:C70 blend. The DTCPB film exhibits a strong PL with the peak centered at 698 nm, which is ~20 nm red-shifted as compared to that observed in CHCl3 solution. In addition to the main emission, a weak emission at ~900 nm (inset in Figure 7a) was also detected, which is assigned to the excimer emission due to molecular stacking in solid state.41 Upon introducing C70, the main emission of DTCPB was significantly quenched, giving the dual emission spectrum as shown in the Figure 7b. The peak at 691 nm was assigned to the residual emission from DTCPB monomer, whereas the emission centered at 913 nm was ascribed to the DTCPB excimer emission. This result implies that the excitons in DTCPB are almost completely dissociated at the DTCPB:C70 interface even with the lower C70 ratio (DTCPB:C70 = 1:1). The excimer emission remained in the DTCPB:C70 blend indicates the less efficient exciton dissociation between DTCPB excimer and C70. This result is consistent with the reported observation of mixed film composing of DA-A type electronic donor and fullerene acceptor.45 The excimer formation reveals the possibility of forming molecular aggregations and indicates that DTCPB molecules are not uniformly dispersed in the DTCPB:C70 blend. In addition, it is obvious that the formation of excimer leads to a reduced optical energy gap as indicated by the evident red-shift emission as compared to that of monomeric counterpart. The dimeric structure has a new and lower-lying LUMO energy level, which renders the excimer excitons could not be smoothly diffused within the donor domain, and therefore, the excimer emission

remained in the DTCPB:C70 blend accordingly. The PL intensity of the mixed film is substantially decreased by 94.3% as compared to pristine DTCPB film, which again pointed out the efficient exciton separation in DTCPB:C70 blend film and hence high IQE (> 88% between 450 to 604 nm), as shown in Figure 5d.46 ACS Paragon Plus Environment

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Figure 7. (a) and (b) PL spectra of DTCPB neat film and mixed film (DTCPB:C70 = 1:1) with different intensity scale. Inset of (a): zoom-in PL spectrum of DTCPB neat film at long wavelength (800-1000 nm).

The screening on device characteristics concludes that DTCPB is the best donor among these four small molecules (DTCPB, DTCTB, DTCPBO, and DTCTBO). In order to study the stability of the DTCPB-based device and to evaluate the feasibility for the application under daily lighting condition, the active materials DTCPB and C70 were purified by sublimation before they were subject to further device optimization with the more advanced semi-automated vacuum fabrication line. The device was then encapsulated with photo-cured (365 nm) epoxy adhesive, which was typically employed as the protection layer of OLED devices. 47 The optimized results are plotted in Figure S3 and summarized in Table S3 and S4 (Supporting Information). The obtained best device can achieve a PCE up to 6.96%, JSC of 11.36 mA/cm2, VOC of 0.91 V, and FF of 67.28% with the device structure configured as ITO/MoO3 (15 nm)/ DTCPB: C70 (1:2.5, v/v) (80 nm) /BCP (8 nm)/Ag (120 nm), where the active area is 4.0 mm2. The encapsulated device operating with the maximum output condition was then examined for the stability under continuous light soaking with AM 1.5 G solar simulator in an air-conditioned room set at 25 °C for 150 hours. The real-time device characteristics were recorded accordingly, and the results are presented in Figure 8a. As shown, the initial device performances, particularly the VOC, a


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slight decline for the first 25 hr, which is consistent with the burn-in process generally observed in most of OPVs.48,49 After then, the device reaches a stable state with a PCE around 5.0 % for the rest of test times. This result clearly indicates that the device based on DTCPB:C70 blend as an active layer is relatively robust even with enforced light treatment. We believe that the photo stability should be sufficiently enough if we only consider this device for harvesting the energy from the ambient lights. The IPCE spectrum of the optimized and encapsulated device nicely covers the whole emission spectrum of a TLD-840 fluorescent lamp as shown in Figure 8b. The J-V characteristics under different luminous intensities are depicted in Figure 8c, and the data are summarized in Table 5. As indicated, the device exhibits excellent performance, in which a PCE can reach to 15.78% under 232.70 μW cm-2 (800 Lux) irradiation of a TLD-840 fluorescent lamp, giving the maximum output power of 36.63 μW/cm2. The achieved high efficiency can be reasonably attributed to the matched light response and the superior exciton dissociation propensity of DTCPB:C70 blend. To the best of our knowledge, this is for the first time a vacuum-processed OPV showing rather high stability and high efficiency under indoor lighting condition.

Table 5. The characteristics of the DTCPB-based device under a TLD-840 fluorescent lamp with different intensity. Irradiation VOC JSC FF PCE Pin Pmax intensity (lux) (%) (%) (V) (μA cm-2) (μW cm-2) (μW cm-2) 200 0.66 21.69 54.20 13.37 58.02 7.76 400 0.69 41.95 53.11 13.24 116.05 15.37 600 0.70 59.37 65.02 15.52 174.07 27.02 800 0.72 79.23 64.21 15.78 232.10 36.63 1000 0.73 99.18 62.79 15.67 290.12 45.46 1200 0.74 114.78 61.42 14.98 348.15 52.17


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Figure 8. (a) The characteristics of the optimized DTCPB-based device under continuous AM 1.5 G light-soaking. (b) The IPCE spectrum of the optimized DTCPB-based device and emission spectrum of the TLD-840 fluorescent lamp. (c) The J-V profile of DTCPB-based device under TLD-840 fluorescent light with different intensity.

CONCLUSION Extend from our previously reported successful D-A-A type small molecular donors, four new molecules (DTCPB, DTCTB, DTCPBO, DTCTBO) with a weaker electron-withdrawing cyano (CN) group (as compared to that of dicyanovinylene) as the terminal A group, benzothiadiazole (BT) or benzoxadiazole (BO) as central A unit and triarylamine as D have been synthesized and characterized. The physical properties such as absorption ranges and HOMO/LUMO levels can be feasibly tuned by ACS Paragon Plus Environment

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the structural features of central A units (BT vs. BO) and triarylamines (thiophene vs. phenylene). Among them, DTCTBO with the CN-substituted BO together with thiophene-based triarylamine exhibits the most red-shifted absorption, whereas DTCPB with phenylene and BT units shows the most blue-shifted absorption. X-ray analyses indicate that the phenylene-bridged molecules (DTCPB, DTCPBO) show twisted conformation between phenylene ring and BT/BO ring due to ortho-ortho steric interactions, whereas the lack of steric interactions leads thiophene-based molecules (DTCTB, DTCTBO) to have coplanar conformations between thiophene ring and BT/BO ring. In addition, the strong dipolar features lead these molecules to pack with anti-parallel dimeric configuration. There are various dimeric pairs arranging into less order packing in DTCPB and DTCPBO crystals, whereas DTCTB and DTCTBO exhibit only one type dimeric pair structure and much order crystal packing that leads to the higher propensity for the exciton recombinations. The device characteristics concluded that DTCPB is the best electronic donor among these new molecules due to the superior exciton dissociation and the less propensity of geminated recombination supported by the PCE and light intensity correlations and steady-state photoluminescence study. The DTCPB-based device was further optimized with vacuum-purified active materials and semi-automation processes to give an improved PCE up to 6.96% with high stability under AM 1.5 G irradiation. The visible light absorption of DTCPB is highly beneficial for indoor photovoltaics application. The optimized device can perform PCE of 13-16% from 200 lux to 1200 lux under the TLD-840 fluorescent lamp. The results reported in this work indicate that small molecules with strong visible light absorption and limited molecular weight are promising candidates for high efficiency and high stability vacuum-processed OPVs as we consider the energy harvesting of the indoor ambient lighting.

EXPERIMENT Device fabrication In this work, the D-A-A type donor material DTCPB was synthesized in the laboratory. Other materials, MoO3, C70, bathocuproine (BCP), and Al were purchased from Sigma-Aldrich. All the ACS Paragon Plus Environment

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organic materials had been sublimated twice by using our homemade graded purification system with high vacuum circumstance of ~ 8  10-6 torr. Before the device fabrication, the ITO substrate was sequentially soaked in solutions, i.e., acetone, isopropyl, and deionized water. Note that such cleaning process was used in the ultrasonic vibration tank with 5 min and dried by nitrogen with 5 N pressure. For OPV device preparation, the thin-films were deposited by thermal evaporation under a high pressure of < 5 10-6 torr. The thickness of each layer was monitored by in-situ quartz crystal monitor, which was calibrated by a surface profiler (Dektak XT). The size of anode and cathode pads defined our device's active area of 4 mm2. After device fabrication process, the devices were transferred into a nitrogen-filled glove box with low oxygen and moisture (< 0.1 ppm). Then, all devices were appropriately encapsulated using UV-curable epoxy resin (Everwide Chemical Corporation Limited EXC345) and getter-attached cover glass to protect our OPV devices.

Characterization The absorption spectrum of the organic layer was determined using an ultraviolet (UV)-visible spectrophotometer (Thermo Scientific Evolution 220). The electrical characteristics of our OPV devices were characterized using a source meter (Keithley 2401) under dark and AM 1.5G solar illumination (Newport 91160A) at 100 mW cm-2, respectively. To confirm the repeatability of our experiment, the measuring result is averaged by using “8” devices to calculate the standard deviation. The EQE spectra were recorded with a monochromator (Newport 74100) and lock-in amplifier Signal Recovery 7265) chopped at 250 Hz. All other measurements were operated automatically by the software programmed by Labview. For evaluating the electrical characteristics of indoor OPVs, the devices were measured using commercial indoor PV measurement system (CMS-PV101) with a fluorescent lamp (PHILIPS TLD 18W/840) and Keithley 2401, which designed by Center for Measurement Standards, Industrial Technology Research Institute (ITRI, Taiwan). In this measurement system, the light intensity of the fluorescent lamp can be easily controlled from 0 to 2500 lux by programmed software, while the nonuniformity and temporal instability were kept at

New D-A-A-configured Small Molecule Donors for High-Efficiency

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