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Effect of the Molecular Configuration of Perylene Diimide Acceptors on Charge Transfer and Device Performance Jianfei Qu, Zhao Mu, Hanjian Lai, Mo Xie, Longzhu Liu, Wei Lu, Wei Chen, and Feng He ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00277 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Energy Materials

Article

Effect of the Molecular Configuration of Perylene Diimide Acceptors on Charge Transfer and Device Performance Jianfei Qua, Zhao Mua, Hanjian Laia, Mo Xiea, Longzhu Liua, Wei Lua, Wei Chenb,c,* and Feng Hea∗ a

Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055,

P. R. China. b

Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois, 60439, United States

c

Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois, 60637, United States

KEYWORDS: perylene diimides, organic solar cells, non-fullerene acceptor, molecular configuration, steric hindrance, charge transfer

ABSTRACT: Three perylene diimides (PDI)-based small molecules, T2-SePDI2, T3B-SePDI3, and T4B-SePDI4, with different molecular configurations are synthesized. Due to a large steric

ACS Paragon Plus Environment

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hindrance, the molecular configuration of T3B-SePDI3 is the most distorted, followed by T4BSePDI4, while T2-SePDI2 shows the smallest steric hindrance. Inverted bulk heterojunction solar cells based on T3B-SePDI3 and PBDB-T show the highest power conversion efficiency (PCE) of 5.82% with an open-circuit voltage of 0.98 V, a high short-circuit current density of 10.52 mA/cm2, and a fill factor of 56.31%. The PCEs of the T2-SePDI2- and T4B-SePDI4based devices are 4.10% and 5.10%, respectively. The results demonstrate that the molecular configuration of the PDI-based small molecule acceptor is critical and that increasing the steric hindrance is helpful in suppressing aggregation and improving device performance.

INTRODUCTION Bulk heterojunction (BHJ) organic solar cells (OSCs) typically composed of a polymer donor and a fullerene derivative acceptor have attracted increasing attention due to their low-cost, lightweight, and flexibility.[1-5] The power conversion efficiency (PCE) of a single-junction OSC based on fullerene derivatives has been reported to exceed 11%.[6] However, fullerene derivatives possess some deficiencies, such as weak absorption in the visible region, high cost for synthesis and purification, and the difficulty in tuning of the energy levels.[7-10] In contrast, non-fullerene small molecule acceptors can show strong absorption in the visible region, relatively simple synthesis and purification and easy control of their energy levels through tuning of the molecular structure; therefore, such acceptor materials have attracted more and more attention.[11-19] Perylene diimide (PDI) derivatives are the most common acceptor units because of their high electron affinity, high electron mobility, and excellent chemical, thermal, and photochemical stability.[20-28] However, PDI derivatives tend to form microscale or sub-microscale aggregates,


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which are not beneficial to charge separation and transport.[29,30] Two strategies have been adopted to suppress aggregation. One is to link two or three PDI units directly together at the imide position or bay position to reduce the strong π-π interactions.[31-36] The other is to couple several PDI units to other electron-rich units at their α or bay position to afford an A-D-A molecule.[37-45] Most studies have shown that the twisted molecular configuration can effectively inhibit the aggregation of PDI derivatives, which can form homogeneous phase separation morphology after blending with the donor materials, thereby facilitating the separation and transportation of charge, then improving the device performance.[46-48] The PCE of a nonfullerene organic solar cell based on PDI derivatives can exceed 9%.[38,

45]

However, the

relationship between the molecular configuration and device performance is not yet clear. It’s not sure whether the most distorted molecular configuration is beneficial to enhance the device performance, or moderate distorted molecular configuration is better. Herein, we synthesized a dimer, trimer, and tetramer of PDI, named T2-SePDI2, T3BSePDI3, and T4B-SePDI4, respectively. These molecules show very different molecular configurations due to steric hindrance. T3B-SePDI3 shows the largest steric hindrance and consequently the most distorted molecular configuration, with a biggest dihedral angle of 85°, while T2-SePDI2 was much planar, with a smallest dihedral angle of 30°. T4B-SePDI4 showed a moderate dihedral angle of 72°. Non-fullerene OSCs were fabricated using the polymer PBDBT as donor, and three small molecules as acceptors. T3B-SePDI3 acceptor-based solar cells showed the highest PCE of 5.82% compared to the T2-SePDI2 and T4B-SePDI4 based OSCs. The improved device performance achieved using T3B-SePDI3 mainly results from appropriate aggregation, high electron mobility, and good film morphology obtained with the polymer donor PBDB-T.


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RESULTS AND DISCUSSION Synthesis of PDI-Based Small Molecules. The chemical structures of T2-SePDI2, T3BSePDI3, T4B-SePDI4, and PBDB-T are shown in Chart 1. The synthetic routes are shown in Scheme 1. Atomic selenium, with its large outer shell electron cloud, was introduced into the bay position of PDI to enhance orbital overlap and increase the charge carrier mobility.[49-51] T2SePDI2, T3B-SePDI3, and T4B-SePDI4 were synthesized with relatively high yield by Stille coupling

of

[1,12-b,c,d]selenophene-6-bromoperylene

diimide

(2)

with

5,5’-

bis(trimethylstannyl)-2,2’-bithiophene (5), 1,3,5-tris(5-(trimethylstannyl)thiophen-2-yl)benzene (4), and 1,2,4,5-tetrakis(5-(trimethylstannyl)thiophen-2-yl)benzene (3), respectively. The final products were further purified by recycling preparative high-performance liquid chromatography after column chromatography purification on silica gel.


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Chart 1. Chemical structures of T2-SePDI2, T3B-SePDI3, T4B-SePDI4, and PBDB-T. Scheme 1. The synthetic routes to T2-SePDI2, T3B-SePDI3, and T4B-SePDI4.


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Molecular Configuration Study. The optimized molecular configurations were calculated by density functional theory (DFT) at the B3LYP/6-31G (d, p) level. As shown in Figure 1, the two PDI units in T2-SePDI2 show the smallest dihedral angle of 30°, with both units perpendicular to the adjacent thiophene units. For T3B-SePDI3, two of the three PDI units show a relatively co-planar geometry with a dihedral angle of 16°, while the third PDI unit forms a large distorted angle of 67° and 85° with the other two units. The dihedral angle of 85° indicates that the two PDI units are nearly perpendicular. For T4B-SePDI4, the dihedral angles between the two pairs of PDI units, closest to each other, are 18° and 29°, respectively, and the dihedral angles between the two pairs of PDI units farthest from each other are 53° and 72°, respectively. In general, the molecular configuration of T3B-SePDI3 is the most twisted, followed by T4B-SePDI4, while T2-SePDI2 showed the smallest steric hindrance. The distortion in the molecular configuration can significantly influence the aggregation properties in the solid state after blending with the donor material, which can affect the film morphologies, charge carrier mobilities, and photovoltaic properties.

Figure 1. Top and side views of the optimized geometry of T2-SePDI2, T3B-SePDI3 and T4B-


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SePDI4 using DFT calculations at the B3LYP/6-31G (d) level. To simplify the calculation, hexylheptyl substituents were replaced by methylethyl groups. Thermal, Optical and Electrochemical Properties. The thermal properties of three small molecules were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and are shown in Figure S7 in the Support Information. T2-SePDI2, T3BSePDI3 and T4B-SePDI4 exhibited good thermal stability with a decomposition temperature of 399, 402, 401 °C, respectively. T2-SePDI2 showed a melting peak at 275 °C, while no obvious melting peak was observed for T3B-SePDI3 and T4B-SePDI4 due to their relatively large

1.0 (a)

T2-SePDI2 T3B-SePDI3 T4B-SePDI4

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Normalized Absorbance (a.u.)

molecular weights.

Normalized Absorbance (a.u.)

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


1.0 (b)


0.8 0.6 0.4 0.2 0.0 300

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Figure 2. (a) Normalized UV-vis absorption spectra of T2-SePDI2, T3B-SePDI3 and T4BSePDI4 in chlorobenzene solutions. (b) Normalized UV-vis absorption spectra of T2-SePDI2, T3B-SePDI3, and T4B-SePDI4 in films. The ultraviolet visible (UV-vis) absorption spectra of T2-SePDI2, T3B-SePDI3, and T4BSePDI4 measured in chlorobenzene solution (10−5 mol/L) and solid thin films are shown in Figure 2a and 2b. In dilute chlorobenzene solution, T2-SePDI2, T3B-SePDI3, and T4BSePDI4 show strong absorption in the wavelength range of 400-600 nm with a maximum


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extinction coefficient of 8.36×104, 1.24×105, and 1.52×105 L·mol-1·cm-1, respectively. The absorption spectra of T2-SePDI2, T3B-SePDI3 and T4B-SePDI4 films show a slight redshift compared to the spectra recorded in solution. The optical band gaps of T2-SePDI2, T3BSePDI3 and T4B-SePDI4 films are estimated from the onset of absorption (648, 589, and 598 nm) to be 1.91, 2.11, and 2.07 eV, respectively. Compared to T3B-SePDI3 and T4B-SePDI4, the absorption spectra of T2-SePDI2 both in solution and film are significantly redshifted due to its strong intermolecular interaction. The electrochemical properties were characterized using cyclic voltammetry with the films deposited onto a glassy carbon electrode in 0.1 M [n-Bu4N]+[PF6]− acetonitrile solution; the corresponding redox curves are shown in Figure S8 in the Supporting Information. LUMO energy levels were calculated using the equation LUMO = − (4.80 + Ere) eV, where Ere is the reduction onset potential; the ferrocene/ferrocenium (Fc/Fc+) pair was used for calibration. As shown in Table 1, the LUMO energy levels of T2-SePDI2, T3B-SePDI3, and T4B-SePDI4 are estimated to be -3.75, -3.77, and -3.73 eV, respectively. The HOMO energy levels were calculated to be -5.66, -5.88, 5.80 eV, respectively, using the equation EHOMO = ELUMO - Eg. Table 1. Optical and electronic properties of T2-SePDI2, T3B-SePDI3, and T4B-SePDI4.

λ sol max

εsol

λ film max

λ film onset

Ega

ELUMOb

EHOMOc

(nm)

( L·mol-1·cm-1)

(nm)

(nm)

(eV)

(eV)

(eV)

T2-SePDI2

478, 508

8.36×104

482, 510

648

1.91

-3.75

-5.66

T3B-SePDI3

477, 513

1.24×105

481, 513

589

2.11

-3.77

-5.88

T4B-SePDI4

478, 514

1.52×105

479, 515

598

2.07

-3.73

-5.80

Materials

a

The band gap was obtained from the onset of UV-vis absorption in each film. bThe LUMO


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energy level was measured by CV. cThe HOMO energy level was calculated using the following equation: EHOMO = ELUMO - Eg. Photovoltaic Properties of Non-Fullerene OSCs. The photovoltaic properties of T2SePDI2, T3B-SePDI3, and T4B-SePDI4 were investigated using an inverted device structure of ITO/ZnO/PBDB-T:Acceptor/MoO3/Ag. The optimized film thickness for the polymer blend was determined to be 82, 90, and 85 nm, respectively. The characteristic current density-voltage (J-V) curves for the optimized devices are shown in Figure 3a. The photovoltaic parameters of the devices are summarized in Table 2. All three molecules showed the best device performance with a D/A ratio of 1:1 (total concentration of 20 mg/mL), and additive 1,8-diiodooctane (DIO) of 0.3% in o-dichlorobenzene. The PBDB-T:T2-SePDI2-based solar cell device showed the lowest PCE of 4.10% with a VOC of 1.00 V, JSC of 7.88 mA/cm2, and FF of 52.13%. The relatively small JSC is consistent with an observed lower photoluminescence quenching (70%), as shown in Figure S9 in the Supporting Information. The solar cell device with a PBDB-T:T3BSePDI3 blend film gave the highest PCE of 5.82%, with a VOC of 0.98 V, JSC of 10.52 mA/cm2, and FF of 56.31%. In comparison, the PBDB-T:T4B-SePDI4-based device showed a moderate PCE of 5.10% with a VOC of 0.99 V, JSC of 8.90 mA/cm2, and FF of 57.74%. The external quantum efficiency (EQE) of T2-SePDI2-, T3B-SePDI3-, and T4B-SePDI4-based devices were also investigated to evaluate the photoresponse of the blends, as shown in Figure 3b. All devices showed a high photo-electron response ranging from 400 to 700 nm. It is noted that the T3BSePDI3-based device shows a high EQE of 65% approximately 630 nm, which is consistent with the higher JSC observed compared to T2-SePDI2 and T4B-SePDI4. According to the theoretical calculations, T3B-SePDI3 has the most twisted backbone and the worst coplanarity, indicating poor packing. However, the PBDB-T:T3B-SePDI3 blend exhibits the highest PCE of 5.82%.


9


This may indicate that reducing the aggregation of small molecule acceptors based on perylene diimide is favorable for the preparation of high-performance solar cell devices. In order to obtain insights how molecular configuration effect the device performance, we further investigate the electron and hole mobilities, film morphology, crystallization, and bimolecular combination of the blend films. 3

(a)

(b)

60

0 -3

EQE(%)

2

Current density (mA/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

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-6 T2-SePDI2 T3B-SePDI3 T4B-SePDI4

-9 -12

0.0

0.3 0.6 Voltage (V)

0.9

1.2

40

20

0 300


400

500 600 700 Wavelength (nm)

800

Figure 3. J-V characteristics (a) and EQE spectra (b) of the PBDT-T:T2-SePDI2, PBDTT:T3B-SePDI3 and PBDT-T:T4B-SePDI4 devices. Table 2. Photovoltaic performances and charge carrier mobilities of the PBDB-T: Acceptorbased devices.

VOC

JSC

FF

PCEmax

PCEavg

µh

µe

(V)

(mA/cm2)

(%)

(%)

(%)

(cm2·V-1·s-1)

(cm2·V-1·s-1)

T2-SePDI2

1.00

7.88

52.13

4.10

4.00

1.40×10-4

7.49×10-5

1.87

T3B-SePDI3

0.98

10.52

56.31

5.82

5.62

5.52×10-4

6.09×10-4

0.91

T4B-SePDI4

0.99

8.90

57.74

5.10

4.93

2.15×10-4

1.68×10-4

1.28

Acceptors

µh/µe

Hole and Electron Mobilities. The charge transport properties of the blend films were


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investigated by the space charge limited current (SCLC) method. The hole-only device structure of ITO/PEDOT:PSS/blend films/MoO3/Ag and electron-only device structure of ITO/ZnO/blend films/Ca/Al were built and characterized. Typical J-V curves are shown in Figure 4 and the mobility parameters are summarized in Table 2. The hole mobilities (µh) of T2-SePDI2-, T3BSePDI3-, and T4B-SePDI4-based blend films are 1.40×10-4, 5.52×10-4, and 2.15×10-4 cm2·V-1·s1

, respectively, while the electron mobilities (µe) are 7.49×10-5, 6.09×10-4, and 1.68×10-4,

respectively. Although the molecular configuration of T3B-SePDI3 is the most distorted, PBDTT:T3B-SePDI3 blend films show the highest hole and electron mobilities and have a better hole/electron balance compared to PBDT-T:T2-SePDI2 and PBDT-T:T4B-SePDI4 blend films, which results in the best device performance. 4

2

2

10

10

(a)


3


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


2

10

1

10

0

10

0.1


1 VAPPL-VBI (V)

(b)

3

10

2

10

1

10


0

10

-1

10

0.1

1 VAPPL-VBI (V)

Figure 4. Double logarithmic plots of the current density (J) versus voltage (V) curves for the hole-only (a) and electron-only (b) devices. Morphology Investigation. We investigated the morphologies of the blend films by atomic force microcopy (AFM) and Transmission electron microscopy (TEM). As shown in Figure 5, large spherical aggregates are clearly observed in PBDT-T:T2-SePDI2 blend films, indicating the strong tendency of T2-SePDI2 to crystallize due to its more planar molecular configuration,


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which favors the formation of large crystal domains. The spherical aggregates can also be observed in PBDT-T:T4B-SePDI4 blend films but are less in number and relatively smaller in size than the aggregates present in PBDT-T:T2-SePDI2, which implies that the intermolecular interaction of T4B-SePDI4 is weaker than that of T2-SePDI2. It is worth noting that no significant aggregation was observed in PBDT-T:T3B-SePDI3 blend films, which showed a more uniform film morphology. The root-mean-square (RMS) roughness values of T2-SePDI2, T3B-SePDI3, and T4B-SePDI4 blend films are 2.39, 1.16, and 1.47 nm. Owing to the strong molecular interaction of T2-SePDI2, large aggregates were formed, leading to a high RMS value. The intermolecular force of T3B-SePDI3 is the weakest because of the significantly distorted molecular configuration, which means it is not easy to form large aggregates. After blending with the donor material PBDB-T, a uniform phase separation and morphology with the smallest roughness is achieved. TEM images of the three blend films are shown in Figure S10 in the Supporting Information, and all show intermixed blend morphologies, without coarse phaseseparated phases.


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Figure 5. AFM height images (a-c) and phase images (d-f) of PBDT-T:T2-SePDI2 (a, d), PBDT-T:T3B-SePDI3 (b, e), and PBDT-T:T4B-SePDI4 (c, f) blends. Image size: 5×5 µm2.

Figure 6. 2D GIWAXS patterns of the blend films of PBDT-T:T2-SePDI2 (a), PBDT-T:T3BSePDI3 (b), and PBDT-T:T4B-SePDI4 (c), and their corresponding in-plane (d), and out-ofplane (e) line cuts. Grazing incidence wide-angle X-ray scattering (GIWAXS) was employed to further investigate the microstructures. The 2D GIWSXS patterns and the corresponding out-of-plane and in-plane line cuts of the blend films are shown in Figure 6, and the 2D GIWSXS patterns of neat PBDB-T, T2-SePDI2, T3B-SePDI3, and T4B-SePDI4 films are shown in Figure S11 in the Supporting Information. For PBDB-T neat film, the scatter peak at 0.29 Å-1 correspond to the


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formation of a lamellar structure with a d-space of 21.7 Å. The π-π stacking peak, which is locating at 1.69 Å-1 (d = 3.7 Å), can be observed both in the in-plane and out-of-plane direction, indicating the coexistence of edge-on and face-on orientation. There is no obvious π-π stacking peak for T2-SePDI2, T3B-SePDI3, and T4B-SePDI4, which is arising from the twisted molecular configuration. Three blend films all show similar scatter patterns with neat PBDB-T, which implying that the phase separation morphologies of the blend films are mainly depending on the PBDB-T due to its strong molecular interaction. Studies of the Charge Carrier Recombination in the Non-Fullerene OSCs. The charge dissociation probability P(E,T) (P(E,T) = Jph/Jsat) was investigated according to the reported methods to further understand the exciton dissociation, and carrier collection process.[52-54] The photocurrent density Jph is defined as Jph = JL - JD, where JL and JD are the current densities under illumination and in the dark, respectively. The saturation photocurrent density Jsat is equal to the value of Jph at saturation at high reverse voltage, which implies that all the photogenerated excitons are dissociated to free charge carriers and then collected by the corresponding electrodes. Photocurrent density (Jph) versus effective voltage (Veff) characteristics are shown in Figure 7a. Veff is defined as Veff = V0 – V, where V0 is the voltage at Jph = 0. The calculated charge dissociation probability P(E,T) under JSC conditions for PBDT-T:T2-SePDI2, PBDTT:T3B-SePDI3, and PBDT-T:T4B-SePDI4 devices is 91%, 96%, and 93%, respectively. The highest P(E,T) value is obtained for PBDT-T:T3B-SePDI3, which demonstrates more efficient exciton dissociation at the interface between T3B-SePDI3 and PBDT-T, in accordance with the higher JSC of T3B-SePDI3-based devices. The relatively low value of P(E,T) for PBDT-T:T2SePDI2 indicates ineffective exciton dissociation and significant charge recombination. To acquire more insight into the exciton recombination, the JSC as a function of illumination


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intensity was measured and plotted on a logarithmic scale following the literature.[55,

56]

As

shown in Figure 7b, the slope for PBDT-T:T2-SePDI2, PBDT-T:T3B-SePDI3, and PBDTT:T4B-SePDI4 devices is 0.95, 0.97, and 0.96, respectively, indicating that the PBDT-T:T3BSePDI3 devices show weak bimolecular recombination compared to PBDT-T:T2-SePDI2, and PBDT-T:T4B-SePDI4 devices.

10

(a)

(b)

2

2

JSC (mA/cm )

10 Jph (mA/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


1

PBDB-T:T2-SePDI2 PBDB-T:T3B-SePDI3 PBDB-T:T4B-SePDI4

0.1

1

1

PBDB-T:T2-SePDI2 Slope=0.95 PBDB-T:T3B-SePDI3 Slope=0.97 PBDB-T:T4B-SePDI4 Slope=0.96

0.1 1

Veff (V)

10 2 Light Intensity (mW/cm )

100

Figure 7. (a) Photocurrent density (Jph) versus effective voltage (Veff) characteristics of the three devices. (b) Short-circuit current density (JSC) versus light intensity of the three devices. CONCLUSIONS In summary, three novel perylene diimides (PDI)-based non-fullerene small molecule acceptors, T2-SePDI2, T3B-SePDI3, and T4B-SePDI4, containing two, three, and four PDI units, respectively, were successfully synthesized. T3B-SePDI3 had the most distorted molecular configuration, which suppressed the formation of aggregates when blended with PBDB-T. The PBDT-T:T3B-SePDI3 blend films showed most uniform phase separation morphology, and the highest electron and hole mobilities. These advantages lead to the T3B-SePDI3-based OSCs showing the highest PCE of 5.82% compared to the T2-SePDI2- (4.10%) and T4B-SePDI4based (5.10%) devices. From above results we can conclude that: for the PDI-based small


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molecule acceptor materials with similar molecular structures, the greater the dihedral angle between adjacent PDI units, the weaker the capacity for molecular aggregation; thus, a good uniform phase separation morphology could be realized when blended with the donor materials, leading to a high charge carrier mobility, which then gave the best device performance. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Figures and images as detailed in the text. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (F.H.) and [email protected] (W.C.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (51603100,

51773087,

21733005),

the

Shenzhen

Fundamental

Research

program

(JCYJ20150630145302237), and the Shenzhen Peacock program (KQTD20140630110339343). W.C. gratefully acknowledges financial support from the US Department of Energy, Office of Science, Materials Science and Engineering Division.


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Effect of the Molecular Configuration of Perylene ... - ACS Publications

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