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Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells Tianxiang Liang, Liangang Xiao, Ke Gao, Wenzhan Xu, Xiaobin Peng, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15241 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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ACS Applied Materials & Interfaces

Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells Tianxiang Liang, Liangang Xiao, Ke Gao, Wenzhan Xu, Xiaobin Peng,* and Yong Cao State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China. E-mail: [email protected]

ABSTRACT: A porphyrin based molecule DPPEZnP-BzTBO with bulky benzothiophene groups was designed and synthesized as an electron donor material for bulk heterojunction (BHJ) solar cells. The optimized devices under thermal annealing (TA) and then chloroform solvent vapor anneanling (SVA) for 80 s exhibited an outstanding PCE of 9.08%. Contrasted with the smaller thienyl substituted analogues we reported previously, DPPEZnP-BzTBO based BHJ solar cells exhibited a higher open circuit voltage due to the lower highest occupied molecular orbital (HOMO) energy level. The TA post-treatment of the active layers induced the formation of more crystallized components, and the subsequent SVA provided a driving force for the domain growth, resulting in more obvious phase segregation between the donor and the acceptor in nanoscale. Furthermore, the PCEs kept above 95% upon the further SVA treatment within the time range of 60 to 95 s probably because the bulky benzothiophene groups retard the too quick change of crystallinity, providing a wide processing window for the reproducible device fabrication.

KEYWORDS: porphyrins, organic solar cells, bulky benzothiophene groups, large open circuit voltages, solvent vapor annealing

INTRODUCTION Owing to the potentials in developing low-cost, flexible, large-scale and semitransparent solar panels, solution-processed bulk heterojunction (BHJ) organic photovoltaics (OPVs) have received significant attention and progresses in recent years with power conversion efficiencies (PCEs) above 10% have been achieved for not only polymer but also small molecules (SMs).1-4 Among the many effective strategies to increase the short circuit current (JSC), open circuit voltage (VOC), fill factor (FF) and finally PCE by developing novel materials and processing

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methods,5-10 it has been demonstrated that a lower HOMO (the highest occupied molecular orbital) energy level of a donor material is likely to improve the VOC, since a VOC in a BHJ solar cell linearly depends on the difference between the HOMO level of the electron-donor material and the lowest unoccupied molecular orbital (LUMO) energy level of the electron-acceptor material.11-14 However, there is always a trade-off between light harvesting (JSC) and low-lying HOMO levels (VOC ).15,16 Besides, JSC and FF depend strongly on the morphology of the active layer. In order to obtain an better morphology with more crystallinity and more bicontinuous phase separation to provide a larger donor/acceptor interface area for more efficient exciton dissociation and continuous pathways for quicker charge transportation to the electrodes so as to reduce carrier recombination losses in the devices,17,

18

various morphological control methods have been

developed including the selection of appropriate solvent, thermal annealing (TA), solvent vapor annealing (SVA) and solvent additives, etc.19, 20 Under SVA, the molecules in blends gain high driving force to facilitate domain growth in a lower energy state, resulting in more obvious phase segregation between donor/acceptor and/or more ordered self-assembly of the active materials in nanoscale.21 Porphyrins have strong absorptions, unique electronic and redox properties due to their large delocalized π-conjugated system. 22 Conjugated donor-acceptor (D-A) porphyrin structures by connecting acceptor units to a porphyrin core through ethynylene or ethynyl-aromatic bridges have been demonstrated to be an effective strategy to lower the energy bandgaps and facilitate the

intramolecular

charge

transfer,

leading

to

significantly

enhanced

photovoltaic

performance.23,24 To date, the porphyrin small molecules with two types of acceptor units of diketopyrrolopyrrole (DPP) and rhodanine achieved PCEs over 8% for solution-processed BHJ devices.25-28 While the porphyrins with DPP units usually exhibited broader and more intense absorption leading to high JSC values up to 19 mA cm-2, the devices showed relatively small VOC (~0.73 V) values.

26, 27

On the contrary, though those with rhodanine units showed larger VOC

values, the JSC values were only modest.28-34 Therefore, to design a high performed porphyrin OPV molecules, some principles should be considered: 1) Coplanar chemical configuration should be constructed by ethynylene linkages or π-bridges to facilitate the electronic coupling between the donor and the acceptor units and enhance the charge carrier mobility. 2) Suitable electron deficient moieties should be selected to balance the electron push-pull ability. And 3) solubilized alkyl chains and aromatic side chains can influence the energy levels, solubility and aggregation properties, and film morphology, which are also important for photovoltaic performance.

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Since it has been reported that the electron density on the thiophene ring would be decreased when fused with benzene, leading to a deeper HOMO and higher VOC,35 Herein, we introduced slightly electron-deficient benzothiophene groups to the meso positions of a porphyrin core in order to obtain a large VOC but maintain a wide range of absorption, and designed and synthesized

a

porphyrin

small

molecule

5,15-bis(2,5-bis-(2-ethylhexyl)-3,6-di-thienyl-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione5'-yl-ethynyl)-10,20-bis(5-(2-butyloctyl)benzo[b]

thiophen-2-yl)-porphyrin

zinc

(DPPEZnP-BzTBO) (Scheme 1) with the same conjugated backbone of DPP units connected to two of the four porphyrin meso-positions by ethynylene linkages but two more bulky 2-butyloctyl)benzo[b]thiophen-2-yl groups (BzTBO) substituted at the other two meso-positions for OPV. The devices with [6, 6]-phenyl C61-butyric acid methyl ester (PC61BM) as the electron acceptor under thermal annealing and then chloroform SVA for 80 s exhibit an outstanding PCE of 9.08% with a very high FF of 67.54%, a high JSC of 16.82 mA cm-2, and a VOC of 0.80 V. The PCE is higher than that based on DPPEZnP-TEH and also competitive to that based on DPPEZnP-TBO.25,26 It is also found that the PCEs kept above 95% upon chloroform SVA within the range of 60 to 95 s, providing a wide processing window for reproducible device fabrication.



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Br H S HO

Br PPh3, CH2Cl2, NBS

Ni(dppp)Cl2, THF

n-butyllithium, DMF

Mg, I, THF

Ar

N

N H

DCM, TFA

NH

S

S

1, 95%

HN

N

THF

N

4, 27% Ar

CHO

2, 66%

3, 55%

Zn(OAc)2 CH3OH CHCl3 Ar N

N

THF, TBAF

Zn

Si

Zn N

N

Ar

N

N

N Si

N

N

Ar

Ar

N

N

Pd(dppf)Cl2, CuI, TMSE Ar

Ar

8, 85%

Zn

N

N

N

N

CHCl3, NBS

Br

Zn

Br

THF

Ar

Ar

6, 88%

7, 80%

N

5, 98%

Br S Pd(pph3)4, CuI

8

O

+

N N

N

S NEt3, toluene

O

Ar

O

O

N

N

S

N

Zn N

N

O

N

S

S

N

S

O

Ar

9 DPPEZnP-TBO ,40% Ar= S

Scheme 1. The synthetic routes of DPPEZnP-BzTBO.

RESULTS AND DISCUSSION The synthetic routes of DPPEZnP-BzTBO, as well as several key intermediates are summarized

in

Scheme

1.

3-(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyr role-1,4-dione (9) was synthesized according to the reported procedures.23 More synthetic details

are

described

in

the

Supporting

Information.

The

final

target

compound

DPPEZnP-BzTBO was synthesized in 40% yield by Sonogashira coupling reactions. DPPEZnP-BzTBO was purified by gel permeation chromatography (eluent: toluene) and column chromatography on silica gel, and fully characterized by 1H NMR spectroscopy, elemental analyses and MALDI-TOF mass spectrometry (see the Supporting Information). As shown in Fig. 1, the absorption spectra of DPPEZnP-BzTBO in dilute dichloromethane solution and in solid film exhibit broad spectral coverage in the visible and near infrared (IR) region, indicating that π-electrons delocalize throughout the whole backbone efficiently.


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DPPEZnP-BzTBO solution shows absorption peaks at 467 (with an absorption coefficient up to 1.2 ×105 M-1 cm-1), 566 and 721 nm. Compared with those of DPPEZnP-TEH (466, 568, and 727 nm), the replacement of TEH with BzTBO substituents on porphyrin macrocycle causes a blue-shift of Q-band by 6 nm, which is consistent with previously published reports that the porphyrins with more bulky substituents usually showed blue-shifted Q-bands since the less sterically hindered substituents can rotate more freely leading to slightly stronger π conjugation between the porphyrin macrocycle and substituents.36,37 DPPEZnP-BzTBO films exhibit broaden and red-shifted absorption peaks at 472, 571, and 786 nm with a shoulder peak at 727 nm. It should be noted that the red-shift of 786 nm in film is only 65 nm compared to the 721 nm peak in solution while the near IR absorption peak of DPPEZnP-TEH in film was found to be at 802 nm with a larger red-shift of 75 nm, indicating that the intermolecular π-π stacking is less effective in DPPEZnP-BzTBO film due to the more bulky BzTBO substituents. 25, 38, 39 The optical band gap of DPPEZnP-BzTBO was calculated to be 1.40 eV from the onset of the film absorption (884 nm). And the HOMO and the LUMO energy levels were investigated by cyclic voltammetry (Fig. S6, Table S2), and calculated to be −5.17 and −3.75 eV from the onset oxidation (Eox) and reduction (Ere) potentials, respectively.40 The HOMO level was slightly lower than that of DPPEZnP-TEH (−5.14 eV),25 which would be beneficial for obtaining a high VOC in OPVs.41 The electrochemical energy bandgap of DPPEZnP-BzTBO is 1.42 eV, which is in agreement with its optical one.



film solution

1.0

Absorbance(a.u.)

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0.8

0.6

0.4

0.2

0.0 300

400

500

600

700

800

900

1000

Wavelength(nm) Fig. 1. UV-vis absorption spectra of DPPEZnP-BzTBO in dichloromethane solution and in thin films.

Single junction BHJ solar cells were fabricated by solution processing with a conventional structure of ITO/ poly(styrene sulfonate)-doped poly(ethylene-dioxythiophene) (PEDOT:PSS) /DPPEZnP-BzTBO:

PC61BM

/

poly[(9,9-bis(3’-(N,N-dimethylamino)-propyl)-2,7-fluorene)]-alt-2,7-(9,9-dioctylfluorene) (PFN) /Al, and measured under simulated AM 1.5 G illumination (100 mW cm-2). The optimum devices were achieved with 1:1.2 of a DPPEZnP-BzTBO:PC61BM blend ratio from chlorobenzene (CB) containing 7 vol% pyridine (Py) additive. As shown in Fig. 2 and Table 1, the devices fabricated without Py additive and no post-treatment show a poor PCE of only 2.06% with a very low JSC of only 4.51 mA cm-2, a VOC of 0.79 V and FF of 57.74%. Quite similar with the case we reported previously based on DPPEZnP-TBO,27 when 7 vol% of pyridine additive was used, the JSC of DPPEZnP-BzTBO-based devices increased to 6.21 mA cm-2 with an enhanced VOC of 0.91 V but a remarkably reduced FF to 31.06%, indicating that pyridine additive causes an increased interface area but the transport channels are less effective due to the increased mixing with reduced crystallinity for the BHJ films.27 Also similar to DPPEZnP-BzTBO-based devices, the TA post-treatment of the active layers at 110 °C for 10 min do lead to an improved PCE up to 7.07% with a remarkably enhanced JSC up to 15.04 mA


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cm-2 and a FF of 55.35%, which can be ascribed to the formation of more crystallized components induced by the thermal annealing for well-mixed blends.27 To further improve the performance, the thermal annealed active layers were subsequently treated with chloroform SVA. While the FF is sharply improved with the annealing time, the JSC is further enhanced and reached the maximum about 16.8 mA cm-2 when the active layer was annealed for about 60~80 s with a first slightly reduced then constant VOC. Meanwhile, compared with the values of the devices fabricated with pyridine additive, the post treatments of TA and SVA lead to devices with reduced series resistances (Rs) but increased shunt resistances (Rsh) values, which are beneficial for lower leakage current and higher FF. Finally, the optimized devices exhibited a high PCE up to 9.08% with an FF of 67.57%, a JSC of 16.82 mA cm-2 and a VOC of 0.80 V. Notably, in many cases, the SVA time was reported to be 10 to 20 s 5, 42-45

, which can make the processing uncontrollable, though only a few reports discussed the

relationship between the photovoltaic performance and SVA time.46 Herein, the optimized SVA time is 80 s, which is not too long or too short. Furthermore, the PCEs kept above 95% upon SVA treatment within the time range of 60 to 95 s, probably because the bulky benzothiophene groups retard the too quick change of crystallinity, providing a wide processing window for the reproducible device fabrication, which is helpful for the potential production of printed solar cells.42, 47 Table 1. Photovoltaic properties of single layer BHJ solar cells based on DPPEZnP-BzTBO.a

Py

TA

SVA (s)

JSC (mA cm-2)

VOC

FF

(V)

(%)

PCE

PCE

Rs (Ω

Rsh (Ω

(%)b

(%)

cm2)

cm2)

(Ave)

(Max)

0

N

N

4.51

0.79

57.74

1.95

2.06

17.92

621.70

7%

N

N

6.21

0.91

31.06

1.66

1.75

40.16

194.02

7%

Y

N

15.04

0.85

55.35

6.90

7.07

9.91

388.18

7%

Y

30

15.75

0.83

56.71

7.33

7.41

7.49

418.57

7%

Y

60

16.81

0.80

66.23

8.87

8.91

5.48

3755.8

7%

Y

80

16.82

0.80

67.54

9.00

9.08

5.73

4210.4

7%

Y

95

15.81

0.80

69.26

8.66

8.76

4.96

8743.0

7%

Y

110

11.79

0.80

72.80

6.53

6.78

4.93

2877.6

a

Additional device statistics including standard deviations are provided in Table S1),

average PCEs are obtained from 20 devices.


b

the

0

a)

70

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Py Py+TA Py+TA+SVA

b)

60

-10

-0.2

0.0

0.2

0.4

0.6

0.8

As cast Py Py+TA Py+TA+SVA 30S 60S 95S 110S

EQE (%)

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

Current density (mA cm-2)


1.0

50 40 30 20 10 0 300

400

500

600

700

800

900

Wavelength (nm)

Voltage (V)

Fig. 2. (a) Characteristic J−V curves of solar cells based on DPPEZnP-BzTBO under AM 1.5G illumination (100 mW cm-2) and (b) EQE curves of devices without treatment (black), with TA treatment (red), and under optimized condition (blue).

The external quantum efficiency (EQE) curves of the devices without any post-treatment, with TA and TA + SVA are presented in Fig. 2b, from which the JSC values are integrated to be 7.06, 14.79 and 16.30 mA cm-2, respectively, which agree well with the ones measured from the corresponding devices. While the devices with no post treatment show a very poor photocurrent generation response, TA treatment significantly elevates the EQE in the whole region with the maximum value to 55%. For the devices with TA and then SVA, the EQE is further improved from 380 to 650 nm with a maximum value more than 65%, which indicated that the photo electron conversion process is more efficient. In order to further understand the effect of TA and SVA on the photovoltaic performance, UV−vis absorption spectra of the blend films without any post-treatment, with TA and with TA + SVA were measured. As shown in Fig. 3, the blend films without treatment show a broad near IR absorption band at 782 nm with a very weak shoulder at 725 nm. Under TA treatment at 110 °C, the 725 nm shoulder is enhanced, and the 782 nm peak decreased and red-shifted slightly, indicating the formation of some more ordered self-aggregation.25,48 Under further SVA treatment, though a very small red-shift of the near IR peak to 787 nm is seen, the absorption spectrum is quite similar, indicating that the self-aggregation of the porphyrin itself does not change remarkably after the further SVA treatment.

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0.6

0.5

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


Py Py+TA Py+TA+SVA

0.4 0.3

0.2 0.1

0.0 300

400

500

600

700

800

900

Wavelength(nm)

Fig. 3. UV-vis absorption spectra of the blend films of DPPEZnP-BzTBO:PC61BM (1: 1.2 w/w) without treatment, with TA and with TA+SVA (80 s) treatment.

The morphology of the active layers was investigated by atomic force microscopy (AFM). As seen in Fig. 4, the root-mean-square (RMS) roughness is 0.266 nm for the blend films without any post-treatment. Under TA and TA + SVA treatment, the RMS roughnesses increase to 0.861 and 0.862 nm, respectively, indicating that the more ordered molecular self-aggregations are induced by TA but not further by SVA, which is also consistent with the results of the absorption spectra of the blend films. Furthermore, much smaller nanoscale domains are seen for the blend films under TA + SVA than TA only post-treatment possibly because chloroform vapor annealing provides a driving force for the domain growth, resulting in more obvious phase segregation between donor/acceptor of the active layer in nanoscale.49, 50



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Fig. 4. Tapping mode AFM height (a, c, e) and phase (b, d, f) images (1.0×1.0 µm) of the blend films of DPPEZnP-BzTBO: PC61BM (1: 1.2 w/w): (a, b) without treatment, (c, d) with TA treatment and (e, f) with TA + SVA treatment.

Fig. 5. GIXD diffractograms of DPPEZnP-BzTBO pure and blend thin films.

The morphological properties of porphyrins in pure and blends were further characterized by grazing incidence X-ray diffraction (GXID) method (Fig. 5). In pure film, a weak diffraction ring is seen at ∼0.29 Å−1 (2.17nm) and a broad ring is seen at about 1.48 Å−1 (0.42 nm), arising from the alkyl-alkyl distance or (100) reflection and the average π-π distance, respectively. In blends without post-treatment, while the corresponding (100) reflection reduces obviously, indicating that PC61BM destructs the aggregation of porphyrins, a weak diffraction arising from PC61BM is seen at 0.68 Å−1, corresponding to a distance of 0.92 nm. After TA process, the (100) peak intensity slightly enhances, also, SVA processing further increases the intensity of (100) peak, indicating more ordered self-aggregation after TA + SVA treatment.


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10000

a)

Current Density (A m-2)

10000

Current Density (A m-2)

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


1000

100 Py Py+TA Py+TA+SVA

10

b)

1000 100 10

Py Py+TA Py+TA+SVA

1 0.1

1

0.0

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0.5

1.0

1.5

2.0

2.5

3.0

Voltage (V)

Voltage (V)

Fig. 6. J-V characteristics of (a) the hole-only devices and (b) the electron-only devices of DPPEZnP-BzTBO:PC61BM (1: 1.2 w/w) without treatment (black), with TA treatment (red), and with TA + SVA treatment (blue).

Table 2. Hole and electron mobilities based on DPPEZnP-BzTBO:PC61BM blend films. Devices Py Py+TA Py+TA+SVA

µh (cm2 V−1 s−1) −5

µe (cm2 V−1 s−1)

Charge balance (µh /µe)

−5

1.14

3.03 × 10

−4

1.70 × 10

1.94

4.47 × 10−4

2.41 × 10−4

1.85

3.65 × 10

−4

3.20 × 10

Hole-only and electron-only devices were fabricated with almost the same thickness as the solar cells to measure the hole and electron mobilities (µh and µe) of the blend films upon different treatments by the space charge limited current (SCLC) method.51 As shown in Fig. 6 and Table 2, both the µh and µe of the devices were enhanced to 3.03 × 10−4 and 1.70 × 10−4 cm2 V−1 s−1, respectively, after the TA treatment, almost an order of magnitude higher than the corresponding value of the films without any post-treatment (µh and µe are 3.65 × 10−5 and 3.20 × 10−5 cm2 V−1 s−1, respectively). The subsequent SVA further slightly increases the µh and µe to 4.47 × 10−4 and 2.41 × 10−4 cm2 V−1 s−1, respectively, showing more balanced electron/hole mobility than the TA only post-treated films. The enhanced changer mobilities and the better hole/electron balance in the TA + SVA post-treated devices reduce the charge recombination and contribute to the higher JSC and FF values in photovoltaic devices.52



10

Jph (mA cm-2)

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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1

Py Py+TA Py+TA+SVA 0.1

0.01

0.1

1

Veff (V) Fig. 7. Photocurrent density (Jph) versus effective voltage (Veff) curves of the devices without treatment (black), with TA treatment (red), and with TA + SVA treatment (blue).

To further understand the mechanisms responsible for the enhanced performance of the devices under TA and TA + SVA treatments, the dependence of the photocurrent density (Jph) on the effective voltage (Veff) of devices were measured and plotted on a double-logarithmic scale as shown in Fig. 7 (Jph = JL – JD, JL and JD are the current densities under illumination and in dark, respectively. Veff = V0−Va, V0 and Va are the voltages of Jph = 0 and under an applied bias). For the as-cast devices, Jph is very small and shows a nearly linear dependence on the voltage without saturation region, in accordance with the low JSC and FF. For the devices with TA only, Jph increases greatly but has not fully saturated even at Veff = 2.8 V, suggesting that significant charge recombination loses and less efficient interfacial contact.

53

For the devices

with TA + SVA, Jph is the largest among the three devices at a low value of Veff (

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