Realizing Highly Efficient Inverted Photovoltaic Cells by Combination

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Realizing Highly Efficient Inverted Photovoltaic Cells by Combination of Non-Conjugated Small Molecule Zwitterions with Polyethylene Glycol Wenjun Zhang, Changjian Song, Xiaohui Liu, and Junfeng Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04955 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on July 6, 2016

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ACS Applied Materials & Interfaces

Realizing Highly Efficient Inverted Photovoltaic Cells by Combination

of

Non-Conjugated

Small

Molecule

Zwitterions with Polyethylene Glycol

Wenjun Zhang, Changjian Song, Xiaohui Liu, and Junfeng Fang* Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

ABSTRACT: Organic ionic materials have been reported to be efficient cathode interlayer (CIL) materials in polymer solar cells (PSCs), however, most of them are employed in conventional PSCs. For inverted structural device which has better stability, the efficiency is still far from expectation and the report is also limited. In this study, by using non-conjugated zwitterions as the CIL and inverted structure, the power conversion efficiency (PCE) is ~6%, though the PCE can reach of 9.14% in the conventional device. By introducing polyethylene glycol (PEG) into the zwitterions, the PCE of the inverted PSCs was improved ~33% and reach ~8% mainly because of the enhancement of open-circuit voltage (Voc) and fill factor (FF). Further research on device parameters, work functions, morphology of ITO with various CILs, and recombination resistance of the devices indicated that PEG + zwitterion induced not only a lower work function of indium tin oxide (ITO) but also a more uniform morphology of CILs with less contact of photoactive layer with ITO, which induced suppressed charge recombination and higher Voc and FF. Enhanced ability in interface modification of PEG + zwitterion CILs displayed a simple and feasible approach to elevate the performance of inverted PSCs with ionic CILs. KEYWORDS: Polymer solar cells, Inverted and conventional devices, Cathode interlayers, Zwitterions, Charge recombination

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1. INTRODUCTION Because of the potential applications in portable, large-area, low-cost devices by solution processing, polymer solar cells (PSCs) have drawn widespread attention.1-3 Numerous researches have been launched to improve the behaviors of PSCs and the power conversion efficiencies (PCEs) have been developed over 10% in the past decades.4-10 One important approach is tuning of the band gaps and energy levels of the conjugated donor polymers.11-12 Another efficient approach is applying a buffer interlayer between light absorbing layer and electrodes,13 which is conducive to both the performance and stability of PSCs. In order to reduce the work function (WF) of metals or indium tin oxide (ITO), many

alcohol/water-soluble

cathode

interlayer

(CIL)

materials

have

been

developed.14-16 Among them, conjugated ionic organic materials, which usually possess ionic pendant groups, are especially attractive as they are soluble orthogonally to major PSC active materials which facilitates solution process ability. Indeed, they could also form a dipole between the photoactive layer and cathode and facilitate the electron extraction and collection.17-18 Recently we developed non-conjugated small-molecule zwitterions containing pendent sulfobetaine groups as cathode CILs.19 In zwitterionic molecule, positive and negative charges are chemically combined, which makes them immobile under the electric field.20-22 What’s more, small-molecule zwitterions can be synthesized easily without complicated C-C coupling reaction, and their chemical structure are definite and benefit for the device reproducibility. The small-molecule zwitterions exhibited efficient electron-injection ability in PLEDs. However, though the zwitterions could effectively modify the WF of ITO, poor device performance was obtained when these materials were used in inverted PSCs as cathode interlayer.23 According to the literatures, high efficiency PSCs with ionic organic materials as CIL are mostly employed in conventional architectures.24-30 While in inverted PSCs, ionic organic materials are frequently used to further elevate interfacial abilities of n-type metal oxides.7, 18, 31-34 This will complicate device fabrication process and the metal oxide maybe need additional thermal treatment. Even though there are several


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ionic CIL materials successfully applied in the inverted PSCs, neither the PCE is poor with low efficiency conjugated polymer donors as light-absorption materials35-39 nor the conjugated unit with high electron mobility should be adapted,40-42 which limits the structural variety of organic molecules and leads to difficult synthesis or purification. Compared to conventional PSCs, inverted PSCs usually exhibit better vertical composition gradient and higher PCEs and device stability.43 Therefore, it is necessary to explore a novel approach to elevate the efficiency of inverted PSCs with ionic CILs. In this article, simply by cooperating with polyethylene glycol (PEG), the PCE of the inverted PSCs with zwitterions as CILs was improved from about 6% to 8% because of the enhancement of open-circuit voltage (Voc) and fill factor (FF). Compared to composite CILs with n-type metal oxide, the composite CILs consisting of PEG and zwitterions were easily solution processed without thermal treatment. Meanwhile, conventional architecture PSCs using the same zwitterions as CILs achieved a high PCE value over 9.0%.

2. EXPERIMENTAL SECTION 2.1 Device fabrication and characterization. Electron donor material poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b’] dithiophene-2,6-diyl-alt-ethylhexyl-3-uorothithieno[3,4-b]thiophene-2-carboxylate-4, 6-diyl] (PTB7) and electron acceptor material (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) were purchased from 1-material and American Dye Source Inc., respectively. The concentration of zwitterion CIL solutions was 0.5 mg/mL. Mixture of PEG and zwitterion CIL solution was obtained by addition of different amount of PEG (finally concentration of 0.1, 1, and 5 mg/mL) to the zwitterionic solution. Glass substrates patterned with two 3 mm wide ITO bands were adopted as anode in conventional PSCs and cathode in inverted PSCs and cleaned as our former literature.23 To fabricate inverted devices, the glass substrates coated with ITO were delivered into a glove box filled with N2 gas atmosphere and interlayer solution was Subsequently spin-coated on the ITO. Then the active layer with a concentration of



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10:15 mg/mL PTB7:PC71BM (chlorobenzene with 3% volume 1,8-diiodooctane as solvent) was deposited. Finally, a 10 nm MoO3 layer was evaporated as anode interlayer and a 200 nm Al layer were evaporated as top anode. The device area was defined to 0.06 cm2 by a shadow mask. To fabricate conventional devices, poly(3,4-ethylenedioxylenethiphene):poly(styrenesulfonic acid) (PEDOT:PSS) was deposited by spin-casting followed by thermal annealing at 140 oC for 15 min. Subsequently, the substrates were delivered into a glove box and then the photoactive layer of PTB7:PC71BM was also spin-coated. Finally, cathode interlayers were deposited and a 200 nm Al was evaporated as top cathode. 2.2 Device characterization. The current–voltage (I–V) characteristics, spectral response, PCE, ultraviolet photoelectron spectroscopy (UPS), tapping-mode AFM images, contact angle measurements, and external quantum efficiency (EQE) were carried out as our former literature.23 The ITO substrate and ITO/CIL films for UPS test were prepared by a same process as device fabrication condition. Electrochemical impedance spectroscopy (EIS) measurements under dark were conducted on an Autolab Electrochemical Analytical Instrument (ECO CHEMIE, B. V. Utrecht, The Netherlands) by applying a 5 mV ac signal and 0.4 V direct current (dc) voltage over the frequency range of 10-1 – 105 Hz.

3. RESULTS AND DISCUSSION Small-molecule zwitterions S1, S2, and S3 were synthesized in our laboratory and have been described as our former publication.19 To study the interfacial modification ability of small-molecule zwitterionic materials, we employed ITO/CIL/PTB7:PC71BM/MoO3/Al

or

ITO/PEDOT:PSS/PTB7:PC71BM/CIL/Al

device architecture with PTB7:PC71BM blend as the photoactive layer. The optimized concentration of the zwitterionic molecules was 0.5 mg/mL, and the thickness was further optimized by adjust of the spin-coating speed (Figure S2 and Table S1).


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Figure 1. Current density versus voltage (J-V) curves of the inverted (A) and conventional (B) PSCs with zwitterionic or PEG + zwitterion CILs under 100 mW cm-2 AM 1.5 G illumination. Table 1. Device parameters of the inverted PSCs with various CILs. CIL

Voc (v)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rsh (Ω cm2)

None

0.31a (0.27b)

14.97 (9.89)

41.43 (34.43)

1.90 (1.12)

139

S1

0.65 (0.63)

16.52 (16.59)

56.54 (57.06)

6.06 (5.92)

667

S2

0.59 (0.58)

16.48 (16.33)

56.76 (56.66)

5.54 (5.38)

687

S3

0.65 (0.64)

17.14 (16.77)

59.32 (58.61)

6.59 (6.30)

725

S1 + PEG

0.73 (0.73)

16.30 (16.14)

66.82 (64.73)

7.96 (7.61)

1348

S2 + PEG

0.72 (0.73)

16.23 (16.23)

67.44 (65.79)

7.92 (7.75)

1192

S3 + PEG

0.73 (0.73)

16.21 (16.05)

67.47 (66.13)

7.98 (7.71)

2210

PEG

0.69 (0.68)

16.51 (16.51)

64.90 (64.65)

7.36 (7.20)

677

a

Optimal device. b Average value determined from six devices

Devices with zwitterions, S1, S2, and S3, as CILs were first examined and the current density-voltage curves of the PSCs measured under AM 1.5G irradiation (100 mW cm-2) are presented in Figure 1. Consistent with our former research, inverted PSCs with zwitterionic CILs exhibited poor performance, especially with low Voc and FF values. S3 device exhibited the best performance with a PCE of 6.59%, a Voc of 0.65 V, a short circuit current density (Jsc) of 17.14 mA/cm2, and a FF of 59.32% (Table 1). Different from inverted PSCs, conventional devices with zwitterionic CILs



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exhibited outstanding performances (Figure 1B and Table 2). Compared to the device treated with methanol, all of the devices with zwitterions as CILs exhibited an enhancement of PCE with a simultaneous improvement of Voc, Jsc and FF. Especially, devices with S2 CIL exhibited an optimal PCE of 9.12% with an average PCE of 8.82%. The high efficiency of conventional PSCs with zwitterionic CILs implied that the non-conjugated zwitterions, S1, S2, and S3 had a potential for cathode interfacial modification. Since inverted PSCs usually show higher efficiency and device stability, it is meaningful to improve the efficiency of inverted PSCs with zwitterionic CILs. Table 2. Device parameters of the conventional PSCs with different CILs.

a

CIL

Voc (v)

Jsc (mA/cm2)

FF (%)

PCE (%)

MeOH

0.75a (0.74b)

17.40 (17.21)

61.89 (61.28)

8.06 (7.81)

S1

0.76 (0.76)

18.09 (17.79)

64.45 (64.87)

8.84 (8.72)

S2

0.76 (0.76)

17.78 (17.62)

67.24 (65.96)

9.14 (8.82)

S3

0.76 (0.76)

17.76 (17.91)

64.90 (63.49)

8.79 (8.58)

Optimal device. b Average value determined from six devices.

In order to improve device performance, PEG was introduced to the interlayer by mixing with zwitterion in the solution and the optimal concentration of PEG was confirmed to be 0.2 mg/mL (Table S2). After mixture of PEG with zwitterions, the device efficiencies were significantly improved owing to the improvement of Voc and FF. Device with S3 + PEG as CIL exhibited a PCE of 7.98%, with a Voc of 0.73 V, a Jsc of 16.21 mA/cm2, and a FF of 67.47%. In order to clarify the effect of PEG on the device parameters, PSCs with PEG as CIL were also fabricated. The concentration of PEG was chosen as 0.2 mg/mL, which is same with that of composite CILs. With PEG as CIL, the device exhibited a PCE of 7.36%, with a Voc of 0.69 V, a Jsc of 16.51 mA/cm2, and a FF of 64.90%. It could be seen that the PCE of PEG device was higher than that of zwitterion devices but lower than that of PEG + zwitterion devices. And the Voc and FF possessed a same tendency as the PCE. These results indicated that the elevated performance of PEG + zwitterion CIL devices was ascribed to the


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combining function of zwitterion and PEG.

To further illuminate the reason of the higher Voc of PEG + zwitterion CIL devices, ultraviolet photoelectron spectroscopy (UPS) were used to measure the WFs of ITO and ITO with CIL. UPS are usually utilized to component and electronic state analysis of sample surface. After UV radiation, photoelectrons would generate, collected and then analyze. Because the UV radiation is only able to excite electrons from the valence levels of atoms, UPS is typically utilized to measure the WF of a metal and the vacuum level and highest occupied molecular orbital (HOMO) positions of organic semiconductors. Figure 2 shows representative UPS spectra of ITO and ITO/CIL. The WFs are determined according to the following equations: WF = hν – ESE, where hν represents the incident photon energy (21.22 eV), ESE represents the secondary edge position. The WF of the ITO film was determined to be 4.5 eV, which was pretty lower than the LUMO level of PC71BM (-4.3 eV) and induced an obstacle for electron extraction and low device performance. On the contrary, after coating a CIL, the secondary electron cut-off of ITO was shift to a higher binding energy. This indicated that the WF of ITO/CIL is lower than that of ITO, which will result in a more efficient electron extraction from the photoactive layer to the cathode.42 The WF of ITO/S1, ITO/S2 and ITO/S3 was 4.2 eV, 4.3 eV and 4.1 eV respectively. Further more, the WF of ITO with PEG + S1, PEG + S2, and PEG + S3 CIL was even lower, with a value of 4.0 eV, 4.1 eV and 4.0 eV respectively. It was also observed that the WF of ITO/PEG was 4.3 eV, which is same with that of ITO/S2 and slightly higher than ITO/S1 and ITO/S3. The lower WFs of ITO/PEG + zwitterions were consistent with the higher Voc of the devices with PEG + zwitterion CILs. However, the higher Voc of PEG device than that of the zwitterion devices seems conflicting to the similar WFs of ITO/PEG and ITO/zwitterions. This indicated that the higher Voc of devices with PEG + zwitterion CILs was not only ascribed to the lower WF of the decorated ITO. In fact, as our former research,23 the WF of ITO/zwitterion was yet low enough for the Ohmic contact of photoactive layer, in which condition the device Voc was be independent of the WF of the cathode.45



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Figure 2. UPS spectra in high binding energy region of ITO, ITO/zwitterions, ITO/PEG + zwitterions, and ITO/PEG.

Then atomic force microscopy (AFM) was employed to determine the morphology of the ITO with zwitterionic CILs or PEG + zwitterion CILs. As shown in Figure 3, ITO coated with S1, S2, and S3 possessed a roughness root-mean-square (RMS) of 3.97 nm, 3.67 nm, and 3.40 nm, respectively. After being coated with S1 + PEG, S2 + PEG, and S3 + PEG, a smoother surface was obtained with a decreased RMS of 3.16 nm, 3.55 nm, and 2.90 nm, respectively. On account of the smoother surface of PEG + zwitterion CILs, the outline of ITO was blurry and ITO was covered more homogenous. This would lead to a less contact of ITO and photoactive layer, and thus less combination of charge carriers on the cathode interface.29 What’s more, the smoother surface of ITO/PEG + zwitterion CIL than ITO/zwitterion was consistent with the larger shunt resistance (Rsh). Without a CIL, the bare ITO-based device showed a small Rsh of 139 Ω cm2. With zwitterions S1, S2, and S3 as CILs, the device delivered a moderate Rsh of 667 Ω cm2, 687 Ω cm2, and 725 Ω cm2 respectively. Furthermore, the devices with PEG + zwitterion CILs exhibited the largest Rsh, for S1 + PEG, S2 + PEG, and S3 + PEG device of 1348 Ω cm2, 1192 Ω cm2, and 2210 Ω cm2 respectively. The increased Rsh indicated a good interfacial modification to boost charge injection and suppress charge recombination. Since charge recombination processes could also have an adverse effect on Voc and FF,46-47 the suppressed charge recombination induced by the less contact of ITO cathode and photoactive layer could


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be an important reason for the higher Voc and FF of PEG + zwitterion devices than that of zwitterionic CIL alone devices.

Figure 3. Tapping-mode AFM height images (2 µm × 2 µm) of (A) ITO/S1, (B) ITO/S2, (C) ITO/S3, (D) ITO/(S1 + PEG), (E) ITO/(S2 + PEG), and (F) ITO/(S3 + PEG).

Since the morphology of photoactive layer was sensitive to the surface energy of CIL, which could also result in the various device performances, contact angle of water and morphology of photoactive layer on ITO with zwitterionic CILs or PEG + zwitterion CILs were examined. Although the water contact angles for ITO/PEG + zwitterion CILs were slightly larger than those for ITO/zwitterionic CILs (Table S3), the photoactive layer morphology on various CILs was almost the same (Figure S3). This meant that different wetting properties induced by the combination of PEG with zwitterionic CILs did not take great effect on the device parameters. The slightly decreased Jsc of PEG + zwitterion devices than zwitterion devices may be ascribed to the slightly different wetting properties. To gain a deeper comprehension of the device parameters enhancement after employing the PEG + zwitterions as CILs, the J-V curves of the devices under dark



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conditions were carried out and presented in Figure 4. In the positive bias regime (0 to 1.5 V), the devices with PEG + zwitterion CILs exhibited higher onset voltages than those of zwitterion devices, suggesting that application of PEG + zwitterion CILs actually increased the device built-in potential and being consistent with the lower WFs of ITO with PEG + zwitterion CILs. What’s more, in the negative bias regime (-0.5 to 0 V), the reverse and leakage currents of the devices with PEG + zwitterion CILs were all greatly reduced compared to those of the zwitterion devices. Literatures have been mentioned that introduction of water-soluble conjugated polymers as CILs in PSCs could enhance the device Voc by reducing reverse dark current.48-49 It was apparent that the reverse dark current density of the devices with PEG + zwitterion CILs in the negative regime was significantly reduced, which could also dedicate to the Voc enhancement of PEG + zwitterion devices than zwitterion devices (from 0.65 V to 0.73 V). This was in agreement with the less contact of ITO cathode with photoactive layer and larger Rsh in PEG + zwitterion CIL devices than those in zwitterion devices.

Figure 4. J-V curves of the inverted PSCs with and without CIL under dark.

The previous research have strongly indicated that the combination of PEG with zwitterionic CILs could suppress charge recombination of inverted PSCs, electrochemical impedance spectra (EIS) were then carried out to directly measure the recombination resistance. Figure 5 presents Nyquist plots of the obtained impedance for the inverted devices under dark condition. Typically, the low-frequency arc is a


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measure of the charge recombination in solar cells.50 To quantitatively determine the device recombination rates, an equivalent circuit was employed to fitted the impedance spectra. According to the literature, we assumed that the device could be equivalent to a circuit with a parallel recombination resistance (Rrec) and chemical capacitance.51 It was obtained that the device without CIL had a very small Rrec value of 103 Ω, which was responsible for the pretty low PCE of the device. With S1, S2, and S3 as CIL, the devices had a moderate Rrec value of 15 kΩ, 14 kΩ and 18 kΩ respectively. Furthermore, With PEG + S1, PEG + S2, and PEG + S3 as CIL, the devices had the highest Rrec value of 222 kΩ, 246 kΩ and 317 kΩ respectively. The Rrec of the inverted devices was greatly increased when the zwitterions were combined with PEG, which revealed a reduced charge recombination by the introduction of PEG into zwitterionic CILs. This result further confirmed that the combination of PEG with zwitterions as CILs was benefit for the block of charge carrier combination on the cathode interface.

Figure 5. Nyquist plots of the inverted PSCs without (A) and with CIL (B, C, and D) under certain dc voltage (0.4 V) in the dark.



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Conventional devices with PEG + zwitterion CILs were also investigated. However, different from the inverted devices, the PEG + zwitterion CILs did not induce performance enhancement (Table S4). With conventional architecture, the devices without CIL already exhibited a high PCE (8.06%), which indicated that the charge recombination induced by contact of photoactive layer with Al cathode was negligible. Therefore, PEG + zwitterion CILs could not enhance the Voc but raise the thickness of CIL and induced a decrease of the PCE. This result gave an indirect evidence of PEG in reducing of charge recombination and enhancing of PCE in the inverted PSCs.

4. CONCLUSIONS In summary, highly efficient PSCs with conventional and inverted structures were simultaneously accomplished by taking advantage of CILs based on small-molecule zwitterions. With PTB7:PC71BM as photoactive materials, the optimal PCE of conventional devices and inverted devices was 9.14% and 7.98% respectively. Moreover, the behaviors of inverted polymer solar cells based on zwitterionic CILs was obviously improve simply by mixture of PEG with zwitterion, which was largely due to the enhancement of Voc and FF. Compared to the zwitterionic CILs, the PEG + zwitterion CILs induced not only a lower work function of cathode but also a more uniform morphology of CILs with less contact of photoactive layer with cathode, which induced suppressed charge recombination and higher Voc and FF. The combination of PEG with zwitterions suggests a simple and helpful approach to elivate the performance of inverted PSCs with ionic CILs. Also, the research of the impact of CIL morphology on device Voc and FF will help a deeper understanding on interfacial layers.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications


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website at DOI: Device parameters of PSCs with various thicknesses of zwitterionic CILs and PEG, contact angles of water on ITO/CILs, device parameters of the conventional PSCs with PEG and S2 + PEG as CIL, J-V spectra of the conventional devices with various concentration of S2, EQE curves of the conventional and inverted devices, and AFM images of photoactive layer.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected].

ACKNOWLEDGMENTS

Thanks for the support by National Natural Science Foundation of China (No. 51273208, 51403222, 61474125), Zhejiang Provincial Natural Science Foundation of China (LR14E030002), and Ningbo Science and Technology Bureau (2014B82010). We also thank the support of National Young Top-Notch Talent Program of China.

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Realizing Highly Efficient Inverted Photovoltaic Cells by Combination

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