Impact of additive residue on the photodegradation of high performance polymer solar cells

Accepted Manuscript Impact of additive residue on the photodegradation of high performance polymer solar cells Xusheng Zhao, Jing Zhao, Rong Wu, Debei Liu, Gang Wang, Ping Li, Lijia Chen, Linna Zhu, Baofu Ding, Qunliang Song PII:

S1566-1199(17)30218-5

DOI:

10.1016/j.orgel.2017.05.021

Reference:

ORGELE 4092

To appear in:

Organic Electronics

Received Date: 8 February 2017 Revised Date:

8 May 2017

Accepted Date: 8 May 2017

Please cite this article as: X. Zhao, J. Zhao, R. Wu, D. Liu, G. Wang, P. Li, L. Chen, L. Zhu, B. Ding, Q. Song, Impact of additive residue on the photodegradation of high performance polymer solar cells, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.05.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

Additive residue accelerates PTB7 degradation after 2 hours illumination, resulting in worse device performance.

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However, this accelerated degradation would gradually disappear with exhausting of additive residue.

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Impact of additive residue on the photodegradation of high performance

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polymer solar cells

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Xusheng Zhaoa,b, Jing Zhaoa, Rong Wuc, Debei Liua,b, Gang Wanga,b, Ping Lid, Lijia Chene, Linna Zhua, Baofu Dinga and Qunliang Songa,b *

5

a

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Southwest University, Chongqing 400715, P. R. China

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b

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Energy, Chongqing 400715, P. R. China

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c

Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy,

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Chongqing Key Laboratory for Advanced Materials and Technologies of Clean

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Key Laboratory of Solid-state Physics and Devices, School of Physical Science and

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Technology, Xinjiang University, Urumqi 830046, China.

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d

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Zunyi 563002, P. R. China e College of Physics and Electronic Engineering,Chongqing Normal University, Chong qing 401331, P. R. China

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School of Physics and Mechanical &Electrical Engineering, Zunyi Normal College,

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* Corresponding author. E-mail address: [email protected]

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Abstract

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Key words:

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1. Introduction

Solvent additives are indispensable to achieve highly efficient organic solar cells. The additive residue is

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unavoidable especially when the devices are prepared at room temperature and atmospheric pressure. In this paper, we introduce 1,10-diiododecane (DID) as the additive, which has high boiling point, and investigate the effects of additive residue on the photodegradation of organic materials and photoelectric properties of solar cells after light illumination. The iodine from the residue of DID in the active layer could be confirmed by X-ray photoelectron spectroscope (XPS) measurements. Structural changes in the films upon illumination are

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probed using Fourier Transform Infrared Spectrometer (FTIR). The residual DID is found to dramatically decrease the photostability of the active layer and device performance under light illumination compared with those without additive residue, which are exemplified in current density–voltage (J-V) and electrochemical impedance

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measurements. Furthermore, the absorption of the film with additive residue is unchanged after light illumination, indicating that the conjugation of the polymer is not affected by the residue.

Solvent additives, photodegradation, photostability, light, organic solar cells. Besides power conversion efficiency (PCE), cost and lifetime are also very important for commercializing photovoltaic device. Organic solar cells (OPVs) have continuously improved during the past decades. In particular, the PCE of polymer solar cells has reached about 11% [1-5] for single junction cell and 12% [6-8] for multiple junction cell. In order to improve the efficiency, many methods have been applied, such as thermal annealing [9, 10] or solvent annealing [11, 12]. However, for the materials which are not capable of annealing or the improvement of device performance is not obvious after annealing, solvent additives have more advantages. While solvent additives can potentially lead to problems because of additive residue after device fabrication, the influence of additive residue can be ignored if the additives are removed during traditional vacuum processing. As the PCE

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of OPVs is over 10%, many efforts have been devoted to large scale and low cost fabrication of OPVs for

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2. Experimental

industrialization[13-16]. Due to the inherent flexibility of OPVs, the low cost preparation of large scale polymer solar cells at low temperature and atmospheric pressure becomes feasible, for example the roll-to-roll technique without vacuum equipment. However, the low cost technology will inevitably bring about the additive residue for highly efficient OPVs, because the additives are difficult to remove without vacuum treatment due to their high boiling points. So understanding the degradation of device performance with additive residue under light illumination is very important to advance the commercialization of OPVs. Several papers have reported that the

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additive residue can accelerate the photodegradation of organic film, resulting in great changes of film absorption after illumination [17-19]. To our best knowledge, there is no report of the effects of additive residue on device photodegradation because a reference device is hard to be found. The reference device should have almost the same initial absorption, morphology and performance as the device to be studied. In

this

paper,

1,10-diiododecane

(DID)

is

used

as

an

additive

for

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Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thie no[3,4-b]thiophenediyl}) (PTB7) : [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM) system. Iodine signal originating from DID is clearly detected by X-ray photoelectron spectroscope (XPS) systems in the film of PTB7:PC71BM spincoated from the precursor solution with DID additive. We found 1,8-diiodooctane (DIO) can be

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removed by exposing the film to a high vacuum, while DID can not be taken away by the same treatment due to its high boiling point. The residual DID in the film is found to accelerate degradation after 2 hours illumination compared to those without additive residue. Chemical structural changes in the films upon illumination are confirmed using Fourier Transform Infrared Spectrometer (FTIR). DID is unstable during long time illumination and could photolyze into iodooctane radicals which can subsequently initiate photooxidation. As discussed in this context, radical-induced degradation of the PTB7 likely commences by hydrogen abstraction on the solubilizing side chains, proceeds to oxidize to form unstable peroxide intermediate product and then generates stable final

2.1. Materials

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photooxidation products.

PTB7 and PC71BM were purchased from One-material Chemscitech Inc. All the other materials used in the

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experiments, including solvent additives of DIO and DID, polyethylenimine, 80% ethoxylated (PEIE) (Mw = 70,000 g/mol) dissolved in H2O with a concentration of 35–40 wt%, and chlorobenzene (anhydrous, 99.8%), were purchased from Sigma–Aldrich. All of these materials were used as received without further treatment.

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2.2. Film and device fabrication

The configuration of the inverted PSCs was as follows: ITO/PEIE/PTB7:PC71BM+additives/MoO3/Ag, with additives being DIO or DID. The patterned ITO glass substrates (sheet resistance of 20 Ω/square) were cleaned by sequential ultrasonic treatment in detergent, acetone, alcohol and deionized water. A single PEIE layer was spin-coated on ITO at 4000 rpm for 60 s, and then heated in air at 100 °C for 30 min. The active layers deposited on the PEIE layers were formed by spin coating a solution of PTB7:PC71BM (1:1.5 w/w) in a mixed solvent of chlorobenzene (CB) with additives (25 mg/ml), at a speed of 900 rpm for 45 s inside a nitrogen filled glove box. It is important to note that the mole concentrations of DIO or DID in the PTB7:PC71BM solution were kept the same of 0.148 mmol (equal to 3 Vol% DIO/CB). The samples were then transferred into a vacuum chamber, in which 6 nm thick of MoO3 and 100 nm thick of Ag electrode were thermally evaporated at rates of 0.01 and 0.1 nm s-1, respectively, through a shadow mask at a base pressure of around 5×10-5 Pa. The device area, defined by the overlap between the ITO and Ag electrodes, was 9 mm2. All the films used for Atomic force microscopy (AFM), XPS, FTIR characterization were prepared in the same way as devices. The only difference was no MoO3/Ag

ACCEPTED MANUSCRIPT electrode on those films. 2.3. Device characterization The UV–vis absorption spectra for the same active film after different illuminations in the glove box was measured using a Shimadzu UV-21011C spectrometer. The current density–voltage (J–V) characteristics were measured under 100 mW cm-2 (AM 1.5 G) simulated sunlight using Keithley 2400 in conjunction with a Newport solar simulator (94043A). The external quantum efficiency (EQE) was calculated from the photocurrent measurement under monochromatic illuminations of different wavelengths, generated using a 150 W xenon lamp and a

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monochromator. The light intensity for each wavelength was measured by a calibrated Si detector. AFM images were obtained in air using Dimension ICON, operating in tapping mode. Infrared spectrum were measured by NICOLET 6700. Additive residue was measured using an XPS 250Xi. The electrochemical impedance measurements were performed with an electrochemical workstation (CHI 660D) in the frequency range of 100 Hz-100 kHz. The bias potential was set at 0 V with AC potential amplitude of 5 mV under the light. The current

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density–voltage (J-V), electrochemical impedance spectroscopy (EIS), EQE measurements were conducted for the same device in glove box. We placed fresh devices for ten hours in the darkness to stabilize the efficiency before light illumination. Each device was restored in the darkness for fifteen minutes after long time illumination before measuring J-V. The device for UV-vis was different from the one for J-V measurement but with the same treatment. For films used for FTIR characterization, a mirror was used to mimic the reflection effect of the Ag electrode

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during light illumination. All above mentioned films and devices were stored or illuminated in the glove box except when occasionally taken out for for AFM, XPS, FTIR and UV-Vis measurements.

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3. Results and discussion

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Figure 1. XPS spectra of iodine elements obtained from the surfaces of the PTB7:PC71BM films spincoated on

In order to study the impact of additive residue on the photodegradation of PTB7 and its corresponding device performance, additives DID and DIO are used for comparison. DIO and DID belong to the homologous series.

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DID just has two more CH2 compared with DIO in the structure, while they typically have a fixed set of functional groups that give them similar chemical and physical properties. The main reason to choose DID and DIO for comparison is the structure similarity but different boiling points. DID has higher boiling point (197-200℃) than DIO (167-169℃), which means DID might remain in while DIO would escape from the device after high vacuum treatment. Thus DIO-based devices can be used as a reference to elucidate the effect of additive residue. In order to

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verify the above statement of residual DID and removal of DIO after vacuum treatment, XPS measurements were

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conducted, as shown in Figure 1.

PEIE from precursors without any additives and with DIO or DID additives, respectively.

ACCEPTED MANUSCRIPT Two peaks at energies of ~620 eV and ~632 eV correspond to the binding energies of I3d. The I3d peaks are clearly observed in the PTB7:PC71BM film with DID, indicating the presence of residual DID, while such peaks can not be detected in the films with DIO or without additives, as shown in Figure 1. Due to the chemical and physical similarity of DID and DIO, it would be expected that morphology, optical properties and electrical properties would be similar for the device processing with DIO or DID. If so, the device with DIO additive which has no

device degradation when it is exposed under light illumination.

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residue in the final device can be used as reference to elucidate the impact of additive residue, herein DID, on the

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Figure 2. AFM images of PTB7:PC71BM films without and with additives of DIO or DID over a scan area of 5.0

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additive residue, vacuum treatment for 1.5 hours at 5×10-5 Pa is used to ensure that DIO is removed, as reported in

µm ×5.0 µm. Images (a, c, e) are height images and (b, d, f) are phase images: (a, b) without additives, (c, d) with

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DIO, (e, f) with DID.

As shown in figure 2, the surface morphology of PTB7:PC71BM film changes a lot after adding additives, but both

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surface roughness and phase configuration are quite similar for the DIO and DID films. Note that, to control the

the paper. In Figure 2a and 2b for the film without additives, uniformly distributed film with large domain sizes in the range from 50 nm to 300 nm are observed, which represent the PC71BM-rich domains, showing a root mean square (RMS) about 8.8 nm. The black region among domains is filled with the polymer PTB7 matrix. These observed domains and the dispersed polymers represent the typical morphology for PTB7:PC71BM films without any additives. Due to the poor mutual solubility between PTB7 and PC71BM, the fullerenes tend to aggregate to form different domains, and this behavior is consistent with the results in the previous work [20, 21]. After adding additives, either DIO or DID, the RMS drops from about 8.8 nm to 1.5 nm, as shown in Figure 2c and 2e. The fuzzy boundary between the PTB7-rich region and the PC71BM-rich domains means that the donor and the acceptor are mixed better than those without additives, as shown in Figure 2d and 2f. The morphologies and phase images are almost the same for the two films with additives. Therefore, in combination with the above discussion, it can be concluded that DIO and DID have the same effect on film morphology before light illumination.

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Figure 3. Absorption spectra of the developed PSCs with DIO or DID after different durations under one sun

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illumination. The number in the legend indicates the hours of light illumination

In order to verify the optical properties, the absorption of the films prepared with DIO or DID was measured (shown in Figure 3). Figure 3 shows the UV–Vis absorption spectra of the PTB7:PC71BM layers with the solvent additives DIO or DID after different illumination durations. The almost overlapped absorption spectra indicate that the films have similar composition ratio and thickness. It is well known that PC71BM contributes two absorption peaks at ~350 nm and ~450 nm in the short-wavelength region, whereas the absorption over the long-wavelength region (from 550 nm to 750 nm) is governed by the PTB7 polymer. Two absorption peaks at ~620 and ~680 nm are typically associated with the π– π* transition characteristic of the PTB7 polymer. After different durations of

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light irradiation, no obvious absorption change can be detected for DIO or DID films. Figure 3 thus indicates that the π-conjugated backbone of PTB7 is not changed after at least thirty hours of illumination, no matter in the absence or presence of the additive residue.

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Figure 4. J-V characteristics of the OPVs with DIO or DID under 1 sun illumination (a) and their corresponding EQEs (b). The number in the legend indicates the hours of light illumination.

ACCEPTED MANUSCRIPT Table 1. Photovoltaic performance of PTB7:PC71BM devices with DIO or DID after different duration of illuminations. PDR represents photodegradation rate (1-(PCEn/PCE0))

additive

Illumination time (h)

Voc(V)

Jsc (mA/cm2)

FF (%)

PDR (%) (PCE (%))

0

0.73/0.74

17.4/16.9

63.0/60.4

0.0 (7.94)/0.0(7.54)

2

0.73/0.73

16.2/15.7

54.0/49.6

20.0 (6.35)/24.4(5.70)

6

0.70/0.70

15.6/13.1

43.9/35.4

39.4 (4.81)/57.2(3.23)

12

0.69/0.67

13.3/9.3

35.9/32.8

18

0.68/0.65

12.1/8.1

34.3/32.3

30

0.66/0.64

10.0/5.9

32.9/31.6

DIO/DID

58.7 (3.28)/72.8(2.05) 64.7 (2.80)/77.6(1.69) 72.7 (2.17)/84.1(1.20)

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Due to the same morphology and absorption before illumination, the J-V and EQE measurements show similar initial device performance for devices with DIO or DID, however subtle differences may be caused by the DID residue as shown in Figure 4. Figure 4 also shows the performances of devices with DIO or DID after different

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illumination durations, which are summarized in table 1, including the open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and PCE. Note that Voc, Jsc, FF and PCE for the devices all decreased with increasing illumination time. When choosing the device performance before illumination as reference, the photodegradation rate for DID device is almost the same as DIO one in the first two hours and then DID device photodegradates faster than DIO device after prolonged illumination, as shown in both J-V and EQE measurements. This accelerated degradation can be explained as the result of residual DID, which is one of the degradation reasons in those devices. The faster performance decrease for the device with DID after 2 hours illumination indicates that the residual DID should take part in the role of degradation. The EQE response in the short

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wavelength range degrades faster relative to the longer wavelength region. The reason for this phenomenon is not clear. Excitons generated from light with different wavelengths have different binding energy. The faster degradation might come from the more prominent quenching by the photodegradation product on the looser bounded exciton generated from short wavelength light.

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Figure 5. Nyquist plots and corresponding equivalent circuit for the developed PSC devices with DIO or DID under light after different illumination durations. The number in the legend indicates the hours of light illumination.

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additive

Illumination

R0(Ω)

R1(Ω)

C1(nF)

R2(Ω)

C2(nF)

τ2(µs)

0

7/34

355/700

23.1/12.6

7425/6333

3.6/4.5

27.1/28.7

2

31/58

1185/1921

7.2/4.5

4119/2527

6.4/9.9

26.4/25.0

6

49/55

877/227

5.6/8.7

2355/1107

7.3/5.6

17.2/6.2

12

68/20

402/114

30.6/7.0

1004/1267

5.0/5.4

5.03/6.2

duration (h)

Table 2. Resistances, capacitances, carrier lifetimes fitted from the equivalent model for the developed

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PTB7:PC71BM PSC devices with DIO or DID under light after different illumination durations.

Note: In this study, 5 batch devices have been fabricated and compared. Though the lifetime for each batch are

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different, the trend of faster degradation for device with DID additive is the same for all batches.

It is well known that EIS is a powerful tool for investigating charge transport and charge extraction on electrode surfaces in solar cell [22-24]. In order to better understand the charge transport in the devices and recombination at the interfaces, AC impedance measurements after illumination for devices with additives of DIO or DID are conducted, as compared in Figure 5. The irregular semicircle observed in Figure 5 is due to the co-existence of two or more regular semicircles, indicating that there are at least two pairs of resistance–capacitance loops in the equivalent circuit model. Figure 5 illustrates the equivalent circuit from which the parameters are fitted and shown in Table 2. Generally, for a given PSC, the BHJ between PTB7 and PC71BM and the interface between the active

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layer and the electrode can contribute to a large semicircle in the low frequency range and a small semicircle in the high frequency range of the Nyquist plot, respectively. The interface part, the resistance (R1) and capacitance (C1) are linked to the total resistance and chemical capacitance at the interfaces between the BHJ layer and the electrodes. For the BHJ part, the resistance (R2) and the capacitance (C2) represent a combination of the recombination resistance and chemical capacitance of the interface between PTB7 and PC71BM inside the bulk of

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the BHJ layer [25], respectively. In addition, the resistance of electrodes (R0) is added in series with the above two RC loops. Table 2 lists the calculated values of R0, R1, R2, C1, C2 as well as the carrier lifetime inside the bulk given by τ2=C2×R2, indicating recombination degree under light, the shorter the lifetime the more serious recombination [26]. Because the photodegradation caused by additive residue mainly occurs in the active layer,

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DIO/DID

here we only study the carrier lifetime. The changes of the τ2 for the two devices with DIO and DID additive are very small after 2 hours illumination. However the τ2 of the device with DID becomes much shorter after 6 hours illumination, which is consistent with the variation of efficiency. After 12 hours illumination, the carrier lifetime for the two devices converge to the short value. The possible reason for this phenomenon is that the charge recombination is not the dominant process to affect the device performance after long irradiation. These results (as shown in Figure 5 and Table 2) indicate that DID residue does play its role in degradation of device performance, especially after long time illumination, through enhancing the recombination rate in the active layer (as evidenced by the lower R2 and the shorter τ2). This is the reason for photodegradation rate of 57.2% for the device with DID residue, contrast to 39.4% of the device without additive residue after 6 hours illumination.

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Figure 6. FTIR spectra vs irradiation durations for PTB7:PC71BM film processed with DIO (top) and with DID (bottom). The number in the legend indicates the hours of light illumination.

Finally, to understand why there is no change in the optical properties but significant changes in electrical properties after different illumination durations, we use FTIR to monitor specific chemical changes in the polymer

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films after light irradiation. Figure 6 shows FTIR spectra of PTB7:PC71BM films processed with DIO or DID after exposure to simulated sunlight in the glove box. Vacuum treatment at 5×10-5 Pa is conducted for the two devices to remove DIO while small amount of DID is left in the film, as shown in Figure 1. Figure 6 shows FTIR spectra in three special regions: 1) region from 2700 to 3500 cm-1 corresponds to absorptions by hydroxyl (-OH) groups and aliphatic hydrogen (-CH2 and –CH3) groups; 2) region from 1630 to 1800 cm−1 corresponds to carbonyl (−C=O)

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stretches; 3) region from 1350 to 1575 cm−1 is where conjugated ring stretches takes place. Because measurement is not conducted in-situ, quantitative analysis of the peak intensity makes no sense. The FTIR peak position and peak shape of both regions from 1350 to 1575 cm−1 and 1630 to 1800 cm−1 are almost unchanged after light illumination for both devices, as shown in Figure 6. However, a broad weak hydroxyl peak centered at around

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3200 cm−1 appears for film with DID additive after 6 hours illumination, indicating the formation of -OH. This peak has also been observed in several other polymer systems [27] after long time illumination. However, in this study, the broad peak at around 3200 cm−1 disappears after 12 hours illumination, which means that the newly accumulated -OH due to DID residue is gradually changed to other products without -OH.

ACCEPTED MANUSCRIPT Scheme 1. Oxidation mechanism of PTB7 alkyl side chain, PTB7 can been abbreviated by -BDT-TT- with an

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ethylhexyloxyl side chain

Recently the photo-oxidation of highly efficient conjugated polymer is well studied. There have been several reports on the mechanisms of photo-oxidation of low bandgap conjugated polymers [28, 29]. Although the

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mechanism details for our system have not been unraveled, following the approach of others [30-32], we propose the radical initiated mechanism for oxidation of PTB7 based on our experimental results, as shown in Scheme 1. In the presence of a radical species, such as iodooctane radical, abstraction of the most acidic hydrogen which attached to the α-carbon of the alkyl side chain pendant on the BDT backbone unit most likely occurs. Conjugation of the polymer backbone provides some stabilization for radical polymer species, as the initiation reaction shown

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in Scheme 1; however, these products are still highly reactive with oxygen and can propagate more radical polymer formation, as propagation reaction shown in Scheme 1. Some other more stable peroxide polymers can be further formed automatically. In our case, the source for radicals is DID, although all the above mentioned reactions can still occur in the absence of DID residue with a lower rate. We speculate that DID dissociates into iodooctane and

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iodine radicals (•I). •I is unstable and will terminate with another •I to form I2, while the iodooctane radical is somewhat stabilized by the hydrocarbon chain in the presence of PTB7, which is likely to initiate hydrogen abstraction from the polymer side chain leading to the formation of unstable chain oxidation products with -C-O-OH with the participation of oxygen. Unstable peroxide will continue to form radicals under illumination. If cage reaction occurs, those formed unstable peroxide radicals will form the final stable ester products, and if β-scission occurs, those radicals will form the ketone and the oxygen free radical. Further termination reactions will consume those radicals to propagate into the conjugated polymer units. Our FTIR spectra confirm the above reaction processes. For the first 2 hour light illumination, DID initiated radicals have not been accumulated enough to accelerate the oxidation of PTB7, resulting in almost the same degree of decline in PCE degradation rate compared the device without residue. But up to 6-hour light irradiation, more and more DID molecules are decomposed to accumulate iodooctane radicals, thus speeding up the generation of PTB7 radicals which proceeds to be oxidized to form -C-O-OH, an unstable peroxide intermediate product, corresponding to a new broad FTIR

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peak located around 3200 cm-1. The faster PCE degradation rate after 6 hours irradiation for the DID device thus is believed to be caused by those unstable peroxide intermediate product. Further light irradiation causes disappearance of the -C-O-OH absorption at around 3200 cm-1. Because the limited supply of DID, the iodooctane radicals and then the unstable intermediate product (-C-O-OH) are depleted by the continuing reaction to generate the stable product without –OH after long irradiation.

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4. Conclusion

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Acknowledgments

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References

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We have shown that the PTB7:PC71BM device with or without DID residue have different stability under light illumination. The PCE decreases at the same speed for the first 2 hours illumination and then degrades faster for the one with DID residue under further light illumination. The fast PCE decrease of PTB7:PC71BM devices under light illumination is mainly caused by photo-oxidation of PTB7. The accumulation of radicals which initiates by DID residual in the film accelerates the oxidation of PTB7, then accelerates the photodegradation of the device

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after 2 hours light illumination. The reactions occur on the side chains of PTB7, so there is no change in UV-vis absorption even though the device performance drops a lot. This study shows that the additive residue is harmful

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for OPVs commercialization if it is not removed during the roll-to-roll fabrication process.

This work was supported by National Natural Science Foundation of China (Grant 11274256, 11504036), Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011), Xinjiang Outstanding Young Scholars Foundation(QN2015YX004) and Guizhou Province Key Laboratory ( SSKH[21]55，

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QKHJC[33]1162).

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Highlights 1. The effect of additive residue on the photodegradation was investigated.

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2. Additive residue accelerates the oxidation of PTB7 under illumination. 3. The oxidation of PTB7 results in less carrier generation.

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4. Additive residue has little effect on the absorption after illumination.