Development of non-halogenated binary solvent systems for high performance bulk-heterojunction organic solar cells

Organic Electronics 16 (2015) 95–100

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Development of non-halogenated binary solvent systems for high performance bulk-heterojunction organic solar cells Choong-Do Park a,b, Tyler Fleetham b, Jian Li b,⇑ a b

Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, USA School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ, USA

a r t i c l e

i n f o

Article history: Received 5 June 2014 Received in revised form 14 October 2014 Accepted 16 October 2014 Available online 29 October 2014 Keywords: Bulk heterojunction Photovoltaic Hansen solubility parameter Binary solvent system Non-halogenated solvents

a b s t r a c t Halogenated solvents such as dichlorobenzene (DCB) are commonly used in the fabrication of bulk-heterojunction (BHJ) organic solar cells. However, most halogenated solvents are very toxic. This report provides a route to eliminate halogenated solvents based on Hansen Solubility Parameters (HSPs) for manufacturing high efficiency BHJ solar cells. The device performances fabricated from several different binary solvent systems were compared to that from DCB. For the investigation of different gel behavior of solvent mixtures, Hansen solubility parameters and viscosity were mainly considered for the selection of binary solvent mixture candidates. Upon the addition of 20 vol.% benzaldehyde (BA) to p-xylene (pXL), the device performances were improved in power conversion efficiency (PCE) from 3.1% to 3.8% and external quantum efficiency (EQE) from 63% to 70%. The solar cell devices fabricated using p-XL/BA binary solvent mixtures exhibited PCE comparable to that from DCB. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction There have been tremendous efforts for the development of solution processable high performance bulk-heterojunction (BHJ) organic solar cells for the last couple decades. In order to improve device performances by modifying the morphology of the organic active layers [1], solvent annealing [2–4], thermal annealing [4–6], and solvent additives [7,8] have been utilized for the fabrication of solution-processed bulk-heterojunction solar cells. The most commonly used solvents in the fabrication of solution processable solar cells tend to be halogenated solvents due to their superior performance [9]. However, most of the halogenated solvents used in the fabrication process are toxic which prevents their usage in large scale organic solar cell manufacturing. Also, nearly all halogenated sol-

⇑ Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.orgel.2014.10.025 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

vents have a global warming potential greater than that of carbon dioxide. Thus, in order to eliminate halogenated solvents in the solution processing of BHJ solar cells, an alternate processing route based on Hansen Solubility Parameters (HSPs) has been introduced in our previous work [10], which can be utilized to predict molecular affinities, solubility, and solubility-related phenomena [11,12]. In order to select good candidates for binary solvent mixtures, Hansen Solubility Parameters (HSPs), describe the total cohesion energy, E, by three contributions: the dispersion interactions, Ed, permanent dipole-permanent dipole molecular inter actions, Ep, and the hydrogen bonding molecular interactions, Eh. This energy is directly related to the solubility parameters (d), which are the square root of the cohesion energy density (E/V) [13].

d2 ¼ d2d þ d2p þ d2h

ð1Þ

C.-D. Park et al. / Organic Electronics 16 (2015) 95–100

Fig. 1 illustrates the HSPs for a number of different solvents. The solvents that can dissolve the P3HT and PCBM mixture (chloroform [CHCl 3], ortho-dichlorobenzene [o-DCB], chlorobenzene [CB], biphenyl, mesitylene [MS], carbon tetrachloride [CCl4], diethylbenzene, ethylbenzene, toluene and xylene [XL]) are grouped together in HSPs space that define the interaction radius (R0) wherein solvents dissolve the polymer and solvents outside of the volume defined by R0 will not dissolve the polymer. As DCB tends to yield devices with better performance, a mixture that matches its solubility parameters (dd = 19.2, dp = 6.3, dh = 3.3 MPa1/2) would be desired. Based on the HSPs selection rules, the composition of 27–73 vol.% MS–AP with dd = 19.2, dp = 6.3, dh = 2.9 MPa1/2 was predicted. However, in our previous report we showed that solutions using MS–AP systems exhibited a gel behavior that led to thin film morphologies which were different than those from DCB solutions and consequently, the best device performance was observed from far from the predicted composition at a 80–20 vol.% MS–AP mixture [10]. These discrepancies might be due to the mismatch of several liquid properties such as viscosity, electron density, and the normal boiling point of the component solvents. Furthermore, such behavior may provide a unique opportunity for the formation of well-defined polymer and PCBM domains due to the formation of the solid network of the polymer. In order to study the effect of several liquid properties on device performance, we developed other binary solvent systems that also exhibit gel behavior but possess solution properties similar to those of halogenated solvents through the tuning of solubility and viscosity of the solutions. MS–AP, pXL–AP, and pXL–BA combinations were selected as binary systems to study considering viscosity, density, and boiling point along with the HSPs of the component solvents. 2. Experimental details 2.1. Materials and device fabrication PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate)) was purchased from Baytron AG (Clevios P VP AI 4083), P3HT (poly(3-hexylthiophene)) was purchased from Rieke Metals (4002E), PCBM (phenylC61-butyric acid methyl ester) was purchased from Solenne B.V., and BCP (bathocuproine) was purchased from TCI Co. DCB (99%), MS (99.0%), AP (99%), p-XL (99%), and BA (99%) were all purchased from Sigma–Aldrich and stored in a dry nitrogen glove box. 2.2. Device fabrication Bulk-heterojunction solar cells based on P3HT/PCBM blend were fabricated in the following simple device stack structure: ITO/PEDOT:PSS/P3HT:PCBM/BCP/Al. Patterned ITO (Indium tin oxide) substrates were scrubbed and cleaned with sequential sonication of deionized water, acetone, and isopropyl alcohol followed by UV-zone treatment for 40 min. The cleaned ITO-coated glass substrates were modified by spin-coating a thin layer (40 nm) of PED-

ethyl lactate

12

Hydrogen Bonding 1/2 eters (MPa ) Solubility Param

96

10 8

PMEA dioxane

6

CHCl3

0 16

cyclohexanone o-DCB

4 2

methyl ethyl ketone pyridine

xylene

toluene

ethylbenzene

CB

acetophenone (AP)

30-70vol.% MS/AP

80-20vol.% MS/AP diethylbenzene CCl4 mesitylene(MS) biphenyl

18 Dis per sio 20 Pa nS ram olu ete b ilit rs ( MP 1 y a /2 )

8

10

6

y ilit 1/2 ) lub Pa o S (M lar rs Po ete m ra Pa 4

2 22

0

Fig. 1. Hansen Solubility Parameters (HSP) for selected solvents and solvent mixtures. Solvents in red are considered as good solvents for P3HT/PCBM blend and in blue are non-solvents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

OT:PSS (Clevios, 4083) and cured at 190 °C for 30 min in air. P3HT and PCBM were dissolved in a 1:1 ratio to 1.0 wt% in mixtures of MS–AP, p-XL–AP and p-XL–BA by heating and stirring for 2 h at 75 °C in a nitrogen atmosphere. These solutions were then cooled slowly to ambient temperature until the systems form gel-type solutions. For comparison devices, the P3HT:PCBM blend (1.5 wt%, 1:1 weight ratio) was dissolved in DCB and stirred for 24 h at 40 °C. The polymer blended films were spin-coated in a nitrogen-filled glove box onto the PEDOT:PSS modified substrate. The spin conditions were modified for each solution to maintain a thickness of 60 nm for all the active layers. The as-cast films were dried overnight in nitrogen atmosphere at ambient temperature. After drying, the samples were thermally annealed at 140 °C for 30 min. Finally, BCP (14 nm) and Al cathode (100 nm) layers were deposited in a vacuum thermal evaporation chamber built by Travato Man. Inc. on top of the active layer and each device had an active area of 0.04 cm2.

2.3. Device characterization The current density–voltage characteristics were measured using a solar simulator (Newport, 150 W) with an AM 1.5G filter in a nitrogen filled glovebox at room temperature. The intensity of illumination was set for 100 mW/cm2 using a Si reference Hamamatsu cell (Model C24 S1787-04) calibrated by the National Renewable Energy Laboratory (NREL), USA. The spectral mismatch of the solar simulator was estimated to be less than 10%. External quantum efficiency data were measured with monochromatic light by varying the excitation wavelength from 300 to 1100 nm at intervals of 10 nm, under a white bias light of 100 mW cm2 (Optronics Laboratories, Inc.). The surface morphology of the thin films was measured using atomic force microscopy (Park Systems XE-150)

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C.-D. Park et al. / Organic Electronics 16 (2015) 95–100 Table 1 Various liquid properties of the solvents selected in the work.

B.P. (°C) Density (g/cm3) Molar mass (g/mol) Viscosity at 25 °C (cP) [16]

DCB

MS

p-XL

AP

BA

180 1.3 147.01 1.32

164.7 0.86 120.19 0.67

138 0.86 (p) 106.16 0.61

202 1.028 120.15 1.62

178.1 1.0415 106.12 1.42

operating in non-contact mode with 2 lm  2 lm scan size. The film thickness was measured using a Variable Angle Spectroscopic Ellipsometer (VASE M-2000, J.A. Woollam Co.).

the volume of liquid phase which coalesces it through surface tension effects [14]. When a solution of P3HT is heated, the solubility of P3HT will be increased, and then as the solution is slowly cooled down to ambient temperature, amorphous and crystalline P3HT will be precipitated out forming a three-dimensional network within the liquid [15]. Thus, the lower solubility of P3HT in MS makes it easier for the P3HT to precipitate out in the MS–AP mixture than in p-XL–AP system which leads to a more rapid formation of the gel structure in MS–AP, around 2 h. Conversely, the higher solubility of P3HT in p-XL means that the P3HT cannot rapidly precipitate upon cooling and the growth of three-dimensional network takes 2–10 days to achieve a gel type consistency. In addition to the solubility of P3HT which affects the rate of precipitation out of the solution, high viscosities are necessary to support the growth of a structured gel encompassing the whole solution, rather than discrete compact polymer particles. Thus, the more rapid formation of the gel-type solution in MS may also be due in part to its higher viscosity (0.67 cP) than p-XL (0.61 cP), although it is suspected that this factor

3. Results and discussion The various liquid properties of solvent candidates used in this work are shown in Table 1. For a primary solvent, pXL was chosen due to its better solubility of P3HT than that of MS. Benzaldehyde (BA) was selected as a secondary component for its similar boiling point and viscosity compared to those of DCB. The gel behavior of MS–AP binary system was compared with those of p-XL–AP and p-XL– BA systems. While the MS–AP binary solvent system needed only 2 h of cooling time at ambient temperature, p-XL–AP binary systems required at least 2 days to form a gel structure. The difference in the formation of gel structure can be explained by the combination of two factors, namely, solubility and viscosity. Gels consist of a solid three-dimensional network (or agglomeration) that spans

(a)

(b) 0

-5

2

J (mA/cm )

Pure p-XL 90-10 vol.% p-XL-AP 80-20 vol.% p-XL-AP 70-30 vol.% p-XL-AP

2

J (mA/cm )

0

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2.5 2 days aged 10 days aged 20 days aged

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% AP

Fig. 2. The J–V characteristics under illumination of 100 mW cm2 (AM 1.5G) for devices fabricated from P3HT:PCBM solutions in the p-XL–AP binary solvent mixtures aged (a) 3 days, (b) 10 days, and (c) 20 days. (d) The plot of power conversion efficiency (PCE) as a function of the amount of acetophenone (AP) for the devices. Devices were not fabricated for 100 vol.% p-XL solutions since the gel-type solution was not yet formed at 2 days.

C.-D. Park et al. / Organic Electronics 16 (2015) 95–100

The results for p-XL–AP system suggest that there is an optimal concentration (up to 20 vol.% AP) and aging time (10 days) for the best device performance. For comparison, these conditions were applied to p-XL–BA system. As is expected, the introduction of less viscose BA (1.42 cP) compared to AP (1.63 cP) delayed the formation of gel structure in the p-XL–BA binary system. While the p-XL–AP system required 2 days, the p-XL–BA system needed 3 days of aging time. The devices fabricated from the 80–20 vol.% p-XL–BA solution, the 80–20 vol.% p-XL–AP solution, as well devices fabricated from solutions of pure MS, pure p-XL, and the standard DCB, are shown in Fig. 3a. The device performances fabricated from the MS, p-XL, and DCB exhibited the similar performances with previously reported results [15,17]. As expected, the J–V curve illustrates the significant difference in the short circuit current for the device fabricated from the p-XL in comparison to DCB [9]. By adding 20 vol.% BA to p-XL, however, Jsc increased from 7.8 mA cm2 to 9.2 mA cm2, RSA decreased from 3.0 X cm2 to 2.6 X cm2 (refer to Table 2), and the fill

0

(a)

pure MS pure p-XL pure DCB 80-20 vol.% MS-AP 80-20 vol.% p-XL-AP 80-20 vol.% p-XL-BA

-2 2

J (mA/cm )

is minor to the solubility difference [16]. To illustrate this point, the pure p-XL solution required 10 days for the formation of a gel structure but the addition of the more viscose AP (1.63) to p-XL reduced the aging time by several days to as low as 2 days for the 70–30 vol.% p-XL–AP system. Thus, the formation of the gel type solution, and in turn, the morphology of the films, depends on both the solubility of the P3HT in the solution and the viscosity. To determine the effect of solution aging on the device performance, devices were fabricated from the various p-XL–AP binary solvent mixtures after 2 days, 10 days and 20 days of aging time. The device performance data are given in Fig. 2. For devices fabricated from 2 days aged solutions, as the amount of AP increases, the short circuit current (Jsc) gradually increased and the open circuit voltage (Voc) remained relatively unchanged leading to the highest efficiencies for the 70–30 vol.% p-XL–AP solutions. Devices were not fabricated for 100 vol.% p-XL solutions since the gel-type solution was not yet formed at 2 days. For the solutions aged 10 days the Jsc increased for devices from all solutions. Devices using 100 vol.% p-XL, 90–10 vol.% p-XL–AP, 80–20 vol.% p-XL–AP, and 70–30 vol.% p-XL–AP demonstrated Jsc values of 8.4 mA cm2, 8.7 mA cm2, 9.1 mA cm2, and 8.2 mA cm2, respectively. The highest open circuit voltage of 0.63 V was achieved for the 100 vol.% p-XL and only decreased to about 0.6 at 30 vol.% AP. Further aging up to 20 days leads to large decreases in Jsc at all solvent ratios yet the Voc remains at 0.6 V for all 4 solutions. These changes in device performance of the p-XL/AP system can be summarized as a plot of the power conversion efficiency (PCE) as a function of the amount of AP added and aging time, shown in Fig. 2d. For the devices fabricated from the solutions aged 2 days, the PCE values were increased with the amount of AP. When the solutions were aged for 10 days, the PCE values were significantly improved for all compositions compared to those fabricated from the 2 days aged solutions and peaked at 20 vol.% AP. Further aging of the solutions, however, only decreased the device performances for all compositions. These results indicate that there is an optimal concentration and aging time for the best device performance achieving 3.68% with a 10 days aged 80–20 vol.% p-XL–AP mixture. The increasing trend in the PCE values with the addition of AP and the increase in aging time is mainly due to the increase in Jsc. Thus, it can be related to the formation of P3HT crystals in the p-XL/AP binary solvent mixtures. First, as the amount of AP increases, the formation of P3HT crystals form at a faster rate due to both the higher viscosity of AP and the lower solubility of P3HT, resulting in higher short circuit current. Similarly, aging the binary solvent mixtures helps improve device performance by increasing the amount of time for crystalline P3HT to precipitate [15]. However, the degradation of device performance observed when more than 20 vol.% of AP is added or the binary solvent mixture is aged more than 10 days needs to be explained. One possible explanation is that the P3HT precipitates in the 20 days aged solutions or high AP ratio for 10 days aged solutions, results in large domain size, and in turn, decreased device performances. Nevertheless, a number of other solution and processing parameters are likely to affect device performances as well.

-4

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pure MS pure p-XL pure DCB MS-AP p-XL-AP p-XL-BA

0.6

EQE

98

0.4

0.2

0.0 200

300

400

500

600

700

800

900

Wavelength (nm) Fig. 3. (a) The J–V characteristics under illumination of 100 mW cm2 (AM 1.5G) and (b) external quantum efficiency (EQE) measurements data for devices fabricated from MS, p-XL, DCB, 80–20 vol.% MS–AP mixture, 80–20 vol.% p-XL–AP mixture, and 80–20 vol.% p-XL–BA mixture.

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C.-D. Park et al. / Organic Electronics 16 (2015) 95–100 Table 2 Summary of device performance for various BHJ solar cell devices in the work.

Thickness (nm) Jsca (mA cm2) Voc (V) RSA (X cm2) FF (%) PCE (%) EQE (%) Jscb (mA cm2) a b

MS

80–20 vol.% MS/AP

p-XL

80–20 vol.% p-XL/AP

80–20 vol.% p-XL/BA

DCB

62 ± 5.4 4.68 ± 0.76 0.56 ± 0.04 3.44 ± 0.17 58.9 ± 1.3 1.53 ± 0.16 3.45 ± 1.8 5.08 ± 0.68

59 ± 4.7 8.88 ± 0.67 0.58 ± 0.02 2.81 ± 0.14 66.1 ± 1.4 3.38 ± 0.13 68.9 ± 1.3 9.04 ± 0.58

76 ± 3.2 7.8 ± 0.76 0.61 ± 0.03 2.97 ± 0.16 63.3 ± 2.2 3.1 ± 0.28 63.2 ± 1.4 8.1 ± 0.45

87 ± 4.5 9.09 ± 0.20 0.59 ± 0.02 2.71 ± 0.16 66.8 ± 2.1 3.68 ± 0.25 69.4 ± 1.8 9.54 ± 0.32

88 ± 5.6 9.23 ± 0.54 0.59 ± 0.03 2.6 ± 0.18 68.7 ± 2.3 3.75 ± 0.24 69.8 ± 1.6 9.68 ± 0.53

82 ± 1.8 9.15 ± 0.15 0.63 ± 0.01 2.73 ± 0.13 68.6 ± 2.0 3.92 ± 0.18 74.2 ± 1.2 10.4 ± 0.16

Jsc: current density measured under simulated solar illumination. Jsc: current density calculated based on EQE measurement.

factor increased from 63% to 69%, which are comparable to that of devices fabricated from DCB. These changes in J–V characteristics provide improved performance in PCE from 3.1% to 3.8% for p-XL and 80–20 vol.% p-XL–BA, respectively and is also superior to the 80–20 vol.% p-XL–AP device. The increase in Jsc is further illustrated in Fig. 3b where the external quantum efficiency (EQE) increases from a maximum of 63% at 500 nm for the device from p-XL up to 70% for the p-XL–BA solvent mixture. The improved device performance of the p-XL–BA mixed solvent systems can be attributed to morphological change of the P3HT/PCBM thin film blend caused by the slower evaporation rate of the high boiling point component (BA) upon spin coating and drying process, because the morphology of the P3HT:PCBM is largely dependent on the solution processing conditions [1–3,6,10,17]. Table 2 summarizes the performances of the devices from the solvent systems used in this work: pure MS, pXL, 80–20 vol.% MS–AP, 80–20 vol.% p-XL–AP, 80–20 vol.% p-XL–BA and pure DCB. The device performance is highly reproducible with at least 8 devices tested for each to calculate the standard deviations in performance characteristics. For the p-XL–BA mixture, the PCE varies between 3.6% and 3.8%; for comparison, the performance for the devices from DCB varies between 4.0% and 4.2%. However, the efficiency for P3HT:PCBM solar cells is reported to depend on the active layer thickness for layers less than approximately 150 nm thick [18]. Thus, due to the difficulties to match similar drying and spin coating conditions for the different solvent systems, there was a limitation to fabricate the devices with similar the film thickness. Nevertheless, the overall performance obtained from the p-XL–BA binary solvent system and DCB appears to be comparable. Further optimization of this work may enable the use of non-halogenated binary solvent mixture systems for scale-up fabrication of high performance of BHJ organic solar cells.

4. Conclusion In conclusion, this work provides a route for the fabrication of solution processable BHJ organic solar cells using non-halogenated binary solvent mixtures. It was demonstrated that solubility and viscosity have an important role in forming gel behavior in the prepared solutions and the precipitation of crystalline P3HT networks were dependent

on both the solvent ratio and aging time. A PCE of 3.68% was achieved for optimal conditions of 10 days aging for a 80–20 vol.% p-XL–AP mixture. Furthermore, devices fabricated from the 10 days aged 80–20 vol.% p-XL–BA binary solvent mixtures showed improved PCE from 3.1% to 3.8% and EQE from 63% to 70% compared to a pure p-XL solution. The improved device performance of the p-XL–BA mixed solvent systems is attributed to an increase in Jsc due to the morphological evolution of the P3HT/PCBM active layer caused by adding less viscose and high boiling point component (BA) of the binary solvent mixture system. The overall performance of the device fabricated from the non-halogenated p-XL–BA binary solvent mixture is comparable to that of a standard device from DCB. These results suggest that it is possible to fabricate high performance organic BHJ solar cells using non-halogenated binary solvent mixtures. Nevertheless, the toxicity of this specific mixed solvent system (p-XL–BA) is still undesirable for large scale use [19,20]. Thus, the further optimization of non-halogenated binary solvent mixture systems which use even more benign solvents could greatly benefit the scale-up fabrication of high performance BHJ organic solar cells. Acknowledgements The authors thank the National Science Foundation (CHE-0748867) and Advanced Photovoltaics Center for partial support of this work. References [1] A.J. Moulé, K. Meerholz, Morphology control in solution-processed bulk-heterojunction solar cell mixtures, Adv. Funct. Mater. 19 (19) (2009) 3028–3036. [2] E. Verploegen, C.E. Miller, K. Schmidt, Z. Bao, M.F. Toney, Manipulating the morphology of P3HT–PCBM bulk heterojunction blends with solvent vapor annealing, Chem. Mater. 24 (20) (2012) 3923–3931. [3] X. Yang, J. Loos, S.C. Veenstra, W.J.H. Verhees, M.M. Wienk, J.M. Kroon, M.A.J. Michels, R.A.J. Janssen, Nanoscale morphology of highperformance polymer solar cells, Nano Lett. 5 (4) (2005) 579–583. [4] Y. Zhao, Z. Xie, Y. Qu, Y. Geng, L. Wang, Solvent-vapor treatment induced performance enhancement of poly(3hexylthiophene):methanofullerene bulk-heterojunction photovoltaic cells, Appl. Phys. Lett. 90 (4) (2007). 043504-N.PAG. [5] S. Guo, M.A. Ruderer, M. Rawolle, V. Körstgens, C. Birkenstock, J. Perlich, P. Müller-Buschbaum, Evolution of lateral structures during the functional stack build-up of P3HT:PCBM-based bulk heterojunction solar cells, ACS Appl. Mater. Interf. 5 (17) (2013) 8581–8590.

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