Conflicted Effects of a Solvent Additive on PTB7:PC71BM Bulk

Feb 25, 2015 - Recently, polymer–fullerene based bulk heterojunction (BHJ) solar cells, which contain blends of poly({4,8-bis[(2-ethylhexyl)oxy]benz...
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Conflicted Effects of a Solvent Additive on PTB7:PC71BM Bulk Heterojunction Solar Cells Wanjung Kim,†,∥ Jung Kyu Kim,† Eunchul Kim,‡ Tae Kyu Ahn,‡ Dong Hwan Wang,*,§ and Jong Hyeok Park*,∥ †

School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea § School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul 156-756, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea S Supporting Information *

ABSTRACT: Recently, polymer−fullerene based bulk heterojunction (BHJ) solar cells, which contain blends of poly({4,8bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), have been widely studied due to exhibiting high power conversion efficiency (PCE) and well-defined nanomorphology. Because of the short exciton diffusion pathway (less than 10 nm) in organic thin films, the optimization of PTB7:PC71BM BHJ with optimized morphology is very important between the donor and acceptor. In order to increase nanoscale phase separation, the chemical additives of 1,8-diiodooctane (DIO) have been used in PTB7:PC71BM blend systems. However, the mechanism studies of DIO in BHJ solar cells and its effectiveness on device stability are unclear. In this study, we fabricated polymer solar cells (PSCs) based on PTB7:PC71BM BHJ with various DIO concentrations to investigate not only correlation between device performances and different morphologies, but also the influence of additives on device stabilities. Positive effects of DIO, which were induced by efficient charge separation in BHJ at optimized blending ratio, are proved by the results of time-resolved photoluminescence (TRPL), and negative effects of DIO on a device stability have been investigated according to the ISOS-D-1 protocol.

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order to gain the highest PCE for PSCs, various π-conjugated p o ly m e r s s uc h as P 3H T, 3 − 6 P C D T B T , 7 − 1 1 a n d PCPDTBT12−16 have been combined with C60-based fullerene or C71-based fullerene materials in BHJ PSCs. Nowadays, poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]-

INTRODUCTION

Nowadays, polymer solar cells (PSCs) have been widely studied by promising research groups due to their effective advantages such as light weight, low cost, high flexibility, and simple fabrication processes. The power conversion efficiency (PCE) of polymer−fullerene based bulk heterojunction (BHJ) solar cells, containing internetworks of π-conjugated polymer and fullerene derivative, has dramatically increased, approaching 9.2% for single and 10.6% for tandem junction devices.1,2 In © 2015 American Chemical Society

Received: November 3, 2014 Revised: February 25, 2015 Published: February 25, 2015 5954

DOI: 10.1021/jp510996w J. Phys. Chem. C 2015, 119, 5954−5961

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additives are much higher than that of the host solvent, such as 1,8-dibromooctane (bp ∼ 271 °C) and 1,8-octanedithiol (bp ∼ 270 °C), it is difficult to eliminate these processing additives after nanoscale phase separation. Many researchers have studied the morphology of BHJ films composed of PTB7:PC71BM including DIO to investigate the composition and aggregated domain size of components with various measurements such as grazing incidence X-ray scattering (GIWAS), near edge X-ray absorption fine structure (NEXAFS), resonant soft X-ray scattering (R-SoXs), photoconductive atomic microscopy (photoconductive-AFM), and smallangle X-ray scattering (SAXS).32,34,36 However, many studies only focused on the morphology change between the PTB7:PC71BM BHJ films without DIO or with optimized concentration DIO (3 vol % DIO in chlorobenzene (CB)), which showed the highest PCE of PSCs. However, changes in morphology and charge separation characterization were not directly correlated when DIO was added in BHJ solar cells. In addition, effectiveness of DIO on BHJ device stability is still unclear. In this study, we controlled the amount of processing additive in the BHJ solution and investigated the BHJ morphology evolution at deficient, optimized, and excess DIO conditions through photocurrent−voltage (J−V) measurements, incident photon to charge carrier efficiency (IPCE) measurement, atomic force microscopy (AFM), contact angle measurement, and time-resolved photoluminescence (TRPL) (see Figure 1b for device schematic in this study). Furthermore, we investigated the relationship between the photophysical responses and nanoscale morphologies. Finally, we studied the device stability of PSCs composed of a BHJ layer having PTB7:PC71BM added with various DIO concentrations in air condition to study the influences of DIO on not only the initial PCE, but also the PCE degradation.

dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) as π-conjugated polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (see Figure 1a for material structure) have been widely used as

Figure 1. (a) Molecular structures of PTB7, PC71BM, and DIO. (b) Schematic of PTB7/PC71BM BHJ solar cells used in the study.

active materials in PSCs, approaching PCE 7−10%.17−28 PC71BM, a type of C70-based fullerene derivative, is generally used to enhance photoabsorption efficiently in a broad lightwavelength range compared to C60-based materials such as PC61BM. As previously reported in many studies,29,30 the phase-separated nanoscale morphology of BHJ blends is very important for efficient charge separation and transport due to the limited exciton diffusion length of organic materials (≤10 nm). This has enabled PCE enhancements of PSCs by controlling the nanoscale morphology of the active layer, satisfying the BHJ donor/acceptor domain length scale of ≤10 nm with good polymer−fullerene miscibility. Therefore, many researchers have focused on the morphology control of PTB7:PC71BM blends to optimize the device efficiency via increasing the high interfacial area between PTB7 and PC71BM. However, PC71BM could aggregate easily and form a large domain size of ∼100−200 nm diameter,31−33 including small spherelike structures of 20−60 nm diameter.34 In order to solve the problem of the aggregation of C60 or C70-based fullerene derivatives in polymer−fullerene systems, chemical additives, especially di(R)alkanes with various functional groups (R), have been added to the BHJ blends solution.12,35 Various di(R)alkanes with various functional groups (R) have been used as processing additives because they satisfy the requirements of processing additives: (1) nonreacting materials with either the polymer or the fullerene, (2) selective solubility of one of the components, and (3) higher boiling point than the host solvent. Among them, 1,8diiodooctane (DIO) is generally used as a processing additive in PSCs because DIO has proper boiling point (bp ∼ 168 °C) compared to its host solvent (chlorobenzene, b.p ∼132 °C). If the boiling point of processing an additive is lower than that of the host solvent, like 1,8-dichlorooctane (bp ∼ 115 °C), it is difficult to expect the positive effects from the additive because the processing additive might be evaporated before the host solvent. On the other hand, if the boiling points of processing

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RESULTS AND DISCUSSION Device Fabrication and Characteristics. The J−V characteristics of the PSC devices prepared from adding various DIO concentrations (0, 1, 3, and 8 vol %) in CB solvent of BHJ solution are shown in Figure 2a and the parameters of the solar cell are shown in Table 1. Without using DIO, the open circuit voltage (VOC) was 0.73 V, but decreased as DIO concentration increased. VOC of PSCs decreased down to 0.71 V when 8 vol % DIO was used as the processing additive. However, in contrast to VOC, the fill factor (FF) increased from 44.61% to 65.43%. In both the VOC and FF tendency, no significant difference was observed between the PSCs with 3 and 8 vol % DIO contents in the solvent. Whereas, we found that the short-circuit current density (JSC) of the devices increased from 0 vol % DIO concentration (10.46 mA/ cm2) to 3 vol % DIO concentration (16.42 mA/cm2), but decreased to 15.48 mA/cm2 when 8 vol % DIO concentration was used. As a result, therefore, the PCE of the device without using DIO is only 3.42%, and increased to 4.12% (with DIO 1 vol %) and 7.83% (with DIO 3 vol %), but decreased to 7.23% when 8 vol % DIO was used. Figure 2b shows the dark current−voltage characteristics through which we found that the series resistances (RS) and shunt resistances (RSh) of the PSCs controlled the amount of DIO. In Table 1, the RS of the PSCs processed with DIO decreased from 0 vol % DIO concentration (1.90 Ω·cm2) to 3 vol % DIO concentration (1.22 Ω·cm2) and 8 vol % DIO concentration (1.25 Ω·cm2). On the other hand, the RSh of the PSCs processed with DIO 5955

DOI: 10.1021/jp510996w J. Phys. Chem. C 2015, 119, 5954−5961

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The Journal of Physical Chemistry C

the BHJ film increased, the RS of PSCs decreased, and the RSh of PSCs increased and became saturated. Figure 2c shows the external quantum efficiency (EQE) spectrum of the PSCs with various DIO concentrations. In Figure 2c, the EQE shows similar variation to JSC of the PSCs with various DIO concentrations; as the DIO contents in the solvent were increased, the EQE of PSCs increased, and decreased to DIO 8 vol % contents in solvent after reaching maximum in the DIO 3 vol % condition. However, the IPCE curves of the PSCs differed from the DIO concentration. In order to analyze the IPCE graph shape specifically, we normalized the EQE of the PSCs and Supporting Information Figure S1 shows the normalized EQE of the PSCs with various DIO concentrations. As shown in the figure, as the DIO concentration in the solvent was increased, the normalized EQE of the PSCs decreased in the wavelength of 600 to 700 nm, but increased in the wavelength of 400 to 600 nm. Interestingly, the IPCE curves of the PSCs of the PTB7:PC71BM exhibited a blue shift with increasing DIO concentration. Correlating the PSCs characteristics, the following experimental results explain the influences of DIO on the morphology of PTB7:PC71BM and charge carrier dynamics, including carrier transport and recombination in active layer. Relation between Morphology of BHJ Film and Charge Carrier Dynamics. In order to characterize the morphology of the PTB7:PC71BM composite BHJ films with various DIO contents in the solvent, we used atomic force microscopy (AFM) with the tapping mode. For an accurate comparison with the device characteristics, all BHJ films were prepared under the same condition as the PSC device preparation. Figure 3 shows the topography (left) and phage images (right) of the BHJ films with various DIO concentrations (0, 1, 3, and 8 vol %) in the solvent. The surface roughness characteristics including Ra, RMS, and average height are shown in Table 2. In order to prevent the additional morphology change with time according to the residual DIO contents in film, all films were dried fully in vacuum chamber after the formation of a film. In the BHJ film without the processing additive, the PC71BM was aggregated and formed large domains of 100−200 nm. This result is similar to previous researches.31−34 Because of the large PC71BM domains, the roughness average (Ra), the root-mean-square (RMS), and the average height of BHJ without DIO were 7.32, 8.49, and 19.58 nm, respectively (Figure 3a and b). However, in the BHJ films with DIO, the large domain of the aggregated PC71BM started to disappear, which could support that the DIO acts as a good solvent for PC71BM and a poor solvent for PTB7.37 In the BHJ film processed with 1 vol %

Figure 2. PSCs performance of PTB7:PC71BM with various DIO concentrations (0, 1, 3, and 8 vol %): (a) J−V characteristics of the PSCs under 1 sun illumination, (b) dark J−V characteristics of the PSCs, and (c) EQE of the PSCs.

increased from 0 vol % DIO concentration (1.60 kΩ·cm2) to 3 vol % DIO concentration (3.16 kΩ·cm2) and 8 vol % DIO concentration (3.15 kΩ·cm2). As the amount of DIO added to

Table 1. J−V Characteristics under 1 Sun Illumination of PTB7:PC71BM PSCs with Various DIO Concentrations (0, 1, 3, and 8 vol %)a VOC [V] DIO 0 vol % PTB7:PC71BM DIO 1 vol % PTB7:PC71BM DIO 3 vol % PTB7:PC71BM DIO 8 vol % PTB7:PC71BM a

0.73 ± (0.73) 0.71 ± (0.72) 0.71 ± (0.72) 0.71 ± (0.71)

0.01 0.01 0.01 0.01

JSC [mA/cm2] 10.47 ± (10.46) 13.26 ± (13.17) 16.50 ± (16.42) 15.55 ± (15.48)

0.10 0.10 0.06 0.10

FF [%] 43.74 ± (44.61) 41.94 ± (43.42) 66.36 ± (66.38) 65.52 ± (65.43)

0.93 1.36 0.08 0.47

PCE [%] 3.36 ± (3.42) 3.96 ± (4.12) 7.76 ± (7.83) 7.20 ± (7.23)

0.07 0.16 0.07 0.06

RS [Ω·cm2]

RSh [kΩ·cm2]

1.90

1.60

2.36

0.97

1.22

3.16

1.25

3.15

The data shown are the average values obtained from six devices with standard deviation. The data in parentheses are the highest values. 5956

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the PTB7 aggregates. Figure 3f shows the phase image of the active layer PC71BM, of which it was very difficult to find the PC71BM aggregates. We could therefore expect that the 3 vol % DIO concentration in CB is the optimized condition in terms of maximizing the interface between PTB7 and PC71BM. This is similar to results from previous research31−34 in that the 3 vol % DIO concentration in CB eliminates PC71BM aggregates and intermixes PTB7:PC71BM well. Finally, when the DIO was added to the active layer in a more than optimized condition (8 vol % in CB), we could find that the Ra, RMS, and average height of the BHJ film start to increase again, as shown in Figure 3g and h. We suggest that the excess DIO acts as a cosolvent competing with the host solvent, CB. Excess DIO could remain after forming optimized morphology between PTB7:PC 71 BM. Due to the remaining DIO, PC 71 BM redissolved and formed the large PC71BM domains. Comparing the phase images of BHJ between 3 vol % DIO and 8 vol % DIO, we could find the PC71BM domains. In order to understand the surface characteristics of the BHJ films, we measured the contact angle of the BHJ films without and with various DIO concentrations with water droplets. In our previous study,38 PTB7 had a higher hydrophobic property than that of PC71BM and a higher contact angle measured by water droplets (PTB7, 98.2°; PC71BM, 77.1°). In Figure S2 in the Supporting Information, the PTB7:PC71BM films show various contact angles of 90.3°, 91.4°, 92.8°, and 91.3°, with various DIO concentrations (0, 1, 3, and 8 vol % in CB), respectively. Through this study, we could estimate the surface composition and property of the BHJ films indirectly. After spin-coating, the BHJ film processed with 0 vol % DIO showed the lowest contact angle value, closest to PTB7’s due to aggregated PC71BM domains in the PTB7 matrix. When the contents of DIO increased from 0 to 3%, the surface property of the BHJ films changed from hydrophilic to hydrophobic relatively, indicating that the surface became richer in PTB7 than PC71BM molecules compared to BHJ film processed with 0 vol % DIO. This was because aggregated PC71BM molecules could be penetrated into PTB7 aggregates. Due to increased content of PTB7 in the surface of BHJ film after penetrating PC71BM molecules into PTB7 aggregates, the BHJ film processed with DIO 1, 3 vol % showed higher contact angle value than that of the BHJ film without DIO. However, the contact angle of BHJ film with 8 vol % DIO decreased again, indicating that the surface became richer PC71BM than PTB7 compared to the BHJ film processed with 3 vol % DIO. This phenomenon showed that the PC71BM molecules were reaggregated due to the cosolvent effect due to the excess DIO and this matched well with the AFM observation. By comparing the morphologies and their corresponding device characteristics, we could conclude that the nanoscale phase separation controlled by additive concentration gains insight related to the increased JSC, FF and PCE of the devices. Figure 4a and b shows the time profile of PL intensity of the BHJ film without and with various DIO concentrations. Generally, time-correlated single photon counting (TCSPC) was conducted to analyze the efficient charge extraction at the device without and with processing additive in PSCs.39 In Figure 4c and Table 3, the PL lifetime decreased from 106 ps in the BHJ with 0 vol % DIO to 77 ps in the BHJ with 1 vol % and 56 ps in the BHJ with 3 vol % DIO, respectively, while there was further increase of the BHJ film with 8 vol % DIO around 80 ps. Thus, the BHJ film processed with 3 vol % DIO showed the highest exciton quenching efficiency and efficient charge

Figure 3. AFM images of PTB7:PC71BM films with various DIO concentrations. Images (a, c, e, g) are height images and (b, d, f, h) are phase images: (a, b) PTB7:PC71BM film with DIO 0 vol %, (c, d) PTB7:PC71BM film with DIO 1 vol %, (e, f) PTB7:PC71BM film with DIO 3 vol %, and (g, h) PTB7:PC71BM film with DIO 8 vol %.

Table 2. AFM Data of PTB7:PC71BM Films with Various DIO Concentrations (0, 1, 3, and 8 vol%) DIO DIO DIO DIO

0 1 3 8

vol vol vol vol

% % % %

PTB7:PC71BM PTB7:PC71BM PTB7:PC71BM PTB7:PC71BM

Ra (nm)

RMS (nm)

avg height (nm)

7.32 7.11 3.86 11.20

8.49 9.14 4.81 13.90

19.58 43.35 18.37 42.37

DIO, as shown in Figure 3c and d, the PC71BM domain size slightly decreased and connected with each domain and formed the “ant’s nest” structure. Because of this structure, the surface roughness and average height of the BHJ film with 1 vol % DIO increased; the Ra, RMS, and average height were 7.11, 9.14, and 43.35 nm, respectively, compared to the BHJ film without DIO. This result shows that small amount of additive in BHJ could not remove aggregated PC71BM domains fully and makes the PC71BM molecules connected each other, resulting in high surface roughness and average height. When more DIO was added to have 3 vol % in the CB, as shown in Figure 3e and f we could observe the smoothest surface of the active layer (Ra, 3.86 nm; RMS, 4.81 nm; average height, 18.37 nm). In this condition, the large aggregated PC71BM molecules could be removed fully and the PC71BM molecules could integrate into 5957

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respectively. The decrease of FF of PSC devices caused the decay of PCE, as shown in Figure 5d. The relative amount of decay of PCE was similar to FFs. This tendency might be due to the difference in the active layer morphologies with various DIO concentrations. Figure 6 shows the height and phase AFM images of the BHJ layer surface with various DIO concentrations stored in air condition after 300 h; PTB7:PC71BM film with 0 vol % DIO (Figure 6a and b), 1 vol % DIO (Figure 6c and d), 3 vol % DIO (Figure 6e and f), and 8 vol % DIO (Figure 6g and h). In Figure 6, the morphology of BHJ film in air after 300 h was similar to the initial morphology state of the BHJ films in Figure 3. Presumably, the homogeneous morphology between PTB7 and PC71BM by the addition of DIO would make more PTB7, which is more easily degraded by O2 and H2O molecules being exposed to the air compared to the BHJ layer without DIO or with small amount of DIO. When the PTB7 was exposed to the air, there are two possible degradation mechanisms. One is the organoaluminum barrier layer generation between conjugated polymer and aluminum due to highly reactive aluminum in the absence of O2 and H2O molecules.41 However, in our study, this phenomenon could not occur because the TiOx layer was inserted between the active layer and aluminum electrode. The other possible case is that the weakened shielding effect of PC71BM layer which could prevent the penetration of O2 and H2O molecules into the active PTB7 polymers. Actually, in previous research,42 the bilayer PSCs shows more stable performance than the BHJ PSCs in the air condition due to the effect of PCBM layer as shielding layer. As mentioned previously, BHJ film processed with DIO showed an increased PTB7 content on the BHJ film surface because large aggregated PC71BM domains dissolved and integrated into PTB7 domains. As a result, the BHJ layer processed with optimized DIO content could be degraded more easily in the air.43−46 However, in Figure 5, the PSC devices processed with 8 vol % DIO showed more decrease of FF compared to the device with the optimized DIO concentration, although the BHJ surface with 8 vol % DIO became richer in PC71BM than PTB7 in comparison to the BHJ film processed with 3 vol % DIO as mentioned before. Comparing the surface morphologies in Figure 3e and g, this phenomenon could occur due to rough surface caused by better contact of O2 and H2O molecules with PTB7 molecules. Through this, we found that the additive in the BHJ layer not only affects the initial photoresponse of PSCs, but also device stability.

Figure 4. Time profiles of PL of the PTB7:PC71BM films with various DIO concentrations (0, 1, 3, and 8 vol %).

Table 3. PL Lifetimes of the PTB7:PC71BM Films with Various DIO Concentrations (0, 1, 3, and 8 vol %) t (ps) DIO DIO DIO DIO

0 1 3 8

vol vol vol vol

% % % %

PTB7:PC71BM PTB7:PC71BM PTB7:PC71BM PTB7:PC71BM

106 77 56 80

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CONCLUSION In this work, we controlled the amount of solvent additive in BHJ solution and compared the BHJ film morphology with deficient, optimized, and excess DIO in terms of the photovoltaic response and device stability. In terms of the initial photovoltaic response, the PTB7:PC71BM BHJ film should form a nanomorphology to increase the interfacial area between the PTB7 and PC71BM molecules to overcome the short-exciton diffusion length. Therefore, in the case of a deficient DIO condition, the photoresponse of the PSCs is too low because the PTB7 and PC71BM molecules could not form the BHJ nanomorphology from the aggregated PC71BM molecule. Whereas, when more DIO was added to the BHJ solution than the optimized condition, the PCE of PSC decreased because the PC71BM molecules reaggregated due to the excess DIO. In terms of the device stability, the PSCs processed with DIO showed poor stability. Especially, as the

separation, which is consistent with the morphology and device characteristics results. Stability Test in the Air Condition. The stability tests of PSCs composed of a BHJ layer with added DIO were performed using ISOS-D-1 protocol (shelf, in air condition).40 Figure 5 and Table 4 show the decay behaviors of VOC, JSC, FF, and PCE of PTB7:PC 71 BM PSCs with various DIO concentrations (0, 1, 3, and 8 vol %) stored in air for 300 h. In the stability data in air condition, no significant difference was observed in the VOC and JSC at various DIO concentrations compared to the initial state, as shown in Figure 5a and b. However, Figure 5c shows that as DIO concentration increases, the FF of all PSC devices decreases more after 300 h in air compared to the initial state. The FF of PSC devices processed with 3 and 8 vol % DIO concentration decreased rapidly compared to the one of devices without DIO (43.74% to 33.24%) from 66.36% to 30.17% and from 65.52% to 26.32%, 5958

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Figure 5. Stability data of PTB7:PC71BM PSCs with various DIO concentrations (0, 1, 3, and 8 vol %) stored in air for 300 h: (a) VOC, (b) JSC, (c) FF, and (d) PCE.

Table 4. J−V Characteristics of PTB7:PC71BM PSCs with Various DIO Concentrations (0, 1, 3, and 8 vol%) Stored in Air for 300 ha JSC [mA/cm2]

VOC [V] initial DIO 0 vol % PTB7:PC71BM DIO 1 vol % PTB7:PC71BM DIO 3 vol % PTB7:PC71BM DIO 8 vol % PTB7:PC71BM a

0.73 0.71 0.71 0.71

± ± ± ±

0.01 0.01 0.01 0.01

final 0.67 0.67 0.64 0.66

± ± ± ±

0.01 0.01 0.01 0.01

10.47 13.26 16.50 15.55

± ± ± ±

0.10 0.10 0.06 0.10

FF [%]

final

initial

9.18 12.19 14.91 13.83

± ± ± ±

initial 0.12 0.10 0.02 0.14

43.74 41.94 66.36 65.52

± ± ± ±

0.93 1.36 0.08 0.47

PCE [%] final

33.24 27.95 30.17 26.32

± ± ± ±

initial 0.60 0.73 0.57 0.34

3.36 3.96 7.76 7.20

± ± ± ±

0.07 0.16 0.07 0.06

final 2.04 2.30 3.02 2.42

± ± ± ±

0.05 0.07 0.04 0.02

The data shown are the average values obtained from 6 devices with standard deviation.

the ITO glass at 4000 rpm for 30 s, and the sample was heated at 150 °C for 10 min on a hot plate. For the devices with PTB7:PC71BM, the active layer solution was spin-coated on the PEDOT:PSS layer. The thickness of the PTB7:PC71BM layer was 100 nm for all devices. A polymeric TiOx layer47 was then spin-coated on the active layer, and an Al cathode with a thickness of 100 nm was deposited on the TiOx layer under a 2 × 10−6 Torr vacuum in a thermal evaporator. Film Characterization. The thickness of the PSC layers was determined using an Alpha-step analyzer (KLA-Tencor, Alpha-Step IQ Surface Profiler). The film morphology and roughness were investigated using AFM (Veeco diINNOVA 840-012-711, tapping mode). The absorbance spectra of the PTB7:PC71BM films with various DIO concentrations were obtained with a UV−vis spectrophotometer (UV-2401 PC, Shimadzu). PL lifetimes were measured on a conventional time-correlated single photon counting (TCSPC) system (PicoQuant GmbH, FluoTime200). The light source was a picosecond diode laser of 670 nm with a 4 MHz repetition (PicoQuant GmbH, LDH-P-670, power: 0.3 mW). The instrumental response function (IRF) was ca. 168 ps (fwhm). The cutoff filters were applied to the emission light to remove any trace of scattering and the emission light was aligned with a magic angle (54.7°) to the excitation laser pulse. PL lifetimes

DIO content was increased, the device stability decreased steadily because the PTB7 molecules could be exposed to the air surface by diminishing the shielding layer, PC71BM layer between under metal electrode. This means that using the solvent additive in the BHJ film not only affects PCE of the PSCs, but also affects the device stability. Therefore, the optimized condition of the solvent additive should consider PCE and the stability of PSCs simultaneously.

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EXPERIMENTAL SECTION Preparation of Chemicals. For the active layer, a mixture of PTB7:PC71BM (poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid methyl ester) at a weight ratio of 1:1.5 was dissolved in a mixed solvent of chlorobenzene (CB)/1,8-diiodooctane (DIO). In that case, DIO was added to the CB at various concentrations (1, 3, and 8 vol %). PEDOT:PSS (poly(3,4ethylenedioxythiophene):poly(styrenesulfonate), Clevios P VP AI4083) was diluted with 2-propanol before use. Fabrication of Polymer Solar Cell Devices. Indium tin oxide (ITO) glass substrates were cleaned sequentially using acetone and 2-propanol in a sonic bath. The ITO glass was then cleaned with UV/ozone cleaner (Altech LTS) for 10 min. After cleaning, the PEDOT:PSS solution was spin-coated on 5959

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Article

AUTHOR INFORMATION

Corresponding Authors

*(D.H.W.) E-mail: [email protected]. Tel: +82-2-820-5074. *(J.H.P.) E-mail: [email protected]. Tel: +82-2-2123-5760. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A2A1A09014038, 2009-0083540, 2014M3A7B4051747, 2014R1A1A1002419).

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Figure 6. AFM images of PTB7:PC71BM films with various DIO concentrations after 300 h in air condition. Images (a, c, e, g) are height images and (b, d, f, h) are phase images: (a, b) PTB7:PC71BM film with DIO 0 vol %, (c, d) PTB7:PC71BM film with DIO 1 vol %, (e, f) PTB7:PC71BM film with DIO 3 vol %, and (g, h) PTB7:PC71BM film with DIO 8 vol %.

were fitted with a multiexponential model using Fluofit (PicoQuant GmBH). Device Measurement. To confirm the accuracy of the device area, the active layer regions of the PSCs were examined using a video microscope (Sometech, SV-35) with an aperture placed on top of the cell; the aperture area was 11.430 mm2. The current−voltage (J−V) characteristics of the PSCs were measured with a Keithley model 2400 source measuring unit under AM 1.5 white light (100 mW/cm2) illumination. A solar simulator (Oriel Sol 3A, class AAA) with a filtered 450 W xenon lamp was employed as the light source, and proper adjustments were made with a Si reference cell (VLSI standards, Oriel P/N 91150 V) in order to produce a 1 sunlight intensity of 100 mW/cm2.

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ASSOCIATED CONTENT

S Supporting Information *

Normalized EQE of the PSCs and contact angle data of PTB7:PC71BM films with various DIO concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org/. 5960

DOI: 10.1021/jp510996w J. Phys. Chem. C 2015, 119, 5954−5961

Article

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