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Enhancing the performance of polymer solar cells using CuPc nanocrystals as additives
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 204001 (http://iopscience.iop.org/0957-4484/26/20/204001) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 204001 (7pp)
doi:10.1088/0957-4484/26/20/204001
Enhancing the performance of polymer solar cells using CuPc nanocrystals as additives Yajie Zhang and Zhixiang Wei 1
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China E-mail: [email protected] Received 30 November 2014, revised 1 February 2015 Accepted for publication 2 February 2015 Published 27 April 2015 Abstract
There is an increasing interest in the use of different nanoparticles as additives in polymer solar cells for enhancing the light absorption of active layers as well as their power conversion efficiency (PCE). In this paper, we report a PCE enhancement by simply adding copper phthalocyanine (CuPc) nanocrystals into photovoltaic devices based on a poly(3hexylthiophene) (P3HT): fullerene system. Two kinds of device structure were studied: the first one is a CuPc nanocrystal suspension spin coated on the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-coated substrate; the second one is the CuPc nanocrystal suspension added into the active layer solutions. It is proved that incorporating organic semiconductor nanocrystals into the active layer can help trap light and enhance the crystallinity of the active layers, thus improving the device performance. This strategy might be generally compatible with a broad range of organic photovoltaic materials and offers an effective approach to enhance the device performance. S Online supplementary data available from stacks.iop.org/NANO/26/204001/mmedia Keywords: organic photovoltaic, organic semiconductors, nanocrystals, additives (Some figures may appear in colour only in the online journal) 1. Introduction
approaches have been taken to enhance the light absorption without increasing the thickness of the active layer so to avoid the increase in charge recombination. Recently, an efficient strategy was developed by using different nanoparticles as fillers in PSCs for enhancing light absorption of the active layers as well as their power conversion efficiency (PCE) [19]. Metal nanoparticles are the most frequently used additives due to their localized surface plasmon resonance for efficient light trapping in the active layer, thus enhancing the photon absorption without a need for a thick film. Various metal nanoparticles can be adopted, such as Au, Ag [20], Au/Ag alloy [19], and Al [21]. Organic semiconductor crystals have high crystallinity and charge carrier mobility, and their absorption can be tuned by various molecular structures. However, there has been almost no investigation on the influence of adding organic semiconductor nanocrystals into PSCs until now. Here we report a cooperative PCE enhancement of over 10% by simply adding copper phthalocyanine (CuPc)
Constructing efficient photovoltaic devices comprising polymer solar cells (PSC) [1, 2], hybrid solar cells [3, 4], and even ternary solar cells [5–7] based on a bulk heterojunction (BHJ) configuration has been attracting enormous attention due to their light weight, flexibility, and solution processability. Multidisciplinary efforts have been taken to achieve a high PCE of solar cells, such as rational designs of low-bandgap conjugated polymers [8–14], optimization of film morphology [15, 16], and development of new device architectures [17, 18]. As a result, BHJ solar cells have been reported with a PCE higher than 10% to date [17]. Although obvious progress has been obtained on the PSCs, there are still various limitations that block further enhancement of device performance. For instance, enhancing the light absorption ability of the active layer is a great challenge [19], and it is well known that reducing the recombination of charge carriers is a linchpin for increasing device efficiency. Therefore, different 0957-4484/15/204001+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2015) 204001
Y Zhang and Z Wei
Figure 1. (a) Chemical structures and (b) energy levels of P3HT, CuPc, PC71BM, IC60BA, and IC70BA. (c) Illustration of the P3HT: fullerene-based solar cells with reference structure, CuPc modified structure (CuPc/P3HT: fullerene), and CuPc mixed structure (CuPc:P3HT: fullerene).
nanocrystals into poly(3-hexylthiophene) (P3HT):fullerene derivative-based photovoltaic devices. P3HT is the most representative conjugated polymer donor, with the advantages of high hole mobility and good crystallinity, which makes it easy to form a nanoscale interpenetrating network with fullerene- derivative acceptors. In addition, good reproducibility of the photovoltaic performance with an optimal active-layer thickness of ∼200 nm is also very attractive for the investigation of adding relatively big nanocrystals. Three systems, including P3HT:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), P3HT:indene-C60 bisadduct (IC60BA), and P3HT:indene-C70 bisadduct (IC70BA), were selected. CuPc nanocrystals were selected as the additive due to their low cost, high charge carrier mobility [22, 23], and excellent photovoltaic property [24, 25]. We show that the incorporation of CuPc nanocrystals exhibits the enhancement of the light absorption and the improvement of the crystallinity of the active layer simultaneously, which result in an increase of the PCE of devices.
spin coated on the PEDOT:PSS-coated substrate (CuPc/ P3HT:fullerene), while in the second one, a CuPc nanocrystal solvent was added into the active layer solutions (CuPc: P3HT:fullerene). 2.1. Materials
P3HT (4002-E) was purchased from Rieke Metals, Inc., and used as received. PC71BM was purchased from American Dye Source, Inc. (ADS). IC60BA [26] and IC70BA [27] were synthesized according to the published procedure. CuPc was purchased from Aldrich and has been purified three times with the physical vapor transport method before using. 2.2. Photovoltaic device fabrication
For the first type of PSCs, a sandwich structure of ITO/ PEDOT:PSS/ CuPc/P3HT:fullerene/Ca/Al was constructed (figure 1(c)). Patterned indium tin oxide (ITO) glass with a sheet resistance of 10 Ω sq−1 was purchased from CSG Holding Co., Ltd (China). The ITO glass was cleaned by a sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, then treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 15 min. A CuPc chloroform suspension (2 mg ml−1) was spin coated on PEDOT:PSS modified ITO glass at 2000 rpm for 30 s. The blend of P3HT:PC71BM (1:1, W/W, 34 mg ml−1 for P3HT:PC71BM), P3HT:IC60BA (1:1, W/W, 34 mg ml−1 for P3HT:IC60BA), and P3HT:IC70BA (1:1, W/W, 34 mg ml−1 for P3HT:IC70BA) were dissolved in orthodichlorobenzene and spin-coated on a CuPc nanocrystal modified substrate (CuPc/P3HT:fullerene). For the second type of device structure, CuPc nanocrystals were mixed with
2. Experiment section Figure 1(a) shows the molecular structure of P3HT, CuPc, PC71BM, IC60BA, and IC70BA. The corresponding energy levels of the components are shown in figure 1(b). The CuPc nanoribbons were produced by physical vapor deposition first [22, 23]. Then they were scraped from the substrate, immersed in solvent, and finally sonicated to obtain a CuPc nanocrystal suspension. Two kinds of device structures were studied with the conventional device structure (ITO/PEDOT: PSS/P3HT:fullerene/Ca/Al) as the reference, as shown in figure 1(c). In the first type, a CuPc nanocrystal solution was 2
Nanotechnology 26 (2015) 204001
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Figure 2. J–V curves of the (a) P3HT:PC71BM-, (b) P3HT:IC60BA-, and (c) P3HT:IC70BA-based systems with comparison of P3HT: fullerene, CuPc/P3HT:fullerene, and CuPc:P3HT:fullerene.
P3HT:fullerene solutions (CuPc:P3HT:fullerene) (figure 1(c)). The weight ratio of CuPc nanocrystals was selected as 0.5%. When the weight ratio is higher than 0.5%, the PCE of the devices decreases. The prepared samples were annealed at 150 °C for 10 min before the vacuum deposition of a metal negative electrode. The active area of PSC is 0.04 cm2. Ca (20 nm) and Al (80 nm) cathodes were thermal evaporated in a glove box at a chamber pressure of ∼4.0 × 10−6 torr.
based on CuPc/P3HT:PC71BM and CuPC:P3HT:PC71BM, were obviously improved compared with a conventional P3HT:PC71BM structure. The highest short circuit current (Jsc) is 10.27 mA cm−2, along with an open circuit voltage (Voc) of 0.63 V and a high fill factor of 64.58%, yields an impressive PCE of 4.57% for CuPc/P3HT:PC71BM-based solar cells. After incorporation of CuPc nanocrystals, the Voc remained nearly the same; the obvious enhancement came from the Jsc increase (from 9.68 to 10.27 mA cm−2). The noticeably upward trend in Jsc suggests that the efficiency of light harvesting in the polymer blend might be improved with incorporation of CuPc nanocrystals. The same phenomenon was also found in P3HT:IC60BA- and P3HT:IC70BA-based systems. The PCE was improved from 6.08 to 6.34% in the P3HT:IC60BA system and from 5.6% to 6.18% in the P3HT: IC70BA system. EQE measurements (figure 3) of three different systems were conducted to better elucidate the reason for the improvement of the Jsc. Figure 3(a) depicts the corresponding EQE spectra of the P3HT:PC71BM-based system. When CuPc nanocrystals were introduced, the EQE increased at the wavelength range from 480 to 650 nm. The maximum EQE value for referenced P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPC:P3HT:PC71BM devices are 60.7%, 62.4%, and 67.7%, respectively. The integrated Jsc value from the EQE 9.42 mA cm−2, and spectrum are 9.30 mA cm−2, −2 9.75 mA cm , respectively. The deviations between the integral current densities and the Jsc read from J–V measurements are under 10%, indicating a good consistency of the photovoltaic results. For the EQE value of the P3HT: IC60BA and P3HT:IC70BA systems, the integrated Jsc were also calculated, and they were also in good accordance with the photovoltaic results. To better illustrate the Jsc enhancement, we also performed UV–vis absorption measurements of solar cells with and without CuPc nanocrystals. The absorption intensities of three P3HT:fullerene systems were normalized with respect to film thickness. The absorption spectrum is shown in figure 4. The absorption spectrum of CuPc is shown in figure S3. It is observed that the absorption of the three P3HT:fullerene systems are enhanced after incorporation of CuPc nanocrystals. This result indicated that incorporating organic
2.3. Measurements
The current density/voltage curves, which were detected under an ambient situation, were measured using a Keithley 2400 source-measure unit. Photocurrent was measured under AM 1.5G illumination at 100 mW cm−2 using a Newport Thermal Oriel 91159A solar simulator. Light intensity was calibrated with a Newport oriel PN 91150V Si-based solar cell. The external quantum efficiency (EQE) measurements of the devices were performed in air with an Oriel Newport system (Model 66902). Grazing incidence wide-angle x-ray scattering (GIWAXS) measurements were carried out using a small-angle x-ray scattering system (XEUSS, FRANCE Xenocs SA). The morphologies of the active layers were characterized by transmission electron microscopy (TEM) (TEM, Tecnai G2 F20 U-TWIN, FEI Co., USA). UV–vis absorption spectra were recorded using a UV–vis spectrophotometer (Lambda 950, Perkin-Elmer, USA). The thicknesses of the active layer were detected by an Alpha-atepD120 stylus profilometer (Kla-Tencor). The morphology of CuPc nanoribbons was characterized by scanning electron microscopy (SEM) (Hitachi S-4800).
3. Results and discussion Figure 2 shows the typical current density versus voltage (J–V) characteristics of solar cells under AM 1.5G illumination at 100 mW cm−2. Different device fabrication conditions were tested, and the device performance data are summarized in table 1. Figure 2(a) shows the J–V characteristics of a P3HT:PC71BM-based system. The PCEs of the devices, 3
Nanotechnology 26 (2015) 204001
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Figure 3. EQE curves of the (a) P3HT:PC71BM-, (b) P3HT:IC60BA-, and (c) P3HT:IC70BA-based systems with comparison of P3HT: fullerene, CuPc/P3HT:fullerene, and CuPc:P3HT:fullerene.
Figure 4. UV–vis absorption spectra of the active layer corresponding to (a) P3HT:PC71BM (b) P3HT:IC60BA (c) P3HT:IC70BA based system with comparison of P3HT: fullerene, CuPc/P3HT:fullerene and CuPc:P3HT:fullerene. Table 1. Summary of the fabrication conditions and performance of the BHJ solar cells.
Donor:Acceptor
PCE (%)
VOC(V)
JSC(mA cm−2)
FF
PCE best (%)
P3HT:PC71BM CuPc/P3HT:PC71BM CuPc:P3HT:PC71BM P3HT:IC60BA CuPc/P3HT:IC60BA CuPc:P3HT/:IC60BA P3HT: IC70BA CuPc/P3HT:IC70BA CuPc:P3HT:IC70BA
3.98 ± 0.04 4.30 ± 0.07 4.52 ± 0.05 5.98 ± 0.10 6.08 ± 0.05 6.20 ± 0.14 5.1 ± 0.50 6.03 ± 0.09 6.11 ± 0.07
0.63 ± 0.01 0.63 ± 0.01 0.63 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01
9.50 ± 0.18 9.80 ± 0.14 10.4 ± 0.13 9.07 ± 0.11 9.22 ± 0.13 9.90 ± 0.13 8.96 ± 0.10 10.0 ± 0.07 9.78 ± 0.07
0.63 ± 0.02 0.68 ± 0.02 0.69 ± 0.02 0.71 ± 0.02 0.71 ± 0.02 0.68 ± 0.01 0.67 ± 0.01 0.65 ± 0.02 0.67 ± 0.02
4.02 4.37 4.57 6.08 6.15 6.34 5.6 6.12 6.18
To prove that, we studied the molecular stacking of active layers through GIWAXS. Figures 5(a)–(c) present the two-dimensional GIWAXS patterns of the P3HT:PC71BMbased system. Figures 5(d) and (e) present the out-of-plane and in-plane cuts of two-dimensional GIWAXS patterns of the P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPc:P3HT: PC71BM. The first diffraction peak (100) was observed in the out-of-plane cut and located at q ≈ 0.39 Å−1. Strong (h00) peaks were found at q ≈ h × 0.39 Å−1 for h = 1, 2, and 3, which corresponds to the (h00) peaks of P3HT. The full-width at half-maximum (FWHM) of the scattering peak, Δq, correlates to the crystalline domain size (synonymous of coherence length of the single crystalline domains along the specified
semiconductor nanocrystals into the active layer can enhance the absorption of the active layer due to CuPc nanocrystals trapping light into the active layer and their complementary absorption range. It is in accordance with the Jsc increase of the corresponding systems. As for the P3HT:PC71BM and P3HT:IC60BA systems, the CuPc nanocrystal mixed structures are more obvious for the light absorption enhancement, while in the P3HT:IC70BA system, the CuPc nanocrystal modified structure is better for trapping light in the active layer. This suggests that device structure is one important factor to decide the light absorption, and CuPc nanocrystalinduced molecular packing might also be important for light absorption. 4
Nanotechnology 26 (2015) 204001
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Figure 5. 2D-GIWAXS patterns of the (a) P3HT:PC71BM, (b) CuPc/P3HT:PC71BM, and (c) CuPc:P3HT:PC71BM. (d) out-of-plane, (e) inplane GIWAXS measurements of the P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPc:P3HT:PC71BM.
Figure 6. (a) SEM image of CuPc nanoribbons. TEM images of (b) P3HT:P71BM active layer and (c) CuPc:P3HT:PC71BM active layer. Table 2. Summary of the fabrication conditions and the GIWAXS parameters of the BHJ solar cells.
Configuration P3HT:PC71BM CuPc/P3HT:PC71BM CuPc:P3HT:PC71BM P3HT:IC60BA CuPc/P3HT:IC60BA CuPc:P3HT:IC60BA P3HT: IC70BA CuPc/P3HT:IC70BA CuPc:P3HT:IC70BA
(100) FWHM
La (nm) (100)
(010) FWHM
La (nm) (010)
0.51 0.49 0.46 0.49 0.48 0.50 0.51 0.50 0.503
11.08 11.38 12.23 11.54 11.78 11.31 11.08 11.30 11.24
3.23 2.67 2.54 3.20 2.82 2.09 2.68 2.38 2.48
1.75 2.11 2.23 1.76 2.00 2.7 2.11 2.38 2.28
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Nanotechnology 26 (2015) 204001
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direction), which could be calculated by Scherrer’s equation [28, 29]. After calculating the FWHM of the (100) peaks using the peak-differentiation-imitating analysis method, the synonymous of the coherence length (La) along the alkyl chain was calculated. Though the FWHM difference of the (100) peaks of the P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPc:P3HT:PC71BM systems are not obvious, there are still domain size increasing when introducing CuPc nanocrystals into the organic solar cells (OSCs). Their La values are 11.08 nm, 11.38 nm, and 12.23 nm, respectively. On the other hand, the (010) peak at q ≈ 1.63 Å−1 is relatively highly oriented in the in-plane direction (figure 5(e)). Since the (h00) peaks are not seen in the in-plane cuts, it indicates the edge-on orientation of P3HT molecules with respect to the substrate. The (010) peak position corresponds to the π–π stacking distance. The single crystalline domain sizes along the molecules’ π–π stacking direction could also be calculated with the FWHM of the (010) peaks, which are 1.75 nm, 2.11 nm, and 2.23 nm for P3HT:PC71BM, CuPc/P3HT: PC71BM, and CuPC:P3HT:PC71BM, respectively. The increasing domain size proved the crystallinity enhancement of the polymer in the systems with CuPc nanocrystals. We also investigated the P3HT:IC60BA and P3HT: IC70BA systems at the same time (as shown in figures S1 and S2). The corresponding parameters are shown in table 2. Their crystallinity is also enhanced with the introduction of CuPc nanocrystals. Especially for the CuPc/P3HT:IC70BA system, the domain size along the π–π stacking direction is 2.38 nm, which is much bigger than the CuPc:P3HT:IC70BA system. This proved that the former has a better crystallinity, in accordance with the relatively higher Jsc of the CuPc/P3HT IC70BA-based OSC. To further study the morphology of the active layer, SEM and TEM measurements were also carried out for the active layers. Figure 6(a) shows the SEM images of the CuPc nanoribbons obtained by physical vapor deposition. After the CuPc nanoribbons were ultrasonicated and mixed into the P3HT:fullerene-based active layers, they were expected to act as a crystal nucleus to induce crystallization of the molecules. CuPc nanoribbon-induced crystallization has been reported in small molecules’ growth with physical vapor deposition [23, 30]. In the present system, the P3HT:PC71BM active layer shows bicontinuous nanomorphology (figure 6(b)), and the bright and dark regions correspond to donors and PC71BM-rich domains, respectively [8, 31, 32]. After incorporating CuPc nanocrystals (figure 6(c)), the morphology of the active layer is not changed obviously, while some latticelike structures are observed around the CuPc nanocrystals (dark point). This indicates that the crystallinity of the active layer might be increased surround the CuPc nanocrystals.
P3HT:PC71BM-, P3HT:IC60BA-, and P3HT:IC70BA-based systems, respectively, by combining CuPc nanocrystals with a surface modification or mixing strategy. We showed that this approach not only enables increased light absorption of active layer materials but also increased their crystallinity, resulting in high current density as well as high PCE. The method is generally compatible with a broad range of PSC materials. The results of this study offer an effective approach to enhance the efficiency of organic BHJ solar cells. Acknowledgments We acknowledge financial support by the National Natural Science Foundation of China (Grant Nos. 61106054 and 21125420), the Ministry of Science and Technology of China (Grant No. 2011CB932300), and the Chinese Academy of Sciences.
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Enhancing the performance of polymer solar cells using CuPc nanocrystals as additives
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 204001 (http://iopscience.iop.org/0957-4484/26/20/204001) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 61.129.42.30 This content was downloaded on 02/05/2015 at 14:54
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 204001 (7pp)
doi:10.1088/0957-4484/26/20/204001
Enhancing the performance of polymer solar cells using CuPc nanocrystals as additives Yajie Zhang and Zhixiang Wei 1
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China E-mail: [email protected] Received 30 November 2014, revised 1 February 2015 Accepted for publication 2 February 2015 Published 27 April 2015 Abstract
There is an increasing interest in the use of different nanoparticles as additives in polymer solar cells for enhancing the light absorption of active layers as well as their power conversion efficiency (PCE). In this paper, we report a PCE enhancement by simply adding copper phthalocyanine (CuPc) nanocrystals into photovoltaic devices based on a poly(3hexylthiophene) (P3HT): fullerene system. Two kinds of device structure were studied: the first one is a CuPc nanocrystal suspension spin coated on the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-coated substrate; the second one is the CuPc nanocrystal suspension added into the active layer solutions. It is proved that incorporating organic semiconductor nanocrystals into the active layer can help trap light and enhance the crystallinity of the active layers, thus improving the device performance. This strategy might be generally compatible with a broad range of organic photovoltaic materials and offers an effective approach to enhance the device performance. S Online supplementary data available from stacks.iop.org/NANO/26/204001/mmedia Keywords: organic photovoltaic, organic semiconductors, nanocrystals, additives (Some figures may appear in colour only in the online journal) 1. Introduction
approaches have been taken to enhance the light absorption without increasing the thickness of the active layer so to avoid the increase in charge recombination. Recently, an efficient strategy was developed by using different nanoparticles as fillers in PSCs for enhancing light absorption of the active layers as well as their power conversion efficiency (PCE) [19]. Metal nanoparticles are the most frequently used additives due to their localized surface plasmon resonance for efficient light trapping in the active layer, thus enhancing the photon absorption without a need for a thick film. Various metal nanoparticles can be adopted, such as Au, Ag [20], Au/Ag alloy [19], and Al [21]. Organic semiconductor crystals have high crystallinity and charge carrier mobility, and their absorption can be tuned by various molecular structures. However, there has been almost no investigation on the influence of adding organic semiconductor nanocrystals into PSCs until now. Here we report a cooperative PCE enhancement of over 10% by simply adding copper phthalocyanine (CuPc)
Constructing efficient photovoltaic devices comprising polymer solar cells (PSC) [1, 2], hybrid solar cells [3, 4], and even ternary solar cells [5–7] based on a bulk heterojunction (BHJ) configuration has been attracting enormous attention due to their light weight, flexibility, and solution processability. Multidisciplinary efforts have been taken to achieve a high PCE of solar cells, such as rational designs of low-bandgap conjugated polymers [8–14], optimization of film morphology [15, 16], and development of new device architectures [17, 18]. As a result, BHJ solar cells have been reported with a PCE higher than 10% to date [17]. Although obvious progress has been obtained on the PSCs, there are still various limitations that block further enhancement of device performance. For instance, enhancing the light absorption ability of the active layer is a great challenge [19], and it is well known that reducing the recombination of charge carriers is a linchpin for increasing device efficiency. Therefore, different 0957-4484/15/204001+07$33.00
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Figure 1. (a) Chemical structures and (b) energy levels of P3HT, CuPc, PC71BM, IC60BA, and IC70BA. (c) Illustration of the P3HT: fullerene-based solar cells with reference structure, CuPc modified structure (CuPc/P3HT: fullerene), and CuPc mixed structure (CuPc:P3HT: fullerene).
nanocrystals into poly(3-hexylthiophene) (P3HT):fullerene derivative-based photovoltaic devices. P3HT is the most representative conjugated polymer donor, with the advantages of high hole mobility and good crystallinity, which makes it easy to form a nanoscale interpenetrating network with fullerene- derivative acceptors. In addition, good reproducibility of the photovoltaic performance with an optimal active-layer thickness of ∼200 nm is also very attractive for the investigation of adding relatively big nanocrystals. Three systems, including P3HT:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), P3HT:indene-C60 bisadduct (IC60BA), and P3HT:indene-C70 bisadduct (IC70BA), were selected. CuPc nanocrystals were selected as the additive due to their low cost, high charge carrier mobility [22, 23], and excellent photovoltaic property [24, 25]. We show that the incorporation of CuPc nanocrystals exhibits the enhancement of the light absorption and the improvement of the crystallinity of the active layer simultaneously, which result in an increase of the PCE of devices.
spin coated on the PEDOT:PSS-coated substrate (CuPc/ P3HT:fullerene), while in the second one, a CuPc nanocrystal solvent was added into the active layer solutions (CuPc: P3HT:fullerene). 2.1. Materials
P3HT (4002-E) was purchased from Rieke Metals, Inc., and used as received. PC71BM was purchased from American Dye Source, Inc. (ADS). IC60BA [26] and IC70BA [27] were synthesized according to the published procedure. CuPc was purchased from Aldrich and has been purified three times with the physical vapor transport method before using. 2.2. Photovoltaic device fabrication
For the first type of PSCs, a sandwich structure of ITO/ PEDOT:PSS/ CuPc/P3HT:fullerene/Ca/Al was constructed (figure 1(c)). Patterned indium tin oxide (ITO) glass with a sheet resistance of 10 Ω sq−1 was purchased from CSG Holding Co., Ltd (China). The ITO glass was cleaned by a sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, then treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 15 min. A CuPc chloroform suspension (2 mg ml−1) was spin coated on PEDOT:PSS modified ITO glass at 2000 rpm for 30 s. The blend of P3HT:PC71BM (1:1, W/W, 34 mg ml−1 for P3HT:PC71BM), P3HT:IC60BA (1:1, W/W, 34 mg ml−1 for P3HT:IC60BA), and P3HT:IC70BA (1:1, W/W, 34 mg ml−1 for P3HT:IC70BA) were dissolved in orthodichlorobenzene and spin-coated on a CuPc nanocrystal modified substrate (CuPc/P3HT:fullerene). For the second type of device structure, CuPc nanocrystals were mixed with
2. Experiment section Figure 1(a) shows the molecular structure of P3HT, CuPc, PC71BM, IC60BA, and IC70BA. The corresponding energy levels of the components are shown in figure 1(b). The CuPc nanoribbons were produced by physical vapor deposition first [22, 23]. Then they were scraped from the substrate, immersed in solvent, and finally sonicated to obtain a CuPc nanocrystal suspension. Two kinds of device structures were studied with the conventional device structure (ITO/PEDOT: PSS/P3HT:fullerene/Ca/Al) as the reference, as shown in figure 1(c). In the first type, a CuPc nanocrystal solution was 2
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Figure 2. J–V curves of the (a) P3HT:PC71BM-, (b) P3HT:IC60BA-, and (c) P3HT:IC70BA-based systems with comparison of P3HT: fullerene, CuPc/P3HT:fullerene, and CuPc:P3HT:fullerene.
P3HT:fullerene solutions (CuPc:P3HT:fullerene) (figure 1(c)). The weight ratio of CuPc nanocrystals was selected as 0.5%. When the weight ratio is higher than 0.5%, the PCE of the devices decreases. The prepared samples were annealed at 150 °C for 10 min before the vacuum deposition of a metal negative electrode. The active area of PSC is 0.04 cm2. Ca (20 nm) and Al (80 nm) cathodes were thermal evaporated in a glove box at a chamber pressure of ∼4.0 × 10−6 torr.
based on CuPc/P3HT:PC71BM and CuPC:P3HT:PC71BM, were obviously improved compared with a conventional P3HT:PC71BM structure. The highest short circuit current (Jsc) is 10.27 mA cm−2, along with an open circuit voltage (Voc) of 0.63 V and a high fill factor of 64.58%, yields an impressive PCE of 4.57% for CuPc/P3HT:PC71BM-based solar cells. After incorporation of CuPc nanocrystals, the Voc remained nearly the same; the obvious enhancement came from the Jsc increase (from 9.68 to 10.27 mA cm−2). The noticeably upward trend in Jsc suggests that the efficiency of light harvesting in the polymer blend might be improved with incorporation of CuPc nanocrystals. The same phenomenon was also found in P3HT:IC60BA- and P3HT:IC70BA-based systems. The PCE was improved from 6.08 to 6.34% in the P3HT:IC60BA system and from 5.6% to 6.18% in the P3HT: IC70BA system. EQE measurements (figure 3) of three different systems were conducted to better elucidate the reason for the improvement of the Jsc. Figure 3(a) depicts the corresponding EQE spectra of the P3HT:PC71BM-based system. When CuPc nanocrystals were introduced, the EQE increased at the wavelength range from 480 to 650 nm. The maximum EQE value for referenced P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPC:P3HT:PC71BM devices are 60.7%, 62.4%, and 67.7%, respectively. The integrated Jsc value from the EQE 9.42 mA cm−2, and spectrum are 9.30 mA cm−2, −2 9.75 mA cm , respectively. The deviations between the integral current densities and the Jsc read from J–V measurements are under 10%, indicating a good consistency of the photovoltaic results. For the EQE value of the P3HT: IC60BA and P3HT:IC70BA systems, the integrated Jsc were also calculated, and they were also in good accordance with the photovoltaic results. To better illustrate the Jsc enhancement, we also performed UV–vis absorption measurements of solar cells with and without CuPc nanocrystals. The absorption intensities of three P3HT:fullerene systems were normalized with respect to film thickness. The absorption spectrum is shown in figure 4. The absorption spectrum of CuPc is shown in figure S3. It is observed that the absorption of the three P3HT:fullerene systems are enhanced after incorporation of CuPc nanocrystals. This result indicated that incorporating organic
2.3. Measurements
The current density/voltage curves, which were detected under an ambient situation, were measured using a Keithley 2400 source-measure unit. Photocurrent was measured under AM 1.5G illumination at 100 mW cm−2 using a Newport Thermal Oriel 91159A solar simulator. Light intensity was calibrated with a Newport oriel PN 91150V Si-based solar cell. The external quantum efficiency (EQE) measurements of the devices were performed in air with an Oriel Newport system (Model 66902). Grazing incidence wide-angle x-ray scattering (GIWAXS) measurements were carried out using a small-angle x-ray scattering system (XEUSS, FRANCE Xenocs SA). The morphologies of the active layers were characterized by transmission electron microscopy (TEM) (TEM, Tecnai G2 F20 U-TWIN, FEI Co., USA). UV–vis absorption spectra were recorded using a UV–vis spectrophotometer (Lambda 950, Perkin-Elmer, USA). The thicknesses of the active layer were detected by an Alpha-atepD120 stylus profilometer (Kla-Tencor). The morphology of CuPc nanoribbons was characterized by scanning electron microscopy (SEM) (Hitachi S-4800).
3. Results and discussion Figure 2 shows the typical current density versus voltage (J–V) characteristics of solar cells under AM 1.5G illumination at 100 mW cm−2. Different device fabrication conditions were tested, and the device performance data are summarized in table 1. Figure 2(a) shows the J–V characteristics of a P3HT:PC71BM-based system. The PCEs of the devices, 3
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Figure 3. EQE curves of the (a) P3HT:PC71BM-, (b) P3HT:IC60BA-, and (c) P3HT:IC70BA-based systems with comparison of P3HT: fullerene, CuPc/P3HT:fullerene, and CuPc:P3HT:fullerene.
Figure 4. UV–vis absorption spectra of the active layer corresponding to (a) P3HT:PC71BM (b) P3HT:IC60BA (c) P3HT:IC70BA based system with comparison of P3HT: fullerene, CuPc/P3HT:fullerene and CuPc:P3HT:fullerene. Table 1. Summary of the fabrication conditions and performance of the BHJ solar cells.
Donor:Acceptor
PCE (%)
VOC(V)
JSC(mA cm−2)
FF
PCE best (%)
P3HT:PC71BM CuPc/P3HT:PC71BM CuPc:P3HT:PC71BM P3HT:IC60BA CuPc/P3HT:IC60BA CuPc:P3HT/:IC60BA P3HT: IC70BA CuPc/P3HT:IC70BA CuPc:P3HT:IC70BA
3.98 ± 0.04 4.30 ± 0.07 4.52 ± 0.05 5.98 ± 0.10 6.08 ± 0.05 6.20 ± 0.14 5.1 ± 0.50 6.03 ± 0.09 6.11 ± 0.07
0.63 ± 0.01 0.63 ± 0.01 0.63 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01 0.86 ± 0.01
9.50 ± 0.18 9.80 ± 0.14 10.4 ± 0.13 9.07 ± 0.11 9.22 ± 0.13 9.90 ± 0.13 8.96 ± 0.10 10.0 ± 0.07 9.78 ± 0.07
0.63 ± 0.02 0.68 ± 0.02 0.69 ± 0.02 0.71 ± 0.02 0.71 ± 0.02 0.68 ± 0.01 0.67 ± 0.01 0.65 ± 0.02 0.67 ± 0.02
4.02 4.37 4.57 6.08 6.15 6.34 5.6 6.12 6.18
To prove that, we studied the molecular stacking of active layers through GIWAXS. Figures 5(a)–(c) present the two-dimensional GIWAXS patterns of the P3HT:PC71BMbased system. Figures 5(d) and (e) present the out-of-plane and in-plane cuts of two-dimensional GIWAXS patterns of the P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPc:P3HT: PC71BM. The first diffraction peak (100) was observed in the out-of-plane cut and located at q ≈ 0.39 Å−1. Strong (h00) peaks were found at q ≈ h × 0.39 Å−1 for h = 1, 2, and 3, which corresponds to the (h00) peaks of P3HT. The full-width at half-maximum (FWHM) of the scattering peak, Δq, correlates to the crystalline domain size (synonymous of coherence length of the single crystalline domains along the specified
semiconductor nanocrystals into the active layer can enhance the absorption of the active layer due to CuPc nanocrystals trapping light into the active layer and their complementary absorption range. It is in accordance with the Jsc increase of the corresponding systems. As for the P3HT:PC71BM and P3HT:IC60BA systems, the CuPc nanocrystal mixed structures are more obvious for the light absorption enhancement, while in the P3HT:IC70BA system, the CuPc nanocrystal modified structure is better for trapping light in the active layer. This suggests that device structure is one important factor to decide the light absorption, and CuPc nanocrystalinduced molecular packing might also be important for light absorption. 4
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Figure 5. 2D-GIWAXS patterns of the (a) P3HT:PC71BM, (b) CuPc/P3HT:PC71BM, and (c) CuPc:P3HT:PC71BM. (d) out-of-plane, (e) inplane GIWAXS measurements of the P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPc:P3HT:PC71BM.
Figure 6. (a) SEM image of CuPc nanoribbons. TEM images of (b) P3HT:P71BM active layer and (c) CuPc:P3HT:PC71BM active layer. Table 2. Summary of the fabrication conditions and the GIWAXS parameters of the BHJ solar cells.
Configuration P3HT:PC71BM CuPc/P3HT:PC71BM CuPc:P3HT:PC71BM P3HT:IC60BA CuPc/P3HT:IC60BA CuPc:P3HT:IC60BA P3HT: IC70BA CuPc/P3HT:IC70BA CuPc:P3HT:IC70BA
(100) FWHM
La (nm) (100)
(010) FWHM
La (nm) (010)
0.51 0.49 0.46 0.49 0.48 0.50 0.51 0.50 0.503
11.08 11.38 12.23 11.54 11.78 11.31 11.08 11.30 11.24
3.23 2.67 2.54 3.20 2.82 2.09 2.68 2.38 2.48
1.75 2.11 2.23 1.76 2.00 2.7 2.11 2.38 2.28
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direction), which could be calculated by Scherrer’s equation [28, 29]. After calculating the FWHM of the (100) peaks using the peak-differentiation-imitating analysis method, the synonymous of the coherence length (La) along the alkyl chain was calculated. Though the FWHM difference of the (100) peaks of the P3HT:PC71BM, CuPc/P3HT:PC71BM, and CuPc:P3HT:PC71BM systems are not obvious, there are still domain size increasing when introducing CuPc nanocrystals into the organic solar cells (OSCs). Their La values are 11.08 nm, 11.38 nm, and 12.23 nm, respectively. On the other hand, the (010) peak at q ≈ 1.63 Å−1 is relatively highly oriented in the in-plane direction (figure 5(e)). Since the (h00) peaks are not seen in the in-plane cuts, it indicates the edge-on orientation of P3HT molecules with respect to the substrate. The (010) peak position corresponds to the π–π stacking distance. The single crystalline domain sizes along the molecules’ π–π stacking direction could also be calculated with the FWHM of the (010) peaks, which are 1.75 nm, 2.11 nm, and 2.23 nm for P3HT:PC71BM, CuPc/P3HT: PC71BM, and CuPC:P3HT:PC71BM, respectively. The increasing domain size proved the crystallinity enhancement of the polymer in the systems with CuPc nanocrystals. We also investigated the P3HT:IC60BA and P3HT: IC70BA systems at the same time (as shown in figures S1 and S2). The corresponding parameters are shown in table 2. Their crystallinity is also enhanced with the introduction of CuPc nanocrystals. Especially for the CuPc/P3HT:IC70BA system, the domain size along the π–π stacking direction is 2.38 nm, which is much bigger than the CuPc:P3HT:IC70BA system. This proved that the former has a better crystallinity, in accordance with the relatively higher Jsc of the CuPc/P3HT IC70BA-based OSC. To further study the morphology of the active layer, SEM and TEM measurements were also carried out for the active layers. Figure 6(a) shows the SEM images of the CuPc nanoribbons obtained by physical vapor deposition. After the CuPc nanoribbons were ultrasonicated and mixed into the P3HT:fullerene-based active layers, they were expected to act as a crystal nucleus to induce crystallization of the molecules. CuPc nanoribbon-induced crystallization has been reported in small molecules’ growth with physical vapor deposition [23, 30]. In the present system, the P3HT:PC71BM active layer shows bicontinuous nanomorphology (figure 6(b)), and the bright and dark regions correspond to donors and PC71BM-rich domains, respectively [8, 31, 32]. After incorporating CuPc nanocrystals (figure 6(c)), the morphology of the active layer is not changed obviously, while some latticelike structures are observed around the CuPc nanocrystals (dark point). This indicates that the crystallinity of the active layer might be increased surround the CuPc nanocrystals.
P3HT:PC71BM-, P3HT:IC60BA-, and P3HT:IC70BA-based systems, respectively, by combining CuPc nanocrystals with a surface modification or mixing strategy. We showed that this approach not only enables increased light absorption of active layer materials but also increased their crystallinity, resulting in high current density as well as high PCE. The method is generally compatible with a broad range of PSC materials. The results of this study offer an effective approach to enhance the efficiency of organic BHJ solar cells. Acknowledgments We acknowledge financial support by the National Natural Science Foundation of China (Grant Nos. 61106054 and 21125420), the Ministry of Science and Technology of China (Grant No. 2011CB932300), and the Chinese Academy of Sciences.
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