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Efficient ternary blend polymer solar cells with a bipolar diketopyrrolopyrrole small molecule as cascade material
Organic Electronics 25 (2015) 219–224
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Efficient ternary blend polymer solar cells with a bipolar diketopyrrolopyrrole small molecule as cascade material Wenqing Liu, Hangqi Shi, Weifei Fu, Lijian Zuo, Ling Wang, Hongzheng Chen ⇑ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 27 May 2015 Received in revised form 18 June 2015 Accepted 25 June 2015
Keywords: Polymer solar cells Ternary blend Bipolar small molecule P3HT
a b s t r a c t We present a ternary strategy to enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (PSCs) with a bipolar small molecule as cascade material. A bipolar diketopyrrolopyrrole small molecule (F(DPP)2B2), as the second electron acceptor, was incorporated into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric-acidmethyl-ester (PC61BM) blend to fabricate ternary blend PSCs. The introduction of the bipolar compound F(DPP)2B2 can not only broaden the light absorption of the active layer because of its absorption in near infrared region but also play a bridging role between P3HT and PC61BM due to the cascaded energy level structure, thus improving the charge separation and transportation. The optimized ternary PSC with 5 wt% F(DPP)2B2 content delivered a high PCE of 3.92% with a short-circuit current density (Jsc) of 9.63 mA cm 2, an open-circuit voltage (Voc) of 0.62 V and a fill factor (FF) of 64.90%, showing an 23% improvement of PCE as compared to the binary systems based on P3HT:PC61BM (3.18%) or P3HT:F(DPP)2B2 (3.17%). The results indicate that the ternary PSCs with a bipolar compound have the potential to surpass high-performance binary PSCs after carefully device optimization. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been envisioned to be a promising energy conversion technology which offers the opportunity for cheap, flexible, transparent, lightweight and mass-producible devices to harvest solar energy [1–4]. In recent years, researchers have paid a great deal of attention to the research and development of BHJ PSCs, leading to a considerable progress in power conversion efficiency (PCE) that has exceeded 10% for single-junction cells [5–7]. However, despite the significant breakthrough achieved, further performance enhancement is required to ensure a bright industrial future for BHJ PSCs. It is generally believed that the photovoltaic performance of BHJ PSCs is still limited by many factors, including low charge carrier mobility and insufficient light absorption. In order to overcome the absorption limitation, various effective strategies have been carried out, such as the development of novel narrow bandgap photoactive materials [8–10], plasmonics based light trapping approaches [11–13] and the innovation of tandem configurations [14–17]. Recently, an elegant alternative strategy named the ternary systems containing two donors and one acceptor (or one donor ⇑ Corresponding author. E-mail address: [email protected] (H. Chen). http://dx.doi.org/10.1016/j.orgel.2015.06.043 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.
and two acceptors) are attracting more and more interests due to their potentials to expand the spectral absorption range of organic semiconductors and improve the photon harvesting, thereby providing an effective route in achieving a higher short-circuit current density (Jsc) and thus a higher PCE [18]. Besides, the ternary blend PSCs can maintain the simplicity of the processing conditions used for single active-layer cells rather than the demanding challenging processing of multi-junction solar cells for spectrally broad light harvesting. Through persistent investigation [19–24], to date, PCEs of exceeding 8% have been achieved for ternary blend PSCs. For instance, Yu et al. reported a ternary blend PSC containing two donor polymers of polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) and poly-3-oxothieno[3,4-d] isothiazole-1,1-dioxide/benzo dithiophene (PID2), and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as an acceptor with a PCE of 8.22% [25]. Zhan et al. used indene-C60 bisadduct (ICBA) as a second electron-cascade acceptor material in PSCs based on the PTB7:PC71BM blend and achieved a best PCE of 8.24% [26]. Wei et al. designed and fabricated a new ternary organic solar cell, which contains a donor–acceptor type polymer and a novel small molecule. The synergistic effect of small molecules and polymers in ternary system led to a maximum PCE of 8.40% at the optimized ratio of ternary blend [27]. These high efficiency records of ternary solar cells are all based on the highly efficient low bandgap polymers as host donors which are
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expensive and frequently complex to scale up. On the other hand, poly(3-hexylthiophene) (P3HT), as the most-studied and most-used donor material for PSCs, possess many advantages such as high crystallinity, relatively good charge-transport properties, defined quality and reasonable price [28]. More importantly, BHJ PSCs based on blend of P3HT and PCBM can be easily processed and made relatively thick (300 nm) film, which are important for fabrication of large area flexible solar modules [29–31]. Due to all these advantages, further improving P3HT-based solar cells by ternary concept has continued to an area of immense interests. So far, various types of materials such as low bandgap polymers, small molecules, dyes or nanoparticles have been utilized as a third component in ternary P3HT-based solar cells with PCEs of 3–5% [32–35]. Despite of the considerable efforts devoted to the ternary blend PSCs, there are rather a limited number of material systems that have been shown to deliver an enhanced PCE. Moreover, the third components added were commonly functioned as the additional donor material for extending the light harvesting in the solar spectrum rather than the acceptor. The effect of acceptors, especially bipolar materials, as the third component in ternary blend PSCs on photovoltaic performance is still not fully understood. Recently, we designed and synthesized a new diketopyrrolopyrrole small molecule (F(DPP)2B2) with bipolar charge-transporting property [36]. This molecule shows intense absorption in the range of 550–700 nm which is complement well with that of P3HT and exhibits suitable the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels matching perfectly those of PCBM and P3HT. Furthermore, when it is used as either an acceptor blended with P3HT or a donor blended with PCBM, the resulting organic solar cells demonstrate good PCEs over 3% [36]. In this work, F(DPP)2B2 was introduced as a cascade material into P3HT:[6,6]-phenyl-C61-butyric-acidmethyl-ester (PC61BM) based PSCs to form a ternary system. Due to its high absorption coefficient and intense absorption in the near infrared range, F(DPP)2B2 can compensate the light harvesting of P3HT in the solar spectrum for an improved Jsc. On the other hand, the open-circuit voltage (Voc) can be increased by the addition of F(DPP)2B2 because of its higher LUMO energy levels relative to PC61BM. Meanwhile, the cascaded energy level structure of P3HT:F(DPP)2B2:PC61BM can also enhance the exciton dissociation and the charge carrier transportation. As a result, the obtained optimal ternary PSC with 5 wt% content of F(DPP)2B2 exhibits a high PCE of 3.92%, which is 23% improvement of efficiency as compared to that of 3.18% for the binary P3HT:PC61BM devices. We demonstrated, for the first time, efficient ternary PSCs with a bipolar small molecule as the cascade material and we attributed the enhanced performance of the ternary PSCs to a synergistic effect of both the improved light absorption and the better charge separation and transportation induced by the third bipolar component. 2. Experimental 2.1. Materials P3HT with a molecular weight (Mn) of 40,000 Da was obtained from Rieke Metals. PC60BM was purchased from 1-material. F(DPP)2B2 was synthesized in our laboratory according to our previous reported procedure [36]. All the solvents were commercially available and used as received. 2.2. Device fabrication The devices were fabricated on commercially available indium tin oxide (ITO)-coated glass substrates. Glass/ITO substrates were
pre-cleaned with a detergent, ultrasonicated in deionized water, acetone and isopropyl alcohol consecutively every 15 min, and then dried in an oven. After treatment in an ultraviolet ozone generator for 15 min on the substrates, a thin layer of 40 nm PEDOT:PSS was spin-coated on the substrates at 3000 rpm for 40 s and baked at 140 °C for 10 min. The ternary blends of P3HT, PC61BM, and F(DPP)2B2 with different ratios were dissolved in chlorobenzene with a total concentration of 40 mg ml 1 and stirred for 12 h before use. The weight ratio of F(DPP)2B2 in electron acceptors (PC61BM and F(DPP)2B2) is 0, 2.5, 5, 10, 15, 20, or 30 wt%, and the weight ratio of the donor to the acceptor was kept the constant as 1:0.8. The ternary blend solution was spin-coated at 1000 rpm for 60 s in a nitrogen glovebox to form a photoactive layer. Subsequently, a 100 nm thick aluminum layer was evaporated through a shadow mask to define the active layer area of the devices (5.4 mm2) and form the top cathode. Finally, all the devices were treated with post-annealing at 200 °C for 5 min before testing. 2.3. Film and device characterizations The absorption spectra were recorded on a UV-Visible spectrophotometer (UV-2450, Shimadzu Corporation, Japan). The AFM images were taken by a Veeco Multimode atomic force microscopy in the tapping mode. The steady-state PL spectra were taken on a FluoroMax-4 HORIBA Jobin Yvon spectrofluorometer. The X-ray diffraction patterns were recorded at a scan rate of 2 deg/min on the Rigaku D/max-2550PC X-ray diffractometer with Cu Ka radiation (1.5406 nm). The J–V curves were measured with Keithley 236 measurement source units at room temperature in air. The photocurrent was measured under a calibrated solar simulator (Abet 300W) at 100 mW cm 2, and the light intensity was calibrated with a standard Si photovoltaic reference cell. External quantum efficiency (EQE) spectrum was measured with Stanford lock-in amplifier 8300 unit. 3. Results and discussion The chemical structures of P3HT, F(DPP)2B2 and PC61BM are shown in Fig. 1a. The cascade energy levels of the three materials are presented in Fig. 1b. We first fabricated the ternary blend BHJ solar cells based on the mixtures of P3HT:F(DPP)2B2:PC61BM and the control binary P3HT:PC61BM based BHJ solar cell under the same conditions with a conventional structure of ITO/PEDOT: PSS/Active layer/Al (as illustrated in Fig. 1c). The overall donor to acceptor ratio was kept at 1:0.8 in this work. Fig. 1d shows the typical current density versus voltage (J–V) characteristics of the binary and ternary PSCs under AM 1.5 G illumination at 100 mW cm 2 and the related photovoltaic parameters are summarized in Table 1. The reference device based on P3HT:PC61BM exhibited an average PCE of 3.18% with an open-circuit voltage (Voc) of 0.62 V, a Jsc of 8.27 mA cm 2 and a fill factor (FF) of 61.95%. As described in Fig. 1d, it is apparent that the Voc, Jsc, and FF of the ternary PSCs are significantly affected by the weight ratio (2.5, 5, 10, 15, 20 and 30 wt%) of F(DPP)2B2 in electron acceptor. The Voc increased slightly with increasing F(DPP)2B2 content due to the higher LUMO energy level of F(DPP)2B2 relative to that of PC61BM. The Jsc was increased in the beginning with the increase of F(DPP)2B2 ratio (620 wt%) in acceptors and then decreased as the F(DPP)2B2 ratio exceed 20 wt%. Meanwhile, the FF and PCE were also increased first and then decreased with increasing the F(DPP)2B2 content. A best PCE of 3.92% was achieved for the ternary PSCs with 5 wt% F(DPP)2B2 content in the acceptor, which is attributed to the simultaneous enhancement of Jsc (9.63 vs 8.27 mA cm 2) and FF (64.90% vs 61.95%) compared with the
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Fig. 1. (a) Chemical structures of organic materials P3HT, PC61BM, and F(DPP)2B2. (b) Energy levels of all materials used in device. (c) A schematic illustration of device structure. (d) J–V characteristics of devices with the structure ITO/PEDOT:PSS/P3HT:F(DPP)2B2:PC61BM/Al with different weight ratios of F(DPP)2B2 in acceptors under the illumination of an AM 1.5G solar simulator, 100 mW/cm2.
Table 1 Summary of J-V parameters of PSC devices based on P3HT:F(DPP)2B2:PC61BM blend with different weight ratios of F(DPP)2B2 in PC61BM.
a
P3HT: F(DPP)2B2:PC61BM
Voc [V]
Jsc [mA/ cm2]
Calculated Jsc [mA/cm2]
FF %
12.5:0:10 12.5:0.25:9.75 12.5:0.5:9.5 12.5:1.0:9.0 12.5:1.5:8.5 12.5:2.0:8.0 12.5:3.0:7.0 12.5:12.5:0a
0.62 0.62 0.63 0.63 0.64 0.65 0.65 1.18
8.27 8.35 9.63 9.71 9.96 10.06 9.02 5.35
7.18 7.55 8.65 8.72 8.97 9.05 8.05 4.49
61.95 62.79 64.90 62.64 60.09 56.88 51.79 50.20
PCE [%] Best
Average
3.18 3.25 3.92 3.81 3.83 3.74 3.05 3.17
3.14 3.24 3.87 3.79 3.72 3.71 2.98 -
Under the best device optimizing conditions according to the Ref. [36].
control binary PSC based on P3HT:PC61BM (3.18%). This value of PCE is also higher than that (3.17%) of the binary PSC based on P3HT:F(DPP)2B2 under the best device optimizing conditions according to the literature [36]. When increasing the F(DPP)2B2 content to 20 wt%, the Jsc was improved to the highest value of 10.06 mA cm 2 with an average PCE of 3.74%, which is higher than the reference device. It is noted that the ternary PSCs can maintain a significantly enhanced efficiency with a wide range of F(DPP)2B2 contents from 2.5 to 20 wt%, which can be attributed to the greatly improved Jsc despite of the slightly decreased FF. To find out the reason for the change of Jsc in devices, UV–Vis absorption spectra of the ternary and binary blend films were firstly examined. Fig. 2a reveals the UV–Vis absorption spectra of the P3HT and F(DPP)2B2 films. It is found that P3HT film has a relative strong absorption peak centered at 520 nm and a shoulder absorption peak at 600 nm. While F(DPP)2B2 has a strong absorption in the region from 500 to 700 nm with a maximum absorption peak located at 670 nm. A great absorption spectral overlap is existed for P3HT and F(DPP)2B2, indicating the broadened light absorption of the ternary blends. Then we examined the normalized absorption of the ternary blend films with different
F(DPP)2B2 concentrations to study the change in absorption after incorporation of the third F(DPP)2B2 component. It is obvious shown in Fig. 2b that the relative absorption intensity of the ternary films from 550 to 700 nm is enhanced with increasing the F(DPP)2B2 content in the host P3HT:PC61BM blend. Meanwhile, the absorption intensity between 450 and 550 nm from P3HT is kept constant, which indicates that the p p stacking of P3HT is not disturbed much by the incorporation of F(DPP)2B2 [37]. However, an apparent decrease between 300 and 450 nm from P3HT and PC61BM is observed when the F(DPP)2B2 concentration exceeds 10 wt%, which is attributed to the distorted P3HT molecular arrangement and the decrease of PC61BM content [38], along with the increase of F(DPP)2B2 content. Accordingly, we measured the external quantum efficiency (EQE) of the binary and ternary PSC devices. As shown in Fig. 2c, the trend of EQE is similar to that of Jsc, The calculated Jsc values from the integration of the EQE spectra are close to the values obtained from J V curves with the average deviations under 1.5%, which indicates that the experimental results presented in this work are reliable. The EQE is enhanced in the 550–700 nm range with the increase of F(DPP)2B2 content due to the enhanced light absorption, which is consistent with the absorption results of the ternary system. Meanwhile, a significantly enhancement of EQE is also displayed in the 300–550 nm range for the devices with 2.5–20 wt% F(DPP)2B2 contents. However, this enhancement in spectral is not resulted from the absorption enhancement since F(DPP)2B2 addition does not cause any obvious increase in absorption of the ternary blend film at this region as shown in Fig. 2a. This result suggests that the bipolar small molecule F(DPP)2B2 not only plays a role of second donor component in the ternary system for extending the absorption spectrum but also acts as an cascade electron acceptor bridged between P3HT and PC61BM, thus providing more routes for the charge transfer at the donor/acceptor interface. To further check whether the blend of P3HT, F(DPP)2B2, and PC61BM can form a proper donor–acceptor energy cascade for
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Fig. 2. (a) UV–Vis absorption spectra of neat P3HT and F(DPP)2B2 films. (b) UV–Vis absorption spectra of P3HT:F(DPP)2B2:PC61BM blend films with different F(DPP)2B2 contents. (c) EQE curves of the ternary P3HT:F(DPP)2B2:PC61BM PSC devices with different F(DPP)2B2 contents.
Table 2 Hole and electron mobilities of P3HT:F(DPP)2B2:PC61BM films with different weight ratios of F(DPP)2B2 in PC61BM obtained from SCLC method. P3HT: F(DPP)2B2:PC61BM 12.5:0:10 12.5:0.25:9.75 12.5:0.5:9.5 12.5:1.0:9.0 12.5:1.5:8.5 12.5:2.0:8.0 12.5:3.0:7.0
Fig. 3. Photoluminescence spectra of pure P3HT and P3HT:F(DPP)2B2:PC61BM ternary blend films with different weight ratios of F(DPP)2B2 in PC61BM excited at 490 nm light. The inset is the enlarged photoluminescence spectra of the ternary blend films with different F(DPP)2B2 contents.
OSCs, steady-state fluorescence quenching of the blends were compared based on the photoluminescence (PL) spectra. The PL spectra of neat P3HT, the binary blend of P3HT and PC61BM, and the ternary blend films under the excitation of 490 nm light are shown in Fig. 3. The PL emission peak of neat P3HT is located at 645 nm with a narrow emission range from 625 to 800 nm. A remarkable reduction of photoluminescence is observed for the P3HT:PC61BM binary blend, indicating that efficient charge transfer occurs at the interface of donor acceptor pairs. Interestingly, as shown in the inset, the relative PL emission intensity of P3HT: PC61BM blend gradually decreased in the range of 625–800 nm with the increase of F(DPP)2B2 content, indicating an improved charge transfer for the ternary blend. In addition, since F(DPP)2B2 has a strong absorption in the region from 500 to 700 nm, which overlaps the PL emission
lh (cm2 V
1
1.26 10 1.25 10 1.06 10 7.76 10 6.84 10 5.63 10 4.39 10
4 4 4 5 5 5 5
s
1
)
le (cm2 V
1
7.21 10 5.94 10 4.32 10 3.12 10 2.61 10 2.12 10 6.98 10
5 5 5 5 5 5 6
s
1
)
lh/le 1.75 2.11 2.45 2.49 2.62 2.66 6.28
peak of P3HT, the reduction of photoluminescence for the ternary blend may also be partly caused by the absorbance of F(DPP)2B2. This result confirms that F(DPP)2B2 can act as an efficient cascade material for the ternary PSCs. The space charge limited current (SCLC) method [39] was employed to investigate the charge transport properties of P3HT:F(DPP)2B2:PC61BM blend films with different weight ratios of F(DPP)2B2 in fullerene. Hole-only and electron-only devices were fabricated using the following device structures respectively: ITO/PEDOT:PSS/P3HT:F(DPP)2B2:PC61BM/MoO3/Al and Al/P3HT: F(DPP)2B2: PC61BM/Al. Fig. 4a and b show the J0.5–V characteristics of the hole-only and the electron-only devices. The mobility values are calculated from the equations reported in the literature [39] and are summarized in Table 2. The binary blend of P3HT:PC61BM exhibits a hole mobility of 1.26 10 4 cm2 V 1 s 1 and an electron mobility of 7.21 10 5 cm2 V 1 s 1. With increasing the fraction of F(DPP)2B2 in fullerene, both of the hole and electron mobilities are decreased slightly for the ternary blends of P3HT:F(DPP)2B2:PC61BM. This phenomenon is reasonable since it is natural that a lower PC61BM concentration is less beneficial to form percolated electron transport pathways [40]. Moreover, we
Fig. 4. J–V characteristics under dark for (a) hole-only and (b) electron-only devices based on P3HT:F(DPP)2B2:PC61BM blends with different weight ratios of F(DPP)2B2 in PC61BM.
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Fig. 5. AFM height (a)–(g) and phase (h)–(n) images of P3HT:F(DPP)2B2:PC61BM films with different weight ratios of F(DPP)2B2 spin-coated on ITO/PEDOT:PSS substrates: (a) and (h) 0%, (b) and (i) 2.5%, (c) and (j) 5%, (d) and (k) 10%, (e) and (l) 15%, (f) and (m) 20%, and (g) and (n) 30%.
by XRD, as shown in Fig. 6. The binary P3HT:PC61BM blend film exhibits only one diffraction peak centered at 5.4°, which is assigned to the ordered P3HT molecular arrangement. The pristine F(DPP)2B2 film exhibits a very weak diffraction peak at 8.5°. After adding different weight ratios of F(DPP)2B2 into the binary blend, no obvious changes in the intensity and location of the diffraction peak are observed for the ternary blend films. Therefore, the effect of F(DPP)2B2 on the crystallinity of the blend films is supposed to be negligible. These observations indicate that introduction of F(DPP)2B2 at low concentrations has no significant effect on the morphology of blend films, from which we rule out that the morphology is the main issue responsible for the change of device performance.
Fig. 6. XRD spectra of pristine F(DPP)2B2 and P3HT:F(DPP)2B2:PC61BM films with different weight ratios of F(DPP)2B2.
find that the ternary blends exhibit relatively similar and balanced hole/electron transport when the F(DPP)2B2 content is below 20%, which are corresponding to their good device performance. The lower hole or electron mobility and the unbalanced hole/electron transport are the main reasons for the worse performance of the 30% F(DPP)2B2 ternary blend. To comprehensively illustrate the effect of F(DPP)2B2 ratios on the performance of the ternary PSCs, the surface morphology of the blend films was investigated by atomic force microscopy (AFM). Fig. 5 shows the height and the phase AFM images of the blend films with different F(DPP)2B2 contents. All the films were prepared under the same conditions as the active layers in the devices. The root-mean-square (r.m.s.) roughnesses of the P3HT:F(DPP)2B2:PC61BM films with 0%, 2.5%, 5%, 10%, 15%, 20%, 30% weight ratios of F(DPP)2B2 in PC61BM are 0.850, 0.932, 0.961, 0.969, 0.990, 1.043, and 1.076 nm, respectively. Although the roughness of the blend films increases with the increase of F(DPP)2B2 content, it should be noticed that blending F(DPP)2B2 into P3HT:PC61BM up to 30% does not cause any significant change in the surface roughness of the host blend. All the blend films display the similarly uniform morphology and the obvious donor-acceptor phase separation with the smallest domain in the scale of 10–20 nm, which provides enough interfaces for efficient exciton dissociation and charge carrier transport, thus leading to a high efficiency. It should be noticed that the AFM investigation is only limited on the surface morphology and phase separation of films. To better understand the morphology change of the ternary blends and its effect on the performance of devices, the crystalline structures of the ternary blend films were characterized
4. Conclusions We have successfully developed a novel ternary PSC system with improved efficiency by using a bipolar diketopyrrolopyrrole small molecule F(DPP)2B2 as energy cascade material. A best PCE of 3.92% is obtained in the ternary P3HT:F(DPP)2B2:PC61BM PSCs with a 5 wt% F(DPP)2B2 ratio in acceptor, corresponding to about 23% improvement compared with that of 3.18% PCE for P3HT:PC61BM based cell. The performance improvement of the ternary PSCs can be attributed to the simultaneous improvement of the light harvesting, exciton dissociation, and charge carrier transport by the introduction of the third bipolar component. Our results demonstrate that the ternary strategy is a promising way towards high performance polymer solar cells and the use of the bipolar small molecule as energy cascade material may open up new design directions for novel ternary systems.
Acknowledgements This work was supported by the Major State Basic Research Development Program (2014CB643503), the National Natural Science Foundation of China (Grants 51261130582 and 91233114), and the program for Innovative Research Team in University of Ministry of Education of China (IRT13R54).
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Efficient ternary blend polymer solar cells with a bipolar diketopyrrolopyrrole small molecule as cascade material Wenqing Liu, Hangqi Shi, Weifei Fu, Lijian Zuo, Ling Wang, Hongzheng Chen ⇑ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 27 May 2015 Received in revised form 18 June 2015 Accepted 25 June 2015
Keywords: Polymer solar cells Ternary blend Bipolar small molecule P3HT
a b s t r a c t We present a ternary strategy to enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (PSCs) with a bipolar small molecule as cascade material. A bipolar diketopyrrolopyrrole small molecule (F(DPP)2B2), as the second electron acceptor, was incorporated into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric-acidmethyl-ester (PC61BM) blend to fabricate ternary blend PSCs. The introduction of the bipolar compound F(DPP)2B2 can not only broaden the light absorption of the active layer because of its absorption in near infrared region but also play a bridging role between P3HT and PC61BM due to the cascaded energy level structure, thus improving the charge separation and transportation. The optimized ternary PSC with 5 wt% F(DPP)2B2 content delivered a high PCE of 3.92% with a short-circuit current density (Jsc) of 9.63 mA cm 2, an open-circuit voltage (Voc) of 0.62 V and a fill factor (FF) of 64.90%, showing an 23% improvement of PCE as compared to the binary systems based on P3HT:PC61BM (3.18%) or P3HT:F(DPP)2B2 (3.17%). The results indicate that the ternary PSCs with a bipolar compound have the potential to surpass high-performance binary PSCs after carefully device optimization. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been envisioned to be a promising energy conversion technology which offers the opportunity for cheap, flexible, transparent, lightweight and mass-producible devices to harvest solar energy [1–4]. In recent years, researchers have paid a great deal of attention to the research and development of BHJ PSCs, leading to a considerable progress in power conversion efficiency (PCE) that has exceeded 10% for single-junction cells [5–7]. However, despite the significant breakthrough achieved, further performance enhancement is required to ensure a bright industrial future for BHJ PSCs. It is generally believed that the photovoltaic performance of BHJ PSCs is still limited by many factors, including low charge carrier mobility and insufficient light absorption. In order to overcome the absorption limitation, various effective strategies have been carried out, such as the development of novel narrow bandgap photoactive materials [8–10], plasmonics based light trapping approaches [11–13] and the innovation of tandem configurations [14–17]. Recently, an elegant alternative strategy named the ternary systems containing two donors and one acceptor (or one donor ⇑ Corresponding author. E-mail address: [email protected] (H. Chen). http://dx.doi.org/10.1016/j.orgel.2015.06.043 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.
and two acceptors) are attracting more and more interests due to their potentials to expand the spectral absorption range of organic semiconductors and improve the photon harvesting, thereby providing an effective route in achieving a higher short-circuit current density (Jsc) and thus a higher PCE [18]. Besides, the ternary blend PSCs can maintain the simplicity of the processing conditions used for single active-layer cells rather than the demanding challenging processing of multi-junction solar cells for spectrally broad light harvesting. Through persistent investigation [19–24], to date, PCEs of exceeding 8% have been achieved for ternary blend PSCs. For instance, Yu et al. reported a ternary blend PSC containing two donor polymers of polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) and poly-3-oxothieno[3,4-d] isothiazole-1,1-dioxide/benzo dithiophene (PID2), and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as an acceptor with a PCE of 8.22% [25]. Zhan et al. used indene-C60 bisadduct (ICBA) as a second electron-cascade acceptor material in PSCs based on the PTB7:PC71BM blend and achieved a best PCE of 8.24% [26]. Wei et al. designed and fabricated a new ternary organic solar cell, which contains a donor–acceptor type polymer and a novel small molecule. The synergistic effect of small molecules and polymers in ternary system led to a maximum PCE of 8.40% at the optimized ratio of ternary blend [27]. These high efficiency records of ternary solar cells are all based on the highly efficient low bandgap polymers as host donors which are
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expensive and frequently complex to scale up. On the other hand, poly(3-hexylthiophene) (P3HT), as the most-studied and most-used donor material for PSCs, possess many advantages such as high crystallinity, relatively good charge-transport properties, defined quality and reasonable price [28]. More importantly, BHJ PSCs based on blend of P3HT and PCBM can be easily processed and made relatively thick (300 nm) film, which are important for fabrication of large area flexible solar modules [29–31]. Due to all these advantages, further improving P3HT-based solar cells by ternary concept has continued to an area of immense interests. So far, various types of materials such as low bandgap polymers, small molecules, dyes or nanoparticles have been utilized as a third component in ternary P3HT-based solar cells with PCEs of 3–5% [32–35]. Despite of the considerable efforts devoted to the ternary blend PSCs, there are rather a limited number of material systems that have been shown to deliver an enhanced PCE. Moreover, the third components added were commonly functioned as the additional donor material for extending the light harvesting in the solar spectrum rather than the acceptor. The effect of acceptors, especially bipolar materials, as the third component in ternary blend PSCs on photovoltaic performance is still not fully understood. Recently, we designed and synthesized a new diketopyrrolopyrrole small molecule (F(DPP)2B2) with bipolar charge-transporting property [36]. This molecule shows intense absorption in the range of 550–700 nm which is complement well with that of P3HT and exhibits suitable the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels matching perfectly those of PCBM and P3HT. Furthermore, when it is used as either an acceptor blended with P3HT or a donor blended with PCBM, the resulting organic solar cells demonstrate good PCEs over 3% [36]. In this work, F(DPP)2B2 was introduced as a cascade material into P3HT:[6,6]-phenyl-C61-butyric-acidmethyl-ester (PC61BM) based PSCs to form a ternary system. Due to its high absorption coefficient and intense absorption in the near infrared range, F(DPP)2B2 can compensate the light harvesting of P3HT in the solar spectrum for an improved Jsc. On the other hand, the open-circuit voltage (Voc) can be increased by the addition of F(DPP)2B2 because of its higher LUMO energy levels relative to PC61BM. Meanwhile, the cascaded energy level structure of P3HT:F(DPP)2B2:PC61BM can also enhance the exciton dissociation and the charge carrier transportation. As a result, the obtained optimal ternary PSC with 5 wt% content of F(DPP)2B2 exhibits a high PCE of 3.92%, which is 23% improvement of efficiency as compared to that of 3.18% for the binary P3HT:PC61BM devices. We demonstrated, for the first time, efficient ternary PSCs with a bipolar small molecule as the cascade material and we attributed the enhanced performance of the ternary PSCs to a synergistic effect of both the improved light absorption and the better charge separation and transportation induced by the third bipolar component. 2. Experimental 2.1. Materials P3HT with a molecular weight (Mn) of 40,000 Da was obtained from Rieke Metals. PC60BM was purchased from 1-material. F(DPP)2B2 was synthesized in our laboratory according to our previous reported procedure [36]. All the solvents were commercially available and used as received. 2.2. Device fabrication The devices were fabricated on commercially available indium tin oxide (ITO)-coated glass substrates. Glass/ITO substrates were
pre-cleaned with a detergent, ultrasonicated in deionized water, acetone and isopropyl alcohol consecutively every 15 min, and then dried in an oven. After treatment in an ultraviolet ozone generator for 15 min on the substrates, a thin layer of 40 nm PEDOT:PSS was spin-coated on the substrates at 3000 rpm for 40 s and baked at 140 °C for 10 min. The ternary blends of P3HT, PC61BM, and F(DPP)2B2 with different ratios were dissolved in chlorobenzene with a total concentration of 40 mg ml 1 and stirred for 12 h before use. The weight ratio of F(DPP)2B2 in electron acceptors (PC61BM and F(DPP)2B2) is 0, 2.5, 5, 10, 15, 20, or 30 wt%, and the weight ratio of the donor to the acceptor was kept the constant as 1:0.8. The ternary blend solution was spin-coated at 1000 rpm for 60 s in a nitrogen glovebox to form a photoactive layer. Subsequently, a 100 nm thick aluminum layer was evaporated through a shadow mask to define the active layer area of the devices (5.4 mm2) and form the top cathode. Finally, all the devices were treated with post-annealing at 200 °C for 5 min before testing. 2.3. Film and device characterizations The absorption spectra were recorded on a UV-Visible spectrophotometer (UV-2450, Shimadzu Corporation, Japan). The AFM images were taken by a Veeco Multimode atomic force microscopy in the tapping mode. The steady-state PL spectra were taken on a FluoroMax-4 HORIBA Jobin Yvon spectrofluorometer. The X-ray diffraction patterns were recorded at a scan rate of 2 deg/min on the Rigaku D/max-2550PC X-ray diffractometer with Cu Ka radiation (1.5406 nm). The J–V curves were measured with Keithley 236 measurement source units at room temperature in air. The photocurrent was measured under a calibrated solar simulator (Abet 300W) at 100 mW cm 2, and the light intensity was calibrated with a standard Si photovoltaic reference cell. External quantum efficiency (EQE) spectrum was measured with Stanford lock-in amplifier 8300 unit. 3. Results and discussion The chemical structures of P3HT, F(DPP)2B2 and PC61BM are shown in Fig. 1a. The cascade energy levels of the three materials are presented in Fig. 1b. We first fabricated the ternary blend BHJ solar cells based on the mixtures of P3HT:F(DPP)2B2:PC61BM and the control binary P3HT:PC61BM based BHJ solar cell under the same conditions with a conventional structure of ITO/PEDOT: PSS/Active layer/Al (as illustrated in Fig. 1c). The overall donor to acceptor ratio was kept at 1:0.8 in this work. Fig. 1d shows the typical current density versus voltage (J–V) characteristics of the binary and ternary PSCs under AM 1.5 G illumination at 100 mW cm 2 and the related photovoltaic parameters are summarized in Table 1. The reference device based on P3HT:PC61BM exhibited an average PCE of 3.18% with an open-circuit voltage (Voc) of 0.62 V, a Jsc of 8.27 mA cm 2 and a fill factor (FF) of 61.95%. As described in Fig. 1d, it is apparent that the Voc, Jsc, and FF of the ternary PSCs are significantly affected by the weight ratio (2.5, 5, 10, 15, 20 and 30 wt%) of F(DPP)2B2 in electron acceptor. The Voc increased slightly with increasing F(DPP)2B2 content due to the higher LUMO energy level of F(DPP)2B2 relative to that of PC61BM. The Jsc was increased in the beginning with the increase of F(DPP)2B2 ratio (620 wt%) in acceptors and then decreased as the F(DPP)2B2 ratio exceed 20 wt%. Meanwhile, the FF and PCE were also increased first and then decreased with increasing the F(DPP)2B2 content. A best PCE of 3.92% was achieved for the ternary PSCs with 5 wt% F(DPP)2B2 content in the acceptor, which is attributed to the simultaneous enhancement of Jsc (9.63 vs 8.27 mA cm 2) and FF (64.90% vs 61.95%) compared with the
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Fig. 1. (a) Chemical structures of organic materials P3HT, PC61BM, and F(DPP)2B2. (b) Energy levels of all materials used in device. (c) A schematic illustration of device structure. (d) J–V characteristics of devices with the structure ITO/PEDOT:PSS/P3HT:F(DPP)2B2:PC61BM/Al with different weight ratios of F(DPP)2B2 in acceptors under the illumination of an AM 1.5G solar simulator, 100 mW/cm2.
Table 1 Summary of J-V parameters of PSC devices based on P3HT:F(DPP)2B2:PC61BM blend with different weight ratios of F(DPP)2B2 in PC61BM.
a
P3HT: F(DPP)2B2:PC61BM
Voc [V]
Jsc [mA/ cm2]
Calculated Jsc [mA/cm2]
FF %
12.5:0:10 12.5:0.25:9.75 12.5:0.5:9.5 12.5:1.0:9.0 12.5:1.5:8.5 12.5:2.0:8.0 12.5:3.0:7.0 12.5:12.5:0a
0.62 0.62 0.63 0.63 0.64 0.65 0.65 1.18
8.27 8.35 9.63 9.71 9.96 10.06 9.02 5.35
7.18 7.55 8.65 8.72 8.97 9.05 8.05 4.49
61.95 62.79 64.90 62.64 60.09 56.88 51.79 50.20
PCE [%] Best
Average
3.18 3.25 3.92 3.81 3.83 3.74 3.05 3.17
3.14 3.24 3.87 3.79 3.72 3.71 2.98 -
Under the best device optimizing conditions according to the Ref. [36].
control binary PSC based on P3HT:PC61BM (3.18%). This value of PCE is also higher than that (3.17%) of the binary PSC based on P3HT:F(DPP)2B2 under the best device optimizing conditions according to the literature [36]. When increasing the F(DPP)2B2 content to 20 wt%, the Jsc was improved to the highest value of 10.06 mA cm 2 with an average PCE of 3.74%, which is higher than the reference device. It is noted that the ternary PSCs can maintain a significantly enhanced efficiency with a wide range of F(DPP)2B2 contents from 2.5 to 20 wt%, which can be attributed to the greatly improved Jsc despite of the slightly decreased FF. To find out the reason for the change of Jsc in devices, UV–Vis absorption spectra of the ternary and binary blend films were firstly examined. Fig. 2a reveals the UV–Vis absorption spectra of the P3HT and F(DPP)2B2 films. It is found that P3HT film has a relative strong absorption peak centered at 520 nm and a shoulder absorption peak at 600 nm. While F(DPP)2B2 has a strong absorption in the region from 500 to 700 nm with a maximum absorption peak located at 670 nm. A great absorption spectral overlap is existed for P3HT and F(DPP)2B2, indicating the broadened light absorption of the ternary blends. Then we examined the normalized absorption of the ternary blend films with different
F(DPP)2B2 concentrations to study the change in absorption after incorporation of the third F(DPP)2B2 component. It is obvious shown in Fig. 2b that the relative absorption intensity of the ternary films from 550 to 700 nm is enhanced with increasing the F(DPP)2B2 content in the host P3HT:PC61BM blend. Meanwhile, the absorption intensity between 450 and 550 nm from P3HT is kept constant, which indicates that the p p stacking of P3HT is not disturbed much by the incorporation of F(DPP)2B2 [37]. However, an apparent decrease between 300 and 450 nm from P3HT and PC61BM is observed when the F(DPP)2B2 concentration exceeds 10 wt%, which is attributed to the distorted P3HT molecular arrangement and the decrease of PC61BM content [38], along with the increase of F(DPP)2B2 content. Accordingly, we measured the external quantum efficiency (EQE) of the binary and ternary PSC devices. As shown in Fig. 2c, the trend of EQE is similar to that of Jsc, The calculated Jsc values from the integration of the EQE spectra are close to the values obtained from J V curves with the average deviations under 1.5%, which indicates that the experimental results presented in this work are reliable. The EQE is enhanced in the 550–700 nm range with the increase of F(DPP)2B2 content due to the enhanced light absorption, which is consistent with the absorption results of the ternary system. Meanwhile, a significantly enhancement of EQE is also displayed in the 300–550 nm range for the devices with 2.5–20 wt% F(DPP)2B2 contents. However, this enhancement in spectral is not resulted from the absorption enhancement since F(DPP)2B2 addition does not cause any obvious increase in absorption of the ternary blend film at this region as shown in Fig. 2a. This result suggests that the bipolar small molecule F(DPP)2B2 not only plays a role of second donor component in the ternary system for extending the absorption spectrum but also acts as an cascade electron acceptor bridged between P3HT and PC61BM, thus providing more routes for the charge transfer at the donor/acceptor interface. To further check whether the blend of P3HT, F(DPP)2B2, and PC61BM can form a proper donor–acceptor energy cascade for
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Fig. 2. (a) UV–Vis absorption spectra of neat P3HT and F(DPP)2B2 films. (b) UV–Vis absorption spectra of P3HT:F(DPP)2B2:PC61BM blend films with different F(DPP)2B2 contents. (c) EQE curves of the ternary P3HT:F(DPP)2B2:PC61BM PSC devices with different F(DPP)2B2 contents.
Table 2 Hole and electron mobilities of P3HT:F(DPP)2B2:PC61BM films with different weight ratios of F(DPP)2B2 in PC61BM obtained from SCLC method. P3HT: F(DPP)2B2:PC61BM 12.5:0:10 12.5:0.25:9.75 12.5:0.5:9.5 12.5:1.0:9.0 12.5:1.5:8.5 12.5:2.0:8.0 12.5:3.0:7.0
Fig. 3. Photoluminescence spectra of pure P3HT and P3HT:F(DPP)2B2:PC61BM ternary blend films with different weight ratios of F(DPP)2B2 in PC61BM excited at 490 nm light. The inset is the enlarged photoluminescence spectra of the ternary blend films with different F(DPP)2B2 contents.
OSCs, steady-state fluorescence quenching of the blends were compared based on the photoluminescence (PL) spectra. The PL spectra of neat P3HT, the binary blend of P3HT and PC61BM, and the ternary blend films under the excitation of 490 nm light are shown in Fig. 3. The PL emission peak of neat P3HT is located at 645 nm with a narrow emission range from 625 to 800 nm. A remarkable reduction of photoluminescence is observed for the P3HT:PC61BM binary blend, indicating that efficient charge transfer occurs at the interface of donor acceptor pairs. Interestingly, as shown in the inset, the relative PL emission intensity of P3HT: PC61BM blend gradually decreased in the range of 625–800 nm with the increase of F(DPP)2B2 content, indicating an improved charge transfer for the ternary blend. In addition, since F(DPP)2B2 has a strong absorption in the region from 500 to 700 nm, which overlaps the PL emission
lh (cm2 V
1
1.26 10 1.25 10 1.06 10 7.76 10 6.84 10 5.63 10 4.39 10
4 4 4 5 5 5 5
s
1
)
le (cm2 V
1
7.21 10 5.94 10 4.32 10 3.12 10 2.61 10 2.12 10 6.98 10
5 5 5 5 5 5 6
s
1
)
lh/le 1.75 2.11 2.45 2.49 2.62 2.66 6.28
peak of P3HT, the reduction of photoluminescence for the ternary blend may also be partly caused by the absorbance of F(DPP)2B2. This result confirms that F(DPP)2B2 can act as an efficient cascade material for the ternary PSCs. The space charge limited current (SCLC) method [39] was employed to investigate the charge transport properties of P3HT:F(DPP)2B2:PC61BM blend films with different weight ratios of F(DPP)2B2 in fullerene. Hole-only and electron-only devices were fabricated using the following device structures respectively: ITO/PEDOT:PSS/P3HT:F(DPP)2B2:PC61BM/MoO3/Al and Al/P3HT: F(DPP)2B2: PC61BM/Al. Fig. 4a and b show the J0.5–V characteristics of the hole-only and the electron-only devices. The mobility values are calculated from the equations reported in the literature [39] and are summarized in Table 2. The binary blend of P3HT:PC61BM exhibits a hole mobility of 1.26 10 4 cm2 V 1 s 1 and an electron mobility of 7.21 10 5 cm2 V 1 s 1. With increasing the fraction of F(DPP)2B2 in fullerene, both of the hole and electron mobilities are decreased slightly for the ternary blends of P3HT:F(DPP)2B2:PC61BM. This phenomenon is reasonable since it is natural that a lower PC61BM concentration is less beneficial to form percolated electron transport pathways [40]. Moreover, we
Fig. 4. J–V characteristics under dark for (a) hole-only and (b) electron-only devices based on P3HT:F(DPP)2B2:PC61BM blends with different weight ratios of F(DPP)2B2 in PC61BM.
W. Liu et al. / Organic Electronics 25 (2015) 219–224
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Fig. 5. AFM height (a)–(g) and phase (h)–(n) images of P3HT:F(DPP)2B2:PC61BM films with different weight ratios of F(DPP)2B2 spin-coated on ITO/PEDOT:PSS substrates: (a) and (h) 0%, (b) and (i) 2.5%, (c) and (j) 5%, (d) and (k) 10%, (e) and (l) 15%, (f) and (m) 20%, and (g) and (n) 30%.
by XRD, as shown in Fig. 6. The binary P3HT:PC61BM blend film exhibits only one diffraction peak centered at 5.4°, which is assigned to the ordered P3HT molecular arrangement. The pristine F(DPP)2B2 film exhibits a very weak diffraction peak at 8.5°. After adding different weight ratios of F(DPP)2B2 into the binary blend, no obvious changes in the intensity and location of the diffraction peak are observed for the ternary blend films. Therefore, the effect of F(DPP)2B2 on the crystallinity of the blend films is supposed to be negligible. These observations indicate that introduction of F(DPP)2B2 at low concentrations has no significant effect on the morphology of blend films, from which we rule out that the morphology is the main issue responsible for the change of device performance.
Fig. 6. XRD spectra of pristine F(DPP)2B2 and P3HT:F(DPP)2B2:PC61BM films with different weight ratios of F(DPP)2B2.
find that the ternary blends exhibit relatively similar and balanced hole/electron transport when the F(DPP)2B2 content is below 20%, which are corresponding to their good device performance. The lower hole or electron mobility and the unbalanced hole/electron transport are the main reasons for the worse performance of the 30% F(DPP)2B2 ternary blend. To comprehensively illustrate the effect of F(DPP)2B2 ratios on the performance of the ternary PSCs, the surface morphology of the blend films was investigated by atomic force microscopy (AFM). Fig. 5 shows the height and the phase AFM images of the blend films with different F(DPP)2B2 contents. All the films were prepared under the same conditions as the active layers in the devices. The root-mean-square (r.m.s.) roughnesses of the P3HT:F(DPP)2B2:PC61BM films with 0%, 2.5%, 5%, 10%, 15%, 20%, 30% weight ratios of F(DPP)2B2 in PC61BM are 0.850, 0.932, 0.961, 0.969, 0.990, 1.043, and 1.076 nm, respectively. Although the roughness of the blend films increases with the increase of F(DPP)2B2 content, it should be noticed that blending F(DPP)2B2 into P3HT:PC61BM up to 30% does not cause any significant change in the surface roughness of the host blend. All the blend films display the similarly uniform morphology and the obvious donor-acceptor phase separation with the smallest domain in the scale of 10–20 nm, which provides enough interfaces for efficient exciton dissociation and charge carrier transport, thus leading to a high efficiency. It should be noticed that the AFM investigation is only limited on the surface morphology and phase separation of films. To better understand the morphology change of the ternary blends and its effect on the performance of devices, the crystalline structures of the ternary blend films were characterized
4. Conclusions We have successfully developed a novel ternary PSC system with improved efficiency by using a bipolar diketopyrrolopyrrole small molecule F(DPP)2B2 as energy cascade material. A best PCE of 3.92% is obtained in the ternary P3HT:F(DPP)2B2:PC61BM PSCs with a 5 wt% F(DPP)2B2 ratio in acceptor, corresponding to about 23% improvement compared with that of 3.18% PCE for P3HT:PC61BM based cell. The performance improvement of the ternary PSCs can be attributed to the simultaneous improvement of the light harvesting, exciton dissociation, and charge carrier transport by the introduction of the third bipolar component. Our results demonstrate that the ternary strategy is a promising way towards high performance polymer solar cells and the use of the bipolar small molecule as energy cascade material may open up new design directions for novel ternary systems.
Acknowledgements This work was supported by the Major State Basic Research Development Program (2014CB643503), the National Natural Science Foundation of China (Grants 51261130582 and 91233114), and the program for Innovative Research Team in University of Ministry of Education of China (IRT13R54).
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