- Email: [email protected]
Semitransparent, non-fullerene and flexible all-plastic solar cells
Polymer 107 (2016) 108e112
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Semitransparent, non-fullerene and flexible all-plastic solar cells Yifan Wang a, c, Boyu Jia a, Fei Qin b, Yao Wu a, Wei Meng b, Shuixing Dai a, Yinhua Zhou b, **, Xiaowei Zhan a, * a
Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing, 100871, China b Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China c Institute of Biomedical Materials and Engineering, Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 4 November 2016 Accepted 8 November 2016 Available online 9 November 2016
Semitransparent, non-fullerene and flexible all-plastic organic solar cell (OSC) based on a blend of poly(3-hexylthiophene) (P3HT) and non-fullerene acceptor IDT-2BR is fabricated using conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as bottom electrode and film-transfer laminated PEDOT:PSS as top electrode. The solar cell shows average visible transmittance of ca. 50%, which can be potentially used for electricity-generating windows. The all-plastic device shows lower power conversion efficiency (PCE) of 2.88% than the traditional inverted device (4.2%), owing to much higher transmittance and lower conductivity of PEDOT:PSS. Our non-fullerene system P3HT:IDT-2BR shows better performance than fullerene system P3HT:PC61BM (PCE ¼ 2.2%) in all-plastic OSCs. Furthermore, the non-fullerene OSC shows better bending stability than the fullerene counterpart. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Organic solar cell Non-fullerene All-plastic
1. Introduction Organic solar cells (OSCs) have drawn lots of attention in the past few years because of their advantages over their traditional inorganic counterparts, such as low-cost and fast roll-to-roll production, light weight and flexibility. Recently, the power conversion efficiency (PCE) of the OSCs has exceeded 11% [1e4]. Semitransparent OSC devices are promising candidates for electricitygenerating window applications due to the advantages of both light transmission and photovoltaic properties [5e10]. Buildings have large surface areas for capturing sunlight and consume huge amount of energy for lighting, temperature adjusting and powering other electronics. As a result, generating clean energy by their own via integrating low-cost OSC windows could reduce the expense of energy and decrease pollution on an appreciable scale. In laboratories, indium-tin-oxide (ITO) is typically utilized as transparent electrode on rigid glass substrates. However, ITO is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhan), [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.polymer.2016.11.015 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
expensive and brittle. Developing alternative cost-effective transparent electrodes to realize mechanically stable OSCs on flexible substrates is necessary [11]. Several promising ITO alternatives have been reported, such as highly conductive poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) [12e17] thin semitransparent layers [18e21] or metal nanowires [7,22e27], graphene [28e30], etc. Conducting polymer PEDOT:PSS has some advantages, such as easy fabrication, excellent mechanical flexibility, excellent thermal stability and high transparency in the visible spectral region [31]. By utilizing film-transfer technique, highly conductive PEDOT:PSS (PH1000) can be deposited on organic layers as top electrodes [12,14,32]. Transfer lamination can avoid wetting issue and film damage caused by spin coating of nonorthogonal solvents or other additive wet-deposition method. Furthermore, this processing method can easily pattern the top PEDOT:PSS electrode as compared with the spin coating technique. Non-fullerene acceptors have received increasing attention and have developed at a fast speed in the recent years [33e42]. OSCs based on non-fullerene acceptors have achieved PCEs of up to 9e11% [43e46]. We reported that a small molecule IDT-2BR (Fig. 1) is a promising non-fullerene acceptor and OSCs based on poly(3hexylthiophene) (P3HT):IDT-2BR exhibit much better efficiencies
Y. Wang et al. / Polymer 107 (2016) 108e112
109
Fig. 1. Device structure (a) and photo (b) of fullerene-free all-plastic solar cell; chemical structure of IDT-2BR (c).
than their counterpart based on P3HT:PC61BM [47]. In this work, we demonstrate the first example for semitransparent, flexible all-plastic solar cells based on non-fullerene OSCs (Fig. 1a and b). P3HT: IDT-2BR is chosen as the active layer. The bottom electrode is a patterned PH1000 film, modified by an ultrathin layer of polyethylenimine (PEIE). A single PH1000-T film prepared by film-transfer lamination method is used as the top electrode. The non-fullerene OSCs show average visible transmittance (AVT) of ca. 50%, which meets the requirement of the transmittance of electricity-generating windows. The all-plastic fullerene-free OSCs exhibit a PCE of 2.88%, higher than the P3HT:PC61BM based OSCs with similar thickness and transparency (2.2%). Furthermore, the non-fullerene OSCs show better mechanical stability than their fullerene counterparts.
2. Experimental section 2.1. Fabrication of photovoltaic cells We fabricated inverted OSCs based on different electrodes and substrates: glass/ITO/PEIE/active layer (A)/MoO3/Ag; glass/ITO/ PEIE/A/PH1000-T; PES/PH1000/PEIE/A/PH1000-T. PH1000 is PEDOT:PSS PH1000 (Heraeus) prepared by spin-coating method as the bottom electrode; PH1000-T is PEDOT:PSS PH1000 prepared by film-transfer lamination as the top electrode. Bottom electrode preparation: 1) ITO as bottom electrode: the patterned ITO glass with a sheet resistance of 10 U/sq was precleaned in an ultrasonic bath of deionized water, acetone and isopropanol (each for 10 min). 2) PH1000 as bottom electrode: first, polydimethylsiloxane (PDMS) was spin coated onto glass at 3000 rpm for 30 s from a mixture of base and crosslinker (1: 1, weight ratio, PP2-OE41, Gelest Inc.) and cured on a hot plate in air at 80 C for 1 h. The thickness of PDMS on glass is 20 mm. Then, polyethersulfone resin (PES) (i-components Co. Ltd.) substrates were adhered onto PDMS-coated glass substrates. For the bottom PH1000 electrode preparation and patterning, a piece of PDMS was put down on half of the PES substrate surface as a shadow mask and then the PES substrate with PDMS shadow mask were treated by 60 s air plasma treatment (PDC-002, Harrick) to tune half of the PES substrate surface hydrophilic whereas the other half remained hydrophobic. The PDMS mask was removed after the plasma treatment. High conductivity PH1000 with 5 wt% ethylene glycol (SigmaeAldrich) and 0.5% surfactant polyethylene glycol 2,5,8,11tetramethyl-6-dodecyne-5,8-diol ether (PEG-TmDD, TOYNOL
Superwet-340, Surfy-Chem T&D) was spin coated onto the PES substrates at a speed of 1000 rpm for 30 s with an acceleration of 1000 rpm s1 and then annealed at 140 C for 10 min on a hot plate in air. PH1000 film was only deposited on half the PES substrate because the aqueous solution only wetted the half of the PES substrate exposed to plasma treatment. The cathode buffer layer PEIE (Sigma-Aldrich, Mw ¼ 75 000 g mol1) was spin coated on PH1000 or ITO at 5000 rpm for 1 min in a N2-filled glove box from a 2-methoxyethanol (Sigma-Aldrich) solution (0.1 wt%) and annealed at 100 C for 10 min. Active layer preparation: 1) P3HT:IDT-2BR (A1) as active layer: the P3HT:IDT-2BR (16.7:10 mg mL1) mixture in o-dichlorobenzene (o-DCB) was spin-coated on the PEIE layer at 1500 rpm for 1 min (70 nm thick) and annealed at 130 C for 15 min. 2) P3HT:PC61BM (A2) as active layer: the P3HT:PC61BM (20:20 mg mL1) mixture in o-DCB was spin-coated on the PEIE layer at 700 rpm for 1 min (200 nm thick) or 2000 rpm for 1 min (70 nm thick) and then annealed at 150 C for 10 min. Top electrode preparation: 1) MoO3/Ag as top electrode: a MoO3 (ca. 10 nm) and Ag layer (ca. 80 nm) was evaporated onto the surface of the active layer under vacuum (ca. 105 Pa). 2) PH1000-T as top electrode: first, a piece of PDMS (1e2 mm thick) was attached to a glass substrate and exposed to O2 plasma for 60 s to tune its surface hydrophilicity. PH1000 with 5 wt% ethylene glycol and 0.1% PEG-TmDD was spin coated onto the PDMS at 1000 rpm for 30 s and dried in air for 5 min. Before transfer, samples of devices without top electrodes were exposed to a flash of O2 plasma for about 10 s. Then, the PDMS with PH1000 was cut into ca. 2 mmwide finger-electrode shapes and transferred onto the active layer face down with PH1000 contacting the photoactive layer. Then, the top PDMS was slowly peeled off and PH1000-T was left on the active layer to finish the PH1000-T lamination process. Ag paint (Leitsilber 200, Ted Pella Inc.) was applied onto PH1000 for electrical contact during the measurement. The cells were annealed in a N2-filled glove box at 110 C for 5 min to dry the PH1000-T top electrode. The device area was measured and calculated precisely for each sample with a reference scale under an optical microscope.
2.2. Characterization Current density-voltage (J-V) characteristics were measured inside a N2-filled glove box by using a source meter (2400, Keithley Instruments) controlled by a LabVIEW program in the dark and under white light illumination (100 mW cm2). The light intensity
110
Y. Wang et al. / Polymer 107 (2016) 108e112
was calibrated using a silicon photodiode with KG5 filter (S1133, Hamamatsu). The external quantum efficiency (EQE) and internal quantum efficiency (IQE) spectra were recorded using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated by using a standard single crystal Si photovoltaic cell. Thin-film UV-vis transmission curve was recorded on a JASCO V-570 spectrophotometer. 3. Results and discussion Fig. 2 illustrates the procedure for fabrication of the all-plastic solar cells. This procedure is briefly summarized as follows (more details are provided in Experimental section): (1) PH1000, PEIE and the active layer were spin coated on a piece of PES substrate successively; (2) PH1000 was spin coated on a PDMS substrate; (3) PDMS/PH1000 was put onto the active layer face down; (4) the top PDMS was slowly peeled off and PH1000-T was left on the active layer to finish the lamination process. The transmittance of the bottom electrodes and the whole devices are shown in Fig. 3. The average visible transmittance (AVT), which means the average transmittance of the solar cells in the visible region (370e740 nm), is summarized in Table 1. The PH1000 electrode (on PES substrate) shows similar excellent transmittance to ITO electrode (on glass substrate) with AVT over 80%, which is supposed to be a basic requirement for transparent electrodes. An AVT of 25% is generally considered as the benchmark for window applications [48]. While our devices with both ITO and PEDOT as bottom electrode show AVT of ca. 50%, fully competent for window applications. Fig. 4a shows the optimized current density-voltage (J-V) characteristics of inverted OSCs based on P3HT:IDT-2BR with different electrodes and substrates. The detailed photovoltaic properties are summarized in Table 2. The inverted device with ITO as cathode and MoO3/Ag as anode shows better performance (PCE ¼ 4.2%) than conventional devices processed without additive (PCE ¼ 3.72%) [47] , since the inverted structure exhibits better vertical phase separation than the conventional one. When we apply transfer laminated PH1000 instead of MoO3/Ag as anode, the devices show VOC of 0.84 V, JSC of 6.23 mA cm2, FF of 61.5%, and PCE of 3.22%. The decreased PCE is related to the lower JSC and FF. As compared with the devices with MoO3/Ag anode, the decreased FF value is attributed to the increased series resistance introduced by the lower conductivity of PH1000 relative to MoO3/Ag. The smaller JSC value is attributed to the much higher transmittance of PH1000 compared with the metal electrode, which results in the absence of the additional current generated by the second pass of unabsorbed incident light in the active layer after reflection from the back metal electrode. The all-plastic device processed on flexible substrate (PES) with spin-coated PH1000 as cathode and PH1000-T as anode shows an even smaller PCE (2.88%), which is mostly due to the decreased FF (57.9%). This is also ascribed to the addition of another
Fig. 2. Fabrication procedure of the fullerene-free all-plastic solar cells.
Fig. 3. Transmittance spectra of different electrodes and devices.
Table 1 The AVT of different electrodes and devices.
AVT (%) a b
ITO/glass
PH1000/PES
ITO devicea
PH1000 deviceb
83.7
83.4
53.2
48.1
The device structure is glass/ITO/PEIE/P3HT:IDT-2BR/PH1000-T. The device structure is PES/PH1000/PEIE/P3HT:IDT-2BR/PH1000-T.
PH1000 layer leading to an even larger series resistance. The EQE spectra match the tested JSC (Fig. 4b). The IQE is the product of charge transfer, charge transport and charge extraction efficiencies. The charge transfer and charge transport efficiency of the three kinds of devices is similar due to the same active layer. Thus, the difference in IQE values of these three devices is attributed to their different charge extraction efficiency. The IQE values (Fig. 4c) slightly decrease from traditional device to device with PH1000-T as top electrode and finally to all-plastic device, which is attributed to the different conductivity of electrodes. The difference in IQE is smaller than that of EQE among the three kinds of devices, indicating that reflection of the back electrode is the main factor affecting the device performance. We compare the all-plastic OSCs based on non-fullerene and fullerene acceptors. Fig. 5a shows the J-V characteristics of allplastic OSCs based on P3HT:IDT-2BR and P3HT:PC61BM. In order to obtain a comparable transmittance, we set the same thickness of the active layer of the fullerene system as the optimized nonfullerene system (70 nm). All-plastic OSC based on P3HT:PC61BM (70) shows a smaller PCE of 2.2% (VOC ¼ 0.59 V, JSC ¼ 6.82 mA cm2, FF ¼ 53.6%) than the non-fullerene system. Although the devices based on P3HT:PC61BM with optimized thickness (200 nm) exhibit a higher PCE of 2.74% but decreased transmittance than the device with 70 nm thickness. The optimized fullerene device shows slightly lower PCE than the non-fullerene system. The EQE spectra (Fig. 5b) prove the JSC difference of the devices with different active layers. Bending test of the devices is performed using a roller with a bending radius of 5.5 mm, in order to evaluate the mechanical properties of P3HT: IDT-2BR and P3HT: PC61BM based all-plastic OSCs. The non-fullerene device shows slight decrease by a factor of 12.1% of the original PCE after 250 bending cycles (Fig. 6), demonstrating the excellent ductility of the PEDOT electrodes and the organic active layers. The fullerene device also shows good bending stability with a decrease of 23% of the original PCE, higher than the non-fullerene device, indicating that the non-fullerene device is mechanically more stable compared with the traditional fullerene device.
Y. Wang et al. / Polymer 107 (2016) 108e112
111
Fig. 4. a) J-V characteristics, b) EQE and c) IQE of OSCs based on P3HT:IDT-2BR (A1) with different electrodes and substrates.
Table 2 Photovoltaic properties of inverted OSCs with different structures (numbers in the brackets are thickness of the active layers). Structure
Area (mm2)
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
Glass/ITO/PEIE/P3HT: IDT-2BR/MoO3/Ag Glass/ITO/PEIE/P3HT:IDT-2BR/PH1000-T PES/PH1000/PEIE/P3HT:IDT-2BR/PH1000-T PES/PH1000/PEIE/P3HT: PC61BM(70)/PH1000-T PES/PH1000/PEIE/P3HT: PC61BM(200)/PH1000-T
4.00 5.61 4.29 4.89 5.21
0.85 0.84 0.84 0.59 0.61
7.02 6.23 5.93 6.82 7.41
70.0 61.5 57.9 53.6 60.6
4.20 3.22 2.88 2.20 2.74
Fig. 5. a) J-V characteristics and b) EQE of all-plastic OSCs based on different acceptors. Numbers in the brackets are thickness of the active layers.
electricity-generating windows. The all-plastic fullerene-free OSC exhibits a PCE of 2.88%, higher than the fullerene OSC based on P3HT:PC61BM (2.2%) with similar thickness and transparency. Furthermore, we test the mechanical stability of the non-fullerene and fullerene all-plastic OSCs, both systems show excellent bending stability, while the non-fullerene device is slightly better. Acknowledgements We thank the 973 Program (2013CB834702) and NSFC (91433114) for financial support. References
Fig. 6. Mechanical testing (PCE vs. bending cycles) of all-plastic OSCs based on P3HT: IDT-2BR (70 nm) and P3HT:PC61BM (70 nm).
4. Conclusions We fabricate a novel semitransparent all-plastic OSC based on a non-fullerene system of P3HT:IDT-2BR. The solar cell shows AVT of ca. 50%, which meets the requirement of the transmittance of
[1] J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Efficient organic solar cells processed from hydrocarbon solvents, Nat. Energy 1 (2016) 15027. [2] C.C. Chen, W.H. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, An efficient triple-junction polymer solar cell having a power conversion efficiency exceeding 11%, Adv. Mater. 26 (2014) 5670e5677. [3] A.R.B.M. Yusoff, D. Kim, H.P. Kim, F.K. Shneider, W.J. da Silva, J. Jang, Energy Environ. Sci. 8 (2015) 303e316. [4] J.C. Aguirre, S.A. Hawks, A.S. Ferreira, P. Yee, S. Subramaniyan, S.A. Jenekhe, S.H. Tolbert, B.J. Schwartz, A high efficiency solution processed polymer inverted triple-junction solar cell exhibiting a power conversion efficiency of 11.83%, Adv. Energy Mater. 5 (2015) 1402020. [5] Z. Liu, J. Li, Z.-H. Sun, G. Tai, S.-P. Lau, F. Yan, The application of highly doped single-layer graphene as the top electrodes of semitransparent organic solar
112
Y. Wang et al. / Polymer 107 (2016) 108e112
cells, ACS Nano 6 (2012) 810e818. [6] F. Pastorelli, P. Romero-Gomez, R. Betancur, A. Martinez-Otero, P. MantillaPerez, N. Bonod, J. Martorell, Adv. Energy Mater. 5 (2015) 1400614. [7] J. Krantz, T. Stubhan, M. Richter, S. Spallek, I. Litzov, G.J. Matt, E. Spiecker, C.J. Brabec, Enhanced light harvesting in semitransparent organic solar cells using an optical metal cavity configuration, Adv. Funct. Mater. 23 (2013) 1711e1717. [8] F. Guo, S. Chen, Z. Chen, H. Luo, Y. Gao, T. Przybilla, E. Spiecker, A. Osvet, K. Forberich, C.J. Brabec, Printed smart photovoltaic window integrated with an energy-saving thermochromic layer, Adv. Opt. Mater. 3 (2015) 1524e1529. [9] J.-Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Semitransparent organic photovoltaic cells with laminated top electrode, Nano Lett. 10 (2010) 1276e1279. [10] C.-Y. Chang, L. Zuo, H.-L. Yip, C.-Z. Li, Y. Li, C.-S. Hsu, Y.-J. Cheng, H. Chen, A.K.Y. Jen, Highly efficient polymer tandem cells and semitransparent cells for solar energy, Adv. Energy Mater. 4 (2013) 1301645. [11] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photon. 6 (2012) 809e817. [12] J. Tong, S. Xiong, Z. Li, F. Jiang, L. Mao, W. Meng, Y. Zhou, Vacuum-free and metal electrode-free organic tandem solar cells, Appl. Phys. Lett. 106 (2015) 053306. [13] Z. Li, F. Qin, T. Liu, R. Ge, W. Meng, J. Tong, S. Xiong, Y. Zhou, Optical properties and conductivity of PEDOT: PSS films treated by polyethylenimine solution for organic solar cells, Org. Electron. 21 (2015) 144e148. [14] Y. Zhou, T.M. Khan, J.W. Shim, A. Dindar, C. Fuentes-Hernandez, B. Kippelen, All-plastic solar cells with a high photovoltaic dynamic range, J. Mater. Chem. A 2 (2014) 3492e3497. [15] L. Yin, Z. Zhao, F. Jiang, Z. Li, S. Xiong, Y. Zhou, PEDOT: PSS top electrode prepared by transfer lamination using plastic wrap as the transfer medium for organic solar cells, Org. Electron. 15 (2014) 2593e2598. [16] W. Zhang, B. Zhao, Z. He, X. Zhao, H. Wang, S. Yang, H. Wu, Y. Cao, High-efficiency ITO-free polymer solar cells using highly conductive PEDOT: PSS/ surfactant bilayer transparent anodes, Energy Environ. Sci. 6 (2013) 1956e1964. [17] M. Vosgueritchian, D.J. Lipomi, Z. Bao, Highly conductive and transparent PEDOT: PSS films with a fluorosurfactant for stretchable and flexible transparent electrodes, Adv. Funct. Mater. 22 (2012) 421e428. [18] G. Zhao, W. Wang, T.-S. Bae, S.-G. Lee, C. Mun, S. Lee, H. Yu, G.-H. Lee, M. Song, J. Yun, Stable ultrathin partially oxidized copper film electrode for highly efficient flexible solar cells, Nat. Commun. 6 (2015) 8830. [19] H. Kang, S. Jung, S. Jeong, G. Kim, K. Lee, Polymer-metal hybrid transparent electrodes for flexible electronics, Nat. Commun. 6 (2015) 6503. [20] J. Zou, C.-Z. Li, C.-Y. Chang, H.-L. Yip, A.K.Y. Jen, Interfacial engineering of ultrathin metal film transparent electrode for flexible organic photovoltaic cells, Adv. Mater. 26 (2014) 3618e3623. [21] J. Huang, C.-Z. Li, C.-C. Chueh, S.-Q. Liu, J.-S. Yu, A.K.Y. Jen, 10.4% Power Conversion Efficiency of ITO-Free organic photovoltaics through enhanced light trapping configuration, Adv. Energy Mater. 5 (2015) 1500406. [22] K. Zilberberg, F. Gasse, R. Pagui, A. Polywka, A. Behrendt, S. Trost, €rrn, T. Riedl, Highly robust indium-free transparent R. Heiderhoff, P. Go conductive electrodes based on composites of silver nanowires and conductive metal oxides, Adv. Funct. Mater. 24 (2014) 1671e1678. [23] J.H. Yim, S.-Y. Joe, C. Pang, K.M. Lee, H. Jeong, J.-Y. Park, Y.H. Ahn, J.C. de Mello, S. Lee, Fully solution-processed semitransparent organic solar cells with a silver nanowire cathode and a conducting polymer anode, ACS Nano 8 (2014) 2857e2863. [24] J. Song, J. Li, J. Xu, H. Zeng, Superstable transparent conductive Cu@Cu4Ni nanowire elastomer composites against oxidation, bending, stretching, and twisting for flexible and stretchable optoelectronics, Nano Lett. 14 (2014) 6298e6305. [25] S. Han, S. Hong, J. Ham, J. Yeo, J. Lee, B. Kang, P. Lee, J. Kwon, S.S. Lee, M.Y. Yang, S.H. Ko, Fast plasmonic laser nanowelding for a Cu-nanowire percolation network for flexible transparent conductors and stretchable electronics, Adv. Mater. 26 (2014) 5808e5814. [26] H. Wu, D. Kong, Z. Ruan, P.-C. Hsu, S. Wang, Z. Yu, T.J. Carney, L. Hu, S. Fan, Y. Cui, A transparent electrode based on a metal nanotrough network, Nat. Nano 8 (2013) 421e425. [27] R.V. Salvatierra, C.E. Cava, L.S. Roman, A.J.G. Zarbin, ITO-free and flexible organic photovoltaic device based on high transparent and conductive polyaniline/carbon nanotube thin films, Adv. Funct. Mater. 23 (2013) 1490e1499.
[28] X. Wang, G. Shi, Flexible graphene devices related to energy conversion and storage, Energy Environ. Sci. 8 (2015) 790e823. [29] K. Parvez, R. Li, S.R. Puniredd, Y. Hernandez, F. Hinkel, S. Wang, X. Feng, K. Müllen, Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes for organic electronics, ACS Nano 7 (2013) 3598e3606. [30] Z. Liu, J. Li, F. Yan, Package-free flexible organic solar cells with graphene top electrodes, Adv. Mater. 25 (2013) 4296e4301. [31] Z. Li, G. Ma, R. Ge, F. Qin, X. Dong, W. Meng, T. Liu, J. Tong, F. Jiang, Y. Zhou, K. Li, X. Min, K. Huo, Y. Zhou, Free-standing conducting polymer films for high-performance energy devices, Angew. Chem. Int. Ed. 55 (2016) 979e982. [32] F. Jiang, T. Liu, S. Zeng, Q. Zhao, X. Min, Z. Li, J. Tong, W. Meng, S. Xiong, Y. Zhou, Metal electrode-free perovskite solar cells with transfer-laminated conducting polymer electrode, Opt. Express 23 (2015) A83eA91. [33] X. Zhan, Z.A. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, A high-mobility electron-transport polymer with broad absorption and its use in field-effect transistors and all-polymer solar cells, J. Am. Chem. Soc. 129 (2007) 7246e7247. [34] Y. Lin, X. Zhan, Non-fullerene acceptors for organic photovoltaics: an emerging horizon, Mater. Horiz. 1 (2014) 470e488. [35] Y. Lin, X. Zhan, Oligomer molecules for efficient organic photovoltaics, Acc. Chem. Res. 49 (2016) 175e183. [36] Y. Lin, X. Zhan, Designing efficient non-fullerene acceptors by tailoring extended fused-rings with electron-deficient groups, Adv. Energy Mater. 5 (2015) 1501063. [37] L. Gao, Z.-G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei, Y. Li, All-polymer solar cells based on absorption-complementary polymer donor and acceptor with high power conversion efficiency of 8.27%, Adv. Mater. 28 (2016) 1884e1890. [38] Y. Lin, Q. He, F. Zhao, L. Huo, J. Mai, X. Lu, C.-J. Su, T. Li, J. Wang, J. Zhu, Y. Sun, C. Wang, X. Zhan, A facile planar fused-ring electron acceptor for As-Cast polymer solar cells with 8.71% efficiency, J. Am. Chem. Soc. 138 (2016) 2973e2976. [39] D. Meng, D. Sun, C. Zhong, T. Liu, B. Fan, L. Huo, Y. Li, W. Jiang, H. Choi, T. Kim, J.Y. Kim, Y. Sun, Z. Wang, A.J. Heeger, High-performance solution-processed non-fullerene organic solar cells based on selenophene-containing perylene bisimide acceptor, J. Am. Chem. Soc. 138 (2016) 375e380. [40] Y. Zhong, M.T. Trinh, R. Chen, G.E. Purdum, P.P. Khlyabich, M. Sezen, S. Oh, H. Zhu, B. Fowler, B. Zhang, W. Wang, C.-Y. Nam, M.Y. Sfeir, C.T. Black, M.L. Steigerwald, Y.-L. Loo, F. Ng, X.Y. Zhu, C. Nuckolls, Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells, Nat. Commun. 6 (2015) 8242. [41] Y. Lin, J. Wang, Z.-G. Zhang, H. Bai, Y. Li, D. Zhu, X. Zhan, An electron acceptor challenging fullerenes for efficient polymer solar cells, Adv. Mater. 27 (2015) 1170e1174. [42] Y. Lin, Z.-G. Zhang, H. Bai, J. Wang, Y. Yao, Y. Li, D. Zhu, X. Zhan, High-performance fullerene-free polymer solar cells with 6.31% efficiency, Energy Environ. Sci. 8 (2015) 610e616. [43] H. Bin, Z.-G. Zhang, L. Gao, S. Chen, L. Zhong, L. Xue, C. Yang, Y. Li, Nonfullerene polymer solar cells based on alkylthio and fluorine substituted 2DConjugated polymers reach 9.5% efficiency, J. Am. Chem. Soc. 138 (2016) 4657e4664. [44] Y. Lin, F. Zhao, Q. He, L. Huo, Y. Wu, T.C. Parker, W. Ma, Y. Sun, C. Wang, D. Zhu, A.J. Heeger, S.R. Marder, X. Zhan, High-performance electron acceptor with thienyl side chains for organic photovoltaics, J. Am. Chem. Soc. 138 (2016) 4955e4961. €s, F. Gao, J. Hou, Fullerene-free [45] W. Zhao, D. Qian, S. Zhang, S. Li, O. Ingana polymer solar cells with over 11% efficiency and excellent thermal stability, Adv. Mater. 28 (2016) 4734e4739. [46] Y. Lin, T. Li, F. Zhao, L. Han, Z. Wang, Y. Wu, Q. He, J. Wang, L. Huo, Y. Sun, C. Wang, W. Ma, X. Zhan, Structure evolution of oligomer fused-ring electron acceptors toward high efficiency of as-cast polymer solar cells, Adv. Energy Mater. 6 (2016), 1600854. [47] Y. Wu, H. Bai, Z. Wang, P. Cheng, S. Zhu, Y. Wang, W. Ma, X. Zhan, A planar electron acceptor for efficient polymer solar cells, Energy Environ. Sci. 8 (2015) 3215e3221. [48] N. Drolet, Organic Photovoltaic: Efficiency and Lifetime Challenges for Commercial Viability, 2012 MRS Spring Meeting & Exhibit, Moscone West Convention Center, Marriott Marquis, San Francisco, CA, 2012.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Semitransparent, non-fullerene and flexible all-plastic solar cells Yifan Wang a, c, Boyu Jia a, Fei Qin b, Yao Wu a, Wei Meng b, Shuixing Dai a, Yinhua Zhou b, **, Xiaowei Zhan a, * a
Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing, 100871, China b Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China c Institute of Biomedical Materials and Engineering, Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 4 November 2016 Accepted 8 November 2016 Available online 9 November 2016
Semitransparent, non-fullerene and flexible all-plastic organic solar cell (OSC) based on a blend of poly(3-hexylthiophene) (P3HT) and non-fullerene acceptor IDT-2BR is fabricated using conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as bottom electrode and film-transfer laminated PEDOT:PSS as top electrode. The solar cell shows average visible transmittance of ca. 50%, which can be potentially used for electricity-generating windows. The all-plastic device shows lower power conversion efficiency (PCE) of 2.88% than the traditional inverted device (4.2%), owing to much higher transmittance and lower conductivity of PEDOT:PSS. Our non-fullerene system P3HT:IDT-2BR shows better performance than fullerene system P3HT:PC61BM (PCE ¼ 2.2%) in all-plastic OSCs. Furthermore, the non-fullerene OSC shows better bending stability than the fullerene counterpart. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Organic solar cell Non-fullerene All-plastic
1. Introduction Organic solar cells (OSCs) have drawn lots of attention in the past few years because of their advantages over their traditional inorganic counterparts, such as low-cost and fast roll-to-roll production, light weight and flexibility. Recently, the power conversion efficiency (PCE) of the OSCs has exceeded 11% [1e4]. Semitransparent OSC devices are promising candidates for electricitygenerating window applications due to the advantages of both light transmission and photovoltaic properties [5e10]. Buildings have large surface areas for capturing sunlight and consume huge amount of energy for lighting, temperature adjusting and powering other electronics. As a result, generating clean energy by their own via integrating low-cost OSC windows could reduce the expense of energy and decrease pollution on an appreciable scale. In laboratories, indium-tin-oxide (ITO) is typically utilized as transparent electrode on rigid glass substrates. However, ITO is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhan), [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.polymer.2016.11.015 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
expensive and brittle. Developing alternative cost-effective transparent electrodes to realize mechanically stable OSCs on flexible substrates is necessary [11]. Several promising ITO alternatives have been reported, such as highly conductive poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) [12e17] thin semitransparent layers [18e21] or metal nanowires [7,22e27], graphene [28e30], etc. Conducting polymer PEDOT:PSS has some advantages, such as easy fabrication, excellent mechanical flexibility, excellent thermal stability and high transparency in the visible spectral region [31]. By utilizing film-transfer technique, highly conductive PEDOT:PSS (PH1000) can be deposited on organic layers as top electrodes [12,14,32]. Transfer lamination can avoid wetting issue and film damage caused by spin coating of nonorthogonal solvents or other additive wet-deposition method. Furthermore, this processing method can easily pattern the top PEDOT:PSS electrode as compared with the spin coating technique. Non-fullerene acceptors have received increasing attention and have developed at a fast speed in the recent years [33e42]. OSCs based on non-fullerene acceptors have achieved PCEs of up to 9e11% [43e46]. We reported that a small molecule IDT-2BR (Fig. 1) is a promising non-fullerene acceptor and OSCs based on poly(3hexylthiophene) (P3HT):IDT-2BR exhibit much better efficiencies
Y. Wang et al. / Polymer 107 (2016) 108e112
109
Fig. 1. Device structure (a) and photo (b) of fullerene-free all-plastic solar cell; chemical structure of IDT-2BR (c).
than their counterpart based on P3HT:PC61BM [47]. In this work, we demonstrate the first example for semitransparent, flexible all-plastic solar cells based on non-fullerene OSCs (Fig. 1a and b). P3HT: IDT-2BR is chosen as the active layer. The bottom electrode is a patterned PH1000 film, modified by an ultrathin layer of polyethylenimine (PEIE). A single PH1000-T film prepared by film-transfer lamination method is used as the top electrode. The non-fullerene OSCs show average visible transmittance (AVT) of ca. 50%, which meets the requirement of the transmittance of electricity-generating windows. The all-plastic fullerene-free OSCs exhibit a PCE of 2.88%, higher than the P3HT:PC61BM based OSCs with similar thickness and transparency (2.2%). Furthermore, the non-fullerene OSCs show better mechanical stability than their fullerene counterparts.
2. Experimental section 2.1. Fabrication of photovoltaic cells We fabricated inverted OSCs based on different electrodes and substrates: glass/ITO/PEIE/active layer (A)/MoO3/Ag; glass/ITO/ PEIE/A/PH1000-T; PES/PH1000/PEIE/A/PH1000-T. PH1000 is PEDOT:PSS PH1000 (Heraeus) prepared by spin-coating method as the bottom electrode; PH1000-T is PEDOT:PSS PH1000 prepared by film-transfer lamination as the top electrode. Bottom electrode preparation: 1) ITO as bottom electrode: the patterned ITO glass with a sheet resistance of 10 U/sq was precleaned in an ultrasonic bath of deionized water, acetone and isopropanol (each for 10 min). 2) PH1000 as bottom electrode: first, polydimethylsiloxane (PDMS) was spin coated onto glass at 3000 rpm for 30 s from a mixture of base and crosslinker (1: 1, weight ratio, PP2-OE41, Gelest Inc.) and cured on a hot plate in air at 80 C for 1 h. The thickness of PDMS on glass is 20 mm. Then, polyethersulfone resin (PES) (i-components Co. Ltd.) substrates were adhered onto PDMS-coated glass substrates. For the bottom PH1000 electrode preparation and patterning, a piece of PDMS was put down on half of the PES substrate surface as a shadow mask and then the PES substrate with PDMS shadow mask were treated by 60 s air plasma treatment (PDC-002, Harrick) to tune half of the PES substrate surface hydrophilic whereas the other half remained hydrophobic. The PDMS mask was removed after the plasma treatment. High conductivity PH1000 with 5 wt% ethylene glycol (SigmaeAldrich) and 0.5% surfactant polyethylene glycol 2,5,8,11tetramethyl-6-dodecyne-5,8-diol ether (PEG-TmDD, TOYNOL
Superwet-340, Surfy-Chem T&D) was spin coated onto the PES substrates at a speed of 1000 rpm for 30 s with an acceleration of 1000 rpm s1 and then annealed at 140 C for 10 min on a hot plate in air. PH1000 film was only deposited on half the PES substrate because the aqueous solution only wetted the half of the PES substrate exposed to plasma treatment. The cathode buffer layer PEIE (Sigma-Aldrich, Mw ¼ 75 000 g mol1) was spin coated on PH1000 or ITO at 5000 rpm for 1 min in a N2-filled glove box from a 2-methoxyethanol (Sigma-Aldrich) solution (0.1 wt%) and annealed at 100 C for 10 min. Active layer preparation: 1) P3HT:IDT-2BR (A1) as active layer: the P3HT:IDT-2BR (16.7:10 mg mL1) mixture in o-dichlorobenzene (o-DCB) was spin-coated on the PEIE layer at 1500 rpm for 1 min (70 nm thick) and annealed at 130 C for 15 min. 2) P3HT:PC61BM (A2) as active layer: the P3HT:PC61BM (20:20 mg mL1) mixture in o-DCB was spin-coated on the PEIE layer at 700 rpm for 1 min (200 nm thick) or 2000 rpm for 1 min (70 nm thick) and then annealed at 150 C for 10 min. Top electrode preparation: 1) MoO3/Ag as top electrode: a MoO3 (ca. 10 nm) and Ag layer (ca. 80 nm) was evaporated onto the surface of the active layer under vacuum (ca. 105 Pa). 2) PH1000-T as top electrode: first, a piece of PDMS (1e2 mm thick) was attached to a glass substrate and exposed to O2 plasma for 60 s to tune its surface hydrophilicity. PH1000 with 5 wt% ethylene glycol and 0.1% PEG-TmDD was spin coated onto the PDMS at 1000 rpm for 30 s and dried in air for 5 min. Before transfer, samples of devices without top electrodes were exposed to a flash of O2 plasma for about 10 s. Then, the PDMS with PH1000 was cut into ca. 2 mmwide finger-electrode shapes and transferred onto the active layer face down with PH1000 contacting the photoactive layer. Then, the top PDMS was slowly peeled off and PH1000-T was left on the active layer to finish the PH1000-T lamination process. Ag paint (Leitsilber 200, Ted Pella Inc.) was applied onto PH1000 for electrical contact during the measurement. The cells were annealed in a N2-filled glove box at 110 C for 5 min to dry the PH1000-T top electrode. The device area was measured and calculated precisely for each sample with a reference scale under an optical microscope.
2.2. Characterization Current density-voltage (J-V) characteristics were measured inside a N2-filled glove box by using a source meter (2400, Keithley Instruments) controlled by a LabVIEW program in the dark and under white light illumination (100 mW cm2). The light intensity
110
Y. Wang et al. / Polymer 107 (2016) 108e112
was calibrated using a silicon photodiode with KG5 filter (S1133, Hamamatsu). The external quantum efficiency (EQE) and internal quantum efficiency (IQE) spectra were recorded using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated by using a standard single crystal Si photovoltaic cell. Thin-film UV-vis transmission curve was recorded on a JASCO V-570 spectrophotometer. 3. Results and discussion Fig. 2 illustrates the procedure for fabrication of the all-plastic solar cells. This procedure is briefly summarized as follows (more details are provided in Experimental section): (1) PH1000, PEIE and the active layer were spin coated on a piece of PES substrate successively; (2) PH1000 was spin coated on a PDMS substrate; (3) PDMS/PH1000 was put onto the active layer face down; (4) the top PDMS was slowly peeled off and PH1000-T was left on the active layer to finish the lamination process. The transmittance of the bottom electrodes and the whole devices are shown in Fig. 3. The average visible transmittance (AVT), which means the average transmittance of the solar cells in the visible region (370e740 nm), is summarized in Table 1. The PH1000 electrode (on PES substrate) shows similar excellent transmittance to ITO electrode (on glass substrate) with AVT over 80%, which is supposed to be a basic requirement for transparent electrodes. An AVT of 25% is generally considered as the benchmark for window applications [48]. While our devices with both ITO and PEDOT as bottom electrode show AVT of ca. 50%, fully competent for window applications. Fig. 4a shows the optimized current density-voltage (J-V) characteristics of inverted OSCs based on P3HT:IDT-2BR with different electrodes and substrates. The detailed photovoltaic properties are summarized in Table 2. The inverted device with ITO as cathode and MoO3/Ag as anode shows better performance (PCE ¼ 4.2%) than conventional devices processed without additive (PCE ¼ 3.72%) [47] , since the inverted structure exhibits better vertical phase separation than the conventional one. When we apply transfer laminated PH1000 instead of MoO3/Ag as anode, the devices show VOC of 0.84 V, JSC of 6.23 mA cm2, FF of 61.5%, and PCE of 3.22%. The decreased PCE is related to the lower JSC and FF. As compared with the devices with MoO3/Ag anode, the decreased FF value is attributed to the increased series resistance introduced by the lower conductivity of PH1000 relative to MoO3/Ag. The smaller JSC value is attributed to the much higher transmittance of PH1000 compared with the metal electrode, which results in the absence of the additional current generated by the second pass of unabsorbed incident light in the active layer after reflection from the back metal electrode. The all-plastic device processed on flexible substrate (PES) with spin-coated PH1000 as cathode and PH1000-T as anode shows an even smaller PCE (2.88%), which is mostly due to the decreased FF (57.9%). This is also ascribed to the addition of another
Fig. 2. Fabrication procedure of the fullerene-free all-plastic solar cells.
Fig. 3. Transmittance spectra of different electrodes and devices.
Table 1 The AVT of different electrodes and devices.
AVT (%) a b
ITO/glass
PH1000/PES
ITO devicea
PH1000 deviceb
83.7
83.4
53.2
48.1
The device structure is glass/ITO/PEIE/P3HT:IDT-2BR/PH1000-T. The device structure is PES/PH1000/PEIE/P3HT:IDT-2BR/PH1000-T.
PH1000 layer leading to an even larger series resistance. The EQE spectra match the tested JSC (Fig. 4b). The IQE is the product of charge transfer, charge transport and charge extraction efficiencies. The charge transfer and charge transport efficiency of the three kinds of devices is similar due to the same active layer. Thus, the difference in IQE values of these three devices is attributed to their different charge extraction efficiency. The IQE values (Fig. 4c) slightly decrease from traditional device to device with PH1000-T as top electrode and finally to all-plastic device, which is attributed to the different conductivity of electrodes. The difference in IQE is smaller than that of EQE among the three kinds of devices, indicating that reflection of the back electrode is the main factor affecting the device performance. We compare the all-plastic OSCs based on non-fullerene and fullerene acceptors. Fig. 5a shows the J-V characteristics of allplastic OSCs based on P3HT:IDT-2BR and P3HT:PC61BM. In order to obtain a comparable transmittance, we set the same thickness of the active layer of the fullerene system as the optimized nonfullerene system (70 nm). All-plastic OSC based on P3HT:PC61BM (70) shows a smaller PCE of 2.2% (VOC ¼ 0.59 V, JSC ¼ 6.82 mA cm2, FF ¼ 53.6%) than the non-fullerene system. Although the devices based on P3HT:PC61BM with optimized thickness (200 nm) exhibit a higher PCE of 2.74% but decreased transmittance than the device with 70 nm thickness. The optimized fullerene device shows slightly lower PCE than the non-fullerene system. The EQE spectra (Fig. 5b) prove the JSC difference of the devices with different active layers. Bending test of the devices is performed using a roller with a bending radius of 5.5 mm, in order to evaluate the mechanical properties of P3HT: IDT-2BR and P3HT: PC61BM based all-plastic OSCs. The non-fullerene device shows slight decrease by a factor of 12.1% of the original PCE after 250 bending cycles (Fig. 6), demonstrating the excellent ductility of the PEDOT electrodes and the organic active layers. The fullerene device also shows good bending stability with a decrease of 23% of the original PCE, higher than the non-fullerene device, indicating that the non-fullerene device is mechanically more stable compared with the traditional fullerene device.
Y. Wang et al. / Polymer 107 (2016) 108e112
111
Fig. 4. a) J-V characteristics, b) EQE and c) IQE of OSCs based on P3HT:IDT-2BR (A1) with different electrodes and substrates.
Table 2 Photovoltaic properties of inverted OSCs with different structures (numbers in the brackets are thickness of the active layers). Structure
Area (mm2)
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
Glass/ITO/PEIE/P3HT: IDT-2BR/MoO3/Ag Glass/ITO/PEIE/P3HT:IDT-2BR/PH1000-T PES/PH1000/PEIE/P3HT:IDT-2BR/PH1000-T PES/PH1000/PEIE/P3HT: PC61BM(70)/PH1000-T PES/PH1000/PEIE/P3HT: PC61BM(200)/PH1000-T
4.00 5.61 4.29 4.89 5.21
0.85 0.84 0.84 0.59 0.61
7.02 6.23 5.93 6.82 7.41
70.0 61.5 57.9 53.6 60.6
4.20 3.22 2.88 2.20 2.74
Fig. 5. a) J-V characteristics and b) EQE of all-plastic OSCs based on different acceptors. Numbers in the brackets are thickness of the active layers.
electricity-generating windows. The all-plastic fullerene-free OSC exhibits a PCE of 2.88%, higher than the fullerene OSC based on P3HT:PC61BM (2.2%) with similar thickness and transparency. Furthermore, we test the mechanical stability of the non-fullerene and fullerene all-plastic OSCs, both systems show excellent bending stability, while the non-fullerene device is slightly better. Acknowledgements We thank the 973 Program (2013CB834702) and NSFC (91433114) for financial support. References
Fig. 6. Mechanical testing (PCE vs. bending cycles) of all-plastic OSCs based on P3HT: IDT-2BR (70 nm) and P3HT:PC61BM (70 nm).
4. Conclusions We fabricate a novel semitransparent all-plastic OSC based on a non-fullerene system of P3HT:IDT-2BR. The solar cell shows AVT of ca. 50%, which meets the requirement of the transmittance of
[1] J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Efficient organic solar cells processed from hydrocarbon solvents, Nat. Energy 1 (2016) 15027. [2] C.C. Chen, W.H. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, An efficient triple-junction polymer solar cell having a power conversion efficiency exceeding 11%, Adv. Mater. 26 (2014) 5670e5677. [3] A.R.B.M. Yusoff, D. Kim, H.P. Kim, F.K. Shneider, W.J. da Silva, J. Jang, Energy Environ. Sci. 8 (2015) 303e316. [4] J.C. Aguirre, S.A. Hawks, A.S. Ferreira, P. Yee, S. Subramaniyan, S.A. Jenekhe, S.H. Tolbert, B.J. Schwartz, A high efficiency solution processed polymer inverted triple-junction solar cell exhibiting a power conversion efficiency of 11.83%, Adv. Energy Mater. 5 (2015) 1402020. [5] Z. Liu, J. Li, Z.-H. Sun, G. Tai, S.-P. Lau, F. Yan, The application of highly doped single-layer graphene as the top electrodes of semitransparent organic solar
112
Y. Wang et al. / Polymer 107 (2016) 108e112
cells, ACS Nano 6 (2012) 810e818. [6] F. Pastorelli, P. Romero-Gomez, R. Betancur, A. Martinez-Otero, P. MantillaPerez, N. Bonod, J. Martorell, Adv. Energy Mater. 5 (2015) 1400614. [7] J. Krantz, T. Stubhan, M. Richter, S. Spallek, I. Litzov, G.J. Matt, E. Spiecker, C.J. Brabec, Enhanced light harvesting in semitransparent organic solar cells using an optical metal cavity configuration, Adv. Funct. Mater. 23 (2013) 1711e1717. [8] F. Guo, S. Chen, Z. Chen, H. Luo, Y. Gao, T. Przybilla, E. Spiecker, A. Osvet, K. Forberich, C.J. Brabec, Printed smart photovoltaic window integrated with an energy-saving thermochromic layer, Adv. Opt. Mater. 3 (2015) 1524e1529. [9] J.-Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Semitransparent organic photovoltaic cells with laminated top electrode, Nano Lett. 10 (2010) 1276e1279. [10] C.-Y. Chang, L. Zuo, H.-L. Yip, C.-Z. Li, Y. Li, C.-S. Hsu, Y.-J. Cheng, H. Chen, A.K.Y. Jen, Highly efficient polymer tandem cells and semitransparent cells for solar energy, Adv. Energy Mater. 4 (2013) 1301645. [11] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photon. 6 (2012) 809e817. [12] J. Tong, S. Xiong, Z. Li, F. Jiang, L. Mao, W. Meng, Y. Zhou, Vacuum-free and metal electrode-free organic tandem solar cells, Appl. Phys. Lett. 106 (2015) 053306. [13] Z. Li, F. Qin, T. Liu, R. Ge, W. Meng, J. Tong, S. Xiong, Y. Zhou, Optical properties and conductivity of PEDOT: PSS films treated by polyethylenimine solution for organic solar cells, Org. Electron. 21 (2015) 144e148. [14] Y. Zhou, T.M. Khan, J.W. Shim, A. Dindar, C. Fuentes-Hernandez, B. Kippelen, All-plastic solar cells with a high photovoltaic dynamic range, J. Mater. Chem. A 2 (2014) 3492e3497. [15] L. Yin, Z. Zhao, F. Jiang, Z. Li, S. Xiong, Y. Zhou, PEDOT: PSS top electrode prepared by transfer lamination using plastic wrap as the transfer medium for organic solar cells, Org. Electron. 15 (2014) 2593e2598. [16] W. Zhang, B. Zhao, Z. He, X. Zhao, H. Wang, S. Yang, H. Wu, Y. Cao, High-efficiency ITO-free polymer solar cells using highly conductive PEDOT: PSS/ surfactant bilayer transparent anodes, Energy Environ. Sci. 6 (2013) 1956e1964. [17] M. Vosgueritchian, D.J. Lipomi, Z. Bao, Highly conductive and transparent PEDOT: PSS films with a fluorosurfactant for stretchable and flexible transparent electrodes, Adv. Funct. Mater. 22 (2012) 421e428. [18] G. Zhao, W. Wang, T.-S. Bae, S.-G. Lee, C. Mun, S. Lee, H. Yu, G.-H. Lee, M. Song, J. Yun, Stable ultrathin partially oxidized copper film electrode for highly efficient flexible solar cells, Nat. Commun. 6 (2015) 8830. [19] H. Kang, S. Jung, S. Jeong, G. Kim, K. Lee, Polymer-metal hybrid transparent electrodes for flexible electronics, Nat. Commun. 6 (2015) 6503. [20] J. Zou, C.-Z. Li, C.-Y. Chang, H.-L. Yip, A.K.Y. Jen, Interfacial engineering of ultrathin metal film transparent electrode for flexible organic photovoltaic cells, Adv. Mater. 26 (2014) 3618e3623. [21] J. Huang, C.-Z. Li, C.-C. Chueh, S.-Q. Liu, J.-S. Yu, A.K.Y. Jen, 10.4% Power Conversion Efficiency of ITO-Free organic photovoltaics through enhanced light trapping configuration, Adv. Energy Mater. 5 (2015) 1500406. [22] K. Zilberberg, F. Gasse, R. Pagui, A. Polywka, A. Behrendt, S. Trost, €rrn, T. Riedl, Highly robust indium-free transparent R. Heiderhoff, P. Go conductive electrodes based on composites of silver nanowires and conductive metal oxides, Adv. Funct. Mater. 24 (2014) 1671e1678. [23] J.H. Yim, S.-Y. Joe, C. Pang, K.M. Lee, H. Jeong, J.-Y. Park, Y.H. Ahn, J.C. de Mello, S. Lee, Fully solution-processed semitransparent organic solar cells with a silver nanowire cathode and a conducting polymer anode, ACS Nano 8 (2014) 2857e2863. [24] J. Song, J. Li, J. Xu, H. Zeng, Superstable transparent conductive Cu@Cu4Ni nanowire elastomer composites against oxidation, bending, stretching, and twisting for flexible and stretchable optoelectronics, Nano Lett. 14 (2014) 6298e6305. [25] S. Han, S. Hong, J. Ham, J. Yeo, J. Lee, B. Kang, P. Lee, J. Kwon, S.S. Lee, M.Y. Yang, S.H. Ko, Fast plasmonic laser nanowelding for a Cu-nanowire percolation network for flexible transparent conductors and stretchable electronics, Adv. Mater. 26 (2014) 5808e5814. [26] H. Wu, D. Kong, Z. Ruan, P.-C. Hsu, S. Wang, Z. Yu, T.J. Carney, L. Hu, S. Fan, Y. Cui, A transparent electrode based on a metal nanotrough network, Nat. Nano 8 (2013) 421e425. [27] R.V. Salvatierra, C.E. Cava, L.S. Roman, A.J.G. Zarbin, ITO-free and flexible organic photovoltaic device based on high transparent and conductive polyaniline/carbon nanotube thin films, Adv. Funct. Mater. 23 (2013) 1490e1499.
[28] X. Wang, G. Shi, Flexible graphene devices related to energy conversion and storage, Energy Environ. Sci. 8 (2015) 790e823. [29] K. Parvez, R. Li, S.R. Puniredd, Y. Hernandez, F. Hinkel, S. Wang, X. Feng, K. Müllen, Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes for organic electronics, ACS Nano 7 (2013) 3598e3606. [30] Z. Liu, J. Li, F. Yan, Package-free flexible organic solar cells with graphene top electrodes, Adv. Mater. 25 (2013) 4296e4301. [31] Z. Li, G. Ma, R. Ge, F. Qin, X. Dong, W. Meng, T. Liu, J. Tong, F. Jiang, Y. Zhou, K. Li, X. Min, K. Huo, Y. Zhou, Free-standing conducting polymer films for high-performance energy devices, Angew. Chem. Int. Ed. 55 (2016) 979e982. [32] F. Jiang, T. Liu, S. Zeng, Q. Zhao, X. Min, Z. Li, J. Tong, W. Meng, S. Xiong, Y. Zhou, Metal electrode-free perovskite solar cells with transfer-laminated conducting polymer electrode, Opt. Express 23 (2015) A83eA91. [33] X. Zhan, Z.A. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, A high-mobility electron-transport polymer with broad absorption and its use in field-effect transistors and all-polymer solar cells, J. Am. Chem. Soc. 129 (2007) 7246e7247. [34] Y. Lin, X. Zhan, Non-fullerene acceptors for organic photovoltaics: an emerging horizon, Mater. Horiz. 1 (2014) 470e488. [35] Y. Lin, X. Zhan, Oligomer molecules for efficient organic photovoltaics, Acc. Chem. Res. 49 (2016) 175e183. [36] Y. Lin, X. Zhan, Designing efficient non-fullerene acceptors by tailoring extended fused-rings with electron-deficient groups, Adv. Energy Mater. 5 (2015) 1501063. [37] L. Gao, Z.-G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei, Y. Li, All-polymer solar cells based on absorption-complementary polymer donor and acceptor with high power conversion efficiency of 8.27%, Adv. Mater. 28 (2016) 1884e1890. [38] Y. Lin, Q. He, F. Zhao, L. Huo, J. Mai, X. Lu, C.-J. Su, T. Li, J. Wang, J. Zhu, Y. Sun, C. Wang, X. Zhan, A facile planar fused-ring electron acceptor for As-Cast polymer solar cells with 8.71% efficiency, J. Am. Chem. Soc. 138 (2016) 2973e2976. [39] D. Meng, D. Sun, C. Zhong, T. Liu, B. Fan, L. Huo, Y. Li, W. Jiang, H. Choi, T. Kim, J.Y. Kim, Y. Sun, Z. Wang, A.J. Heeger, High-performance solution-processed non-fullerene organic solar cells based on selenophene-containing perylene bisimide acceptor, J. Am. Chem. Soc. 138 (2016) 375e380. [40] Y. Zhong, M.T. Trinh, R. Chen, G.E. Purdum, P.P. Khlyabich, M. Sezen, S. Oh, H. Zhu, B. Fowler, B. Zhang, W. Wang, C.-Y. Nam, M.Y. Sfeir, C.T. Black, M.L. Steigerwald, Y.-L. Loo, F. Ng, X.Y. Zhu, C. Nuckolls, Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells, Nat. Commun. 6 (2015) 8242. [41] Y. Lin, J. Wang, Z.-G. Zhang, H. Bai, Y. Li, D. Zhu, X. Zhan, An electron acceptor challenging fullerenes for efficient polymer solar cells, Adv. Mater. 27 (2015) 1170e1174. [42] Y. Lin, Z.-G. Zhang, H. Bai, J. Wang, Y. Yao, Y. Li, D. Zhu, X. Zhan, High-performance fullerene-free polymer solar cells with 6.31% efficiency, Energy Environ. Sci. 8 (2015) 610e616. [43] H. Bin, Z.-G. Zhang, L. Gao, S. Chen, L. Zhong, L. Xue, C. Yang, Y. Li, Nonfullerene polymer solar cells based on alkylthio and fluorine substituted 2DConjugated polymers reach 9.5% efficiency, J. Am. Chem. Soc. 138 (2016) 4657e4664. [44] Y. Lin, F. Zhao, Q. He, L. Huo, Y. Wu, T.C. Parker, W. Ma, Y. Sun, C. Wang, D. Zhu, A.J. Heeger, S.R. Marder, X. Zhan, High-performance electron acceptor with thienyl side chains for organic photovoltaics, J. Am. Chem. Soc. 138 (2016) 4955e4961. €s, F. Gao, J. Hou, Fullerene-free [45] W. Zhao, D. Qian, S. Zhang, S. Li, O. Ingana polymer solar cells with over 11% efficiency and excellent thermal stability, Adv. Mater. 28 (2016) 4734e4739. [46] Y. Lin, T. Li, F. Zhao, L. Han, Z. Wang, Y. Wu, Q. He, J. Wang, L. Huo, Y. Sun, C. Wang, W. Ma, X. Zhan, Structure evolution of oligomer fused-ring electron acceptors toward high efficiency of as-cast polymer solar cells, Adv. Energy Mater. 6 (2016), 1600854. [47] Y. Wu, H. Bai, Z. Wang, P. Cheng, S. Zhu, Y. Wang, W. Ma, X. Zhan, A planar electron acceptor for efficient polymer solar cells, Energy Environ. Sci. 8 (2015) 3215e3221. [48] N. Drolet, Organic Photovoltaic: Efficiency and Lifetime Challenges for Commercial Viability, 2012 MRS Spring Meeting & Exhibit, Moscone West Convention Center, Marriott Marquis, San Francisco, CA, 2012.