High-Performance Polymer Solar Cells Employing Rhodamines as

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High Performance Polymer Solar Cells Employing Rhodamines as Cathode Interface Layers Wang Li, Zhiyang Liu, Rongjuan Yang, Qian Guan, Weigang Jiang, Amjad Islam, Tao Lei, Ling Hong, Ruixiang Peng, and Ziyi Ge ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07855 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Applied Materials & Interfaces

High Performance Polymer Solar Cells Employing Rhodamines as Cathode Interface Layers Wang Li†‡, Zhiyang Liu†‡, Rongjuan Yang†, Qian Guan†‡, Weigang Jiang†‡, Amjad Islam†‡, Tao Lei†‡, Ling Hong†‡, Ruixiang Peng†, Ziyi Ge*† †Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, P.R. China. E-mail: [email protected] ‡University of Chinese Academy of Sciences, Beijing, 100049, P.R.China Highlights: 1.Two eco-friendly and low-cost rhodamine dyes (named BRB and RB) were used as cathode interface layers. 2.BRB-based device realized an excellent power conversion efficiency of 10.39% which is 42.3% higher than that of Ca-based device (7.30%). 3.BRB and RB significantly improved the current density and fill factor of the devices.

Keywords: cathode interface layers, polymer solar cells, solution processed, conjugated zwitterions, rhodamines Abstract：： The development of simple and water/alcohol soluble interfacial materials is crucial for the cost-effective fabrication process of polymer solar cells (PSCs). Herein, highly efficient PSCs are reported employing water/alcohol soluble and low-cost rhodamines as cathode interfacial layers (CILs). The results reveal that rhodamine-based CILs can reduce the work function (WF) of Al cathode and simultaneously increase the open-circuit voltage (Voc), current density (Jsc), fill factor (FF) and the power conversion efficiency (PCE) of PSCs. The solution processed rhodamine-based PSCs demonstrated a remarkable PCE of 10.39%, which is one of the best efficiencies reported for PTB7:PC71BM-based PSCs so far. The efficiency is also 42.3% higher than the vacuum deposited Ca-based device (PCE of 7.30%), and 21.5% higher than the complicated solution-processable polymeric electrolyte PFN-based device (PCE of 8.55%). Notably, rhodamines are very economical and have been extensively used as 1 ACS Paragon Plus Environment


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dyes in industry. Our work indicates that rhodamines have shown a strong potential as CILs compared to their counterparts in large-area fabrication process of PSCs.

1. Introduction Solar cells are electronic devices in which conversion of sun-light directly into electricity takes place and are considered as a very promising natural source of energy. Polymer solar cells (PSCs) have multiple benefits, such as low-cost solution processability and tunability of optoelectronic properties on mechanically flexible substrates.1 During the past years, bulk heterojunction PSCs based on a combination of conjugated polymers as donors (D) with acceptors (A) (fullerene/non-fullerene derivatives) have made a great progress, a PCE

of

over 10% have already been achieved,2-11 among which most devices contained vacuum deposited interfacial layers such as MoO3 and LiF,12-14 limiting their commercialization with low cost and large area. The routes to upgrade the device performance include synthesis of efficient materials, such as photoactive layers,15,16 charge-transport layers,17-19 morphology as well as light control.20 -22 Optimization of interfacial layers is an efficient way to improve PCE of PSCs.23 Cathode interface layers (CILs) with the low WFs could adjust the WF of Al cathode to match the lowest unoccupied molecular orbital (LUMO) energy levels of acceptors, which is beneficial to enhance the extraction and transportation of charges at the interface in the PSCs.24 Moreover, an exemplary interface demands good Ohmic contact with the least-possible resistance and high charge separation to avoid carriers from meeting the opposite electrodes.25 For PSCs, Ca is commonly employed as a buffer layer to tune the WF of Al cathode but that is rapidly oxidized in air, resulting in low device performance of PSCs. Therefore, many attempts have been made to apply several CILs in PSCs to replace Ca, that include semiconducting metal oxides,26-29 metal salts, metal complexes,30,31 carbon-based materials,24 2 ACS Paragon Plus Environment

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hybrids and composites,32,33.34 Most of them work well in optimizing the device performance, but they usually need high thermal treatment (over 150℃) or vacuum deposition,26-31 which obstruct the application of large-scale roll-to-roll fabrication. To overcome these obstacles, water/alcohol soluble polymer/small-molecule interfacial layers35-42 have gained a lot of attention due to their solution processable application at low temperature. For example, poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN) as a CIL has increased the PCE from 5.0% to 8.37% for PSCs based on polymer thieno[3,4b]thiophene/benzodithiophene (PTB7) active layer,39 and non-conjugated small molecule 4,4(((methyl(4-sulphonatobutyl)ammonio)bis(propane-3,1-diyl))bis(dimethyl-ammoniumdiyl)) bis-(butane-1-sulphonate)(MASPBS) as a CIL of PSCs based on PTB7 active layer has increased the PCE from 8% to 10%.5 Nevertheless, the pursuit of simple existing interfacial materials is of more significance to reduce the fabrication cost of PSCs, compared to other complicated ones demanding difficult synthesis. Rhodamine-dye materials are potential candidate to be used as CILs because of their conjugated zwitterionic structure and good water/alcohol solubility as well as very low cost, although rhodamines were still barely used as CILs. In this contribution, we reported the application of two rhodamines (shown in Figure 1a) as CILs of PSCs and showed excellent photovoltaic performance of the devices. They are conjugated zwitterions with positive and negative charges and have proper energy level structure which is suitable to be used as a modified layer for the Al cathode. The device architectures were illustrated in Figure 1c, a combination of the low-band-gap semiconducting PTB7 and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) was utilized as the photoactive material. All the devices adopted rhodamines as CILs illustrated notable improvements in photovoltaic properties. The best performance of the device was based on BRB with a PCE of 10.39%, which is 42.3% higher than its counterpart (Ca-based device, 7.30%) and 21.5% higher than that of the device using PFN as CIL (8.55%). Intriguingly, the 3 ACS Paragon Plus Environment


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efficiency is also one of the best PCEs reported for PTB7:PC71BM-based PSCs. Moreover, rhodamines are commercially available with a low price, which is beneficial to reduce the cost and promote the commercialization of PSCs. Our discovery provides a new avenue to develop low-cost interfacial materials at large scale for efficient polymer solar cells.

Figure 1. a) Structures of BRB, RB, PTB7 and PC71BM. b) Energy level diagram of the device. c) Schematic diagram of the structure of PTB7:PC71BM PSCs.

2. Experimental Section 2.1 Materials and Methods: Indium-doped tin oxide (ITO) glass sheet (1.2 mm thick, ≤15 Ω/square, transmittance>90%) was bought from Nippon Sheet Glass Company, Ltd. PEDOT:PSS (Baytron P VP Al 4083) was bought from the Baytron Company. Butyl Rhodamine B (BRB) was purchased from Aladdin. Rhodamine B (RB,>99%) was purchased from Germany DR. PFN, PTB7 and PC71BM were respectively purchased from 1-Material. 1,8-diiodooctane (DIO) was received from Acros. All of them were used as received, a 4 ACS Paragon Plus Environment

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solution of PTB7:PC71BM with a weight ratio of 1: 1.5 (total concentration of 25 mg ml-1) in a chlorobenzene/1,8-diiodoctane (97 : 3 vol%) was prepared and stirred at 60 ℃ overnight. PFN was dissolved in methanol/acetic acid (95:5 vol%) with a concentration of 1 mg ml-1. 2.2 Device fabrication: The conventional device configuration was ITO/PEDOT: PSS/PTB7: PC71BM/interface layer/Al. ITO was washed by consecutive sonication in soap deionized water, deionized water, acetone, and isopropanol for 40 min at each step, followed by drying in a nitrogen stream and then an UV-ozone treatment of 30 min. A PEDOT:PSS film (35 nm) was spin-coated onto the ITO substrate after filtration by a 0.45 µm filter, and annealed at 100 ℃ for 30 min. Then the PTB7:PC71BM mixed solvent was deposited through spin-coating at 2000 rpm for 2 min to get a 110 nm thick film, then placed the devices in vacuum for 1 hour. The two rhodamines were dissolved in a solution of methanol/water (96:4 vol%) respectively and deposited through spin-coating at 4000 rpm for one minute. The thickness of cathode interface layer was optimized through the concentration of the solution. A Ca (20 nm)/Al (100 nm) or Al (100 nm) electrode was evaporated through a shadow mask to get 4 mm2 active area of the devices. 2.3 Characterization and measurement: Cyclic voltammetry (CV) measurements were performed in acetonitrile with 0.1 M tetra-n-buty-lammoniumhexafluoro-phosphate (nBu4NPF6) as the supporting electrolyte using a scan rate of 100 mV s-1, the working, counter and reference electrode was glass carbon, platinum wire and Ag/Ag+, respectively. Kratos ULTRADLD UPS/XPS system was used to measure ultraviolet photoelectron spectroscopy (UPS). Perkin-Elmer Lambda 950 spectrophotometer was used to record the absorption and reflex spectra. The current-voltage (J-V) characteristics of the PSCs without encapsulation were measured in a N2 -filled glove box using an AM 1.5 G solar simulator (Newport-Oriel® Sol3A 450W) and a Keithley 2440 source-measure unit. The intensity of the simulated solar light was standardized by a standard Si photodiode detector which was calibrated at the National Renewable Energy Laboratory (NREL). A Keithley 2440 Source Measure Unit was 5 ACS Paragon Plus Environment


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used to recorded the device parameters. The data of external quantum efficiency (EQE) were obtained using a Newport-Oriel® IQE 200™. The thickness of the films were measured by Dektak 150. The surface images and roughness were calculated on an atomic force microscope (Veeco Dimension 3100V) and scanning electron microscope (Hitachi S-4800).

3. Results and Discussion

3.1 Electrical and Optical properties The difference of two rhodamines with the same conjugated structure is their benzene ring connecting with different functional groups. BRB contains butyl ester group while RB contains carboxyl group. They are readily soluble in polar solvents, like water and methanol, which is feasible to employ solution-processable method. To investigate the absorption properties, these dyes were placed on quartz glass through spin-coating and measured their UV-vis spectra. The characteristics of absorption are shown in Figure S1. They exhibit similar absorption range and similar absorption curve, but BRB shows a slightly blue shift in comparison with RB. The absorption spectra of PTB7:PC71BM layers with BRB or RB layer were shown in Figure S2. The result indicated that the BRB and RB slightly influence the absorption spectra of active layer. Through Cyclic voltammetry (CV) measurements (Figure 2a), the frontier molecular orbitals of the molecules were determined. The HOMO energy levels were calculated using the equation HOMO=− (Eox,onset + 4.71) eV, where Eox,onset is the onset potential of the first oxidation peaks versus Ag/Ag+.[38] Eg=hc/λabs onset. λabs onset is the wavelength of the onset absorption from infrared to ultraviolet. The electrochemical data of the two rhodamines are displayed in Table 1. The LUMO of DRB and RB are 3.54 eV, 3.42 eV, respectively, which are higher than the LUMO of PC71BM. The change of the WF of Al was tested by UPS and the results are shown in Figure 2b. It shows that two rhodamines could effectively adjust the WFs of Al cathode and match the 6 ACS Paragon Plus Environment

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LUMO energy level of PC71BM, which is conducive to transfer and collect electron between active layer and cathode. BRB caused the biggest change in WF of Al with a ∆E of 0.44 eV, and the change of WF of Al modified by RB was 0.29 eV, due to the strong dipole moment in rhodamines. The strong dipole moment facilitates the electron collection by twisting the vacuum level of the metal up compared to the active layer.43

(b)

(a) 1.4 BRB RB

1.2

Al RB/Al BRB/Al

Intensity / a.u.

1.0

Current / mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0.0 -0.2 0.0

0.5

1.0

1.5

Potential vs. Ag/Ag+ / V

2.0

18.0

17.8

17.6

17.4

17.2

17.0

16.8

16.6

Binding energy / eV

Figure 2. a) CVs for BRB, RB. b) UPS spectra for Al with various treatment.

Table 1. Photophysical and electrochemical data of BRB and RB. Eox,onset a) [V](vs. Ag/Ag+)

CIL

λabs onset b) [nm]

Energy [eV] (vs. vacuum) HOMO

LUMO

Eg

BRB

+0.86

610

5.57

3.54

2.03

RB

+0.68

628

5.39

3.42

1.97

a)

CV measurements are performed in acetonitrile containing 0.1 M Bu4NPF6 at a scan rate of 10 mV/S. The potential values are quoted with respect to the Ag/Ag+ QRE. b) The onset absorption wavelength of absorption spectra of the solid-state films.

Lower WF of Al electrode makes it easier to transport and collect electron. Previous reports proved that polar molecules could improve energy level alignment and device performance by provide interfacial dipole on the conducting surface.44,45 3.2 Photovoltaic performances Devices were fabricated to investigate the effects of the two rhodamine CILs, the configuration

of

devices

were

ITO/PEDOT:PSS/PTB7:PC71BM/Rhodamine/Al.

In

comparison, devices with Ca and Al as electrode were also fabricated. Treatment of the active layer with methanol can passivate the surface traps and hence increases the surface charge 7 ACS Paragon Plus Environment


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density of active layer.46,47 Methanol was used as solvent to dissolve the rhodamines, the contrasting devices were fabricated with methanol treatment as well as without methanol treatment. Figure S3a-b show the J–V characteristics of the devices respectively based on BRB and RB with different solution concentration. The best concentration of BRB and RB was 0.25 mg ml-1 and 0.5 mg ml-1. Figure 3a presents J–V characteristics of the PSCs based on different CILs. As showed in Table S1 and Table S2, The photovoltaic performance data of the devices were summarized in Table 2. For the BRB-based devices, the highest PCE of 10.39% is obtained with a Voc of 0.76 V, a Jsc of 18.01 mA cm-2 and a high fill factor (FF) of 75.79%. The PCE of BRB-based device is 42.3% higher than that of the Ca-based device (PCE 7.3%) and 21.5% higher than the PFN-based device (PCE 8.55%). Compared with the Ca-based devices, the Voc was slightly increased from 0.74 V to 0.76 V. Jsc was significantly increased from 15.14 mA cm-2 to 18.02 mA cm-2. It indicates that the devices based on BRB have excellent effect on electron transfer and collection. Notably, the FF was dramatically enhanced from 70.78% to 75.79%, which was seldom observed from PSCs based on the pure PTB7:PC71BM blend active layer. RB also exhibited good performance in optimizing the device resulting in Voc of 0.76 V, Jsc of 16.91 mA cm-2, FF of 72.76 and a final PCE of 9.37%. The EQE test curve was shown in Figure 3e, the highest EQE reached 80% at 625 nm of BRB-based devices. The calculated Jsc of the devices was listed in Table S3. Overall, two rhodamines simultaneously improve the Voc, Jsc, FF and PCE of the devices.

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(a)5

(b) Ca MeOH/Ca PFN BRB RB

0.01 0 1E-3

MeOH/Ca PFN

-5

Current / A

J / mA cm-2

Ca

BRB RB

-10

-15

1E-4 1E-5 1E-6 1E-7

-20 0.0

0.2

0.4

0.6

0.8

1E-8 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Voltage / V

Voltage / V

(d) 103

104

J / mA cm2

J / mA cm2

(c) Ca MeOH/Ca PFN SRB RB

103

10

Ca MeOH/Ca PFN BRB RB

2

101

10

2

100 0

1

2

3

4

0

1

Voltage / V

2

Voltage / V

3

4

(e) 80

60

EQE / %

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Ca MeOH/Ca PFN BRB RB

40

20

0 300

400

500

600

700

800

Wavelength / nm

Figure 3. J–V curves for various interfacial treatments as indicated: a) under AM 1.5G 100 mW cm−2, b) under dark, c) electron-only devices and d) hole-only devices. e)EQE spectra of devices based on various CILs.

The enhancement in the device performance of PSCs by employing rhodamines as CILs can be presumed in terms of the series resistance (Rs) and shunt resistance (Rsh) of the devices43. Lower Rs means better Ohmic contact between the photoactive layer and Al electrode, and higher Rsh implies better suppression of leakage current of the PSCs. As shown in Table 2, there is a positive correlation between the value of Rsh/Rs and the PCE of devices. 9 ACS Paragon Plus Environment


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For Ca/Al cathode devices, the Rs was decreased from 7.19 Ω cm2 to 5.40 Ω cm2 while Rsh was increased from 594 Ω cm2 to 0.98 kΩ cm2 after methanol treatment, which matches well with the enhancement of photovoltaic performance of the devices after methanol treatment. The Rs of the BRB device was 3.21 Ω cm2 and its Rsh reached a high value of 1.43 kΩ cm2. Such a lowest Rs and a highest Rsh of BRB-CIL devices indicated that the BRB provided excellent Ohmic contact between photoactive layer and Al electrode, and effectively decreased leakage current of devices at the same time. The RB-CIL devices also presented relative lower Rs and relative higher Rsh compared with Ca/Al devices. These results implied that rhodamines CILs can efficiently tune the Rs and Rsh of the PSCs. Table 2. Photovoltaic results of PSC devices with different CILs under the illumination of AM 1.5G 100 mW cm−2. All the photovoltaic parameters were averaged from more than 30 samples. FF [%]

PCEmax [%]

PCEavg [%]

Rs [Ω cm2]

Rsh [kΩ cm2]

0.74

Jsc (Javg sc) [mA cm-2] 15.14 (14.94)

65.08 (63.08)

7.30

7.09 ± 0.24

7.19

0.59

MeOH/Ca

0.75

15.31 (15.16)

70.78 (70.55)

8.09

8.06 ± 0.03

5.40

0.98

PFN

0.75

16.01 (15.83)

71.56 (71.18)

8.55

8.47 ± 0.09

3.86

1.09

BRBa)

0.76

18.02 (17.57)

75.79 (75.47)

10.39

9.96 ± 0.41

3.21

1.43

0.76

16.91 (16.71)

72.76 (72.96)

9.37

9.24 ± 0.12

3.50

1.18

CIL

Voc [V]

Ca

RB

b)

Rhodamines are dissolved in methanol (4% water) to get solutions with concentration of RB 0.5 mg/ml.

a)

BRB 0.25 mg/ml. b)

Rhodamines have pretty good effect in the application as CIL of PSCs. The two rhodamines have similar structure except for the little difference in the side chain at the benzene, in which BRB contains butyl ester group while RB possesses carboxyl group. They have similar electrical and optical properties but they exhibited large difference in improving the devices performance. According to the UPS tests and J-V measurements, the parameters of these devices are highly consistent with the WF of the Al. The WF was decreased by 0.44 eV for BRB-modified Al, which is more suitable to transfer electron from active layer. As for RB sample, the WF of Al was decreased by 0.29 eV. Obviously, BRB modifies the Al more effective than RB, resulting in better photovoltaic performance with PCE up to 10.39%.


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The charge carrier mobilities were calculated through the space charge limited current (SCLC) model and tested in the dark to study the charge transport properties and the J–V characteristics are shown in Figure 3c-d. The electron-only devices with the configuration of ITO/Al/PTB7:PC71BM/CIL/Al were fabricated to obtain electron mobility. Electron mobility was measured by fitting to the Mott-Gurney law. J=9εrε 0µV2/8L3.48 As listed in Table S4, the electron mobility was increased from 2.78×10-4 cm2v-1s-1 for Ca/Al devices to 10.6×10-4 cm2 V-1 s-1 for BRB-CIL devices. As for the RB-CIL device, the electron mobility is 8.9×10-4 cm2 V-1 s-1. Hole-only devices were constructed with the structure of ITO/PEDOT:PSS/PTB7: PC71BM /CIL/MoO3/Ag. The values of hole mobility were also improved from 3.60×10-5 cm2 V-1 s-1 of pristine film to 9.18×10-5 cm2 V-1 s-1 of BRB-CIL devices. when BRB and RB were employed as CILs, both the electron and hole mobility were increased, especially for BRBCIL devices, which agrees well with the increase trend of the photovoltaic parameters as listed in Table 2. High carrier mobility means less time of the carrier reached the electrode, which decreases the probability of charge recombination and increases the charge collection efficiency46. The surface morphology of PTB7:PC71BM blend film treated with several CILs was investigated through AFM as shown in Figure 4. All the samples were prepared with the same procedure before CILs deposition. The pristine PTB7:PC71BM blend film exhibited a flat surface with the root mean square roughness (RMS) of 1.91nm. After methanol treatment, RMS was 1.99 nm which was similar to pristine one. After spin-coating CILs, it was decreased to 1.62 nm and 1.79 nm for BRB and RB respectively. Surface of active film became smoother after deposition of rhodamines CILs, illustrating pretty good film-forming property for the two rhodamines, which was also demonstrated by the SEM images as shown in Figure S4.



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Figure 4. 5×5 µm2 AFM images of the PTB7:PC71BM film with various interfacial treatments: a) without treatment, b) methanol treatment, c) BRB, d) RB.

4. Conclusion In conclusion, we have presented two commercially available conjugated zwitterion of rhodamines which can be employed as cathode interface layer by solution processing for PSCs and gained high improvement in photovoltaic performance. They offered good Ohmic contact between photoactive layer and cathode and modified WF of Al effectively. Meanwhile, they increased charge carrier mobility and improved charge-collection efficiency. Voc, Jsc, FF and the PCE of the devices were simultaneously improved by employing rhodamines as CILs. The PCE of devices based on each rhodamine/Al cathode were 10.39% and 9.37% for devices based on BRB and RB CILs, respectively. The best device using BRB as CIL exhibited a high PCE of 10.39%, illustrating a remarkable improvement compared with Ca-based or PFN-based devices, which is one of the best PCEs reported for PTB7:PC71BM-based PSCs. Our work offers more effective alternatives of cheap commercial rhodamines as CILs for efficient PSCs with potential of cost effective and large area fabrication, instead of complex synthesized interfacial materials with high-cost. 12 ACS Paragon Plus Environment

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Associated content *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: BRB and RB UV-vis absorption curves (Figure S1), Active layer UV-vis absorption curves (Figure S2), J-V characteristics of the devices with various rhodamines concentration (Figure S3), SEM images (Figure S4), Photovoltaic parameters of BRB-based devices (Table S1), Photovoltaic parameters of RB-based devices (Table S2), Calculated Jsc from EQE (Table S3), carrier mobilities (Table S4) Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21574144 and 21674123), National Key Research and Development Plan (2016YFB0401000), Zhejiang Provincial Natural Science Foundation of China (LR16B040002), Ningbo Municipal Science and Technology Innovative Research Team (2015B11002 and 2016B10005), CAS Interdisciplinary Innovation Team, CAS Key Project of Frontier Science Research (QYZDB-SSW-SYS030) and CAS Key Project of International Cooperation (174433KYSB20160065). References [1]

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High-Performance Polymer Solar Cells Employing Rhodamines as

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