Semitransparent flexible organic solar cells employing doped

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Semitransparent flexible organic solar cells employing doped-graphene layers as anode and cathode electrodes Dong Hee Shin, Chan Wook Jang, Ha Seung Lee, Sang Woo Seo, and Suk-Ho Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16730 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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ACS Applied Materials & Interfaces

Semitransparent Flexible Organic Solar Cells Employing Doped-Graphene Layers as Anode and Cathode Electrodes Dong Hee Shin†, Chan Wook Jang†, Ha Seung Lee, Sang Woo Seo, and Suk-Ho Choi* Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin 17104, Korea

ABSTRACT Semi-transparent flexible photovoltaic cells are advantageous for effective use of solar energy in many areas such as building-integrated solar-power generation and portable photovoltaic chargers. We report semitransparent and flexible organic solar cells (FOSCs) with high aperture, composed of doped graphene layers, ZnO, P3HT:PCBM, and PEDOT:PSS as anode/cathode transparent conductive electrodes (TCEs), electron transport layer, photoactive layer, and hole transport layer, respectively, fabricated based on simple solution processing. The FOSCs do not only harvest solar energy from ultraviolet-visible region but are also less sensitive to near-infrared photons, indicating semi-transparency. For the anode/cathode TCEs, graphene is doped with bis(trifluoromethanesulfonyl)-amide or triethylene tetramine, respectively. Power conversion efficiency (PCE) of 3.12% is obtained from the fundamental FOSC structure, and the PCE is further enhanced to 4.23% by adding an Al reflective mirror on the top or bottom side of the FOSCs. The FOSCs also exhibit remarkable mechanical flexibilities through bending tests for various curvature radii.

Keywords: graphene, organic solar cell, flexibility, doping, semitransparent, reflective mirror †

Two authors have contributed equally to this study. Corresponding author: [email protected]

*

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1. Introduction

Use of electronics has become widespread for the past decades and will continue to expand indefinitely in the future. As electronic devices become more miniaturized and energy-efficient, wireless, foldable, or even wearable electronics/photonics will soon be available. For the realization of these expectations, integrated photovoltaic chargers should be provided as a practical solution to the energy supply. In particular, it is highly desirable to integrate a photovoltaic cell directly onto or near the surface of the device, which requires the cell to be visually unobstructed for easy manufacture or adhesion onto any substrate. For meeting the requirement, it is inevitable to acquire materials having excellent electrical, optical, and mechanical properties suitable for transparent, light-weight, and flexible substrates/electrodes/active layers/charge transport films/dopants. Flexible photovoltaic cells can be applied to building integrated photovoltaics, automotive integrated photovoltaics, portable and indoor electronics. In addition, curved cells produced higher total electrical energy per surface area than flat cells.1 Currently, various organic donor and acceptor compounds with different absorption bands are used to optimize the transmittance of an optical device in visible region.2-7 Flexible organic solar cells (FOSCs) typically require small amount of active material, coupled with suitable substrates and electrodes, to provide an opportunity for realizing low-cost flexible and transparent energy harvesting.8-10 To date, most organic solar cells (OSCs) consist of an organic layer sandwiched between an indium tin oxide (ITO) anode and a metal cathode and electron/hole transport layers (ETLs/HTLs).11-19 Recently, many studies have been done for increasing the absorption of light transmitted through the ITO-based OSCs.12-19, thereby enhancing their efficiency. Since ITO and metal electrodes are both disadvantageous in view of light transmittance, rigidity, and flexibility, they are not suitable for semitransparent


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FOSCs applications. Therefore, ITO is generally used as a bottom electrode whilst metal nanowire, conductive polymer, metal thin film, and graphene (GR) are used as top electrodes for semitransparent FOSCs. However, the transmittances and aperture ratios of the top electrodes are still far from the practical standards.20,21 Among them, GR has recently attracted much attention as a next-generation flexible electrode due to its high transparency and elasticity.22,23 Chemical-vapor-deposition (CVD)-grown GR has been widely employed for various photoelectric devices so far because it can be prepared in large area.24 If GR is used for both-side electrodes of FOSCs, they may be produced to be transparent and flexible with high aperture ratio. However, it is not still easy to obtain high-performance GR transparent conductive electrodes (TCEs) because the sheet resistance (Rs) of pristine GR (PG) is much larger than ITO. Approaches to reduce Rs of GR while maintaining its excellent transmittance should be developed for the actual use of GR TCEs in optoelectronic devices.2527

Multi-layer or doped GR TCEs have been frequently employed for reducing sheet

resistance of graphene.28-35 Recently, the use of dopants such as bis(trifluoromethanesulfonyl)-amide (TFSA) or triethylene tetramine (TETA) has successfully produced p- or n-type GR TCEs with low resistance, high transparency, and long-term stability.32-35 Here, we explore a solution to overcome the aforementioned challenges for semitransparent FOSCs. High-performance semitransparent FOSCs are achieved using PG layers doped with TFSA and TETA as anode and cathode TCEs, respectively. In addition, we study how to further enhance the performance of the FOSCs by using an Al reflective mirror. Finally, we demonstrate high flexibility of the semitransparent FOSCs through bending tests for various curvature radii (R).

2. Experimental section


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Preparation of materials Single-layer PG layers of 2.5 x 2.5 cm2 size were transferred on polyethylene terephthalate (PET) substrates by conventional method after they were grown on Cu foils by CVD under flow of the gas mixture of CH4 and H2 at 1000 oC, as described in our previous work.36 TFSA powder/TETA molecules were dissolved in nitromethane/ethanol to prepare TFSA/TETA solution at a molar fraction of 20/0.2 mM, respectively. For doping, the solution was dropped on the whole surface of the GR sheet, and after 2 min elapsed, it was spincoated at 2500 rpm for 1 min. Fabrication of semitransparent FOSCs To prepare the FOSCs, PEDOT:PSS was spin-coated onto the TFSA-doped GR (TFSA/GR) sheet/PET at 2500 rpm for 60s to form PEPOT:PSS/TFSA/GR/PET stack, and subsequently annealed at 130 oC for 15 min in air. This procedure was repeated two times to make the PEDOT: PSS layer uniform. On the other hand, ZnO-nanoparticles layer was spincoated on the TETA-doped GR (TETA/GR) on PET at 3000 rpm for 30 s to form the ZnO/TETA/GR/PET stack. The ZnO nanoparticles were prepared according to literature procedures.37,38 This process was repeated three times to obtain continuously-smooth film. To prepare bulk-heterojunction (BHJ) P3HT:PCBM solution, 25-mg P3HT and 25-mg PCBM were mixed with 1-ml 1,2-dichlorobenzene, and the solution was then spin-coated on the ZnO/TETA/GR/PET stack at 700 rpm for 60 s in a nitrogen atmosphere, and then annealed two

times

at

120

°C

for

2

h

and

at

110

°C

for

10

min

to

obtain

P3HT:PCBM/ZnO/TETA/GR/PET stack. After the completion of the two separate stacks, they were coupled to fabricate the FOSCs. Finally, some polymer adhesives were attached throughout the corners of the jointed region to firmly combine the two parts. Characterization


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The transmittance, sheet resistance, and work function of GR were measured by ultraviolet-visible-near infrared (NIR) optical spectroscopy (Varian, Cary 5000), 4 probe van der Pauw method (Dasol eng, model FPP-HS8-40K), and Kevin probe force microscopy (Park systems, model XE100), respectively. The calibration of the work function was done by using gold as a reference. Raman spectra of the GR sheets were measured in a micro-Raman spectroscopy system with a 532-nm laser of ~1 mW as the excitation source. The surface morphologies of pristine and doped GR were analyzed by atomic probe force microscopy (AFM, Park systems). The atomic bonding states of the TFSA/GR and TETA/GR were characterized by X-ray photoelectron spectroscopy (XPS) using Al ka line of 1486.6 eV. Current density-voltage (J–V) characteristics of the solar cells were taken with a Keithley 2400 source meter under illumination of 1 Sun (100 mWcm-2 AM 1.5G) in air. External quantum efficiency (EQE) was measured under short circuit conditions while the cells were illuminated by a light source system with monochromator. The illumination area was fixed to 7 mm2. A commercially-available Al reflective mirror was used to enhance the reflection of the light.

3. Results and discussion

Figure 1a shows a schematic structure for a typical semitransparent FOSC employing PEDOT: PSS and PCBM as HTL and ETL, respectively. The photoactive layer of the FOSC is a BHJ blend consisting of P3HT and PCBM. Figure 1b shows transmittance spectra of ZnO, PEDOT:PSS, and P3HT:PCBM. The photoresponse of the BHJ film is very strong in the wavelength (λ) range of 300 to 650 nm, but is negligible at λ > ~650 nm. On the other hand, the transmittance becomes larger at λ > ~500 nm, and is maximized to be 84% at 700 nm. The transmittances of both HTL and ETL are also more than 85% at λ ≥ 700 nm. The spectral


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coverage of the BHJ film ensures harvesting photons in ultraviolet (UV)-visible region whilst most of the photons in NIR region are transmitted through the film, very useful for semitransparent FOSCs. For the FOSCs, the bottom and top electrodes should be also transparent, low-resistive, and flexible. GR TCEs possibly meet all of these requirements, but are limited in their use for high-performance FOSCs due to high sheet resistance of GR. Many studies have been done on doping of GR with various dopants to lower the sheet resistance, but the doping brought about problems such as reduction of transmittance and degradation of long-term stability. To relieve these problems, TFSA/GR and TETA/GR were employed as the top and bottom TCEs of the FOSCs, respectively in this work. Figure 1c compares transmittance spectra of PG, TFSA/GR, and TETA/GR TCEs on PET substrates. Here, the baseline was set to air. By doping with the two dopants, the transmittances slightly decrease in the visible region, but these properties are much better compared to the treatments with other dopants, as previously reported.39,40 The inset of Figure 1c shows the excellent transparency of the TFSA/GR and TETA/GR. Root-mean-square roughness was estimated to be ~0.58, ~0.86, and ~0.98 nm on PG, TFSA/GR, and TETA/GR, respectively by AFM height profiles (see Supporting Information, Figure S1). These results indicate smoother surfaces of TFSA/GR and TETA/GR compared to GR layers doped with other dopants,39,40 which explains why the TFSA/GR and TETA/GR exhibit high transmittances. The XPS peaks corresponding to the C 1s, N 1s, and O 1s core levels are evident for the doped GR layers (see Supporting Information, Figure S1), meaning successful doping of GR with TFSA or TETA dopant. Raman G/2D peaks of TFSA/GR and TETA/GR are blue-shifted by 16/9 and 9/7 cm-1, respectively, compared to those of PG (see Supporting Information, Figure S2), which can be attributed to the change of the electronic configurations of GR by doping with TFSA or TETA. These results further demonstrate successful formation of TFSA or TETA dopant in GR, consistent with previous results.32-35


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The sheet resistance (Rs) of GR doped with TFSA or TETA on PET is 185 or 220 Ω/sq, respectively, as shown in Figure 1d. It should be noted that the Rs is greatly reduced by doping (Rs = ~775 Ω/sq for PG). The work functions (WFs) of PG, TFSA/GR, and TETA/GR TCEs are 4.56 ± 0.04, 4.88 ± 0.02, and 4.49 ± 0.03, respectively, as shown in Figure 1d. In other words, the TFSA- or TETA- dopant tends to accept or donate charges from other materials, respectively, thereby making GR p- or n-type, compatible with anode or cathode TCE for OSCs, as reported before.33,34,41 Based on these results, we produced semitransparent FOSCs based on solution processes. Figure 2a shows a photograph of a typical highly-transparent FOSC, through which the

buildings

are

clearly

seen.

The

FOSC

consists

of

TFSA/GR/PEDOT:PSS/P3HT:PCBM/ZnO/TETA/GR, as shown in Figure 1a. Together with well-known band energies of ZnO, PEDOT:PSS, and P3HT:PCBM, the measured work functions of the doped-GR TCEs, as shown in Figure 1d, were used for describing the band structures of the FOSC, as shown in Figure 2b. Figure 2c shows J-V curves of a typical semitransparent FOSC, measured under simulated AM 1.5G illumination with an intensity of 100 mW-cm-2. When the light is illuminated from the TFSA/GR side, the FOSC exhibits 0.616 V open-circuit voltage (Voc), 9.84 mA cm-2 current density (Jsc), 54.51 % fill factor (FF), and 3.30 % PCE. In the opposite-side illumination, Voc = 0.614 V, Jsc = 9.45 mA cm-2, FF = 53.76 %, and PCE = 3.12 %. These results are summarized in Table 1. On the other hand, FOSCs containing PG TCEs on their both ends show 1.39 - 1.53 % PCEs, resulting from high Rs of PG despite the high transmittance (see Supporting Information, Figure S3). We believe that high PCE can be expected if the light transmitted through a semitransparent FOSC is well reflected back to the device. We measured the transmittance spectrum of a typical FOSC, as shown in Figure 2d. The transmittance of the FOSC is 30 40 % in the range of 400 - 550 nm, and reaches a maximum of 70 % at ~ 650 nm,


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demonstrating the semitransparency of the FOSC. Based on these results, we tried to enhance the performance by reflecting the light transmitted through the device with an Al reflective mirror, which can reflect the light effectively in the UV-visible region. (see Supporting Information, Figure S4) Figure 3a explains how the light is transmitted through the semitransparent FOSC can be reflected by an Al reflective mirror, thereby enhancing the EQE. The EQE is the product of the light harvesting efficiency, charge injection/transfer efficiency, and charge collection efficiency. The Jsc obtained by the integration of the EQE spectra are 9.74 and 9.45 mA cm-2 in the I and II configurations, respectively, consistent with those estimated in the J-V curves, as shown in Table 1, based on the strong correlation between Jsc and EQE.42 By placing the Al reflective mirror on the TETA (III) or TFSA (IV) side, the EQE was remarkably enhanced in a wide wavelength range from 400 to 600 nm, as shown in Figure 3b, possibly resulting from the increase in the light harvesting efficiency. The Jsc obtained from the EQE spectra are 12.57 and 11.94 mA cm-2 in the III and IV configurations, respectively. The measured Jsc, Voc, FF, and PCE in the III/V schemes are 12.88/12.23 mA cm-2, 0.613/0.610 V, 53.55/53.74 %, and 4.23/4.01 %, respectively, as shown in Table 1. Overall, the PCE increases by about 30% by using the reflective mirror. We also evaluated the operating stability of the FOSC through repeated bending tests for various R to check its functionality as a practical flexible power source. The inner or outer bending of the FOSC induces compressive or tensile stress on the GR electrode, respectively. Figure 4 shows normalized PCE of an FOSC as functions of inner/outer bending R for R = ∞, 12, 10, 8, and 6 mm. At the inner/outer R = 12, 10, 8, and 6 mm, the FOSC lost only 0.52/0.55, 0.70/0.77, 0.91/0.93, and 1.22/1.24 % of its original PCE, respectively, very encouraging in view of the applications of the FOSCs in flexible optoelectronic devices.


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These results further suggest that the photons transmitted through the semi-transparent FOSCs can be utilized in broad optical area including the energy generation.

4. Conclusion

High-performance semitransparent FOSCs were successfully fabricated based on simple solution processes by employing NIR-less-sensitive photoactive layer, TFSA/GR and TETA/GR TCEs. The PCEs of 3.30 and 3.12 % were achieved by illumination through TFSA/GR and TETA-GR sides, respectively, with the devices exhibiting high transparency: 30 – 50% from 400 to 600 nm and 70% above 650 nm. The PCE was about 30% enhanced by reflecting the light through the device with an Al reflective mirror. The FOSCs maintained more than 99% of the original PCE value even after bending tests at radii of 12 to 6 mm. These results are very promising in that the flexible semi-transparent FOSCs can be further developed for their use as add-on components for multi-junction photovoltaic devices, smart windows, building-integrated photovoltaics, and others.


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ASSOCIATED CONTENT Supporting Information Figures S1–S4. This

material is available

free

of

charge

via

the

Internet

at

http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions S. W. Seo deposited graphene sheets by CVD. D. H. Shin and C. W. Jang prepared and characterized solar cells. H. S. Lee analyzed the characteristics of doped graphene. S. -H. Choi initiated, supervised the work, and wrote the paper. All authors discussed the results and commented on the manuscript.

Funding Sources This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A2B3006054). Notes The authors declare no competing financial interests.


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Wolf C.; Seo H.-K.; Ahn J.-H.; Lee T.-W. On-Fabrication Solid-State N-Doping of Graphene by an Electron-Transporting Metal Oxide Layer for Efficient Inverted Organic Solar Cells. Adv. Energy Mater. 2016, 6, 1600172. (29) Kim H.; Bae S.-H.; Han T.-H.; Lim K.-G.; Ahn J.-H.; Lee T.-W. Organic solar cells using CVD-grown graphene electrodes. Nanotechnology 2014, 25, 014012. (30) Han T.-H.; Kim H.; Kwon S.-J.; Lee T.-W. Graphene-based flexible electronic devices. Mater. Sci. Eng. R 2017, 118, 1-43. (31) Han T.-H.; Kwon S.-J.; Li N.; Seo H.-K.; Xu W.; Kim K. S.; Lee T.-W. Versatile pType Chemical Doping to Achieve Ideal Flexible Graphene Electrodes. Angew. Chem. Int. Ed. 2016, 55, 6197-6201. (32) Tongay, S.; Berke, K.; Lemaitre, M.; Nasrollahi, Z.; Tanner, D. B.; Hebard, A. F.; Appleton, B. R. Stable Hole Doping of Graphene for Low Electrical Resistance and High Optical Transparency. Nanotechnology 2011, 22, 425701. (33)Kim, D.; Lee, D.; Lee, Y.; Jeon, Y. Work-Function Engineering of Graphene Anode by Bis(trifluoromethanesulfonyl)amide Doping for Efficient Polymer Light-Emitting Diodes. Adv. Funct. Mater. 2013, 23, 5049–5055. (34) Kim, Y.; Ryu, J.; Park, M.; Kim, E. S.; Yoo, J. M.; Park, J.; Kang, J. H.; Hong, B. H. Vapor-Phase Molecular Doping of Graphene for High-Performance Transparent Electrodes. ACS Nano 2014, 8, 868-874. (35) Jo, I.; Kim, Y.; Moon, J.; Park, S.; Moon, J. S.; Park, W. B.; Lee, J. S.; Hong, B. H. Stable n-Type Doping of Graphene via High-Molecular-Weight Ethylene Amines. Phys. Chem. Chem. Phys. 2015, 17, 29492-29495. (36) Jang, C. W.; Kim, J. H.; Kim, J. M.; Shin, D. H.; Kim, S.; Choi, S.-H. Rapid-ThermalAnnealing Surface Treatment for Restoring the Intrinsic Properties of Graphene FieldEffect Transistors. Nanotechnology 2013, 24, 405301. (37) Beek, W. J. E.; Wienk, M. M.; Kemerink, M.; Yang, X.; Janssen, R. A. J. Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells. J. Phys. Chem. B 2005, 109, 9505-9516.


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Table 1. Photovoltaic parameters of the organic solar cells under different conditions. Light reflection No mirror

With mirror

Illumination side

Voc (V)

Jsc (mA/cm )

FF (%)

PCE (%)

TFSA-GR

0.616

9.84

54.51

3.30

TETA-GR

0.614

9.45

53.76

3.12

TFSA-GR

0.613

12.88

53.55

4.23

TETA-GR

0.610

12.23

53.74

4.01

2


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Page 18 of 22

Figure captions

Figure 1. (a) Schematic of the device architecture for a typical solution-processed semitransparent FOSC and (b) Transmission spectra of PEDOT:PSS, ZnO, and P3HT:PCBM and absorption spectrum of P3HT:PCBM. (c) Transmittance spectra of PG, TETA/GR, and TFSA/GR TCEs. The inset images show the excellent transparency of TETA/ GR and TFSA/GR on PET substrates. (d) Sheet resistances and work functions of PG, TETA/GR, and TFSA/GR TCEs.

Figure 2. (a) Photograph of a typical semitransparent FOSC and (b) its energy band structure. (c) J-V curves of the FOSCs by illumination from TETA/GR and TFSA/GR TCE sides. (d) Transmission spectrum of a typical FOSC.

Figure 3. (a) Photovoltaic characterization of the FOSCs under different conditions. (I) and (II) Incident light beam is illuminated from TFSA/GR and TETA/GR TCE sides without a mirror, respectively. (III) and (IV) Incident light beam is illuminated from TFSA/GR and TETA/GR TCE sides while an Al reflective mirror is placed at the opposite side. (b) EQE spectra measured under the four different conditions, as described in (a). (c) J-V curves of the FOSCs with an Al reflective mirror under illumination from TFSA/GR and TETA/GR sides.

Figure 4. Changes of normalized PCEs as functions of bending cycles at R = ∞, 12, 10, 8, and 6 mm for FOSCs under (a) inner bending and (b) outer bending. The insets describe the bending tests and show a real image of a typical FOSC.


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Figure 1.

Figure 2.


19


(a)

Ⅱ

Ⅰ

(b)

Ⅳ Active layer

TFSA-GR TCE

Al Reflective mirror TFSA side, No mirror TETA side, No mirror TFSA side, with mirror TETA side, with mirror

100

EQE (%)

Ⅲ

TETA-GR TCE

80 60 40 20 0 300

400

500 600 700 800 Wavelength (nm)

900

(c)

10

2

Current density (mA/cm )

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

Page 20 of 22

illumination side, with mirror

5

TETA/GR TFSA/GR

0 -5 -10 -15

-0.4

0.0 0.4 Voltage (V)

0.8

Figure 3.


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Figure 4.


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Table of contents


22

Semitransparent flexible organic solar cells employing doped

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