4% Efficient Polymer Solar Cells on Paper Substrates - American

Article pubs.acs.org/JPCC

4% Efficient Polymer Solar Cells on Paper Substrates Lucia Leonat,*,† Matthew Schuette White,† Eric Daniel Głowacki,† Markus Clark Scharber,† Tino Zillger,‡ Julia Rühling,‡ Arved Hübler,‡ and Niyazi Serdar Sariciftci† †

Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, 4040 Linz, Austria Institute for Print and Media Technology, Chemnitz University of Technology 09107 Chemnitz, Saxony, Germany

‡

ABSTRACT: Lightweight, biodegradable, and inexpensive materials are desirable to enable low-cost photovoltaics. Paper fulfills these requirements and is compatible with standard roll-to-roll printing processes. Here we report 4% efficient organic solar cells fabricated on paper substrates with a zinc-coating acting as the back contact and evaporated MoO3/Ag/MoO3 semitransparent top electrodes. We demonstrate that the rough surface of paper does not preclude its use in highefficiency organic photovoltaics.

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transparent top-contact device geometries.11 Nonetheless, there have been recent reports of printed organic solar cells on paper.4 These reports have focused on printing techniques, using gravure, flexographic, and vapor printing methods, demonstrating flexible, foldable modules and cells with efficiency approaching 1.3%. In this work, we focus on the incorporation of several state-of-the-art OPV materials into the paper-based solar cells, bringing their power conversion efficiency over 4%. The main goal of our work is to build cheap, disposable solar cells. In this case, it becomes crucial to correlate the substrate degradability with the lifetime of the entire device. For example, polyethylene terephtalate (PET) is durable and chemically and thermally stable, but it also shows a slow decomposition of more than 20 years.12 The issue of plastic pollution has evolved to become a threat to global ecology, especially because of its resistance to degradation and its proliferation in industry. Our solar cells show that the challenges of paper substrates do not inherently limit the efficiency of the solar cells and provide some insight into pathways for further improvement.

INTRODUCTION There are many reasons why natural, abundant, recyclable, and environmentally friendly materials are highly desirable for use in the field of organic electronics.1,2 For organic photovoltaic (OPV) devices, the easiest component to replace with a biodegradable material is the supporting substrate. The substrate must provide mechanical support but has no intrinsic electronic function, and it constitutes the bulk (over 99%) of the device in terms of both weight and volume for common OPV devices.3 Paper offers many potential advantages as a substrate material, including low cost, abundance, biodegradability, and many tunable physical properties. In recent years paper has been used as a substrate for different electronic applications such as solar cells,3,4 transistors,5,6 sensors, biological assays, RF antennas, batteries, circuit boards, and smart packaging labels.7 The biggest advantage in using paper as a substrate is that it enables large scale fabrication of devices with relatively inexpensive processes such as printing.8 However, the paper surface presents several challenges for fabrication of OPV devices including high roughness, poor chemical and mechanical barrier properties, and capillary action of liquid materials into its porous structure.9 Moreover, because of the voids created between the cellulosic fibers, mineral fillers, sizers, and clays are used to fill the pores, followed by the addition of different coatings of pigments and fluorescent whitening agents to improve the roughness and the whiteness of the paper surface.10,11 All these additives can severely affect the fabrication of any device on a paper substrate especially if solution processes are involved. For this reason, organic electronic devices on paper have been successful using only dry techniques or a protective coating.4−12 Additionally, the majority of paper products are not transparent and so require © XXXX American Chemical Society

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EXPERIMENTAL SECTION Materials. For this work, we used double-sided coated image printing paper, type 135 g/m2. A commercially available thin polypropylene foil (∼15 μm) with homogeneous zinc coating (∼50 nm) is glued to the paper by cold foil lamination.13 Metal-coated plastic foils, especially aluminumcoated foils, are available commercially and commonly used for food labeling and packaging where gas and/or liquid Special Issue: Michael Grätzel Festschrift Received: December 30, 2013

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Figure 1. (a) Chemical structures of the materials, (b) picture of the device, (c) energy level diagram of the components of the inverted device on paper, and (d) schematic structure of the device.

permeation is the most important property.14,15 A thin zinc oxide (ZnO) layer is formed by natural oxidation of the Zn, which works as an electron-transporting layer. Squares of 1.5 × 1.5 cm2 were cut from this paper and then adhered to rigid glass slides to ease handling during the following device fabrication steps. The Zn foil was patterned with acetic acid. These substrates were then treated in argon plasma for 10−20 min at 50 W. For devices using indium tin oxide (ITO) substrates, slides of 1.5 × 1.5 cm2 were sonicated in acetone for 15 min, isopropanol for 15 min, Hellmanex solution for 30 min, then rinsed thoroughly with deionized water, dried in nitrogen flux and ultimately treated by oxygen plasma for 5 min at 50 W. Polyethylenimine (PEIE) solution was prepared by mixing 0.27 mg/mL polyethylenimine from Aldrich with 0.44 mg/mL glycerol diglycidyl ether from Aldrich in n-butanol. Glycerol diglycidyl ether is a well-known cross-linker used in membranerelated research, and it has been reported that cross-linked PEIE-layers were generally more robust.16 Poly(3-hexylthiophene) (P3HT) was obtained from Plextronics and purified by repeated precipitation. (6,6)-Phenyl-C70-butyric acid methyl ester (PC70BM) was purchased from Solenne BV. Thieno[3,4b]thiophene/benzodithiophene (PTB7) was purchased from 1Material, Chemscitech Inc.

Device Fabrication. For inverted structured devices, approximately 10 nm of PEIE was deposited by spin-coating on the clean substrates which were subsequently annealed at 110 °C for 5−10 min (Figure 1). A solution of polymer− fullerene blend of 1:1.5 by weight (10 mg/mL of PTB7 and 15 mg/mL PC70BM) in chlorobenzene was prepared and used for all sample preparation. The PTB7:PC70BM blend was mixed with 1,8- diiodoctane (97:3% by volume) and deposited by spin-coating at 1500 rpm for 20 s and then at 4000 rpm for another 20 s, giving an approximately 80 nm thick layer. A short thermal treatment on a hot plate at 110 °C for 2 min was applied to the samples, followed by a methanol treatment by spin-coating methanol solvent on top of the PTB7:PC70BM active layer at 2500 rpm for 40 s. Treatments using polar solvents are common in interface engineering and have been shown to improve photovoltaic devices by optimizing the phase separation in bulk heterojunction active layers.17 A solution of 1:1 P3HT (30 mg/mL) and PC70BM (15 mg/ mL) in chlorobenzene was spun using the same spin-coating recipe (1500 rpm for 20 s and then at 4000 rpm for 20s) resulting in an approximately 60 nm thick layer, followed by a 10 min annealing at 130 °C. The samples were then transferred to a nitrogen-filled glovebox for top contact deposition. For the samples with paper substrate, the top contact was deposited B

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followed by 13 nm of Ag (sheet resistance of ∼5 ohm sq−1) and 25 nm of MoO3 to minimize reflection losses due to index mismatch at the air interface. By combining these materials, we have significantly improved the performance of solar cells on paper. The J−V characteristics are plotted in Figure 2a, and the device parameters are reported in Table 1. The Jsc values in Table 1 were standardized with measured EQE, shown in Figure 2b. The P3HT-based solar cells demonstrate that the choice of electrodes, including the PEIE and the MAM stack, improve the FF of the cell to 54%, compared to previously reported 37%.4 Incorporating the lowbandgap PTB7, we show PCE of 4.1%, which is more than three times higher than previously reported paper-based cells.4 We processed a batch of 45 paper-based solar cells according to the optimized recipe, using PTB7. Of these, 4 cells were defective, giving a 91% success rate. The working cells had Voc= 707 ± 33 mV, Jsc= 7.06 ± 1.15 mA/cm2, FF = 45.4 ± 4.5%, and PCE = 2.9 ± 0.7%, measured at 80 mW/cm2 and not calibrated by integrated EQE. A histogram showing the distribution of resulting device efficiencies is shown in Figure 2c. Small differences in Voc between the paper- and ITO-based PTB7 devices are not considered to be statistically significant. The best PCE values on paper substrates are low, by roughly a factor of 2, compared to the benchmark set by He et al.21 To explain the discrepancy, we turn to a more detailed discussion of the individual components of the device. At macroscopic scale (millimeter scale), there are relatively large roughness patterns, average roughness ∼ 1.5 μm, because of the different elasticity of the polypropylene foil relative to the paper substrate and the gluing on the glass slides (for easier handling during fabrication), but these do not hinder the fabrication of the solar cells. Zn-Coated Paper. There are several techniques used to coat a paper surface with a metallic layer such as vacuum thermal evaporation, electron beam deposition, and transfer techniques. They have been industrially developed in recent years because metallized paper is intensively used for packaging and labeling.15 But our trials to deposit a thin layer (100 nm) of copper by thermal vacuum evaporation in a laboratory scale turned out to be inefficient because the layer did not decrease the roughness of the paper and it did not act as a mechanical barrier layer for the paper substrate. Instead, we used a zinc metalized polypropylene film which serves several purposes: the polypropylene foil smooths the paper surface and prevents capillary action of the processing inks into the porous structure of the paper, while Zn serves as a metallic electrode for the device. The support paper provides mechanical and thermal stability to the metalized propylene film; average roughness of the paper surface was ∼3.5 μm. The surface of Zn was exposed to air, forming a natural zinc oxide (ZnO) thin layer acting as an electron-conducting layer, having an average surface roughness of 60 nm at micrometric scale. Figure 3 shows an optical microscopy image of the pp−Zn-foil surface. A slight “light-soaking” effect was observed during the measurements of the solar cells on paper substrates under the solar simulator, which increased the current densities by 5% over the course of several minutes. This effect is caused by the thin, native ZnO layer and may be explained by the increase in ZnO conductivity with low-wavelength light exposure.22 Because the ZnO in the paper solar cells is behind the polymer active layer, it “sees” less light exposure. This effect may be amplified and could be one of the reasons for the low FF in the previously reported devices.4 The overall efficiency variations

using a shadow mask by vacuum thermal evaporation as follows: 8 nm MoO3 followed by 13 nm of Ag and then 25 nm of MoO3.18 The active area of the devices is 0.1 cm2. Similarly, for the samples with glass/ITO substrate, the top contact was fabricated by evaporating 8 nm of MoO3 followed by Ag, 100 nm. Device Characterization. The devices were first characterized in the dark and under illumination of a solar simulator (AM 1.5 Global spectrum with 80 mW/cm2 intensity). I−V characteristics were recorded using a Keithley 2400 source meter. All I−V measurements were carried out in a nitrogenfilled glovebox. Layer thicknesses were measured with a Dektak-XT stylus profilometer. The absorption and transmission spectra were recorded using an ultraviolet−visible spectrophotometer (PerkinElmer Lambda 1050). For this, we prepared glass slides previously cleaned in hellmanex and rinsed in deionized water and deposited thin layers of the polymer− fulerene mixture by spin-coating or thermally evaporating molybdenum oxide (MAM) stacks using recipes the same as those for solar cells. External quantum efficiency (EQE) measurements were performed in a nitrogen-filled glovebox using a home-built setup including a monochromator, chopper, lock-in amplifier, xenon arc lamp, and a reference Si photodiode. Short-circuit current Jsc values were standardized by integrating the product of EQE and AM 1.5 spectrum.

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RESULTS AND DISCUSSION Device Efficiency. The goal of this work was to incorporate state-of-the-art OPV materials while overcoming the challenges presented by paper substrates. The first layer to coat the substrate must accomplish two functions: it must serve as a conductive, metallic electrode and prevent the capillary action of the subsequent solution processes into the paper. Evaporation of metals onto the paper results in a highly porous film with low conductivity and is therefore not a suitable method. Cold-foil lamination of a compact polypropylene−zinc foil (pp−Zn) is a common roll-to-roll technique that meets both of the specified criteria. The pp−Zn foil has a native ZnO layer, which is a well-known electron selective contact for OPV devices.19 Previously reported paper-based solar cells incorporate this same Zn/ZnO foil system and show poor fill factor (FF).4 To improve on this, we apply the recently reported PEIE treatment to the ZnO surface.20 For the active layer of the BHJ-based OPV cell, we compare the standard semiconducting polymer P3HT with the state-ofthe-art PTB7. The PTB7 polymer has produced the highestefficiency polymer-based single-junction OPV cell. He et al.21 demonstrated the upper limit of what can be expected using this material, with Jsc = 17.46 mA/cm2, Voc= 0.754 V, and FF = 70%, resulting in an efficiency of 9.2%. We use these values as our target to identify the primary losses in our devices. The final challenge in constructing these solar cells is utilizing a transparent top contact. The Zn layer already functions as an electron-selective contact, so we must utilize a hole-selective material. Previous work on paper employed poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)4,10 which has a severe trade-off between conductivity and transparency. The MAM transparent holeselective contact proposed by Jin et. al18 was shown to have transmittance of 80% at 520 nm and sheet resistance of ∼5 ohm/cm2. We tested several layer thicknesses, but for our devices the optimized MAM stack structure is 8 nm of MoO3, C

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Table 1. Devices Parameters for Champion Solar Cells on Paper Compared to Solar Cells on Glass/ITO Substrates device structure paper/Zn/PEIE/P3HT:PC70BM/ MoO3 8 nm/Ag 13 nm/MoO3 25 nm Paper/Zn/PEIE/PTB7:PC70BM/ MoO3 8 nm/Ag 13 nm/MoO3 25 nm ITO/PEIE/PTB7:PC70BM/MoO3 8 nm/ Ag 100 nm

Jsc (mA/cm2)

Voc (mV)

FF (%)

PCE (%)

5.5

590

54

1.8

10.6

710

55

4.1

12.5

680

61

5.2

Figure 3. Optical micrograph of the pp−Zn/ZnO surface, scale 50 μm (a) and 1 mm (b).

However, the FF of our ITO-based device is still below the 72% benchmark reported in the literature. Optimizing the BHJ blend morphology while maintaining good coverage of the rough defects of the substrate surface will be key to improving the FF. MAM Top Contact. For the top contact we needed a system that can be easily patterned in a laboratory scale. PEDOT:PSS is processable from solution by spin-coating, but patterning from solution is difficult to achieve in a laboratory scale. In addition, because of the roughness of the paper substrate, it is very challenging to form a conducting yet transparent thin film that can act as a top contact electrode. Moreover, in inverted devices, it has been observed that the major cause of device failure occurs at the interface between the active layer and PEDOT:PSS; the PEDOT-rich phase is responsible for most of the interface degradation in the presence of oxygen.23 A reasonable alternative is to use a stack consisting of a thin layer of silver sandwiched between two layers of molybdenum oxide (MAM).18 The transparency of the contact is extremely important for the solar cell performance. Figure 4 shows the transmittance spectrum of the optimized MAM structure plotted with the optical absorption spectra of the two active layer systems. The

Figure 2. (a) Current density−voltage (J−V) curves of inverted cells on paper/pp−Zn/ZnO with P3HT:PC 70 BM (red line) and PTB7:PC70BM (blue line) and on glass/ITO with PTB7:PC70BM (green line), active area 0.1 cm2, in the dark (dashed lines) and under AM 1.5G illumination from a calibrated solar simulator with irradiation intensity of 80 mW/cm2; (b) external quantum efficiency (EQE) of the three devices; (c) histogram of the power conversion efficiency (PCE) of the device batch. The cells were built according to the optimized paper/PTB7:PC70BM structure and were measured at 80 mW/cm2 (80% sun intensity).

observed here are small, but this indicates that the Zn/ZnO/ PEIE electrode is still subject to some of the pitfalls of the ZnO electron transport layer. This may be a limiting mechanism in the FF of the devices, as is evident in the higher FF (61%) of the samples with ITO/PEIE and MoO3/Ag electrodes.

Figure 4. Optical transmittance spectra of the MAM stack (MoO3, 8 nm; Ag, 13 nm; MoO3, 25 nm) on glass and normalized absorbance spectra for P3HT:PC70BM (red line) and PTB7:PC70BM (blue line). D

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(7) Steckl, A. J. Circuits on Cellulose. IEEE Spectrum 2013, 50, 48− 61. (8) Rolland, J.; Mourey, D. Paper as a Novel Material Platform for Devices. MRS Bull. 2013, 38, 299−305. (9) Tobjörk, D.; Ö sterbacka, R. Paper Electronics. Adv. Mater. (Weinheim, Ger.) 2011, 23, 1935−1961. (10) Barr, M. C.; Rowehl, J. A.; Lunt, R. R.; Xu, J.; Wang, A.; Boyce, C. M.; Im, S. G.; Bulović, V.; Gleason, K. K.; Barr, B. M. C.; et al. Direct Monolithic Integration of Organic Photovoltaic Circuits on Unmodified Paper. Adv. Mater. (Weinheim, Ger.) 2011, 23, 3499− 3505. (11) Barr, M. C.; Howden, R. M.; Lunt, R. R.; Bulović, V.; Gleason, K. K. Top-Illuminated Organic Photovoltaics on a Variety of Opaque Substrates with Vapor-Printed Poly(3,4-ethylenedioxythiophene) Top Electrodes and MoO3 Buffer Layer. Adv. Energy Mater. 2012, 2, 1404− 1409. (12) Webb, H.; Arnott, J.; Crawford, R.; Ivanova, E. Plastic Degradation and Its Environmental Implications with Special Reference to Poly(ethylene terephthalate). Polymers (Basel, Switz.) 2012, 5, 1−18. (13) Ali, M.; Prakash, D.; Zillger, T.; Singh, P. K.; Hübler, A. C. Printed Piezoelectric Energy Harvesting Device. Adv. Energy Mater. 2014, 4, 1300427. (14) Weiss, J. Parameters That Influence the Barrier Properties of Metallized Polyester and Polypropylene Films. Thin Solid Films 1991, 204, 203−216. (15) Ramade, C.; Silvestre, S.; Pascal-Delannoy, F.; Sorli, B. Thin Film HF RFID Tag Deposited on Paper by Thermal Evaporation. Int. J. Radio Freq. Identif. Technol. Appl. 2012, 4, 49−66. (16) Udum, Y.; Denk, P.; Adam, G.; Apaydin, D. H.; Nevosad, A.; Teichert, C.; S. White, M.; S. Sariciftci, N.; Scharber, M. C. Inverted Bulk-Heterojunction Solar Cell with Cross-Linked Hole-Blocking Layer. Org. Electron. 2014, 15, 997−1001. (17) Zhou, H.; Zhang, Y.; Seifter, J.; Collins, S. D.; Luo, C.; Bazan, G. C.; Nguyen, T.-Q.; Heeger, A. J. High-Efficiency Polymer Solar Cells Enhanced by Solvent Treatment. Adv. Mater. (Weinheim, Ger.) 2013, 25, 1646−1652. (18) Jin, H.; Tao, C.; Velusamy, M.; Aljada, M.; Zhang, Y.; Hambsch, M.; Burn, P. L.; Meredith, P. Efficient, Large Area ITO-and-PEDOTFree Organic Solar Cell Sub-Modules. Adv. Mater. (Weinheim, Ger.) 2012, 24, 2572−2577. (19) White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Inverted Bulk-Heterojunction Organic Photovoltaic Device Using a Solution-Derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517. (20) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; et al. A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. Science 2012, 336, 327−332. (21) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 591−595. (22) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai, T.-H.; Reynolds, J. R.; So, F. High-Efficiency Inverted Dithienogermole−Thienopyrrolodione-Based Polymer Solar Cells. Nat. Photonics 2011, 6, 115−120. (23) Norrman, K.; Madsen, M. V.; Gevorgyan, S. a; Krebs, F. C. Degradation Patterns in Water and Oxygen of an Inverted Polymer Solar Cell. J. Am. Chem. Soc. 2010, 132, 16883−16892. (24) Krebs, F. C. Roll-to-Roll Fabrication of Monolithic Large-Area Polymer Solar Cells Free from Indium-Tin-Oxide. Sol. Energy Mater. Sol. Cells 2009, 93, 1636−1641. (25) Kim, K.; Ahn, S. Il; Choi, K. C. Simultaneous Synthesis and Patterning of Graphene Electrodes by Reactive Inkjet Printing. Carbon 2014, 66, 172−177.

peak transmittance of the MAM stack is roughly 80% at a wavelength of 504 nm. This aligns nicely with the peak absorption of P3HT. However, the transmittance at 700 nm, the peak absorption of the PTB7 blend, is only 60%. Therefore, we see significant loss in the EQE at longer wavelengths, which is clearly seen in Figure 2b. Optimizing the JSC of the device will require over 90% in the full spectrum of the cell absorption. Alternative electrodes for transparent contact should be employed. There are several options, including semitransparent silver nanowire electrodes, composite electrodes based on printed metal grid and a semitransparent conducting polymer, titanium metal or Ag/ZnO electrodes,24 and reduced graphene oxide (RGO) films.25 Many of these options offer good transparency and are solution processable. Exploring them will be required to improve efficiency and enable complete fabrication of this type of solar cells through printing techniques.

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CONCLUSIONS We have successfully demonstrated 4% efficient solar cells on paper substrates using a transparent top contact system. By incorporating state-of-the-art polymers and electrode materials, we produce a 3-fold improvement in the efficiency of paperbased OPV. We identify the transparency of the top contact as the primary limiting factor in the cell performance. This research shows that high efficiencies are within reach for printable photovoltaic devices on paper.

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AUTHOR INFORMATION

Corresponding Author

*Johannes Kepler University, Linz Institute for Organic Solar Cells, Altenbergerstraße 69, A4040 Linz, Austria. E-mail: Lucia. [email protected]. Tel.: +43 732 2468 1213. Fax: +43 732 2468 8770. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the seventh framework European Union Project 134600 “Disposable ultra-cheap printed paper photovoltaics - DUC3PV” and the Austrian Fund for Advancement of Science Wittgenstein Prize scheme (Z222-N19 Solare Energieumwandlung).

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REFERENCES

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