- Email: [email protected]
Mechanically robust, ITO-free, 4.8% efficient, all-solution processed organic solar cells on flexible PET foil
Solar Energy Materials & Solar Cells 130 (2014) 317–321
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Mechanically robust, ITO-free, 4.8% efficient, all-solution processed organic solar cells on flexible PET foil Felix Nickel a,n, Thomas Haas b, Eduard Wegner a, Daniel Bahro a, Sayedus Salehin a, Oliver Kraft b, Patric A. Gruber b, Alexander Colsmann a,n a b
Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany Institute for Applied Materials, Karlsruhe Institute of Technology (KIT), PO Box 3640, 76021 Karlsruhe, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2014 Received in revised form 24 June 2014 Accepted 1 July 2014 Available online 7 August 2014
We present all-solution processed, indium tin oxide-free organic solar cells on mechanically flexible polyethylene terephthalate (PET) substrates with power conversion efficiencies up to 4.8%. The mechanical properties of the devices are dictated by the electrodes from either metal-organic silver inks or a combination of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) and silver nanowires. Both electrodes can sustain high tensile strains. The influence of mechanical strain on the solar cell performance was experimentally studied in situ by measuring the J–V curves of elongated samples under illumination. At a strain of 14%, we still observed 90% of the initial device power output. At higher strain, crack formation within the electrodes was identified as the origin for device failure. & 2014 Elsevier B.V. All rights reserved.
Keywords: Polymer solar cell ITO-free All-solution processing Mechanical flexible Printed electronics
1. Introduction Solution processibility and mechanical flexibility are frequently promoted advantages of organic solar cells, which are supposed to enable high volume production in roll-to-roll processes [1]. Their low weight and mechanical flexibility make them suitable for applications on shaped surfaces, for building integration or for mobile electronics. However, it is often not considered that small bending radii can cause high tensile stress within the device and that mechanically flexible devices require special designs. On the contrary, the focus of most research and development is on achieving high power conversion efficiencies (PCE) utilizing expensive and brittle indium tin oxide (ITO) that has been vacuum deposited on rigid glass substrates. In order to exploit the mechanical properties of organic solar cells, they have to be fabricated on inexpensive and flexible substrates such as polyethylene terephthalate (PET) foils employing printable flexible transparent electrodes. So far, highly conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) has been well investigated to replace the brittle and costly ITO [2–4]. Additional printed silver grids or silver nanowire (AgNW) networks help to further enhance the lateral conductivity, to match
n
Corresponding authors. Tel.: þ 49 721 608 48587; fax: þ 49 721 608 42590. E-mail addresses: [email protected] (F. Nickel), [email protected] (A. Colsmann). http://dx.doi.org/10.1016/j.solmat.2014.07.005 0927-0248/& 2014 Elsevier B.V. All rights reserved.
the electronic properties of ITO [5–14] and to improve the mechanical stability of the devices [15–17]. In this work, we fabricated organic solar cells all from solution. The silver bottom electrodes exhibited very low surface roughness to enable subsequent functional layer deposition. The transparent top electrodes comprised a 200 nm PEDOT:PSS layer and a AgNW mesh. With respect to mechanical flexibility, the electrodes play a key role as they tend to be damaged first under mechanical strain [16]. Our mechanical reliability tests primarily focused on the tensile behavior since various studies have shown that mechanical device degradation is dominated by tensile strain instead of compression strain [8,18,19]. We benchmarked the mechanical reliability of the solar cells and their electrodes against state-ofthe-art solar cells comprising ITO electrodes.
2. Experimental details For the preparation of the all-solution processed solar cells, we doctor bladed a 220 nm (715 nm) silver electrode on a 175 mm thick PET foil (Melinex ST506, DuPont Teijin Films Ltd.) utilizing a commercially available metal-organic decomposition (MOD) silver ink (TEC-PR-010, InkTec). The electrode was patterned photolithographically with iron (III) nitride. A 35 nm electron extraction layer was spin cast from zinc oxide nanoparticles (Nanograde Ltd.) atop the Ag electrode. Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]
318
F. Nickel et al. / Solar Energy Materials & Solar Cells 130 (2014) 317–321
thieno[3,4-b]thiophenediyl]] (PTB7, 1-Material Inc.) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM, Solenne BV, 99%) were dissolved 2:3 in chlorobenzene at a concentration of 40 g/L together with 4 vol% 1,8-diiodooctane and spun atop the ZnO to form the 180 nm (75 nm) photoactive layer. Following the same route, we also fabricated solar cells from poly[[2,60 -4,8-di(5-ethylhexylthienyl) benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PBDTT-FTTE, 1-Material Inc., commercial name PCE-10) and PC71BM. In order to define the area of the top electrode, sticky tape was applied on top of the active layer. Then the top electrode was spin cast from solution comprising PEDOT:PSS (Clevios PH1000, Heraeus Precious Metals GmbH & Co KG), 5 vol% dimethyl sulfoxide and 8 vol% wetting agent. To enhance the conductivity of the polymer electrode, silver nanowires (AgNW-115, Seashell Technology Llc.) were drop cast from 1.25 g/L isopropyl alcohol dispersion, followed by an annealing step at 110 1C for 5 min. Two device layouts were designed to meet the different requirements of the experimental setups: The current density–voltage (J–V) curves of the solar cells were measured on an active area of 0.8 0.5 cm2 whereas the samples for tensile stress testing exhibited an active area of 0.25 0.4 cm2. The mechanical degradation of the electrodes was investigated with a compact modified micro-translation stage (M-112, PI GmbH & Co. KG) for in situ tensile tests in a scanning electron microscope (SEM). For the in situ measurements of the relative change of the electrode resistance versus strain, we utilized a tensile/compression stage (10 kN Mz.Mb, Kammrath & Weiss GmbH). The samples were electrically contacted via specifically designed sample clamps and the voltage was measured (Keithley 2182A, Keithley Instruments Inc.) versus strain at a constant current of 50 mA (Keithley 6220, Keithley Instruments Inc.) using a four-point probing setup. The initial distance between the clamps before the application of strain was 8 mm. Transmission spectra were recorded in a Cary 5000 UV–vis–NIR spectrophotometer (Agilent Technologies Inc.) equipped with a 150 mm integrating sphere. The organic solar cells were characterized under illumination from a spectrally monitored solar simulator (Oriel 300W, ASTM G-17303, AM1.5, Newport Corp.). The layer thicknesses were measured with a profilometer (Dektak XT, Bruker Corp.). A 3D optical profiler (plm neox, Sensofar Corp.) was used in confocal mode to capture the topography of the MOD silver layer, i.e. the bottom electrode. We assessed the peak-to-valley roughness of the bottom electrode instead of the more common arithmetic average or root mean square to capture peaks that may account for device shunting.
(“flexibility”) of the device are ruled by the electrodes, we deliberately focused on the investigation of the electrode properties. First, we assessed the electrical properties of the opaque and unstrained silver electrodes that were deposited from a MOD silver ink onto PET substrates by doctor blading. Since the MOD silver layers partly reproduce the texture of the PET substrates, the PET surface has to be very clean and dust free to ensure a smooth silver layer surface. As the surface smoothness of these electrodes plays a crucial role to avoid shunts in the devices, we analyzed the topography of the MOD silver layers on PET with a confocal microscope (Fig. S1 in the Supplementary information). The smooth bottom electrode with a peak-to-valley roughness Rpv o 40 nm enabled the fabrication of organic solar cells with active layer thicknesses below 200 nm which is important for yielding good power conversion efficiencies with most highly efficient photo-active materials that exhibit low charge carrier mobilities. The sheet resistance (Rsheet) of the 220 nm (715 nm) silver layers ranged between 0.3 and 0.5 mΩ/sq. To investigate the properties of the unstrained PEDOT:PSS (200 nm)/AgNW (1.8 mg/cm2) top electrode, the electrode was also applied onto a PET substrate, hence being uncoupled from the photovoltaic device. Despite the fairly low sintering temperature, which was applied to avoid thermal damage of the active layer, the samples showed a good sheet resistances Rsheet ¼60 Ω/sq (75 Ω/ sq). We further measured the PEDOT:PSS/AgNW electrode transparency T500 nm ¼76% at a wavelength of λ ¼500 nm with a UV–vis absorption spectrometer. Both, optical and electrical properties compared to the transmission and sheet resistance of ITO covered PET (T500 nm ¼80%, Rsheet ¼60 Ω/sq, Sigma-Aldrich) that we used for reference (Fig. S2).
3. Results and discussion For the experiments discussed herein, we investigated organic solar cells with an all-solution processed PET/Ag/ZnO/PTB7: PC71BM/PEDOT:PSS/AgNW device architecture (see Fig. 1). Since preliminary experiments indicated that the mechanical properties
Fig. 2. Relative change of the resistance (ΔR/R0) versus strain (εeng) for solution processed MOD silver (Ag) and PEDOT:PSS/AgNW electrodes on PET foil in comparison to the predicted change based on geometric considerations, a commercially available ITO electrode and a vacuum deposited Ag electrode.
Fig. 1. (a) Device architecture and (b) chemical structures of PBDTT-FTTE and PTB7. (c) Photo of the all-solution processed, mechanically flexible solar cell.
F. Nickel et al. / Solar Energy Materials & Solar Cells 130 (2014) 317–321
After investigating the properties of the as prepared electrode samples, we examined the changes of the electrical properties under strain. Therefore, we performed in situ tensile tests on both the MOD silver and the PEDOT:PSS/AgNW electrodes, respectively. As suggested by Lu et al. [20], based on purely geometric considerations, i.e. the change of the film dimension (length and thickness) under strain, the relative change of resistance (ΔR/R0)
319
can be calculated as follows:
ΔR R0
¼ 2εeng þ εeng2
ð1Þ
where R0 represents the initial resistance and ΔR¼R R0 describes the absolute change of resistance. εeng is the engineering strain. Fig. 2 depicts ΔR/R0 measured in situ versus the engineering strain
Fig. 3. SEM images of solution processed (a) MOD silver layers and (b) PEDOT:PSS/AgNW electrodes at different strain. In MOD silver electrodes, we observed first cracks at strains εeng 49%. The silver nanowires started to fracture at strains εeng between 3% and 6% whereas no cracks were observed in the PEDOT:PSS layer.
Fig. 4. (a) J–V curves of all-solution processed organic solar cells on PET foil comprising PTB7:PC71BM or PBDTT-FTTE:PC71BM, respectively. (b) Picture of the tensile test setup under the solar simulator.
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F. Nickel et al. / Solar Energy Materials & Solar Cells 130 (2014) 317–321
for the MOD silver electrode and the PEDOT:PSS/AgNW electrode as well as, for reference, an ITO electrode, a vacuum processed silver layer and the theoretic prediction. The reference ITO electrode already failed above εeng ¼1% due to crack formation as observed in ITO electrodes before. [21,22] In contrast, ΔR/R0 for the MOD silver layer well followed the theoretic prediction up to εeng E8%. The deviation from the prediction towards higher εeng originates from the formation of cracks within the silver layer as observed in the SEM images in Fig. 3a. Whereas the MOD silver electrode exhibited an intact surface at low εeng, we observed first microcracks at tensile strains of εeng ¼ 9%. Towards higher strain (here exemplified at εeng ¼12%), we found notable damage of the silver surface. However, despite this layer damage, the silver electrode still exhibited 75% of its initial conductivity. We observed about the same behavior for evaporated reference silver electrodes, demonstrating the very good mechanical properties of the solution processed MOD silver layers. In contrast, the tensile strain much more affected the PEDOT:PSS/AgNW electrodes. The PEDOT:PSS/ AgNWs electrode did not show any significant change of ΔR/R0 for strain below εeng ¼3%. At a strain of εeng ¼6%, first fractures of the silver nanowires became visible in the SEM pictures in Fig. 3b that further evolved towards higher strain. Concomitantly, ΔR/R0 increased to 0.75 for εeng ¼ 12%. The outstanding mechanical properties of the MOD silver layer and the PEDOT:PSS/AgNW electrodes were ideal prerequisites for the fabrication of mechanically flexible organic solar cells as depicted in Fig. 1a. Fig. 4a shows the current density–voltage (J–V) curves of the respective PTB7:PC71BM and PBDTT-FTTE:PCB71M solar cells as well as devices comprising PTB7:PC71BM reference solar cells with an ITO bottom electrode and a thermally evaporated molybdenum oxide/silver top electrode. The electrical key performance data is summarized in Table 1. PTB7 solar cells showed similar opencircuit voltages Voc ¼ 72777 mV as compared to the ITO reference devices (Voc ¼732 mV711 mV). The fill factor FF¼5371% and the short circuit current density Jsc ¼10.770.7 mA/cm2 were slightly lower than in the reference devices exhibiting Jsc ¼ 11.670.2 mA/ cm2 and FF¼ 5671%, which we attribute to the low process temperature for the deposition of the PEDOT:PSS/AgNWs electrode. We note that annealing can improve the performance of the PEDOT: PSS/AgNW electrode, however, at the same time reducing the device power conversion efficiencies due to an adverse effect on the photoactive layer. Still, hero devices exhibited remarkable PCEs of η ¼4.2% and a maximum device output power Pmax ¼1.7 mW (on 0.8 0.5 cm2). In order to demonstrate the universality of the device architecture, we further fabricated solar cells comprising the photoactive polymer PBDTT-FTTE for light harvesting. The device PCE improved to η ¼ 4.8% exhibiting Voc ¼ 78074 mV, Jsc ¼ 11.470.3 mA/ cm2, FF¼5271% and a maximum device output power Pmax ¼ 1.9 mW. For the following device strain tests, however, we deliberately focus on PTB7 as absorber material. Finally, we tested the mechanical reliability of the all-solution processed devices by measuring the solar cell's performance under illumination versus tensile strain in situ. Therefore, we placed the micro tensile tester under the solar simulator, successively increased the strain and recorded J–V curves at least every Δεeng ¼2% (Fig. S3). Again, we observe an immediate failure of the ITO reference device
under strain. In contrast, we found that the strain does neither affect the Voc of the all-solution processed solar cells nor the device short circuit current Isc, but both remained stable throughout the experiment (Fig. 5a). We note that Jsc was reduced upon sample elongation and hence upon an increase of the effective device area. Since the area inherently changes during strain experiments, Jsc and PCE are
Table 1 Average key performance data of 3 devices.
Fig. 5. (a) The Voc's of both solar cells remain unchanged even at strains beyond 10%. The ISC of the ITO reference device drops to zero at εeng 4 2% whereas the ISC of the all-solution device remains about constant. (b) Influence of strain on fill factor and the solar cell output power. Whereas the reference solar cell with ITO electrode already fails at substrate elongations εeng 41%, the all-solution processed device can withstand strain up to εeng ¼ 14%. (c) Estimated series resistance of the allsolution processed solar cell as derived from the I–V curve in comparison to the combined resistance of the individually measured electrodes (MOD silver and PEDOT:PSS/AgNW) versus strain.
Device
Voc (mV) Jsc (mA/cm2) FF (%)
7277 7 10.7 7 0.7 PTB7:PC71BM (all-solution) PTB7:PC71BM (ITO-PET) 7327 11 11.6 7 0.2 PBDTT-FTTE:PC71BM (all-solution) 780 7 4 11.4 7 0.3
PCE (%)
537 1% 4.17 0.1 567 1 4.7 7 0.1 527 1 4.6 7 0.1
F. Nickel et al. / Solar Energy Materials & Solar Cells 130 (2014) 317–321
not meaningful parameters anymore. Instead we assessed the short circuit current (Isc) and the device output power henceforth. Fig. 5b depicts the impact of strain on the fill factor and the device output power of a typical solar cell. Upon a tensile strain of εeng ¼14% the output power droped below 90% (rel.) of the initial output power. The direct comparison of the Voc, Isc, FF and device output power allows the conclusion that the fill factor is the main failure reason for the all-solution processed devices at high strain. In order to further analyze the origin of the fill factor degradation, we estimated the influence of our previously measured electrode degradation on the device properties by assessing the series resistance as derived from the I–V curves following the single-diode model: dV nkB=q ¼ þ Rs d I I sc þ I V=Rsh
ð2Þ
where n is the ideality factor, q the elementary charge, kB the Boltzmann constant, Rs the series resistance and Rsh the shunt resistance [23]. Accordingly, the performance parameters of the solar cells above as derived from the I–V curves allow an estimation of Rs versus strain. The comparison of Rs with the combined series resistance of the two electrodes in Fig. 5c shows that Rs is dominated by the electrodes. This in turn indicates that the degradation and decomposition of the electrodes under strain was also the main failure mechanism of the device and that device decomposition or delamination, as described by Dupont et al., may play only a minor role [24].
4. Conclusion In conclusion, we have fabricated efficient all-solution processed solar cells comprising MOD silver bottom and PEDOT:PSS/ AgNWs top electrodes on flexible PET substrates exhibiting PCEs of up to 4.8%. The mechanical reliability of the all-solution processed solar cells was investigated by assessing I–V curves that were recorded while applying tensile strain. Whereas conventional solar cells comprising ITO electrodes failed already at εeng 41%, the allsolution processed devices sustained tensile strains up to 14%. With respect to bending and for a substrate thickness of 175 mm such tensile strains correspond to a sample bending radius of about 0.6 mm. Additional tensile tests of the individual electrode materials enabled assignment of the solar cell degradation to the tensile failure of the electrodes.
Acknowledgments This work was carried out within the projects “DELICIOUS”, funded by the Baden-Württemberg Stiftung, and “POPUP”, funded by the German Federal Ministry for Education and Research (BMBF) under contract no. 03EK3501H. It was supported by the Helmholtz Virtual Institute VI-530. F.N. was supported by the Karlsruhe School of Optics and Photonics (KSOP).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2014.07. 005.
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Mechanically robust, ITO-free, 4.8% efficient, all-solution processed organic solar cells on flexible PET foil Felix Nickel a,n, Thomas Haas b, Eduard Wegner a, Daniel Bahro a, Sayedus Salehin a, Oliver Kraft b, Patric A. Gruber b, Alexander Colsmann a,n a b
Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany Institute for Applied Materials, Karlsruhe Institute of Technology (KIT), PO Box 3640, 76021 Karlsruhe, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2014 Received in revised form 24 June 2014 Accepted 1 July 2014 Available online 7 August 2014
We present all-solution processed, indium tin oxide-free organic solar cells on mechanically flexible polyethylene terephthalate (PET) substrates with power conversion efficiencies up to 4.8%. The mechanical properties of the devices are dictated by the electrodes from either metal-organic silver inks or a combination of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) and silver nanowires. Both electrodes can sustain high tensile strains. The influence of mechanical strain on the solar cell performance was experimentally studied in situ by measuring the J–V curves of elongated samples under illumination. At a strain of 14%, we still observed 90% of the initial device power output. At higher strain, crack formation within the electrodes was identified as the origin for device failure. & 2014 Elsevier B.V. All rights reserved.
Keywords: Polymer solar cell ITO-free All-solution processing Mechanical flexible Printed electronics
1. Introduction Solution processibility and mechanical flexibility are frequently promoted advantages of organic solar cells, which are supposed to enable high volume production in roll-to-roll processes [1]. Their low weight and mechanical flexibility make them suitable for applications on shaped surfaces, for building integration or for mobile electronics. However, it is often not considered that small bending radii can cause high tensile stress within the device and that mechanically flexible devices require special designs. On the contrary, the focus of most research and development is on achieving high power conversion efficiencies (PCE) utilizing expensive and brittle indium tin oxide (ITO) that has been vacuum deposited on rigid glass substrates. In order to exploit the mechanical properties of organic solar cells, they have to be fabricated on inexpensive and flexible substrates such as polyethylene terephthalate (PET) foils employing printable flexible transparent electrodes. So far, highly conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) has been well investigated to replace the brittle and costly ITO [2–4]. Additional printed silver grids or silver nanowire (AgNW) networks help to further enhance the lateral conductivity, to match
n
Corresponding authors. Tel.: þ 49 721 608 48587; fax: þ 49 721 608 42590. E-mail addresses: [email protected] (F. Nickel), [email protected] (A. Colsmann). http://dx.doi.org/10.1016/j.solmat.2014.07.005 0927-0248/& 2014 Elsevier B.V. All rights reserved.
the electronic properties of ITO [5–14] and to improve the mechanical stability of the devices [15–17]. In this work, we fabricated organic solar cells all from solution. The silver bottom electrodes exhibited very low surface roughness to enable subsequent functional layer deposition. The transparent top electrodes comprised a 200 nm PEDOT:PSS layer and a AgNW mesh. With respect to mechanical flexibility, the electrodes play a key role as they tend to be damaged first under mechanical strain [16]. Our mechanical reliability tests primarily focused on the tensile behavior since various studies have shown that mechanical device degradation is dominated by tensile strain instead of compression strain [8,18,19]. We benchmarked the mechanical reliability of the solar cells and their electrodes against state-ofthe-art solar cells comprising ITO electrodes.
2. Experimental details For the preparation of the all-solution processed solar cells, we doctor bladed a 220 nm (715 nm) silver electrode on a 175 mm thick PET foil (Melinex ST506, DuPont Teijin Films Ltd.) utilizing a commercially available metal-organic decomposition (MOD) silver ink (TEC-PR-010, InkTec). The electrode was patterned photolithographically with iron (III) nitride. A 35 nm electron extraction layer was spin cast from zinc oxide nanoparticles (Nanograde Ltd.) atop the Ag electrode. Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]
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F. Nickel et al. / Solar Energy Materials & Solar Cells 130 (2014) 317–321
thieno[3,4-b]thiophenediyl]] (PTB7, 1-Material Inc.) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM, Solenne BV, 99%) were dissolved 2:3 in chlorobenzene at a concentration of 40 g/L together with 4 vol% 1,8-diiodooctane and spun atop the ZnO to form the 180 nm (75 nm) photoactive layer. Following the same route, we also fabricated solar cells from poly[[2,60 -4,8-di(5-ethylhexylthienyl) benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PBDTT-FTTE, 1-Material Inc., commercial name PCE-10) and PC71BM. In order to define the area of the top electrode, sticky tape was applied on top of the active layer. Then the top electrode was spin cast from solution comprising PEDOT:PSS (Clevios PH1000, Heraeus Precious Metals GmbH & Co KG), 5 vol% dimethyl sulfoxide and 8 vol% wetting agent. To enhance the conductivity of the polymer electrode, silver nanowires (AgNW-115, Seashell Technology Llc.) were drop cast from 1.25 g/L isopropyl alcohol dispersion, followed by an annealing step at 110 1C for 5 min. Two device layouts were designed to meet the different requirements of the experimental setups: The current density–voltage (J–V) curves of the solar cells were measured on an active area of 0.8 0.5 cm2 whereas the samples for tensile stress testing exhibited an active area of 0.25 0.4 cm2. The mechanical degradation of the electrodes was investigated with a compact modified micro-translation stage (M-112, PI GmbH & Co. KG) for in situ tensile tests in a scanning electron microscope (SEM). For the in situ measurements of the relative change of the electrode resistance versus strain, we utilized a tensile/compression stage (10 kN Mz.Mb, Kammrath & Weiss GmbH). The samples were electrically contacted via specifically designed sample clamps and the voltage was measured (Keithley 2182A, Keithley Instruments Inc.) versus strain at a constant current of 50 mA (Keithley 6220, Keithley Instruments Inc.) using a four-point probing setup. The initial distance between the clamps before the application of strain was 8 mm. Transmission spectra were recorded in a Cary 5000 UV–vis–NIR spectrophotometer (Agilent Technologies Inc.) equipped with a 150 mm integrating sphere. The organic solar cells were characterized under illumination from a spectrally monitored solar simulator (Oriel 300W, ASTM G-17303, AM1.5, Newport Corp.). The layer thicknesses were measured with a profilometer (Dektak XT, Bruker Corp.). A 3D optical profiler (plm neox, Sensofar Corp.) was used in confocal mode to capture the topography of the MOD silver layer, i.e. the bottom electrode. We assessed the peak-to-valley roughness of the bottom electrode instead of the more common arithmetic average or root mean square to capture peaks that may account for device shunting.
(“flexibility”) of the device are ruled by the electrodes, we deliberately focused on the investigation of the electrode properties. First, we assessed the electrical properties of the opaque and unstrained silver electrodes that were deposited from a MOD silver ink onto PET substrates by doctor blading. Since the MOD silver layers partly reproduce the texture of the PET substrates, the PET surface has to be very clean and dust free to ensure a smooth silver layer surface. As the surface smoothness of these electrodes plays a crucial role to avoid shunts in the devices, we analyzed the topography of the MOD silver layers on PET with a confocal microscope (Fig. S1 in the Supplementary information). The smooth bottom electrode with a peak-to-valley roughness Rpv o 40 nm enabled the fabrication of organic solar cells with active layer thicknesses below 200 nm which is important for yielding good power conversion efficiencies with most highly efficient photo-active materials that exhibit low charge carrier mobilities. The sheet resistance (Rsheet) of the 220 nm (715 nm) silver layers ranged between 0.3 and 0.5 mΩ/sq. To investigate the properties of the unstrained PEDOT:PSS (200 nm)/AgNW (1.8 mg/cm2) top electrode, the electrode was also applied onto a PET substrate, hence being uncoupled from the photovoltaic device. Despite the fairly low sintering temperature, which was applied to avoid thermal damage of the active layer, the samples showed a good sheet resistances Rsheet ¼60 Ω/sq (75 Ω/ sq). We further measured the PEDOT:PSS/AgNW electrode transparency T500 nm ¼76% at a wavelength of λ ¼500 nm with a UV–vis absorption spectrometer. Both, optical and electrical properties compared to the transmission and sheet resistance of ITO covered PET (T500 nm ¼80%, Rsheet ¼60 Ω/sq, Sigma-Aldrich) that we used for reference (Fig. S2).
3. Results and discussion For the experiments discussed herein, we investigated organic solar cells with an all-solution processed PET/Ag/ZnO/PTB7: PC71BM/PEDOT:PSS/AgNW device architecture (see Fig. 1). Since preliminary experiments indicated that the mechanical properties
Fig. 2. Relative change of the resistance (ΔR/R0) versus strain (εeng) for solution processed MOD silver (Ag) and PEDOT:PSS/AgNW electrodes on PET foil in comparison to the predicted change based on geometric considerations, a commercially available ITO electrode and a vacuum deposited Ag electrode.
Fig. 1. (a) Device architecture and (b) chemical structures of PBDTT-FTTE and PTB7. (c) Photo of the all-solution processed, mechanically flexible solar cell.
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After investigating the properties of the as prepared electrode samples, we examined the changes of the electrical properties under strain. Therefore, we performed in situ tensile tests on both the MOD silver and the PEDOT:PSS/AgNW electrodes, respectively. As suggested by Lu et al. [20], based on purely geometric considerations, i.e. the change of the film dimension (length and thickness) under strain, the relative change of resistance (ΔR/R0)
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can be calculated as follows:
ΔR R0
¼ 2εeng þ εeng2
ð1Þ
where R0 represents the initial resistance and ΔR¼R R0 describes the absolute change of resistance. εeng is the engineering strain. Fig. 2 depicts ΔR/R0 measured in situ versus the engineering strain
Fig. 3. SEM images of solution processed (a) MOD silver layers and (b) PEDOT:PSS/AgNW electrodes at different strain. In MOD silver electrodes, we observed first cracks at strains εeng 49%. The silver nanowires started to fracture at strains εeng between 3% and 6% whereas no cracks were observed in the PEDOT:PSS layer.
Fig. 4. (a) J–V curves of all-solution processed organic solar cells on PET foil comprising PTB7:PC71BM or PBDTT-FTTE:PC71BM, respectively. (b) Picture of the tensile test setup under the solar simulator.
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for the MOD silver electrode and the PEDOT:PSS/AgNW electrode as well as, for reference, an ITO electrode, a vacuum processed silver layer and the theoretic prediction. The reference ITO electrode already failed above εeng ¼1% due to crack formation as observed in ITO electrodes before. [21,22] In contrast, ΔR/R0 for the MOD silver layer well followed the theoretic prediction up to εeng E8%. The deviation from the prediction towards higher εeng originates from the formation of cracks within the silver layer as observed in the SEM images in Fig. 3a. Whereas the MOD silver electrode exhibited an intact surface at low εeng, we observed first microcracks at tensile strains of εeng ¼ 9%. Towards higher strain (here exemplified at εeng ¼12%), we found notable damage of the silver surface. However, despite this layer damage, the silver electrode still exhibited 75% of its initial conductivity. We observed about the same behavior for evaporated reference silver electrodes, demonstrating the very good mechanical properties of the solution processed MOD silver layers. In contrast, the tensile strain much more affected the PEDOT:PSS/AgNW electrodes. The PEDOT:PSS/ AgNWs electrode did not show any significant change of ΔR/R0 for strain below εeng ¼3%. At a strain of εeng ¼6%, first fractures of the silver nanowires became visible in the SEM pictures in Fig. 3b that further evolved towards higher strain. Concomitantly, ΔR/R0 increased to 0.75 for εeng ¼ 12%. The outstanding mechanical properties of the MOD silver layer and the PEDOT:PSS/AgNW electrodes were ideal prerequisites for the fabrication of mechanically flexible organic solar cells as depicted in Fig. 1a. Fig. 4a shows the current density–voltage (J–V) curves of the respective PTB7:PC71BM and PBDTT-FTTE:PCB71M solar cells as well as devices comprising PTB7:PC71BM reference solar cells with an ITO bottom electrode and a thermally evaporated molybdenum oxide/silver top electrode. The electrical key performance data is summarized in Table 1. PTB7 solar cells showed similar opencircuit voltages Voc ¼ 72777 mV as compared to the ITO reference devices (Voc ¼732 mV711 mV). The fill factor FF¼5371% and the short circuit current density Jsc ¼10.770.7 mA/cm2 were slightly lower than in the reference devices exhibiting Jsc ¼ 11.670.2 mA/ cm2 and FF¼ 5671%, which we attribute to the low process temperature for the deposition of the PEDOT:PSS/AgNWs electrode. We note that annealing can improve the performance of the PEDOT: PSS/AgNW electrode, however, at the same time reducing the device power conversion efficiencies due to an adverse effect on the photoactive layer. Still, hero devices exhibited remarkable PCEs of η ¼4.2% and a maximum device output power Pmax ¼1.7 mW (on 0.8 0.5 cm2). In order to demonstrate the universality of the device architecture, we further fabricated solar cells comprising the photoactive polymer PBDTT-FTTE for light harvesting. The device PCE improved to η ¼ 4.8% exhibiting Voc ¼ 78074 mV, Jsc ¼ 11.470.3 mA/ cm2, FF¼5271% and a maximum device output power Pmax ¼ 1.9 mW. For the following device strain tests, however, we deliberately focus on PTB7 as absorber material. Finally, we tested the mechanical reliability of the all-solution processed devices by measuring the solar cell's performance under illumination versus tensile strain in situ. Therefore, we placed the micro tensile tester under the solar simulator, successively increased the strain and recorded J–V curves at least every Δεeng ¼2% (Fig. S3). Again, we observe an immediate failure of the ITO reference device
under strain. In contrast, we found that the strain does neither affect the Voc of the all-solution processed solar cells nor the device short circuit current Isc, but both remained stable throughout the experiment (Fig. 5a). We note that Jsc was reduced upon sample elongation and hence upon an increase of the effective device area. Since the area inherently changes during strain experiments, Jsc and PCE are
Table 1 Average key performance data of 3 devices.
Fig. 5. (a) The Voc's of both solar cells remain unchanged even at strains beyond 10%. The ISC of the ITO reference device drops to zero at εeng 4 2% whereas the ISC of the all-solution device remains about constant. (b) Influence of strain on fill factor and the solar cell output power. Whereas the reference solar cell with ITO electrode already fails at substrate elongations εeng 41%, the all-solution processed device can withstand strain up to εeng ¼ 14%. (c) Estimated series resistance of the allsolution processed solar cell as derived from the I–V curve in comparison to the combined resistance of the individually measured electrodes (MOD silver and PEDOT:PSS/AgNW) versus strain.
Device
Voc (mV) Jsc (mA/cm2) FF (%)
7277 7 10.7 7 0.7 PTB7:PC71BM (all-solution) PTB7:PC71BM (ITO-PET) 7327 11 11.6 7 0.2 PBDTT-FTTE:PC71BM (all-solution) 780 7 4 11.4 7 0.3
PCE (%)
537 1% 4.17 0.1 567 1 4.7 7 0.1 527 1 4.6 7 0.1
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not meaningful parameters anymore. Instead we assessed the short circuit current (Isc) and the device output power henceforth. Fig. 5b depicts the impact of strain on the fill factor and the device output power of a typical solar cell. Upon a tensile strain of εeng ¼14% the output power droped below 90% (rel.) of the initial output power. The direct comparison of the Voc, Isc, FF and device output power allows the conclusion that the fill factor is the main failure reason for the all-solution processed devices at high strain. In order to further analyze the origin of the fill factor degradation, we estimated the influence of our previously measured electrode degradation on the device properties by assessing the series resistance as derived from the I–V curves following the single-diode model: dV nkB=q ¼ þ Rs d I I sc þ I V=Rsh
ð2Þ
where n is the ideality factor, q the elementary charge, kB the Boltzmann constant, Rs the series resistance and Rsh the shunt resistance [23]. Accordingly, the performance parameters of the solar cells above as derived from the I–V curves allow an estimation of Rs versus strain. The comparison of Rs with the combined series resistance of the two electrodes in Fig. 5c shows that Rs is dominated by the electrodes. This in turn indicates that the degradation and decomposition of the electrodes under strain was also the main failure mechanism of the device and that device decomposition or delamination, as described by Dupont et al., may play only a minor role [24].
4. Conclusion In conclusion, we have fabricated efficient all-solution processed solar cells comprising MOD silver bottom and PEDOT:PSS/ AgNWs top electrodes on flexible PET substrates exhibiting PCEs of up to 4.8%. The mechanical reliability of the all-solution processed solar cells was investigated by assessing I–V curves that were recorded while applying tensile strain. Whereas conventional solar cells comprising ITO electrodes failed already at εeng 41%, the allsolution processed devices sustained tensile strains up to 14%. With respect to bending and for a substrate thickness of 175 mm such tensile strains correspond to a sample bending radius of about 0.6 mm. Additional tensile tests of the individual electrode materials enabled assignment of the solar cell degradation to the tensile failure of the electrodes.
Acknowledgments This work was carried out within the projects “DELICIOUS”, funded by the Baden-Württemberg Stiftung, and “POPUP”, funded by the German Federal Ministry for Education and Research (BMBF) under contract no. 03EK3501H. It was supported by the Helmholtz Virtual Institute VI-530. F.N. was supported by the Karlsruhe School of Optics and Photonics (KSOP).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2014.07. 005.
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