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All-solution processed organic solar cells with top illumination
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 4 5 3
All-solution processed organic solar cells with top illumination
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Bhushan R. Patil, Santhosh Shanmugam, Jean-Pierre Teunissen, Yulia Galagan ⇑
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Holst Centre – Solliance, P.O. Box 8550, 5605KN Eindhoven, The Netherlands
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a r t i c l e
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Article history: Received 15 December 2014 Received in revised form 23 February 2015 Accepted 27 February 2015 Available online xxxx Keywords: Organic solar cells Top illumination All-solution processed Non-transparent substrate
a b s t r a c t All-solution processed organic solar cells with inverted device architecture were demonstrated. Devices contain opaque bottom electrodes and semitransparent top electrodes, resulting in top illuminated devices. Nanoparticles-based Ag ink was used for inkjet printing both top and bottom electrodes. Semi-transparent top electrode consists of high conductivity PEDOT:PSS and Ag current collecting grids. Printed electrodes were compared to evaporated Ag electrodes (both top and bottom) and to ITO electrode in terms of transmittance, roughness, sheet resistance and device performance. All-solution processed devices with top illumination have average PCE of 2.4%, using P3HT:PCBM as photoactive layer. Top-illuminated devices with inverted architecture and bottom-illuminated device with conventional architecture, containing the identical layers, but in the reverse sequence, were then compared. Performed studies have revealed an advantage of inverted cell architecture. Ó 2015 Published by Elsevier B.V.
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1. Introduction
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Organic photovoltaics (OPVs) are a perfect complement to photovoltaic (PV) technology because of promising features such as low cost [1], high flexibility, and solution roll-to-roll processing [2–4]. One of the biggest challenges in roll-to-roll fabrication of OPVs is the replacement of Indium-Tin-Oxide (ITO) as a transparent bottom electrode. The main arguments to the replacement of ITO are the brittle nature and comparatively higher sheet resistance on flexible substrates. In addition to that a highly sophisticated vacuum fabrication processes and patterning of ITO are required. Owing to these bottlenecks, today enormous investigations are being made around the world to find an ideal replacement to ITO in OPVs. In the past decade, a lot of alternatives have been proposed. Researchers have explored many of the potential conductive coatings of ultra-thin metal films [5,6], highly conducting polymers [7], carbon nanotubes (CNTs) [8], graphene [9], metal nano-wires [10,11] etc. However, most of the proposed alternatives have sheet resistance comparable to or higher than ITO. To reduce the sheet resistance of electrodes, current collecting grids were introduced as an integral part of transparent electrodes [12–15]. Disadvantage of this approach is a topology of the grids, however this issue can be overcome by embedding the current collecting grids into substrates [16,17].
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⇑ Corresponding author. E-mail address: [email protected] (Y. Galagan).
An alternative approach, intensively presented in last few years, is manufacturing OPVs with top illumination, containing nontransparent bottom electrodes [18–25]. The advantage of this approach is very low sheet resistance of the electrodes, allowing to make large area devices [23]. Moreover, this approach does not have issues with topology of the grids and provide homogeneous surface for the deposition of subsequent layers. This approach has been intensively explored by the group from Fraunhofer ISE, where non-transparent bottom contact of Chromium/Aluminum/Chromium (Cr/Al/Cr) was evaporated through the shadow mask [18–21]. Janssen et al. have reported tandem polymer solar cells with non-transparent vacuum evaporated Chromium/Gold (Cr/Au) electrodes having top illumination [24]. Organic solar cells containing evaporated Al–Au bottom contacts with top illumination were demonstrated on stainless steel substrates by Chen et al. [22]. The device stack was electrically insulated from the conducting substrate by spin coating a photoresist. However, sometimes the electrical properties of the metal substrate were utilized and served as one of the electrodes in the OPV stack. Such type of OPV devices were demonstrated on stainless steel [23,25], titanium coated steel [25] or chromium coated aluminum [25]. Although direct deposition of OPV stack on metal substrates allows all-solution processing the following layers, this type of the devices is better suitable for the lab applications than for manufacturing large area modules, because of the interconnection issues and safety issues. The upscaling of OPV modules with top illumination requires a Roll-to-Roll deposition of the non-transparent bottom electrode. Roll-to-Roll printing and coating is the most attractive deposition
http://dx.doi.org/10.1016/j.orgel.2015.02.028 1566-1199/Ó 2015 Published by Elsevier B.V.
Please cite this article in press as: B.R. Patil et al., All-solution processed organic solar cells with top illumination, Org. Electron. (2015), http://dx.doi.org/ 10.1016/j.orgel.2015.02.028
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method which can guaranty low cost manufacturing. Silver (Ag) is considered as a potential candidate for Roll-to-Roll solution processing, suitable for the application in OPV devices. Thin layer of printed Ag provides low sheet resistance, allowing large area devices and has high flexibility. Solution processed opaque bottom metallic electrode was demonstrated by doctor blading of organometallic silver precursor ink [26,27]. However, for the application of this silver coating as an electrode in the OPV devices, the post structuring either with laser ablation [26] or photolithography [27] was required. Krebs et al. [28] have demonstrated R2R processed semitransparent bottom Ag electrode with 25–30% transmittance at 550 nm. However, due to the low transmittance of such electrode, top illumination would be preferred. Later, the same group has demonstrated slot die coated non-transparent Ag film deposited from organometallic complexes and employed in the processing of top-illuminated ITO-free polymer solar cells in single-junction and tandem structures [29]. The usage of nanoparticles-based Ag inks as an opaque bottom electrode also was shown [30,31], however all-solution processed devices demonstrated efficiency below 1%. Moreover, for top-illuminated devices a semitransparent top electrode is required. A lot of studies have been done on the optimization of thermally evaporated top contacts in the organic solar cells adapted for non-transparent substrates [23,32,33]. However, because the ultimate goal is all-solution processing, the investigation of printed alternatives are in the focus for many research groups. Combination of PEDOT:PSS with printed current collecting grids is considered as a promising alternative. However, as has been shown by Angmo et al., to prevent solvent penetration from the ink during solution processing of Ag, either thick PEDOT:PSS layer (that limits the transparency of the electrode) [34], or thick photoactive is required [29]. To overcome this problem aerosol jet printed grids have been demonstrated [19,35]. The advantage of this approach is that the most of the solvents were evaporated during the deposition, allowing to use relatively thin PEDOT:PSS. Recently our group has demonstrated PEDOT:PSS formulation with improved nature allowing to make only 40 nm layer of PEDOT:PSS which completely prevents the penetration of the solvents from Ag inks deposited either by inkjet or screen printing and provides high transmittance of the electrode [13]. In this study, we have investigated the potential of inkjet printed silver layer, deposited from Ag nanoparticles ink, as a non-transparent bottom electrode and inkjet printed Ag grids as a top electrode for organic solar cells with top illumination. Allsolution processed ITO-free organic solar cells with inverted architecture were demonstrated.
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2. Experimental
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The substrates for fabricating OPV devices were either 30 30 mm glass slides (Eagle XG, Corning Inc., USA) or ITO covered glass substrates (sheet resistance of 10 X/h) patterned by photolithography (Naranjo, the Netherlands). Substrates were cleaned with several rinsing steps including ultrasonic treatment with Extran industrial detergent, deionized (DI) water and isopropanol (IPA). Bottom Ag electrode was made either by thermal evaporation or inkjet printing of Ag nano-particles ink. Evaporation was carried out using a shadow mask in a vacuum thermal evaporation chamber at base pressure below 2 10 6 mbar inside N2 environment. Ag slug for evaporation was purchased from Sigma–Aldrich Corporation. The Ag nano-particles ink (SunTronic U5603) was purchased from Sun chemicals, UK. The Ag ink was inkjet printed by DMP-2800 (Dimatix-Fujifilm Inc., USA) printer, equipped with a 10 pL cartridge (DMC-11610, Dimatix-Fujifilm Inc., USA). Printing
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was performed with a drop space of 20 lm, a voltage of 20 V, a print head temperature of 30 °C, a frequency of 10 kHz and a customized waveform. After printing, the samples were sintered for 30 min at 130 °C in an hot air oven. Alternative annealing for the printed Ag electrode can be photonic flash sintering, which can provide the resistance of the electrode even lower [36], than after 30 min of thermal annealing. The thicknesses of bottom electrodes: ITO, evaporated Ag and ink-jet printed Ag layers were 135, 100 and 250–300 nm, respectively. To fabricate electron transport layer (ETL), a zinc oxide (ZnO) nanoparticles (np) solution in acetone was used. Synthesis of ZnO np was performed using a modified hydrothermal condensation process [37] where nano-particles (diameter of 1–5 nm) were re-dispersed in acetone at a concentration of 10 mg/ml. The solution was spin coated in air at 1000 rpm for 1 min with 5000 rpm/s acceleration. Thickness of ZnO layer was measured in the range of 30–40 nm. Photoactive layer (PAL) solution was prepared by mixing poly(3-hexylthiophene) (P3HT, Plexcore 2100, Plextronics Inc., USA) and [6,6] phenyl-C61-butyric acid methyl ester (PC60BM, 99%, Solenne B.V., The Netherlands) with weight ratio of 1:1 and at a concentration of 26 mg/ml in ortho-dichlorobenzene (oDCB). Solution was prepared in ambient conditions and kept stirring for 16 h at 90 °C. PAL was fabricated by spin coating the solution at 1000 rpm for 1 min with 1000 rpm/s acceleration inside nitrogen (N2) glove box and annealed at 130 °C for 10 min. The measured PAL thickness was approximately of 240 nm. A highly conductive PEDOT:PSS (Orgacon S315, Agfa-Gevaert N.V., Belgium) was used as an hole transport layer (HTL). PEDOT:PSS (approx. 120 nm) was spin coated at 1000 rpm for 1 min with 1000 rpm/s acceleration and dried in air for 2 min at 120 °C. Then, PEDOT:PSS layer was annealed inside N2 glove box at 120 °C for 8 min. A top electrode was either thermally evaporated or inkjet printed, using the same method and materials as for preparation of bottom Ag electrode. The top electrode has a current collecting grids pattern. The thicknesses of evaporated and inkjet printed top grids electrodes were 200 and 250–300 nm, respectively. Sintering of top electrode was done for 20 min at 130 °C in an hot air oven. Longer sintering time might increase conductivity of inkjet printed Ag electrode. However, it can lead to a degradation of the photoactive and PEDOT:PSS layers, because sintering is performed in air. As a result, the performance of the photovoltaic devices decreases. The optimal sintering condition has been found to be between 10 and 20 min at 130 °C [13]. The grid electrode consists of three lines with a pitch size of 2 mm. The width of evaporated grids was exactly 150 lm, providing the grids surface coverage of 7.5%. While, the width of inkjet printed grid lines was in average of 250 lm, providing the grids surface coverage of 12.5%. Because a top PEDOT:PSS/Ag-grid electrode does not have a well-defined area, an illumination mask was used to have precise active area of 0.25 cm2. Current–density voltage (JV) characteristics and external quantum efficiency (EQE) measurements were performed in a N2 glove box with a source meter (Keithley 2400) between 2 and 2 V using simulated AM 1.5 global solar radiations (100 mW/cm2) with a halogen lamp. Power conversion efficiencies were calculated using the short-circuit current density obtained from the EQE measurement. The average performance of at least 8–10 identical devices is reported. The layers thicknesses were measured with an optical profilometer Wyko NT9100 (Veeco, Mannheim, Germany). The surface roughness was analyzed with optical interferometry. The measurements were performed with a Bruker NPFlex 3D Optical Profiler, using the phase-shifting interferometry mode with green light. AFM analysis was performed with Park NX10 system.
Please cite this article in press as: B.R. Patil et al., All-solution processed organic solar cells with top illumination, Org. Electron. (2015), http://dx.doi.org/ 10.1016/j.orgel.2015.02.028
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Fig. 1. Schematic illustration of a reference ITO-based device and top-illuminated device with opaque bottom electrode.
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3. Results and discussion
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The objective of this study was to fabricate all-solution processed OPV devices with top illumination. In this approach a standard ITO electrode was substituted by non-transparent inkjet printed Ag layer, as shown in Fig. 1. This type of devices is illuminated from the top side, which requires semitransparent top electrode. While the reference ITO-based devices can be measured by illumination from both sides. In order to reach the objective of the study, the reference semitransparent ITO-based devices were fabricated. Semitransparent top (back) electrode consisted highly conducting PEDOT:PSS and Ag current collecting grids. Current collecting grids were initially evaporated with a shadow mask and later substituted by printed version. Evaporated grids have a pitch size of 2 mm and a width of 150 lm, resulting in 7.5% area coverage. Inkjet printing of Ag
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grids was performed using SunTronic U5603 nanoparticle ink [13]. This ink has very low contact angle on PEDOT:PSS surface resulting in an extra spread of the ink. It leads to a high surface coverage, which is undesirable for the top illumination devices. The printing was performed with a dot pitch of 20 lm. The set width of printed Ag grid lines was 100 lm, which results in an actual width of 200–250 lm. A total grids coverage with a pitch size of 2 mm was of about 12.5%. This combination of inkjet printed Ag grids and PEDOT:PSS provides sheet resistance of 2–3 X/h [38]. Semitransparent ITO-based devices with evaporated and inkjet printed top (back) grids electrode were measured by illumination from both sides. The JV curves of the devices are shown in Fig. 2 and photovoltaic parameters are summarized in Table 1. Device with evaporated grids have slightly lower FF for illumination from bottom (ITO side), but slightly higher Jsc, compared to top illumination (Ag grids side). The same trend in FF difference for top and bottom illumination is also observed in the device with inkjet printed Ag grids. However, top and bottom illumination of the device with inkjet printed Ag grids shows quite big difference in Jsc, The device with inkjet printed Ag grids shows a significant drop in Jsc when illuminated from top (Ag side), compared to bottom illumination, which can be explained by the difference in transmittance due to grids coverage, 7.5% versus 12.5% for evaporated and inkjet printed grids, respectively. Transmission spectra of semitransparent electrodes are shown in Fig. 3. The front ITO side of the device contains glass substrate, therefore the transmittance of glass was also taken into account. While, the back PEDOT/A-grid side of the device does not have substrate material, therefor influence of the substrate was eliminated by baseline corrections. The Voc of the devices with printed Ag electrode was slightly lower compare to the devices with evaporated back contact. An ideal interface requires the formation of Ohmic contact with minimum resistance. Non-Ohmic contacts lead to variety of interfacial effects including charge transfer, dipole formation, and the formation of interface states [15], which negatively affect Voc [39]. Next step towards the objective was a replacement of ITO electrode by non-transparent Ag film. Ag layer with thickness of
Fig. 2. JV curves of reference ITO-based OPV devices with evaporated and ink jet printed Ag grids top electrode. Devices measured with front (bottom) and back (top) illumination.
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Table 1 Average photovoltaic parameters of the devices. Bottom electrode
Illumination
Top grids electrode
Voc (V)
Jsc (mA/cm2)
FF (%)
g (%)
Jsc
ITO ITO ITO ITO Ev. Ag IJP Ag IJP Ag
Bottom Top Bottom Top Top Top Top
Ev. Ag Ev. Ag IJP Ag IJP Ag Ev. Ag Ev. Ag IJP Ag
0.549 0.562 0.539 0.529 0.563 0.571 0.564
7.74 7.56 7.93 6.89 8.52 7.74 7.11
62.9 65.2 59.6 63.1 62.4 62.5 62.6
2.7 2.8 2.6 2.3 3.0 2.8 2.5
7.81 7.52 8.25 6.51 8.21 7.62 6.77
Fig. 3. Transmission spectra of semitransparent electrodes. 274 275 276 277
100 nm was deposited via thermal evaporation using shadow mask with the same pattern as ITO electrode. Devices with evaporated bottom Ag have Jsc higher than identical devices with ITO electrode (Table 1), which are 8.21 and 7.52 mA/cm2, respectively, for the
EQE
(mA/cm2)
gEQE (%) 2.7 2.8 2.7 2.2 2.9 2.7 2.4
devices with evaporated top grids electrodes. Increased Jsc attributed to the reflectivity of Ag electrode. Inkjet printed layer of silver served as a bottom electrode has a thickness of 300 nm, with resistivity of 1.88 10–7 X m, which provides sheet resistance of 0.63 X/h. Devices with inkjet printed bottom Ag electrode and evaporated top grids electrode have average PCE of 2.72%, while devices with two evaporated electrodes have average PCE of 2.88% (see Fig. 4 and Table 1). The difference in the performance occurs due to difference in Jsc, which are 7.62 and 8.21 mA/cm2, for the devices with ink jet printed and evaporated bottom Ag electrode, respectively. The difference in Jsc can be explained by the morphology of the electrode materials (see Fig. 5). Evaporated Ag layer has highly crystalline and therefore rougher structure (root mean square roughness Rms = 13.65 nm), allowing the creation of surface plasmon polaritons (SPPs) [40]. It is well known that integration of metallic plasmonic nanostructures into electrodes material can be very beneficial for OPV devices [39,40], due to improved light trapping. Inkjet printed Ag nanoparticles has average diameter of 40 nm and therefor form smoother surface (Rms = 8.05 nm). Moreover, presence of residual organic materials in Ag nanoparticles layer can reduce reflectivity of the electrode. Nevertheless, the surface plasmons of metal nanoparticles can be used for the fine tuning of light trapping on OPV devices, because interaction of surface
Fig. 4. JV curves of top illuminated OPV devices.
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Fig. 5. AFM images (phase and roughness) of evaporated (a) and inkjet printed (b) Ag electrode.
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plasmons is very sensitive to the size and shape of the particles [43,44]. The layer uniformity of evaporated and inkjet printed Ag films were measured with optical interferometry. The resulting height images and line sections obtained from the height images are presented in Fig. 6. The figure depicts printing defects in inkjet printed layer, while the evaporated Ag film has very smooth profile. The roughness values, root mean square height (Rms), were 0.37 and 1.9 nm for evaporated Ag and inkjet printed Ag, respectively. The roughness profile of inkjet printed Ag film has slightly striped pattern due to merging of individual printed lines at inkjet printing process. However, no spikes or holes we found in the inkjet printing layer, making it fully compatible for the manufacturing OPV devices. Finally, all-solution processed devices with inkjet printed top (grids) and bottom (full area) Ag electrodes were fabricated. Devices with two inkjet printed electrodes have lower Jsc and
PCE compare to the device with evaporated top grids electrode (see Fig. 4), due to grids surface coverage. However, these devices have slightly higher PCE compared to ITO-based device with inkjet printed top electrode (see Table 1), due to reflectivity of Ag. Devices with two inkjet printed Ag electrodes have average PCE of 2.4%, which can be further improved by increasing of transparency (grids surface coverage) of top electrode, improving the quality and reflectivity of bottom inkjet printed Ag electrode and optimization of layer thicknesses in the devices, which can be done via optical modeling. The all-solution processed devices in this study have so-called inverted cell architecture and have absolutely the same layer sequence as reported earlier [15], but only in reverse order (see Fig. 7a). Typical JV curves of such devices are shown in Fig. 7b. The JV curves show big difference in Voc for conventional and inverted devices. The difference in Voc between regular and inverted devices have been observed numerous times [45,46]. However
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Fig. 6. Optical interferometry images and roughness of evaporated (a) and inkjet printed (b) Ag films.
Fig. 7. (a) Schematic illustration of all-solution processed organic solar cells with inverted and conventional architecture, and (b) JV curves of these devices.
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in most of the cases the difference can be explained by the usage of different electron and hole transport materials and different contact materials. Petoukhoff et al. [45] have calculated the electronic
and optical performance parameters for various metal electrodes, with and without metal oxide/fluoride coatings for inverted and conventional bulk-heterojunction organic photovoltaic devices. They have concluded that the presence of strong interface dipoles (at the cathode or anode of the conventional or inverted devices respectively), which may form a strong interacting layer, reduces the energy loss for the charge collection at the respective electrode, and increased Voc. Boix et al. [46] systematically observed that the open-circuit voltage Voc at 1 sun illumination results higher for inverted cells than that achieved by regular structures in DVoc 30–50 mV (both types of devices contain PEDOT:PSS, as hole transport layer, and ZnO, as electron transport layer). This shift correlates with the displacement of the flat-band potential Vfb extracted from Mott Schottky capacitance analysis. A coherent picture is provided that states the hole Fermi level of the polymer highest occupied molecular orbital as an energy reference for both Voc and Vfb. The study connects the position of the hole Fermi level to the p-doping character of the active layer that is influenced by the film morphology through vertical phase segregation. Finally, Voc is determined by the quasi Fermi level splitting of the electron and hole and in organic solar cells maximized by the energy of the charge transfer state, provided that the electrodes form Ohmic contacts [39,47,48]. For non-Ohmic contacts, Voc will be less and limited by the work function of the electrode [39]. We note that the nature of electronic contact formation between the electrode and active layer can be complicated since a wide variety of interfacial effects including charge transfer, dipole formation, and the formation of interface states can occur depending on the type and strength of interactions between the two materials and the type of contact formation [49]. These conclusions agrees very well with our results. In the devices containing absolutely the same layers (all-solution processes conventional and inverted devices as shown in Fig. 7) the difference in Voc can be explained only by the order of layers deposition. Vertical phase separation in P3HT:PCM layer suggests a significant increase of the P3HT crystallinity at the top regions of the blend films with additional PCBM segregated to the bottom interface [50–52]. The vertical phase separation is attributed to the surface energy difference of the components and their interactions
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with the substrates material. This inhomogeneous distribution of the donor and acceptor components significantly affects the photovoltaic device performance and makes the inverted device structure a promising choice. However, at the current stage of research this has been proven only for P3HT:PCBM as a photoactive material, other absorber materials have to be further investigated.
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4. Conclusions
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All-solution processed organic solar cells with inverted device architecture were demonstrated. Conventional ITO electrode was substituted by non-transparent inkjet printed layer of Ag. Devices with top illumination were fabricated. Semitransparent top electrode consists of highly conducting PEDOT:PSS and inkjet printed current collecting grids. All-solution processed devices with two inkjet printed Ag electrodes have average PCE of 2.4%, which can be further improved by increasing of transparency (grids surface coverage) of top electrode, improving the quality and reflectivity of bottom inkjet printed Ag electrode and optimization of layer thicknesses in the devices. Comparison of all-solution processed devices with conventional and inverted architectures revealed a big difference in Voc due to the difference in the morphology of P3HT:PCBM layer through vertical phase segregation. This significantly affects PCE and makes the inverted device structure a promising choice.
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[41,42].
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 4 5 3
All-solution processed organic solar cells with top illumination
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Bhushan R. Patil, Santhosh Shanmugam, Jean-Pierre Teunissen, Yulia Galagan ⇑
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Holst Centre – Solliance, P.O. Box 8550, 5605KN Eindhoven, The Netherlands
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a r t i c l e
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Article history: Received 15 December 2014 Received in revised form 23 February 2015 Accepted 27 February 2015 Available online xxxx Keywords: Organic solar cells Top illumination All-solution processed Non-transparent substrate
a b s t r a c t All-solution processed organic solar cells with inverted device architecture were demonstrated. Devices contain opaque bottom electrodes and semitransparent top electrodes, resulting in top illuminated devices. Nanoparticles-based Ag ink was used for inkjet printing both top and bottom electrodes. Semi-transparent top electrode consists of high conductivity PEDOT:PSS and Ag current collecting grids. Printed electrodes were compared to evaporated Ag electrodes (both top and bottom) and to ITO electrode in terms of transmittance, roughness, sheet resistance and device performance. All-solution processed devices with top illumination have average PCE of 2.4%, using P3HT:PCBM as photoactive layer. Top-illuminated devices with inverted architecture and bottom-illuminated device with conventional architecture, containing the identical layers, but in the reverse sequence, were then compared. Performed studies have revealed an advantage of inverted cell architecture. Ó 2015 Published by Elsevier B.V.
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1. Introduction
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Organic photovoltaics (OPVs) are a perfect complement to photovoltaic (PV) technology because of promising features such as low cost [1], high flexibility, and solution roll-to-roll processing [2–4]. One of the biggest challenges in roll-to-roll fabrication of OPVs is the replacement of Indium-Tin-Oxide (ITO) as a transparent bottom electrode. The main arguments to the replacement of ITO are the brittle nature and comparatively higher sheet resistance on flexible substrates. In addition to that a highly sophisticated vacuum fabrication processes and patterning of ITO are required. Owing to these bottlenecks, today enormous investigations are being made around the world to find an ideal replacement to ITO in OPVs. In the past decade, a lot of alternatives have been proposed. Researchers have explored many of the potential conductive coatings of ultra-thin metal films [5,6], highly conducting polymers [7], carbon nanotubes (CNTs) [8], graphene [9], metal nano-wires [10,11] etc. However, most of the proposed alternatives have sheet resistance comparable to or higher than ITO. To reduce the sheet resistance of electrodes, current collecting grids were introduced as an integral part of transparent electrodes [12–15]. Disadvantage of this approach is a topology of the grids, however this issue can be overcome by embedding the current collecting grids into substrates [16,17].
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⇑ Corresponding author. E-mail address: [email protected] (Y. Galagan).
An alternative approach, intensively presented in last few years, is manufacturing OPVs with top illumination, containing nontransparent bottom electrodes [18–25]. The advantage of this approach is very low sheet resistance of the electrodes, allowing to make large area devices [23]. Moreover, this approach does not have issues with topology of the grids and provide homogeneous surface for the deposition of subsequent layers. This approach has been intensively explored by the group from Fraunhofer ISE, where non-transparent bottom contact of Chromium/Aluminum/Chromium (Cr/Al/Cr) was evaporated through the shadow mask [18–21]. Janssen et al. have reported tandem polymer solar cells with non-transparent vacuum evaporated Chromium/Gold (Cr/Au) electrodes having top illumination [24]. Organic solar cells containing evaporated Al–Au bottom contacts with top illumination were demonstrated on stainless steel substrates by Chen et al. [22]. The device stack was electrically insulated from the conducting substrate by spin coating a photoresist. However, sometimes the electrical properties of the metal substrate were utilized and served as one of the electrodes in the OPV stack. Such type of OPV devices were demonstrated on stainless steel [23,25], titanium coated steel [25] or chromium coated aluminum [25]. Although direct deposition of OPV stack on metal substrates allows all-solution processing the following layers, this type of the devices is better suitable for the lab applications than for manufacturing large area modules, because of the interconnection issues and safety issues. The upscaling of OPV modules with top illumination requires a Roll-to-Roll deposition of the non-transparent bottom electrode. Roll-to-Roll printing and coating is the most attractive deposition
http://dx.doi.org/10.1016/j.orgel.2015.02.028 1566-1199/Ó 2015 Published by Elsevier B.V.
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method which can guaranty low cost manufacturing. Silver (Ag) is considered as a potential candidate for Roll-to-Roll solution processing, suitable for the application in OPV devices. Thin layer of printed Ag provides low sheet resistance, allowing large area devices and has high flexibility. Solution processed opaque bottom metallic electrode was demonstrated by doctor blading of organometallic silver precursor ink [26,27]. However, for the application of this silver coating as an electrode in the OPV devices, the post structuring either with laser ablation [26] or photolithography [27] was required. Krebs et al. [28] have demonstrated R2R processed semitransparent bottom Ag electrode with 25–30% transmittance at 550 nm. However, due to the low transmittance of such electrode, top illumination would be preferred. Later, the same group has demonstrated slot die coated non-transparent Ag film deposited from organometallic complexes and employed in the processing of top-illuminated ITO-free polymer solar cells in single-junction and tandem structures [29]. The usage of nanoparticles-based Ag inks as an opaque bottom electrode also was shown [30,31], however all-solution processed devices demonstrated efficiency below 1%. Moreover, for top-illuminated devices a semitransparent top electrode is required. A lot of studies have been done on the optimization of thermally evaporated top contacts in the organic solar cells adapted for non-transparent substrates [23,32,33]. However, because the ultimate goal is all-solution processing, the investigation of printed alternatives are in the focus for many research groups. Combination of PEDOT:PSS with printed current collecting grids is considered as a promising alternative. However, as has been shown by Angmo et al., to prevent solvent penetration from the ink during solution processing of Ag, either thick PEDOT:PSS layer (that limits the transparency of the electrode) [34], or thick photoactive is required [29]. To overcome this problem aerosol jet printed grids have been demonstrated [19,35]. The advantage of this approach is that the most of the solvents were evaporated during the deposition, allowing to use relatively thin PEDOT:PSS. Recently our group has demonstrated PEDOT:PSS formulation with improved nature allowing to make only 40 nm layer of PEDOT:PSS which completely prevents the penetration of the solvents from Ag inks deposited either by inkjet or screen printing and provides high transmittance of the electrode [13]. In this study, we have investigated the potential of inkjet printed silver layer, deposited from Ag nanoparticles ink, as a non-transparent bottom electrode and inkjet printed Ag grids as a top electrode for organic solar cells with top illumination. Allsolution processed ITO-free organic solar cells with inverted architecture were demonstrated.
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2. Experimental
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The substrates for fabricating OPV devices were either 30 30 mm glass slides (Eagle XG, Corning Inc., USA) or ITO covered glass substrates (sheet resistance of 10 X/h) patterned by photolithography (Naranjo, the Netherlands). Substrates were cleaned with several rinsing steps including ultrasonic treatment with Extran industrial detergent, deionized (DI) water and isopropanol (IPA). Bottom Ag electrode was made either by thermal evaporation or inkjet printing of Ag nano-particles ink. Evaporation was carried out using a shadow mask in a vacuum thermal evaporation chamber at base pressure below 2 10 6 mbar inside N2 environment. Ag slug for evaporation was purchased from Sigma–Aldrich Corporation. The Ag nano-particles ink (SunTronic U5603) was purchased from Sun chemicals, UK. The Ag ink was inkjet printed by DMP-2800 (Dimatix-Fujifilm Inc., USA) printer, equipped with a 10 pL cartridge (DMC-11610, Dimatix-Fujifilm Inc., USA). Printing
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was performed with a drop space of 20 lm, a voltage of 20 V, a print head temperature of 30 °C, a frequency of 10 kHz and a customized waveform. After printing, the samples were sintered for 30 min at 130 °C in an hot air oven. Alternative annealing for the printed Ag electrode can be photonic flash sintering, which can provide the resistance of the electrode even lower [36], than after 30 min of thermal annealing. The thicknesses of bottom electrodes: ITO, evaporated Ag and ink-jet printed Ag layers were 135, 100 and 250–300 nm, respectively. To fabricate electron transport layer (ETL), a zinc oxide (ZnO) nanoparticles (np) solution in acetone was used. Synthesis of ZnO np was performed using a modified hydrothermal condensation process [37] where nano-particles (diameter of 1–5 nm) were re-dispersed in acetone at a concentration of 10 mg/ml. The solution was spin coated in air at 1000 rpm for 1 min with 5000 rpm/s acceleration. Thickness of ZnO layer was measured in the range of 30–40 nm. Photoactive layer (PAL) solution was prepared by mixing poly(3-hexylthiophene) (P3HT, Plexcore 2100, Plextronics Inc., USA) and [6,6] phenyl-C61-butyric acid methyl ester (PC60BM, 99%, Solenne B.V., The Netherlands) with weight ratio of 1:1 and at a concentration of 26 mg/ml in ortho-dichlorobenzene (oDCB). Solution was prepared in ambient conditions and kept stirring for 16 h at 90 °C. PAL was fabricated by spin coating the solution at 1000 rpm for 1 min with 1000 rpm/s acceleration inside nitrogen (N2) glove box and annealed at 130 °C for 10 min. The measured PAL thickness was approximately of 240 nm. A highly conductive PEDOT:PSS (Orgacon S315, Agfa-Gevaert N.V., Belgium) was used as an hole transport layer (HTL). PEDOT:PSS (approx. 120 nm) was spin coated at 1000 rpm for 1 min with 1000 rpm/s acceleration and dried in air for 2 min at 120 °C. Then, PEDOT:PSS layer was annealed inside N2 glove box at 120 °C for 8 min. A top electrode was either thermally evaporated or inkjet printed, using the same method and materials as for preparation of bottom Ag electrode. The top electrode has a current collecting grids pattern. The thicknesses of evaporated and inkjet printed top grids electrodes were 200 and 250–300 nm, respectively. Sintering of top electrode was done for 20 min at 130 °C in an hot air oven. Longer sintering time might increase conductivity of inkjet printed Ag electrode. However, it can lead to a degradation of the photoactive and PEDOT:PSS layers, because sintering is performed in air. As a result, the performance of the photovoltaic devices decreases. The optimal sintering condition has been found to be between 10 and 20 min at 130 °C [13]. The grid electrode consists of three lines with a pitch size of 2 mm. The width of evaporated grids was exactly 150 lm, providing the grids surface coverage of 7.5%. While, the width of inkjet printed grid lines was in average of 250 lm, providing the grids surface coverage of 12.5%. Because a top PEDOT:PSS/Ag-grid electrode does not have a well-defined area, an illumination mask was used to have precise active area of 0.25 cm2. Current–density voltage (JV) characteristics and external quantum efficiency (EQE) measurements were performed in a N2 glove box with a source meter (Keithley 2400) between 2 and 2 V using simulated AM 1.5 global solar radiations (100 mW/cm2) with a halogen lamp. Power conversion efficiencies were calculated using the short-circuit current density obtained from the EQE measurement. The average performance of at least 8–10 identical devices is reported. The layers thicknesses were measured with an optical profilometer Wyko NT9100 (Veeco, Mannheim, Germany). The surface roughness was analyzed with optical interferometry. The measurements were performed with a Bruker NPFlex 3D Optical Profiler, using the phase-shifting interferometry mode with green light. AFM analysis was performed with Park NX10 system.
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Fig. 1. Schematic illustration of a reference ITO-based device and top-illuminated device with opaque bottom electrode.
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3. Results and discussion
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The objective of this study was to fabricate all-solution processed OPV devices with top illumination. In this approach a standard ITO electrode was substituted by non-transparent inkjet printed Ag layer, as shown in Fig. 1. This type of devices is illuminated from the top side, which requires semitransparent top electrode. While the reference ITO-based devices can be measured by illumination from both sides. In order to reach the objective of the study, the reference semitransparent ITO-based devices were fabricated. Semitransparent top (back) electrode consisted highly conducting PEDOT:PSS and Ag current collecting grids. Current collecting grids were initially evaporated with a shadow mask and later substituted by printed version. Evaporated grids have a pitch size of 2 mm and a width of 150 lm, resulting in 7.5% area coverage. Inkjet printing of Ag
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grids was performed using SunTronic U5603 nanoparticle ink [13]. This ink has very low contact angle on PEDOT:PSS surface resulting in an extra spread of the ink. It leads to a high surface coverage, which is undesirable for the top illumination devices. The printing was performed with a dot pitch of 20 lm. The set width of printed Ag grid lines was 100 lm, which results in an actual width of 200–250 lm. A total grids coverage with a pitch size of 2 mm was of about 12.5%. This combination of inkjet printed Ag grids and PEDOT:PSS provides sheet resistance of 2–3 X/h [38]. Semitransparent ITO-based devices with evaporated and inkjet printed top (back) grids electrode were measured by illumination from both sides. The JV curves of the devices are shown in Fig. 2 and photovoltaic parameters are summarized in Table 1. Device with evaporated grids have slightly lower FF for illumination from bottom (ITO side), but slightly higher Jsc, compared to top illumination (Ag grids side). The same trend in FF difference for top and bottom illumination is also observed in the device with inkjet printed Ag grids. However, top and bottom illumination of the device with inkjet printed Ag grids shows quite big difference in Jsc, The device with inkjet printed Ag grids shows a significant drop in Jsc when illuminated from top (Ag side), compared to bottom illumination, which can be explained by the difference in transmittance due to grids coverage, 7.5% versus 12.5% for evaporated and inkjet printed grids, respectively. Transmission spectra of semitransparent electrodes are shown in Fig. 3. The front ITO side of the device contains glass substrate, therefore the transmittance of glass was also taken into account. While, the back PEDOT/A-grid side of the device does not have substrate material, therefor influence of the substrate was eliminated by baseline corrections. The Voc of the devices with printed Ag electrode was slightly lower compare to the devices with evaporated back contact. An ideal interface requires the formation of Ohmic contact with minimum resistance. Non-Ohmic contacts lead to variety of interfacial effects including charge transfer, dipole formation, and the formation of interface states [15], which negatively affect Voc [39]. Next step towards the objective was a replacement of ITO electrode by non-transparent Ag film. Ag layer with thickness of
Fig. 2. JV curves of reference ITO-based OPV devices with evaporated and ink jet printed Ag grids top electrode. Devices measured with front (bottom) and back (top) illumination.
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Table 1 Average photovoltaic parameters of the devices. Bottom electrode
Illumination
Top grids electrode
Voc (V)
Jsc (mA/cm2)
FF (%)
g (%)
Jsc
ITO ITO ITO ITO Ev. Ag IJP Ag IJP Ag
Bottom Top Bottom Top Top Top Top
Ev. Ag Ev. Ag IJP Ag IJP Ag Ev. Ag Ev. Ag IJP Ag
0.549 0.562 0.539 0.529 0.563 0.571 0.564
7.74 7.56 7.93 6.89 8.52 7.74 7.11
62.9 65.2 59.6 63.1 62.4 62.5 62.6
2.7 2.8 2.6 2.3 3.0 2.8 2.5
7.81 7.52 8.25 6.51 8.21 7.62 6.77
Fig. 3. Transmission spectra of semitransparent electrodes. 274 275 276 277
100 nm was deposited via thermal evaporation using shadow mask with the same pattern as ITO electrode. Devices with evaporated bottom Ag have Jsc higher than identical devices with ITO electrode (Table 1), which are 8.21 and 7.52 mA/cm2, respectively, for the
EQE
(mA/cm2)
gEQE (%) 2.7 2.8 2.7 2.2 2.9 2.7 2.4
devices with evaporated top grids electrodes. Increased Jsc attributed to the reflectivity of Ag electrode. Inkjet printed layer of silver served as a bottom electrode has a thickness of 300 nm, with resistivity of 1.88 10–7 X m, which provides sheet resistance of 0.63 X/h. Devices with inkjet printed bottom Ag electrode and evaporated top grids electrode have average PCE of 2.72%, while devices with two evaporated electrodes have average PCE of 2.88% (see Fig. 4 and Table 1). The difference in the performance occurs due to difference in Jsc, which are 7.62 and 8.21 mA/cm2, for the devices with ink jet printed and evaporated bottom Ag electrode, respectively. The difference in Jsc can be explained by the morphology of the electrode materials (see Fig. 5). Evaporated Ag layer has highly crystalline and therefore rougher structure (root mean square roughness Rms = 13.65 nm), allowing the creation of surface plasmon polaritons (SPPs) [40]. It is well known that integration of metallic plasmonic nanostructures into electrodes material can be very beneficial for OPV devices [39,40], due to improved light trapping. Inkjet printed Ag nanoparticles has average diameter of 40 nm and therefor form smoother surface (Rms = 8.05 nm). Moreover, presence of residual organic materials in Ag nanoparticles layer can reduce reflectivity of the electrode. Nevertheless, the surface plasmons of metal nanoparticles can be used for the fine tuning of light trapping on OPV devices, because interaction of surface
Fig. 4. JV curves of top illuminated OPV devices.
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Fig. 5. AFM images (phase and roughness) of evaporated (a) and inkjet printed (b) Ag electrode.
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plasmons is very sensitive to the size and shape of the particles [43,44]. The layer uniformity of evaporated and inkjet printed Ag films were measured with optical interferometry. The resulting height images and line sections obtained from the height images are presented in Fig. 6. The figure depicts printing defects in inkjet printed layer, while the evaporated Ag film has very smooth profile. The roughness values, root mean square height (Rms), were 0.37 and 1.9 nm for evaporated Ag and inkjet printed Ag, respectively. The roughness profile of inkjet printed Ag film has slightly striped pattern due to merging of individual printed lines at inkjet printing process. However, no spikes or holes we found in the inkjet printing layer, making it fully compatible for the manufacturing OPV devices. Finally, all-solution processed devices with inkjet printed top (grids) and bottom (full area) Ag electrodes were fabricated. Devices with two inkjet printed electrodes have lower Jsc and
PCE compare to the device with evaporated top grids electrode (see Fig. 4), due to grids surface coverage. However, these devices have slightly higher PCE compared to ITO-based device with inkjet printed top electrode (see Table 1), due to reflectivity of Ag. Devices with two inkjet printed Ag electrodes have average PCE of 2.4%, which can be further improved by increasing of transparency (grids surface coverage) of top electrode, improving the quality and reflectivity of bottom inkjet printed Ag electrode and optimization of layer thicknesses in the devices, which can be done via optical modeling. The all-solution processed devices in this study have so-called inverted cell architecture and have absolutely the same layer sequence as reported earlier [15], but only in reverse order (see Fig. 7a). Typical JV curves of such devices are shown in Fig. 7b. The JV curves show big difference in Voc for conventional and inverted devices. The difference in Voc between regular and inverted devices have been observed numerous times [45,46]. However
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Fig. 6. Optical interferometry images and roughness of evaporated (a) and inkjet printed (b) Ag films.
Fig. 7. (a) Schematic illustration of all-solution processed organic solar cells with inverted and conventional architecture, and (b) JV curves of these devices.
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in most of the cases the difference can be explained by the usage of different electron and hole transport materials and different contact materials. Petoukhoff et al. [45] have calculated the electronic
and optical performance parameters for various metal electrodes, with and without metal oxide/fluoride coatings for inverted and conventional bulk-heterojunction organic photovoltaic devices. They have concluded that the presence of strong interface dipoles (at the cathode or anode of the conventional or inverted devices respectively), which may form a strong interacting layer, reduces the energy loss for the charge collection at the respective electrode, and increased Voc. Boix et al. [46] systematically observed that the open-circuit voltage Voc at 1 sun illumination results higher for inverted cells than that achieved by regular structures in DVoc 30–50 mV (both types of devices contain PEDOT:PSS, as hole transport layer, and ZnO, as electron transport layer). This shift correlates with the displacement of the flat-band potential Vfb extracted from Mott Schottky capacitance analysis. A coherent picture is provided that states the hole Fermi level of the polymer highest occupied molecular orbital as an energy reference for both Voc and Vfb. The study connects the position of the hole Fermi level to the p-doping character of the active layer that is influenced by the film morphology through vertical phase segregation. Finally, Voc is determined by the quasi Fermi level splitting of the electron and hole and in organic solar cells maximized by the energy of the charge transfer state, provided that the electrodes form Ohmic contacts [39,47,48]. For non-Ohmic contacts, Voc will be less and limited by the work function of the electrode [39]. We note that the nature of electronic contact formation between the electrode and active layer can be complicated since a wide variety of interfacial effects including charge transfer, dipole formation, and the formation of interface states can occur depending on the type and strength of interactions between the two materials and the type of contact formation [49]. These conclusions agrees very well with our results. In the devices containing absolutely the same layers (all-solution processes conventional and inverted devices as shown in Fig. 7) the difference in Voc can be explained only by the order of layers deposition. Vertical phase separation in P3HT:PCM layer suggests a significant increase of the P3HT crystallinity at the top regions of the blend films with additional PCBM segregated to the bottom interface [50–52]. The vertical phase separation is attributed to the surface energy difference of the components and their interactions
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with the substrates material. This inhomogeneous distribution of the donor and acceptor components significantly affects the photovoltaic device performance and makes the inverted device structure a promising choice. However, at the current stage of research this has been proven only for P3HT:PCBM as a photoactive material, other absorber materials have to be further investigated.
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All-solution processed organic solar cells with inverted device architecture were demonstrated. Conventional ITO electrode was substituted by non-transparent inkjet printed layer of Ag. Devices with top illumination were fabricated. Semitransparent top electrode consists of highly conducting PEDOT:PSS and inkjet printed current collecting grids. All-solution processed devices with two inkjet printed Ag electrodes have average PCE of 2.4%, which can be further improved by increasing of transparency (grids surface coverage) of top electrode, improving the quality and reflectivity of bottom inkjet printed Ag electrode and optimization of layer thicknesses in the devices. Comparison of all-solution processed devices with conventional and inverted architectures revealed a big difference in Voc due to the difference in the morphology of P3HT:PCBM layer through vertical phase segregation. This significantly affects PCE and makes the inverted device structure a promising choice.
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[41,42].
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References
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