Organic Photovoltaic Cells with All Inkjet Printed ...

Organic Photovoltaic Cells with All Inkjet Printed Layers and Freedom of Form Tamara M. Eggenhuisen1, Yulia Galagan1, Anne Biezemans2, Michiel Coenen1, Jan Gilot1, Pim Groen1,3, and Ronn Andriessen1 1

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Holst Centre, Eindhoven, 5656AE, The Netherlands ECN Solar Energy, Eindhoven, 5656AE, The Netherlands 3 Delft University, 2629HS, The Netherlands

Abstract — Large volume production of organic photovoltaics by roll-to-roll compatible techniques is a field of intensive research. Inkjet printing is a well-known deposition technique in the graphical and textile industry, and has several advantages for the production of OPV as it is contactless and has economic materials use. More importantly, cells and modules can be directly patterned during R2R production and by digital fabrication of OPV altering the cell or module design does not require changes of hardware. This makes inkjet printing suitable for OPV with unconventional shapes, but also allows for customizable large scale production. Therefore, inkjet printing offers the flexibility required at this stage of technological and market development of OPV. We have been able, for the first time, to create fully inkjet printed OPVs with a performance of more than 75% of its reference prepared by spin coating and evaporation. Large areas were printed in single passes with an industrial printer head using non-halogenated solvents only. An inverted OPV stack of 6 layers was printed using 4 types of inks. ITO was replaced by an inkjet printed Ag current collecting grid combined with highly conducting PEDOT:PSS. In this contribution we will discuss the additive effect of printing multiple layers on the OPV performance. Furthermore, the performance of cells of different shapes and sizes (up to 6.5 cm2) will be discussed. This work confirms the potential of inkjet printing for OPV as well as printed electronics in general. Index Terms — organic solar cells, inkjet printing, large area, customization.

I. INTRODUCTION Organic photovoltaics (OPVs) have several benefits over conventional silicon PV, such as freedom of shape, semitransparency, light weight and the possibility of high volume production with a favorable energy payback time. Currently, OPV appear in selected applications such as portable electronic devices. However, large area OPV films are foreseen to contribute to energy harvesting when integrated in facades, buildings, greenhouses or in public transportation. For large area OPV modules on flexible substrates, roll-to-roll (R2R) production will allow for high enough production throughput. On lab scale, solution processing is commonly performed using spin coating as the main deposition technique, however R2R production requires alternative deposition techniques and

industrially compatible synthesis procedures for large availability of material. Several technologies are available to deposit one or multiple layers of OPV, e.g. slot die coating, curtain coating, gravure printing, screen printing, spray coating or inkjet printing. Inkjet printing has shown its versatility in the past decades, and is an industrial production technique in the graphical, textile and ceramics industry. Being a drop-on-demand deposition technique it has an economic material use and allows for direct patterning of separate cells and modules during R2R production, making post-processing steps unnecessary. Furthermore, when using digital fabrication altering the cell or module layout does not require (often) expensive replacement of hardware. Most lab-scale inkjet printing apparatus have a limited number of nozzles, making the printing of large areas slow. Here we use an industrial printer head with a printing width of 3.5 cm, allowing printing of large areas in a single pass at speeds compatible to R2R web speeds. We aim to demonstrate the feasibility of inkjet printing for the full digital fabrication of OPV. We here evaluate stepwise the effect of printing consecutively six electro-active layers in an inverted OPV stack starting from a glass substrate. A combination of Ag grids and highly conductive PEDOT replaces ITO as transparent electrode. Subsequently ZnO, photoactive layer (PAL) and PEDOT are printed as core of the stack and are finally finished by full area Ag. II. EXPERIMENTAL Glass substrates were cleaned in several rinsing, ultrasound treatment and scrubbing with Teepol steps. The SunTronic U 5603 silver nanoparticle ink from Sun Chemical was used for the inkjet printing of the conductive grids and full area electrode. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Agfa, Orgacon-IJ 1005) was provided by Agfa. ZnO nanoparticles were synthesized in house via published procedure.[1] Poly(3-hexylthiophene) (P3HT, Merck, Lisicon SP001, Mw~19 kg/mol,) and [6,6] phenyl C61-butyric acid methyl ester (PCBM, 99%, Solenne BV) were used as received.

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b Ag (ev) MoO3 (ev) PAL (sc) ZnO (sc) ITO Substrate

c Ag (ev) PEDOT (ijp) PAL (ijp) ZnO (sc) ITO Substrate

d Ag (ev) MoO3 (ev) PAL (sc) ZnO (ijp) PEDOT (ijp) Ag Ag Substrate

e Ag (ijp) PEDOT (ijp) PAL (ijp) ZnO (ijp) PEDOT (ijp) Ag Ag Substrate

Ag (ijp) PEDOT (ijp) PAL (ijp) ZnO (sc) ITO Substrate

Fig. 1: Schematic representation of the different stacks tested to come to a fully inkjet printed solar cell: a) reference spin coated cell, b) inkjet printed photoactive and hole transporting layer, c) inkjet printed bottom electrode, d) inkjet printed top electrode, e) fully inkjet printed organic solar cell.

III. CONSECUTIVE INKJET PRINTED LAYERS Most lab-scale solar cells currently produced use spin coating and evaporation as most convenient techniques for testing different materials in an OPV stack. For OPV to become an industrial product, several aspects of an OPV stack need to be exchanged for large area compatibility with high throughput processes and economic material usage. Here we describe a stepwise the transition towards an all inkjet printed solar cell. As a reference cell a glass/ITO substrate was spin coated with ZnO and PAL and completed with the evaporation of a MoO3/Ag top electrode (Fig. 1a). In a first step the spin coating of the PAL and the evaporation of the hole transporting layer (MoO3) were replaced by 2 inkjet printed layers: the photoactive layer from a non-chlorinated solvent system and PEDOT (Fig. 1b). Hardly any loss in performance was observed (-8%) (Fig. 2). The loss in current is caused by a change in light distribution in the cell due to a thicker PEDOT compared to MoO3. A second step was the replacement of ITO by an alternative transparent electrode. ITO has a limited sheet resistance (~15Ω/□) causing resistive losses in solar cells when the cell width exceeds 1 cm. With the combination of Ag grids and highly conductive PEDOT much larger cell widths are achievable without significant loss in performance which in terms of upscaling allow for higher geometrical fill factors of

modules.[2] By inkjet printing of 3 layers (Ag, PEDOT and ZnO) some current is lost due to the blocking of light by the Ag grid and the absorbing PEDOT compared to ITO/ZnO. Therefore the current is reduced compared to the reference cell (-13%), but the overall performance of open-circuit voltage and fill factor remains similar (Fig. 2). 6 4 2

J (mA/cm2)

Inkjet printing of Ag was performed using a FujuFilm Dimatix Materials Printer (DMP 2831). Inkjet printing of all other layers (PEDOT:PSS, ZnO and photoactive layer) was performed on a LP50 printing platform (Pixdro, OTB) using an industrial printer head (3.5 cm width, 360 DPI) and nonhalogenated solvents only. Layer thicknesses were obtained by Dektak profilometry. Cross sections for analysis with transmission electron microscopy were prepared using a Nova 200 Nanolab Small Dual Beam. Current–voltage curves were measured using simulated AM 1.5 global solar irradiation (100 mW/cm2), using a halogen lamp with filters for 1 cm2 devices and a WXS-300S-50 solar simulator (WACOM Electric Co.) for the large area modules.

SC reference IJP PAL/PEDOT IJP Ag/PEDOT/ZnO IJP PAL/PEDOT/Ag All IJP

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V (V) Fig. 2: Current-voltage characteristics of the different stacks tested.

The third step is to replace the evaporated top MoO3/Ag electrode by an inkjet printed PEDOT and Ag combination. Now the current is somewhat lower than the cell (b) because the reflectivity of an inkjet printed Ag layer is lower than that of an evaporated layer (-12%) (Fig. 2). In a final step all the previous steps were combined in a fully inkjet printed solar cell (6 consecutive layers). It is worth to mention here that in this stack design extra caution has been taken to the 3D cell design because of a potential PEDOT/PEDOT interaction. When two highly conductive PEDOT layers are applied were one layer overlaps the other like in a tile configuration often used in striped slot die coating, there is a large change of a connection between the two layers due to imperfections (like coffee stain, particles or dust).[3] Therefore the design of the layers was in such a way that the photoactive layer was used as an insulating layer

between the two PEDOT layers with a larger area coverage. The top PEDOT was printed smaller to avoid contacting with the coffee stain edges of the bottom PEDOT. In some cases shunt burning helped to restore the cell performance. The all inkjet printed cell showed a very nice performance with similar open-circuit voltage and fill factor like the previous inkjet printed cells (f). The loss in current is now the combination of all previously mentioned losses. However the all inkjet printed cell shows an efficiency which is still more than 75% of the spin coated and evaporated cell. IV. FREEDOM OF SHAPE The real advantage of inkjet printing over other large area deposition technologies is the drop-on-demand functionality that allows for a completely digital freedom of shape. As a proof of concept a special all inkjet printed Christmas card was prepared with Christmas trees and the text “Merry Christmas” printed with Ag (Fig. 4). Here the same layers were inkjet printed as described before. The total active area of the tree was 6.5 cm2. The performance of the tree was low because of the non-optimal series resistance of this inkjet printed Ag grid, but the tree was working nevertheless (Fig. 3). Also leafs with the Ag grid veins as electrode were processed to compile in an artificial tree. This can be seen in the background of the Christmas card (Fig. 4).

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V. CONCLUSIONS

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Upscaling of organic solar cells asks for alternative techniques than spin coating and evaporation. In this article all layers in a traditional cell stack are replaced by in total 6 inkjet printed layers. Despite a loss in current due to modified light distribution, reflection of Ag and absorption of PEDOT, the performance of the all inkjet printed cell was more than 75% of the reference cell due to a stable open-circuit voltage and fill factor. With this technology leafs and Christmas trees were realized on larger area. These findings proof the potential of inkjet printing as an industrial viable technique for large area, customized organic solar cells.

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J (mA/cm2)

Fig. 4: Examples of the potential of freedom of shape by inkjet printing: (clockwise) design of all layers in a Christmas tree; OPV leaf with Ag grid veins; all inkjet printed OPV Christmas card; all inkjet printed Christmas tree.

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V (V) Fig. 3: Current-voltage characteristics of an all inkjet printed organic solar cell Christmas tree.

REFERENCES [1] W. J. E. Beek, M. M. Wienk, M. Kemerink, X. Yang and R. A. J. Janssen, “Hybrid Zinc Oxide Conjugated Polymer Bulk Heterojunction Solar Cells”, Journal of Physical Chemistry B, vol. 109, pp. 9505-9516, 2005. [2] Y. Galagan, B. Zimmermann, E. W. C. Coenen, M. Jørgensen, D. M. Tanenbaum, F. C. Krebs, H. Gorter, S. Sabik, L. H. Slooff, S. C. Veenstra, J. M. Kroon and R. Andriessen, “Current Collecting Grids for ITO-Free Solar Cells”, Advanced Energy Materials, vol. 2, pp. 103-110. 2012. [3] T. T. Larsen-Olsen, R. S. Søndergaard, K. Norrman, M. Jorgensen and F. C. Krebs, “All Printed Transparent Electrodes Through an Electrical Switching Mechanism: A Convincing Alternative to Indium-Tin-Oxide, Silver and Vacuum”, Energy & Environmental Science, vol. 5, pp. 9467-9471, 2012.