Abundance, fast manufacture, and low cost are what ideally epitomize organic and polymer photovoltaics. However, they have remained esoteric (in physical form) almost since their inception and though they have been extensively studied they cannot be said to be generally available to the public with the exception of a few samples. It is obvious that to qualify as a technology, polymer photovoltaics have to be generally available in significant quantities. We recently reported a fast, efficient combined printing and coating method1 that enabled roll-to-roll processing of the polymer solar cell stack directly onto almost any flexible material, which ideally comprises a thin flexible barrier substrate.
Herein we describe the fabrication of 20 928 small modules (10.0×14.2 cm2) directly on barrier foil by employing a newly designed front electrode grid. This type of encapsulation results from efficient edge sealing by laser-cutting of the final modules. These “freeOPV” modules are, as the name suggests, made freely available to anyone who registers on our scientific website.2 The general idea behind the establishment of such a program is that the power of analysis is closely linked to the amount of available data and we thus encourage feedback from any technical or scientific study regardless of its nature. The website will furthermore function as a platform through which new materials can be evaluated in the context of this new module.
The indium-tin oxide (ITO)-free solar cell modules were prepared by using previously described procedures,1 although this current work was performed at higher speeds and with a module design specific to this purpose. A few distinct advances and differences are described in the following paragraphs. An illustration of the complete solar-cell stack is shown in Figure 1.
The front silver grid of the solar cell is processed by flexoprinting at high speed (20 m min−1). We have previously reported the use of a hexagonal front grid made of silver in combination with highly conductive poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as a transparent electrode, but more-detailed studies have shown that the presence of small electrical shorts in the solar cell is much more pronounced in the areas where the silver front electrode grid and the silver back electrode grid overlap. Ideally a design should be developed for which no overlap occurs, but considering the current control of registration (horizontally and in the web direction) and the unpredictable thermal shrinkage and stretching (in the cross-web and in the web direction respectively) of the foil during the process, it is not currently possible to handle such precision at sufficiently high speeds. As an alternative approach, a comb structure with slants of ±5° for the respective grids has been chosen. In such a design there is only one region of overlap (or a maximum of two) thus minimizing the number of likely electrical shorts in the structure. Figure 2 a and b shows light-beam-induced current (LBIC) and dark lock-in thermography (DLIT) images of a module for which three of the eight cells are heavily shunted. In the thermographic image the shunted silver comb lines light up due to dissipation of heat at the short circuit located there. Careful analysis of the DLIT image using DLIT microscopy reveals just one point of contact. This technique (Figure 2 c) enables extremely high resolution in the thermal image. The grid line is 100 µm wide and the pixel size in the infrared image is 3 µm. We found this new IR microscopy technique to be of exceptional value for the development of the grid electrode.
An extremely important factor in the operation of organic solar cells and modules is to ensure that the cells are properly encapsulated, while retaining the essential access to the electrodes. The previous method was to protect the organic solar cells by applying a barrier foil containing a pressure-sensitive adhesive over the active area while leaving the electrodes exposed for external access.3 However, such sealing is very sensitive to the slow diffusion from the edges of the seal (a distance of a few millimeters). As an alternative, we present a method for encapsulating the solar cells by using a UV-curable adhesive (DELO Katiobond LP655). Access to the electrodes is subsequently achieved by piercing the finished solar cells through the area where a thick conductor is printed (we have employed both carbon and silver) with a metal push button (Figure 3).
The adhesive was applied to the encapsulation barrier foil by flexoprinting (30 cm3 m−2 anilox cylinder) and this foil was fed into a nip together with the solar cells where the combined foil was subsequently exposed to UV light from an array of twelve lamps. The lamination process was performed at a web speed of 2 m min−1. The area containing the extra-thick silver layer (“Ag connector” in Figure 1) was used to make electrical contact after lamination by piercing a nickel-free metal connector through the foil.
The finished solar cells were finally cut into individual units by laser cutting using a 90 W roll-to-roll CO2 laser with a laser speed of 4.5 m min−1. Besides the obvious issue of speed an additional advantage of laser cutting is that it minimizes the mechanical stress at the edges of the solar cell that would certainly be present if the cells were cut by conventional means using a knife. It is furthermore reasonable to assume that melting of the substrate at the edges actually seals the multilaminate further and avoids introducing a delamination defect/fracture that has a tendency to propagate. Figure 4 shows a photo of the laser-cutting process (a movie showing the process can be found in the Supporting Information).
Despite the high speed of production the precision and accuracy in each step of coating, printing, encapsulating, and cutting provides high consistency in the device performance with a very low percentage of defective or malfunctioning devices. Figure 5 a shows the distribution of the photovoltaic parameters for 80 samples randomly chosen from the roll. The I–V curve of a typical sample is shown in Figure 5 b.
The results of short-term stability measurements in accordance with the International Summit on OPV Stability (ISOS-L and ISOS-D)4 suggest that this new generation of samples is approximately as stable as its roll-to-roll processed predecessors, which have been shown to exceed 10 000 h lifetime under outdoor exposure conditions.5 As a result of the aforementioned full encapsulation, the cells also exhibit an extremely long shelf life that allows for the samples to be shipped across long distances without degradation of the performance. However, due to their highly flexible nature the cells can be sensitive towards constant handling, excessive flexing, mechanical stresses, and heating, which may introduce flaws in the encapsulation and deteriorate the stability. Thus, the lifetime of such a sample is linked to its use, application, and handling during shipment. For the first experiment announced on the website, we aim to establish how the modules are affected by shipment without packaging (i.e., by sending them as a postcard). Initial results are promising and the interested reader can still participate in this study.
As mentioned above, the aim of distributing freeOPV samples is to generate a platform from which the roll-to-roll processing technology can be evaluated. Such evaluation is most efficiently performed with the technology freely available for everyone. Organic solar cell research has been conducted for more than 25 years now with the vision of mass-produced flexible roll-to-roll processed solar cells, but only a few researchers have actually had a flexible organic solar cell in their hands. It is our belief that the research progress is best evaluated by using comparisons among results with the same processing origin and this platform is intended to provide such an origin. For the same reasons the transparent ITO-free substrate Flextrode2 is also freely available.
All freeOPV samples are equipped with a 2D barcode giving them a unique ID and full tracability. It will thus be possible to retrieve and reference all information on the processing and handling of a given cell and the platform will allow the receiver to give feedback on his/her specific cell through the website.2 The purpose of this is to develop a methodology for processing, testing, and distributing an enormous amount of solar cells with minimal influence from a human operator while maintaining full traceability. The software we developed for this purpose can be experienced on the website. The 2D barcode can be scanned with any modern mobile device; for those without access to such a device there is also a code that can be typed in for extraction of the information. Again the purpose of the effort is to organize a fully automated platform for handling every aspect of the module along the value chain (preparation, distribution, service, and decommission).
The platform is furthermore thought of as a possible vector for testing of new materials in a roll-to-roll context. Very few research groups have access to roll-to-roll processing equipment for testing of their new materials and the platform will provide a means to do so. The module structure described here is sufficiently refined to enable development of new active materials and interface layers for this structure; pending success new results can be rapidly integrated in future freeOPV samples, again in a way that everyone can test and see for themselves. We have thus chosen an initial standard with this first freeOPV sample and a direct comparison can be performed by substituting just one or more of the components.
We have devised an efficient method to prepare small, flexible, ITO-free polymer solar-cell modules directly on barrier foil. The method is in principle generic and though we have exemplified the modules here with P3HT:PCBM as the commonly known active material, this methodology can also serve as a generic platform for the development of new and more-effective materials combinations, both with respect to performance and stability. The modules are true to the art in the sense that they are flexible, prepared using fast printing and coating methods, and of such a low cost that they can be made freely available to the public through a website. In fact the postage of the solar cell is significantly more expensive than the solar cell itself. All conceivable scientific or technical studies are encouraged and welcomed regardless of their nature.