• Energy Research

Abstract Patterns for fully printed polymer solar cells are presented that inherently enable scaling of the power output with single point electrical energy connection is presented. Connection is made to only one end of the printed foil that can be rolled out for light energy harvesting. The power level is simply increased/decreased by increasing/decreasing the length of the foil with a corresponding increase/decrease in operating voltage. The current flow runs in both directions along the printed foil thus alleviating the need for post process addition of complex busbar topologies. The power conversion takes place in a HVDC–DC converter that is tailored specifically for operation with polymer solar cells by regulation on the input side. The system charges a lithium-polymer battery thus enabling storage of 82 Wh for a printed OPV foil measuring 0.305 m×9 m having a nominal power output of at least 15 W (AM1.5G, 1000 W m −2 ). As a demonstration we present a scalable fully integrated and compact power unit for mobile applications comprising solar energy harvesting OPV modules, power conversion and storage. Applications possible include electrical charging of mobile devices, illumination using LED lamps and low mechanical power applications such as pumping water.

Abstract Inflection point behaviour is often observed in the current–voltage (IV) curve of polymer solar cells. This phenomenon is examined in the context of flexible roll-to-roll (R2R) processed polymer solar cells in a large series of devices with a layer structure of: PET–ITO–ZnO–P3HT:PCBM–PEDOT:PSS–Ag. The devices were manufactured using a combination of slot-die coating and screen printing; they were then encapsulated by lamination using a polymer based barrier material. All manufacturing steps were carried out in ambient air. The freshly prepared devices showed a consistent inflection point in the IV curve and a corresponding poor performance or lack of photovoltaic behaviour. Upon exposure to 1000 Wm−2 illumination at ca. 85 °C and repeated IV scans (photo-annealing) the inflection point gradually disappeared, and performance drastically increased over time. The characteristics and stability of this “photo-annealing” behaviour was further investigated by studying the effects of several key factors: temperature, illumination and atmosphere. The results consistently showed that the inflection point is a dynamic phenomenon which can be removed under specific conditions. Subsequently, chemical characterization of device interfaces was carried out in order to identify possible chemical processes that are related to photo-annealing. A possible mechanism based on ZnO photoconductivity, photooxidation and redistribution of oxygen inside the cell is proposed, and it is anticipated that the findings are applicable to various other device structures based on semi-conducting oxides. The findings may have influences on the possibilities and scale-up of polymer solar technologies.

AbstractA solution‐processed silver film is employed in the processing of top‐illuminated indium‐tin‐oxide (ITO)‐free polymer solar cells in single‐ and double‐junction (tandem) structures. The nontransparent silver film fully covers the substrate and serves as the bottom electrode whereas a PEDOT:PSS/Ag grid forms the semitransparent top electrode. All layers are roll‐coated/printed on a flexible substrate by using only two techniques: slot–die coating for up to 11 consecutive layers and flexo‐printing for the last Ag grid layer. The slot–die coated Ag film is compared to an evaporated Ag film in terms of surface morphological and topographical properties and to ITO in terms of flexibility. The slot–die coated Ag film demonstrates extremely low roughness (a root‐mean‐square roughness of 3 nm was measured over 240×320 μm2 area), is highly conductive (<1 Ω/□), highly flexible, and cost‐effective in comparison to other reported metal films applied in polymer solar cells. Such properties result in high fill factors exceeding 50 % in both single and tandem structures on large‐area devices (1 cm2) and the corresponding efficiencies exceed 2 %.

Abstract We demonstrate a method for the preparation of multijunction polymer solar cells without the use of vacuum evaporation methods or indium tin oxide (ITO). The entire layer stack is prepared by printing or coating of each layer. The number of layers typically employed in complete devices exceeds ten and to efficiently identify layers and interfaces that are not robust we developed a double sided illumination method and demonstrate how layer thicknesses can be optimized with respect to the roll processing in the aim of achieving functional tandem devices. The devices were prepared directly on barrier foil and were later encapsulated. In this study the same active material comprising poly-3-hexylthiophene (P3HT) and phenyl-C 61 -butyric acid methyl ester ([60]PCBM) was employed using nanoparticle based zinc oxide for electron selectivity and several different PEDOT:PSS formulations for hole selectivity, electrode- and recombination layer formation. A novel slanted comb silver grid electrode structure was employed to enable efficient double sided illumination and minimize shunts. The operational stability of the tandem devices evaluated under ISOS-D-2 conditions demonstrated less variation in stability between devices than similar single junctions prepared in the same manner for reference. We demonstrate lifetime studies for 480 h without any sign of degradation and estimate that the tandem or multijunction polymer solar cells are as stable as single junctions.

Abstract We report manufacture of fully printed and coated polymer solar cells on a small scale roll-to-roll coater representing the intermediate scale between laboratory and pilot scale. We highlight the enormous span in scale between the laboratory scale and the intended industrial scale by a factor of >100.000 and detail how this enormous scale must be covered by equipment that follow the scale. Especially the intermediate scale between equipment that can fit inside a fume cupboard and the typical pilot equipment with a footprint having the size of a large room presents a challenge that comprises some of the most critical steps in the scaling process. We describe the development of such a machine that comprise web guiding, tension control and surface treatment in a compact desk size that is easily moved around and also detail the development of a small cassette based flexographic unit for back electrode printing that is parsimonious in terms of ink usage and more gentle than laboratory scale flexo units where the foil transport is either driven by the flexo unit or the flexo unit is driven by the foil transport. We demonstrate fully operational flexible polymer solar cell manufacture using this new roll and roll-to-roll (R3) approach and compare with the existing methods.

Accurate characterization and reporting of organic photovoltaic (OPV) device performance reniains one of the important challenges in the field. The large spread among the efficiencies of devices with the same structure reported by different groups is significantly caused by different procedures and equipment used during testing. The presented article addresses this issue by offering a new method of device testing using "suitcase sample" approach combined with outdoor testing that limits the diversity of the equipment, and a strict measurement protocol. A round robin outdoor characterization of roll-to-roll coated OPV cells and modules conducted among 46 laboratories worldwide is presented, where the samples and the testing equipment were integrated in a compact suitcase that served both as a sample transportation tool and as a holder and test equipment during testing. In addition, an internet based coordination was used via plasticphotovoltaics.org that allowed fast and efficient communication among participants and provided a controlled reporting format for the results that eased the analysis of the data. The reported deviations among the laboratories were limited to 5% when compared to the Si reference device integrated in the suitcase and were up to 8% when calculated using the local irradiance data. Therefore, this method offers a fast, cheap and efficient tool for sample sharing and testing that allows conducting outdoor measurements of OPV devices in a reproducible manner. (C) 2014 Elsevier B.V. All rights reserved.

The investigation of degradation of seven distinct sets (with a number of individual cells of n $ 12) of state of the art organic photovoltaic devices prepared by leading research laboratories with a combination of imaging methods is reported. All devices have been shipped to and degraded at Risø DTU up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. Imaging of device function at different stages of degradation was performed by laser-beam induced current (LBIC) scanning; luminescence imaging, specifically photoluminescence (PLI) and electroluminescence (ELI); as well as by lock-in thermography (LIT). Each of the imaging techniques exhibits its specific advantages with respect to sensing certain degradation features, which will be compared and discussed here in detail. As a consequence, a combination of several imaging techniques yields very conclusive information about the degradation processes controlling device function. The large variety of device architectures in turn enables valuable progress in the proper interpretation of imaging results—hence revealing the benefits of this large scale cooperation in making a step forward in the understanding of organic solar cell aging and its interpretation by state-of-the-art imaging methods.